OBJECTIVE: Our objective was to determine whether RUNX3 tumor suppressor is inactivated in endometrial carcinoma. METHODS: We have investigated 24 endometrial carcinomas, 3 endometrial carcinoma cell lines, and 9 normal endometria for genetic and epigenetic alterations of RUNX3. Reverse-transcription PCR (RT-PCR), methylation-specific PCR (MS-PCR) analysis, and loss of heterozygosity (LOH) analysis were performed. We also tested RUNX3 protein expression by immunohistochemistry. RESULTS: Using RT-PCR technique, we observed a significant loss of RUNX3 mRNA expression in nine of 24 endometrial carcinomas (38%) and in all 3-cell lines (100%). In contrast, all nine of the normal endometria showed an abundant expression of RUNX3 mRNA. Methylation-specific PCR (MS-PCR) analysis of the CpG islands of RUNX3 showed the promoter region to be hypermethylated in 18 of 21 analyzed carcinomas (86%), whereas only two of nine normal endometria (22%) were methylated (p<0.01). By using two polymorphic microsatellite markers, D1S199 and D1S1676, we detected 1p36 LOH in 7 of 21 carcinomas (33%). We observed a significant relationship between the loss of RUNX3 mRNA expression and this regional LOH (p<0.01). Immunohistochemical staining showed that RUNX3 protein expression was lost in 12 of 21 endometrial carcinomas (57%). We observed a significantly more frequent loss of RUNX3 protein expression in the histologically higher-grade tumors (Grade 3) than in Grade 1 or 2 tumors (p<0.01). CONCLUSION: These findings indicate that RUNX3 inactivation may play an important role in carcinogenesis of the endometrium, especially in high-grade endometrial carcinoma.