Anthracycline Therapy Modifies Immune Checkpoint Signaling in the Heart

Cancer survival rates have increased significantly because of improvements in therapy regimes and novel immunomodulatory drugs. Recently, combination therapies of anthracyclines and immune checkpoint inhibitors (ICIs) have been proposed to maximize neoplastic cell removal. However, it has been speculated that a priori anthracycline exposure may prone the heart vulnerable to increased toxicity from subsequent ICI therapy, such as an anti-programmed cell death protein 1 (PD1) inhibitor. Here, we used a high-dose anthracycline mouse model to characterize the role of the PD1 immune checkpoint signaling pathway in cardiac tissue using flow cytometry and immunostaining. Anthracycline treatment led to decreased heart function, increased concentration of markers of cell death after six days and a change in heart cell population composition with fewer cardiomyocytes. At the same time point, the number of PD1 ligand (PDL1)-positive immune cells and endothelial cells in the heart decreased significantly. The results suggest that PD1/PDL1 signaling is affected after anthracycline treatment, which may contribute to an increased susceptibility to immune-related adverse events of subsequent anti-PD1/PDL1 cancer therapy.

Programmed cell death protein 1 (PD1) deficiency induces cardiac inflammation during baseline conditions and ischemia/reperfusion injury

Background

The Programmed cell death protein 1 (PD1) immune checkpoint is densely expressed on cardiac endothelial cells. The increasing clinical application of immune checkpoint inhibitor therapy targeting PD1 for the treatment of advanced malignancies has revealed profound cardiovascular side effects that have recently paralleled in a preclinical model. However, the effects of PD1 deficiency during baseline conditions and myocardial injury are so far unknown.

Purpose

This study aims to assess the impact of PD1 deficiency on myocardia immunity, and to evaluate the relevance of PD1 signalling in cardiac disease.

Methods

C57BL/6J wild-type mice and C57BL/6J Pdcd1−/− mice were purchased and bred at the animal facility. In-vivo ischaemia/reperfusion (I/R) injury was applied to assess the response in cardiac injury. In brief, mice were anesthetised followed by lateral thoracotomy and ligation of the left coronary artery for 45 min. For flow cytometry, hearts were removed and subjected to an enzymatic digestion. Single cell solutions were stained with different antibody panels to assess cardiac immune cells and the expression of programmed cell death protein 1 ligand 1 (PDL1). Western blot was conducted after homogenization of snap-frozen heart tissue using specific primary and corresponding secondary antibodies. Immunofluorescence and conventional haematoxylin and eosin stain from 4 μm sections were used to visualize the distribution of PDL1 in cardiac tissue.

Results

While no overt cardiac phenotype was observed in Pdcd1−/− mice, a profound upregulation of pro-inflammatory cytokines was determined during baseline conditions, including interleukin (IL) 1α IL4, and extracellular-signal regulated kinase (ERK) 1/2. NADPH oxidase 1 (NOX1) which is involved in production of reactive oxygen species and endothelial injury response was downregulated in Pdcd1−/− mice. Following I/R injury, a significant decline in endothelial PDL1 expression was observed, which was attributed to changes in the area at risk, as shown by immunofluorescence staining. The infarct size following in-vivo I/R injury was not altered upon PD1-deficiency as determined by TTC staining. However, flow cytometry determined increased cell numbers of distinct leukocyte subsets during reperfusion-related inflammation.

Conclusion

Deficiency of PD1/PDL1 signalling shows distinct inflammatory changes in cardiac tissue at baseline and I/R injury. The results indicate that relevant PD1-related detrimental effects are not limited to complications from cancer therapy but can be expected in various forms of cardiovascular disease, hence requiring further investigations.

Funding Acknowledgement

Type of funding sources: Public hospital(s). Main funding source(s): Department of Cardiology and Vascular Medicine, West German Heart and Vascular Center, University Hospital Essen, Germany

Targeting early stages of cardiotoxicity from anti-PD1 immune checkpoint inhibitor therapy

AIMS

Cardiac immune-related adverse events (irAEs) from immune checkpoint inhibition (ICI) targeting programmed death 1 (PD1) are of growing concern. Once cardiac irAEs become clinically manifest, fatality rates are high. Cardio-oncology aims to prevent detrimental effects before manifestation of severe complications by targeting early pathological changes. We therefore aimed to investigate early consequences of PD1 inhibition for cardiac integrity to prevent the development of overt cardiac disease.

