Cooperating Oncogenes and Their Targets in LMO2-Induced T-Cell Leukemia

Abstract 2458

LIM domain Only 2 (LMO2) is an important driver in acute T-cell lymphoblastic leukemia. Enforced expression of LMO2 in mouse models induces T-ALL with long latency suggesting that it requires cooperating oncogenic mutations for leukemia to fully manifest. To identify cooperating oncogenes, we analyzed spontaneous T-ALLs induced by retroviral insertional mutation, and T-ALLs that developed in B6.CD2-Lmo2 transgenic mice. First, we noted that the CD2-Lmo2 transgenic T-ALLs exclusively upregulated the class II basic helix-loop-helix transcription factor, Lyl1, whereas the retrovirally-induced T-ALLs showed upregulation of Tal1. We attributed this difference to the stage at which the Lmo2 activating mutations occurred. Nevertheless, we found that LYL1 protein upregulation is functionally important, as it is part of a macromolecular complex containing LMO2 that binds tandem E boxes as described for TAL1. LMO2 and LYL1 are known to be co-expressed in human T-ALL, but we found that they are part of a larger gene expression signature comprised of NMYC, HHEX, MEF2C, and BCL2 genes. The latter genes are remarkable because they are oncogenic when overexpressed; and, Mef2c and Nmyc are concordant insertions in independent tumors driven by Lmo2 or Hhex in retroviral insertional mutagenesis models. In fact, insertion site analysis suggests that Lmo2 and Hhex may actually operate in the same pathway and that Hhex may be a target of Lmo2. We tested this genetic result by chromatin immunoprecipitation and reporter assays and discovered that Lmo2 and its binding partners, Ldb1 and Lyl1, occupy an enhancer in the first intron of Hhex in T-ALL cells. Furthermore, LMO2 or LYL1 knockdown downregulated expression of HHEX in T-ALL. HHEX activation has significant functional consequences in T-ALL. We found that its upregulation is associated with T-ALL treatment resistance, clinical latency, and differential activation of Notch1 targets. These data offer important insight into the pathogenesis of Lmo2-induced T-ALL.