Telomerase Upregulation Induces Progression of Mouse BrafV600E-Driven Thyroid Cancers and Triggers Nontelomeric Effects

Mutations in the promoter of the telomerase reverse transcriptase (TERT) gene are the paradigm of a cross-cancer alteration in a noncoding region. TERT promoter mutations (TPM) are biomarkers of poor prognosis in cancer, including thyroid tumors. TPMs enhance TERT transcription, which is otherwise silenced in adult tissues, thus reactivating a bona fide oncoprotein. To study TERT deregulation and its downstream consequences, we generated a Tert mutant promoter mouse model via CRISPR/Cas9 engineering of the murine equivalent locus (Tert-123C>T) and crossed it with thyroid-specific BrafV600E-mutant mice. We also employed an alternative model of Tert overexpression (K5-Tert). Whereas all BrafV600E animals developed well-differentiated papillary thyroid tumors, 29% and 36% of BrafV600E+Tert-123C>T and BrafV600E+K5-Tert mice progressed to poorly differentiated cancers at week 20, respectively. Tert-upregulated tumors showed increased mitosis and necrosis in areas of solid growth, and older animals displayed anaplastic-like features, that is, spindle cells and macrophage infiltration. Murine TPM increased Tert transcription in vitro and in vivo, but temporal and intratumoral heterogeneity was observed. RNA-sequencing of thyroid tumor cells showed that processes other than the canonical Tert-mediated telomere maintenance role operate in these specimens. Pathway analysis showed that MAPK and PI3K/AKT signaling, as well as processes not previously associated with this tumor etiology, involving cytokine, and chemokine signaling, were overactivated. These models constitute useful preclinical tools to understand the cell-autonomous and microenvironment-related consequences of Tert-mediated progression in advanced thyroid cancers and other aggressive tumors carrying TPMs.

IMPLICATIONS

Telomerase-driven cancer progression activates pathways that can be dissected and perhaps therapeutically exploited.

Genomic and Transcriptomic Characteristics of Metastatic Thyroid Cancers with Exceptional Responses to Radioactive Iodine Therapy

PURPOSE

The determinants of response or resistance to radioiodine (RAI) are unknown. We aimed to identify genomic and transcriptomic factors associated with structural responses to RAI treatment of metastatic thyroid cancer, which occur infrequently, and to test whether high MAPK pathway output was associated with RAI refractoriness.

EXPERIMENTAL DESIGN

Exceptional response to RAI was defined as reduction of tumor volume based on RECIST v1.1. We performed a retrospective case-control study of genomic and transcriptomic characteristics of exceptional responders (ER; n = 8) versus nonresponders (NR; n = 16) matched by histologic type and stage at presentation on a 1:2 ratio.

RESULTS

ER are enriched for mutations that activate MAPK through RAF dimerization (RAS, class 2 BRAF, RTK fusions), whereas NR are associated with BRAFV600E, which signals as a monomer and is unresponsive to negative feedback. ER have a lower MAPK transcriptional output and a higher thyroid differentiation score (TDS) than NR (P < 0.05). NR are enriched for 1q-gain (P < 0.05) and mutations of genes regulating mRNA splicing and the PI3K pathway. BRAFV600E tumors with 1q-gain have a lower TDS than BRAFV600E/1q-quiet tumors and transcriptomic signatures associated with metastatic propensity.

CONCLUSIONS

ER tumors have a lower MAPK output and higher TDS than NR, whereas NR have a high frequency of BRAFV600E and 1q-gain. Molecular profiling of thyroid cancers and further functional validation of the key findings discriminating ER from NR may help predict response to RAI therapy.

Association of Treatment Strategies and Tumor Characteristics With Overall Survival Among Patients With Anaplastic Thyroid Cancer: A Single-Institution 21-Year Experience

Importance

Survival outcomes for anaplastic thyroid cancer (ATC), the most aggressive subtype of thyroid cancers, have remained poor. However, targeted therapies and immunotherapies present new opportunities for treatment of this disease. Evaluations of survival outcomes over time with new multimodal therapies are needed for optimizing treatment plans.

Objective

To evaluate the association of treatment strategies and tumor characteristics with overall survival (OS) among patients with ATC.

Design, Setting, and Participants

This retrospective case series study evaluated the survival outcomes stratified by treatment strategies and tumor characteristics among patients with ATC treated at a tertiary level academic institution from January 1, 2000, to December 31, 2021. Demographic, tumor, treatment, and outcome characteristics were analyzed. Kaplan-Meier method and log rank test modeled OS by treatment type and tumor characteristics. Data were analyzed in May 2022.

Main Outcomes and Measures

Overall survival (OS).

Results

The study cohort comprised 97 patients with biopsy-proven ATC (median [range] age at diagnosis, 70 [38-93] years; 60 (62%) female and 85 [88%] White individuals; 59 [61%] never smokers). At ATC diagnosis, 18 (19%) patients had stage IVA, 19 (20%) had stage IVB, and 53 (55%) had stage IVC disease. BRAF status was assessed in 38 patients; 18 (47%) had BRAF-V600E variations and 20 (53%), BRAF wild type. Treatment during clinical course included surgery for 44 (45%) patients; chemotherapy, 41 (43%); definitive or adjuvant radiation therapy, 34 (RT; 35%); and targeted therapy, 28 (29%). Median OS for the total cohort was 6.5 (95% CI, 4.3-10.0) months. Inferior OS was found in patients who did not receive surgery (hazard ratio [HR], 2.12; 95% CI, 1.35-3.34; reference, received surgery), chemotherapy (HR, 3.28; 95% CI, 1.99-5.39; reference, received chemotherapy), and definitive or adjuvant RT (HR, 2.47; 95% CI, 1.52-4.02; reference, received definitive/adjuvant RT). On multivariable analysis, age at diagnosis (HR, 1.03; 95% CI, 1.01-1.06), tumor stage IVC (HR, 2.65; 95% CI, 1.35-5.18), and absence of definitive or adjuvant RT (HR, 1.90; 95% CI, 1.01-3.59) were associated with worse OS.

Conclusions and Relevance

This retrospective single-institution study found that lower tumor stage, younger age, and the ability to receive definitive or adjuvant RT were associated with improved OS in patients with ATC.

Telomerase reactivation induces progression of mouse Braf V600E -driven thyroid cancers without telomere lengthening

Mutations in the promoter of the telomerase reverse transcriptase ( TERT ) gene are the paradigm of a cross-cancer alteration in a non-coding region. TERT promoter mutations (TPMs) are biomarkers of poor prognosis in several tumors, including thyroid cancers. TPMs enhance TERT transcription, which is otherwise silenced in adult tissues, thus reactivating a bona fide oncoprotein. To study TERT deregulation and its downstream consequences, we generated a Tert mutant promoter mouse model via CRISPR/Cas9 engineering of the murine equivalent locus (Tert -123C>T ) and crossed it with thyroid-specific Braf V600E -mutant mice. We also employed an alternative model of Tert overexpression (K5-Tert). Whereas all Braf V600E animals developed well-differentiated papillary thyroid tumors, 29% and 36% of Braf V600E +Tert -123C>T and Braf V600E +K5-Tert mice progressed to poorly differentiated thyroid cancers at week 20, respectively. Braf+Tert tumors showed increased mitosis and necrosis in areas of solid growth, and older animals from these cohorts displayed anaplastic-like features, i.e., spindle cells and macrophage infiltration. Murine Tert promoter mutation increased Tert transcription in vitro and in vivo , but temporal and intra-tumoral heterogeneity was observed. RNA-sequencing of thyroid tumor cells showed that processes other than the canonical Tert-mediated telomere maintenance role operate in these specimens. Pathway analysis showed that MAPK and PI3K/AKT signaling, as well as processes not previously associated with this tumor etiology, involving cytokine and chemokine signaling, were overactivated. Braf+Tert animals remained responsive to MAPK pathway inhibitors. These models constitute useful pre-clinical tools to understand the cell-autonomous and microenvironment-related consequences of Tert-mediated progression in advanced thyroid cancers and other aggressive tumors carrying TPMs.

Yap governs a lineage-specific neuregulin1 pathway-driven adaptive resistance to RAF kinase inhibitors

BACKGROUND

Inactivation of the Hippo pathway promotes Yap nuclear translocation, enabling execution of a transcriptional program that induces tissue growth. Genetic lesions of Hippo intermediates only identify a minority of cancers with illegitimate YAP activation. Yap has been implicated in resistance to targeted therapies, but the mechanisms by which YAP may impact adaptive resistance to MAPK inhibitors are unknown.

METHODS

We screened 52 thyroid cancer cell lines for illegitimate nuclear YAP localization by immunofluorescence and fractionation of cell lysates. We engineered a doxycycline (dox)-inducible thyroid-specific mouse model expressing constitutively nuclear YAP^S127A, alone or in combination with endogenous expression of either Hras^G12V or Braf^V600E. We also generated cell lines expressing dox-inducible sh-miR-E-YAP and/or YAP^S127A. We used cell viability, invasion assays, immunofluorescence, Western blotting, qRT-PCRs, flow cytometry and cell sorting, high-throughput bulk RNA sequencing and in vivo tumorigenesis to investigate YAP dependency and response of BRAF-mutant cells to vemurafenib.

RESULTS

We found that 27/52 thyroid cancer cell lines had constitutively aberrant YAP nuclear localization when cultured at high density (NU-YAP), which rendered them dependent on YAP for viability, invasiveness and sensitivity to the YAP-TEAD complex inhibitor verteporfin, whereas cells with confluency-driven nuclear exclusion of YAP (CYT-YAP) were not. Treatment of BRAF-mutant thyroid cancer cells with RAF kinase inhibitors resulted in YAP nuclear translocation and activation of its transcriptional output. Resistance to vemurafenib in BRAF-mutant thyroid cells was driven by YAP-dependent NRG1, HER2 and HER3 activation across all isogenic human and mouse thyroid cell lines tested, which was abrogated by silencing YAP and relieved by pan-HER kinase inhibitors. YAP activation induced analogous changes in BRAF melanoma, but not colorectal cells.

CONCLUSIONS

YAP activation in thyroid cancer generates a dependency on this transcription factor. YAP governs adaptive resistance to RAF kinase inhibitors and induces a gene expression program in BRAF^V600E-mutant cells encompassing effectors in the NRG1 signaling pathway, which play a central role in the insensitivity to MAPK inhibitors in a lineage-dependent manner. HIPPO pathway inactivation serves as a lineage-dependent rheostat controlling the magnitude of the adaptive relief of feedback responses to MAPK inhibitors in BRAF-^V600E cancers.

