Microsatellite Instability And Promoter Methylation Of Mismatch Repair Genes In Nasopharyngeal Carcinoma

Mismatch repair(MMR)defects are found in almostly different cancers. hMSH2 and hMLH1 gene play an superlatively important role in mismatch repair processes.It was reported to be inactivated by its promoter methylation or gene mutation in general human cancers.Tumors defective in their MMR system accumulate mutations at the level of microsatellites and are called microsatellite instability(MSI).The instability of non-coding microsatellites is a good indicator of the MSI status.On the other hand,instability leads to a frameshift within the gene containing the repeat.The consequence is the inactivation of this gene.In the present study,seven microsatellite polymorphic markers were selected to detect MSI for 54 cases of nasopharyngeal carcinoma(NPC)by PCR-SSCP,The total frequency of MSI was 70.37%,positive rate of replication errors was 44.44%.It is possibly suggested that NPC is a tumor with MMR defects,we also found that NPC with MSI are characterized by earlier stage and lower lymph node metastasis.To identify NPC is a tumor with MMRdefects.we investigated the correlation of hMSH2 and hMLH1 expression with its promoter methylation in NPC.Expression of hMSH2 was down-regulated in 80%of NPC cell lines. Decreased hMSH2 expression was also observed in NPC primary tumors compared with normal nasopharyngeal epithelia.But hMLH1expression was no statistical difference in NPC cell lines and NPC primary tumors compared with NP69 cell line and normal nasopharyngeal epithelia.Promoter methylation of hMSH2 could be detected in 80%of the hMSH2 -silenced NPC cell lines and 75.9%of primary tumors,Promoter methylation of hMLH1was also detected in 80%of NPC cell lines and 42.59%of primary tumors,but not in any of the normal nasopharyngeal epithelia tissues,hMSH2-methylated but not hMLH1-methylated cases showed a significantly lower level expression than unmethylated cases.Loss of hMSH2 gene expression can be greatly restored by the methyltransferase inhibitor 5-aza-dC in NPC cell lines,we also found that hMSH2 and hMLH1promoter methylation have no correlation with the clinicopathological characteristics of the patients with NPC.Does MSI in NPC have direct correlation with MMR defects? we investigated the correlation of MSI in NPC with hMSH2 and hMLH1 promoter methylation,We found that hMSH2 but not hMLH1 promoter methylation has obvious correlation with MSI.In conclusion,NPC is a tumor with MMR defects,NPC with MSI are characterized by earlier stage and lower lymph node metastasis,hMSH2,a candidate tumor suppressor gene(TSG),is frequently inactivated by its promoter methylation and this aberrant methylation closely correlates with MSI in NPC.