Addressing a Pre-Clinical Pipeline Gap: Development of the Pediatric Acute Myeloid Leukemia Patient-Derived Xenograft Program at Texas Children's Hospital at Baylor College of Medicine

The survival rate of pediatric acute myeloid leukemia (pAML) is currently around 60%. While survival has slowly increased over the past few decades, the development of novel agents likely to further improve survival for this heterogeneous patient population has been limited by gaps in the pAML pre-clinical pipeline. One of the major hurdles in evaluating new agents for pAML is the lack of pAML patient-derived xenograft (PDX) models. Unlike solid tumors and other types of leukemias, AML is notoriously hard to establish in mouse models, likely due in part to the need for specific human microenvironment elements. Our laboratory at TCH/BCM addressed this gap by establishing a systematic PDX workflow, leveraging advanced immunodeficient hosts and capitalizing on our high volume of pAML patients and close coordination between labs and clinical sections. Patients treated at TCH are offered the chance to participate in specimen banking protocols that allow blood and bone marrow collection as well as the collection of relevant clinical data. All patients who consent and have samples available are trialed for PDX development. In addition, samples from the Children's Oncology Group (COG) are also trialed for PDX generation. Serially transplanting PDX models are validated using short tandem repeat (STR) and characterized using both targeted DNA/RNA next generation sequencing and RNAseq. As of March 2023, this systematic approach has resulted in 26 serially transplanting models. Models have been shared with requesting labs to facilitate external pAML pre-clinical studies. Available PDX models can be located through the BCM PDX Portal. We expect our growing PDX resource to make a significant contribution to expediting the testing of promising novel therapeutics for pAML.

ALK Fusion in an Adolescent with Acute Myeloid Leukemia: A Case Report and Review of the Literature

Activating mutations and fusions of the ALK oncogene have been identified as drivers in a number of malignancies. Crizotinib and subsequent ALK tyrosine kinase inhibitors have improved treatment outcomes for these patients. In this paper, we discuss the case of an adolescent patient with acute myeloid leukemia, who was identified to have an activating ALK fusion, which is a rare finding and has never been reported in cases of AML without monosomy 7. Crizotinib was added to this patient's frontline therapy and was well tolerated. In cases of more common gene alterations, existing data supports the use of targeted agents as post-HSCT maintenance therapy; however, crizotinib was not able to be used post-HSCT for this patient due to the inability to obtain insurance coverage.

IL-10 and TNFα are associated with decreased survival in low-risk pediatric acute myeloid leukemia; a children's oncology group report

Pediatric acute myeloid leukemia (AML) is a devastating disease with a high risk of relapse. Current risk classification designates patients as high or low risk (LR) based on molecular features and therapy response. However, 30% of LR patients still suffer relapse, indicating a need for improvement in risk stratification. Cytokine levels, such as IL-6 and IL-10, have been shown to be prognostic in adult AML but have not been well studied in children. Previously, we reported elevated IL-6 levels in pediatric AML bone marrow to be associated with inferior prognosis. Here, we expanded our investigation to assess cytokine levels in diagnostic peripheral blood plasma (PBP) of pediatric AML patients and determined correlation with outcome. Diagnostic PBP was obtained from 80 patients with LR AML enrolled on the Children's Oncology Group AAML1031 study and normal PBP from 11 controls. Cytokine levels were measured and correlation with clinical outcome was assessed. IL-6, TNFα, MIP-3a, and IL-1β were significantly higher in AML patients versus controls when corrected by the Bonferroni method. Furthermore, elevated TNFα and IL-10 were significantly associated with inferior outcomes. Our data demonstrate that in diagnostic PBP of LR pediatric AML patients, certain cytokine levels are elevated as compared to healthy controls and that elevated TNFα and IL-10 are associated with inferior outcomes, supporting the idea that an abnormal inflammatory state may predict poor outcomes. Studies are needed to determine the mechanisms by which these cytokines impact survival, and to further evaluate their use as prognostic biomarkers in pediatric AML.

Repurposing Atovaquone as a Therapeutic against Acute Myeloid Leukemia (AML): Combination with Conventional Chemotherapy Is Feasible and Well Tolerated

Survival of pediatric AML remains poor despite maximized myelosuppressive therapy. The pneumocystis jiroveci pneumonia (PJP)-treating medication atovaquone (AQ) suppresses oxidative phosphorylation (OXPHOS) and reduces AML burden in patient-derived xenograft (PDX) mouse models, making it an ideal concomitant AML therapy. Poor palatability and limited product formulations have historically limited routine use of AQ in pediatric AML patients. Patients with de novo AML were enrolled at two hospitals. Daily AQ at established PJP dosing was combined with standard AML therapy, based on the Medical Research Council backbone. AQ compliance, adverse events (AEs), ease of administration score (scale: 1 (very difficult)-5 (very easy)) and blood/marrow pharmacokinetics (PK) were collected during Induction 1. Correlative studies assessed AQ-induced apoptosis and effects on OXPHOS. PDX models were treated with AQ. A total of 26 patients enrolled (ages 7.2 months-19.7 years, median 12 years); 24 were evaluable. A total of 14 (58%) and 19 (79%) evaluable patients achieved plasma concentrations above the known anti-leukemia concentration (>10 µM) by day 11 and at the end of Induction, respectively. Seven (29%) patients achieved adequate concentrations for PJP prophylaxis (>40 µM). Mean ease of administration score was 3.8. Correlative studies with AQ in patient samples demonstrated robust apoptosis, OXPHOS suppression, and prolonged survival in PDX models. Combining AQ with chemotherapy for AML appears feasible and safe in pediatric patients during Induction 1 and shows single-agent anti-leukemic effects in PDX models. AQ appears to be an ideal concomitant AML therapeutic but may require intra-patient dose adjustment to achieve concentrations sufficient for PJP prophylaxis.

