Microsatellite instability in unselected Sardinian patients with endometrial cancer: screening for hMLH1/hMSH2 and PTEN germline mutations

P. Baldinu1, Ph.D., A. Cossu2, M.D., A. Manca1, Ph.D., M.P. Satta1, M. Pisano1, Ph.D., M. Casula1, Ph.D., S. Dessole3, M.D., A. Pintus2, M.D., F. Tanda2, M.D., and G. Palmieri1, M.D.

1Institute of Molecular Genetics, C.N.R., Loc. Tramariglio, 07040 Santa Maria La Palma Sassari; 2Institute of Pathology, and 3Department of Obstetric & Gynecology, University of Sassari, Viale San Pietro 10, 07100 Sassari; Italy.

Aim: 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]. Correlation with some clinicopathological features (tumor grading, disease stage, disease-free survival and overall survival) revealed no significant difference among the two groups (MSI+ or MSI-) of EC patients. Conclusions: Epigenetic mechanisms inactivating the MMR genes seem to play the major role in endometrial cancers with the mutator phenotype. Predictive value as prognostic factor was demonstrated for disease stage only.

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Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Prognostic Markers.