BRAFV600E Expression in Thyrocytes Causes Recruitment of Immunosuppressive STABILIN-1 Macrophages

Simple Summary

Incidence of thyroid cancer, including papillary thyroid cancer, is rapidly increasing. Oncogenes, such as the BRAF^V600E, have been identified, and their effect on thyroid cancer cells have been studied in vitro and in mouse models. What is less understood is the impact of these mutations on thyroid cancer microenvironment and, in turn, the effect of changes in the microenvironment on tumor progression. We investigated the modifications in the cellular composition of thyroid cancer microenvironment using an inducible mouse model. We focused on a subpopulation of macrophages, expressing the STABILIN-1 protein, recruited in the thyroid tumor microenvironment following BRAF^V600E expression. CRISPR/Cas9 genetic inactivation of Stablin-1 did not change macrophage recruitment but highlighted the immunosuppressive role of STABILIN-1-expressing macrophages. The identification of a similar subpopulation of STABILIN-1 macrophages in human thyroid diseases supports a conserved role for these macrophages and offers an opportunity for intervention.

Abstract

Papillary thyroid carcinoma (PTC) is the most frequent histological subtype of thyroid cancers (TC), and BRAF^V600E genetic alteration is found in 60% of this endocrine cancer. This oncogene is associated with poor prognosis, resistance to radioiodine therapy, and tumor progression. Histological follow-up by anatomo-pathologists revealed that two-thirds of surgically-removed thyroids do not present malignant lesions. Thus, continued fundamental research into the molecular mechanisms of TC downstream of BRAF^V600E remains central to better understanding the clinical behavior of these tumors. To study PTC, we used a mouse model in which expression of BRAF^V600E was specifically switched on in thyrocytes by doxycycline administration. Upon daily intraperitoneal doxycycline injection, thyroid tissue rapidly acquired histological features mimicking human PTC. Transcriptomic analysis revealed major changes in immune signaling pathways upon BRAF^V600E induction. Multiplex immunofluorescence confirmed the abundant recruitment of macrophages, among which a population of LYVE-1+/CD206+/STABILIN-1+ was dramatically increased. By genetically inactivating the gene coding for the scavenger receptor STABILIN-1, we showed an increase of CD8+ T cells in this in situ BRAF^V600E-dependent TC. Lastly, we demonstrated the presence of CD206+/STABILIN-1+ macrophages in human thyroid pathologies. Altogether, we revealed the recruitment of immunosuppressive STABILIN-1 macrophages in a PTC mouse model and the interest to further study this macrophage subpopulation in human thyroid tissues.

Mitonuclear genotype remodels the metabolic and microenvironmental landscape of Hürthle cell carcinoma

Hürthle cell carcinomas (HCCs) display two exceptional genotypes: near-homoplasmic mutation of mitochondrial DNA (mtDNA) and genome-wide loss of heterozygosity (gLOH). To understand the phenotypic consequences of these genetic alterations, we analyzed genomic, metabolomic, and immunophenotypic data of HCC and other thyroid cancers. Both mtDNA mutations and profound depletion of citrate pools are common in HCC and other thyroid malignancies, suggesting that thyroid cancers are broadly equipped to survive tricarboxylic acid cycle impairment, whereas metabolites in the reduced form of NADH-dependent lysine degradation pathway were elevated exclusively in HCC. The presence of gLOH was not associated with metabolic phenotypes but rather with reduced immune infiltration, indicating that gLOH confers a selective advantage partially through immunosuppression. Unsupervised multimodal clustering revealed four clusters of HCC with distinct clinical, metabolomic, and microenvironmental phenotypes but overlapping genotypes. These findings chart the metabolic and microenvironmental landscape of HCC and shed light on the interaction between genotype, metabolism, and the microenvironment in cancer.

Abstract 986: RBM10 loss in thyroid cancer leads to aberrant splicing of cytoskeletal and extracellular matrix mRNAs and increased metastatic fitness

NGS studies implicate dysregulation of the splicing machinery in the development of diverse cancers. The X-chromosome RBM10 gene encodes an RNA-binding protein that modulates transcriptome-wide cassette exon splicing. Truncation and missense RBM10 mutations are enriched (11%) in non-anaplastic thyroid cancers of patients who died of metastatic disease. MSK-MET, an integrated pan-cancer cohort of tumor genomic and clinical outcome data showed RBM10 alterations associate with metastatic burden in thyroid cancer. Within the MSK-IMPACT thyroid cancer cohort, 44% of RBM10 alterations co-occur with RAS mutations. We developed a murine Rbm10 floxed allele, which results in a non-functional transcript. Thyroid-specific Rbm10 inactivation through Tpo-Cre did not induce a phenotype. When crossed with FR-HrasG12V mice, which upon recombination generate endogenous expression of HrasG12V, Hras/Rbm10 mice developed thyroid cancers, ~ 20% of which were metastatic to lung. Cell lines derived from these tumors showed high penetrance of lung metastases after tail vein or orthotopic implantation into the thyroid. We identified splicing targets of RBM10 by high depth RNAseq of 5 isogenic human thyroid cancer cell lines (2 RBM10-null with dox-induced RBM10; 3 RBM10 WT with RBM10 shRNA). The common abnormalities associated with RBM10 loss were exon inclusion events. Ingenuity Pathway Analysis of global transcriptomes in RBM10 isogenic human thyroid cancer cell lines showed enrichment in pro-migratory, aberrant integrin and RHO/RAC signaling expression signatures. Enriched GO analysis confined to genes subject to aberrant splicing by RBM10 loss extended these findings, with the top terms being cell adhesion, cytoskeleton, cadherin and integrin binding. RBM10 loss was associated with increased cell migration velocity in vitro as measured by time lapse imaging. Key cytoskeletal and extracellular matrix genes subject to exon inclusion events included vinculin (VCL), tenascin C (TNC), CD44, fibronectin (FN1) and tropomyosin 1 (TPM1). Isoform-specific knockdown of the VCL splice inclusion cassette exon that leads to illegitimate expression of metavinculin (mVCL) reduced cell migration, whereas isoform-specific knockdown of TNC and CD44 exon inclusion events reduced cell invasiveness in vitro. Rho GTPases transduce signals from the ECM to integrin receptors to regulate cell adhesion, migration, and invasiveness. Consistent with this, RBM10 overexpression in RBM10 null cells reduced RAC1-GTP levels. Finally, RBM10 re-expression in RBM10 null cells reversed metastatic competency in vivo. In conclusion, RBM10 loss alters the ratio of cassette exon inclusion events in a subset of transcripts that regulate interactions between the ECM and the cytoskeleton, leading to RHO/RAC activation and governing a process favoring increased cell movement and metastatic competence.

Citation Format: Gnana P. Krishnamoorthy, Anthony Glover, Brian Untch, Mahesh Saqcena, Dina Vukel, Katherine Berman, Omar Abdel-Wahab, Robert K. Bradley, Jeffrey A. Knauf, James A. Fagin. RBM10 loss in thyroid cancer leads to aberrant splicing of cytoskeletal and extracellular matrix mRNAs and increased metastatic fitness [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 986.

Abstract 913: Tert mutant promoter mouse model induces cancer progression in BrafV600E-driven thyroid tumors: A novel tool to understand the biology of telomerase-reactivated cancers

Hotspot mutations in the proximal promoter of the telomerase reverse transcriptase (TERT) gene are the first cross-cancer alterations lying in a gene regulatory region. TERT promoter mutations (TPMs) are enriched in advanced thyroid tumors and constitute markers of disease severity. TPMs enhance TERT transcription, which is otherwise silenced in adult tissues, thus reactivating a bone fide oncoprotein. To study TERT deregulation and its downstream consequences in a biologically accurate model, we generated a Tert-mutant promoter mouse model via CRISPR/Cas9 editing of the equivalent murine locus and crossed these animals with thyroid-specific BrafV600E-mutant mice. BrafV600E animals develop highly penetrant papillary thyroid tumors (PTC) by week 5, but do not progress. In contrast, BrafV600E+TertMUT animals showed an increased incidence of poorly differentiated thyroid cancers (PDTC) by 20 weeks (30% vs. 0% in BrafV600E; chi-squared P= 0.03), mimicking those exhibited by an alternative transgenic model of Tert overexpression (BrafV600E+K5-Tert; 36% PDTCs). Mouse Tert promoter mutation increased Tert transcription in vitro and in vivo, as reported in patients’ tumors carrying TPMs. Braf+Tert animals partially responded to MAPK pathway inhibition (dabrafenib plus trametinib), showing that MAPK signaling remains relevant in these specimens. Interestingly, RNA sequencing of Tert-reactivated murine thyroid tumors showed unique transcriptomic profiles (compared to BrafV600E alone), suggesting that downstream effects, other than the canonical Tert-mediated telomere maintenance, operate in cancers harboring TPMs. These cancer models of telomerase reactivation provide excellent pre-clinical settings to understand the regulatory mechanisms and biological underpinnings of TPM-induced progression of thyroid and other tumors, and to explore novel therapeutic strategies.

Citation Format: Inigo Landa, Jingzhu Hao, Bin Xu, Joseph Giacalone, Zach Herbert, Maria A. Blasco, Jeffrey A. Knauf, Ronald Ghossein, James A. Fagin. Tert mutant promoter mouse model induces cancer progression in BrafV600E-driven thyroid tumors: A novel tool to understand the biology of telomerase-reactivated cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 913.

Characterization of Subtypes of BRAF-Mutant Papillary Thyroid Cancer Defined by Their Thyroid Differentiation Score

CONTEXT

The BRAFV600E mutation has been associated with more advanced clinical stage in papillary thyroid cancer (PTC) and decreased responsiveness to radioiodine (RAI). However, some BRAF mutant PTCs respond to RAI and have an indolent clinical behavior suggesting the presence of different subtypes of BRAF mutant tumors with distinct prognosis.

OBJECTIVE

To characterize the molecular and clinical features of 2 subtypes of BRAF-mutant PTCs defined by their degree of expression of iodine metabolism genes.

DESIGN

227 BRAF-mutant PTCs from the Cancer Genome Atlas Thyroid Cancer study were divided into 2 subgroups based on their thyroid differentiation score (TDS): BRAF-TDShi and BRAF-TDSlo. Demographic, clinico-pathological, and molecular characteristics of the 2 subgroups were compared.

RESULTS

Compared to BRAF-TDShi tumors (17%), BRAF-TDSlo tumors (83%) were more frequent in blacks and Hispanics (6% vs 0%, P = 0.035 and 12% vs 0%, P = 0.05, respectively), they were larger (2.95 ± 1.7 vs 2.03 ± 1.5, P = 0.002), with more tumor-involved lymph nodes (3.9 ± 5.8 vs 2.0 ± 4.2, P = 0.042), and a higher frequency of distant metastases (3% vs 0%, P = 0.043). Gene set enrichment analysis showed positive enrichment for RAS signatures in the BRAF-TDShi cohort, with corresponding reciprocal changes in the BRAF-TDSlo group. Several microRNAs (miRs) targeting nodes in the transforming growth factor β (TGFβ)-SMAD pathway, miR-204, miR-205, and miR-144, were overexpressed in the BRAF-TDShi group. In the subset with follow-up data, BRAF-TDShi tumors had higher complete responses to therapy (94% vs 57%, P < 0.01) than BRAF-TDSlo tumors.

CONCLUSION

Enrichment for RAS signatures, key genes involved in cell polarity and specific miRs targeting the TGFβ-SMAD pathway define 2 subtypes of BRAF-mutant PTCs with distinct clinical characteristics and prognosis.

