Nanoscopic analysis of 53BP1, BRCA1 and Rad51 reveals new insights in temporal progression of DNA-repair and pathway choice.

The accumulation and spatial distribution of 53BP1, BRCA1 and Rad51, key proteins in DNA double-strand break (DSB) repair, was investigated with high temporal resolution over a time span of 24 h, using STED nanoscopy. DNA lesions were induced by irradiation with high-LET (linear energy transfer) α-particles. We show that 53BP1 IRIF formation occurs quickly in almost all cells and after about 6 h the fraction of 53BP1 IRIF positive cells slowly declines. Against the expectations BRCA1 and Rad51 IRIF formation is only shortly delayed but with the maximum of cells showing foci after 6 and 8 h after irradiation. At this stage, almost all IRIF in a given Rad51-positive cell show Rad51 accumulation, suggesting that repair via homologous recombination is attempted at almost all residual DSB sites. The frequency of BRCA1 IRIF positive cells increases much earlier and remains high after Rad51 positive cells start to decline, supporting models claiming that functional roles of BRCA1 change over time. Correlation analysis showed a high degree of correlation of Rad51 with BRCA1, while the exclusion of 53BP1 from the actual resection-zone is demonstrated by anti-correlation of Rad51 and 53BP1. Interestingly, these correlation and anti-correlation patterns exhibit complementary temporal variation.