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Supplementary Figures S1-S4. Suppplementary S1: Western Blot analysis o study the effect of the combinatory therapy on DDR Supplementary S2: Evaluation of the percentage of hyper damaged cells among the leukemic compartment Supplementary S3: Cell cycle analysis on OCI-AML2 treated cells Supplementary S4: Model of the mechanism of action of the combinatory therapy PARPi and 5FU

The existing treatments to cure acute leukemias seem to be nonspecific and suboptimal for most patients, drawing attention to the need of new therapeutic strategies. In the last decade the anticancer potential of poly ADP-ribose polymerase (PARP) inhibitors became apparent and now several PARP inhibitors are being developed to treat various malignancies. So far, the usage of PARP inhibitors has been mainly focused on the treatment of solid tumors and not too much about their efficacy on leukemias is known. In this study we test, for the first time on leukemic cells, a combined therapy that associates the conventional chemotherapeutic agent fluorouracil (5FU), used as a source of DNA damage, and a PARP inhibitor, rucaparib. We demonstrate the efficacy and the specificity of this combined therapy in killing both acute myeloid leukemia and acute lymphoid leukemia cells in vitro and in vivo. We clearly show that the inhibition of DNA repair induced by rucaparib is synthetic lethal with the DNA damage caused by 5FU in leukemic cells. Therefore, we propose a new therapeutic strategy able to enhance the cytotoxic effect of DNA-damaging agents in leukemia cells via inhibiting the repair of damaged DNA. Mol Cancer Ther; 14(4); 889–98. ©2015 AACR.

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Alterations of the tumor suppressor gene TP53 are found in different cancers, in particular in carcinomas of adults. In pediatric acute lymphoblastic leukemia (ALL), TP53 mutations are infrequent but enriched at relapse. As in most cancers, mainly DNA-binding domain missense mutations are found, resulting in accumulation of mutant p53, poor therapy response, and inferior outcome. Different strategies to target mutant p53 have been developed including reactivation of p53’s wildtype function by the small molecule APR-246. We investigated TP53 mutations in cell lines and 62 B-cell precursor ALL samples and evaluated the activity of APR-246 in TP53-mutated or wildtype ALL. We identified cases with TP53 missense mutations, high (mutant) p53 expression and insensitivity to the DNA-damaging agent doxorubicin. In TP53-mutated ALL, APR-246 induced apoptosis showing strong anti-leukemia activity. APR-246 restored mutant p53 to its wildtype conformation, leading to pathway activation with induction of transcriptional targets and re-sensitization to genotoxic therapy in vitro and in vivo. In addition, induction of oxidative stress contributed to APR-246-mediated cell death. In a preclinical model of patient-derived TP53-mutant ALL, APR-246 reduced leukemia burden and synergized strongly with the genotoxic agent doxorubicin, leading to superior leukemia-free survival in vivo. Thus, targeting mutant p53 by APR-246, restoring its tumor suppressive function, seems to be an effective therapeutic strategy for this high-risk group of TP53-mutant ALL.

Targeting BCL-2, a key regulator of survival in B-cell malignancies including precursor B-cell acute lymphoblastic leukemia, has become a promising treatment strategy. However, given the redundancy of anti-apoptotic BCL-2 family proteins (BCL-2, BCL-XL, MCL-1), single targeting may not be sufficient. When analyzing the effects of BH3-mimetics selectively targeting BCL-XL and MCL-1 alone or in combination with the BCL-2 inhibitor venetoclax, heterogeneous sensitivity to either of these inhibitors was found in ALL cell lines and in patient-derived xenografts. Interestingly, some venetoclax-resistant leukemias were sensitive to the MCL-1-selective antagonist S63845 and/or BCL-XL-selective A-1331852 suggesting functional mutual substitution. Consequently, co-inhibition of BCL-2 and MCL-1 or BCL-XL resulted in synergistic apoptosis induction. Functional analysis by BH3-profiling and analysis of protein complexes revealed that venetoclax-treated ALL cells are dependent on MCL-1 and BCL-XL, indicating that MCL-1 or BCL-XL provide an Achilles heel in BCL-2-inhibited cells. The effect of combining BCL-2 and MCL-1 inhibition by venetoclax and S63845 was evaluated in vivo and strongly enhanced anti-leukemia activity was found in a pre-clinical patient-derived xenograft model. Our study offers in-depth molecular analysis of mutual substitution of BCL-2 family proteins in acute lymphoblastic leukemia and provides targets for combination treatment in vivo and in ongoing clinical studies.

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive hematological cancer characterized by skewed epigenetic patterns, raising the possibility of therapeutically targeting epigenetic factors in this disease. Here we report that among different cancer types, epigenetic factor TET1 is highly expressed in T-ALL and is crucial for human T-ALL cell growth in vivo. Knockout of TET1 in mice and knockdown in human T cell did not perturb normal T-cell proliferation, indicating that TET1 expression is dispensable for normal T-cell growth. The promotion of leukemic growth by TET1 was dependent on its catalytic property to maintain global 5-hydroxymethylcytosine (5hmC) marks, thereby regulate cell cycle, DNA repair genes, and T-ALL associated oncogenes. Furthermore, overexpression of the Tet1-catalytic domain was sufficient to augment global 5hmC levels and leukemic growth of T-ALL cells in vivo. We demonstrate that PARP enzymes, which are highly expressed in T-ALL patients, participate in establishing H3K4me3 marks at the TET1 promoter and that PARP1 interacts with the TET1 protein. Importantly, the growth related role of TET1 in T-ALL could be antagonized by the clinically approved PARP inhibitor Olaparib, which abrogated TET1 expression, induced loss of 5hmC marks, and antagonized leukemic growth of T-ALL cells, opening a therapeutic avenue for this disease.

The FOXO1 transcription factor plays an essential role in the regulation of proliferation and survival programs at early stages of B-cell differentiation. Here, we show that tightly regulated FOXO1 activity is essential for maintenance of B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Genetic and pharmacological inactivation of FOXO1 in BCP-ALL cell lines produced a strong antileukemic effect associated with CCND3 downregulation. Moreover, we demonstrated that CCND3 expression is critical for BCP-ALL survival and that overexpression of CCND3 protected BCP-ALL cell lines from growth arrest and apoptosis induced by FOXO1 inactivation. Most importantly, pharmacological inhibition of FOXO1 showed antileukemia activity on several primary, patient-derived, pediatric ALL xenografts with effective leukemia reduction in the hematopoietic, lymphoid, and central nervous system organ compartments, ultimately leading to prolonged survival without leukemia reoccurrence in a preclinical in vivo model of BCP-ALL. These results suggest that repression of FOXO1 might be a feasible approach for the treatment of BCP-ALL.

In contrast to well-established hierarchical concepts of tumor stem cells, leukemia-initiating cells in B-cell precursor acute lymphoblastic leukemia have not yet been phenotypically identified. Different subpopulations, as defined by surface markers, have shown equal abilities to reconstitute leukemia upon transplantation into immunodeficient mice. Using a non-obese diabetes/severe combined immunodeficiency human acute lymphoblastic leukemia mouse model and cell cycle analysis annotating cells to distinct cycle phases, we functionally characterized leukemia-initiating cells and found that cells in all stages of the cell cycle are able to reconstitute leukemia in vivo, with early cycling cells (G1blow population) exhibiting the highest leukemia-initiating potential. Interestingly, cells of the G2/M compartment, i.e. dividing cells, were less effective in leukemia reconstitution. Moreover, G1blow cells were more resistant to spontaneous or drug-induced cell death in vitro, were enriched for stem cell signatures and were less metabolically active, as determined by lower levels of reactive oxygen species, compared to G2/M stage cells. Our data provide new information on the biological properties of leukemia-initiating cells in B-cell precursor acute lymphoblastic leukemia and underline the concept of a stochastic model of leukemogenesis.

