BRCA1185delAG tumors may acquire therapy resistance through expression of RING-less BRCA1

Synthetic lethality: the road to novel therapies for breast cancer

When the BRCA1 and BRCA2 tumour suppressor genes were identified in the early 1990s, the immediate implications of mapping, cloning and delineating the sequence of these genes were that individuals in families with a BRCA gene mutation could be tested for the presence of a mutation and their risk of developing cancer could be predicted. Over time though, the discovery of BRCA1 and BRCA2 has had a much greater influence than many might have imagined. In this review, we discuss how the discovery of BRCA1 and BRCA2 has not only provided an understanding of the molecular processes that drive tumourigenesis but also reignited an interest in therapeutically exploiting loss-of-function alterations in tumour suppressor genes.

53BP1 ablation rescues genomic instability in mice expressing 'RING-less' BRCA1

BRCA1 mutations strongly predispose affected individuals to breast and ovarian cancer, but the mechanism by which BRCA1 acts as a tumor suppressor is not fully understood. Homozygous deletion of exon 2 of the mouse Brca1 gene normally causes embryonic lethality, but we show that exon 2-deleted alleles of Brca1 are expressed as a mutant isoform that lacks the N-terminal RING domain. This "RING-less" BRCA1 protein is stable and efficiently recruited to the sites of DNA damage. Surprisingly, robust RAD51 foci form in cells expressing RING-less BRCA1 in response to DNA damage, but the cells nonetheless display the substantial genomic instability. Genomic instability can be rescued by the deletion of Trp53bp1, which encodes the DNA damage response factor 53BP1, and mice expressing RING-less BRCA1 do not show an increased susceptibility to tumors in the absence of 53BP1. Genomic instability in cells expressing RING-less BRCA1 correlates with the loss of BARD1 and a defect in restart of replication forks after hydroxyurea treatment, suggesting a role of BRCA1-BARD1 in genomic integrity that is independent of RAD51 loading.

BRCA1 loses the ring but lords over resistance

Germline breast cancer 1 (BRCA1) variants are associated with a high risk of breast and ovarian cancers. Many BRCA1-mediated cancers are initially responsive to platinum-based therapy; however, resistance commonly develops. The BRCA1185delAG mutation is common in the Ashkenazi Jewish population and has been thought to result in loss of function due to the introduction of a stop codon in the 5' region of the BRCA1 transcript. Two studies in this issue of the JCI reveal that the BRCA1185delAG mutation results in the production of BRCA1 that lacks the N-terminal really interesting new gene (RING) domain. RING-less BRCA1 was shown to directly mediate chemoresistance, while maintaining some homologous recombination function. These results provide important insight into BRCA1 function and indicate that other truncated proteins could arise through similar alterations in codon usage.