Lmo2 Induces T-Cell Leukemia with Epigenetic Deregulation of CD4.

The LIM domain Only-2 (Lmo2) oncogene is deregulated in the majority of human T-cell acute lymphoblastic leukemias (T-ALL). Enforced, constitutive expression of Lmo2 in transgenic mice using the T-cell-specific Cd2 promoter/enhancer induces T-ALL with median latency of 200 days and 100% penetrance. These transgenic mice have a pre-leukemic differentiation block at the double negative stage (CD4⁻ CD8⁻) of T-cell differentiation. Despite this, T-ALLs in these mice arise from various stages of T-cell differentiation. To explore the relationship between T-cell development and Lmo2 oncogene function, we established cell lines from four independent T-cell tumors representing the double negative (DN), double positive (DP), and single positive (SP) stages. Two of the lines showed variegated expression of CD4 antigen. One line, 32080, was striking in that CD4 was expressed in 50% of cells whereas CD8 was uniformly present. The cell line expressed markers consistent with the intermediate single positive stage (ISP) to double positive (DP) stage transition. We sorted the 32080 line for highly CD4 positive and negative cells and observed them in culture. After one week, both sorted populations showed variegated CD4 expression. We determined CD4 expression was regulated at the transcriptional level and not due to post-transcriptional effects and that cell replication was necessary to re-establish CD4 variegation. Importantly, variegated CD4 expression was dependent on Lmo2 because its knockdown resulted in decreased CD4 expression. HDAC inhibition by trichostatin A had the reciprocal effect of inducing more CD4 expression suggesting that chromatin modification was playing a role. The CD4 locus was analyzed by chromatin immunoprecipitation and histone H3 methylation patterns consistent with silencing were found in the CD4 negative sorted cells but not in the CD4 positive population. The 32080 cell line is a striking model of ISP to DP T-cell plasticity and invokes a novel mechanism by which Lmo2 functions.