Abstract 3479: UBTF tandem duplications (UBTF-TD) in childhood AML: Enrichment in FLT3-ITD and association with clinical outcome

Childhood AML is an aggressive disease with high rates of failures and poor survival. We have demonstrated that the molecular landscape of AML in children is distinct, and co-occurrence of variants modulate outcomes. Recent discovery of tandem duplication (TD) of the UBTF gene in AML, with enrichment in FLT3-ITD has implicated yet another mutation whose cooperation with FLT3-ITD may modify outcome. Here, we provide a comprehensive evaluation of UBTF-TD in de novo AML and define its clinical implications within FLT3-ITD patients. Initial interrogation of transcriptome data from 1,158 children enrolled on COG AAML1031 identified 50 cases of UBTF-TD (4.3%). Overwhelming majority of UBTF-TD cases were observed in FLT3-ITD cases (77%), vs. that of 1.2% in those without FLT3-ITD (p<0.001). Given extreme enrichment of UBTF-TD in FLT3-ITD, we inquired whether cooperation of UBTF-TD and FLT3-ITD creates a distinct clinical entity. To this end we screened diagnostic DNA from 400 FLT3-ITD patients treated on three consecutive CCG/COG trials (COG AAML1031, COG AAML0531, and CCG-2961) by PCR and fragment analysis. UBTF-TD was identified in 61 FLT3-ITD cases (15.3%).

The data presented here forth focuses on evaluation of implications of UBTF-TD in FLT3-ITD positive patients only. Within the FLT3-ITD patients, initial correlation of UBTF-TD with demographics, disease characteristics, and associated genomic variants was conducted. Patients with and without UBTF-TD had a similar median age at diagnosis (p=0.322), lower diagnostic WBC (p=0.010) and higher marrow blast % (p<0.001). There was a stark paucity of cooperating variants that commonly co-occur with FLT3-ITD, with a single NPM1 mutation (1.6% vs. 29%, p<0.001) and no NUP98 fusions (0% vs. 23%, p<0.001). There was a significant enrichment of WT1 mutations, with 45% UBTF-TD patients with a WT1 mutation (FLT3-ITD/UBTF-TD/WT1), vs. 11% in UBTF-WT (p<0.001). Trisomy 8 (Tri8) was seen in 15% of UBTF-TD. Patients with UBTF-TD had a lower CR rate (44% vs. 60%, p = 0.018), and Higher MRD rate (38% vs. 21%, p<0.001). Patients with and without UBTF-TD had an EFS of 28% vs. 42% (p=0.047) with a corresponding OS of 40% and 57% (p=0.019). Given enrichment of WT1 mutations and Tri8 in patients with UBTF-TD, we studied the outcome UBTF-TD patients in the context of these two variants. FLT3-ITD/UBTF-TD/WT1 patients had a 5-year EFS of 17% vs. 38% for similar patients without WT1 mutations (p=0.0062). Patients with UBTF-TD with additional Tri8 had a similarly poor outcome with an EFS of 23% with a corresponding OS of 33%, providing a distinct high risk UBTF-TD cohort (+WT1 or Tri8), whereas the remaining UBTF-TD patients had a more favorable outcome with EFS and OS of 64% and 86%, respectively (p<0.0001, and p<0.0001). UBTF-TD is a novel genomic entity with high enrichment in patients with FLT3-ITD and a distinct clinical outcome driven by cooperating WT1 mutation and Tri8.

Citation Format: Leila Robinson, Amanda Leonti, Todd A. Alonzo, Yi-Cheng Wang, Michele S. Redell, Rhonda E. Ries, Jenny L. Smith, Tiffany A. Hylkema, Quy Le, E Anders Kolb, Richard Aplenc, Xiaotu Ma, Jeffrey Klco, Katherine Tarlock, Soheil Meshinchi. UBTF tandem duplications (UBTF-TD) in childhood AML: Enrichment in FLT3-ITD and association with clinical outcome [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 3479.

Comparison of the Transcriptomic Signatures in Pediatric and Adult CML

Simple Summary

To investigate whether pediatric and adult chronic myeloid leukemia (CML) have unique molecular characteristics, we studied the transcriptomic signature of pediatric and adult CML cells using high-throughput RNA sequencing. We identified differentially expressed genes and pathways unique to pediatric CML cells compared to adult CML cells. The Rho pathway was significantly dysregulated in pediatric CML cells compared to adult CML cells, suggesting the potential importance in the pathogenesis of pediatric CML. Our study is the first to compare transcriptome profiles of CML across different age groups. A better understanding of the biology of CML across different ages may inform future treatment approaches.

