Environmental and Molecular Mutagenesis (EMM) Editor's Choice - January 2012
The Editor's Choice for January is “BRCA1 Requirement for the Fidelity of Plasmid DNA Double-Strand Break Repair in Cultured Breast Epithelial Cells” by Eric G. Thompson, Hanna Fares, and Kathleen Dixon.
BRCA1 is a tumor suppressor gene whose product repairs double-strand breaks in DNA, thereby protecting the genome. However, BRCA1 can impact error-prone repair pathways, like microhomology-mediated end joining. Noting that women with one rather than two functional copies of the BRCA1 gene were at greater risk of developing breast and ovarian cancers, Thompson and colleagues investigated the relationship between the amount of BRCA1 protein in cultured breast epithelial cells and degree of error-prone repair occurring in those cells. To accomplish this, the investigators first had to design and create several different biological reagents. They expressed BRCA1 in cells lacking the protein and also modulated the level of BRCA1 in BRCA1-expressing cells using lentivirus transduction of small hairpin RNAs specific for BRCA1. They generated plasmids for measuring microhomology-mediated end joining by creating constructs in which the open reading frame for the enhanced green fluorescence protein was interrupted by a DNA sequence containing a restriction cleavage site flanked by microhomologies. Restriction enzyme cleavage between the regions of homology generated a double-strand break and repair of the break using the microhomologies could be monitored as the appearance of fluorescent cells. Thompson and colleagues prepared additional constructs to serve as reporters for non-homologous end joining. Using these tools, Thompson and colleagues showed that even relatively small decreases in cellular levels of BRCA1 resulted in increased microhomology-mediated end joining. The addition of mirin, an inhibitor of the meiotic recombination enzyme 11 (Mre11) exonuclease, further decreased the frequency of microhomology-mediated end joining, showing Mre11 is required for this type of repair. The investigators found fewer mutations induced by non-homologous end joining in BRCA1-proficient as compared to BRCA1-deficient cells. This repair was not affected by mirin, indicating that non-homologous end joining does not require Mre11 exonuclease activity. Thus, this intricate set of experiments allowed the authors to conclude that BRCA1 protects cells from error-prone repair of double-strand breaks by suppressing microhomology-mediated end joining and inhibiting mutagenesis during non-homologous end joining. Environ. Mol. Mutagen. 53:32-43 (2012) Published 2012 Wiley-Liss, Inc.