The Role of Chaf1b in malignant and normal hematopoiesis

Changes in the chromatin landscape directly affect hematopoietic homeostasis and disease. While the mechanisms of histone modification have been extensively studied in hematopoiesis, there is a lack of research devoted to studying the mechanisms behind histone assembly into nucleosomes on DNA as well as the chaperones responsible. Chromatin Assembly Factor 1B (CHAF1B) is the p60 subunit of the Chromatin Assembly Factor-1 (CAF1) complex responsible for delivering newly-synthesized H3/H4 heterodimers to the replication fork during the cell cycle. CHAF1B, which is encoded on chromosome 21, is amplified in a subset of human leukemias and is trisomic in Down syndrome-related blood cancers. The effects of CHAF1B amplification are not known, even though its amplification in AML correlates with dismal prognosis. My preliminary data show that overexpression of CHAF1B is sufficient to induce hematopoietic colony replating and drive entry of hematopoietic stem and progenitor cells (HSPCs) into the cell cycle. While overexpression of CHAF1B alone is not sufficient to drive leukemia formation in vivo, its overexpression can cooperate with other leukemic drivers, such as MLL-AF9, to enhance the severity and progression of leukemia in vivo. This suggests CHAF1B has a direct role in supporting a cell-intrinsic pro-leukemic environment. Therefore, I hypothesize that CHAF1B is required for maintenance of nucleosome architecture around key self-renewal genes and that aberrant overexpression of CHAF1B promotes leukemogenesis by altering the chromatin accessibility of these genes. I will study the requirements of CHAF1B in both normal and malignant hematopoiesis using my newly-developed inducible Chaf1b knockout mouse model. In Aim 1, I will delete Chaf1b from HSPCs in vitro and in vitro and measure the effects on normal hematopoiesis and leukemogenesis. In Aim 2, I will determine the effects of CHAF1B overexpression on normal and malignant hematopoiesis in vitro, and also in vivo by transplantation. In Aim 3, I will determine the mechanisms by which CHAF1B alters chromatin accessibility through ATAC-seq and how changes in CHAF1B levels drive subsequent changes in the chromatin landscape through ChIP-seq and gene transcription by RNA-seq. The information from this proposal will substantially contribute to the field of chromatin biology, hematopoiesis and leukemia.