Chapman group

Chromatin and genome integrity

We aim to discover the function of molecular events propagated in chromatin upon DNA damage detection, and how defects in these events manifest in immune-deficiency and cancer in humans.

Fig for Brochure
(Click to enlarge) BRCA1 and 53BP1 regulate DNA double strand break repair pathway choice. BRCA1 (green) and 53BP1 (red) enrich in mutually exclusive yet associated subnuclear volumes at sites of DNA damage as determined by 3D structured illumination super-resolution microscopy (OMX, Applied Precision). BRCA1 enrichment at DNA damage sites in S-phase (pictured) antagonizes 53BP1 association with chromatin to enable DSB repair by homologous recombination.

DNA double-strand breaks (DSBs) are a highly toxic form of DNA damage, which if not properly repaired can result in mutations and genomic translocations. However, DSBs are also required during the specialized recombination events that generate diversity in our immune systems.  For this reason cellular responses to DSBs are tightly regulated in a cell type and cell cycle dependent manner.

Two core regulators of DNA repair pathway choice are the BRCA1 and 53BP1 tumour suppressor proteins. Unlike core components of the DNA repair machinery that interact with or enzymatically process the DNA, these proteins exert their influence indirectly, interacting with large regions of chromatin spanning single DSBs. However, the actual activities of these proteins and nature of chromatin changes that are brought about by their enrichment at DNA damage sites remain undefined.

Recent research has revealed that BRCA1 functions to antagonize 53BP1-dependent DNA repair activities during DNA replication, to ensure that DSBs are repaired accurately by homologous recombination. Furthermore, an inability to counteract 53BP1 results in the chromosomal instability and tumour predisposition evident in cellular and mouse models of Brca1-deficiency, respectively. Our recent work has focused on understanding the opposing molecular roles of the BRCA1 and 53BP1 proteins in regulating DNA double-strand break repair pathway choice. We have also recently identified RIF1 as the major effector protein during 53BP1-dependent non-homologous end joining, a process crucial for humoral immunity that also drives genomic instability in cells lacking functional BRCA1. 

Using transgenic mouse models and a combination of cell biology, biochemical, genomic and proteomic approaches, we are investigating the alterations that occur within DSB-associated chromatin as a result of the activities of these core proteins and other newly identified components of the DNA damage response. Moreover, we hope to work out how such changes determine DNA double-strand break repair fate, to better understand why a breakdown in these processes results in disease and cancer predisposition in humans.