DNA mismatch repair is essential to the maintenance of genome integrity, and as such, it is not surprising that loss of mismatch repair is a feature of many cancers. Inherited defects in DNA mismatch repair dramatically increase risk for malignancies. Somatic inactivation of a mismatch repair gene (mutation or epigenetic) confers a mutator phenotype. We will investigate the causes and consequences of defective DNA mismatch repair in endometrial cancers. We will determine how the loss of specific components of the DNA mismatch repair system influences the mutation rate and mutational spectrum in vivo. We will test whether inherited genetic factors contribute to risk for epigenetic inactivation of the MLH1 DNA mismatch repair gene. By careful clinical and molecular classification of endometrial tumors we will determine how mismatch repair defects contribute to inherited and sporadic forms of endometrial cancer, and what clinical and pathologic features are associated with specific mismatch repair defects. The aims are: 1. To define the mutator phenotype in endometrial cancers with defective DNA mismatch repair we will conduct a large-scale mutation analysis in primary endometrial cancers. Comparing tumors with different defects in DNA mismatch repair and tumors with normal mismatch repair will allow us to answer key questions regarding in vivo a) mutational rates and b) mutational spectra associated with specific mismatch repair defects. These data will provide insights into how defective DNA mismatch repair contributes to the genetic decline of the tumor cell, and begin to explain the strong selection for the mismatch repair mutator phenotype in endometrial cancers. 2. To test for association between genetic variants in MLH1 and genes involved in DNA methylation and MLH1 promoter methylation in endometrial cancers Epigenetic silencing of MLH1 is the most frequent cause of defective DNA mismatch repair, with 1 in 5 endometrial cancers exhibiting aberrant MLH1 methylation. A recent study of patients with sporadic Beckwith-Wiedemann syndrome showed inherited genetic variation is associated with risk for abnormal methylation, linking genotype and the epigenetic state. We hypothesize that inherited factors contribute to risk for MLH1 methylation in endometrial carcinoma. Association studies will be undertaken to test the hypothesis that variation in MLH1 and genes involved in DNA methylation contribute to risk for aberrant MLH1 promoter methylation in endometrial cancers. 3. To further elucidate the causes and consequences of defective DNA mismatch repair in endometrial cancers we will prospectively characterize endometrial cancers for defects in DMMR and correlate molecular features with clinicopathologic variables. We will also determine whether DMMR genes other than MSH2, MSH6 and MLH1 play significant roles in endometrial tumorigenesis. These studies will define the relationship between defective DNA mismatch repair and the disease state.