MLL is an epigenetic regulatory protein that is mutated in a subset of leukemias with a poor prognosis and few therapeutic options. The epigenetic pathways and perturbations involved in MLL leukemia pathogenesis are complex, and the effectiveness of targeted molecular therapies not yet established. The studies proposed in this renewal application investigate the pathologic contributions of ASH1L, an epigenetic regulatory factor not previously implicated in MLL leukemia, and will define its role as a potential therapeutic target. Substantial preliminary data show that recruitment of MLL at target genes involved in leukemogenesis is dependent on ASH1L, a histone methyltransferase that specifically di-methylates histone H3 on lysine 36 (H3K36me2) associated with active promoters. Furthermore, ASH1L is required for leukemic transformation by MLL oncoproteins in mouse models of acute myeloid leukemia. A component of the MLL complex (LEDGF) specifically binds H3K36me2 suggesting that ASH1L may serve a key upstream role for recruitment or retention of MLL at its target genes. However, the specific mechanisms involved and the consequences for human leukemia remain to be determined. The proposed studies will address the hypothesis that ASH1L serves a crucial role in the MLL oncogenic pathway by establishing a chromatin environment enriched for H3K36 di-methyl at specific target promoters to facilitate binding of the MLL protein complex that perturbs gene expression in leukemia cells. Studies in the first specific aim will establish the functional roles of ASH1L and histone H3K36 di- methylation in the pathogenesis of MLL leukemia using shRNA technology in pre-clinical and human leukemia cell model systems. These studies will define which leukemia subtypes are dependent on ASH1L, and characterize the deleterious consequences of its inhibition to provide the basis for a rational therapeutic strategy in leukemia. Studies in the second aim will use chromatin immunoprecipitation techniques to establish the genome- wide distribution of histone modifications, determine which marks and factors are selectively dependent on ASH1L, and establish their respective mechanistic roles in defining the epigenomic states required for aberrant gene expression in MLL leukemia cells. Studies in the third specific aim will employ structure-function and unbiased proteomics approaches to characterize ASH1L heterologous interactions that direct its activity to the chromatin of leukemia- associated target genes. Taken together, the proposed studies will provide significant insights into the molecular mechanisms of a novel epigenetic regulatory pathway in leukemia cells, and facilitate efforts to specifically target the pathway to achieve more efficacious therapies.
The studies in this application investigate epigenetic regulatory pathways that sustain a characteristic genetic subtype of leukemia with alterations of the MLL oncogene. Specific writers, readers and erasers of the histone code will be studied for their roles in regulating the localization, assembly and functions of oncoprotein-associated transcriptional machinery at crucial target genes. A greater understanding of the unique functions for these factors in cancer versus normal cells will facilitate efforts to selectively target leukemia to achieve more efficacious therapies.
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