Introduction One of the criteria of impaired DNA repair is microsatellite instability (MSI) resulting from functional insufficiency of the mismatched nucleotide repair (MMR) system, a complex of proteins (MLH-1, PMS- 2, MSH-2, MSH-6). No data on the study of MSI in chronic endometritis (CE) were found in the available literature.The aim of the study was to determine the structural features of microsatellite instability in the endometrium in female patients with chronic inflammation of the uterine mucosa.Materials and methods Group I consisted of 30 women with morphologically confirmed high-grade CE; Group II consisted of 30 patients with low-grade CE; Group III consisted of 30 women who sought pregnancy planning and had histologically unchanged endometrium. The degree of CE in patients in groups I and II was variable. We analyzed the expression levels of MLH-1-, MSH- 2-, MSH-6-, and PMS-2-proteins in the endometrium by estimating the staining area of nuclei and cytoplasm of the affected cells over the entire slice area. Nonparametric statistical methods with Mann-Whitney test were used. The value of probability of error was set at 0.05.Results There was a statistically significant decrease in the level of MMR protein expression in the endometrial samples from the Group I patients compared to the same indices in the Group II and III women. No statistically significant results were found when analyzing the level of MMR protein expression depending on the severity degree of CE.Discussion There was a statistically significant decrease in the expression level of the markers studied (MLH-1, PMS-2, MSH-2, MSH-6) in endometrial specimens from patients with low-activity CE compared to uterine mucosa biopsy specimens from highly active CE and mid-stage endometrial secretion phase specimens. The described morphological features of the uterine mucosa in patients with low-activity CE are consistent with the findings of other authors. The literature provides indications of structural features of MSI in pre-tumor and tumor processes in the uterine mucosa.Conclusion Endometrial samples with low activity and various degrees of CE show statistically significantly lower expression levels of MLH-1, PMS-2, MSH-2, MSH-6 when compared to biopsy specimens from highly active CE and normal endometrial samples, which may indicate pathogenetic heterogeneity in the development of inflammation in the endometrium.
