The breast cancer susceptibility gene 1 (BRCA1) is a key mediator for maintaining genomic stability (1). Females carrying BRCA1 pathogenic variants have a higher risk of developing breast and ovarian cancer (2). Additionally, a recent study showed that mice with a loss of Brca1 function in oocytes had a reduced litter size (3). To investigate Brca1 function in embryogenesis and fetal development, we assessed whether loss of Brca1 function in mouse oocytes impacts fertilisation, blastocyst formation and pregnancy.
Postnatal day (PN) 60 wild type (WT: n=7) and Brca1 conditional knockout (cKO: n=6) mice were hormone stimulated (10IU PMSG, 10IU hCG) and ovulated oocytes were fertilised in vitro. No difference was observed in oocyte yield, fertilisation and blastocyst formation rates between WT and cKO mice. WT and cKO females were also paired with C57/BL6 adult males overnight for natural mating, generating WT embryos in the control dams, and Brca1 heterozygous embryos in the cKO dams (n=4-8 dams/genotype/timepoint). Implantation sites were collected and assessed on embryonic day (E) 6, E12 and E18. No difference was observed in the number of viable implantation sites at E6 and E12 between genotypes, although cKO mice had fewer viable implantation sites at E18 compared to WT (p=0.0183). There were no differences in resorption rates between genotypes at any timepoint.
These data show that Brca1 is not required by oocytes for meiotic maturation, ovulation in response to hormonal stimulation, or the developmental competence of embyros after in vitro fertilisation. In vivo, fewer implantation sites at E18, but not E6 or E12, indicates pregnancy loss during late gestation. Studies are ongoing to further explore the role of Brca1 in oocyte quality and how late gestation is affected by losing one copy of Brca1.