Abstract

Children with chronic myeloid leukemia (CML) tend to present with higher white blood counts and larger spleens than adults with CML, suggesting that the biology of pediatric and adult CML may differ. To investigate whether pediatric and adult CML have unique molecular characteristics, we studied the transcriptomic signature of pediatric and adult CML CD34+ cells and healthy pediatric and adult CD34+ control cells. Using high-throughput RNA sequencing, we found 567 genes (207 up- and 360 downregulated) differentially expressed in pediatric CML CD34+ cells compared to pediatric healthy CD34+ cells. Directly comparing pediatric and adult CML CD34+ cells, 398 genes (258 up- and 140 downregulated), including many in the Rho pathway, were differentially expressed in pediatric CML CD34+ cells. Using RT-qPCR to verify differentially expressed genes, VAV2 and ARHGAP27 were significantly upregulated in adult CML CD34+ cells compared to pediatric CML CD34+ cells. NCF1, CYBB, and S100A8 were upregulated in adult CML CD34+ cells but not in pediatric CML CD34+ cells, compared to healthy controls. In contrast, DLC1 was significantly upregulated in pediatric CML CD34+ cells but not in adult CML CD34+ cells, compared to healthy controls. These results demonstrate unique molecular characteristics of pediatric CML, such as dysregulation of the Rho pathway, which may contribute to clinical differences between pediatric and adult patients.

Aberrantly low STAT3 and STAT5 responses are associated with poor outcome and an inflammatory gene expression signature in pediatric acute myeloid leukemia

The relapse rate for children with acute myeloid leukemia is nearly 40% despite aggressive chemotherapy and often stem cell transplant. We sought to understand how environment-induced signaling responses are associated with clinical response to treatment. We previously reported that patients whose AML cells showed low G-CSF-induced STAT3 activation had inferior event-free survival compared to patients with stronger STAT3 responses. Here, we expanded the paradigm to evaluate multiple signaling parameters induced by a more physiological stimulus. We measured STAT3, STAT5 and ERK1/2 responses to G-CSF and to stromal cell-conditioned medium for 113 patients enrolled on COG trials AAML03P1 and AAML0531. Low inducible STAT3 activity was independently associated with inferior event-free survival in multivariate analyses. For inducible STAT5 activity, those with the lowest and highest responses had inferior event-free survival, compared to patients with intermediate STAT5 responses. Using existing RNA-sequencing data, we compared gene expression profiles for patients with low inducible STAT3/5 activation with those for patients with higher inducible STAT3/5 signaling. Genes encoding hematopoietic factors and mitochondrial respiratory chain subunits were overexpressed in the low STAT3/5 response groups, implicating inflammatory and metabolic pathways as potential mechanisms of chemotherapy resistance. We validated the prognostic relevance of individual genes from the low STAT3/5 response signature in a large independent cohort of pediatric AML patients. These findings provide novel insights into interactions between AML cells and the microenvironment that are associated with treatment failure and could be targeted for therapeutic interventions.

Modulating TNFα activity allows transgenic IL15-Expressing CLL-1 CAR T cells to safely eliminate acute myeloid leukemia

Background

C-type lectin-like molecule 1 (CLL-1) is highly expressed in acute myeloid leukemia (AML) but is absent in primitive hematopoietic progenitors, making it an attractive target for a chimeric antigen receptor (CAR) T-cell therapy. Here, we optimized our CLL-1 CAR for anti-leukemic activity in mouse xenograft models of aggressive AML.

Methods

First, we optimized the CLL-1 CAR using different spacer, transmembrane and costimulatory sequences. We used a second retroviral vector to coexpress transgenic IL15. We measured the effects of each construct on T cell phenotype and sequential (recursive) co culture assays with tumor cell targets to determine the durability of the anti tumor activity by flow cytometry. We administered CAR T cells to mice engrafted with patient derived xenografts (PDX) and AML cell line and determined anti tumor activity by bioluminescence imaging and weekly bleeding, measured serum cytokines by multiplex analysis. After euthanasia, we examined formalin-fixed/paraffin embedded sections. Unpaired two-tailed Student’s t-tests were used and values of p<0.05 were considered significant. Survival was calculated using Mantel-Cox log-rank test.

Results

In vitro, CLL-1 CAR T cells with interleukin-15 (IL15) were less terminally differentiated (p<0.0001) and had superior expansion compared with CD28z-CD8 CAR T cells without IL15 (p<0.001). In both AML PDX and AML cell line animal models, CLL-1 CAR T coexpressing transgenic IL15 initially expanded better than CD28z-CD8 CAR T without IL15 (p<0.0001), but produced severe acute toxicity associated with high level production of human tumor necrosis factor α (TNFα), IL15 and IL2. Histopathology showed marked inflammatory changes with tissue damage in lung and liver. This acute toxicity could be managed by two strategies, individually or in combination. The excessive TNF alpha secretion could be blocked with anti-TNF alpha antibody, while excessive T cell expansion could be arrested by activation of an inducible caspase nine safety switch by administration of dimerizing drug. Both strategies successfully prolonged tumor-free survival.

Conclusion

Combinatorial treatment with a TNFα blocking antibody and subsequent activation of the caspase-9 control switch increased the expansion, survival and antileukemic potency of CLL-1 CAR T-cells expressing transgenic IL15 while avoiding the toxicities associated with excessive cytokine production and long-term accumulation of activated T-cells.