Enhancing Radioiodine Incorporation in BRAF-Mutant, Radioiodine-Refractory Thyroid Cancers with Vemurafenib and the Anti-ErbB3 Monoclonal Antibody CDX-3379: Results of a Pilot Clinical Trial

Background: Oncogenic activation of mitogen-activated protein kinase (MAPK) signaling is associated with radioiodine refractory (RAIR) thyroid cancer. Preclinical models suggest that activation of the receptor tyrosine kinase erbB-3 (HER3) mitigates the MAPK pathway inhibition achieved by BRAF inhibitors in BRAF^V600E mutant thyroid cancers. We hypothesized that combined inhibition of BRAF and HER3 using vemurafenib and the human monoclonal antibody CDX-3379, respectively, would potently inhibit MAPK activation and restore radioactive iodine (RAI) avidity in patients with BRAF-mutant RAIR thyroid cancer. Methods: Patients with BRAF^V600E RAIR thyroid cancer were evaluated by thyrogen-stimulated iodine-124 (¹²⁴I) positron emission tomography-computed tomography (PET/CT) at baseline and after 5 weeks of treatment with oral vemurafenib 960 mg twice daily alone for 1 week, followed by vemurafenib in combination with 1000 mg of intravenous CDX-3379 every 2 weeks. Patients with adequate ¹²⁴I uptake on the second PET/CT then received therapeutic radioactive iodine (¹³¹I) with vemurafenb+CDX-3379. All therapy was discontinued two days later. Treatment response was monitored by serum thyroglobulin measurements and imaging. The primary endpoints were safety and tolerability of vemurafenib+CDX-3379, as well as the proportion of patients after vemurafenb+CDX-3379 therapy with enhanced RAI incorporation warranting therapeutic ¹³¹I. Results: Seven patients were enrolled; six were evaluable for the primary endpoints. No grade 3 or 4 toxicities related to CDX-3379 were observed. Five patients had increased RAI uptake after treatment; in 4 patients this increased uptake warranted therapeutic ¹³¹I. At 6 months, 2 patients achieved partial response after ¹³¹I and 2 progression of disease. Next-generation sequencing of 5 patients showed that all had co-occurring telomerase reverse transcriptase promoter alterations. A deleterious mutation in the SWItch/Sucrose Non-Fermentable (SWI/SNF) gene ARID2 was discovered in the patient without enhanced RAI avidity after therapy and an RAI-resistant tumor from another patient that was sampled off-study. Conclusions: The endpoints for success were met, providing preliminary evidence of vemurafenib+CDX-3379 safety and efficacy for enhancing RAI uptake. Preclinical data and genomic profiling in this small cohort suggest SWI/SNF gene mutations should be investigated as potential markers of resistance to redifferentiation strategies. Further evaluation of vemurafenib+CDX-3379 as a redifferentiation therapy in a larger trial is warranted (ClinicalTrials.gov: NCT02456701).

Co-inhibition of SMAD and MAPK signaling enhances 124I uptake in BRAF-mutant thyroid cancers

Constitutive MAPK activation silences genes required for iodide uptake and thyroid hormone biosynthesis in thyroid follicular cells. Accordingly, most BRAFV600E papillary thyroid cancers (PTC) are refractory to radioiodide (RAI) therapy. MAPK pathway inhibitors rescue thyroid-differentiated properties and RAI responsiveness in mice and patient subsets with BRAFV600E-mutant PTC. TGFB1 also impairs thyroid differentiation and has been proposed to mediate the effects of mutant BRAF. We generated a mouse model of BRAFV600E-PTC with thyroid-specific knockout of the Tgfbr1 gene to investigate the role of TGFB1 on thyroid-differentiated gene expression and RAI uptake in vivo. Despite appropriate loss of Tgfbr1, pSMAD levels remained high, indicating that ligands other than TGFB1 were engaging in this pathway. The activin ligand subunits Inhba and Inhbb were found to be overexpressed in BRAFV600E-mutant thyroid cancers. Treatment with follistatin, a potent inhibitor of activin, or vactosertib, which inhibits both TGFBR1 and the activin type I receptor ALK4, induced a profound inhibition of pSMAD in BRAFV600E-PTCs. Blocking SMAD signaling alone was insufficient to enhance iodide uptake in the setting of constitutive MAPK activation. However, combination treatment with either follistatin or vactosertib and the MEK inhibitor CKI increased 124I uptake compared to CKI alone. In summary, activin family ligands converge to induce pSMAD in Braf-mutant PTCs. Dedifferentiation of BRAFV600E-PTCs cannot be ascribed primarily to activation of SMAD. However, targeting TGFβ/activin-induced pSMAD augmented MAPK inhibitor effects on iodine incorporation into BRAF tumor cells, indicating that these two pathways exert interdependent effects on the differentiation state of thyroid cancer cells.

SWI/SNF Complex Mutations Promote Thyroid Tumor Progression and Insensitivity to Redifferentiation Therapies

Mutations of subunits of the SWI/SNF chromatin remodeling complexes occur commonly in cancers of different lineages, including advanced thyroid cancers. Here we show that thyroid-specific loss of Arid1a, Arid2, or Smarcb1 in mouse BRAF^V600E-mutant tumors promotes disease progression and decreased survival, associated with lesion-specific effects on chromatin accessibility and differentiation. As compared with normal thyrocytes, BRAF^V600E-mutant mouse papillary thyroid cancers have decreased lineage transcription factor expression and accessibility to their target DNA binding sites, leading to impairment of thyroid-differentiated gene expression and radioiodine incorporation, which is rescued by MAPK inhibition. Loss of individual SWI/SNF subunits in BRAF tumors leads to a repressive chromatin state that cannot be reversed by MAPK pathway blockade, rendering them insensitive to its redifferentiation effects. Our results show that SWI/SNF complexes are central to the maintenance of differentiated function in thyroid cancers, and their loss confers radioiodine refractoriness and resistance to MAPK inhibitor-based redifferentiation therapies. SIGNIFICANCE: Reprogramming cancer differentiation confers therapeutic benefit in various disease contexts. Oncogenic BRAF silences genes required for radioiodine responsiveness in thyroid cancer. Mutations in SWI/SNF genes result in loss of chromatin accessibility at thyroid lineage specification genes in BRAF-mutant thyroid tumors, rendering them insensitive to the redifferentiation effects of MAPK blockade.This article is highlighted in the In This Issue feature, p. 995.

Vemurafenib Redifferentiation of BRAF Mutant, RAI-Refractory Thyroid Cancers

CONTEXT

BRAFV600E mutant thyroid cancers are often refractory to radioiodine (RAI).

OBJECTIVES

To investigate the utility and molecular underpinnings of enhancing lesional iodide uptake with the BRAF inhibitor vemurafenib in patients with RAI-refractory (RAIR).

DESIGN

This was a pilot trial that enrolled from June 2014 to January 2016.

SETTING

Academic cancer center.

PATIENTS

Patients with RAIR, BRAF mutant thyroid cancer.

INTERVENTION

Patients underwent thyrotropin-stimulated iodine-124 (124I) positron emission tomography scans before and after ~4 weeks of vemurafenib. Those with increased RAI concentration exceeding a predefined lesional dosimetry threshold (124I responders) were treated with iodine-131 (131I). Response was evaluated with imaging and serum thyroglobulin. Three patients underwent research biopsies to evaluate the impact of vemurafenib on mitogen-activated protein kinase (MAPK) signaling and thyroid differentiation.

MAIN OUTCOME MEASURE

The proportion of patients in whom vemurafenib increased RAI incorporation to warrant 131I.

RESULTS

Twelve BRAF mutant patients were enrolled; 10 were evaluable. Four patients were 124I responders on vemurafenib and treated with 131I, resulting in tumor regressions at 6 months. Analysis of research tumor biopsies demonstrated that vemurafenib inhibition of the MAPK pathway was associated with increased thyroid gene expression and RAI uptake. The mean pretreatment serum thyroglobulin value was higher among 124I responders than among nonresponders (30.6 vs 1.0 ng/mL; P = 0.0048).

CONCLUSIONS

Vemurafenib restores RAI uptake and efficacy in a subset of BRAF mutant RAIR patients, probably by upregulating thyroid-specific gene expression via MAPK pathway inhibition. Higher baseline thyroglobulin values among responders suggest that tumor differentiation status may be a predictor of vemurafenib benefit.

Mouse Models as a Tool for Understanding Progression in BrafV600E-Driven Thyroid Cancers

The development of next generation sequencing (NGS) has led to marked advancement of our understanding of genetic events mediating the initiation and progression of thyroid cancers. The NGS studies have confirmed the previously reported high frequency of mutually-exclusive oncogenic alterations affecting BRAF and RAS proto-oncogenes in all stages of thyroid cancer. Initially identified by traditional sequencing approaches, the NGS studies also confirmed the acquisition of alterations that inactivate tumor protein p53 (TP53) and activate phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) in advanced thyroid cancers. Novel alterations, such as those in telomerase reverse transcriptase (TERT) promoter and mating-type switching/sucrose non-fermenting (SWI/SNF) complex, are also likely to promote progression of the BRAF^V600E-driven thyroid cancers. A number of genetically engineered mouse models (GEMM) of BRAF^V600E-driven thyroid cancer have been developed to investigate thyroid tumorigenesis mediated by oncogenic BRAF and to explore the role of genetic alterations identified in the genomic analyses of advanced thyroid cancer to promote tumor progression. This review will discuss the various GEMMs that have been developed to investigate oncogenic BRAF^V600E-driven thyroid cancers.

Comprehensive Genetic Characterization of Human Thyroid Cancer Cell Lines: A Validated Panel for Preclinical Studies

PURPOSE

Thyroid cancer cell lines are valuable models but have been neglected in pancancer genomic studies. Moreover, their misidentification has been a significant problem. We aim to provide a validated dataset for thyroid cancer researchers.

EXPERIMENTAL DESIGN

We performed next-generation sequencing (NGS) and analyzed the transcriptome of 60 authenticated thyroid cell lines and compared our findings with the known genomic defects in human thyroid cancers.

RESULTS

Unsupervised transcriptomic analysis showed that 94% of thyroid cell lines clustered distinctly from other lineages. Thyroid cancer cell line mutations recapitulate those found in primary tumors (e.g., BRAF, RAS, or gene fusions). Mutations in the TERT promoter (83%) and TP53 (71%) were highly prevalent. There were frequent alterations in PTEN, PIK3CA, and of members of the SWI/SNF chromatin remodeling complex, mismatch repair, cell-cycle checkpoint, and histone methyl- and acetyltransferase functional groups. Copy number alterations (CNA) were more prevalent in cell lines derived from advanced versus differentiated cancers, as reported in primary tumors, although the precise CNAs were only partially recapitulated. Transcriptomic analysis showed that all cell lines were profoundly dedifferentiated, regardless of their derivation, making them good models for advanced disease. However, they maintained the BRAF^V600E versus RAS-dependent consequences on MAPK transcriptional output, which correlated with differential sensitivity to MEK inhibitors. Paired primary tumor-cell line samples showed high concordance of mutations. Complete loss of p53 function in TP53 heterozygous tumors was the most prominent event selected during in vitro immortalization.

