Clinical relevance of pathogenic germline variants in mismatch repair genes in Chinese breast cancer patients

The prevalence and clinical relevance of pathogenic germline variants in MMR genes have not been investigated in large series of breast cancers. In this study, we screened the germline variants in MMR genes in 8085 consecutive Chinese breast cancer patients, and investigated the MMR/PD-L1 protein expression and tumor mutation burden (TMB) of breast tumors from MMR variant carriers. We found that 15 of 8085 patients (0.19%) carried a pathogenic germline variant in MMR genes. Compared with non-carriers, MMR variant carriers might have worse recurrence-free survival (unadjusted hazard ratios [HR] = 2.70, 95% CI: 1.12-6.49, P = 0.027) and distant recurrence-free survival (unadjusted HR = 3.24, 95% CI: 1.45-7.22, P= 0.004). More importantly, some of the breast cancers from MMR carriers displayed MMR protein loss (5/13), TMB-high (2/10), and PD-L1 positive expression (9/13). This study showed that MMR variant carriers were rare in breast cancer. They might have worse survival and part of them might benefit from immunotherapy.