In contrast, hMLH1 and hMSH2 were absent or had extremely low expression at estrogen levels ranging from 20 to 60 pg/mL, but some cell growth still occurred. Therefore, cells dividing in a low-estrogen environment are more likely CB-839 nmr to accumulate genetic errors due to low repair activity and may be at high risk for carcinogenesis. Based on these results, Miyamoto et al.[8] suggested that the incidence of growth-induced genetic errors should be low in young women with high estrogen levels and sufficient repair activity of MMR proteins, making carcinogenesis unlikely. In older women with lower estrogen

but an atrophic endometrium, carcinogenesis would also be unlikely because of the absence of cell growth. However, under perimenopausal R788 clinical trial conditions, the carcinogenic risk would

be increased because sufficient estrogen is present to promote cell division, but MMR activity is low. This intermediate status was defined as the cancer window (Fig. 1). The mismatch repair (MMR) system is responsible for repairing base mismatches that arise during DNA replication. Typical MMR proteins include hMLH1, hMSH2, hPMS2, hMSH3 and hMSH6. Genes encoding these proteins are called MMR genes and aberrations in these genes prevent correct repair of mismatched bases, resulting in DNA strands with different lengths. This phenomenon occurs in microsatellite regions of the human genome and is referred to as microsatellite instability (MSI). Microsatellites or short tandem repeats (STR) are repeating sequences of one to five base pairs of DNA, such as CA and CAG. Some STR

occur in regions encoding phosphatase and tensin homolog deleted on chromosome ten (PTEN), a lipid phosphatase that is a tumor suppressor gene; TGF-βR2 and IGF2R, which are associated with inhibition of cell proliferation; K-ras, which is involved in cell proliferation; and BAX, which is related to apoptosis induction. Therefore, MSI is implicated in carcinogenesis.[9] Aberrations in MMR genes are involved in carcinogenesis of type I endometrial cancer. These aberrations are caused by epigenetic changes independent of the DNA sequence, that is, gene inactivation by aberrant hypermethylation of promoter regions. Such inactivation of MMR genes permits accumulation of gene mutations and leads to carcinogenesis. 3-oxoacyl-(acyl-carrier-protein) reductase In endometrial cancer, carcinogenesis most frequently involves aberrant methylation of hMLH1 and mutation of hMLH1 is detected in 30% of cases. Mutations of hMLH1 are also found in atypical endometrial hyperplasia, which suggests that hMLH1 is implicated in the early stage of carcinogenesis.[10, 11] Muraki et al.[12] reported aberrant hMLH1 hypermethylation in 40.4% of patients with endometrial cancer and found significantly reduced hMLH1 protein levels in these patients (P < 0.01). However, none of the four cancer-related genes were aberrantly methylated in the normal endometrium. MMR genes are also causative genes in Lynch syndrome (hereditary nonpolyposis colorectal cancer).