JAK signaling pathway members in 27.7% of T-ALL samples screened; an observation with therapeutic potential. Statistically significant pairwise associations were found between different mutations, indicating the presence of functional interactions among different pathways in T-ALL pathogenesis. Of significance, we found a mutually exclusive relationship between IL7R-JAK mutations and the presence of TAL1/LMO2 rearrangements. We also identified positive correlations among IL7R-JAK mutations and mutations/deletions in PHF6 and members of the PRC2 complex. Our findings begin to unravel the diversity of genetic lesions that are implicated in the development of T-ALL. Methods DNA samplesT-ALL samples from patients (n=155: 111 children, 44 adults) were collected from various institutions (Online Supplementary Table S1). The diagnosis of T-ALL was based on morphology, cytochemistry and immunophenotyping according to World Health Organization criteria. Genomic DNA was isolated from bone marrow (either fixed or fresh bone marrow cells). 5,21,22 To investigate the prognostic relevance of IL7R, JAK1 and JAK3 mutations, Sanger sequencing was used to screen for mutations in these three genes in an independent cohort of 78 T-ALL patients. Those patients were all enrolled into the United Kingdom (UK) Children's Cancer and Leukaemia Group (CCLG) ALL2003 trial. 23This study was approved by the ethics committees of the institutes involved and informed consent was obtained from the participants. Samples and clinical data were stored in accordance with the declaration of Helsinki. Capture designSureDesign software was used to design two slightly different Haloplex capture assays (Table 1). The total amplicon number for design A was 23,127 with a region size of 472.006 kbp and a predicted target coverage of >99%. For design B the total amplicon number was 19,694 with a region size of 418.373 kbp and a predicted target coverage of >99%. For this study, 80 samples were processed with design A and 75 samples with design B. In both assays, the coding exons of selected genes (based on RefSeq, CCDS and VEGA databases) were targeted with an extra ten bases upstream and downstream. Targeted regions comprised the coding sequence of genes that were either recently identified as recurrently mutated in ALL or other hematologic malignancies (known driver genes) or were similar to known oncogenes (candidate driver genes) to be sequenced. 18,19,24,25 For statistical analyses we only considered the 115 genes that were sequenced in both Haloplex designs (Online Supplementary Table S2). Library preparation and sequencing were performed as described in the Online Supplementary Material. Data analysesIn NextGENe software (v2.2.1, Softgenetics, State College, PA, USA), we performed the following steps: (i) the fastQ output file was converted into a FASTA file to eliminate reads that were not "paired" and that did not meet the criteria of the default settings; (ii) reads from the converted unique FASTA file were aligned to the reference genome (...

Key Points• JAK3 pseudokinase mutants require JAK1 for their transforming potential.• JAK3 mutants cause T-ALL in a mouse bone marrow transplant model and respond to tofacitinib, a JAK3-selective inhibitor.JAK3 is a tyrosine kinase that associates with the common g chain of cytokine receptors and is recurrently mutated in T-cell acute lymphoblastic leukemia (T-ALL). We tested the transforming properties of JAK3 pseudokinase and kinase domain mutants using in vitro and in vivo assays. Most, but not all, JAK3 mutants transformed cytokine-dependent Ba/F3 or MOHITO cell lines to cytokine-independent proliferation. JAK3 pseudokinase mutants were dependent on Jak1 kinase activity for cellular transformation, whereas the JAK3 kinase domain mutant could transform cells in a Jak1 kinase-independent manner.

T-cell acute lymphoblastic leukemia (T-ALL) is a high-risk subtype of acute lymphoblastic leukemia (ALL) with gradually improved survival through introduction of intensified chemotherapy. However, therapy-resistant or refractory T-ALL remains a major clinical challenge. Here, we evaluated B-cell lymphoma (BCL)-2 inhibition by the BH3 mimetic ABT-199 as a new therapeutic strategy in humanT-ALL. The T-ALL cell line LOUCY, which shows a transcriptional program related to immature T-ALL, exhibited high in vitro and in vivo sensitivity for ABT-199 in correspondence with high levels of BCL-2. In addition, ABT-199 showed synergistic therapeutic effects with different chemotherapeutic agents including doxorubicin, L-asparaginase, and dexamethasone. Furthermore, in vitro analysis of primary patient samples indicated that some immature, TLX3-orHOXA-positive primary T-ALLs are highly sensitive to BCL-2 inhibition, whereas TAL1 driven tumors mostly showed poor ABT-199 responses. Because BCL-2 shows high expression in early T-cell precursors and gradually decreases during normal T-cell differentiation, differences in ABT-199 sensitivity could partially be mediated by distinct stages of differentiation arrest between different molecular genetic subtypes of human T-ALL. In conclusion, our study highlights BCL-2 as an attractive molecular target in specific subtypes of human T-ALL that could be exploited by ABT-199