METHODS AND RESULTS

We investigated cardiac-specific consequences from anti-PD1 therapy in a combined biochemical and in vivo phenotyping approach. Mouse hearts showed broad expression of the ligand PDL1 on cardiac endothelial cells as a main mediator of immune-crosstalk. Using a novel melanoma mouse model, we assessed that anti-PD1 therapy promoted myocardial infiltration with CD4+ and CD8+ T cells, the latter being markedly activated. Left ventricular (LV) function was impaired during pharmacological stress, as shown by pressure-volume catheterization. This was associated with a dysregulated myocardial metabolism, including the proteome and the lipidome. Analogous to the experimental approach, in patients with metastatic melanoma (n = 7) receiving anti-PD1 therapy, LV function in response to stress was impaired under therapy. Finally, we identified that blockade of tumour necrosis factor alpha (TNFα) preserved LV function without attenuating the anti-cancer efficacy of anti-PD1 therapy.

CONCLUSIONS

Anti-PD1 therapy induces a disruption of cardiac immune homeostasis leading to early impairment of myocardial functional integrity, with potential prognostic effects on the growing number of treated patients. Blockade of TNFα may serve as an approach to prevent the manifestation of ICI-related cardiotoxicity.

Superiority of focused ion beam-scanning electron microscope tomography of cardiomyocytes over standard 2D analyses highlighted by unmasking mitochondrial heterogeneity

BACKGROUND

Cardioprotection by preventing or repairing mitochondrial damage is an unmet therapeutic need. To understand the role of cardiomyocyte mitochondria in physiopathology, the reliable characterization of the mitochondrial morphology and compartment is pivotal. Previous studies mostly relied on two-dimensional (2D) routine transmission electron microscopy (TEM), thereby neglecting the real three-dimensional (3D) mitochondrial organization. This study aimed to determine whether classical 2D TEM analysis of the cardiomyocyte ultrastructure is sufficient to comprehensively describe the mitochondrial compartment and to reflect mitochondrial number, size, dispersion, distribution, and morphology.

METHODS

Spatial distribution of the complex mitochondrial network and morphology, number, and size heterogeneity of cardiac mitochondria in isolated adult mouse cardiomyocytes and adult wild-type left ventricular tissues (C57BL/6) were assessed using a comparative 3D imaging system based on focused ion beam-scanning electron microscopy (FIB-SEM) nanotomography. For comparison of 2D vs. 3D data sets, analytical strategies and mathematical comparative approaches were performed. To confirm the value of 3D data for mitochondrial changes, we compared the obtained values for number, coverage area, size heterogeneity, and complexity of wild-type cardiomyocyte mitochondria with data sets from mice lacking the cytosolic and mitochondrial protein BNIP3 (BCL-2/adenovirus E1B 19-kDa interacting protein 3; Bnip3^-/- ) using FIB-SEM. Mitochondrial respiration was assessed on isolated mitochondria using the Seahorse XF analyser. A cardiac biopsy was obtained from a male patient (48 years) suffering from myocarditis.

RESULTS

The FIB-SEM nanotomographic analysis revealed that no linear relationship exists for mitochondrial number (r = 0.02; P = 0.9511), dispersion (r = -0.03; P = 0.9188), and shape (roundness: r = 0.15, P = 0.6397; elongation: r = -0.09, P = 0.7804) between 3D and 2D results. Cumulative frequency distribution analysis showed a diverse abundance of mitochondria with different sizes in 3D and 2D. Qualitatively, 2D data could not reflect mitochondrial distribution and dynamics existing in 3D tissue. 3D analyses enabled the discovery that BNIP3 deletion resulted in more smaller, less complex cardiomyocyte mitochondria (number: P < 0.01; heterogeneity: C.V. wild-type 89% vs. Bnip3^-/- 68%; complexity: P < 0.001) forming large myofibril-distorting clusters, as seen in human myocarditis with disturbed mitochondrial dynamics. Bnip3^-/- mice also show a higher respiration rate (P < 0.01).

