4292

MLH1

mutL homolog 1;MLH1;mutL homolog 1

DNA mismatch repair protein Mlh1|mutL homolog 1, colon cancer, nonpolyposis type 2

3p21.3

protein-coding

Aberrant hypermethylation of gene promoter regions is one of the mechanisms for inactivation of tumour suppressor genes in many human cancers including breast carcinoma. In the current study, we aimed to assess by MSP, the methylation pattern of two cancer-related genes involved in DNA repair: hMLH1 (mutL homolog 1, colon cancer, nonpolyposis type 2 (E. coli) and BRCA1 (breast cancer 1, early onset) in 78 primary breast cancers from Tunisian patients. The methylation frequencies were 24.36% for hMLH1 and 46% for BRCA1. BRCA1 methylation correlated with age at diagnosis (P = .015) and 5-years disease free survival (P = .016) while hMLH1 methylation was more frequent in larger tumors (P = .002) and in presence of distant metastasis (P = .004). Furthermore, methylation of hMLH1 significantly correlated with high level of P53 expression (P = .006) and with overall survival (P = .015) suggesting that silencing of hMLH1 through aberrant promoter methylation could be used as a poor prognosis indicator in breast cancer.

Mononucleotide repeat sequences are particularly prone to frameshift mutations in tumors with biallelic inactivation of the mismatch repair (MMR) genes MLH1 or MSH2. In these tumors, several genes harboring mononucleotide repeats in their coding region have been proposed as targets involved in tumor progression, among which are also the MMR genes MSH3 and MSH6. We have analyzed the expression of the MSH3 and MSH6 proteins by immunohistochemistry in 31 colorectal carcinomas in which MLH1 was inactivated. Loss of MSH3 expression was identified in 15 tumors (48.5%), whereas all tumors expressed MSH6. Frameshift mutations at coding microsatellites were more frequent in MSH3 (16 of 31) than in MSH6 (3 of 31; Fishers exact test, P < 0.001). Frameshift mutations and allelic losses of MSH3 were more frequent in MSH3-negative tumors compared with those with normal expression (22 mutations in 30 alleles versus 8 mutations in 28 alleles; chi(2), P = 0.001). Biallelic inactivation was evident or inferred for 60% of MSH3-negative tumors but none of the tumors with normal MSH3 expression. In contrast, we did not identify frameshift mutations in the (A)8 tract of MSH3 in a control group of 18 colorectal carcinomas in which the MMR deficiency was based on the inactivation of MSH2. As it has been suggested that mutations of MSH3 might play a role in tumor progression, we studied the association between MSH3 expression and disease stage assessed by lymph node and distant metastases status. Dukes stages C and D were more frequent in primary tumors with loss of MSH3 expression (9 of 13), compared with tumors with retained expression (1 of 14; Fishers exact test, P = 0.001), suggesting that MSH3 abrogation may be a predictor of metastatic disease or even favor tumor cell spread in MLH1-deficient colorectal cancers.

OBJECTIVE: To investigate the expression and clinical significance of mismatch repair genes hMLH1 and hMSH2 in sporadic colorectal carcinoma tissues. METHODS: The expression of hMLH1 and hMSH2 proteins was detected in the 63 sporadic colorectal carcinoma samples by immunohistochemical staining, including tumor tissue, adjacent tissue at 3 cm from the carcinoma, and normal tissue at 10 cm away from the tumor. RESULTS: The positive rate of hMLH1 protein expression in the 63 normal colorectal tissues, adjacent tissues and sporadic colorectal carcinoma tissues was 95.2%, 85.7% and 81.0%, respectively. The positive rate of hMLH1 protein expression was significantly lower in the tumor than in normal colorectal tissues (P < 0.05). The positive rate of hMSH2 protein in the 63 normal colorectal tissues, adjacent tissues and sporadic colorectal carcinoma tissues were 76.2%, 66.7% and 52.4%, respectively. The positive rate of hMSH2 protein expression was significantly lower in the tumor than in normal colorectal tissues (P < 0.01). The positive rate of hMLH1 protein expression was significantly higher in the tumor tissue of patients aged younger than 60 years (100%) than that in patients >/= 60 years (75.0%, P < 0.05). The positive rate of hMLH1 protein expression in the tumor tissue accompanied by lymphatic metastasis was 50.0%, significantly lower than that (93.3%) in tumors without lymphatic metastasis (P < 0.05). The positive rate of hMSH2 protein expression in the tumor tissue of patients aged younger than 60 years was 80.0%, significantly higher than that (43.8%) in the cases >/= 60 years (P < 0.05). The positive rate of hMSH2 protein expression in the tumor tissues with invasion reaching to the intestinal serosa (61.5%) was significantly higher than that (37.5%) in the tumors invading to submucosa or muscular layer (P < 0.05). There was a positive correlation between the expressions of hMLH1 and hMSH2 proteins in the sporadic colorectal carcinomas. CONCLUSION: There is a certain loss of expression of hMLH1 and hMSH2 proteins in sporadic colorectal carcinoma, and is correlated with the age of patients, lymphatic metastasis and different depth of cancer invasion. HMLH1 and hMSH2 may be used as a useful laboratory marker in clinical judgement of occurrence and development of sporadic colorectal carcinoma.