Clinicopathological and Survival Characteristics of Mismatch Repair Status and PD-1 Expression in Serous Ovarian Cancer.

Objective: To evaluate the clinicopathological characteristics of mismatch repair (MMR) deficiency and its clinical outcomes by performing immunohistochemistry (IHC) for MMR genes in the serous ovarian cancer (SOC) tumour sections.

Study Design: A retrospective case-control study. Place and Duration of the Study: Gynecology Department of Kanuni Sultan Süleyman Training and Research Hospital, and Department of Medical Oncology of Medipol University, between March 2001 and January 2020.

Methodology: IHC was carried out for MLH1, MSH2, MSH6, and PMS2 on full-section slides from 127 SOCs to evaluate the MMR status. MMR-negative and MMR-low groups together were defined as MMR deficient and called microsatellite instability-high (MSI-H). The MSI status and expression of programmed cell death-1 (PD-1) were compared in SOCs with different MMR statuses.

Results: A significantly higher frequency of MMR-deficient SOCs was diagnosed at early stages compared with the patients in the MSS group (38.6% and 20.6%, respectively, p=0.022). The frequency of cases with PD-1 expression was significantly higher in the MSI-H group (76.2%) than in the MSS counterparts (58.8%, p=0.028). Patients in the MSI-H group had significantly longer DFS (25.6 months) and OS (not reached) than those in the MSS group (16 months and 48.9 months, p=0.039 and p=0.026, respectively).

Conclusion: MSI-H SOCs were diagnosed at an earlier stage as compared to MMR proficient cases. The presence of PD-1 expression was significantly higher in cases presenting MMR deficiency compared with MMR-proficient cases. MSI status was significantly associated with DFS and OS.

Key Words: Serous ovarian cancer, Microsatellite instability, Mismatch repair deficiency.