CONCLUSIONS

Here, we demonstrate the need of 3D analyses for the characterization of mitochondrial features in cardiac tissue samples. Hence, we observed that BNIP3 deletion physiologically acts as a molecular brake on mitochondrial number, suggesting a role in mitochondrial fusion/fission processes and thereby regulating the homeostasis of cardiac bioenergetics.

Ca2+ functions as a molecular switch that controls the mutually exclusive complex formation of pyridoxal phosphatase with CIB1 or calmodulin

Pyridoxal 5'-phosphate (PLP) is an essential cofactor for neurotransmitter metabolism. Pyridoxal phosphatase (PDXP) deficiency in mice increases PLP and γ-aminobutyric acid levels in the brain, yet how PDXP is regulated is unclear. Here, we identify the Ca2+ - and integrin-binding protein 1 (CIB1) as a PDXP interactor by yeast two-hybrid screening and find a calmodulin (CaM)-binding motif that overlaps with the PDXP-CIB1 interaction site. Pulldown and crosslinking assays with purified proteins demonstrate that PDXP directly binds to CIB1 or CaM. CIB1 or CaM does not alter PDXP phosphatase activity. However, elevated Ca2+ concentrations promote CaM binding and, thereby, diminish CIB1 binding to PDXP, as both interactors bind in a mutually exclusive way. Hence, the PDXP-CIB1 complex may functionally differ from the PDXP-Ca2+ -CaM complex.

Synergistic activity of BET inhibitor MK-8628 and PLK inhibitor Volasertib in preclinical models of medulloblastoma

Medulloblastoma is the most prevalent central nervous system tumor in children. Targeted treatment approaches for patients with high-risk medulloblastoma are needed as current treatment regimens are not curative in many cases and cause significant therapy-related morbidity. Medulloblastoma harboring MYC amplification have the most aggressive clinical course and worst outcome. Targeting the BET protein BRD4 has significant anti-tumor effects in preclinical models of MYC-amplified medulloblastoma, however, in most cases these are not curative. We here assessed the therapeutic efficacy of the orally bioavailable BRD4 inhibitor, MK-8628, in preclinical models of medulloblastoma. MK-8628 showed therapeutic efficacy against in vitro and in vivo models of MYC-amplified medulloblastoma by inducing apoptotic cell death and cell cycle arrest. Gene expression analysis of cells treated with MK-8628 showed that anti-tumor effects were accompanied by significant repression of MYC transcription as well as disruption of MYC-regulated transcriptional programs. Additionally, we found that targeting of MYC protein stability through pharmacological PLK1 inhibition showed synergistic anti-medulloblastoma effects when combined with MK-8628 treatment. Thus, MK-8628 is effective against preclinical high-risk medulloblastoma models and its effects can be enhanced through simultaneous targeting of PLK1.

Targeting tachykinin receptors in neuroblastoma

Neuroblastoma is the most common extracranial tumor in children. Despite aggressive multimodal treatment, high-risk neuroblastoma remains a clinical challenge with survival rates below 50%. Adding targeted drugs to first-line therapy regimens is a promising approach to improve survival in these patients. TACR1 activation by substance P has been reported to be mitogenic in cancer cell lines. Tachykinin receptor (TACR1) antagonists are approved for clinical use as an antiemetic remedy since 2003. Tachykinin receptor inhibition has recently been shown to effectively reduce growth of several tumor types. Here, we report that neuroblastoma cell lines express TACR1, and that targeting TACR1 activity significantly reduced cell viability and induced apoptosis in neuroblastoma cell lines. Gene expression profiling revealed that TACR1 inhibition repressed E2F2 and induced TP53 signaling. Treating mice harboring established neuroblastoma xenograft tumors with Aprepitant also significantly reduced tumor burden. Thus, we provide evidence that the targeted inhibition of tachykinin receptor signaling shows therapeutic efficacy in preclinical models for high-risk neuroblastoma.