CONCLUSIONS

This cell line resource will help inform future preclinical studies exploring tumor-specific dependencies.

Establishment and Characterization of Four Novel Thyroid Cancer Cell Lines and PDX Models Expressing the RET/PTC1 Rearrangement, BRAFV600E, or RASQ61R as Drivers

Cancer cell lines are critical models to study tumor progression and response to therapy. In 2008, we showed that approximately 50% of thyroid cancer cell lines were redundant or not of thyroid cancer origin. We therefore generated new authenticated thyroid cancer cell lines and patient-derived xenograft (PDX) models using in vitro and feeder cell approaches, and characterized these models in vitro and in vivo. We developed four thyroid cancer cell lines, two derived from 2 different patients with papillary thyroid cancer (PTC) pleural effusions, CUTC5, and CUTC48; one derived from a patient with anaplastic thyroid cancer (ATC), CUTC60; and one derived from a patient with follicular thyroid cancer (FTC), CUTC61. One PDX model (CUTC60-PDX) was also developed. Short tandem repeat (STR) genotyping showed that each cell line and PDX is unique and match the original patient tissue. The CUTC5 and CUTC60 cells harbor the BRAF (V600E) mutation, the CUTC48 cell line expresses the RET/PTC1 rearrangement, and the CUTC61 cells have the HRAS (Q61R) mutation. Moderate to high levels of PAX8 and variable levels of NKX2-1 were detected in each cell line and PDX. The CUTC5 and CUTC60 cell lines form tumors in orthotopic and flank xenograft mouse models. IMPLICATIONS: We have developed the second RET/PTC1-expressing PTC-derived cell line in existence, which is a major advance in studying RET signaling. We have further linked all cell lines to the originating patients, providing a set of novel, authenticated thyroid cancer cell lines and PDX models to study advanced thyroid cancer.

EIF1AX and RAS Mutations Cooperate to Drive Thyroid Tumorigenesis through ATF4 and c-MYC

Translation initiation is orchestrated by the cap binding and 43S preinitiation complexes (PIC). Eukaryotic initiation factor 1A (EIF1A) is essential for recruitment of the ternary complex and for assembling the 43S PIC. Recurrent EIF1AX mutations in papillary thyroid cancers are mutually exclusive with other drivers, including RAS. EIF1AX mutations are enriched in advanced thyroid cancers, where they display a striking co-occurrence with RAS, which cooperates to induce tumorigenesis in mice and isogenic cell lines. The C-terminal EIF1AX-A113splice mutation is the most prevalent in advanced thyroid cancer. EIF1AX-A113splice variants stabilize the PIC and induce ATF4, a sensor of cellular stress, which is co-opted to suppress EIF2α phosphorylation, enabling a general increase in protein synthesis. RAS stabilizes c-MYC, an effect augmented by EIF1AX-A113splice. ATF4 and c-MYC induce expression of amino acid transporters and enhance sensitivity of mTOR to amino acid supply. These mutually reinforcing events generate therapeutic vulnerabilities to MEK, BRD4, and mTOR kinase inhibitors. SIGNIFICANCE: Mutations of EIF1AX, a component of the translation PIC, co-occur with RAS in advanced thyroid cancers and promote tumorigenesis. EIF1AX-A113splice drives an ATF4-induced dephosphorylation of EIF2α, resulting in increased protein synthesis. ATF4 also cooperates with c-MYC to sensitize mTOR to amino acid supply, thus generating vulnerability to mTOR kinase inhibitors. This article is highlighted in the In This Issue feature, p. 151.

Hgf/Met activation mediates resistance to BRAF inhibition in murine anaplastic thyroid cancers

Anaplastic thyroid carcinomas (ATCs) have a high prevalence of BRAF and TP53 mutations. A trial of vemurafenib in nonmelanoma BRAFV600E-mutant cancers showed significant, although short-lived, responses in ATCs, indicating that these virulent tumors remain addicted to BRAF despite their high mutation burden. To explore the mechanisms mediating acquired resistance to BRAF blockade, we generated mice with thyroid-specific deletion of p53 and dox-dependent expression of BRAFV600E, 50% of which developed ATCs after dox treatment. Upon dox withdrawal there was complete regression in all mice, although recurrences were later detected in 85% of animals. The relapsed tumors had elevated MAPK transcriptional output, and retained responses to the MEK/RAF inhibitor CH5126766 in vivo and in vitro. Whole-exome sequencing identified recurrent focal amplifications of chromosome 6, with a minimal region of overlap that included Met. Met-amplified recurrences overexpressed the receptor as well as its ligand Hgf. Growth, signaling, and viability of Met-amplified tumor cells were suppressed in vitro and in vivo by the Met kinase inhibitors PF-04217903 and crizotinib, whereas primary ATCs and Met-diploid relapses were resistant. Hence, recurrences are the rule after BRAF suppression in murine ATCs, most commonly due to activation of HGF/MET signaling, which generates exquisite dependency to MET kinase inhibitors.

Tipifarnib Inhibits HRAS-Driven Dedifferentiated Thyroid Cancers

Of the three RAS oncoproteins, only HRAS is delocalized and inactivated by farnesyltransferase inhibitors (FTI), an approach yet to be exploited clinically. In this study, we treat mice bearing Hras-driven poorly differentiated and anaplastic thyroid cancers (Tpo-Cre/Hras^G12V/p53^flox/flox ) with the FTI tipifarnib. Treatment caused sustained tumor regression and increased survival; however, early and late resistance was observed. Adaptive reactivation of RAS-MAPK signaling was abrogated in vitro by selective RTK (i.e., EGFR, FGFR) inhibitors, but responses were ineffective in vivo, whereas combination of tipifarnib with the MEK inhibitor AZD6244 improved outcomes. A subset of tumor-bearing mice treated with tipifarnib developed acquired resistance. Whole-exome sequencing of resistant tumors identified a Nf1 nonsense mutation and an activating mutation in Gnas at high allelic frequency, supporting the on-target effects of the drug. Cell lines modified with these genetic lesions recapitulated tipifarnib resistance in vivo This study demonstrates the feasibility of targeting Ras membrane association in cancers in vivo and predicts combination therapies that confer additional benefit.Significance: Tipifarnib effectively inhibits oncogenic HRAS-driven tumorigenesis and abrogating adaptive signaling improves responses. NF1 and GNAS mutations drive acquired resistance to Hras inhibition, supporting the on-target effects of the drug. Cancer Res; 78(16); 4642-57. ©2018 AACR.

Oncogene-induced senescence and its evasion in a mouse model of thyroid neoplasia

Here we describe a conditional doxycycline-dependent mouse model of RET/PTC3 (NCOA4-RET) oncogene-induced thyroid tumorigenesis. In these mice, after 10 days of doxycycline (dox) administration, RET/PTC3 expression induced mitogen activated protein kinase (MAPK) stimulation and a proliferative response which resulted in the formation of hyperplastic thyroid lesions. This was followed, after 2 months, by growth arrest accompanied by typical features of oncogene-induced senescence (OIS), including upregulation of p16INK4A and p21CIP, positivity at the Sudan black B, activation of the DNA damage response (DDR) markers γH2AX and pChk2 T68, and induction of p53 and p19ARF. After 5 months, about half of thyroid lesions escaped OIS and formed tumors that remained dependent on RET/PTC3 expression. This progression was accompanied by activation of AKT-FOXO1/3a pathway and increased serum TSH levels.

Genomic Alterations in Fatal Forms of Non-Anaplastic Thyroid Cancer: Identification of MED12 and RBM10 as Novel Thyroid Cancer Genes Associated with Tumor Virulence

Purpose: Patients with anaplastic thyroid cancer (ATC) have a very high death rate. In contrast, deaths from non-anaplastic thyroid (NAT) cancer are much less common. The genetic alterations in fatal NAT cancers have not been reported.Experimental Design: We performed next-generation sequencing of 410 cancer genes from 57 fatal NAT primary cancers. Results were compared with The Cancer Genome Atlas study (TCGA study) of papillary thyroid cancers (PTCs) and to the genomic changes reported in ATC.Results: There was a very high prevalence of TERT promoter mutations, comparable with that of ATC, and these co-occurred with BRAF and RAS mutations. A high incidence of chromosome 1q gain was seen highlighting its importance in tumor aggressiveness. Two novel fusion genes DLG5-RET and OSBPL1A-BRAF were identified. There was a high frequency of mutations in MED12 and these were mutually exclusive to TERT promoter mutations and also to BRAF and RAS mutations. In addition, a high frequency of mutations in RBM10 was identified and these co-occurred with RAS mutations and PIK3CA mutations. Compared with the PTCs in TCGA, there were higher frequencies of mutations in TP53, POLE, PI3K/AKT/mTOR pathway effectors, SWI/SNF subunits, and histone methyltransferases.Conclusions: These data support a model, whereby fatal NAT cancers arise from well-differentiated tumors through the accumulation of key additional genetic abnormalities. The high rate of TERT promoter mutations, MED12 mutations, RBM10 mutations, and chromosome 1q gain highlight their likely association with tumor virulence. Clin Cancer Res; 23(19); 5970-80. ©2017 AACR.

NADPH Oxidase NOX4 Is a Critical Mediator of BRAFV600E-Induced Downregulation of the Sodium/Iodide Symporter in Papillary Thyroid Carcinomas

AIMS

The BRAF^V600E oncogene, reported in 40%-60% of papillary thyroid cancer (PTC), has an important role in the pathogenesis of PTC. It is associated with the loss of thyroid iodide-metabolizing genes, such as sodium/iodide symporter (NIS), and therefore with radioiodine refractoriness. Inhibition of mitogen-activated protein kinase (MAPK) pathway, constitutively activated by BRAF^V600E, is not always efficient in resistant tumors suggesting that other compensatory mechanisms contribute to a BRAF^V600E adaptive resistance. Recent studies pointed to a key role of transforming growth factor β (TGF-β) in BRAF^V600E-induced effects. The reactive oxygen species (ROS)-generating NADPH oxidase NOX4, which is increased in PTC, has been identified as a new key effector of TGF-β in cancer, suggestive of a potential role in BRAF^V600E-induced thyroid tumors.

RESULTS

Here, using two human BRAF^V600E-mutated thyroid cell lines and a rat thyroid cell line expressing BRAF^V600E in a conditional manner, we show that NOX4 upregulation is controlled at the transcriptional level by the oncogene via the TGF-β/Smad3 signaling pathway. Importantly, treatment of cells with NOX4-targeted siRNA downregulates BRAF^V600E-induced NIS repression. Innovation and Conclusion: Our results establish a link between BRAF^V600E and NOX4, which is confirmed by a comparative analysis of NOX4 expression in human (TCGA) and mouse thyroid cancers. Remarkably, analysis of human and murine BRAF^V600E-mutated thyroid tumors highlights that the level of NOX4 expression is inversely correlated to thyroid differentiation suggesting that other genes involved in thyroid differentiation in addition to NIS might be silenced by a mechanism controlled by NOX4-derived ROS. This study opens a new opportunity to optimize thyroid cancer therapy. Antioxid. Redox Signal. 26, 864-877.