In acute lymphoblastic leukemia (ALL), the most frequent malignancy in children and adolescents, deregulated cell death pathways contribute to leukemia development and therapy failure. Apoptosis (programmed cell death) is controlled at the mitochondrial level by different pro- and anti-apoptotic regulators. Molecules of the BCL-2 family are key in regulating intrinsic apoptosis signaling. Pro-apoptotic BH3-only proteins such as BIM and BID activate pro-death proteins like BAX and BAK leading to cell death, while anti-apoptotic BCL-2 family members including BCL-2, MCL-1 and BCL-XL sequester pro-apoptotic molecules, thereby preventing pro-death protein activation and apoptosis induction. Small molecule inhibitors have been developed, which bind to the anti-apoptotic molecules BCL-2, MCL-1 and BCL-XL leading to cell death induction. In particular inhibition of BCL-2, a key regulator of survival in B-cell malignancies including BCP-ALL, by the specific inhibitor venetoclax (VEN) has shown substantial, clinical anti-tumor activity. However, in BCP-ALL heterogeneous sensitivities for VEN have been described suggesting that other BCL-2 family members like MCL-1 and BCL-XL interfere with BCL-2 inhibition thereby counteracting VEN activity leading to poor treatment response. In this study, we compared the effects of inhibition of the anti-apoptotic BCL-2 family members BCL-2 (venetoclax, VEN), MCL-1 (S63845) and BCL-XL (A-1331852), investigated molecular mechanisms and determinants of inhibitor sensitivity, and addressed potential synergistic activity upon simultaneous blockage of BCL-2 together with MCL-1 or BCL-XL in BCP-ALL. First, we investigated the activity of the single inhibitors to induce cell death (positive staining for propidium iodide) in 7 BCP-ALL cell lines and a series of 27 BCP-ALL patient-derived xenograft samples (PDX). Titrating increasing concentrations, we estimated half-maximal effective concentrations (EC 50) for each inhibitor showing heterogenous responses of individual samples to the different inhibitors. Importantly, sensitivities to either inhibitor were not associated with leukemia characteristics including recurrent genetic alterations described in BCP-ALL. We also did not observe similar sensitivities to the inhibitors of individual samples, however some VEN insensitive samples showed sensitivity to MCL-1 and/or BCL-XL-inhibition, suggesting functional substitution of the anti-apoptotic regulators. Next, we investigated anti-apoptotic addictions of BCP-ALL cells to BCL-2 family proteins upon VEN treatment. Analyzing apoptosis signaling after exposure to VEN and BH3-peptides, we identified that ALL cells adapt their anti-apoptotic addiction to MCL-1 and BCL-XL as an escape strategy from VEN-induced cell death. Analyzing protein complexes by co-immunoprecipitation, we found that exposure of ALL cells to VEN rapidly led to reduced BIM/BCL-2 and compensatory increased BIM/MCL-1 complexes. Conversely, S63845 reduced BIM/MCL-1 complexes and led to increased BIM/BCL-2 binding. Importantly, both protein complexes could be effectively disrupted by combination treatment with VEN and S63845, which resulted in release of BIM and promotion of apoptosis signaling. Based on our mechanistic findings, we evaluated BH3-mimetic combinations for cell death induction using multi-dose matrix assays to calculate synergy metrics based on the Bliss independence model. Analyzing combination effects in seven BCP-ALL cell lines and four PDX samples, we identified positive mean synergy scores of VEN with S63845 and A-1331852 in all cases. However, the most synergistic area of the multi-dose-response matrices differed among different leukemias, indicating different extents of addictions and thereby synergies at different drug concentrations. Finally, combined BCL-2 and MCL-1 inhibition was evaluated in vivo in a PDX model of KMT2A-ENL positive pro-B ALL. Combination treatment of VEN with S63845 led to reduced leukemia loads in spleen, bone marrow and CNS as compared to single agent treatment. Taken together, we found heterogeneous responses of BCP-ALL samples to BH3-mimetics antagonizing BCL-2, MCL-1 and BCL-XL. The ability of leukemia cells to adapt their anti-apoptotic dependency from BCL-2 to MCL-1 or BCL-XL can be used as target for combination therapy, demonstrating synergistic activity in PDX samples ex vivo and in vivo. Disclosures No relevant conflicts of interest to declare.

In acute lymphoblastic leukemia (ALL), central nervous system (CNS) involvement is a major clinical concern. Despite nondetectable CNS leukemia in many cases, prophylactic CNS-directed conventional intrathecal chemotherapy is required for relapse-free survival, indicating subclinical CNS manifestation in most patients. However, CNS-directed therapy is associated with long-term sequelae, including neurocognitive deficits and secondary neoplasms. Therefore, molecular mechanisms and pathways mediating leukemia-cell entry into the CNS need to be understood to identify targets for prophylactic and therapeutic interventions and develop alternative CNS-directed treatment strategies. In this study, we analyzed leukemia-cell entry into the CNS using a primograft ALL mouse model. We found that primary ALL cells transplanted onto nonobese diabetic/severe combined immunodeficiency mice faithfully recapitulated clinical and pathological features of meningeal infiltration seen in patients with ALL. ALL cells that had entered the CNS and were infiltrating the meninges were characterized by high expression of vascular endothelial growth factor A (VEGF). Although cellular viability, growth, proliferation, and survival of ALL cells were found to be independent of VEGF, transendothelial migration through CNS microvascular endothelial cells was regulated by VEGF. The importance of VEGF produced by ALL cells in mediating leukemia-cell entry into the CNS and leptomeningeal infiltration was further demonstrated by specific reduction of CNS leukemia on in vivo VEGF capture by the anti-VEGF antibody bevacizumab. Thus, we identified a mechanism of ALL-cell entry into the CNS, which by targeting VEGF signaling may serve as a novel strategy to control CNS leukemia in patients, replacing conventional CNS-toxic treatment.

Second mitochondria���derived activator of caspase (SMAC) mimetics (SMs) targeting inhibitor of apoptosis proteins (IAPs) activate cell death pathways, and are currently being evaluated in clinical trials. Their successful therapeutic implementation requires upfront identification of patients who could benefit from a SM���based treatment but biomarkers for SM sensitivity have not yet been described. Here, we analyzed the intrinsic activity of two monovalent (AT406 and LCL161) and two bivalent (Birinapant and BV6) SMs on unselected patient���derived pediatric precursor B���cell acute lymphoblastic leukemia (BCP���ALL) identifying a subset of patient samples to be particularly sensitive to SM���induced cell death. This subset was defined by a characteristic gene expression signature with 127 differentially regulated genes, amongst them TNFRSF1A encoding TNFR1, and a critical role of TNFR1 in SM���induced cell death in sensitive BCP���ALL was confirmed on the functional level. Interestingly, samples with intermediate or low sensitivity to SMs were sensitized to SM���induced cell death by inhibition of caspases using zVAD.fmk or Emricasan, a pan���caspase inhibitor in clinical trials. When we compared our expression data to published data sets, we identified an overlap of four genes to be commonly differentially regulated in SM���sensitive BCP���ALL, that is, TSPAN7, DIPK1C, MTX2 and, again, TNFRSF1A. Functional testing revealed that this set of genes identified samples with high sensitivity to SM treatment. In summary, our data suggest using this gene signature as biomarker predicting response to SM treatment and point to the development of new combinatorial treatments consisting of SMs and pan���caspase inhibitors for a successful clinical implementation of SMs in treatment of BCP���ALL, What's new? SMAC mimetics can activate cell death pathways and are currently undergoing clinical trials for treatment of advanced solid tumors and multiple myeloma. Successful therapeutic implementation would require upfront identification of patients most likely to benefit, but biomarkers for SMAC mimetics sensitivity have not yet been described. Here, the authors identified a highly sensitive subset of B���cell precursor acute lymphoblastic leukemia (BCP���ALL) primografts that showed a characteristic gene expression pattern consisting in high TSPAN7, DIPK1C, and TNFRSF1A and low MTX2. The gene signature could potentially be used in the clinic as a biomarker predicting response to SMAC mimetics treatment., publishedVersion

Deregulated cell death pathways contribute to leukemogenesis and treatment failure in B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Intrinsic apoptosis signaling is regulated by different proapoptotic and antiapoptotic molecules: proapoptotic BCL-2 homology domain 3 (BH3) proteins activate prodeath molecules leading to cellular death, while antiapoptotic molecules including B-cell lymphoma 2 (BCL-2) prevent activation of prodeath proteins and counter-regulate apoptosis induction. Inhibition of these antiapoptotic regulators has become a promising strategy for anticancer treatment, but variable anticancer activities in different malignancies indicate the need for upfront identification of responsive patients. Here, we investigated the activity of the BCL-2 inhibitor venetoclax (VEN, ABT-199) in B-cell precursor acute lymphoblastic leukemia and found heterogeneous sensitivities in BCP-ALL cell lines and in a series of patient-derived primografts. To identify parameters of sensitivity and resistance, we evaluated genetic aberrations, gene-expression profiles, expression levels of apoptosis regulators, and functional apoptosis parameters analyzed by mitochondrial profiling using recombinant BH3-like peptides. Importantly, ex vivo VEN sensitivity was most accurately associated with functional BCL-2 dependence detected by BH3 profiling. Modeling clinical application of VEN in a preclinical trial in a set of individual ALL primografts, we identified that leukemia-free survival of VEN treated mice was precisely determined by functional BCL-2 dependence. Moreover, the predictive value of ex vivo measured functional BCL-2 dependence for preclinical in vivo VEN response was confirmed in an independent set of primograft ALL including T- and high risk-ALL. Thus, integrative analysis of the apoptosis signaling indicating mitochondrial addiction to BCL-2 accurately predicts antileukemia activity of VEN, robustly identifies VEN-responsive patients, and provides information for stratification and clinical guidance in future clinical applications of VEN in patients with ALL.