RITA displays anti-tumor activity in medulloblastomas independent of TP53 status

Current therapy of medulloblastoma, the most common malignant brain tumor of childhood, achieves 40–70% survival. Secondary chemotherapy resistance contributes to treatment failure, where TP53 pathway dysfunction plays a key role. MDM2 interaction with TP53 leads to its degradation. Reactivating TP53 functionality using small-molecule inhibitors, such as RITA, to disrupt TP53-MDM2 binding may have therapeutic potential. We show here that RITA decreased viability of all 4 analyzed medulloblastoma cell lines, regardless of TP53 functional status. The decrease in cell viability was accompanied in 3 of the 4 medulloblastoma cell lines by accumulation of TP53 protein in the cells and increased CDKN1A expression. RITA treatment in mouse models inhibited medulloblastoma xenograft tumor growth. These data demonstrate that RITA treatment reduces medulloblastoma cell viability in both in vitro and in vivo models, and acts independently of cellular TP53 status, identifying RITA as a potential therapeutic agent to treat medulloblastoma.

The GSK461364 PLK1 inhibitor exhibits strong antitumoral activity in preclinical neuroblastoma models

Polo-like kinase 1 (PLK1) is a serine/threonine kinase that promotes G2/M-phase transition, is expressed in elevated levels in high-risk neuroblastomas and correlates with unfavorable patient outcome. Recently, we and others have presented PLK1 as a potential drug target for neuroblastoma, and reported that the BI2536 PLK1 inhibitor showed antitumoral actvity in preclinical neuroblastoma models. Here we analyzed the effects of GSK461364, a competitive inhibitor for ATP binding to PLK1, on typical tumorigenic properties of preclinical in vitro and in vivo neuroblastoma models. GSK461364 treatment of neuroblastoma cell lines reduced cell viability and proliferative capacity, caused cell cycle arrest and massively induced apoptosis. These phenotypic consequences were induced by treatment in the low-dose nanomolar range, and were independent of MYCN copy number status. GSK461364 treatment strongly delayed established xenograft tumor growth in nude mice, and significantly increased survival time in the treatment group. These preclinical findings indicate PLK1 inhibitors may be effective for patients with high-risk or relapsed neuroblastomas with upregulated PLK1 and might be considered for entry into early phase clinical trials in pediatric patients.

Targeting MYCN-Driven Transcription By BET-Bromodomain Inhibition

PURPOSE

Targeting BET proteins was previously shown to have specific antitumoral efficacy against MYCN-amplified neuroblastoma. We here assess the therapeutic efficacy of the BET inhibitor, OTX015, in preclinical neuroblastoma models and extend the knowledge on the role of BRD4 in MYCN-driven neuroblastoma.

EXPERIMENTAL DESIGN

The efficacy of OTX015 was assessed in in vitro and in vivo models of human and murine MYCN-driven neuroblastoma. To study the effects of BET inhibition in the context of high MYCN levels, MYCN was ectopically expressed in human and murine cells. The effect of OTX015 on BRD4-regulated transcriptional pause release was analyzed using BRD4 and H3K27Ac chromatin immunoprecipitation coupled with DNA sequencing (ChIP-Seq) and gene expression analysis in neuroblastoma cells treated with OTX015 compared with vehicle control.

RESULTS

OTX015 showed therapeutic efficacy against preclinical MYCN-driven neuroblastoma models. Similar to previously described BET inhibitors, concurrent MYCN repression was observed in OTX015-treated samples. Ectopic MYCN expression, however, did not abrogate effects of OTX015, indicating that MYCN repression is not the only target of BET proteins in neuroblastoma. When MYCN was ectopically expressed, BET inhibition still disrupted MYCN target gene transcription without affecting MYCN expression. We found that BRD4 binds to super-enhancers and MYCN target genes, and that OTX015 specifically disrupts BRD4 binding and transcription of these genes.

CONCLUSIONS

We show that OTX015 is effective against mouse and human MYCN-driven tumor models and that BRD4 not only targets MYCN, but specifically occupies MYCN target gene enhancers as well as other genes associated with super-enhancers. Clin Cancer Res; 22(10); 2470-81. ©2015 AACR.

