Hypothesis: Homologous Recombination Depends on Parallel Search

BRCA1 and RNAi factors promote repair mediated by small RNAs and PALB2-RAD52

Strong connections exist between R-loops (three-stranded structures harbouring an RNA:DNA hybrid and a displaced single-strand DNA), genome instability and human disease^1-5. Indeed, R-loops are favoured in relevant genomic regions as regulators of certain physiological processes through which homeostasis is typically maintained. For example, transcription termination pause sites regulated by R-loops can induce the synthesis of antisense transcripts that enable the formation of local, RNA interference (RNAi)-driven heterochromation⁶. Pause sites are also protected against endogenous single-stranded DNA breaks by BRCA1⁷. Hypotheses about how DNA repair is enacted at pause sites include a role for RNA, which is emerging as a normal, albeit unexplained, regulator of genome integrity⁸. Here we report that a species of single-stranded, DNA-damage-associated small RNA (sdRNA) is generated by a BRCA1-RNAi protein complex. sdRNAs promote DNA repair driven by the PALB2-RAD52 complex at transcriptional termination pause sites that form R-loops and are rich in single-stranded DNA breaks. sdRNA repair operates in both quiescent (G0) and proliferating cells. Thus, sdRNA repair can occur in intact tissue and/or stem cells, and may contribute to tumour suppression mediated by BRCA1.

DNA Repair: The Search for Homology

The repair of chromosomal double-strand breaks (DSBs) by homologous recombination is essential to maintain genome integrity. The key step in DSB repair is the RecA/Rad51-mediated process to match sequences at the broken end to homologous donor sequences that can be used as a template to repair the lesion. Here, in reviewing research about DSB repair, I consider the many factors that appear to play important roles in the successful search for homology by several homologous recombination mechanisms. See also the video abstract here: https://youtu.be/vm7-X5uIzS8.

The Good and Bad of RNA:DNA Hybrids in Double-Strand Break Repair

Transcription and genome stability have somewhat of a love-hate relationship. In a recent issue of Cell, Ohle et al. (2016) demonstrate a previously unappreciated mechanism by which transcription and RNA contribute to genome stability.