Survival of patients with pediatric acute lymphoblastic leukemia (ALL) after allogeneic hematopoietic stem cell transplantation (allo-SCT) is mainly compromised by leukemia relapse, carrying dismal prognosis. As novel individualized therapeutic approaches are urgently needed, we performed whole-exome sequencing of leukemic blasts of 10 children with post-allo-SCT relapses with the aim of thoroughly characterizing the mutational landscape and identifying druggable mutations. We found that post-allo-SCT ALL relapses display highly diverse and mostly patient-individual genetic lesions. Moreover, mutational cluster analysis showed substantial clonal dynamics during leukemia progression from initial diagnosis to relapse after allo-SCT. Only very few alterations stayed constant over time. This dynamic clonality was exemplified by the detection of thiopurine resistance-mediating mutations in the nucleotidase NT5C2 in 3 patients' first relapses, which disappeared in the post-allo-SCT relapses on relief of selective pressure of maintenance chemotherapy. Moreover, we identified TP53 mutations in 4 of 10 patients after allo-SCT, reflecting acquired chemoresistance associated with selective pressure of prior antineoplastic treatment. Finally, in 9 of 10 children's post-allo-SCT relapse, we found alterations in genes for which targeted therapies with novel agents are readily available. We could show efficient targeting of leukemic blasts by APR-246 in 2 patients carrying TP53 mutations. Our findings shed light on the genetic basis of post-allo-SCT relapse and may pave the way for unraveling novel therapeutic strategies in this challenging situation.

Acute lymphoblastic leukemia (ALL) is the most common malignancy of childhood. Relapse is associated with poor prognosis, requiring further investigation on high-risk leukemias. MicroRNAs (miRNAs) are involved in the regulation of physiological processes. Deregulated miRNAs have been described to play a role in the initiation and progression of leukemia. Here, we investigated miRNA expression profiles comparing leukemia samples of patients with good and poor outcome, also characterized by slow or rapid engraftment, respectively, upon transplantation into NOD/SCID mice. We found downregulation of the miR-497~195 cluster in B-cell precursor (BCP)-ALL patient-derived xenograft samples with rapid engraftment and early relapse. Accordingly, in an independent cohort, patients with a high expression of miR-497 or miR-195 showed higher event-free survival. We identified promoter methylation as the mechanism responsible for miR-497~195 downregulation, as it was associated with low miR-497~195 levels in xenografts and treatment with the demethylating agent Decitabine increased miR-497~195 expression in BCP-ALL cell lines. To study the role of miR-497~195 in BCP-ALL, we stably overexpressed the cluster in three xenograft samples. MiR-497~195 overexpression impaired engraftment of leukemic cells in NOD/SCID mice, as compared to control cells, prolonging recipient survival. Gene expression profiling and ex vivo analysis of miR-497~195 overexpressing xenograft cells showed that inhibition of cell proliferation is a mechanism responsible for the miRNA cluster tumor suppressive role. Mechanistically, CDK4 and CCND3 were downregulated at mRNA and protein levels upon miR-497~195 overexpression, leading to decreased RB1 phosphorylation, reduced transcription of E2F target genes, and inhibition of the entry in S phase. CDK4 mediated G1/S transition is also inhibited by CDKN2A and CDKN2B, which are deleted in 20% of BCP-ALL. Deletion of CDKN2A/B and concomitant low miR-497~195 expression were particularly associated with a high proportion of early relapse cases in our cohort, indicating that impaired regulation of G1/S transition is critical for ALL aggressiveness. Importantly, targeting this pathway with the CDK4/CDK6 inhibitor Palbociclib impaired ex vivo growth of BCP-ALL xenograft samples, indicating a possible therapeutic strategy. Altogether, we showed that the tumor-suppressive cluster miR-497~195 is downregulated by epigenetic repression in BCP-ALL and that low expression is associated with early relapse and rapid engraftment. Low miR-497~195 expression might cooperate with deletion of CDKN2A/B promoting tumor progression, through loss of CCND3/CDK4-dependent control of cell cycle regulation. Citation Format: Elena Boldrin, Enrico Gaffo, Judith Boer, Salih Demir, Julia Zinngrebe, Rainer Claus, Christoph Plass, Monique L. den Boer, Klaus-Michael Debatin, Geertruy te Kronnie, Stefania Bortoluzzi, Lüder Hinrich Meyer. MicroRNA-497~195 cluster suppresses acute lymphoblastic leukemia growth by targeting CCND3/CDK4 and inhibiting cell cycle progression [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2541.

The existing treatments to cure acute leukemias seem to be nonspecific and suboptimal for most patients, drawing attention to the need of new therapeutic strategies. In the last decade the anticancer potential of poly ADP-ribose polymerase (PARP) inhibitors became apparent and now several PARP inhibitors are being developed to treat various malignancies. So far, the usage of PARP inhibitors has been mainly focused on the treatment of solid tumors and not too much about their efficacy on leukemias is known. In this study we test, for the first time on leukemic cells, a combined therapy that associates the conventional chemotherapeutic agent fluorouracil (5FU), used as a source of DNA damage, and a PARP inhibitor, rucaparib. We demonstrate the efficacy and the specificity of this combined therapy in killing both acute myeloid leukemia and acute lymphoid leukemia cells in vitro and in vivo. We clearly show that the inhibition of DNA repair induced by rucaparib is synthetic lethal with the DNA damage caused by 5FU in leukemic cells. Therefore, we propose a new therapeutic strategy able to enhance the cytotoxic effect of DNA-damaging agents in leukemia cells via inhibiting the repair of damaged DNA. Mol Cancer Ther; 14(4); 889–98. ©2015 AACR.

Acute lymphoblastic leukemia (ALL) is the most common malignancy in childhood. While improved multi-agent chemotherapy regimens with individualized risk stratification have led to increased survival rates of approximately 80 percent, 20 percent of patients respond poorly to therapy or relapse. Therefore, novel therapeutic avenues are urgently needed to improve treatment outcome, overcome resistance and reduce side effects. Failure to undergo cell death represents a key survival mechanism of cancer cells and results in drug resistance and clonal escape. Since inhibitor of apoptosis proteins (IAPs) are often overexpressed in malignant cells and their overexpression correlates with inferior survival rates, they provide an attractive molecular target for therapeutic intervention. Small molecule inhibitors have been developed that act as SMAC mimetics (SMs) to counteract the cell death inhibitory function of IAPs. SMs can activate and/or modulate cell death pathways, and are currently being evaluated in clinical trials. Their successful therapeutic implementation requires identification of patients who could benefit from a SM-based treatment regimen ideally before start of therapy. Here, we analyzed the intrinsic activity of two monovalent (AT406 and LCL161) and two bivalent (Birinapant or BV6) SMs on 29 unselected patient-derived pediatric precursor B-cell (BCP)-ALL samples and identified a subset of BCP-ALL primografts to be sensitive to SM treatment (n=8). When we compared gene expression of SM-sensitive (n=8) and SM-insensitive (n=6) patient-derived BCP-ALL samples, we identified a characteristic gene expression signature with 127 differentially regulated genes, amongst them upregulation of TNFRSF1A (TNFR1) in the SM-sensitive subset. In line with previous reports, we confirmed a critical role of the TNF/TNFR1-axis for SM-induced cell death in BCP-ALL by functional analysis. Expression of TNFRSF1A alone, however, did not correlate with sensitivity to SM-induced cell death indicating that TNFR1 is not the only factor regulating cell fate decisions in response to SM treatment. To identify potential biomarker genes for prediction of patient response to SM monotherapy in BCP-ALL, we compared differentially regulated genes of SM responders and non-responders from our cohort with data from a published cohort. Interestingly, we found 4 genes to overlap between these two cohorts. Of these 4 genes TSPAN7, FAM69C, and TNFRSF1A were upregulated whereas MTX2 was downregulated in SM-sensitive samples. The signature identified may reflect a particular TNF network. Analysis of expression levels of these 4 genes in BCP-ALL cell lines (Nalm6, Reh, UoCB6 and RS4;11) revealed that Reh cells, sensitive to SM-induced cell death, exhibited the biomarker profile of primograft sensitivity, i.e. upregulation of TSPAN7, FAM69C, TNFRSF1A and downregulation of MTX2. Nalm6 cells resembled the expression pattern of SM-insensitive samples with a downregulation of TSPAN7, FAM69C, TNFRSF1A and an upregulation of MTX2 and were resistant to SM-induced cell death. RS4;11 and UoCB6 cells showed no pattern. Based on these findings we hypothesized that the respective expression patterns of TSPAN7, FAM69C, TNFRSF1A and MTX2 could predict sensitivity to SMs. An extended screen of additional primary BCP-ALL samples for their expression levels of TSPAN7, FAM69C, TNFRSF1A and MTX2 and response to SMs substantiated this hypothesis. In summary, the subset of primary BCP-ALL samples with sensitivity to SMs is characterized by a gene signature with MTX2 low and TSPAN7, FAM69C and TNFRSF1A high. By using this expression profile, sensitivity to SMs in BCP-ALL could be identified in cell lines and additional primografts. Based on these results, we suggest the identified gene expression pattern as a biomarker for selecting patients to be treated by SM monotherapy in clinical trials. Disclosures No relevant conflicts of interest to declare.

