Colorectal Carcinomas With Isolated Loss of PMS2 Staining by Immunohistochemistry

Context.-Isolated loss of PMS2 staining is an uncommon immunophenotype in colorectal carcinomas, accounting for approximately 4% of tumors with microsatellite instability. Limited information regarding these tumors is available in the literature. Objective.-To compare the clinicopathologic features of colorectal carcinomas with isolated PMS2 loss by immunohistochemistry to those with other forms of mismatch repair deficiency. Design.-Ninety-three colorectal carcinomas with isolated PMS2 loss by immunohistochemistry and 193 with other forms of mismatch repair deficiency were identified. Forty (43%) of the isolated PMS2 loss cases and 35 control cases (18%) had a known germline mutation or a clinical diagnosis of Lynch syndrome. Results.-Overall, isolated PMS2-loss tumors occurred in significantly younger patients (P < .001) and in fewer female patients (P = .006). These tumors were significantly less likely to be right-sided (P = .001), high-grade (P = .01), or display histologic features of microsatellite instability (P < .001). The isolated PMS2-loss group also exhibited increased odds of disease-specific death (odds ratio [OR], 3.09; 95% CI, 1.41-6.85; P = .007). When the analysis was restricted to germline mutation/Lynch syndrome cases and controls, no significant differences were detected for age, sex, tumor location, tumor grade, histologic features, or distant metastases, although a trend toward increased odds of disease-specific death in the isolated PMS2-loss group was evident (OR, 3.87; 95% CI, 0.89-27.04; P = .10). Conclusions.-Unusual clinicopathologic features observed in colorectal carcinomas with isolated PMS2 loss are likely related to the high proportion of cases caused by germline mutations. Isolated PMS2-loss tumors may demonstrate more aggressive behavior than other tumors with microsatellite instability, but larger studies are needed to investigate that possibility further.