Sustained ERK inhibition maximizes responses of BrafV600E thyroid cancers to radioiodine

Radioiodide (RAI) therapy of thyroid cancer exploits the relatively selective ability of thyroid cells to transport and accumulate iodide. Iodide uptake requires expression of critical genes that are involved in various steps of thyroid hormone biosynthesis. ERK signaling, which is markedly increased in thyroid cancer cells driven by oncogenic BRAF, represses the genetic program that enables iodide transport. Here, we determined that a critical threshold for inhibition of MAPK signaling is required to optimally restore expression of thyroid differentiation genes in thyroid cells and in mice with BrafV600E-induced thyroid cancer. Although the MEK inhibitor selumetinib transiently inhibited ERK signaling, which subsequently rebounded, the MEK inhibitor CKI suppressed ERK signaling in a sustained manner by preventing RAF reactivation. A small increase in ERK inhibition markedly increased the expression of thyroid differentiation genes, increased iodide accumulation in cancer cells, and thereby improved responses to RAI therapy. Only a short exposure to the drug was necessary to obtain a maximal response to RAI. These data suggest that potent inhibition of ERK signaling is required to adequately induce iodide uptake and indicate that this is a promising strategy for the treatment of BRAF-mutant thyroid cancer.

Abstract 892: Functional characterization of EIF1AX mutations in thyroid cancer predicts for gain of function by increasing translational rate with concomitant derepression of upstream inputs from mTOR

EIF1AX (eukaryotic translation initiation factor 1A) is a component of the translation pre-initiation complex (PIC). Recurrent EIF1AX mutations, first reported in uveal melanomas, are found in ∼1% of papillary thyroid cancers in a mutually exclusive manner with other oncogenic driver events (BRAF, RAS, and oncogenic fusions). By contrast, they are enriched in advanced thyroid cancers (9% of anaplastic and 11% poorly-differentiated thyroid cancers), and are strongly associated with RAS mutations (p&lt;0.0001). EIF1AX mutations cluster in the N-terminal (NTT) or C-terminal tails (CTT). EIF1AX NTT missense mutations in thyroid cancer occur within the first 15 amino acids, whereas the CTT mutation disrupts a splice acceptor site at exon 6 (A113splice). A113splice is the most prevalent defect and is private to this disease, and results in two differentially spliced mRNAs: (1) Cryptic splice variant: by use of a cryptic acceptor site in exon 6 that leads to a 132 AA protein that excludes 12 AA; (2) Truncated splice variant: by retaining intron 5, leading to a 115 AA truncated protein. EIF1AX mutants retain the ability to recruit the ternary complex, as shown by co-IP with EIF2 after ectopic expression of NTT or A113splice EIF1AX mutants in HEK293T cells, or after CRISPR-mediated knock-in of A113splice into Cal62 RAS-mutant thyroid cancer cells. The EIF1AX mutants had greater affinity to EIF5 compared to WT, consistent with a more stable PIC. As translation initiation is a rate-limiting step, the altered affinity of EIF1AX mutants to PIC components could impact the rate of protein synthesis. We tested this by L-azidohomoalanine labeling, which showed contrasting roles of the two A113-generated splice variants expressed at endogenous levels: i.e. the cryptic splice variant increased protein synthetic rate, whereas the truncated splice variant strongly inhibited protein translation. Despite inhibiting translation, the truncated splice variant showed a paradoxical increase in 4EBP1 phosphorylation. Upon A113splice knock-in, where both variants are expressed, translation is increased, which we hypothesize results from the combined effects of 4EBP1 phosphorylation, caused by relief of negative feedback events upstream in the pathway, with increased PIC assembly caused by the cryptic splice variant. We are currently determining whether the altered rate of protein synthesis is global or selective. Of note, cells expressing the cryptic, but not the truncated splice form, showed a transforming phenotype as assessed by soft agar colony formation. As EIF1AX mutations co-occur with RAS mutations in advanced thyroid cancers, it is likely that RAS-induced PI3K-AKT/mTOR signaling may provide a further cooperative benefit and play a key role in disease progression, and in generating specific tumor cell dependencies.

Citation Format: Gnana P. Krishnamoorthy, Inigo Landa, Jeffrey A. Knauf, James Nagarajah, Gunnar Rätsch, Hans-Guido Wendel, James A. Fagin. Functional characterization of EIF1AX mutations in thyroid cancer predicts for gain of function by increasing translational rate with concomitant derepression of upstream inputs from mTOR. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 892.

Genomic and transcriptomic hallmarks of poorly differentiated and anaplastic thyroid cancers

BACKGROUND

Poorly differentiated thyroid cancer (PDTC) and anaplastic thyroid cancer (ATC) are rare and frequently lethal tumors that so far have not been subjected to comprehensive genetic characterization.

METHODS

We performed next-generation sequencing of 341 cancer genes from 117 patient-derived PDTCs and ATCs and analyzed the transcriptome of a representative subset of 37 tumors. Results were analyzed in the context of The Cancer Genome Atlas study (TCGA study) of papillary thyroid cancers (PTC).

RESULTS

Compared to PDTCs, ATCs had a greater mutation burden, including a higher frequency of mutations in TP53, TERT promoter, PI3K/AKT/mTOR pathway effectors, SWI/SNF subunits, and histone methyltransferases. BRAF and RAS were the predominant drivers and dictated distinct tropism for nodal versus distant metastases in PDTC. RAS and BRAF sharply distinguished between PDTCs defined by the Turin (PDTC-Turin) versus MSKCC (PDTC-MSK) criteria, respectively. Mutations of EIF1AX, a component of the translational preinitiation complex, were markedly enriched in PDTCs and ATCs and had a striking pattern of co-occurrence with RAS mutations. While TERT promoter mutations were rare and subclonal in PTCs, they were clonal and highly prevalent in advanced cancers. Application of the TCGA-derived BRAF-RAS score (a measure of MAPK transcriptional output) revealed a preserved relationship with BRAF/RAS mutation in PDTCs, whereas ATCs were BRAF-like irrespective of driver mutation.

CONCLUSIONS

These data support a model of tumorigenesis whereby PDTCs and ATCs arise from well-differentiated tumors through the accumulation of key additional genetic abnormalities, many of which have prognostic and possible therapeutic relevance. The widespread genomic disruptions in ATC compared with PDTC underscore their greater virulence and higher mortality.

FUNDING

This work was supported in part by NIH grants CA50706, CA72597, P50-CA72012, P30-CA008748, and 5T32-CA160001; the Lefkovsky Family Foundation; the Society of Memorial Sloan Kettering; the Byrne fund; and Cycle for Survival.

NF2 Loss Promotes Oncogenic RAS-Induced Thyroid Cancers via YAP-Dependent Transactivation of RAS Proteins and Sensitizes Them to MEK Inhibition

Ch22q LOH is preferentially associated with RAS mutations in papillary and in poorly differentiated thyroid cancer (PDTC). The 22q tumor suppressor NF2, encoding merlin, is implicated in this interaction because of its frequent loss of function in human thyroid cancer cell lines. Nf2 deletion or Hras mutation is insufficient for transformation, whereas their combined disruption leads to murine PDTC with increased MAPK signaling. Merlin loss induces RAS signaling in part through inactivation of Hippo, which activates a YAP-TEAD transcriptional program. We find that the three RAS genes are themselves YAP-TEAD1 transcriptional targets, providing a novel mechanism of promotion of RAS-induced tumorigenesis. Moreover, pharmacologic disruption of YAP-TEAD with verteporfin blocks RAS transcription and signaling and inhibits cell growth. The increased MAPK output generated by NF2 loss in RAS-mutant cancers may inform therapeutic strategies, as it generates greater dependency on the MAPK pathway for viability.

SIGNIFICANCE

Intensification of mutant RAS signaling through copy-number imbalances is commonly associated with transformation. We show that NF2/merlin inactivation augments mutant RAS signaling by promoting YAP/TEAD-driven transcription of oncogenic and wild-type RAS, resulting in greater MAPK output and increased sensitivity to MEK inhibitors.

Switch in signaling control of mTORC1 activity after oncoprotein expression in thyroid cancer cell lines

CONTEXT

Thyroid growth is regulated by TSH and requires mammalian target of rapamycin (mTOR). Thyroid cancers frequently exhibit mutations in MAPK and/or phosphoinositol-3-kinase-related kinase effectors.

OBJECTIVE

The objective of the study was to explore the contribution of RET/PTC, RAS, and BRAF to mTOR regulation and response to mTOR inhibitors.

METHODS

PCCL3 cells conditionally expressing RET/PTC3, HRAS(G12V), or BRAF(V600E) and human thyroid cancer cells harboring mutations of these genes were used to test pathways controlling mTOR and its requirement for growth.

RESULTS

TSH/cAMP-induced growth of PCCL3 cells requires mTOR, which is stimulated via protein kinase A in a MAPK kinase (MEK)- and AKT-independent manner. Expression of RET/PTC3, HRAS(G12V), or BRAF(V600E) in PCCL3 cells induces mTOR but does not entirely abrogate the cAMP-mediated control of its activity. Acute oncoprotein-induced mTOR activity is regulated by MEK and AKT, albeit to differing degrees. By contrast, mTOR was not activated by TSH/cAMP in human thyroid cancer cells. Tumor genotype did not predict the effects of rapamycin or the mTOR kinase inhibitor AZD8055 on growth, with the exception of a PTEN-null cell line. Selective blockade of MEK did not influence mTOR activity of BRAF or RAS mutant cells. Combined MEK and mTOR kinase inhibition was synergistic on growth of BRAF- and RAS-mutant thyroid cancer cells in vitro and in vivo.

CONCLUSION

Thyroid cancer cells lose TSH/cAMP dependency of mTOR signaling and cell growth. mTOR activity is not decreased by the MEK or AKT inhibitors in the RAS or BRAF human thyroid cancer cell lines. This may account for the augmented effects of combining the mTOR inhibitors with selective antagonists of these oncogenic drivers.