Deregulation of cell death pathways is a hallmark of many cancers and contributes to leukemogenesis and treatment failure in B-cell precursor acute lymphoblastic leukemia (BCP-ALL). Different pro- and anti-apoptotic molecules control apoptosis signaling. While pro-apoptotic BCL-2 homology domain 3 (BH3) proteins induce cellular death, apoptosis induction is counter-regulated by anti-apoptotic molecules like B-cell lymphoma 2 (BCL-2). Therefore, inhibition of anti-apoptotic molecules has been developed as therapeutic strategy. Venetoclax (VEN) selectively binds to BCL-2, which leads to the release of pro-apoptotic molecules such as BIM resulting in apoptosis induction. In BCP-ALL, we and others showed preclinical activity of VEN and first clinical trials have started to evaluate VEN in ALL. Despite high activity, resistance can be acquired over time and acquisition of BCL2 mutations has been reported in CLL patients. In this study, we modeled VEN resistance in BCP-ALL and investigated underlying mechanisms in order to identify potential strategies to overcome VEN insensitivity. Starting from the BCP-ALL cell line RS4;11, five parallel VEN insensitive lines were generated by exposure to increasing concentrations of VEN over time (49 passages, over 8 months of continuous treatment). Simultaneously, five control lines were exposed to corresponding concentrations of solvent (DMSO). Measuring half maximal effective concentrations (EC50) over time showed increasing EC50 values from 4 nM to 26.2 µM in all VEN treated lines, reflecting acquired resistance in our model. Importantly, no mutations of the BCL2 gene were identified by sequencing at high coverage (Illumina AmpliSeq, 650- to 5951-fold), excluding acquisition of BCL2 mutations as mechanism of resistance. Next, we assessed expression levels of the mitochondrial apoptosis regulators BCL-2, MCL-1 and BCL-XL by western blot analysis. Comparing VEN insensitive to control cell lines, no differences in expression of the target molecule BCL-2 and similar levels of BCL-XL were observed. Most interestingly, all five VEN insensitive lines showed significant up-regulation of MCL-1 compared to all control lines. We next investigated the dependence of apoptosis signaling on different BCL-2 family members by exposing ALL cells to synthetic BH3-only peptides, which specifically bind to different regulators of mitochondrial apoptosis signaling (BH3-profiling), followed by analysis of apoptosis induction. Interestingly, dependence on BCL-2 was clearly reduced in all VEN insensitive lines indicated by almost lost mitochondrial priming. On the other hand, VEN insensitive ALL cells showed increased dependence on MCL-1. In order to analyze whether our modeled VEN resistance can be overcome by targeting MCL-1, we investigated sensitivity of these cells to the MCL-1 inhibitor S63845. High EC50 values for S63845 were found in all VEN insensitive and corresponding control lines, indicating low anti-MCL-1 activity. Importantly, the combination of S63845 with VEN synergistically induced cell death, showing that acquired VEN insensitivity in BCP-ALL can be overcome by co-targeting BCL-2 and MCL-1. To get insight into the molecular mechanisms underlying the synergism, we treated VEN insensitive ALL cells with VEN, S63845 or both and investigated binding of BIM to either BCL-2 or MCL-1 by immunoprecipitation. In the presence of VEN, clearly lower co-precipitation of BIM with BCL-2 but increased co-precipitation with MCL-1 was observed, indicating that pro-apoptotic BIM displaced from BCL-2 by VEN is sequestered by MCL-1, thereby counter-regulating VEN activity. Inversely, the MCL-1 inhibitor S63845 reduced binding of BIM to MCL-1 but increased BIM binding to BCL-2. Most interestingly, upon combination of both inhibitors, lower BIM binding to both BCL-2 and MCL-1 was found. This indicates that co-targeting MCL-1 can block the sequestration of pro-apoptotic BIM from BCL-2 to MCL-1, overcoming VEN resistance. Taken together, we show that acquired VEN resistance in BCP-ALL is characterized by up-regulated expression of counter-regulatory MCL-1 and can be overcome by simultaneous BCL-2 and MCL-1 inhibition, which prevents sequestration of BIM by MCL-1 after release from BCL-2. Disclosures Tausch: Roche: Consultancy, Honoraria, Speakers Bureau; AbbVie: Consultancy, Honoraria, Other: travel support, Speakers Bureau. Stilgenbauer:Celgene: Consultancy, Honoraria, Research Funding, Speakers Bureau; AbbVie: Consultancy, Honoraria, Research Funding, Speakers Bureau; Amgen: Consultancy, Honoraria, Research Funding, Speakers Bureau; AstraZeneca: Consultancy, Honoraria, Research Funding, Speakers Bureau; Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; Pharmacyclics: Other: Travel support; GSK: Consultancy, Honoraria, Research Funding, Speakers Bureau; Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Hoffmann La-Roche: Consultancy, Honoraria, Research Funding, Speakers Bureau; Gilead: Consultancy, Honoraria, Research Funding, Speakers Bureau.

Apoptosis is controlled by the expression levels and interplay of pro- and anti-apoptotic BCL-2 family proteins. The specific BCL-2 inhibitor Venetoclax (VEN) showed high efficiency in BCL-2 dependent cancers like chronic lymphocytic leukemia (CLL) or mantle cell lymphoma (MCL). Despite its high efficiency in CLL and MCL, refractory disease can develop. BCL-2 mutations have been described to mediate resistance in CLL cases, however these mutations are only found in a proportion of VEN resistant cases and in a fraction of cells. In order to design alternative therapeutic strategies to overcome drug resistance, a better understanding of the mechanisms mediating resistance to VEN is necessary. VEN-resistant (VEN-R) MCL cell lines (MINO and MAVER-1) were generated by chronic exposure to increasing amounts of VEN (up to 3µM). A significant and stable upregulation of BCL-XL mRNA and protein was seen in the MINO and MAVER-1 resistant cell lines (2 and 4 fold increase in mRNA and 2.6 and 4.5 fold increase in protein, respectively). We used BH3 profiling in combination with VEN treatment for 4h to investigate the differences in anti- and pro-apoptotic signaling in parental and VEN-R cell lines. Additionally, sensitivity to VEN was restored upon shRNA-mediated knockdown of BCL-XL. These results confirmed the importance of BCL-XL upregulation in mediating resistance. Furthermore, we did not detect mutations in BCL-2 upon resistance to VEN via targeted NGS, which is in contrast to results obtained in VEN-R CLL patients (Blombery et al., Cancer Discovery 2019 and Tausch et al., Hematologica 2019). However, the results obtained by dynamic BH3-profiling (VEN treatment in combination with BH3 Profiling) suggest that increase in BCL-XL is most likely not the only alteration necessary to render cells resistant to VEN. In addition, reduced activation of pro-apoptotic proteins like BAX and BAK might contribute to resistance to VEN. In order, to investigate if VEN resistance can be overcome by drug mediated inhibition of BCL-XL we used different therapeutic approaches. Combinational treatment with the BCL-XL inhibitor A-1331852 and VEN or the single treatment with Navitoclax, a combined inhibitor of BCL-2, BCL-W and BCL-XL for 48h reduced cell viability in VEN-R MINO and MAVER-1 cell lines. Furthermore, BDA-366, a BH4 domain BCL-2 inhibitor effectively reduced the cell viability after 48h of treatment in a dose dependent manner in both parental and VEN-R cell lines. The binding of BDA-366 to the anti-apoptotic BCL-2 protein leads to a conformational change into a pro-apoptotic molecule by the exposure of the BH3 domain of the protein. Despite mediating apoptosis in a TP53-independent manner, VEN treatment in CLL has been associated with inferior outcome in the presence of TP53 aberrations. In order to address the role of TP53 dysfunction in mediating resistance to VEN, we generated p53 knock out cell lines (N=2) by CRISPR/Cas9 gene editing. This significantly decreased the sensitivity to VEN compared to p53 WT cell lines. Additionally, the sensitivity to BDA-366 was significantly reduced upon knockout of p53, suggesting an interference of p53 downstream of BCL-2. Overall, VEN resistance is mediated by a permanent increase in BCL-XL mRNA and protein level in MCL. Importantly, BDA-366, which converts the anti-apoptotic BCL-2 molecule into a BAX-like death molecule, could be a potential alternative treatment strategy for BCL-2 dependent cancers even when resistant to VEN. Despite mediating apoptosis in a p53 independent manner, VEN seems to be less effective in p53 deficient cells, underlining the importance of further investigations of treatment combinations in these groups. Disclosures Tausch: Roche: Consultancy, Honoraria, Speakers Bureau; AbbVie: Consultancy, Honoraria, Other: travel support, Speakers Bureau. Döhner:AbbVie, Agios, Amgen, Astellas, Astex, Celator, Janssen, Jazz, Seattle Genetics: Consultancy, Honoraria; AROG, Bristol Myers Squibb, Pfizer: Research Funding; Celgene, Novartis, Sunesis: Honoraria, Research Funding. Stilgenbauer:Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Gilead: Consultancy, Honoraria, Research Funding, Speakers Bureau; Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; Hoffmann La-Roche: Consultancy, Honoraria, Research Funding, Speakers Bureau; Pharmacyclics: Other: Travel support; Amgen: Consultancy, Honoraria, Research Funding, Speakers Bureau; AbbVie: Consultancy, Honoraria, Research Funding, Speakers Bureau; AstraZeneca: Consultancy, Honoraria, Research Funding, Speakers Bureau; Celgene: Consultancy, Honoraria, Research Funding, Speakers Bureau; GSK: Consultancy, Honoraria, Research Funding, Speakers Bureau. Schneider:Celgene: Other: travel grant.

