Dot1L-dependent H3K79 methylation facilitates histone variant H2A.Z exchange at DNA double strand breaks and is required for high fidelity, homology-directed DNA repair

In eukaryotic cells, the homology-directed repair (HDR) and non-homologous end joining (NHEJ) pathways are required for the repair of DNA double strand breaks (DSB). The high-fidelity HDR pathway is particularly important for maintenance of genomic stability. In mammals, histone post-translational modifications and histone variant exchange into nucleosomes at sites of DSB generate an open chromatin state necessary for repair to take place. However, the specific contributions of histone modifications to histone variant exchange at DSB sites and the influence of these changes on the DNA repair process and genome stability are incompletely understood. Here we show that Dot1L-catalyzed methylation of H3 histone on lysine 79 (H3K79) is required for efficient HDR of DSB. In cells with DNA DSB either lacking Dot1L or expressing a methylation-dead Dot1L, there is altered kinetics of DNA repair factor recruitment, markedly decreased H2A.Z incorporation at DSB sites, and a specific and profound reduction in HDR, which results in significant genomic instability. These findings demonstrate a new role for Dot1L, identifying it as a critical regulator of the DNA repair process and a steward of genomic integrity.