MLL-AF4 Down-Regulates CDKN1B (P27) Independent of Cell Cycle Progression.

The MLL gene, involved in many chromosomal translocations associated with acute myeloid and lymphoid leukemia, has more than forty known partner genes with which it is able to form in- frame fusions. MLL fusion genes transform hematopoietic cells in vitro, and cause leukemia in mouse models. However, the mechanism is still not clear. Characterizing important downstream target genes may provide rational therapeutic strategies for the treatment of MLL-associated leukemia. We explored potential downstream target genes of the most prevalent MLL fusion protein, MLL-AF4, which is primarily associated with pro-B ALL and is involved in the majority of infant leukemia. To this end, we developed an inducible MLL-AF4 fusion cell line. Overexpression of MLL-AF4 does not lead to increased proliferation in this cell line, but rather, cell growth is slowed compared to similar cell lines inducibly expressing truncated MLL. To try to understand the reason for slower cell growth, we assayed for expression of several CDK inhibitors. We found that in the MLL-AF4 induced cell line, the amount of CDKN1B (cyclin-dependent kinase inhibitor P27) was dramatically decreased both at the RNA and protein levels, in contrast, the levels of CDKN1A (P21) and CDKN2A (P16) were unchanged. Interestingly, we did not observe an increased percentage of cells in S phase of the cell cycle. To explore whether CDKN1B might be a direct target of MLL-AF4, we employed chromatin immunoprecipitation (ChIP) assays and luciferase reporter gene assays. We observed that MLL-AF4 binds to the CDKN1B promoter in vivo and represses CDKN1B promoter activity. Further, we confirmed CDKN1B promoter binding by ChIP assays in the MLL-AF4 leukemia cell line MV4-11. Our results suggest that the CDKN1B may be a downstream target of MLL-AF4, and that MLL-AF4 inhibits CDKN1B expression independent of cell cycle progression.