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Aims: The hallmark of Lynch syndrome (LS) is DNA mismatch repair protein (MMR) deficiency. Recently, MMR deficiency in non-neoplastic colonic crypts has been identified as a novel indicator of LS. We aimed to determine whether MMR-deficient non-neoplastic endometrial glands can distinguish patients with and without LS, and to compare the level of MMR deficiency in the normal endometrium and colon in LS patients., Methods and Results: We evaluated the immunohistochemical expression of MMR proteins in the normal endometrial mucosa from 64 patients, including 34 patients with confirmed LS (17 with endometrial cancer and 17 without cancer), 30 patients with endometrial cancer without LS (10 with tumours with MLH1 promoter hypermethylation and 20 with MMR-proficient tumours), and in the normal colonic mucosa from 30 LS patients. MMR-deficient non-neoplastic endometrial glands were identified in 47% of LS patients and in no patients without LS (P < 0.001). MMR-deficient non-neoplastic glands were more often identified in LS patients with endometrial cancer (65%) than in those without endometrial cancer (29%) (P = 0.04). In contrast to what was seen in the normal colon, MMR-deficient glands in the normal endometrium were seen as large, contiguous groups, ranging in number from two to 101 (87% versus 45%, P = 0.02). MMR-deficient glands were identified at a higher density in the endometrium than in the colon in LS patients (median number of MMR-deficient glands, 22 versus two, P = 0.02)., Conclusions: Our findings indicate that MMR-deficient non-neoplastic endometrial glands constitute an indicator of LS, and that MMR-deficient glands in the endometrium are present in a pattern of contiguous large groups., (© 2021 John Wiley & Sons Ltd.)

The hallmark of Lynch syndrome (LS)-associated neoplasia is DNA mismatch repair protein (MMR) deficiency. Recent studies have demonstrated that histologically normal colonic crypts in patients with LS can exhibit deficient MMR expression. The aim of this study was to determine the feasibility of detecting MMR deficient crypts in random colonoscopic biopsies of normal mucosa in patients with and without LS. Forty-nine patients, including 33 with LS, 12 without LS, and 4 with germline MMR gene variants of uncertain significance (VUS), were prospectively and blindly evaluated by immunohistochemistry for MMR deficient crypts within random normal-appearing mucosal biopsies obtained during surveillance colonoscopy. MMR deficient crypts were identified in 70% (23/33) of patients with LS and in no patients without LS (0/12) (p < 0.001). MMR deficient crypts were identified with nearly equal frequency in both LS patients with and without a cancer history and were associated with germline variants in all four MMR genes and EPCAM. MMR deficient crypts were also identified in LS patients with a history of non-colorectal cancer types, including patients with endometrial cancer, skin sebaceous neoplasms, and renal cancer. Two of the four patients with germline MMR gene VUS had MMR deficient crypts. In conclusion, MMR deficient crypts are a specific biomarker of LS and can be identified in random normal mucosal biopsies in LS patients. Evaluation for MMR deficient crypts in colonoscopic biopsies of normal mucosa can help identify LS patients.

Lynch syndrome is the most common form of hereditary colorectal carcinoma. However, establishing the diagnosis of Lynch syndrome is challenging, and ancillary studies that distinguish between sporadic DNA mismatch repair (MMR) protein deficiency and Lynch syndrome are needed, particularly when germline mutation studies are inconclusive. The aim of this study was to determine if MMR protein-deficient non-neoplastic intestinal crypts can help distinguish between patients with and without Lynch syndrome. We evaluated the expression of MMR proteins in non-neoplastic intestinal mucosa obtained from colorectal surgical resection specimens from patients with Lynch syndrome-associated colorectal carcinoma (n = 52) and patients with colorectal carcinoma without evidence of Lynch syndrome (n = 70), including sporadic MMR protein-deficient colorectal carcinoma (n = 30), MMR protein proficient colorectal carcinoma (n = 30), and "Lynch-like" syndrome (n = 10). MMR protein-deficient non-neoplastic colonic crypts were identified in 19 of 122 (16%) patients. MMR protein-deficient colonic crypts were identified in 18 of 52 (35%) patients with Lynch syndrome compared to only 1 of 70 (1%) patients without Lynch syndrome (p < 0.001). This one patient had "Lynch-like" syndrome and harbored two MSH2-deficient non-neoplastic colonic crypts. MMR protein-deficient non-neoplastic colonic crypts were not identified in patients with sporadic MMR protein-deficient or MMR protein proficient colorectal carcinoma. Our findings suggest that MMR protein-deficient colonic crypts are a novel indicator of Lynch syndrome, and evaluation for MMR protein-deficient crypts may be a helpful addition to Lynch syndrome diagnostics.