Mismatch repair protein deficiency in triple-negative breast carcinomas

Background Breast cancer, particularly triple-negative breast cancer (TNBC), poses a significant global health burden. Chemotherapy was the mainstay treatment for TNBC patients until immunotherapy was introduced. Studies indicate a noteworthy prevalence (0.2% to 18.6%) of mismatch repair protein (MMRP) deficiency in TNBC, with recent research highlighting the potential of immunotherapy for MMRP-deficient metastatic breast cancer. This study aims to identify MMRP deficiency in TNBC patients using immunohistochemistry.Methods A retrospective cohort study design was used and included TNBC patients treated between 2015 and 2021 at King Hussein Cancer Center. Immunohistochemistry was conducted to assess MMRP expressionResults Among 152 patients, 14 (9.2%) exhibited deficient MMR (dMMR). Loss of PMS2 expression was observed in 13 patients, 5 of whom showed loss of MLH1 expression. Loss of MSH6 and MSH2 expression was observed in one patient. The median follow-up duration was 44 (3-102) months. Despite the higher survival rate (80.8%, 5 years) of dMMR patients than of proficient MMR patients (62.3%), overall survival did not significantly differ between the two groups.Conclusion Approximately 9% of TNBC patients exhibit dMMR. dMMR could be used to predict outcomes and identify patients with TNBC who may benefit from immunotherapy.