Introduction/Background About 97–100% of Lynch syndrome (LS) associated EC are mismatch repair deficient (dMMR). Previous studies demonstrated the presence of dMMR glands in normal endometrial tissue from LS patients whereas non-LS patients had only MMR proficient (pMMR) glands. We investigated whether there is an association between dMMR glands in normal endometrial tissue and subsequent development of EC in patients with LS.

Methodology Patients with LS or Lynch-like syndrome in which endometrial tissue was obtained to screen for EC, were retrospectively included. MMR immunohistochemistry was performed and follow-up data was collected regarding development of EC.

Results We included 54 patients in two centers between March 2004 and April 2019. Mean age at time of biopsy was 46 years (SD 9 years) and was not significantly different between the dMMR and pMMR group. Median follow-up time was 29 months (IQR 11–73 months). In the dMMR group (n=10) two patients underwent preventive hysterectomy and three out of remaining eight patients (38%) developed EC. Time between biopsy and developing EC ranged from 10–49 months. Follow-up time of the patients who were still being screened ranged from 19–40 months.

In the pMMR group (n=44) 12 patients underwent a preventive hysterectomy. Two patients out of remaining 32 (6%) developed dMMR EC at 59 and 119 months after normal biopsy.

Conclusion We show that dMMR glands can already be detected in endometrial tissue without any (pre)malignant lesion in patients with LS. Although the numbers are small, patients in which dMMR glands were detected appear to have a higher chance of developing EC within four years compared to patients with only pMMR glands. The presence of dMMR glands in normal tissue could therefore be a potential predictive marker for the development of EC in patients with LS. These findings should be validated prospectively in larger cohorts.

Disclosure Nothing to disclose.