Enhanced Tumoral MLH1-Expression in MLH1-/PMS2-Deficient Colon Cancer Is Indicative of Sporadic Colon Cancer and Not HNPCC

Hereditary Non-Polyposis Colorectal Cancer (HNPCC) is caused by germline mutations of mismatch-repair (MMR) genes MLH1, MSH2, MSH6 and PMS2. MLH1-/PMS2-deficient colorectal carcinomas might be HNPCC-associated but also caused by MLH1-promoter methylation in sporadic colon carcinoma. This study analyzed semiquantitatively whether the MLH1 staining pattern might be indicative of sporadic or HNPCC-associated colorectal cancer. Using a semiquantitative score ranging from 0 (negative) to 12 (maximum immunopositivity) we analyzed MLH1 expression patterns in 130 MLH1-/PMS2-deficient colorectal cancers. The collective consisted of 70 HNPCC-associated colorectal cancers and 60 sporadic colon cancers. In tumor cells of 70 HNPCC-associated colorectal cancers, 64 cases (91.43%) showed no MLH1 staining, 5 cases weak (7.14%) and 1 case (1.43%) stronger staining intensity. In contrast, in tumor cells of 60 sporadic colorectal cancers 45 cases (75.0%) showed no MLH1 staining, 10 cases weak (16.67%) and 5 cases (8.33%) stronger staining intensity to a varying extent. In immunopositive cases, MLH1 showed a characteristic dot-like nuclear staining pattern in the tumor cells. We compared cases with absent to weak MLH1-staining (immunoscores 0 to 2) to cases with elevated immunoscores (3 to 12) detecting a statistically significant difference between HNPCC-associated and sporadic colon cancers (pvalue = 0.0031, Fisher's exact test). Taken together, enhanced tumoral MLH1 expression in MLH1-/PMS2-deficient colorectal carcinomas seems to be indicative of sporadic origin. In contrast, HNPCC-associated colorectal cancer showed absent or very weak MLH1 immunopositivity. Therefore, this semiquantitative and easy to exert MLH1 immunoscore might help to identify sporadic MLH1-/PMS2-deficient colorectal cancer cases prior to time-consuming methylation analysis.