Targeting STAT3 anti-apoptosis pathways with organic and hybrid organic-inorganic inhibitors

Recurrence and drug resistance are major challenges in the treatment of acute myeloid leukemia (AML) that spur efforts to identify new clinical targets and active agents. STAT3 has emerged as a potential target in resistant AML, but inhibiting STAT3 function has proven challenging. This paper describes synthetic studies and biological assays for a naphthalene sulfonamide inhibitor class of molecules that inhibit G-CSF-induced STAT3 phosphorylation in cellulo and induce apoptosis in AML cells. We describe two different approaches to inhibitor design: first, variation of substituents on the naphthalene sulfonamide core allows improvements in anti-STAT activity and creates a more thorough understanding of anti-STAT SAR. Second, a novel approach involving hybrid sulfonamide-rhodium(ii) conjugates tests our ability to use cooperative organic-inorganic binding for drug development, and to use SAR studies to inform metal conjugate design. Both approaches have produced compounds with improved binding potency. In vivo and in cellulo experiments further demonstrate that these approaches can also lead to improved activity in living cells, and that compound 3aa slows disease progression in a xenograft model of AML.

Pediatric myeloid sarcoma: a single institution clinicopathologic and molecular analysis

Myeloid sarcoma (MS) is a neoplastic condition composed of immature myeloid cells involving an extramedullary site. We investigated underlying chromosomal and molecular alterations to assess potential molecular markers of prognosis and outcome in this rare pediatric disease. We conducted a retrospective review of clinicopathologic and cytogenetic data from 33 pediatric patients with MS (ages 1 month-18 years) at our institution over a 32 year period (1984-2016). Tissue-based cancer microarray and targeted next-generation sequencing analysis were performed on six cases. The median age at diagnosis was 2.8 years with a male-to-female ratio of 2.6:1. MS is commonly presented with concomitant marrow involvement (n = 12, 36.4%) or as a recurrence of acute myeloid leukemia (AML; n = 14, 42.4%). The skin (n = 18, 54.5%) and soft tissue (n = 9, 27.3%) were the most common sites of involvement. Twenty-one of 25 samples (84.0%) harbored chromosomal aberrations; KMT2A alterations (n = 10, 40.0%) or complex cytogenetics (n = 7, 28.0%) were most frequent. Mutations in RAS, tyrosine kinase, cell signaling, and chromatin remodeling genes were detected. When compared to pediatric patients with AML without extramedullary involvement (EMI), inferior overall survival (OS) was observed (18.8 months vs. 89.3 months, p = .008). Pediatric patients with MS with non-favorable cytogenetics [abnormalities other than t(8;21), inv(16)/t(16;16), or t(15;17)] had a significantly lower OS compared to patients with AML with non-favorable cytogenetics and no extramedullary involvement (8.0 months vs. 28.1 months, p < .001). Pediatric MS is a rare disease with diverse clinical presentations. Non-favorable cytogenetics may be a poor prognostic marker for pediatric patients with MS and molecular diagnostics can assist with risk stratification and identify potentially actionable targets.

Atovaquone is active against AML by upregulating the integrated stress pathway and suppressing oxidative phosphorylation

Atovaquone, a US Food and Drug Administration-approved antiparasitic drug previously shown to reduce interleukin-6/STAT3 signaling in myeloma cells, is well tolerated, and plasma concentrations of 40 to 80 µM have been achieved with pediatric and adult dosing. We conducted preclinical testing of atovaquone with acute myeloid leukemia (AML) cell lines and pediatric patient samples. Atovaquone induced apoptosis with an EC50 <30 µM for most AML lines and primary pediatric AML specimens. In NSG mice xenografted with luciferase-expressing THP-1 cells and in those receiving a patient-derived xenograft, atovaquone-treated mice demonstrated decreased disease burden and prolonged survival. To gain a better understanding of the mechanism of atovaquone, we performed an integrated analysis of gene expression changes occurring in cancer cell lines after atovaquone exposure. Atovaquone promoted phosphorylation of eIF2α, a key component of the integrated stress response and master regulator of protein translation. Increased levels of phosphorylated eIF2α led to greater abundance of the transcription factor ATF4 and its target genes, including proapoptotic CHOP and CHAC1. Furthermore, atovaquone upregulated REDD1, an ATF4 target gene and negative regulator of the mechanistic target of rapamycin (mTOR), and caused REDD1-mediated inhibition of mTOR activity with similar efficacy as rapamycin. Additionally, atovaquone suppressed the oxygen consumption rate of AML cells, which has specific implications for chemotherapy-resistant AML blasts that rely on oxidative phosphorylation for survival. Our results provide insight into the complex biological effects of atovaquone, highlighting its potential as an anticancer therapy with novel and diverse mechanisms of action, and support further clinical evaluation of atovaquone for pediatric and adult AML.

Management of chronic myeloid leukemia in children and adolescents: Recommendations from the Children's Oncology Group CML Working Group

Chronic myeloid leukemia (CML) accounts for 2-3% of leukemias in children under 15 and 9% in adolescents aged 15-19. The diagnosis and management of CML in children, adolescents, and young adults have several differences compared to that in adults. This review outlines the diagnosis and management of the underlying disease as well as challenges that can occur when dealing with CML in this patient population.