Transformation by Hras(G12V) is consistently associated with mutant allele copy gains and is reversed by farnesyl transferase inhibition

RAS-driven malignancies remain a major therapeutic challenge. The two-stage 7,12-dimethylbenz(a)anthracene (DMBA)/12-o-tetradecanoylphorbol-13-acetate (TPA) model of mouse skin carcinogenesis has been used to study mechanisms of epithelial tumor development by oncogenic Hras. We used mice with a Hras^G12V knock-in allele to elucidate the early events after Hras activation, and to evaluate the therapeutic effectiveness of farnesyltransferase (FTI) inhibition. Treatment of Caggs-Cre/FR-Hras^G12V mice with TPA alone was sufficient to trigger papilloma development with shorter latency and a ~10-fold greater tumor burden than DMBA/TPA-treated WT controls. Hras^G12V allele copy number was increased in all papillomas induced by TPA. DMBA/TPA treatment of Hras^G12V knock-in mice induced an even greater incidence of papillomas, which either harbored Hras^G12V amplification, or developed a Hras^Q61L mutation in the second allele. Laser-capture microdissection of normal skin, hyperplastic skin and papillomas showed that amplification occurred only at the papilloma stage. HRAS mutant allelic imbalance was also observed in human cancer cell lines, consistent with a requirement for augmented oncogenic HRAS signaling for tumor development. The FTI SCH66336 blocks HRAS farnesylation and delocalizes it from the plasma membrane. NRAS and KRAS are not affected as they are alternatively prenylated. When tested in lines harboring HRAS, NRAS or KRAS mutations, SCH66336 delocalized, inhibited signaling and preferentially inhibited growth only of HRAS-mutant lines. Treatment with SCH66336 also induced near-complete regression of papillomas of TPA-treated Hras^G12V knock-in mice. These data suggest that farnesyl transferase inhibitors should be reevaluated as targeted agents for human HRAS-driven cancers, such as those of bladder, thyroid and other epithelial lineages.

Identification of kinase fusion oncogenes in post-Chernobyl radiation-induced thyroid cancers

Exposure to ionizing radiation during childhood markedly increases the risk of developing papillary thyroid cancer. We examined tissues from 26 Ukrainian patients with thyroid cancer who were younger than 10 years of age and living in contaminated areas during the time of the Chernobyl nuclear reactor accident. We identified nonoverlapping somatic driver mutations in all 26 cases through candidate gene assays and next-generation RNA sequencing. We found that 22 tumors harbored fusion oncogenes that arose primarily through intrachromosomal rearrangements. Altogether, 23 of the oncogenic drivers identified in this cohort aberrantly activate MAPK signaling, including the 2 somatic rearrangements resulting in fusion of transcription factor ETS variant 6 (ETV6) with neurotrophic tyrosine kinase receptor, type 3 (NTRK3) and fusion of acylglycerol kinase (AGK) with BRAF. Two other tumors harbored distinct fusions leading to overexpression of the nuclear receptor PPARγ. Fusion oncogenes were less prevalent in tumors from a cohort of children with pediatric thyroid cancers that had not been exposed to radiation but were from the same geographical regions. Radiation-induced thyroid cancers provide a paradigm of tumorigenesis driven by fusion oncogenes that activate MAPK signaling or, less frequently, a PPARγ-driven transcriptional program.

Targeting mTOR in RET mutant medullary and differentiated thyroid cancer cells

Inhibitors of RET, a tyrosine kinase receptor encoded by a gene that is frequently mutated in medullary thyroid cancer, have emerged as promising novel therapies for the disease. Rapalogs and other mammalian target of rapamycin (mTOR) inhibitors are effective agents in patients with gastroenteropancreatic neuroendocrine tumors, which share lineage properties with medullary thyroid carcinomas. The objective of this study was to investigate the contribution of mTOR activity to RET-induced signaling and cell growth and to establish whether growth suppression is enhanced by co-targeting RET and mTOR kinase activities. Treatment of the RET mutant cell lines TT, TPC-1, and MZ-CRC-1 with AST487, a RET kinase inhibitor, suppressed growth and showed profound and sustained inhibition of mTOR signaling, which was recapitulated by siRNA-mediated RET knockdown. Inhibition of mTOR with INK128, a dual mTORC1 and mTORC2 kinase inhibitor, also resulted in marked growth suppression to levels similar to those seen with RET blockade. Moreover, combined treatment with AST487 and INK128 at low concentrations suppressed growth and induced apoptosis. These data establish mTOR as a key mediator of RET-mediated cell growth in thyroid cancer cells and provide a rationale for combinatorial treatments in thyroid cancers with oncogenic RET mutations.

Abstract 3402: Relief of feedback inhibition of HER3 transcription by RAF and MEK inhibitors attenuates their antitumor effects in BRAF mutant thyroid carcinomas

The RAF inhibitor vemurafenib (PLX4032) increases survival in patients with BRAF-mutant metastatic melanoma, but has limited efficacy in patients with colorectal cancers. A trial with vemurafenib for thyroid cancer patients is now in progress.

We compared the response of thyroid and melanoma cell lines harboring BRAFV600E to vemurafenib and found that the majority of thyroid cancer cells were refractory to the inhibitor (IC50 &gt; 1.5 μM). In addition, thyroid cancer cell lines showed a rebound in pERK beginning 6h post-treatment and a transient activation of pAKT. Expression profiling and pRTK array screens of PLX4032-treated cells showed higher expression and activation of HER family receptors, particularly HER2 and HER3. Phosphorylation of HER3 represented the most consistent PLX4032-dependent RTK activated across thyroid cancer cell lines (5/6 lines tested) and was not detectable in any of the melanoma or colorectal cancer cells we investigated. HER3 phosphorylation was also induced by the allosteric MEK inhibitor PD0325901 in thyroid cancers of TPO-Cre/LSL-BrafV600E mice. We identified HER2 as the main HER3 heterodimerization partner, which was activated via a neuregulin-dependent autocrine loop, which was specific to the thyroid cancer lineage, as it was not observed in either melanoma or colorectal BRAF-mutant lines. HER3 transcription was induced by MAPK inhibition, via dissociation of the CTBP1/2 transcriptional repressors from the HER3 gene promoter. Finally, the HER2/HER3 kinase inhibitor lapatinib sensitized BRAF-mutant thyroid cancer cells to growth inhibition by RAF or MEK inhibitors in vitro and in vivo, providing rationale for combination therapies in this disease. Hence, the early response of BRAF-mutant cancers to selective MAPK pathway inhibitors is marked by the relaxation of oncoprotein-driven negative feedback events, which differ between tumors of various lineages, and which predict a requirement for distinct therapeutic strategies.

Citation Format: Cristina Montero-Conde, Sergio Ruiz-Llorente, Jose M. Dominguez, Jeffrey A. Knauf, Agnes Viale, Eric J. Sherman, Mabel Ryder, Ronald A. Ghossein, Neal Rosen, James A. Fagin. Relief of feedback inhibition of HER3 transcription by RAF and MEK inhibitors attenuates their antitumor effects in BRAF mutant thyroid carcinomas. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3402. doi:10.1158/1538-7445.AM2013-3402

Relief of feedback inhibition of HER3 transcription by RAF and MEK inhibitors attenuates their antitumor effects in BRAF-mutant thyroid carcinomas

The RAF inhibitor vemurafenib (PLX4032) increases survival in patients with BRAF-mutant metastatic melanoma, but has limited efficacy in patients with colorectal cancers. Thyroid cancer cells are also comparatively refractory to RAF inhibitors. In contrast to melanomas, inhibition of mitogen-activated protein kinase (MAPK) signaling by PLX4032 is transient in thyroid and colorectal cancer cells. The rebound in extracellular signal-regulated kinase (ERK) in thyroid cells is accompanied by increased HER3 signaling caused by induction of ERBB3 (HER3) transcription through decreased promoter occupancy by the transcriptional repressors C-terminal binding protein 1 and 2 and by autocrine secretion of neuregulin-1 (NRG1). The HER kinase inhibitor lapatinib prevents MAPK rebound and sensitizes BRAF-mutant thyroid cancer cells to RAF or MAP-ERK kinase inhibitors. This provides a rationale for combining ERK pathway antagonists with inhibitors of feedback-reactivated HER signaling in this disease. The determinants of primary resistance to MAPK inhibitors vary between cancer types, due to preferential upregulation of specific receptor tyrosine kinases, and the abundance of their respective ligands.

GLP-1 receptor agonists and the thyroid: C-cell effects in mice are mediated via the GLP-1 receptor and not associated with RET activation

Liraglutide and exenatide are glucagon-like peptide receptor (GLP-1R) agonists used in the treatment of type 2 diabetes. Both molecules have been associated with the development of thyroid C-cell tumors after lifetime exposure in rodents. Previously, it has been reported that these tumors are preceded by increased plasma calcitonin and C-cell hyperplasia. We can now document that the murine C-cell effects are mediated via GLP-1R. Thus, 13 wk of continuous exposure to GLP-1R agonists was associated with marked increases in plasma calcitonin and in the incidence of C-cell hyperplasia in wild-type mice. In contrast, similar effects were not seen in GLP-1R knockout mice. Human C-cell cancer is often caused by activating mutations in the rearranged-during-transfection (RET) protooncogene. We developed an immunohistochemical method to assess RET activation in tissues. Liraglutide dosing to mice was not found to activate RET. Further evaluation of the signaling pathways demonstrated that liraglutide increased ribosomal S6, but not MAPK kinase, phosphorylation. These observations are consistent with effects of GLP-1R agonists on rodent C cells being mediated via mammalian target of rapamycin activation in a RET- and MAPK-independent manner.

Small-molecule MAPK inhibitors restore radioiodine incorporation in mouse thyroid cancers with conditional BRAF activation

Advanced human thyroid cancers, particularly those that are refractory to treatment with radioiodine (RAI), have a high prevalence of BRAF (v-raf murine sarcoma viral oncogene homolog B1) mutations. However, the degree to which these cancers are dependent on BRAF expression is still unclear. To address this question, we generated mice expressing one of the most commonly detected BRAF mutations in human papillary thyroid carcinomas (BRAF(V600E)) in thyroid follicular cells in a doxycycline-inducible (dox-inducible) manner. Upon dox induction of BRAF(V600E), the mice developed highly penetrant and poorly differentiated thyroid tumors. Discontinuation of dox extinguished BRAF(V600E) expression and reestablished thyroid follicular architecture and normal thyroid histology. Switching on BRAF(V600E) rapidly induced hypothyroidism and virtually abolished thyroid-specific gene expression and RAI incorporation, all of which were restored to near basal levels upon discontinuation of dox. Treatment of mice with these cancers with small molecule inhibitors of either MEK or mutant BRAF reduced their proliferative index and partially restored thyroid-specific gene expression. Strikingly, treatment with the MAPK pathway inhibitors rendered the tumor cells susceptible to a therapeutic dose of RAI. Our data show that thyroid tumors carrying BRAF(V600E) mutations are exquisitely dependent on the oncoprotein for viability and that genetic or pharmacological inhibition of its expression or activity is associated with tumor regression and restoration of RAI uptake in vivo in mice. These findings have potentially significant clinical ramifications.

Proto-oncogene PBF/PTTG1IP regulates thyroid cell growth and represses radioiodide treatment

Pituitary tumor transforming gene (PTTG)-binding factor (PBF or PTTG1IP) is a little characterized proto-oncogene that has been implicated in the etiology of breast and thyroid tumors. In this study, we created a murine transgenic model to target PBF expression to the thyroid gland (PBF-Tg mice) and found that these mice exhibited normal thyroid function, but a striking enlargement of the thyroid gland associated with hyperplastic and macrofollicular lesions. Expression of the sodium iodide symporter (NIS), a gene essential to the radioiodine ablation of thyroid hyperplasia, neoplasia, and metastasis, was also potently inhibited in PBF-Tg mice. Critically, iodide uptake was repressed in primary thyroid cultures from PBF-Tg mice, which could be rescued by PBF depletion. PBF-Tg thyroids exhibited upregulation of Akt and the TSH receptor (TSHR), each known regulators of thyrocyte proliferation, along with upregulation of the downstream proliferative marker cyclin D1. We extended and confirmed findings from the mouse model by examining PBF expression in human multinodular goiters (MNG), a hyperproliferative thyroid disorder, where PBF and TSHR was strongly upregulated relative to normal thyroid tissue. Furthermore, we showed that depleting PBF in human primary thyrocytes was sufficient to increase radioiodine uptake. Together, our findings indicate that overexpression of PBF causes thyroid cell proliferation, macrofollicular lesions, and hyperplasia, as well as repression of the critical therapeutic route for radioiodide uptake.

