Post-translational Modifications in DNA Damage Response: a Structural Perspective

Project Summary/Abstract This application is focused on probing how post-translational modifications (PTMs) contribute to the DNA damage response (DDR), a vital process for the maintenance of genomic integrity. The long-term goal is a basic mechanistic understanding of DNA damage repair that will facilitate the development of new therapeutic strategies to combat cancer. In this context, the proposed overall objective is to elucidate how the tumor suppressor E3 ubiquitin ligase BRCA1-BARD1 and DDR protein 53BP1 regulate the cell cycle dependency of DNA double-strand break (DSB) repair pathway selection. BRCA1-BARD1 promotes DSB repair by homologous recombination (HR) during S and G2 phases of the cell cycle whereas 53BP1 inactivates HR and promotes repair by non-homologous end-joining (NHEJ) mostly in G1 phase. Imbalance between HR and NHEJ—for example linked to mutations in BRCA1-BARD1—can result in chromosomal rearrangements that drive oncogenesis. BRCA1-BARD1 and 53BP1 can recognize a similar DSB-dependent PTM, the ubiquitylation of histone H2A at K15 in the nucleosome, the basic subunit of chromatin, suggesting that competition for chromatin association in response to DNA damage could account, at least in part, for the antagonistic activities of BRCA1- BARD1 and 53BP1 with respect to HR. The central hypothesis to be tested is that BRCA1-BARD1, through its ubiquitin ligase activity on the nucleosome, indirectly leads to 53BP1 displacement from damaged chromatin and directly prevents the establishment in newly replicated chromatin of a PTM (i.e., dimethylation of histone H4 K20) required for the chromatin recruitment of 53BP1. The rationale for this work is that it will provide fundamental knowledge about the mechanisms of action of BRCA1-BARD1 and 53BP1. Such knowledge is important for understanding how DSBs are repaired with implications for cancer prevention and the development of new therapies. Two specific aims will be pursued to test the central hypothesis. Aim 1: Probe how BRCA1-BARD1 associates with and modifies chromatin in response to DNA damage. Aim 2: Probe how 53BP1 associates with chromatin and is antagonized by BRCA1-BARD1. In both aims, structural biology approaches will be integrated to characterize BRCA1-BARD1, 53BP1 and a chromatin remodeler that may connect BRCA1-BARD1 and 53BP1 functionally. The structures will inform functional in vitro studies and collaborative cell biology to reveal the chromatin recruitment mechanisms of BRCA1-BARD1 and 53BP1. The proposed research is significant because it is expected to explain how BRCA1-BARD1 opposes the chromatin recruitment and retention of 53BP1 to promote HR DSB repair post-replication. This work will therefore explain how BRCA1-BARD1 and 53BP1 regulate DSB repair pathway selection in a cell cycle-dependent manner. Knowledge gained from this research is expected to stimulate the long-term development of new therapies to treat cancer.