BRCA1 separation-of-function alleles in stalled fork repair and FANCM synthetic lethality

BRCA1 mutant cancers, but not those lacking BRCA2, contain abundant 10 kb (Group 1) tandem duplications (TDs). Group 1 TDs form at a site-specific Tus/Ter replication fork barrier in mouse embryonic stem (mES) cells lacking Brca1 exon 11 (Del11), which are defective for DNA end resection. To explore relationships between DNA end resection and Group 1 TD formation, we analyzed Brca1 coiled coil (CC) domain mutants, in which PALB2 binding and RAD51 loading for homologous recombination are impaired, but DNA end resection is intact. In contrast to Brca1 Del11 mutants, Brca1 CC mutants retain the ability to suppress Tus/Ter-induced TDs. Further, mouse mammary tumors driven by a Brca1 CC mutant, L1363P, do not reveal a Group 1 TD phenotype. FANCM is a TD co-suppressor in Brca1 Del11 mES cells and Fancm loss is synthetic lethal/sick in combination with Brca1 Del11 mutation. In contrast, Fancm deletion is well-tolerated by Brca1 L1363P mES cells. These findings suggest that Group 1 TD formation and Fancm synthetic lethality are linked phenotypes arising from defective BRCA1-mediated DNA end resection.

携带BRCA1突变的癌症（而非BRCA2缺失的癌症）中存在大量10 kb（Group 1）串联重复序列（tandem duplications，TDs）。Group 1 TDs可在缺失BRCA1外显子11（Del11）的小鼠胚胎干细胞（mES细胞）的位点特异性Tus/Ter复制叉屏障处形成，此类细胞存在DNA末端切除缺陷。为探究DNA末端切除与Group 1 TD形成之间的关联，本研究分析了BRCA1卷曲螺旋（CC）结构域突变体：此类突变体中参与同源重组的PALB2结合与RAD51加载过程受损，但DNA末端切除功能完好。与BRCA1 Del11突变体相比，BRCA1 CC结构域突变体仍可抑制Tus/Ter诱导的TDs。此外，由BRCA1 CC结构域突变体L1363P驱动的小鼠乳腺肿瘤，未表现出Group 1 TD表型。FANCM是BRCA1 Del11 mES细胞中的TD共同抑制因子，且Fancm缺失与BRCA1 Del11突变组合后可产生合成致死/致病效应。与之相反，BRCA1 L1363P mES细胞可良好耐受Fancm缺失。上述研究结果表明，Group 1 TD形成与Fancm合成致死效应均为BRCA1介导的DNA末端切除缺陷所引发的关联表型。