Heritable predisposition is an important cause of cancer in children and adolescents. Although a large number of cancer predisposition genes and their associated syndromes and malignancies have already been described, it appears likely that there are more pediatric cancer patients in whom heritable cancer predisposition syndromes have yet to be recognized. In a consensus meeting in the beginning of 2016, we convened experts in Human Genetics and Pediatric Hematology/Oncology to review the available data, to categorize the large amount of information, and to develop recommendations regarding when a cancer predisposition syndrome should be suspected in a young oncology patient. This review summarizes the current knowledge of cancer predisposition syndromes in pediatric oncology and provides essential information on clinical situations in which a childhood cancer predisposition syndrome should be suspected.

Deregulated cell death and survival pathways contribute to leukemogenesis and treatment failure of B-cell precursor acute lymphoblastic leukemia (BCP-ALL) patients. The intrinsic apoptosis pathway is regulated at the mitochondrial level by different pro- and anti-apoptotic molecules. Members of the BCL-2 family are key regulators of mitochondrial apoptosis signaling. Pro-apoptotic BH3-only proteins like BIM and BID activate pro-death proteins such as BAX and BAK leading to cell death. Anti-apoptotic BCL-2 family members including BCL-2, BCL-XL and MCL-1 bind to and sequester pro-apoptotic molecules, prevent activation of pro-death proteins and counter-regulate apoptosis induction. Small molecule inhibitors have been developed that block binding to anti-apoptotic molecules like BCL-2, leading to release of pro-apoptotic proteins and cell death induction. In particular, the BCL-2-specific inhibitor venetoclax (VEN) has demonstrated substantial anti-cancer activity and became an approved drug for the treatment of CLL patients. Investigating different, individual BCP-ALL samples, we and others recently identified heterogeneous sensitivities for VEN, suggesting that BCP-ALL cells might also depend on other pro-survival BCL-2 family proteins including MCL-1, leading to VEN insensitivity and resistance. A novel BH3-mimetic, S63845, that selectively targets MCL-1 has been reported. Here, we assessed the activity of S63845 and addressed a potential synergism of simultaneous blockage of BCL-2 and MCL-1 by VEN and S63845 (S) in BCP-ALL. The activity of the MCL-1 inhibitor was analyzed in a panel of BCP-ALL cell lines (N=6) and a series of primary, patient-derived BCP-ALL primograft samples (N=27) determining half-maximal effective concentrations (EC50) upon exposure to increasing concentrations of S and analysis of cell death induction. We observed heterogeneous sensitivities to S with EC50 values ranging from 16 nM to almost 10 µM. Protein expression of MCL-1 and other BCL-2 family members BCL-2, BCL-XL and BCL-W was assessed by western blot analysis and quantified, however neither association of MCL-1 levels nor expression of the other regulators and S sensitivity was found in cell lines and primograft leukemias. Moreover, we also compared sensitivities for both inhibitors but found independent activities of S and VEN in individual ALL samples. Next, we addressed the role of MCL-1 for VEN sensitivity and generated two MCL-1 knock out BCP-ALL cell lines by CRISPR/Cas9 gene editing. In both lines, clearly increased VEN sensitivities were observed upon depletion of MCL-1, indicating that MCL-1 is contributing to activity of the BCL-2 inhibitor VEN. Based on these findings, we investigated the effects of pharmacological MCL-1 inhibition for VEN sensitivity and incubated all 6 cell lines with VEN and S at increasing concentrations and observed clear synergistic effects upon combined BCL-2 and MCL-1 inhibition indicated by combination indices (CI) below 0.1. Moreover, we investigated 7 primograft BCP-ALL samples and found that MCL-1 inhibition by S clearly synergized with VEN activity (CI < 0.3). To investigate the anti-leukemia activity of co-targeting BCL-2 and MCL-1 in vivo in a pre-clinical setting, a high-risk leukemia derived from an infant, MLL/ENL rearranged pro-B ALL case was transplanted onto NOD/SCID mice. Upon ALL manifestation (presence of >5% human blasts in blood), recipients were treated with either VEN, S, the combination of both, or vehicle for 10 days. After treatment, leukemia loads were analyzed showing significantly reduced loads in the co-treated group as compared to vehicle, VEN or S alone in spleen, bone marrow, and central nervous system (p-values < 0.05), indicating synergistic activity of co-inhibition of BCL-2 and MCL-1 in vivo. Taken together, our data show heterogeneous sensitivity of individual BCP-ALL samples to MCL-1 inhibition by S, which is not associated with MCL-1 protein expression levels or VEN sensitivity. Both, genetic depletion and inhibition of MCL-1 by S synergizes with VEN leading to increased anti-leukemia activity in vitro and ex vivo. Importantly, co-targeting BCL-2 and MCL-1 significantly reduced leukemia infiltration in spleen, BM and CNS in a pre-clinical model of high-risk BCP-ALL, warranting further evaluation and possible clinical application of targeting MCL-1 alone and in combination with BCL-2 inhibition. Disclosures No relevant conflicts of interest to declare.

In many cancers the equilibrium of pro- versus anti-apoptotic BCL-2 proteins is deregulated. BCL-2 inhibitors like Venetoclax (VEN) have been shown to be highly active drugs in BCL-2 dependent cancers like chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL). Despite being highly efficient in cell killing, resistance to VEN can be acquired over time. In addition to understanding the underlying mechanisms of resistance to VEN it is important to identify additional treatment options. BDA-366 is a BCL-2 inhibitor with a different mode of action than the BH3 mimetic VEN. BDA-366 acts by inhibiting the BH4 domain and thereby inducing a conversion of anti-apoptotic BCL-2 into a pro-apoptotic protein. BDA-366 showed high effectivity in inducing apoptosis in CLL cells, in primary as well as in cell lines, while all of the CLL cell lines (n=7) tested were resistant to VEN. Furthermore all of the MCL cell lines (n=5) tested were sensitive to the treatment with BDA-366 while only a subset (3 out of 5) responded to treatment with VEN. In order to investigate whether BDA-366 would be a treatment option for VEN-resistant patients, we generated VEN-resistant MCL cell lines (MINO and MAVER-1) by chronic exposure to the drug. In the resistant cell lines, BCL-2 protein levels were not deregulated. In variance to previous reports in diffuse large B cell lymphoma (DLBCL) (Choudhary et al, Cell Death Dis 2015), resistance in MCL cell lines was not mediated by MCL-1 upregulation. In VEN-resistant MINO cells, MCL-1 expression was similar to the parental cells, while MCL-1 was significantly downregulated in VEN-resistant MAVER-1 cells. In contrast, VEN-resistant MCL cell lines showed BCL-XL upregulation as compared to parental cells, which is in line with results obtained in DLBCL (Choudhary et al, Cell Death Dis 2015). Furthermore, dynamic BH3 profiling validated a dependency on BCL-XL in resistant cells and confirmed that resistance was not mediated by MCL-1. The significance of BCL-XL in mediating resistance to VEN was underlined by additional experiments using navitoclax. In contrast to VEN, navitoclax inhibits BCL-2, BCL-XL and BCL-W and was sufficient to induce apoptosis in both parental and resistant cells. In contrast to the BH3 domain inhibitor VEN, the BCL-2 inhibitor BDA-366 acts by converting BCL-2 into a pro-apoptotic molecule. BDA-366 efficiently induced dose dependent apoptosis in VEN-resistant cells. MINO as well as MINO VEN-resistant cells showed the same sensitivity to BDA-366 while VEN-resistant MAVER-1 cells showed reduced sensitivity to BDA-366 as compared to the parental cells. However, with increased BDA-366 concentrations efficient cell killing was achieved in the VEN-resistant cell lines Overall, these results suggest that VEN-resistance is mostly mediated by permanent upregulation of BCL-XL. BCL-2 levels are not deregulated upon development of resistance to VEN. The inhibition of the BH4 domain and thereby converting BCL-2 into a pro-apoptotic protein proved to be a promising therapeutic option even in cancers with acquired resistance to VEN. Disclosures Döhner: Pfizer: Research Funding; Amgen: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Sunesis: Consultancy, Honoraria, Research Funding; Seattle Genetics: Consultancy, Honoraria; Sunesis: Consultancy, Honoraria, Research Funding; Agios: Consultancy, Honoraria; Pfizer: Research Funding; AROG Pharmaceuticals: Research Funding; Bristol Myers Squibb: Research Funding; Astex Pharmaceuticals: Consultancy, Honoraria; Janssen: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; AROG Pharmaceuticals: Research Funding; Astellas: Consultancy, Honoraria; Astellas: Consultancy, Honoraria; Agios: Consultancy, Honoraria; Astex Pharmaceuticals: Consultancy, Honoraria; Celator: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Celator: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Research Funding; Janssen: Consultancy, Honoraria; Seattle Genetics: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding. Stilgenbauer:Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Boehringer-Ingelheim: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; GSK: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Hoffmann La-Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Genentech: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Genzyme: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; AbbVie: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Mundipharma: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmcyclics: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Sanofi: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