Neuroblastoma in dialog with its stroma: NTRK1 is a regulator of cellular cross-talk with Schwann cells

In neuroblastoma, the most common solid tumor of childhood, excellent prognosis is associated with extensive Schwann cell (SC) content and high-level expression of the neurotrophin receptor, NTRK1/TrkA, which is known to mediate neuroblastoma cell differentiation. We hypothesized that both stromal composition and neuroblastic differentiation are based on bidirectional neuroblastoma-SC interaction. Reanalysis of microarray data from human SY5Y neuroblastoma cells stably transfected with either NTRK1 or NTRK2 revealed upregulation of the mRNA for the SC growth factor, NRG1, in NTRK1-positive cells. Media conditioned by NTRK1-expressing neuroblastoma cells induced SC proliferation and migration, while antibody-based NRG1 neutralization significantly decreased these effects. Vice versa, NRG1-stimulated SC secreted the NTRK1-specific ligand, NGF. SC-conditioned medium activated the NTRK1 receptor in a neuroblastoma cell culture model conditionally expressing NTRK1 and induced differentiation markers in NTRK1-expressing cells. NTRK1 induction in neuroblastoma xenografts mixed with primary SC also significantly reduced tumor growth in vivo. We propose a model for NTRK1-mediated and NRG1-dependent attraction of adjacent SC, which in turn induce neuroblastic differentiation by secretion of the NTRK1-specific ligand, NGF. These findings have implications for understanding the mature and less malignant neuroblastoma phenotype associated with NTRK1 expression, and could assist the development of new therapeutic strategies for neuroblastoma differentiation.

Abstract 4731: Targeting super-enhancer induced gene expression with the novel BRD4 inhibitor OTX015 in preclinical models of MYCN-amplified neuroblastoma

Bromodomain-containing protein 4 (BRD4) functions as an epigenetic reader and binds to promoter super-enhancer regions driving oncogenes such as MYC. Neuroblastomas (NB) harboring MYCN amplifications are highly lethal tumors often resistant to standard chemotherapy. OTX015 is a novel BRD2/3/4 inhibitor currently in clinical Phase Ib studies in hematologic malignancies and solid tumors. We have previously reported that OTX015 displayed in vitro and in vivo antitumor effects, together with MYCN transcription attenuation in NB models (Henssen et al; AACR 2014). Here, we investigated OTX015 targeting of super-enhancer regulated genes in MYCN-amplified NB in vitro and in vivo models. Protein-DNA interactions were analyzed using ChipSeq in IMR 5 cells. We identified super-enhancers associated with a variety of genes of known importance in NB, including MYCN, as well as some previously undescribed genes. OTX015 inhibited cell proliferation in Chp-212, Chp-134, Gimen, IMR-32, NB69, SK-N-AS, SK-N-BE, and SK-N-BE2 NB cell lines after 72 h exposure. OTX015 reduced tumor burden in IMR 5 xenograft mice and in a genetically engineered model of MYCN-amplified NB LSL MYCN;Dbh-iCre, when administered by oral gavage at a dose of 25 mg/kg daily for 3 weeks. Antitumoral effects of OTX015 were coupled with decreased binding of BRD4 to chromatin and subsequent global transcriptional changes. Moreover, OTX015 exposure led to significant transcriptional downregulation of genes associated with super-enhancers, supporting the notion that BRD4 preferentially acts at these chromatin sites. Interestingly, BRD inhibition not only attenuated MYCN transcription but most significantly affected MYCN-regulated transcriptional programs. Ectopic expression of MYCN was not able to abrogate the antitumoral effects of BRD4 inhibition, indicating direct involvement of MYCN in super-enhancer regulated gene expression and possibly explaining the increased susceptibility of MYCN-amplified NB to OTX015 inhibition. We describe here for the first time that BRD inhibition by OTX015 selectively and preferentially targets global super-enhancer induced transcription in MYCN-driven NB. These new insights will serve as a rationale for a clinical trial in pediatric MYCN-amplified NB patients with OTX015.

