Screening of MLH1 and MSH2 mutations in gastric cancer via microsatellite analysis and immunohistochemistry

Microsatellite instability (MSI) is a hallmark of defective DNA mismatch repair (MMR) system. It is frequently involved in the carcinogenesis of various tumours including gastric cancer. This study aimed to determine the MSI status in gastric cancer and its association with MLH1 and MSH2 protein expression. It also aimed to identify mutations in the MLH1 and MSH2 genes. Sixty resected gastric cancers were assessed for microsatellite analysis and immunohistochemistry staining of MLH1 and MSH2 protein. The cases with MSI-positive or loss of MLH1/MSH2 protein expression were then sequenced to identify mutations in the MLH1 and MSH2 genes. Six out of 10 MSI-positive cases showed absence of MLH1 (n=3) or MSH2 (n=3) protein expression. DNA sequencing showed 13 and nine types of mutations for MLH1 and MSH2 genes respectively. All were novel except one aberrant splicing in MLH1. Two pathological mutations including c.1874A>C in MLH1 and c2018G>A in MSH2 were identified and they may have significant clinical implication, which should be further elucidated. In conclusion, IHC staining and microsatellite analysis are complementary in detecting MMR gene mutations. A larger number of subjects with gastric cancer is needed to validate the identified pathological mutations.