Histopathological and genetic features of mismatch repair-deficient high-grade prostate cancer

Mismatch repair (MMR) deficiency is commonly caused by functional inactivation of MLH1, PMS2, MSH2 or MSH6. The morphological and molecular correlates of MMR deficiency have been extensively characterized in certain tumour types such as colorectal and endometrial adenocarcinoma. In contrast, the histological and molecular features of MMR-deficient prostate cancer remain incompletely described. In this study, we evaluated 19 MMR-deficient prostate cancers, including 11 cases without prior systemic treatment.All treatment-naive cases (11 of 11, 100%) were grade group 4-5 and had predominant cribriform and/or solid growth patterns. Solid components (any amount) and tumour infiltrating lymphocytes were 7 cases each (7 of 11, 64%). In 68 MMR-proficient grade group 5 prostate cancers, predominant cribriform or solid growth patterns, solid components (any amount) and tumour infiltrating lymphocytes were seen at significantly lower frequencies (31 of 68, 46%; 9 of 68, 13% and 6 of 62, 9%, respectively; P 0.001 for all comparisons). Molecular evaluation of 19 cases demonstrated that MMR-deficiency was secondary to functional loss of MSH2/MSH6 and MLH1/PMS2 in 15 (79%) and 4 cases (21%), respectively. Definite or probable germline mutations were present in 4 cases (4 of 19, 21%). TMPRSS2::ERG rearrangements were identified in 2 cases (2 of 19, 11%). Recurrent cancer-relevant somatic mutations included (but were not limited to) ATM, TP53, FOXA1, RB1, BRCA2 and PTEN.MMR deficiency was most commonly secondary to inactivation of MSH2/MSH6 in this study. Importantly, MMR-deficient high-grade prostatic adenocarcinomas had morphological features that might be useful to identify selected cases for MMR immunohistochemistry.