Prevalence and clinicopathological/molecular characteristics of mismatch repair protein-deficient tumours among surgically treated patients with prostate cancer in a Japanese hospital-based population

Background: The prevalence and molecular characteristics of deficient mismatch repair prostate cancer in the Japanese population have scarcely been investigated. Methods: Immunohistochemistry for mismatch repair proteins (MLH1, MSH2, MSH6 and PMS2) was performed in formalin-fixed paraffin-embedded sections prepared from resected primary prostate cancers in patients who underwent prostatectomy at our institution between January 2001 and May 2016. Genetic and/or epigenetic alterations of mismatch repair genes were investigated in patients with any loss of mismatch repair protein expression in the tumour. Results: Of the 337 patients, four (1.2%) showed loss of mismatch repair protein expression on immunohistochemistry. All four patients showed loss of both MSH2 and MSH6 protein expression. Genetic testing was performed in two of the four patients, demonstrating no pathogenic germline alterations were present. In each of these two patients, at least one somatic alteration inactivating MSH2 without MSH2 hypermethylation was identified, leading to the diagnosis of supposed 'Lynch-like syndrome'. Patients with deficient mismatch repair prostate cancer were at a significantly higher stage (pT2pN0 vs. pT3-4pN0/pTanypN1, P = 0.02) and had a greater Gleason score (<8 vs. >= 8, P < 0.01) than those with proficient mismatch repair prostate cancer. Conclusions: The prevalence of deficient mismatch repair prostate cancer in the Japanese hospital-based prostatectomized population was extremely low. To improve screening efficacy for deficient mismatch repair prostate cancer, screening candidates can be limited to patients with locally advanced, node-positive and/or Gleason score of 8 or greater prostate cancer. Universal tumour screening for Lynch syndrome seems ineffective in patients with prostate cancer.