Abstract 3550: Lipid rafts contribute to ligand-induced signaling and survival in pediatric AML

Pediatric acute myeloid leukemia (pAML) is an aggressive malignancy with high mortality due to therapy resistance and relapse. We previously reported that the failure of pAML cells to phosphorylate STAT3 upon G-CSF or IL-6 stimulation was associated with inferior survival. The defect in STAT3 signaling was not due to low expression of receptors or of STAT3. We hypothesized that the signaling failure seen in some pAML samples is not specific for STAT3, but rather a generalized dysfunction due to abnormal lipid raft function. Lipid rafts (LR) are highly ordered plasma membrane microdomains rich in glycosphingolipids and cholesterol. Two types of LR - flat planar and invaginated caveolar - contain different membrane proteins and serve distinct but overlapping cellular functions. Both types of LR organize receptors, downstream mediators, and repressors for efficient signaling. The role of LRs in signaling and clinical outcome is not yet established in pAML. Primary samples (n=95) from the Children's Oncology Group (COG) were stimulated with HS5 stromal cell conditioned medium (HS5CM), which contains IL-6 and dozens of physiological ligands. Basal and induced phospho-STAT3 (pSTAT3), pSTAT5, pAKT and pERK1/2 were measured by intracellular flow cytometry, and the fold change in the mean fluorescence intensity (ΔMFI) was calculated. The HS5CM-induced ΔMFI for pSTAT3 was positively correlated with that for pSTAT5 (p&lt;0.0001, r=0.5), and pAKT (p=0.0002, r=0.4). Samples that failed to increase pSTAT3 failed to activate other pathways, suggesting a generalized signaling dysfunction. In AML cells with intact signaling, disruption of LRs with methylβcyclodextrin markedly reduced HS5CM-induced pSTATs, recapitulating the dysfunctional phenotype. LRs were assessed by immunocytochemistry in 29 primary samples with and without HS5CM stimulation. Choleratoxin B, which binds ganglioside GM-1, stained LRs. Antibodies to caveolin-1 (cav-1) marked caveolar LRs. We also stained for gp130, the signaling subunit of the IL-6 receptor. Images were captured by deconvolution fluorescence microscopy. Expression was quantified by pixel intensity for average of 30 cells per condition. Colocalization of gp130 with LR markers was determined by the Manders Overlap Coefficient (MOC). Intensities and MOCs varied between samples. The MOC of gp130 with cav-1 correlated significantly with pSTAT3 ΔMFI (p=0.02, r=0.43). MOC of gp130 with GM-1 did not correlate with pSTAT3 ΔMFI (p=0.85, r=0.03), suggesting that cav-1 facilitates IL-6/STAT3 signaling. Interestingly, patients whose samples demonstrated basal cav-1 and GM-1 intensities below the median had inferior 5-year event-free survival compared to those with intensities above the median (log-rank p=0.079 for cav-1; p=0.013 for GM-1). Our results suggest that LRs contribute to ligand-induced signaling and possibly to chemotherapy response. Further studies of LR biology in AML are warranted.

Citation Format: Padmini Narayanan, Julien Dubrulle, Robert B. Gerbing, Todd A. Alonzo, Fabio Stossi, Michele S. Redell. Lipid rafts contribute to ligand-induced signaling and survival in pediatric AML [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3550.

Assessing the intracellular fate of rhodium(ii) complexes

Rhodium(ii)-fluorophore conjugates have strong rhodium-based fluorescence quenching that can be harnessed to report on a conjugate's cellular uptake and the intracellular decomposition rate. Information gleened from this study allowed the design of an improved STAT3 metalloinhibitor.

Rhodium(II) Proximity-Labeling Identifies a Novel Target Site on STAT3 for Inhibitors with Potent Anti-Leukemia Activity

Nearly 40 % of children with acute myeloid leukemia (AML) suffer relapse arising from chemoresistance, often involving upregulation of the oncoprotein STAT3 (signal transducer and activator of transcription 3). Herein, rhodium(II)-catalyzed, proximity-driven modification identifies the STAT3 coiled-coil domain (CCD) as a novel ligand-binding site, and we describe a new naphthalene sulfonamide inhibitor that targets the CCD, blocks STAT3 function, and halts its disease-promoting effects in vitro, in tumor growth models, and in a leukemia mouse model, validating this new therapeutic target for resistant AML.

Stromal CYR61 Confers Resistance to Mitoxantrone via Spleen Tyrosine Kinase Activation in Human Acute Myeloid Leukaemia

Approximately 50% of children with acute myeloid leukaemia (AML) relapse, despite aggressive chemotherapy. The bone marrow stromal environment protects leukaemia cells from chemotherapy (i.e., stroma-induced chemoresistance), eventually leading to recurrence. Our goal is to delineate the mechanisms underlying stroma-mediated chemoresistance in AML. We used two human bone marrow stromal cell lines, HS-5 and HS-27A, which are equally effective in protecting AML cells from chemotherapy-induced apoptosis in AML-stromal co-cultures. We found that CYR61 was highly expressed by stromal cells, and was upregulated in AML cells by both stromal cell lines. CYR61 is a secreted matricellular protein and is associated with cell-intrinsic chemoresistance in other malignancies. Here, we show that blocking stromal CYR61 activity, by neutralization or RNAi, increased mitoxantrone-induced apoptosis in AML cells in AML-stromal co-cultures, providing functional evidence for its role in stroma-mediated chemoresistance. Further, we found that spleen tyrosine kinase (SYK) mediates CYR61 signalling. Exposure to stroma increased SYK expression and activation in AML cells, and this increase required CYR61. SYK inhibition reduced stroma-dependent mitoxantrone resistance in the presence of CYR61, but not in its absence. Therefore, SYK is downstream of CYR61 and contributes to CYR61-mediated mitoxantrone resistance. The CYR61-SYK pathway represents a potential target for reducing stroma-induced chemoresistance.

