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