Sister chromatid exchanges (SCEs) are considered to be products of homologous recombination repair. The authors show that SCEs can arise independently of homologous recombination due to processing... Show moreSister chromatid exchanges (SCEs) are considered to be products of homologous recombination repair. The authors show that SCEs can arise independently of homologous recombination due to processing of replication intermediates during mitosis.Sister chromatid exchanges (SCEs) are products of joint DNA molecule resolution, and are considered to form through homologous recombination (HR). Indeed, SCE induction upon irradiation requires the canonical HR factors BRCA1, BRCA2 and RAD51. In contrast, replication-blocking agents, including PARP inhibitors, induce SCEs independently of BRCA1, BRCA2 and RAD51. PARP inhibitor-induced SCEs are enriched at difficult-to-replicate genomic regions, including common fragile sites (CFSs). PARP inhibitor-induced replication lesions are transmitted into mitosis, suggesting that SCEs can originate from mitotic processing of under-replicated DNA. Proteomics analysis reveals mitotic recruitment of DNA polymerase theta (POLQ) to synthetic DNA ends. POLQ inactivation results in reduced SCE numbers and severe chromosome fragmentation upon PARP inhibition in HR-deficient cells. Accordingly, analysis of CFSs in cancer genomes reveals frequent allelic deletions, flanked by signatures of POLQ-mediated repair. Combined, we show PARP inhibition generates under-replicated DNA, which is processed into SCEs during mitosis, independently of canonical HR factors. Show less
Pulver, E.M.; Mukherjee, C.; Kamp, G. van de; Roobol, S.J.; Rother, M.B.; Gulden, H. van der; ... ; Jonkers, J. 2021
The BRCA1 tumor suppressor gene encodes a multidomain protein for which several functions have been described. These include a key role in homologous recombination repair (HRR) of DNA double-strand... Show moreThe BRCA1 tumor suppressor gene encodes a multidomain protein for which several functions have been described. These include a key role in homologous recombination repair (HRR) of DNA double-strand breaks, which is shared with two other highrisk hereditary breast cancer suppressors, BRCA2 and PALB2. Although both BRCA1 and BRCA2 interact with PALB2, BRCA1 domain are considered variants of uncertain clinical significance (VUS). Using genetically engineered mice, we show here that a BRCA1 coiled-coil domain VUS, Brca1 p.L1363P, disrupts the interaction with PALB2 and leads to embryonic lethality. Brca1 p.L1363P led to a similar acceleration in the development of Trp53deficient mammary tumors as Brca1 loss, but the tumors showed distinct histopathologic features, with more stable DNA copy number profiles in Brca1 p.L1363P tumors. Nevertheless, Brca1 p.L1363P mammary tumors were HRR incompetent and responsive to cisplatin and PARP inhibition. Overall, these results provide the first direct evidence that a BRCA1 missense variant outside of the RING and BRCT domains increases the risk of Show less
