Abstract PS3-01-16: Gut microbial dysbiosis associates with the early-onset breast cancer in BRCA mutation carriers

Abstract Germline BRCA mutation carriers face a high risk of developing breast cancer, yet individual risk and age of onset vary significantly, influenced by both genetic and environmental factors. The gut microbiota is crucial for host health, regulating immunity, metabolism, and endocrine functions. Dysbiosis—an imbalance of gut microbiota—has been linked to diseases including cancer. We aimed to investigate whether specific gut microbes influence breast cancer risk in BRCA mutation carriers and to explore the underlying mechanisms. First, we studied 27 BRCA mutation carriers with early-onset breast cancer (40 years old) and 8 old carriers (60 years) without cancer. Fecal microbiota were analyzed using full-length 16S rRNA sequencing. The linear discriminant analysis (LDA) effect size (LEfSe) analysis showed that the Eggerthella lenta (E. lenta) and Dorea longicatena (D. longicatena) were the most significantly enriched bacteria in feces of the mutation carriers with early-onset breast cancer, compared to those in the old carriers without breast cancer. As these two bacteria are more common in older individuals, their presence in younger cancer patients suggests an association with cancer, independent of age. Second, we expanded the study to 102 BRCA mutation carriers (54 BRCA1, 48 BRCA2), 64 of whom had breast cancer. Mutation carriers with fecal E. lenta had a significantly earlier age of breast cancer than those without it (median age: 42.2 vs. 52.8 years-old, p0.001), while D. longicatena showed no significant association. Subgroup analyses confirmed the link between E. lenta and early-onset breast cancer in both BRCA1 and BRCA2 groups (p0.05). We then generated antibiotic-induced microbiome depletion (AIMD) model in the brca1-null mouse. The AIMD mice were gavaged with phosphate-buffered saline or E. lenta; mice gavaged with E. lenta developed breast tumors significantly earlier than those without E. lenta (p=0.022). We conducted untargeted metabolomics on urine samples from mutation carriers with early-onset breast cancer and from old cancer-free carriers (age 60). Tryptophan and its downstream metabolites—kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and quinolinic acid—were significantly elevated in early-onset patients and positively correlated with E. lenta levels (p0.05), compared to metabolites in the old cancer-free carriers. In mice fed E. lenta, serum levels of tryptophan/kynurenine pathway metabolites were elevated, too. To assess causality, brca1-null mice were intraperitoneal injected with quinolinic acid, a downstream end-product of the tryptophan/kynurenine pathway. The treated mice developed breast tumors significantly earlier than controls (p0.01). Then, we tested the impact of quinolinic acid in the cells with heterozygous BRCA1 mutation. DNA fiber assay revealed significantly increased replication speed and impaired replication fork protection in the BRCA1 (185delAG/+) MCF10A cell line with co-culture of 10uM quinolinic acid, compared to those without quinolinic acid co-culture. We long-term (6 weeks) treated BRCA1 (185delAG/+) MCF10A cells with 10uM quinolinic acid. Whole exome sequencing revealed the treated cells harboring a significantly elevated homologous recombination deficiency score (by scarHRD software), compared to cells without quinolinic acid co-culture (p0.05). These data suggested quinolinic acid enhanced the replication stress and caused the genomic instability in the human mammary cells harboring BRCA1 mutation. Our study demonstrated that dysbiosis is associated with early-onset breast cancer in BRCA mutation carriers. The E. lenta-derived tryptophan/kynurenine/quinolinic acid pathway induces the replication stress and genomic instability that drive breast cancer. Citation Format: P. Lin, Y. Chen, T. Yen, H. Chang, Y. Chu, Z. Liu, S. Kuo, C. Huang. Gut microbial dysbiosis associates with the early-onset breast cancer in BRCA mutation carriers abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-01-16.