Citation Format: Anton Henssen, Kristina Althoff, Richard Koche, Andrea Odersky, Anneleen Beckers, Frank Speleman, Simon Schäfers, Katleen De Preter, Alexandra Florin, Lukas Heukamp, Annika Spruessel, Kathy Astrahanseff, Natalie Sadowski, Alexander Schramm, Angelika Eggert, Lucile Astorgues-Xerri, Eugenia Riveiro, Esteban Cvitkovic, Johannes H. Schulte. Targeting super-enhancer induced gene expression with the novel BRD4 inhibitor OTX015 in preclinical models of MYCN-amplified neuroblastoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4731. doi:10.1158/1538-7445.AM2015-4731

Mutational dynamics between primary and relapse neuroblastomas

Neuroblastoma is a malignancy of the developing sympathetic nervous system that is often lethal when relapse occurs. We here used whole-exome sequencing, mRNA expression profiling, array CGH and DNA methylation analysis to characterize 16 paired samples at diagnosis and relapse from individuals with neuroblastoma. The mutational burden significantly increased in relapsing tumors, accompanied by altered mutational signatures and reduced subclonal heterogeneity. Global allele frequencies at relapse indicated clonal mutation selection during disease progression. Promoter methylation patterns were consistent over disease course and were patient specific. Recurrent alterations at relapse included mutations in the putative CHD5 neuroblastoma tumor suppressor, chromosome 9p losses, DOCK8 mutations, inactivating mutations in PTPN14 and a relapse-specific activity pattern for the PTPN14 target YAP. Recurrent new mutations in HRAS, KRAS and genes mediating cell-cell interaction in 13 of 16 relapse tumors indicate disturbances in signaling pathways mediating mesenchymal transition. Our data shed light on genetic alteration frequency, identity and evolution in neuroblastoma.

Absence of telomerase reverse transcriptase promoter mutations in neuroblastoma

Maintenance of telomere length is a critical hallmark of malignant transformation. While silenced in somatic cells, telomerase reverse transcriptase (TERT), the catalytic subunit of telomerase, is frequently overexpressed in malignant cells thereby maintaining their telomere length. Specific point mutations in the TERT promoter region have recently been identified in melanoma and other tumor entities resulting in high TERT expression. Neuroblastoma is the most common extracranial tumor of childhood, arising from neural-crest progenitor cells. TERT overexpression has been observed in the majority of neuroblastoma. Taking into consideration that TERT promoter mutations are frequently described in neural-crest-derived tumors such as melanoma, as well as a variety of other neuronal tumors, the present study analyzed the frequency of TERT promoter mutations in primary neuroblastoma and neuroblastoma cell lines. In 131 neuroblastoma primary tumors representing the whole spectrum of neuroblastoma, no TERT promoter mutations were detected. However, in 3 out of 19 neuroblastoma cell lines the previously described C228T TERT promoter mutation was present. In conclusion, the TERT promoter mutations are not a frequent mechanism of TERT overexpression in neuroblastoma.

Abstract 3967: BET protein inhibitor OTX015 has selective anti-tumoral activity in preclinical models of MYCN- amplified neuroblastoma

Neuroblastomas harboring MYCN amplifications are highly lethal tumors. They are often resistant to standard chemotherapy, yet the development of targeted therapies has been hampered by a lack of compounds targeting MYCN. We and others have recently discovered that targeting BET bromodomain proteins, especially BRD4, disrupts epigenetic regulation of MYCN and its targets in neuroblastoma. OTX015, a new BET protein inhibitor, is the first lead into clinical phase I/II trials and has shown promising pharmacological properties in adults. Here, we investigate the preclinical efficacy of OTX015 in MYCN-amplified neuroblastoma.

We tested in vitro OTX015 efficacy in 6 established neuroblastoma (NB) cell lines. We performed cell cycle profiling and analyzed markers for apoptosis and proliferation after 72h-treatment at 500 nM OTX015. The effect of OTX015 on MYCN expression and global MYCN-associated transcriptional activity was assessed by quantitative real time PCR and gene expression microarray profiling, respectively. In vivo efficacy of orally OTX015 was assessed in IMR5 xenografts, a N-MYC driven NB model, using diffent treatment schedules (50mg/kg/day, 100mg/kg/day and 50mg/kg/bidaily).