Abstract 4206: Increased responsiveness to ligand stimulation of the STAT pathway at relapse in acute myelogenous leukemia

Background: We have shown that Stat pathway sensitivity to G-CSF and IL-6, measured by the increase in tyrosine phosphorylated Stat3 (pY-Stat3), is associated with clinical outcome in pediatric AML patients. These results support a possible relationship between chemoresponsiveness and ligand-induced Stat3 response. We hypothesized that consistently altered changes in Stat3 signaling between diagnosis and relapse would represent advantageous adaptations that promote chemotherapy resistance in pediatric AML.

Methods: 25 sample pairs from initial diagnosis and relapse from pediatric AML patients treated on the COG frontline trial AAML0531 were analyzed. After thawing, ≥80% viability was confirmed by Trypan exclusion. Constitutively phosphorylated Stats (pY-Stat3, pS-Stat3, and pY-Stat5), total Stat3 (TStat3), G-CSF receptor, and gp130 were measured in unstimulated cells. Additionally, cells were stimulated for 15 minutes with 10 or 100 ng/ml G-CSF, or 5 or 50 ng/ml IL-6 with 10 or 100 ng/ml soluble IL-6 receptor, respectively, for measurement of ligand-induced pStats. Data were collected on the LSR II (BD) and analyzed with FCS Express 4 (DeNovo). For the ligand-induced pStats, data are expressed as the fold change in mean fluorescence intensity (ΔMFI) of the stimulated sample compared to the corresponding unstimulated sample. Constitutive pStats, receptors, and TStat3 data are expressed as percent in the positive region, as defined by the upper limit of the signal in the isotype control. The Wilcoxon Signed Rank test was used to test for significant differences in parameters between diagnosis and relapse.

Results: 24/25 sample pairs had adequate cell numbers and viability for analysis. At both doses of G-CSF, 21/24 pairs demonstrated an increase in pY-Stat3 ΔMFI. At the lower G-CSF dose, the mean ±SEM ΔMFI increased from 1.41±0.13 to 2.36±0.28 (p=0.0001). At the higher dose, ΔMFI increased from 1.65±0.20 to 2.71±0.33 (p=0.0002). Similarly, at both doses of IL-6, 17/24 pairs demonstrated an increase in pY-Stat3 ΔMFI. At the lower IL-6 dose, the ΔMFI of pY-Stat3 increased from 1.25±0.09 to 1.56±0.16 (p=0.0168). At the higher dose, ΔMFI increased from 1.49±0.17 to 2.01±0.24 (p=0.0051). Only 3/24 pairs showed &gt;15% increase in G-CSF receptor expression, while 10/24 had a &gt;15% increase in gp130 expression. No significant changes were seen in constitutive activity of pStats, or TStat3 expression.

Conclusions: Our data demonstrate that ligand-induced Stat3 signaling pathways evolve to become stronger at relapse in pediatric AML. More robust signaling was not due to increased expression of total Stat3 and was rarely associated with increased receptor expression. Our data suggest that ligand-induced Stat pathway activation may be promoting survival in relapsed AML. Our results provide support for further development and evaluation of targeted agents against this pathway in AML.

Citation Format: Alexandra M. Stevens, Marcos Ruiz, Michele S. Redell. Increased responsiveness to ligand stimulation of the STAT pathway at relapse in acute myelogenous leukemia. [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 4206. doi:10.1158/1538-7445.AM2014-4206

Abstract 4962: Stroma-induced molecules and chemotherapy resistance in acute myeloid leukemia cells

Overall objective: Acute myeloid leukemia (AML) is a devastating malignancy with a relapse rate near 50% in children, despite very toxic chemotherapy. Once a child relapses, the survival rate is very low. Therefore new, rational therapies for AML are desperately needed. Accumulating evidence shows that the bone marrow stroma protects a subset of leukemia cells from chemotherapy, eventually leading to recurrence. Our goal is to delineate the mechanisms underlying stroma-mediated chemotherapy resistance in AML.

Methods, Results & Conclusions: We used a human bone marrow stromal cell line, HS-27A and two human AML cell lines, THP-1 and NB-4. HS-27A cells provide important contact-dependent interactions with AML cells and secrete only a few soluble factors. Our HS-27A cells constitutively express mOrange, to allow discrimination from AML cells after co-culture.

We performed co-culture experiments to determine the ability of stromal cells to confer resistance to chemotherapy. THP-1 and NB-4 cells were cultured alone or co-cultured with HS-27A cells, and treated with etoposide, mitoxantrone or cytarabine for 48 hours before labeling with annexin V for apoptosis assay by FACS. HS-27A cells significantly protected THP-1 and NB-4 cells from all three chemotherapy agents.

