A POLD3/BLM dependent pathway handles DSBs in transcribed chromatin upon excessive RNA:DNA hybrid accumulation

DNA Double Strand breaks in transcriptionally active loci (TC-DSBs) undergo a dedicated repair pathway. Here, the authors show that excessive RNA:DNA hybrid accumulation at TC-DSBs elicits POLD3/BLM-dependent DNA synthesis that induces cell toxicity. Transcriptionally active loci are particularly prone to breakage and mounting evidence suggests that DNA Double-Strand Breaks arising in active genes are handled by a dedicated repair pathway, Transcription-Coupled DSB Repair (TC-DSBR), that entails R-loop accumulation and dissolution. Here, we uncover a function for the Bloom RecQ DNA helicase (BLM) in TC-DSBR in human cells. BLM is recruited in a transcription dependent-manner at DSBs where it fosters resection, RAD51 binding and accurate Homologous Recombination repair. However, in an R-loop dissolution-deficient background, we find that BLM promotes cell death. We report that upon excessive RNA:DNA hybrid accumulation, DNA synthesis is enhanced at DSBs, in a manner that depends on BLM and POLD3. Altogether our work unveils a role for BLM at DSBs in active chromatin, and highlights the toxic potential of RNA:DNA hybrids that accumulate at transcription-associated DSBs.

Automated summary

This study reveals that accumulation of RNA:DNA hybrids at transcriptionally active loci induces DNA synthesis and cell toxicity. Bloom RecQ DNA helicase (BLM) plays a critical role in repair process and is recruited in a transcription-dependent manner.