Repression of TRIM13 by chromatin assembly factor CHAF1B is critical for AML development

Acute myeloid leukemia (AML) is an aggressive blood cancer that stems from the rapid expansion of immature leukemic blasts in the bone marrow. Mutations in epigenetic factors represent the largest category of genetic drivers of AML. The chromatin assembly factor CHAF1B is a master epigenetic regulator of transcription associated with self-renewal and the undifferentiated state of AML blasts. Upregulation of CHAF1B, as observed in almost all AML samples, promotes leukemic progression by repressing transcription of differentiation factors and tumor suppressors. However, the specific factors regulated by CHAF1B and their contributions to leukemogenesis are unstudied. We analyzed RNAseq from mouse MLL-AF9 leukemic cells and bone marrow aspirates representing a diverse collection of pediatric AML samples and identified the E3 ubiquitin ligase TRIM13 as a target of CHAF1B-mediated transcriptional repression associated with leukemogenesis. We found that CHAF1B binds the promoter of TRIM13, resulting in its transcriptional repression. In turn, TRIM13 suppresses self-renewal of leukemic cells by promoting pernicious entry into the cell cycle through its nuclear localization and catalytic ubiquitination of cell cycle-promoting protein CCNA1. Overexpression of TRIM13 initially prompts a proliferative burst in AML cells that is followed by exhaustion, while loss of total TRIM13 or deletion of its catalytic domain enhanced leukemogenesis in AML cell lines and patient-derived xenografts. These data suggest that CHAF1B promotes leukemic development in part by repressing TRIM13 expression, and that this relationship is necessary for leukemic progression. Overall design: Examination of the function of CHAF1B in maintaining leukemic cells. We used Chaf1b-floxed MLL-AF9 leukemic cell clones which were transduced with CreERT2. Knockout was induced by treatment with B-estradiol, and cells were lysed 48 hours after induction for analysis. Floxed cells expressing CreERT2 treated with EtOH were used as clonal controls.