B cell transcription factors define a novel tumor suppressor gene network In ALL

Abstract

Abstract The transcription factors EBF1 and PAX5 are frequently mutated in B cell acute lymphoblastic leukemia (B-ALL). We demonstrated that Pax5+/− x Ebf1+/− compound heterozygous mice develop leukemia with high penetrance. Similar results were seen in Pax5+/− x Ikzf1+/− and Ebf1+/− x Ikzf1+/− mice for B-ALL, or in Tcf1+/− x Ikzf1+/− mice for T cell leukemia. To identify genetic defects that cooperate with Pax5 and Ebf1 compound heterozygosity to initiate leukemia, we carried out a Sleeping Beauty transposon screen. This screen identified a number of cooperating partners including gain-of-function mutations in Stat5 (~65%) and Jak1(~68%), as well as loss-of-function mutations in Cblb (61%)and Myb (32%). These findings highlight the key role of JAK/STAT5 signaling in cooperating with Pax5 and Ebf1 compound haploinsufficiency to drive B cell transformation. Moreover, these studies pointed to unexpected roles for loss of function mutations in CBL-b and MYB in B cell transformation. Subsequent RNA-Seq studies on WT, Pax5+/−, Ebf1+/−, Pax5+/− x Ebf1+/− pre-leukemic, Pax5+/− x Ebf1+/− leukemic cells and Pax5+/− x Ebf1+/− Sleeping Beauty leukemic cells demonstrated upregulation of a PDK1>PLK1>MYC pathway; treatment of Pax5+/− x Ebf1+/− leukemias with PDK1 specific inhibitors blocked their proliferation in vitro. Finally, we identified conserved transcriptional variation in a subset of genes between human leukemias and our mouse B-ALL models. Thus, compound haploinsufficiency for B cell transcription factors likely plays a critical role in transformation of human B cells and suggest that newly developed PDK1 inhibitors may be effective for treating patients characterized by such defects.