Brca2-Null Embryonic Survival is Prolonged on the BALB/c Genetic Background

Women who inherit mutations in the BRCA2 cancer susceptibility gene have an 85% chance of developing breast cancer. The function of the BRCA2 gene remains elusive, but there is evidence to support its role in transcriptional transactivation, tumor suppression, and the maintenance of genomic integrity. Individuals with identical BRCA2 mutations display a different distribution of cancers, suggesting that there are low-penetrance genes that can modify disease outcome. We hypothesized that genetic background could influence embryonic survival of a Brca2 mutation in mice. Brca2-null embryos with a 129/SvEv genetic background (129B2−/−) died before embryonic day 8.5. Transfer of this Brca2 mutation onto the BALB/cJ genetic background (BALB/cB2−/−) extended survival to embryonic day 10.5. These results indicate that the BALB/c background harbors genetic modifiers that can prolong Brca2-null embryonic survival. The extended survival of BALB/cB2−/− embryos enabled us to ask whether transcriptional regulation of the Brca1 and Brca2 genes is interdependent. The interdependence of Brca1 and Brca2 was evaluated by studying Brca2 gene expression in BALB/cB1−/− embryos and Brca1 gene expression in BALB/cB2−/− embryos. Nonisotopic in situ hybridization demonstrated that Brca2 transcript levels were comparable in BALB/cB1−/− embryos and wild-type littermates. Likewise, reverse transcriptase–polymerase chain reactions confirmed Brca1 mRNA expression in embryonic day 8.5 BALB/cB2−/− embryos that was comparable to Brca2-heterozygous littermates. Thus, the Brca1 and Brca2 transcripts are expressed independently of one another in Brca1- and Brca2-null embryos. Mol. Carcinog. 28:174–183, 2000. © 2000 Wiley-Liss, Inc.