MOF and H4 K16 Acetylation Play Important Roles in DNA Damage Repair by Modulating Recruitment of DNA Damage Repair Protein Mdc1

MOF (MYST1) is the major enzyme to catalyze acetylation of histone H4 lysine 16 (K16) and is highly conserved through evolution. Using a conditional knockout mouse model and the derived mouse embryonic fibroblast cell lines, we showed that loss of Mof led to a global reduction of H4 K16 acetylation, severe G2/M cell cycle arrest, massive chromosome aberration, and defects in ionizing radiation-induced DNA damage repair. We further showed that although early DNA damage sensing and signaling by ATM were normal in Mof-null cells, the recruitment of repair mediator protein Mdc1 and its downstream signaling proteins 53bp1 and Brca1 to DNA damage foci was completely abolished. Mechanistic studies suggested thatMof-mediated H4 K16 acetylation and an intact acidic pocket on H2A.X were essential for the recruitment of Mdc1. Removal of Mof and its associated proteins phenocopied a charge-neutralizing mutant of H2A.X. Given the well-charac-terized H4-H2A trans interactions in regulating higher-order chromatin structure, our study revealed a novel chromatin-based mechanism that regulates the DNA damage repair process. In eukaryotes, DNA is packaged with core histones and other nonhistone chromosomal proteins into several orders of chromatin structure with increasing compaction. Many cellular processes, including transcription, DNA replication, and DN