To discover genes in AML cells that are induced by stromal cells and may contribute to chemotherapy resistance, oligonucleotide microarray analysis was done using total RNA extracted from THP-1 or NB-4 cells cultured alone or co-cultured with HS-27A cells. We identified over 1000 genes that were similarly differentially expressed in both AML cell lines by HS-27A co-culture. Among the significantly upregulated genes was cysteine-rich, angiogenic inducer 61 (CYR61). CYR61 is a secreted, extracellular matrix-associated signaling molecule, which has been related to chemotherapy resistance in breast cancer cells and malignant melanoma cells. To validate the microarray results, qRT-PCR was done with RNA from THP-1 cells and NB-4 cells, cultured alone or co-cultured with HS-27A cells. CYR61 expression was increased over 100-fold in those cell lines when co-cultured with HS-27A cells. Next, we examined CYR61 protein expression by immunocytochemistry, with co-staining for myeloperoxidase to confirm that CYR61-expressing cells were AML cells. This study confirmed that CYR61 was highly expressed in THP-1 cells co-cultured with HS-27A cells, but barely detectable in THP-1 cells cultured alone. Further studies are underway to determine if CYR61 contributes to stroma-induced chemotherapy resistance in AML.

This study suggests that CYR61 induced by the microenvironment may promote chemotherapy resistance in AML cells. Elucidating the pathways involved in chemotherapy resistance is likely to result in promising combination therapies to reduce relapse, and thereby improve survival for children with AML.

Citation Format: Xin Long, Laszlo Perlaky, Tsz-Kwong Man, Michele S. Redell. Stroma-induced molecules and chemotherapy resistance in acute myeloid leukemia cells. [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 4962. doi:10.1158/1538-7445.AM2013-4962

Intestinal perforation after treatment of Burkitt's lymphoma: case report and review of the literature

Non-Hodgkin's Lymphoma (NHL) is the most common intestinal malignancy in children, and Burkitt's lymphoma is the most frequently encountered histologic subtype. In pediatric patients, intestinal involvement of the lymphoma is a common finding. As over half of these intestinal tumors are unresectable at the time of presentation, chemotherapy is the mainstay of treatment. However, as the tumor responds to chemotherapy, regression of the tumor in the bowel wall can result in intestinal perforation. We report a unique case of a pediatric patient with lymphoma-related intestinal perforation which was managed with a damage control laparotomy, discuss operative management strategies in these difficult cases, and provide a review of similar cases in the literature.

Synthesis, Activity and Metabolic Stability of Non-Ribose Containing Inhibitors of Histone Methyltransferase DOT1L

Histone methyltransferase DOT1L is a drug target for MLL leukemia. We report an efficient synthesis of a cyclopentane-containing compound that potently and selectively inhibits DOT1L (K_i = 1.1 nM) as well as H3K79 methylation (IC₅₀ ~ 200 nM). Importantly, this compound exhibits a high stability in plasma and liver microsomes, suggesting it is a better drug candidate.

Abstract 2908: Multiparameter FACS analysis of Stat3 and Stat5 signaling in pediatric AML samples

Pediatric acute myeloid leukemia (AML) is a devastating disease with a relapse rate near 50%. New targeted therapies are entering clinical use, yet the pathways affected by these drugs remain poorly understood. Aberrantly active Stat3 and Stat5 are found in AML and have been associated with chemoresistant disease, likely due to increased pro-survival gene expression. Our hypothesis is that analysis of Stat3/5 activation, and expression levels of key proteins in the pathways, will provide insight into mechanisms of aberrant signaling, and will guide the development of targeted therapies. We performed multiparameter FACS analysis of tyrosine- and serine-phosphorylated Stat3 (pY- and pS-Stat3; n=65) and pY-Stat5 (n=47) in pediatric AML samples from the Children's Oncology Group. Constitutive activation was common and quite variable (median pY-Stat3+ events/sample: 38%, range: 3 – 82%; median pY-Stat5+: 34%, range: 5.9 – 77%). There was non-significant correlation between %pY-Stat3+ and %pY-Stat5+ events in unstimulated samples. Constitutive pS-Stat3 was rare (median 2%). As expected, there was a significant correlation between constitutive %pY-Stat3+ and total Stat3 expression (by MFI), with linear correlation coefficient R=0.411 (p&lt;0.001). Most samples were not responsive to G-CSF or IL-6; 38/65 (58%) failed to show at least a 2-fold increase in the pY-Stat3 MFI after treatment with G-CSF (100 ng/ml), and 42/65 (65%) did not respond to IL-6 (50 ng/ml IL-6 + 100 ng/ml soluble IL-6 receptor (sIL6R)). A pY-Stat3 response to only this high dose of G-CSF was seen in 23% of samples, while 15% responded to 10 ng/ml, and only 1 sample responded to 1 ng/ml G-CSF. The patterns of pY-Stat5 responses to G-CSF and of pY-Stat3 responses to IL-6 were similar. The pY-Stat3 and pY-Stat5 responses to G-CSF, as well as the pY-Stat3 responses to G-CSF and to IL-6, were significantly correlated (R=0.911, p&lt;0.001 and R=0.431, p&lt;0.001, respectively), suggesting overlap of these signaling pathways. Surface G-CSFR expression positively correlated with the GCSF-induced pY-Stat3 response (R=0.333, p=0.005). Interestingly, surface gp130 expression negatively correlated with the IL6-induced response (R=−0.349, p&lt;0.005), meaning samples with robust IL-6 responses tended to have fewer gp130+ cells. Expression of the negative regulators SOCS3 and Shp1 were measured by Western blot (n=46) and normalized to the AML cell line Kasumi-1. SOCS3 expression in primary samples was consistently low, whereas Shp1 expression was up to 26-fold higher in primary samples compared to Kasumi-1 cells. Neither SOCS3 nor Shp1 levels varied in proportion to basal pY-Stat3. In summary, multiparameter FACS analysis of baseline and ligand-induced Stat3 and Stat5 activity provides biologic insight into signaling aberrancies in AML, and this knowledge will promote rational design and testing of new therapies.

