The PAF Complex Synergizes with MLL Fusion Proteins at .Hox Loci to Promote Leukemogenesis.

Abstract Abstract 1277 Poster Board I-299 Mixed lineage leukemia (MLL) is a histone H3 lysine 4 methyltransferase that is required to maintain a normal hematopoietic stem cell compartment. MLL functions to maintain expression of HOX genes as well as the HOX co-factor MEIS1, which play significant roles in regulating hematopoiesis. MLL is involved in chromosomal translocations with up to sixty different partners in both acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL). HOXA9 and MEIS1, are directly regulated by MLL fusion proteins and are crucial for MLL fusion protein mediated transformation. The deregulated expression of target genes in AML is dependent on specific protein-protein interactions and functional domains of MLL. For example, the tumor suppressor Menin bridges LEDGF to the extreme N-terminus of MLL and both of these interactions are necessary for transformation. Furthermore, a DNA methyltransferase homology region (CxxC domain) of MLL is essential for binding to non-methylated CpG islands and MLL-fusion protein oncogenesis. We have found that sequences downstream of the CxxC domain, termed the RD2 region, that interact with the Polymerase Associated Factor (PAF) complex are also required for MLL fusion protein mediated transformation. The PAF complex interacts with RNA polymerase II and is required for H2B mono-ubiquitination and subsequent histone H3K4 and H3K79 methylation. Together the PAF complex has been shown to be involved in transcriptional initiation, elongation and termination. Interaction of MLL with the PAF complex is mediated through direct contacts with two subunits: Ctr9 and PAF1. The PAF complex synergizes with MLL-AF9 to augment transcriptional activation of the Hoxa9 promoter. Furthermore, MLL fusion proteins recruit high levels of the PAF complex to the Hoxa9 promoter. Importantly, deletions of the MLL RD2 region that abolish interactions with the PAF complex eliminate MLL-AF9 mediated transformation of mouse bone marrow cells. Transcription of PAF components is dramatically downregulated during differentiation of hematopoietic cells, consistent with recent data showing a requirement for the PAF complex to maintain an embryonic stem cell phenotype. Knock down and transplantation experiments are underway to further define how the PAF complex regulates normal MLL function and cooperates with MLL fusion proteins to promote leukemogenesis. Disclosures No relevant conflicts of interest to declare.