Clinicopathological comparison of colorectal and endometrial carcinomas in patients with Lynch-like syndrome versus patients with Lynch syndrome.

Screening for DNA mismatch repair (MMR) deficiency in colorectal and endometrial carcinomas identifies patients at risk for Lynch syndrome. Some patients with MMR-deficient tumors have no evidence of a germline mutation and have been described as having Lynch-like syndrome. We compared the clinicopathological features of colorectal and endometrial carcinomas in patients with Lynch-like syndrome and Lynch syndrome. Universal screening identified 356 (10.6%) of 3352 patients with colorectal carcinoma and 72 (33%) of 215 patients with endometrial carcinoma with deficient DNA MMR. Sixty-six patients underwent germline mutation analysis with 45 patients (68%) having evidence of a germline MMR gene mutation confirming Lynch syndrome and 21 patients (32%) having Lynch-like syndrome with no evidence of a germline mutation. Most patients with Lynch-like syndrome had carcinoma involving the right colon compared to patients with Lynch syndrome (93% versus 45%; P < .002). All patients with colorectal carcinomas demonstrating isolated loss of MSH6 expression had Lynch syndrome confirmed by germline mutation analysis. Synchronous or metachronous Lynch syndrome-associated carcinoma was more frequently identified in patients with Lynch syndrome compared to Lynch-like syndrome (38% versus 7%; P = .04). There were no significant differences in clinicopathological variables between patients with Lynch-like syndrome and Lynch syndrome with endometrial carcinoma. In summary, 32% of patients with MMR deficiency concerning Lynch syndrome will have Lynch-like syndrome. Our results demonstrate that patients with Lynch-like syndrome are more likely to have right-sided colorectal carcinoma, less likely to have synchronous or metachronous Lynch syndrome-associated carcinoma, and less likely to demonstrate isolated loss of MSH6 expression within their tumor.