Translocations that generate MLL fusion proteins are common causes of human acute leukemias. Aberrant target gene activation is the primary driver of MLL-rearranged leukemogenesis, but the underlying mechanisms remain poorly understood. In the present study, we identified two partners of the MLL fusion complex, PAFc and CBX8, which can interact with a common MLL fusion protein, MLL-AF9, through the N-terminal MLL part and the C-terminal AF9 part, respectively. Our data demonstrate that both PAFc and CBX8 are required for MLL-AF9-mediated transcriptional upregulation and leukemic transformation. The molecular mechanisms for their requirements in the leukemogenic process are different. By chromatin immunoprecipitation, we show that PAFc binds at the MLL target gene (e.g., Hoxa9 and Meis1) loci in MLL-AF9 leukemic cells. Its binding contributes to the recruitment of MLL-AF9 and augments MLL-AF9-mediated transcriptional activation. In contrast, CBX8 promotes the transcriptional activation of MLL-AF9 target genes, not by regulating MLL-AF9 recruitment but may through interacting with other cofactors, such as the histone acetyltransferase TIP60, whose enzymatic activity could potentially facilitate gene transcription. Furthermore, although CBX8 is essential for MLL-AF9 transformation, it is not required for normal hematopoiesis, as shown by the normal viability of hematopoiesis in Cbx8-deficient mice. This suggests targeting CBX8 may be of therapeutic value in treating MLL-rearranged leukemias. In conclusion, our findings demonstrate that both PAFc and CBX8 play essential roles in MLL-AF9-mediated transcriptional regulation and leukemogenesis.
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