Relja Popovic | Search PubMed

PubMed2005

The MLL fusion gene, MLL-AF4, regulates cyclin-dependent kinase inhibitor CDKN1B (p27kip1) expression.

MLL, involved in many chromosomal translocations associated with acute myeloid and lymphoid leukemia, has >50 known partner genes with which it is able to form in-frame fusions. Characterizing important downstream target genes of MLL and of MLL fusion proteins may provide rational therapeutic strategies for the treatment of MLL-associated leukemia. We explored downstream target genes of the most prevalent MLL fusion protein, MLL-AF4. To this end, we developed inducible MLL-AF4 fusion cell lines in different backgrounds. Overexpression of MLL-AF4 does not lead to increased proliferation in either cell line, but rather, cell growth was slowed compared with similar cell lines inducibly expressing truncated MLL. We found that in the MLL-AF4-induced cell lines, the expression of the cyclin-dependent kinase inhibitor gene CDKN1B was dramatically changed at both the RNA and protein (p27kip1) levels. In contrast, the expression levels of CDKN1A (p21) and CDKN2A (p16) were unchanged. To explore whether CDKN1B might be a direct target of MLL and of MLL-AF4, we used chromatin immunoprecipitation (ChIP) assays and luciferase reporter gene assays. MLL-AF4 binds to the CDKN1B promoter in vivo and regulates CDKN1B promoter activity. Further, we confirmed CDKN1B promoter binding by ChIP in MLL-AF4 as well as in MLL-AF9 leukemia cell lines. Our results suggest that CDKN1B is a downstream target of MLL and of MLL-AF4, and that, depending on the background cell type, MLL-AF4 inhibits or activates CDKN1B expression. This finding may have implications in terms of leukemia stem cell resistance to chemotherapy in MLL-AF4 leukemias.

Cell Line↗

PubMed2005

MLL: how complex does it get?

The mixed lineage leukemia (MLL) gene encodes a very large nuclear protein homologous to Drosophila trithorax (trx). MLL is required for the proper maintenance of HOX gene expression during development and hematopoiesis. The exact regulatory mechanism of HOX gene expression by MLL is poorly understood, but it is believed that MLL functions at the level of chromatin organization. MLL was identified as a common target of chromosomal translocations associated with human acute leukemias. About 50 different MLL fusion partners have been isolated to date, and while similarities exist between groups of partners, there exists no unifying property shared by all the partners. MLL gene rearrangements are found in leukemias with both lymphoid and myeloid phenotypes and are often associated with infant and secondary leukemias. The immature phenotype of the leukemic blasts suggests an important role for MLL in the early stages of hematopoietic development. Mll homozygous mutant mice are embryonic lethal and exhibit deficiencies in yolk sac hematopoiesis. Recently, two different MLL-containing protein complexes have been isolated. These and other gain- and loss-of-function experiments have provided insight into normal MLL function and altered functions of MLL fusion proteins. This article reviews the progress made toward understanding the function of the wild-type MLL protein. While many advances in understanding this multifaceted protein have been made since its discovery, many challenging questions remain to be answered.

Animals↗