Microsatellite instability and mutation analysis of candidate genes in unselected Sardinian patients with endometrial cancer

Purpose: Microsatellite instability (MSI) is mostly due to a defective DNA mismatch repair (MMR). Inactivation of the two principal MMR genes, hMLH1 and hMSH2, and the PTEN tumor suppressor gene seems to be involved in endometrial tumorigenesis. In this study, Sardinian patients with endometrial cancer (EC) were analyzed to assess the prevalence of both the mutator phenotype (as defined by the presence of MSI and abnormal MMR gene expression at somatic level) and the hMLH1, hMSH2, and PTEN germline mutations among MSI+ EC cases. Methods: Paraffin-embedded tissue samples from 116 consecutive EC patients were screened for MSI by PCR-based microsatellite analysis. Immunohistochemistry (IHC) with anti-hMLH1 and anti-hMSH2 antibodies was performed on MSI+ tumor tissue sections. Germline DNA was used for mutational screening by DHPLC analysis and automated sequencing. Results: Thirty-nine (34%) EC patients exhibited MSI; among them, 25 (64%) tumor samples showed a negative immunostaining for hMLH1/hMSH2 proteins (referred to as IHC-). No disease-causing mutation within the coding sequences of the hMLH1/hMSH2 and PTEN genes was found in EC cases with the mutator phenotype (MSI+ and IHC-), except for a newly described hMLH1 missense mutation [Ile655Val, observed in 1/27 (4%) cases]. Although MSI was found to be more common among advanced-stage cases as well as to increase as the tumor grading increases, no significant correlation with disease-free survival and overall survival was observed among the two groups (MSI+ or MSI-) of EC patients. Conclusions: In MSI+ EC cases, epigenetic inactivations rather than genetic mutations of the MMR genes seem to be involved in endometrial tumorigenesis. No prognostic value was demonstrated for MSI in endometrial cancer.