Objective To investigate the potential significance of FOXP3 expression in BRCA1/2-mutant breast cancer.Methods A total of 48 BRCA mutation carriers(16 with BRCA1 and 32 with BRCA2)and 78 age-matched non-carriers were included in this study.Immunohistochemistry was used to detect the expression of FOXP3 in breast cancer tissues.The FOXP3 RNA expression in 39 BRCA1,36 BRCA2,and 948 non-carrier breast cancer patients from TCGA-BRCA and the correlation with homologous recombin-ation deficiency scores were evaluated to validate the immunohistochemistry results.Results The FOXP3 positive rate was 43.8%(7/16)in BRCA1 mutation carriers,59.4%(19/32)in BRCA2 mutation carriers,and 9.0%(7/78)in non-carriers.The FOXP3 positive rates in patients with BRCA1/2 mutant breast cancer were significantly higher than those in non-carriers(P=0.002;P<0.001).TCGA-BRCA results showed that the FOXP3 RNA level in BRCA1/2 mutant breast cancer was significantly higher than that in non-carriers(P=0.02,P=0.004).The FOXP3 RNA level was positively correlated with the homologous recombination deficiency score(Spearman R=0.30,P<2.2e-16).Conclusion Patients with BRCA1/2 mutant breast cancers have higher FOXP3expression than non-carriers,and may be more sensitive to immunotherapy.

Objective To compare the clinicopathological characteristics between primary and contralateral cancers in patients with metachronous bilateral breast cancer (MBBC) who carried a BRCA1/2 germline pathogenic variant. Methods A total of 496 BRCA1/2 carriers with primary unilateral breast cancer were included (196 with BRCA1 and 300 with BRCA2). Clinicopathological information of patients was collected, and the median follow-up for the entire cohort was 10.4 years (0.4-20.8 years). Results Among all patients, 31 (15.8%) of the 196 BRCA1 carriers and 49 (16.3%) of the 300 BRCA2 carriers had MBBC, respectively. Among the 31 BRCA1 carriers who developed MBBC, the proportion of triple-negative breast cancer (TNBC) in primary cancer and contralateral cancer was 61.3% and 67.7%, respectively. If the primary cancer of BRCA1-mutated MBBC was TNBC, the probability of the contralateral breast cancer with TNBC was 89.5% (17/19), which was significantly higher than that if the primary cancer was non-TNBC (33.3%, 4/12) (P=0.004). Among the 49 BRCA2 carriers who developed MBBC, the predominant molecular phenotype of the primary and contralateral cancers was HR+ & HER2- (77.6% and 67.3%, respectively; P=0.53). Conclusion Approximately 60% of BRCA1 carriers exhibit TNBC. If a BRCA1 carrier with a TNBC primary breast cancer had an MBBC, the probability of the contralateral breast cancer being TNBC phenotype is almost 89.5%.