Despite superior outcome and survival of patients with B-cell precursor acute lymphoblastic leukemia (BCP-ALL), relapse occurs in 10-20% and is associated with poor outcome, clearly indicating future challenges including reduction of relapse rates and effective treatment of reoccurred leukemia. Deficiencies in cell death and survival pathways have been implicated in therapy failure and treatment resistance in BCP-ALL. Members of the BCL-2 family are key regulators of these pathways and are therefore of interest as therapeutic targets. The small molecule ABT-199 binds selectively to BCL-2, inhibits its anti-apoptotic function and leads to release of pro-apoptotic molecules. Recently, ABT-199 has demonstrated clinical activity, particularly in poor prognosis CLL. However, insensitivity and resistance in different cases clearly emphasize the need of predictive markers for upfront identification of ABT-199 responsive leukemias. Here, we analyzed sensitivity for ABT-199 in a series of individual BCP-ALL samples, addressed mechanisms of resistance and evaluated markers indicating response to ABT-199. Anti-leukemic activities of ABT-199 were investigated in BCP-ALL cell lines (n=6) and patient-derived BCP-ALL primograft samples (n=17), which were established by transplantation of primary patient ALL cells obtained at diagnosis onto NOD/SCID mice. Half maximal inhibitory concentrations (IC50) for ABT-199 were analyzed for each sample. Expression of apoptosis regulating molecules was investigated by western blot analysis and associated with ABT-199 responsiveness. Two MCL-1 deficient ALL cell lines were generated by CRISPR/Cas9 gene editing. Leukemia free-survival of ALL bearing animals was analyzed after in vivoABT-199 treatment. The majority of BCP-ALL samples showed sensitivity for ABT-199 induced cell death in the nanomolar range, both in cell lines (n=4, IC50: 29 - 422 nM) and patient-derived primograft samples (n=10, IC50: 1.7 - 74 nM), while 2 cell lines and 7 primograft leukemias showed insensitivity with IC50 values above 1 µM. ABT-199 binds directly to BCL-2 and upon binding, pro-apoptotic Bcl-2 family molecules like Bim are dislocated from BCL-2 and induce apoptosis. The anti-apoptotic BCL-2 family member MCL-1 is not bound by ABT-199, but sequesters pro-apoptotic molecules dislocated from BCL-2 leading to interruption of apoptosis induction. Therefore, we addressed expression levels of BCL-2 and MCL-1. We found high BCL-2 levels in ABT-199 sensitive and low BCL-2 levels in resistant leukemia samples and an opposite pattern for MCL-1 (high in resistant and low MCL-1 in sensitive ALL), in line with previous reports. Most interestingly, a high ratio of MCL-1 to BCL-2 expression (high MCL-1, low BCL-2) was significantly associated with high IC50 values/resistance (Spearman Rho correlation, p= .01), whereas a low MCL-1/BCL-2 ratio indicated ABT-199 sensitivity. Two of the 6 cell lines showed ABT-199 resistance (IC50 > 1 µM) and high Mcl-1 expression. Effective MCL-1 knock-out in both cell lines led to a clear sensitization for ABT-199 with up to 40-fold reduced IC50 values, clearly indicating MCL-1 as a key mediator of ABT-199 resistance in BCP-ALL. Finally, we also evaluated the anti-leukemia activity of ABT-199 in a preclinical setting in vivo. Two patient-derived leukemias, one with a low MCL-1/BCL-2 ratio of 0.9 and the other with a high ratio of 16.1, indicative of ABT-199 sensitivity or resistance, were transplanted onto NOD/SCID mice and treated with ABT-199 for 10 days after ALL engraftment. Most interestingly, a significantly increased leukemia free survival was observed in ABT-199 as compared to vehicle treated recipients (p Taken together, ABT-199 shows anti-leukemia activity in the majority of BCP-ALL samples, with a strong association of high BCL-2 and low MCL-1 levels with ABT-199 sensitivity. Silencing of MCL-1 clearly revealed a crucial role for MCL-1 as mediator of ABT-199 resistance. Importantly, in vivo evaluation of ABT-199 in a preclinical setting highlighted the predictive value of BCL-2/MCL-1 expression for the identification of patients who would benefit from future BCL-2 directed therapies. Disclosures Stilgenbauer: Genzyme: Consultancy, Honoraria, Other: Travel grants , Research Funding; Genentech: Consultancy, Honoraria, Other: Travel grants , Research Funding; Janssen: Consultancy, Honoraria, Other: Travel grants , Research Funding; Sanofi: Consultancy, Honoraria, Other: Travel grants , Research Funding; Hoffmann-La Roche: Consultancy, Honoraria, Other: Travel grants , Research Funding; Novartis: Consultancy, Honoraria, Other: Travel grants , Research Funding; GSK: Consultancy, Honoraria, Other: Travel grants , Research Funding; Gilead: Consultancy, Honoraria, Other: Travel grants , Research Funding; Pharmacyclics: Consultancy, Honoraria, Other: Travel grants , Research Funding; AbbVie: Consultancy, Honoraria, Other: Travel grants, Research Funding; Mundipharma: Consultancy, Honoraria, Other: Travel grants , Research Funding; Celgene: Consultancy, Honoraria, Other: Travel grants , Research Funding; Amgen: Consultancy, Honoraria, Other: Travel grants, Research Funding; Boehringer Ingelheim: Consultancy, Honoraria, Other: Travel grants , Research Funding.

Although improved treatment of pediatric Acute Lymphoblastic Leukemia (ALL) has become increasingly successful with cure rates of up to 90%, leukemia reoccurrence is clearly associated with inferior prognosis. Treatment failure and resistance in leukemias are considered to involve defects in apoptosis signaling, which forms the basis for apoptogenic treatment approaches such as the use of Bcl-2 inhibitors. Identification of alternative cell death programs like lysosomal membrane permeabilization-induced cell death may provide a novel strategy for the treatment of ALL. Previous studies showed that lysosomal activity, as well as their traffic properties are greatly altered during tumorigenesis. Intralysosomal hydrolases such as cathepsin B, L and D have been implicated in cancer progression and high expression levels are generally associated with poor prognosis. Lysosomotropic agents modulating lysosomal integrity may overcome cell death resistance and might therefore also improve the outcome of high risk or ALL relapse patients. In this study, we investigated the efficacy of the lysosomotropic compound B10, a derivative of Betulinic acid, as well as its combination with the Bcl-2 inhibitor ABT-263 in B-cell precursor- (BCP-) ALL. In BCP-ALL cell lines, B10 showed induction of cell death combined with increased DNA fragmentation. Interestingly, additional treatment with the pan-caspase inhibitor zVAD.fmk only partially rescued B10 triggered loss of cell viability, indicating that B10 is not exclusively inducing caspase-dependent apoptosis, but also induces an additional alternative cell death program. B10 permeabilized lysosomes as indicated by the significant decrease of LysotrackerRED positive populations detected by flow cytometry, and treatment with E-64d, a potent inhibitor of thiol proteases and cathepsins, reduced B10-induced cell death, thus emphasizing the cathepsin-dependent effect of B10. To exclude that B10-induced lysosomal permeabilization might represent a secondary effect, a panel of different substances was tested for their activity on lysosomal membrane integrity. While treatment with dexamethasone, chloroquine, vincristine and asparaginase also showed lysosomal permeabilization, co-treatment with E-64d did not reduce cell death indicating a secondary effect of these compounds on lysosomes. In addition to cell lines, we tested the effect of B10 on 15 primary leukemia samples isolated from ALL bearing mice of established patient-derived NOD/SCID/hu BCP-ALL xenografts. In a majority of these individual patient-derived leukemias, B10 induced cell death that could be inhibited by E-64d, thus demonstrating a lysosomal protease-dependent death program also in primary ALL. In addition to its classical role in triggering the intrinsic mitochondria dependent apoptosis pathway, the pro-apoptotic Bcl-2 family member Bax was previously described to be involved in the regulation of lysosomal membrane integrity, pointing to a possible synergistic effect of B10 and BH3-mimetics. Intriguingly, in cell fractionation assays we observed increased Bax recruitment to lysosomal membranes when B10 is present. Interestingly, co-treatment with the Bcl-2 inhibitor ABT-263, which displaces Bcl-2 from its inhibitory binding to Bax, led to increased lysosomal permeabilization, loss of mitochondrial membrane potential, and caspase activation indicating involvement of the intrinsic apoptosis pathway. The importance of lysosomal Bax recruitment for the activity of ABT-263 and its concomitant effect on lysosomal membrane permeabilization was further supported by Bax knockdown experiments, since induction of lysosomal disruption, release of cathepsins and their subsequent effect on cell death activation by B10 was reduced. Taken together, our findings suggest an important role of lysosomal membrane permeabilization-induced cell death for the activity of Bcl-2 inhibitors such as ABT-263. The combination of BH3 mimetics with lysosomotropic compounds may provide the basis for novel molecular directed treatment strategies. Disclosures No relevant conflicts of interest to declare.