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 2908. doi:10.1158/1538-7445.AM2011-2908

Stat3 isoforms, alpha and beta, demonstrate distinct intracellular dynamics with prolonged nuclear retention of Stat3beta mapping to its unique C-terminal end

Two isoforms of Stat3 (signal transducer and activator of transcription 3) are expressed in cells, alpha (p92) and beta (p83), both derived from a single gene by alternative mRNA splicing. The 55-residue C-terminal transactivation domain of Stat3alpha is deleted in Stat3beta and replaced by seven unique C-terminal residues (CT7) whose function remains uncertain. We subcloned the open reading frames of Stat3alpha and Stat3beta into the C terminus of green fluorescent protein (GFP). Fluorescent microscopic analysis of HEK293T cells transiently transfected with GFP-Stat3alpha or GFP-Stat3beta revealed similar kinetics and cytokine concentration dependence of nuclear accumulation; these findings were confirmed by high throughput microscope analysis of murine embryonic fibroblasts that lacked endogenous Stat3 but stably expressed either GFP-Stat3alpha or GFP-Stat3beta. However, although time to half-maximal cytoplasmic reaccumulation after cytokine withdrawal was 15 min for GFP-Stat3alpha, it was >180 min for GFP-Stat3beta. Furthermore, although the intranuclear mobility of GFP-Stat3alpha was rapid and increased with cytokine stimulation, the intranuclear mobility of GFP-Stat3beta in unstimulated cells was slower than that of GFP-Stat3alpha in unstimulated cells and was slowed further following cytokine stimulation. Deletion of the unique CT7 domain from Stat3beta eliminated prolonged nuclear retention but did not alter its intranuclear mobility. Thus, Stat3alpha and Stat3beta have distinct intracellular dynamics, with Stat3beta exhibiting prolonged nuclear retention and reduced intranuclear mobility especially following ligand stimulation. Prolonged nuclear retention, but not reduced intranuclear mobility, mapped to the CT7 domain of Stat3beta.

Conditional overexpression of Stat3alpha in differentiating myeloid cells results in neutrophil expansion and induces a distinct, antiapoptotic and pro-oncogenic gene expression pattern

Normal neutrophil development requires G-CSF signaling, which includes activation of Stat3. Studies of G-CSF-mediated Stat3 signaling in cell culture and transgenic mice have yielded conflicting data regarding the role of Stat3 in myelopoiesis. The specific functions of Stat3 remain unclear, in part, because two isoforms, Stat3alpha and Stat3beta, are expressed in myeloid cells. To understand the contribution of each Stat3 isoform to myelopoiesis, we conditionally overexpressed Stat3alpha or Stat3beta in the murine myeloid cell line 32Dcl3 (32D) and examined the consequences of overexpression on cell survival and differentiation. 32D cells induced to overexpress Stat3alpha, but not Stat3beta, generated a markedly higher number of neutrophils in response to G-CSF. This effect was a result of decreased apoptosis but not of increased proliferation. Comparison of gene expression profiles of G-CSF-stimulated, Stat3alpha-overexpressing 32D cells with those of cells with normal Stat3alpha expression revealed novel Stat3 gene targets, which may contribute to neutrophil expansion and improved survival, most notably Slc28a2, a purine nucleoside transporter, which is critical for maintenance of intracellular nucleotide levels and prevention of apoptosis, and Gpr65, an acid-sensing, G protein-coupled receptor with pro-oncogenic and antiapoptotic functions.

Targeting transcription factors in cancer: Challenges and evolving strategies

As investigators uncover the molecular pathways leading to cancer, an abundance of potential molecular targets is accumulating. Among these targets, transcription factors are especially promising. This review discusses the reasons for focusing targeting efforts on transcription factors and highlights some salient examples. The advantages and problems with current methods for targeting are summarized. Finally, we discuss the drug delivery technologies in development that may one day make transcription factor targeting a therapeutic reality.:

Targeting transcription factors for cancer therapy

Advances in the molecular biology of oncogenesis have established a key role for transcription factors in malignant transformation. In some cases the activity of the transcription factor itself is altered by mutation. In many other cases, the activity of the transcription factor is affected by mutations in upstream signaling or regulatory proteins. This review highlights four transcription factors--Stat3, Stat5, NF-kappaB, and HIF-1--which are associated with cancer development. The evidence for the involvement of these factors in oncogenesis is reviewed. Further, we examine the efforts to specifically target these transcription factors for therapeutic intervention. Such strategies include using peptidomimetics, antisense oligonucleotides, small molecule inhibitors, and G-quartet oligonucleotides. Inhibition of transcription factor activity may occur at the level of activation, translocation, or DNA binding. Application of these approaches to in vitro and in vivo models of tumorigenesis is discussed.