The tumor suppressor PTEN controls multiple cellular functions, including cell cycle, apoptosis, senescence, transcription, and mRNA translation of numerous genes. In tumor cells, PTEN is frequently inactivated by genetic mutations and epimutations. The aim of this study was to investigate the methylation patterns of the PTEN gene and its pseudogene PTENP1 as potential genetic markers of endometrial hyperplasia (EH) and endometrial carcinoma (EC). Methylation of the 5-terminal regions of the PTEN and PTENP1 sequences was studied using methyl-sensitive PCR of genomic DNA isolated from 57 cancer, 43 endometrial hyperplasia, and normal tissue samples of 24 females aged 17-34 years and 19 females aged 45-65 years, as well as 20 peripheral venous blood samples of EC patients. None of the analyzed DNA samples carried a methylated PTEN gene. On the contrary, the PTENP1 pseudogene was methylated in all analyzed tissues, except for the peripheral blood. Comparison of PTENP1 methylation rates revealed no differences between the EC and EH groups (0.80 p 0.50). In all these groups, the methylation level was high (71-77% in patients vs. 58% in controls). Differences in PTENP1 methylation rates between normal endometrium in young (4%) and middle-aged and elderly (58%) females were significant (p 0.001). These findings suggest that PTENP1 pseudogene methylation may reflect age-related changes in the body and is not directly related to the endometrium pathology under study. It is assumed that, depending on the influence of a methylated PTENP1 pseudogene on PTEN gene expression, the pseudogene methylation may protect against the development of EC and/or serve as a marker of a precancerous condition of endometrial cells.
The processed pseudogene PTENP1 is involved in the regulation of the expression of the PTEN and acts as a tumor suppressor in many types of malignances. In our previous study we showed that PTENP1 methylation is present not only in tumor, but also in normal endometrium tissues of women over 45 years old. Here we used methylation-specific PCR to analyze methylation status of CpG island located near promoter region of PTENP1 in malignant and non-malignant endometrium tissues collected from 236 women of different age groups. To confirm our results, we also analyzed RNA sequencing and microarray data from 431 women with endometrial cancer from TCGA database. We demonstrated that methylation of PTENP1 is significantly increased in older patients. We also found an age-dependent increase in the level of PTENP1 expression in endometrial tissue. According to our data, PTENP1 methylation elevates the level of the pseudogene sense transcript. In turn, a high level of this transcript correlates with a more favorable prognosis in endometrial cancer. The data obtained suggested that PTENP1 methylation is associated with age-related changes in normal and hyperplastic endometrial tissues. We assumed that age-related increase in PTENP1 methylation and subsequent elevation of its expression may serve as a protective mechanism aimed to prevent malignant transformation of endometrial tissue in women during the perimenopause, menopause, and postmenopause periods.
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