Progression of BRAF-induced thyroid cancer is associated with epithelial-mesenchymal transition requiring concomitant MAP kinase and TGFβ signaling

Mice with thyroid-specific expression of oncogenic BRAF (Tg-Braf) develop papillary thyroid cancers (PTCs) that are locally invasive and have well-defined foci of poorly differentiated thyroid carcinoma (PDTC). To investigate the PTC-PDTC progression, we performed a microarray analysis using RNA from paired samples of PDTC and PTC collected from the same animals by laser capture microdissection. Analysis of eight paired samples revealed a profound deregulation of genes involved in cell adhesion and intracellular junctions, with changes consistent with an epithelial-mesenchymal transition (EMT). This was confirmed by immunohistochemistry, as vimentin expression was increased and E-cadherin lost in PDTC compared with adjacent PTC. Moreover, PDTC stained positively for phospho-Smad2, suggesting a role for transforming growth factor (TGF)β in mediating this process. Accordingly, TGFβ-induced EMT in primary cultures of thyroid cells from Tg-Braf mice, whereas wild-type thyroid cells retained their epithelial features. TGFβ-induced Smad2 phosphorylation, transcriptional activity and induction of EMT required mitogen-activated protein kinase (MAPK) pathway activation in Tg-Braf thyrocytes. Hence, tumor initiation by oncogenic BRAF renders thyroid cells susceptible to TGFβ-induced EMT, through a MAPK-dependent process.

Abstract 4290: The farnesyl transferase inhibitor SCH 66336 induces regression of Hras-driven tumors in mice with widespread endogenous expression of HrasG12V

We previously reported that mice with a knock-in HrasG12V allele (Caggs-Cre/FR-HrasG12V) develop skin and forestomach papillomas as well as angiosarcomas with a relatively long latency. In these mice, activation of a single Hras mutant allele was not sufficient for transformation. The two-stage 7,12-dimethylbenz(a)anthracene (DMBA) /12-o-tetradecanoylphorbol-13-acetate (TPA) model of mouse skin carcinogenesis has been used to study mechanisms of epithelial tumor development. Although DMBA-induced HrasQ61L mutations are implicated in tumor formation in this model, it is not clear whether Hras is a tumor initiator, and whether its activity is required for tumor maintenance. To further clarify the role of the Hras gene in the DMBA/TPA model, we treated the skin of HrasG12V mice with the TPA tumor promoter alone, and found that this was sufficient to trigger rapid papilloma development. Latency was shorter, and tumor burden ∼ 10-fold greater than in DMBA/TPA treated WT controls. The mutant Hras allele copy number was increased in all 10 papillomas induced by TPA alone. Interestingly, DMBA/TPA treatment of HrasG12V mice induced an even greater incidence of papillomas: whereas 7/10 randomly selected papillomas induced by DMBA/ TPA in HrasG12V mice had amplified the HrasG12V allele, the remaining 3 had acquired an HrasQ61L mutation in the second Hras allele. Laser-capture microdissection of normal, hyperplastic and papilloma tissues from TPA-treated HrasG12V mice showed that Hras mutant allele amplification occurred only at the papilloma stage. Moreover, HRAS allelic imbalance was also observed in several human cancer cell lines with HRAS mutation (Hth83, C643 and T24). These data indicate that oncogenic Hras is a tumor initiator, and that augmentation of oncogenic Hras signaling is likely required for tumor formation through increased mutant Hras copy number or through mutation in the second Hras allele. SCH66336 is a farnesyl transferase inhibitor (FTI) that blocks the farnesylation of Hras, resulting in Hras delocalization from the plasma membrane. All Ras isoforms are farnesylated, however, upon treatment with FTI, Nras and Kras, but not Hras, are alternatively prenylated by geranylgeranylprenyltransferase-1. When tested in human cell lines harboring HRAS, NRAS or KRAS mutations, the FTI SCH 66336 delocalized only the Hras protein, inhibited its downstream signaling, and preferentially inhibited growth of HRAS-mutant lines. Treatment of TPA-treated HrasG12V mice with SCH 66336 (80mg/kg, b.i.d PO) induced an almost complete regression of Hras-driven papillomas within 10 days. The mechanism by which SCH66336 induced dramatic papilloma regression is under investigation. These observations suggest that FTIs should be re-evaluated as a therapy for human HRAS-driven cancers, such as those of bladder, thyroid and other epithelial lineages.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4290. doi:10.1158/1538-7445.AM2011-4290

Mutational profile of advanced primary and metastatic radioactive iodine-refractory thyroid cancers reveals distinct pathogenetic roles for BRAF, PIK3CA, and AKT1

Patients with poorly differentiated thyroid cancers (PDTC), anaplastic thyroid cancers (ATC), and radioactive iodine-refractory (RAIR) differentiated thyroid cancers have a high mortality, particularly if positive on [(18)F]fluorodeoxyglucose (FDG)-positron emission tomography (PET). To obtain comprehensive genetic information on advanced thyroid cancers, we designed an assay panel for mass spectrometry genotyping encompassing the most significant oncogenes in this disease: 111 mutations in RET, BRAF, NRAS, HRAS, KRAS, PIK3CA, AKT1, and other related genes were surveyed in 31 cell lines, 52 primary tumors (34 PDTC and 18 ATC), and 55 RAIR, FDG-PET-positive recurrences and metastases (nodal and distant) from 42 patients. RAS mutations were more prevalent than BRAF (44 versus 12%; P = 0.002) in primary PDTC, whereas BRAF was more common than RAS (39 versus 13%; P = 0.04) in PET-positive metastatic PDTC. BRAF mutations were highly prevalent in ATC (44%) and in metastatic tumors from RAIR PTC patients (95%). Among patients with multiple metastases, 9 of 10 showed between-sample concordance for BRAF or RAS mutations. By contrast, 5 of 6 patients were discordant for mutations of PIK3CA or AKT1. AKT1_G49A was found in 9 specimens, exclusively in metastases. This is the first documentation of AKT1 mutation in thyroid cancer. Thus, RAIR, FDG-PET-positive metastases are enriched for BRAF mutations. If BRAF is mutated in the primary, it is likely that the metastases will harbor the defect. By contrast, absence of PIK3CA/AKT1 mutations in one specimen may not reflect the status at other sites because these mutations arise during progression, an important consideration for therapies directed at phosphoinositide 3-kinase effectors.

Role of MAPK pathway oncoproteins in thyroid cancer pathogenesis and as drug targets

Constitutive activation of MAPK in cancer occurs through activating mutations or overexpression of upstream effectors in the pathway, primarily of genes encoding receptor tyrosine kinases, RAS and BRAF. Arguably, the evidence for MAPK activation is most compelling in thyroid cancers and in melanomas. In this review we discuss the mechanisms of tumor development by oncogenic BRAF in these two cancer cell lineages, since this kinase signals preferentially through this pathway. We describe recent information on the mediators of BRAF-induced tumor initiation and escape from senescence. In addition, we review the biochemical events implicated in cellular growth triggered by oncogenic BRAF and the determinants of oncogene addiction. The biology of thyroid cancers induced by oncogenic BRAF is quite distinct, both in humans and in mice. There is great interest in using these insights to design rational new therapies, for which it will become crucial to understand the determinants of sensitivity and resistance to compounds designed to block the pathway. In thyroid cancer, this interest is further heightened by new information on the role of activated BRAF and MAPK pathway activation in disrupting iodine transport and thyroid hormonogenesis.

Increased density of tumor-associated macrophages is associated with decreased survival in advanced thyroid cancer

Thyroid cancers are infiltrated with tumor-associated macrophages (TAMs), yet their role in cancer progression is not known. The objectives of this study were to characterize the density of TAMs in well-differentiated (WDTC), poorly differentiated (PDTC), and anaplastic thyroid cancers (ATC) and to correlate TAM density with clinicopathologic parameters. Immunohistochemistry was performed on tissue microarray sections from WDTC (n=33), PDTC (n=37), and ATC (n=20) using macrophage-specific markers. Electronic medical records were used to gather clinical and pathologic data. Follow-up information of PDTC patients was available for 0-12 years. In total, 9 out of 33 WDTC (27%), 20 out of 37 PDTC (54%), and 19 out of 20 ATC (95%) had an increased density of CD68(+) TAMs (> or = 10 per 0.28 mm(2); WDTC versus PDTC, P=0.03; WDTC versus ATC, P<0.0001; PDTC versus ATC, P<0.002). Increased TAMs in PDTC was associated with capsular invasion (P=0.034), extrathyroidal extension (P=0.009), and decreased cancer-related survival (P=0.009) compared with PDTC with a low density of TAMs. In conclusion, the density of TAMs is increased in advanced thyroid cancers. The presence of a high density of TAMs in PDTC correlates with invasion and decreased cancer-related survival. These results suggest that TAMs may facilitate tumor progression. As novel therapies directed against thyroid tumor cell-specific targets are being tested, the potential role of TAMs as potential modulators of the thyroid cancer behavior will need to be considered.

Deoxyribonucleic acid profiling analysis of 40 human thyroid cancer cell lines reveals cross-contamination resulting in cell line redundancy and misidentification

CONTEXT

Cell lines derived from human cancers provide critical tools to study disease mechanisms and develop novel therapies. Recent reports indicate that up to 36% of cell lines are cross- contaminated.

OBJECTIVE

We evaluated 40 reported thyroid cancer-derived cell lines using short tandem repeat and single nucleotide polymorphism array analysis.

RESULTS

Only 23 of 40 cell lines tested have unique genetic profiles. The following groups of cell lines are likely derivatives of the same cell line: BHP5-16, BHP17-10, BHP14-9, and NPA87; BHP2-7, BHP10-3, BHP7-13, and TPC1; KAT5, KAT10, KAT4, KAT7, KAT50, KAK1, ARO81-1, and MRO87-1; and K1 and K2. The unique cell lines include BCPAP, KTC1, TT2609-C02, FTC133, ML1, WRO82-1, 8505C, SW1736, Cal-62, T235, T238, Uhth-104, ACT-1, HTh74, KAT18, TTA1, FRO81-2, HTh7, C643, BHT101, and KTC-2. The misidentified cell lines included the DRO90-1, which matched the melanoma-derived cell line, A-375. The ARO81-1 and its derivatives matched the HT-29 colon cancer cell line, and the NPA87 and its derivatives matched the M14/MDA-MB-435S melanoma cell line. TTF-1 and Pax-8 mRNA levels were determined in the unique cell lines.