Treatment of MYCN-amplified neuroblastoma cells with OTX015 resulted in decreased cell viability, induction of apoptosis and reduced proliferation. Concentrations of 50% inhibition (IC50) ranged between 50nM and 500nM. OTX015 treatment also resulted in an increase in the percentage of cells in G1 phase. This corresponded with the downregulation of MYCN mRNA and protein levels and MYCN-associated transcriptional activity. Interestigly, MYCN amplified cell lines were most sensitive to OTX015 treatment. In contrast, no effect was observed with OTX15 on normal cells. In vivo treatment with OTX015, significantly decreased tumor burden after 4 weeks and prolonged survival as compared to vehicle-treated mice. These preclinical findings highlight the promise of BET bromodomain inhibitors as novel agents for MYCN-driven neuroblastomas and serve as rationale to move forward with early phase clinical trials for children with these highly lethal tumors.

Citation Format: Johannes H. Schulte, Kristina Althoff, Emma Bell, Andrea Odersky, Anneleen Beckers, Frank Speleman, Simon Schäfers, Alexander Schramm, Angelika Eggert, Frank Westermann, Eugenia Riveiro, Esteban Cvitkovic, Anton Henssen. BET protein inhibitor OTX015 has selective anti-tumoral activity in preclinical models of MYCN- amplified neuroblastoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3967. doi:10.1158/1538-7445.AM2014-3967

MiR-137 functions as a tumor suppressor in neuroblastoma by downregulating KDM1A

Neuroblastoma is the most common extracranial solid tumor of childhood, and accounts for ∼15% of all childhood cancer deaths. The histone demethylase, lysine-specific demethylase 1 (KDM1A, previously known as LSD1), is strongly expressed in neuroblastomas, and overexpression correlates with poor patient prognosis. Inducing differentiation in neuroblastoma cells has previously been shown to down regulate KDM1A, and siRNA-mediated KDM1A knockdown inhibited neuroblastoma cell viability. The microRNA, miR-137, has been reported to be downregulated in several human cancers, and KDM1A mRNA was reported as a putative target of miR-137 in colon cancer. We hypothesized that miR-137 might have a tumor-suppressive role in neuroblastoma mediated via downregulation of KDM1A. Indeed, low levels of miR-137 expression in primary neuroblastomas correlated with poor patient prognosis. Re-expressing miR-137 in neuroblastoma cell lines increased apoptosis and decreased cell viability and proliferation. KDM1A mRNA was repressed by miR-137 in neuroblastoma cells, and was validated as a direct target of miR-137 using reporter assays in SHEP and HEK293 cells. Furthermore, siRNA-mediated KDM1A knockdown phenocopied the miR-137 re-expression phenotype in neuroblastoma cells. We conclude that miR-137 directly targets KDM1A mRNA in neuroblastoma cells, and activates cell properties consistent with tumor suppression. Therapeutic strategies to re-express miR-137 in neuroblastomas could be useful to reduce tumor aggressiveness.

Targeted expression of mutated ALK induces neuroblastoma in transgenic mice

Activating anaplastic lymphoma kinase (ALK) mutations were recently detected in most familial and 10% of sporadic neuroblastomas. However, the role of mutated ALK in tumorigenesis remains elusive. We demonstrate that targeted expression of the most frequent and aggressive variant, ALK(F1174L), is tumorigenic in mice. Tumors resembled human neuroblastomas in morphology, metastasis pattern, gene expression, and the presence of neurosecretory vesicles as well as synaptic structures. This ALK-driven neuroblastoma mouse model precisely recapitulated the genetic spectrum of the disease. Chromosomal aberrations were syntenic to those in human neuroblastoma, including 17q gain and MYCN oncogene amplification. Targeted ALK(F1174L) and MYCN coexpression revealed a strong synergism in inducing neuroblastoma with minimal chromosomal aberrations, suggesting that fewer secondary hits are required for tumor induction if both oncoproteins are targeted. Treatment of ALK(F1174L) transgenic mice with the ALK inhibitor TAE-684 induced complete tumor regression, indicating that tumor cells were addicted to ALK(F1174L) activity. We conclude that an activating mutation within the ALK kinase domain is sufficient to induce neuroblastoma development, and ALK inhibitors show promise for treating human neuroblastomas harboring ALK mutations.

