BACKGROUND Philadelphia chromosome–like acute lymphoblastic leukemia (Ph-like ALL) is characterized by a gene-expression profile similar to that of BCR–ABL1–positive ALL, alterations of lymphoid transcription factor genes, and a poor outcome. The frequency and spectrum of genetic alterations in Ph-like ALL and its responsiveness to tyrosine kinase inhibition are undefined, especially in adolescents and adults. METHODS We performed genomic profiling of 1725 patients with precursor B-cell ALL and detailed genomic analysis of 154 patients with Ph-like ALL. We examined the functional effects of fusion proteins and the efficacy of tyrosine kinase inhibitors in mouse pre-B cells and xenografts of human Ph-like ALL. RESULTS Ph-like ALL increased in frequency from 10% among children with standard-risk ALL to 27% among young adults with ALL and was associated with a poor outcome. Kinase-activating alterations were identified in 91% of patients with Ph-like ALL; rearrangements involving ABL1, ABL2, CRLF2, CSF1R, EPOR, JAK2, NTRK3, PDGFRB, PTK2B, TSLP, or TYK2 and sequence mutations involving FLT3, IL7R, or SH2B3 were most common. Expression of ABL1, ABL2, CSF1R, JAK2, and PDGFRB fusions resulted in cytokine-independent proliferation and activation of phosphorylated STAT5. Cell lines and human leukemic cells expressing ABL1, ABL2, CSF1R, and PDGFRB fusions were sensitive in vitro to dasatinib, EPOR and JAK2 rearrangements were sensitive to ruxolitinib, and the ETV6–NTRK3 fusion was sensitive to crizotinib. CONCLUSIONS Ph-like ALL was found to be characterized by a range of genomic alterations that activate a limited number of signaling pathways, all of which may be amenable to inhibition with approved tyrosine kinase inhibitors. Trials identifying Ph-like ALL are needed to assess whether adding tyrosine kinase inhibitors to current therapy will improve the survival of patients with this type of leukemia. (Funded by the American Lebanese Syrian Associated Charities and others.)

Mixed phenotype acute leukaemia (MPAL) is a high-risk subtype of leukaemia with myeloid and lymphoid features, limited genetic characterization, and a lack of consensus regarding appropriate therapy. Here we show that the two principal subtypes of MPAL, T/myeloid (T/M) and B/myeloid (B/M), are genetically distinct. Rearrangement of ZNF384 is common in B/M MPAL, and biallelic WT1 alterations are common in T/M MPAL, which shares genomic features with early T-cell precursor acute lymphoblastic leukaemia. We show that the intratumoral immunophenotypic heterogeneity characteristic of MPAL is independent of somatic genetic variation, that founding lesions arise in primitive haematopoietic progenitors, and that individual phenotypic subpopulations can reconstitute the immunophenotypic diversity in vivo. These findings indicate that the cell of origin and founding lesions, rather than an accumulation of distinct genomic alterations, prime tumour cells for lineage promiscuity. Moreover, these findings position MPAL in the spectrum of immature leukaemias and provide a genetically informed framework for future clinical trials of potential treatments for MPAL.

SUMMARY Alterations of IKZF1, encoding the lymphoid transcription factor IKAROS, are a hallmark of high risk acute lymphoblastic leukemia (ALL), however the role of IKZF1 alterations in ALL pathogenesis is poorly understood. Here we show that in mouse models of BCR-ABL1 leukemia, Ikzf1 and Arf alterations synergistically promote the development of an aggressive lymphoid leukemia. Ikzf1 alterations result in acquisition of stem cell-like features, including self-renewal and increased bone marrow stromal adhesion. Retinoid receptor agonists reversed this phenotype, partly by inducing expression of IKZF1, resulting in abrogation of adhesion and self-renewal, cell cycle arrest and attenuation of proliferation without direct cytotoxicity. Retinoids potentiated the activity of dasatinib in mouse and human BCR-ABL1 ALL, providing an additional therapeutic option in IKZF1-mutated ALL.

Prolonged exposure to thiopurines (eg, mercaptopurine [MP]) is essential for curative therapy in acute lymphoblastic leukemia (ALL), but is also associated with frequent dose-limiting hematopoietic toxicities, which is partly explained by inherited genetic polymorphisms in drug metabolizing enzymes (eg, ). Recently, our group and others identified germ line genetic variants in as another major cause of thiopurine-related myelosuppression, particularly in Asian and Hispanic people. In this article, we describe 3 novel coding variants (p.R34T, p.K35E, and p.G17_V18del) in 5 children with ALL enrolled in frontline protocols in Singapore, Taiwan, and at St. Jude Children's Research Hospital. Patients carrying these variants experienced significant toxicity and reduced tolerance to MP across treatment protocols. Functionally, all 3 variants led to partial to complete loss of NUDT15 nucleotide diphosphatase activity and negatively influenced protein stability. In particular, the p.G17_V18del variant protein showed extremely low thermostability and was completely void of catalytic activity, thus likely to confer a high risk of thiopurine intolerance. This in-frame deletion was only seen in African and European patients, and is the first risk variant identified in non-Asian, non-Hispanic populations. In conclusion, we discovered 3 novel loss-of-function variants in associated with MP toxicity, enabling more comprehensive pharmacogenetics-based thiopurine dose adjustments across diverse populations.