CONCLUSIONS

Many of these human cell lines have been widely used in the thyroid cancer field for the past 20 yr and are not only redundant, but not of thyroid origin. These results emphasize the importance of cell line integrity, and provide the short tandem repeat profiles for a panel of thyroid cancer cell lines that can be used as a reference for comparison of cell lines from other laboratories.

RET/PTC-induced cell growth is mediated in part by epidermal growth factor receptor (EGFR) activation: evidence for molecular and functional interactions between RET and EGFR

RET/PTC rearrangements are one of the genetic hallmarks of papillary thyroid carcinomas. RET/PTC oncoproteins lack extracellular or transmembrane domains, and activation takes place through constitutive dimerization mediated through coiled-coil motifs in the NH(2) terminus of the chimeric protein. Based on the observation that the epidermal growth factor receptor (EGFR) kinase inhibitor PKI166 decreased RET/PTC kinase autophosphorylation and activation of downstream effectors in thyroid cells, despite lacking activity on the purified RET kinase, we proceeded to examine possible functional interactions between RET/PTC and EGFR. Conditional activation of RET/PTC oncoproteins in thyroid PCCL3 cells markedly induced expression and phosphorylation of EGFR, which was mediated in part through mitogen-activated protein kinase signaling. RET and EGFR were found to coimmunoprecipitate. The ability of RET to form a complex with EGFR was not dependent on recruitment of Shc or on their respective kinase activities. Ligand-induced activation of EGFR resulted in phosphorylation of a kinase-dead RET, an effect that was entirely blocked by PKI166. These effects were biologically relevant, as the EGFR kinase inhibitors PKI166, gefitinib, and AEE788 inhibited cell growth induced by various constitutively active mutants of RET in thyroid cancer cells as well as NIH3T3 cells. These data indicate that EGFR contributes to RET kinase activation, signaling, and growth stimulation and may therefore be an attractive therapeutic target in RET-induced neoplasms.

BRAFV600E mutation is associated with preferential sensitivity to mitogen-activated protein kinase kinase inhibition in thyroid cancer cell lines

CONTEXT

Mutually exclusive mutations of RET, RAS, or BRAF are present in about 70% of papillary thyroid carcinomas, whereas only the latter two are seen in poorly differentiated and anaplastic cancers. Although the signal output common to these oncoproteins is ERK, a recent report showed that only BRAF mutations consistently predicted responsiveness to MAPK kinase (MEK) inhibitors.

OBJECTIVES

Here we investigated whether sensitivity to MEK inhibition was determined by oncogene status in 13 human thyroid cancer cell lines: four with BRAF mutations, four RAS, one RET/PTC1, and four wild type.

RESULTS

Growth of BRAF (+) cells was inhibited by the MEK antagonist PD0325901 with an IC(50) of less than 5 nm. By contrast, RAS, RET/PTC1, or wild-type cells had IC(50) of 4 nm to greater than 1000 nm. Sensitivity was not predicted by coexisting mutations in PIK3CA or by PTEN status. Similar effects were obtained with the MEK inhibitor AZD6244. PD0325901 induced a sustained G1/S arrest in BRAF (+) but not BRAF (-) lines. PD0325901 was equipotent at inhibiting pERK1/2 after 2 h, regardless of genetic background, but pERK rebounded at 24 h in most lines. MEK inhibitor resistance was associated with partial refractoriness of pERK to further inhibition by the compounds. AZD6244 was more potent at inhibiting growth of NPA (BRAF +) than Cal62 (KRAS +) xenografts.

CONCLUSION

Thyroid cancers with BRAF mutation are preferentially sensitive to MEK inhibitors, whereas tumors with other MEK-ERK effector pathway gene mutations have variable responses, either because they are only partially dependent on ERK and/or because feedback responses elicit partial refractoriness to MEK inhibition.

The RET Kinase Inhibitor NVP-AST487 Blocks Growth and Calcitonin Gene Expression through Distinct Mechanisms in Medullary Thyroid Cancer Cells

The RET kinase has emerged as a promising target for the therapy of medullary thyroid cancers (MTC) and of a subset of papillary thyroid cancers. NVP-AST487, a N,N'-diphenyl urea with an IC(50) of 0.88 mumol/L on RET kinase, inhibited RET autophosphorylation and activation of downstream effectors, and potently inhibited the growth of human thyroid cancer cell lines with activating mutations of RET but not of lines without RET mutations. NVP-AST487 induced a dose-dependent growth inhibition of xenografts of NIH3T3 cells expressing oncogenic RET, and of the MTC cell line TT in nude mice. MTCs secrete calcitonin, a useful indicator of tumor burden. Human plasma calcitonin levels derived from the TT cell xenografts were inhibited shortly after treatment, when tumor volume was still unchanged, indicating that the effects of RET kinase inhibition on calcitonin secretion were temporally dissociated from its tumor-inhibitory properties. Accordingly, NVP-AST487 inhibited calcitonin gene expression in vitro in TT cells, in part, through decreased gene transcription. These data point to a previously unknown physiologic role of RET signaling on calcitonin gene expression. Indeed, the RET ligands persephin and GDNF robustly stimulated calcitonin mRNA, which was blocked by pretreatment with NVP-AST487. Antagonists of RET kinase activity in patients with MTC may result in effects on plasma calcitonin that are either disproportionate or dissociated from the effects on tumor burden, because RET kinase mediates a physiologic pathway controlling calcitonin secretion. The role of traditional tumor biomarkers may need to be reassessed as targeted therapies designed against oncoproteins with key roles in pathogenesis are implemented.

Inhibitors of Raf kinase activity block growth of thyroid cancer cells with RET/PTC or BRAF mutations in vitro and in vivo

PURPOSE

Papillary thyroid carcinomas are associated with nonoverlapping activating mutations of RET, NTRK, RAS and BRAF, which altogether are present in approximately 70% of cases. We postulated that compounds that inhibit a distal effector in the mitogen-activated protein kinase (MAPK) pathway would inhibit growth and tumorigenicity of human thyroid cancer cell lines with mutations of RET or BRAF.

EXPERIMENTAL DESIGN AND RESULTS

We first examined the effects of AAL-881 and LBT-613, two inhibitors of RAF kinase activity, on RAF-MAPK/extracellular signal-regulated kinase (ERK) kinase (MEK)-ERK activation in thyroid PCCL3 cells after conditional induction of expression of H-RAS(G12V) or BRAF(V600E). Both compounds blocked RAS and RAF-dependent MEK and ERK phosphorylation. They also potently blocked MEK phosphorylation in human thyroid cancer cell lines with either RET/PTC1 (TPC1) or BRAF(V600E) (NPA, ARO, and FRO) mutations. Inhibition of ERK phosphorylation was transient in TPC1 and ARO cells, with recovery of ERK phosphorylation associated with concomitant down-regulation of the MAPK phosphatases MKP-3 and DUSP5. Both compounds inhibited growth of all cell lines, with LBT-613 being approximately 10-fold more potent than AAL-881. TPC1 cells were more sensitive to growth inhibition (IC50 0.1-0.25 and approximately 0.05 micromol/L for AAL-881 and LBT-613, respectively) than BRAF + lines (IC50 2.5-5 and 0.1-0.5 micromol/L, respectively). Growth inhibition was associated with G1 arrest, and induction of cell death. Growth of ARO and NPA tumor xenografts was inhibited by LBT-613 or AAL-881. MEK and ERK phosphorylation was inhibited by both compounds in ARO but not in NPA cell xenografts.

CONCLUSIONS

Compounds that inhibit kinase activity are effective growth inhibitors for poorly differentiated thyroid cancer cell lines with either RET or RAF mutations, and hold promise for treatment of most forms of papillary thyroid carcinoma.

Conditional activation of RET/PTC3 and BRAFV600E in thyroid cells is associated with gene expression profiles that predict a preferential role of BRAF in extracellular matrix remodeling

Papillary thyroid cancers (PTC) are associated with nonoverlapping mutations of genes coding for mitogen-activated protein kinase signaling effectors (i.e., the TK receptors RET or NTRK and the signaling proteins RAS and BRAF). We examined the pattern of gene expression after activation of these oncoproteins in thyroid PCCL3 cells, with the goal of identifying pathways or gene subsets that may account for the phenotypic differences observed in human cancers. We hybridized cDNA from cells treated with or without doxycycline to induce expression of BRAF(V600E), RET/PTC3, or RET/PTC3 with small interfering RNA-mediated knockdown of BRAF, respectively, to slides arrayed with a rat 70-mer oligonucleotide library consisting of 27,342 oligos. Among the RET/PTC3-induced genes, 2,552 did not require BRAF as they were similarly regulated by RET/PTC3 with or without BRAF knockdown and not by expression of BRAF(V600E). Immune response and IFN-related genes were highly represented in this group. About 24% of RET/PTC3-regulated genes were BRAF dependent, as they were similarly modified by RET/PTC3 and BRAF(V600E) but not in cells expressing RET/PTC3 with knockdown of BRAF. A gene cluster coding for components of the mitochondrial electron transport chain pathway was down-regulated in this group, potentially altering regulation of cell viability. Metalloproteinases were also preferentially induced by BRAF, particularly matrix metalloproteinase 3 (MMP3), MMP9, and MMP13. Accordingly, conditional expression of BRAF was associated with markedly increased invasion into Matrigel compared with cells expressing RET/PTC3. The preferential induction of MMPs by BRAF could explain in part the more invasive behavior of thyroid cancers with BRAF mutations.

BRAF mediates RET/PTC-induced mitogen-activated protein kinase activation in thyroid cells: functional support for requirement of the RET/PTC-RAS-BRAF pathway in papillary thyroid carcinogenesis

In human papillary thyroid cancers (PTCs), mutations of RET/PTC, NTRK, RAS, or BRAF are found in about two thirds of cases with practically no overlap, providing genetic evidence that constitutive signaling along RET-RAS-BRAF-MAPK is key to their development. The requirement for BRAF in RET/PTC-mediated MAPK activation and gene expression has not been tested functionally. There are three RAF isoforms: ARAF, BRAF, and CRAF. Compared with the others, ARAF is a much weaker stimulator of MAPK. To determine the key RAF isoform mediating RET/PTC-induced ERK phosphorylation, we stably transfected doxycycline-inducible RET/PTC3-expressing thyroid PCCL3 cells with small interfering RNA vectors to induce selective knockdown of BRAF or CRAF. Conditional RET/PTC3 expression induced comparable ERK phosphorylation in CRAF knockdown and control cells but negligible ERK phosphorylation in BRAF knockdown cells. Selective knockdown of BRAF prevented RET/PTC-dependent down-regulation of the sodium iodide symporter, a gene that confers key biological effects of RET/PTC in PTCs. Moreover, microarray analysis revealed numerous RET/PTC-regulated genes showing requirement of BRAF for appropriate expression. These data indicate that BRAF is required for RET/PTC-induced MAPK activation in thyroid cells and support the notion that BRAF inactivation may be an attractive target for PTCs.