POL? processes ssDNA gaps and promotes replication fork progression in BRCA1-deficient cells

Polymerase theta (POLO) is an error-prone DNA polymerase whose loss is synthetically lethal in cancer cells bearing breast cancer susceptibility proteins 1 and 2 (BRCA1/2) mutations. To investigate the basis of this genetic interaction, we utilized a small-molecule inhibitor targeting the POLO polymerase domain. We found that POLO processes single-stranded DNA (ssDNA) gaps that emerge in the absence of BRCA1, thus promot-ing unperturbed replication fork progression and survival of BRCA1 mutant cells. A genome-scale CRISPR-Cas9 knockout screen uncovered suppressors of the functional interaction between POLO and BRCA1, including NBN, a component of the MRN complex, and cell-cycle regulators such as CDK6. While the MRN complex nucleolytically processes ssDNA gaps, CDK6 promotes cell-cycle progression, thereby exac-erbating replication stress, a feature of BRCA1-deficient cells that lack POLO activity. Thus, ssDNA gap formation, modulated by cell-cycle regulators and MRN complex activity, underlies the synthetic lethality between POLO and BRCA1, an important insight for clinical trials with POLO inhibitors.

Automated summary

Polymerase theta (POLO) is an error-prone DNA polymerase that is synthetically lethal in cancer cells with BRCA1/2 mutations. The study shows that POLO processes single-stranded DNA gaps in the absence of BRCA1, promoting replication fork progression and survival of BRCA1 mutant cells. The research also uncovers suppressors of the POLO-BRCA1 interaction, including NBN and CDK6, which contribute to ssDNA gap formation and exacerbate replication stress in BRCA1-deficient cells.