No abstract

Children with Down syndrome (DS) are prone to development of high-risk B-cell precursor ALL (DS-ALL), which differs genetically from most sporadic pediatric ALLs. Increased expression of cytokine receptor-like factor 2 (CRLF2), the receptor to thymic stromal lymphopoietin (TSLP), characterizes about half of DS-ALLs and also a subgroup of sporadic "Philadelphia-like" ALLs. To understand the pathogenesis of relapsed DS-ALL, we performed integrative genomic analysis of 25 matched diagnosis-remission and -relapse DSALLs. We found that the CRLF2 rearrangements are early events during DS-ALL evolution and generally stable between diagnoses and relapse. Secondary activating signaling events in the JAK-STAT/ RAS pathway were ubiquitous but highly redundant between diagnosis and relapse, suggesting that signaling is essential but that no specific mutations are "relapse driving." We further found that activated JAK2 may be naturally suppressed in 25% of CRLF2 pos DSALLs by loss-of-function aberrations in USP9X, a deubiquitinase previously shown to stabilize the activated phosphorylated JAK2. Interrogation of large ALL genomic databases extended our findings up to 25% of CRLF2 pos , Philadelphia-like ALLs. Pharmacological or genetic inhibition of USP9X, as well as treatment with low-dose ruxolitinib, enhanced the survival of pre-B ALL cells overexpressing mutated JAK2. Thus, somehow counterintuitive, we found that suppression of JAK-STAT "hypersignaling" may be beneficial to leukemic B-cell precursors. This finding and the reduction of JAK mutated clones at relapse suggest that the therapeutic effect of JAK specific inhibitors may be limited. Rather, combined signaling inhibitors or direct targeting of the TSLP receptor may be a useful therapeutic strategy for DS-ALL.C hildren with Down syndrome (DS) are at a markedly increased risk for B-cell precursor acute lymphoblastic leukemia (BCP-ALL) (1). The poor survival of DS-ALL compared with ALL in children without DS ("sporadic" ALL) is related to increased treatment toxicity and to increased incidence of relapse (2). Thus, better therapy is needed for these patients.Previous studies by our group and others revealed differences between the genetics of DS-ALLs and of sporadic ALLs (3-6). The typical cytogenetic subgroups, ETV6-RUNX1 and hyperdiploid ALLs, are less common in DS-ALLs. Acquired somatic activation of the thymic stromal lymphopoietin (TSLP) pathway are present at diagnosis in about half of DS-ALLs. Aberrant expression of cytokine receptor-like factor 2 (CRLF2) in these leukemias is caused by chromosomal rearrangements consisting either of a microdeletion on chromosome X, juxtaposing the promoter of P2RY8 with the coding region of CRLF2, or by a translocation of CRLF2 into the Ig heavy chain (IgH) locus. CRLF2 heterodimerizes with IL7 receptor-α (IL7R) to form the receptor to TSLP (reviewed in ref. 7). TSLP receptors signal by activation of the JAK-STAT pathway. Interestingly, in the majority of DS-ALLs, SignificanceChildren with Down syndrome are at increased risk ...

The cell-context dependency for RNA binding proteins (RBPs) mediated control of stem cell fate remains to be defined. Here we adapt the HyperTRIBE method using an RBP fused to a Drosophila RNA editing enzyme (ADAR) to globally map the mRNA targets of the RBP MSI2 in mammalian adult normal and malignant stem cells. We reveal a unique MUSASHI-2 (MSI2) mRNA binding network in hematopoietic stem cells that changes during transition to multipotent progenitors. Additionally, we discover a significant increase in RNA binding activity of MSI2 in leukemic stem cells compared with normal hematopoietic stem and progenitor cells, resulting in selective regulation of MSI2's oncogenic targets. This provides a basis for MSI2 increased dependency in leukemia cells compared to normal cells. Moreover, our study provides a way to measure RBP function in rare cells and suggests that RBPs can achieve differential binding activity during cell state transition independent of gene expression.