Leukemia is caused by the accumulation of multiple genomic lesions in hematopoietic precursor cells. However, how these events cooperate during oncogenic transformation remains poorly understood. We studied the cooperation between activated JAK3/STAT5 signaling and HOXA9 overexpression, two events identified as significantly co-occurring in T-cell acute lymphoblastic leukemia. Expression of mutant JAK3 and HOXA9 led to a rapid development of leukemia originating from multipotent or lymphoid-committed progenitors, with a significant decrease in disease latency compared with JAK3 or HOXA9 alone. Integrated RNA sequencing, chromatin immunoprecipitation sequencing, and Assay for Transposase-Accessible Chromatin using sequencing (ATAC-seq) revealed that STAT5 and HOXA9 have co-occupancy across the genome, resulting in enhanced STAT5 transcriptional activity and ectopic activation of FOS/JUN (AP1). Our data suggest that oncogenic transcription factors such as HOXA9 provide a fertile ground for specific signaling pathways to thrive, explaining why JAK/STAT pathway mutations accumulate in HOXA9-expressing cells. The mechanism of oncogene cooperation in cancer development remains poorly characterized. In this study, we model the cooperation between activated JAK/STAT signaling and ectopic HOXA9 expression during T-cell leukemia development. We identify a direct cooperation between STAT5 and HOXA9 at the transcriptional level and identify PIM1 kinase as a possible drug target in mutant JAK/STAT/HOXA9-positive leukemia cases. .

Mutations in the interleukin-7 receptor (IL7R) or the Janus kinase 3 (JAK3) kinase occur frequently in T-cell acute lymphoblastic leukemia (T-ALL) and both are able to drive cellular transformation and the development of T-ALL in mouse models. However, the signal transduction pathways downstream of JAK3 mutations remain poorly characterized. Here we describe the phosphoproteome downstream of the JAK3(L857Q)/(M511I) activating mutations in transformed Ba/F3 lymphocyte cells. Signaling pathways regulated by JAK3 mutants were assessed following acute inhibition of JAK1/JAK3 using the JAK kinase inhibitors ruxolitinib or tofacitinib. Comprehensive network interrogation using the phosphoproteomic signatures identified significant changes in pathways regulating cell cycle, translation initiation, mitogen-activated protein kinase and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/AKT signaling, RNA metabolism, as well as epigenetic and apoptotic processes. Key regulatory proteins within pathways that showed altered phosphorylation following JAK inhibition were targeted using selumetinib and trametinib (MEK), buparlisib (PI3K) and ABT-199 (BCL2), and found to be synergistic in combination with JAK kinase inhibitors in primary T-ALL samples harboring JAK3 mutations. These data provide the first detailed molecular characterization of the downstream signaling pathways regulated by JAK3 mutations and provide further understanding into the oncogenic processes regulated by constitutive kinase activation aiding in the development of improved combinatorial treatment regimens.

Background: JAK3, a tyrosine kinase associated to cytokine receptors, is frequently mutated in leukemia. Results: JAK3L857P induces constitutive signaling independently of cytokine receptors and JAK1, in contrast to other JAK mutants. Conclusion: Different JAK mutants signal through distinct mechanisms and show different sensitivity to JAK1-or JAK3-specific inhibitors. Significance: Depending on the JAK residue mutated, patients will require different treatments.

Key Points A subset of T-ALL cases show high expression of hedgehog pathway genes including the SHH ligand and the GLI1 transcription factor. T-ALL samples with high GLI1 expression levels respond to hedgehog inhibitor treatment in vitro and in vivo.

K E Y P O I N T Sl One-third of T-ALL cases with JAK3 mutation harbor 2 JAK3 mutations.l Double JAK3 mutants show stronger signaling than single JAK3 mutants.The Janus kinase 3 (JAK3) tyrosine kinase is mutated in 10% to 16% of T-cell acute lymphoblastic leukemia (T-ALL) cases. JAK3 mutants induce constitutive JAK/STAT signaling and cause leukemia when expressed in the bone marrow cells of mice. Surprisingly, we observed that one third of JAK3-mutant T-ALL cases harbor 2 JAK3 mutations, some of which are monoallelic and others that are biallelic. Our data suggest that wild-type JAK3 competes with mutant JAK3 (M511I) for binding to the common g chain and thereby suppresses its oncogenic potential. We demonstrate that JAK3 (M511I) can increase its limited oncogenic potential through the acquisition of an additional mutation in the mutant JAK3 allele. These double JAK3 mutants show increased STAT5 activation and increased potential to transform primary mouse pro-T cells to interleukin-7-independent growth and were not affected by wild-type JAK3 expression. These data extend our insight into the oncogenic properties of JAK3 mutations and provide an explanation of why progression of JAK3-mutant T-ALL cases can be associated with the accumulation of additional JAK3 mutations. (Blood. 2018;131(4):421-425)