What can yeast tell us about breast cancer?

The positional cloning of the major breast and ovarian cancer susceptibility gene BRCA1 in 1994 marked the start of an intensive search to explain its functions. To date, the protein encoded by BRCA1 has been implicated in several biochemical activities and cellular processes such as the DNA damage response, transcription regulation, and cell cycle progression. Following a long tradition of using heterologous systems to dissect the function of human genes, Lodovichi and colleagues set out to determine whether ectopically expressed human BRCA1 would interact with the DNA repair machinery in Saccharomyces cerevisiae (which does not have a recognizable BRCA1 counterpart). Their approach stemmed from a previous observation when studying BRCA1 missense variants of uncertain clinical significance with the use of a yeast recombination assay. The investigators found that expressing cancerassociated RING or BRCT-domain variants, but not wild type BRCA1 protein, in yeast triggers a spontaneous “hyper-recombination” phenotype. The RING finger domain (located at the amino-terminal region) acts as an E3 ubiquitin ligase and the tandem BRCT domains (located at the carboxy-terminal region) recognize phosphorylated amino acid residues in proteins that are targets of the main DNA damage kinases such as ATM, ATR, and DNA-PK. Both domains are required for BRCA1-mediated tumor suppression as demonstrated by the clustering of cancer-predisposing variants in those domains and exemplified by the well characterized pathogenic variants Cys61Gly (in the RING domain) and Met1775Arg (in the BRCT domains). In the recent study published in Cell Cycle, Lodovichi and colleagues extended the work to test the ability of BRCA1 to affect homologous recombination repair and survival, as well as intracellular localization in yeast following treatment with DNA-damaging agents. Expression of wild-type human BRCA1 in yeast exposed to the alkylating agent methyl methane-sulfonate (MMS) led to a dose-dependent decrease in survival more pronounced than in treated cells not expressing the ectopic BRCA1. Along similar lines, expression of wild type BRCA1 also led to a decrease in intraand inter-chromosomal recombination as measured by reporter genes. Intriguingly, expression of BRCA1 Cys61Gly or Met1775Arg pathogenic variants led to a decrease in survival and homologous recombination rates in MMS-treated cells even more pronounced than in cells expressing the wild-type BRCA1 protein. The authors also employed fluorescence microscopy methods to investigate differences in intracellular localization between yeast transformants with wild type BRCA1 or with benign and pathogenic variants. While normal BRCA1 protein and neutral variants (Arg45Gln and Arg1751Gln) localized to a single spot (focus) in the yeast nucleus in response to alkylating DNA damage, pathogenic variants Cys61GLy and Met1775Arg failed to do so. So, what makes yeast cells more sensitive to the alkylating agent MMS in the presence of BRCA1 and, to an even greater extent, in the presence of its pathogenic variants? Lodovichi and colleagues hypothesize that when BRCA1 mediates DNA damage repair pathways in yeast, it may lead to formation of recombination intermediates which may make the damage less repairable. This suggests that BRCA1 may be perturbing repair in yeast by illegitimately interacting with the yeast DNA damage response (DDR) machinery. Presumably, pathogenic variants still retain the illegitimate interaction but promote further disruption of the DDR. Lodovichi et al. hypothesize that the observed effect could be mediated by BRCA1 interaction with yeast MRE11-RAD50-XRS2 (MRX) complex upon DNA damage. Previous biochemical and co-localization data showing DNA damage-specific interactions between BRCA1 and the human Mre11/Rad50/NBS1 complex makes it an attractive hypothesis. Whether the effect is mediated by the MRX complex or by a different set of proteins is a matter of future investigation. In any event, the investigators now have a yeast experimental model that can be exploited to dissect the molecular functions of BRCA1.