Acute lymphoblastic leukemia (ALL) is the most common pediatric and adolescent malignancy. Although current treatment provides five-year event-free survival, in up to 20% conventional chemotherapy fails resulting in relapse with inferior prognosis. FOXO1 is a member of the forkhead family of transcription factors, which is preferably expressed in B-cells with high expression at the early B-cell stage. FOXOs are involved in several cellular processes including cell death and proliferation, anti-cancer drug resistance and protection from oxidative stress. Since FOXO1 can enhance tumor growth and potentiate metastasis, we aimed to investigate the effects of FOXO1 inactivation on B-cell precursor (BCP)-ALL, including preclinical in vivo evaluation. FOXO1 expression levels were compared among 497 cancer samples using the Genevestigator online software. Expression of FOXO1 in BCP-ALL was significantly higher than in any of the other cancer types. Next, we investigated FOXO1 expression and subcellular localization in 3 BCP-ALL cell lines by cellular fractionation and fluorescent microscopy. Both methods showed localization of FOXO1 in the nucleus, indicating transcriptionally active FOXO1 in BCP-ALL. In order to study the potential anti-tumor effect of FOXO1 repression, we investigated genetically modified, FOXO1 deficient BCP-ALL cell lines (n=5) and observed no cell death induction in control transduced cells, in contrast to a clear reduction of cell viability of up to 80% upon FOXO1 knock-down, clearly indicating dependency of BCP-ALL cells on FOXO1. Moreover, lentiviral mediated FOXO1 knockdown did not induce cell death in the Hodgkin's lymphoma cell line cHL, suggesting a BCP-ALL specific importance for FOXO1. Based on these results indicating the importance of FOXO1 expression for BCP-ALL maintenance, we investigated the feasibility of pharmacological interference with FOXO1. Exposure of 7 BCP-ALL, 4 T-ALL, 3 B-cell NHL, 2 DLBCL and 3 cHL cell lines to the small molecular weight FOXO1 inhibitor AS1842856 showed effectivity in BCP-ALL lines, reflected by significantly higher half maximal inhibitory concentrations (IC50) by MTT test. The most sensitive cell line was the BCP-ALL line RS4;11, while the cHL cell line SUP-HD1 showed insensitivity for FOXO1 inhibition (IC50: 3 nM and 26 µM), again indicating that BCP-ALL is particularly dependent on FOXO1 activity. Caspase 3 cleavage detected upon exposure to AS1842856 showed induction of apoptosis as mechanism of cell death. Furthermore, we evaluated the sensitivity of primary BCP-ALL primograft samples (n=9) exposing the ALL cells to increasing pharmacologically relevant concentrations of AS1842856. The inhibitor increased cell death as measured by flow cytometry (FSC/SSC criteria) in all of the samples tested in a time and dose dependent manner. Importantly, FOXO1 inhibition also showed activity on high risk leukemias including MLL-rearranged and early or second-relapse cases. Moreover, we investigated the in vivo effectivity of AS1842856. Two different patient derived leukemias were transplanted onto NOD/SCID mice and upon leukemia manifestation vehicle or AS1842856 was administered for a time of 11 days. At the end of the experiment, all mice were sacrificed and tumor loads were quantified in spleen, bone marrow and central nervous system (CNS). Importantly, tumor loads of all compartments and spleen sizes were significantly reduced in AS1842856 treated animals (p=0.028, U-test). Moreover, in an early-relapse sample leukemia-free survival upon AS1842856 treatment was evaluated. Mice were treated by vehicle or AS1842856 (n=10/group) during 11 days. Leukemia-free survival was significantly prolonged in mice which received AS1842856 (p=0.003, Log-rank test). Taken together, we show that the active form of FOXO1 is highly expressed in BCP-ALL cells as compared to other cancers, and that viability of BCP-ALL cells is regulated by FOXO1 activity. Importantly, silencing or pharmacological inhibition of FOXO1 induces cell death in BCP-ALL primogafts including high risk cases, both ex vivo and preclinically in vivo. Thus, targeting FOXO1 provides a promising novel strategy for therapeutic intervention in these high-risk subtypes of BCP-ALL. Disclosures No relevant conflicts of interest to declare.

Mutations of the tumor suppressor gene TP53 have been described to be associated with aggressive disease and inferior prognosis in different types of cancer, including hematological malignancies. In acute lymphoblastic leukemia (ALL), TP53 alterations are infrequently found at diagnosis but have recently been described in about 12% of patients at relapse. This suggests an association with therapy resistance in high risk/relapsed ALL and patients with TP53 mutated ALL have in fact an inferior outcome. Small molecule compounds targeting mutated TP53 such as APR-246, initially described as PRIMA-1MET (p53-dependent reactivation and induction of massive apoptosis) leading to apoptosis induction have shown activity in several types of malignancies with mutated TP53. In ALL, however, mutant TP53 has so far not been addressed as a target for therapeutic intervention. In this study, we investigated a large cohort of patient-derived pediatric B cell precursor (BCP)-ALL primograft samples to identify cases with mutated TP53. Further, we analyzed the effects of APR-246 and evaluated its activity on BCP-ALL cell lines and primografts with mutated (mut) orwild type (wt) TP53. Altogether, 62 BCP-ALL primograft samples established from patients at diagnosis (n=53) or relapse (n=9) by transplantation of primary ALL cells onto NOD/SCID mice were screened for TP53 mutations by denaturating high-performance liquid chromatography (dHPLC) followed by Sanger sequencing of exons 4 to 10 to confirm detected mutations. We identified 4 cases with TP53 mut, 3 obtained from diagnosis (5.6%) and one at relapse (11.1%), corresponding to frequencies described in clinical studies. Mutated cases were further analyzed by fluorescence in situ hybridization (FISH), revealing a 17p deletion in one TP53 mut sample. Similarly, we analyzed 6 BCP-ALL cell lines and identified 2 TP53 mut and 4 TP53 wt lines. Exposure of BCP-ALL primograft (TP53 mut n=4, TP53 wt n=4) and cell line (TP53 mut n=2, TP53 wt n=4) samples to the DNA damaging agent doxorubicin showed, as expected, resistance of TP53 mut leukemia cells for cell death induction, reflected by significantly higher half maximal inhibitory concentrations (IC50; TP53 mut 49 and 143 ng/ml, TP53 wt mean 12 ng/ml) and lower induction of cell death (TP53 mut 16 to 23%, TP53 wt 10 to 60%) in TP53 mut ALL, corroborating the tumor-suppressive function of p53 in ALL. We then investigated the sensitivity of BCP-ALL cell lines for cell death induction by APR-246 (kindly provided by Aprea, Stockholm, Sweden). We observed high sensitivity for APR-246 in TP53 mut (IC50: 5 µM for both cell lines) as compared to TP53 wt ALL (mean IC50: 58 µM). DNA fragmentation and Annexin-V/propidium-iodide (PI) positivity revealed apoptosis as mechanism of APR-246 mediated cell death. Reactive oxygen species (ROS) have recently been described to mediate APR-246 induced cell death in multiple myeloma cells. Therefore, we investigated ROS levels by detection of oxidation-specific fluorescence of dichlorodihydrofluorescein diacetate (DCFDA) in ALL cells. Interestingly, ROS quenching by N-acetyl cysteine abolished induction of cell death in TP53 mut but not TP53 wt ALL cells indicating ROS as a mediator of APR-246 induced cell death in TP53 mut ALL. Furthermore, we addressed p53 activation in response to APR-246 by assessing phosphorylation of p53 (p53pSer15) using phosphoflow cytometry. Most interestingly, APR-246 led to 6-fold increased p53pSer15 levels in TP53 mut compared to no activation in TP53 wt leukemia cells, indicating restoration of p53function upon APR-246treatment in BCP-ALL. Based on these findings, we addressed the effectivity of APR-246on primary, patient-derived primografts and compared sensitivities for cell death induction in TP53 mut (n=4) and TP53 wt (n=4) samples. Importantly, the pattern of responsiveness of TP53 mut ALL was also identified in TP53 mut patient-derived ALL samples with induction of significantly higher cell death rates in TP53 mut ALL (TP53 mut 48%, TP53 wt 18%, 5 µM APR-246, 24 h). Taken together, we showed that TP53 mut BCP-ALL can be targeted by APR-246 leading to re-activation of p53, induction of ROS dependent apoptosis and effective leukemia cell killing. Thus, targeting and re-activation of mutated p53 provides a promising novel strategy for therapeutic intervention in this high-risk subtype of BCP-ALL. Disclosures Selivanova: Aprea: Patents & Royalties: APR-246. Tausch:Gilead: Other: Travel support. Stilgenbauer:Gilead: Honoraria, Research Funding.

