53BP1 regulates dna double-strand break repair in a mouse “BRCA-like” model by a non-resection mechanism

DNA end resection is believed to be a step that influences the choice between the two major DNA double-strand break (DSB) repair pathways: homologous recombination (HR) and non-homologous end joining (NHEJ). Deficiencies in either of these pathways can increase predisposition to cancer, a common example being BRCA1 deficient breast and ovarian cancer. Specifically, cells deficient in HR have been considered to be “BRCA-like”. BLM/Dna2 and Exo1 serve as the two main pathways for long-range resection in yeast, but the significance of long-range resection for genome maintenance has not been shown in a primary mammalian model. We found that BlmΔ/ΔExo1-/- cells show synergistic genomic instability compared to single knockout controls, reduced resection, and decreased HR. BlmΔ/ΔExo1-/- cells also show a defect in the G2M checkpoint, attributed to a lack of resected DNA to maintain the checkpoint activation. This suggests that co-deletion of BLM and Exo1 abolishes long-range resection resulting in a “BRCA-like” model. Deleting 53BP1, a protein believed to block resection, restores genomic integrity and HR in BlmΔ/ΔExo1-/- cells, but resection is not restored. This data suggests a role for 53BP1 in restoring HR that is independent of its role in inhibiting DNA end resection.Ph.D.Includes bibliographical referencesby Sarah Marie Misenk