The Role of MLL1 and MLL2 in MLL Fusion Oncoprotein-Initiated Leukemia

The MLL1 histone methyltransferase gene at 11q23 undergoes many distinct chromosomal translocations to yield poor prognosis leukemia. In these MLL-rearranged (MLL-r) leukemias, the remaining wild-type allele is typically expressed, and the fusion oncoprotein lacks the C terminal histone methyltransferase domain. The role of the wild-type, endogenous MLL1 allele in MLL-fusion protein mediated leukemogenesis is not fully understood, with some evidence suggesting an important functional role. On the other hand, the wild-type allele is lost in some patients and in the MLL-r ML2 cell line, suggesting that the wild-type allele is not absolutely required. Here we rigorously address the role of endogenous MLL1 in MLL-r leukemia. Using precise and independent animal models, we show that MLL1 is dispensable for MLL-AF9- or MLL-AF6-mediated acute myelogenous leukemia (AML). In parallel experiments, deletion of Menin, encoding a chromatin targeting cofactor for MLL-fusion proteins, has severe effects on the proliferation and survival of MLL-r leukemia, as previously published. Potential redundancy between MLL family methyltransferases was addressed by co-deleting the closest paralog, Mll2. Surprisingly, deletion of Mll2 by itself had a significant impact on survival of MLL-AF9 leukemia cells, and co-deletion of Mll1 and Mll2 more effectively killed these cells, suggesting a redundant or synthetic lethal relationship between the two genes. A comparison of gene expression and histone modification changes in Mll1, Mll2 and Mll1;Mll2 double knockout leukemia cells showed that 1) global defects in histone H3, lysine 4 methyation can be exclusively attributed to MLL2, 2) MLL1- versus MLL2-dependent genes are largely distinct, and 3) Mll1;Mll2 double knockout leukemia cells exhibit deficiencies in NFκB and integrin ß3 pathways, but exhibit little evidence of deregulation of either MLL1- or MLL-fusion protein target genes. These findings reshape our thinking about the role of endogenous MLL family proteins in MLL-fusion protein leukemogenesis. The data presented here, in conjunction with the minimal effect of Mll2 deletion on hematopoietic stem and progenitor cells make MLL2 an attractive candidate for targeted therapy in MLL-r leukemia.