The mutagenic potential of non-homologous end joining in the absence of the NHEJ core factors Ku70/80, DNA-PKcs and XRCC4-LigIV

Non-homologous end joining (NHEJ), the major pathway of double-strand break (DSB) repair in mammalian cells, comprises two subpathways: one that requires the three core factors Ku70/80, DNA-PKcs and XRCC4/LigIV (DNA-PK-dependent NHEJ) and the other that is independent of these factors. Using a cell-free NHEJ assay, we have investigated the ability of three Chinese hamster ovary (CHO) mutants deficient in Ku80 (xrs6), DNA-PKcs (XR-C1) and XRCC4 (XR-1) in comparison with CHO-K1 wild-type cells to rejoin non-compatible DSB ends. Both NHEJ efficiency and fidelity are strongly reduced in the mutants with xrs6 and XR-1 exhibiting the strongest reduction and XR-C1 displaying a phenotype intermediate between the wild-type and the other two mutants indicating a non-essential but facilitating role of DNA-PKcs in NHEJ. The decrease in fidelity in the mutants is expressed by an increase of deletion junctions formed at microhomologies (microhom) near the DSB (microhomology-mediated non-homologous end joining: microhomNHEJ). Using a novel microhomNHEJ assay, we show that microhom regions of 6-10 bp that are located directly at the DSB termini strongly enhance the mutagenic microhomNHEJ reaction even in the wild type. Due to its error proneness, DNA-PK-independent microhomNHEJ may actively promote genome instability. It will, therefore, be of increasing importance to examine NHEJ fidelity in the context with tumorigenesis and cellular senescence for which we here provide two efficient and reliable tools.

Analysis of DNA double-strand break repair by nonhomologous end joining in cell-free extracts from mammalian cells

Double-strand breaks (DSBs) in genomic DNA are induced by ionizing radiation or radiomimetic drugs, but they also occur spontaneously during the cell cycle at quite significant frequencies. In vertebrate cells, nonhomologous DNA end joining (NHEJ) is considered the major pathway of DSB repair. NHEJ is able to rejoin two broken DNA termini directly end-to-end irrespective of sequence and structure. Genetic studies in various radiosensitive and DSB repair-deficient hamster cell lines have yielded insights into the factors involved in NHEJ. Studies in cell-free systems derived from Xenopus eggs and mammalian cells have allowed the dissection of the underlying mechanisms. In the present chapter, we describe a protocol for the preparation of whole cell extracts from mammalian cells and a plasmid-based in vitro assay that permits the easy analysis of the efficiency and fidelity of DSB repair via NHEJ in different cell types.

Repair of sequence-specific 125I-induced double-strand breaks by nonhomologous DNA end joining in mammalian cell-free extracts

In mammalian cells, nonhomologous DNA end joining (NHEJ) is considered the major pathway of double-strand break (DSB) repair. Rejoining of DSB produced by decay of (125)I positioned against a specific target site in plasmid DNA via a triplex-forming oligonucleotide (TFO) was investigated in cell-free extracts from Chinese hamster ovary cells. The efficiency and quality of NHEJ of the "complex" DSB induced by the (125)I-TFO was compared with that of "simple" DSB induced by restriction enzymes. We demonstrate that the extracts are indeed able to rejoin (125)I-TFO-induced DSB, although at approximately 10-fold decreased efficiency compared with restriction enzyme-induced DSB. The resulting spectrum of junctions is highly heterogeneous exhibiting deletions (1-30 bp), base pair substitutions, and insertions and reflects the heterogeneity of DSB induced by the (125)I-TFO within its target site. We show that NHEJ of (125)I-TFO-induced DSB is not a random process that solely depends on the position of the DSB but is driven by the availability of microhomology patches in the target sequence. The similarity of the junctions obtained with the ones found in vivo after (125)I-TFO-mediated radiodamage indicates that our in vitro system may be a useful tool to elucidate the mechanisms of ionizing radiation-induced mutagenesis and repair.