Manipulating MEF2C: Discovering Novel Drugs to Target ETP-ALL

Ectopic MEF2C expression due to rearrangements have been recurrently found in patients with human early thymocyte progenitor acute lymphoblastic leukemia (ETP-ALL). ETP-ALL is the most immature subtype of T-ALL and is characterized by the expression of myeloid lineage markers and an ETP-like gene expression profile. In normal haematopoiesis, MEF2C expression occurs at immature stages and in myeloid- and B cell lineages but is absent in early T cell progenitors.

Here, we investigate MEF2C as an oncogene for ETP-ALL. First, we validated our initial observations in an independent T-ALL cohort that ETP-ALL patients highly express MEF2C along with BCL2, HHEX, LMO2 and LYL1 in contrast to other T-ALL subtypes. In addition, we have demonstrated that enforced MEF2C expression in the T-lineage drives a bi-phenotypic CD3 ⁺CD19 ⁺ leukemia in mice by inhibiting a T-cell developmental program in favor a B-cell transcriptional program and that resembles human ETP-ALL. This confirms MEF2C as an oncogenic driver of ETP-ALL.

We developed a model system that enabled screening of various drug libraries for compounds that overcome developmental arrest induced by MEF2C-expression, as evidenced by the upregulation of CD3 and TCRγδ. The ETP-ALL-like LOUCY cells express MEF2C as consequence of a del5(q14-qter). Further induction of MEF2C levels (i.e. LOUCY-iMEF2C cells) blocked differentiation of LOUCY cells towards a CD3 ⁺/TCRγδ ⁺ phenotype on OP9-DL1 or DLL4-coated plates in contrast to parental LOUCY cells. This model enabled high-throughput screening of drug libraries using a flow cytometry-based assay for drugs that further promote differentiation of LOUCY-MEF2C cells. We identified various epigenetic inhibitors that promoted differentiation of LOUCY-iMEF2C cells. Unlike AML patients, where HDAC4 was shown to play an important role in the activation of MEF2C and the propagation of the leukemia, we found that different HDACs are involved that can block or promote MEF2C function. This suggests that the role of different HDACs in MEF2C regulation is cell-type or disease dependent.

In addition, over-expression of MEF2C raised cellular resistance towards prednisolone treatment. Conversely, inhibition of MEF2C sensitized cells to prednisolone cytotoxicity, even in the presence of MEF2C-overexpression. Together, these data show that MEF2C is an oncogenic driver of ETP-ALL, and ectopic expression of MEF2C negatively affects steroid sensitivity. Specific targeting of only those HDACs that promote MEF2C activity may provide new therapeutic options in ETP-ALL by inhibiting MEF2C function and enhancing steroid sensitivity.