Acute lymphoblastic leukemia (ALL) is the most frequent malignant disorder in children and adolescents. Despite successful treatment, relapse of the disease remains a major problem and is associated with poor prognosis. This emphasizes the need for novel treatment strategies to be applied in addition to established chemotherapy regimens without increasing general toxicity. Previously, we described a strong association of leukemia cell engraftment of primary patient B cell precursor (BCP) ALL samples transplanted in a NOD/SCID/huALL mouse model and patient outcome. Rapid onset of leukemia related morbidity (time to leukemia, TTLshort) is indicative for patient relapse and characterized by a specific gene expression profile. Among the top differentially regulated genes, the gene coding for CD70 was identified to be significantly up-regulated in TTLshort/high risk ALL. CD70 is a member of the tumor necrosis factor (TNF) family expressed on activated B- and T-lymphocytes and dendritic cells. Binding of CD70 to its receptor CD27 is involved in regulation of T- and B-cells including priming and generation of memory and plasma cells. CD70 has been described to be constitutively expressed on different cancers including hematological malignancies. However, expression and targeting of CD70 in B- cell precursor lymphoblastic leukemia has so far not been investigated. In this study, we addressed expression of CD70 in patient-derived primograft leukemia samples and primary patient specimens obtained at diagnosis from pediatric patients. Furthermore, we evaluated CD70 as a therapeutic target for directed immunotherapy in vitro and in our BCP-ALL xenograft system in vivo. Flow cytometric analyses of CD70 surface expression in all together 19 patient-derived xenograft samples (TTLshort n= 7, TTLlong n=12) revealed a higher expression of CD70 on ALL cells with a TTLshort/early relapse phenotype compared to TTLlongsamples. We also investigated expression of the CD70 receptor CD27 and found no significant difference in surface expression between both TTL subgroups. Moreover, we investigated the transcript expression levels of 198 BCP-ALL specimens obtained at diagnosis. Interestingly, we found a heterogenous expression of CD70 with no association to cytogenetic subgroups, minimal residual disease (MRD) risk classes or patient outcome. Importantly, a significant higher CD70 expression was found in leukemia samples compared to healthy bone marrow controls indicating a general over-expression in BCP-ALL. CD27 however, did not show different transcript expression including healthy bone marrow controls. To take advantage of increased CD70 expression in BCP-ALL, we addressed CD70 as therapeutic target for immunotherapy. Co-culture in vitro experiments of primograft ALL cells with NK cells in the presence of specific anti-CD70 antibodies revealed five-fold increased antibody-dependent cell-mediated cytotoxicity (ADCC) as compared to the respective isotype control. To evaluate the efficacy of CD70 directed immunotherapy, we assessed leukemia development in NOD/SCID mice upon transplantation of primograft ALL with high CD70 expression either incubated with anti-CD70 antibody or the respective isotype control. Most importantly, a marked reduction of leukemia load in peripheral blood, bone marrow and spleens of the animals was detected in anti-CD70 treated cases. This indicates, that CD70 provides an immunotherapeutic target on ALL cells inducing ADCC by NK cells present in NOD/SCID mice. Most interestingly, this effect could be abrogated both by NK-cell depletion (pre-treatment with anti-mouse CD122 antibodies) in the recipient animals and by using NK-cell deprived NSG mice as recipients, confirming that decreased in vivo leukemia growth upon anti-CD70 treatment is mediated by NK-cell induced cytotoxicity of anti-CD70 bearing CD70 positive ALL cells. Taken together, we identified significantly up-regulated CD70 expression in BCP-ALL with varying expression among molecular and prognostic subgroups. BCP-ALL samples with high surface expression of CD70, as detected by flowcytometry, can be targeted by directed immunotherapy with anti-CD70 antibodies leading to efficient NK-cell dependent lysis of leukemia cells in vitro and decreased growth in an in vivo BCP-ALL model. Thus, CD70 provides a novel target for directed immunotherapy of BCP-ALL. Disclosures No relevant conflicts of interest to declare.

A pivotal step during B-cell development is the expression of the precursor B-cell receptor (pre-BCR) by pre-B lymphocytes (cyto-Igµ+, surface-IgM-). The pre-BCR represents an immature form of the BCR and consists of two immunoglobulin heavy chains (IgH), two surrogate light chains (SLC) and the signal transducing adapter proteins Igα and Igβ. A functional pre-BCR drives proliferation of pre-B-cells, ensuring their further differentiation into mature B-cells. By immunophenotype, ~20% of B-cell acute lymphoblastic leukemia (B-ALL) cases originate from the pre-B-cell stage (pre-B-ALL) of lymphocyte development and might therefore also express the pre-BCR. In view of the importance of pre-BCR signaling for normal pre-B-cell development, we hypothesize that it is exploited by pre-B-ALL for malignant growth and proliferation. A hallmark of active pre-BCR signaling is the continuous internalization of pre-BCRs, resulting in low pre-BCR surface expression. Using this phenotype of active pre-BCR signaling (low pre-BCR expression and high phosphorylation of the pre-BCR associated kinases LYN and SYK), we identified pre-BCR+ ALL cell lines (RCH-ACV, SMS-SB and Nalm6) and xenograft expanded patient samples. To study the role of the pre-BCR in these cells, we rendered RCH-ACV and SMS-SB pre-BCR null by using CRISPR/CAS9 gene editing with guide RNAs specific for the hypervariable region (recombined V, D, and J segments) of their expressed IgH allele. As identified by flow cytometry for the pre-BCR, deficient RCH-ACV and SMS-SB cells exhibited reduced viability and impaired proliferation when compared to their pre-BCR+ controls (Figure 1). Pre-BCR- cells showed reduced baseline phosphorylation of CD19, VAV1 and AKT. Interestingly, BTK and ERK phosphorylation were not affected. These results provide evidence for the dependency of pre-BCR+ ALL on pre-BCR signaling and suggest selective involvement of the PI3K-AKT pathway. We also investigated the effects of pharmacological pre-BCR inhibition by treating pre-BCR+ and pre-BCR- ALL cell lines and xenograft expanded primary patient samples with PRT318, a small-molecule inhibitor of spleen tyrosine kinase (SYK). In pre-BCR+ ALL PRT318 blocked cell proliferation and selectively inhibited AKT phosphorylation, thus mimicking the effects of IgH knockout. Pre-BCR- ALL cells were resistant to PRT318. Key effectors of the pre-BCR during normal B-cell development are FOXO transcription factors. In line with this, we found reduced FOXO1 phosphorylation and increased FOXO1 total protein levels after IgH knockout as well as after treatment with PRT318. This was accompanied by an increase in the FOXO1 transcriptional targets p27 and BLNK, suggesting increased FOXO1 transcriptional activity in response to the inhibition of pre-BCR signaling. To study the contribution of FOXO1 to the effects of IgH knockout and SYK inhibition more thoroughly, we expressed constitutively active FOXO1 (FOXO1-3A) in the pre-BCR+ ALL cell line RCH-ACV and consequently assessed its effects on cell proliferation and protein expression. Similar to IgH knockout and PRT318, FOXO1-3A reduced cell proliferation and increased p27 and BLNK protein levels, confirming FOXO1 as an important downstream target of pre-BCR signaling in B-ALL. To identify additional effectors of the pre-BCR in B-ALL we performed gene expression profiling (GEP) to compare pre-BCR+ and pre-BCR- cells of RCH-ACV and SMS-SB. Gene set enrichment analysis (GSEA) showed that IgH knockout resulted in significant enrichment for gene sets associated with down-modulation of MYC activity. This was confirmed by Western blot analysis of MYC total protein levels, and consistent with the finding of reduced MYC protein in PRT318-treated and FOXO1-3A-expressing pre-BCR+ cells, all indicating that pre-BCR signaling modulates MYC activity through a mechanism involving SYK and FOXO1. In conclusion, we provide evidence for the dependence of certain B-ALL subgroups on pre-BCR signaling. According to our data this is mainly due to pre-BCR-induced inactivation of FOXO1 and the subsequent deregulation of MYC. Importantly, pharmacological inhibition of pre-BCR signaling with the SYK inhibitor PRT318 completely reversed these effects, therefore providing a rationale for the use of SYK inhibitors in pre-BCR+ subgroups of B-ALL. Figure 1: Figure 1 Figure 1. Disclosures Coffey: Portola Pharmaceuticals: Employment, Equity Ownership.