# Conditional loss of Brca1 in oocytes causes reduced litter size, ovarian reserve depletion and impaired oocyte in vitro maturation with advanced reproductive age in mice

## TL;DR

Deleting the Brca1 gene in mouse oocytes reduces litter size, depletes ovarian reserves, and impairs oocyte maturation with age, suggesting BRCA1 is important for female fertility.

## Contribution

This study demonstrates that BRCA1 is essential for maintaining oocyte quality and ovarian reserve in mice, and highlights limitations of using AMH as a marker in BRCA1/2 mutation carriers.

## Key findings

- Conditional loss of Brca1 in oocytes leads to reduced litter size and ovarian reserve depletion in mice.
- Oocyte maturation is impaired with advanced maternal age in Brca1 cKO mice.
- Serum AMH levels do not reliably predict ovarian reserve in Brca1 cKO mice or women with BRCA1/2 mutations.

## Abstract

An estimated 1 in 350 women carry germline BRCA1/2 mutations, which confer an increased risk of developing breast and ovarian cancer, and may also contribute to subfertility. All mature, sex steroid-producing ovarian follicles are drawn from the pool of non-renewable primordial follicles, termed the ‘ovarian reserve’. The clinical implications of early ovarian reserve exhaustion extend beyond infertility, to include the long-term adverse health consequences of loss of endocrine function and premature menopause. We aimed to determine whether conditional loss of Brca1 in oocytes impacts ovarian follicle numbers, oocyte quality and fertility in mice with advancing maternal age. We also aimed to determine the utility of AMH as a marker of ovarian function, by assessing circulating AMH levels in mice and women with BRCA1/2 mutations, and correlating this with ovarian follicle counts.

In this study, we addressed a longstanding question in the field regarding the functional consequences of BRCA1 inactivation in oocytes. To recapitulate loss of BRCA1 protein function in oocytes, we generated mice with conditional gene deletion of Brca1 in oocytes using Gdf9-Cre recombinase (WT: Brca1fl/flGdf9+/+; cKO: Brca1fl/flGdf9cre/+).

While the length of the fertile lifespan was not altered between groups after a comprehensive breeding trial, conditional loss of Brca1 in oocytes led to reduced litter size in female mice. Brca1 cKO animals had a reduced ovarian reserve and oocyte maturation was impaired with advanced maternal age at postnatal day (PN)300, compared to WT animals. Serum anti-Müllerian hormone (AMH) concentrations (the gold-standard indirect marker of the ovarian reserve used in clinical practice) were not predictive of reduced primordial follicle number in Brca1 cKO mice versus WT. Furthermore, we found no correlation between follicle number or density and serum AMH concentrations in matched samples from a small cohort of premenopausal women with BRCA1/2 mutations.

Together, our data demonstrate that BRCA1 is a key regulator of oocyte number and quality in females and suggest that caution should be used in relying on AMH as a reliable marker of the ovarian reserve in this context.

This work was made possible through Victorian State Government Operational Infrastructure Support and Australian Government NHMRC IRIISS. This work was supported by funding from the 10.13039/501100000923Australian Research Council (ALW - DE21010037 and KJH - FT190100265), as well as the 10.13039/501100001026National Breast Cancer Foundation (IIRS-22-092) awarded to ALW and KJH. LRA, YML, LT, EOKS and MG were supported by Australian Government Research Training Program Scholarships. LRA, YML and LT were also supported by a Monash Graduate Excellence Scholarship. YC, SG and XC were supported by 10.13039/501100020821Monash Biomedicine Discovery Institute PhD Scholarships. LRA was also supported by a 10.13039/501100001779Monash University ECPF24-6809920940 Fellowship. JMS was supported by 10.13039/501100000925NHMRC funding (2011299). MH was supported by an 10.13039/501100000925NHMRC Investigator Grant (1193838).