Introduction: Ph-like or BCR-ABL1-like B-progenitor acute lymphoblastic leukemia (ALL) is a high-risk subtype characterized by a gene expression profile similar to BCR-ABL1 ALL. The prevalence of Ph-like ALL rises from 10% in standard risk childhood ALL to over 25% in young adults. Next-generation sequencing of Ph-like ALL identified a variety of alterations involving kinase or cytokine receptor genes, including rearrangement, sequence mutation and copy number alterations. Chromosomal rearrangements in about one-third of Ph-like ALL cases create fusion genes of a variety of 5’ partners that involve ABL1-class genes (ABL1, ABL2, CSF1R and PDGFRB) or activate JAK family members (JAK2, TYK2, IL2RB) that are potentially amenable to treatment with ABL1-class or JAK-class tyrosine kinase inhibitors (TKIs). Notably, ABL2 (Abelson-related gene, ARG), a homolog of ABL1, has rarely been identified as a rearrangement partner in ALL. CSF1R (encoding the macrophage colony stimulating receptor) regulates the differentiation of macrophages, and is not normally expressed in lymphocytes. Likewise, rearrangements involving the JAK family member TYK2, the beta chain of the interleukin 2 cytokine receptor (IL2RB), and the neurotrophic tyrosine kinase receptor type 3 (NTRK3), have not been previously described in leukemia. The goals of this study were to assess the role of these kinase alterations in leukemogenesis, to determine the activation of signaling pathways, and to investigate the efficacy of TKIs. Methods: Kinase fusions were expressed in interleukin-3 dependent Ba/F3 cells, and co-expressed with the dominant negative isoform of IKAROS (IK6) in interleukin-7 dependent Arf-/- mouse pre-B cells. Xenograft models of 10 Ph-like ALL tumors - ETV6-ABL1, RANBP2-ABL1, PAG1-ABL2, RCSD1-ABL2, SSBP2-CSF1R, IGH-EPOR, ETV6-NTRK3, ATF7IP-JAK2, PAX5-JAK2 and ZEB2-PDGFRB - were generated by engrafting primary human leukemia cells into NOD-SCID IL2R gamma null (NSG) mice. Activation of kinase signaling was performed using phosphoflow cytometry analysis, and sensitivity to TKIs was assessed ex vivo and in vivo. Results: All kinase fusions (PAG1-ABL2, MYH9-IL2RB, ATF7IP-JAK2, ETV6-NTRK3 or MYB-TYK2) induced cytokine-independent proliferation of Ba/F3 cells. Mice transplanted with Arf-/- pre-B cells co-expressing IK6 and either RCSD1-ABL2 or SSBP2-CSF1R developed pre-B ALL (CD43+, B220+, CD19+, BP-1+ and IgM-) with a median latency of 36 and 40 days respectively, providing evidence that ABL2 and CSF1R fusions contribute to leukemogenesis. In human leukemic cells harvested from xenograft mice we observed distinct patterns of kinase signaling activation and TKI sensitivity for the different fusions. Xenograft cells expressing ABL1-class kinase fusions showed activation of STAT5 that was inhibited with imatinib or dasatinib. Phosphorylation of CRKL, a known target of ABL1 and ABL2, was only observed in cells expressing ABL1/2 fusions. Cells harboring ATF7IP-JAK2, PAX5-JAK2 or IGH-EPOR showed phosphorylation of STAT5 that was attenuated with the JAK2 inhibitor, ruxolitinib. In contrast, cells expressing ETV6-NTRK3 signaled through the MAPK pathway with constitutive pERK1/2 that was inhibited with the ALK-inhibitor, crizotinib. This TKI response profile was confirmed by cytotoxicity assays in xenograft cells, with ABL1-class fusions being sensitive to dasatinib (IC50 range 1-2nM), whilst cases harboring ATF7IP-JAK2 or EPOR rearrangement uniquely responded to ruxolitinib with IC50 values of 500nM and 850nM respectively. Interestingly, in human leukemic cells harboring the ETV6-NTRK3 fusion we observed selective inhibition with both crizotinib and the FLT3 inhibitor, lestaurtinib. Pre-clinical studies on three xenograft models of Ph-like ALL - ETV6-ABL1, RCSD1-ABL2 and SSBP2-CSF1R – showed significantly reduced leukemic burden in dasatinib treated mice (20mg/kg/day p.o) compared to vehicle treated mice. Conclusions: These data provide important insight on new targets of rearrangement in ALL and describe the first engineered mouse models of Ph-like B-ALL. Functional modeling of these alterations is essential to improve the clinical management of Ph-like ALL by identifying patients with specific genomic lesions at diagnosis and directing them to treatment with appropriate TKIs combined with chemotherapy, analogous to current treatment for BCR-ABL1 B-ALL. Disclosures Hunger: Bristol Myers Squibb: Consultancy.