Methylation-associated inactivation of RASSF1A has frequently been observed in several human
malignancies including sporadic colorectal and
gastric cancer. However, nothing is known about the RASSF1A methylation status in the setting of MMR-deficient gastrointestinal tumours. In this study, we analysed systematically alterations in KRAS, BRAF and RASSF1A, in order to define the frequency and the pattern of these genetic/epigenetic alterations in three distinct subsets of MSI gastrointestinal tumours. Further, an association study was performed between RASSF1A methylation and the clinicopathological parameters in order to determine the profile of tumours harbouring this epigenetic event. A total of 56 MSI sporadic gastrointestinal tumours (31 colorectal and 25 gastric) and 20 MSI HNPCC analysed for KRAS/BRAF were analysed for RASSF1A promoter hypermethylation by MSP and
DNA sequencing. No significant differences were found between the frequency of RASSF1A methylation in sporadic MSI colorectal and gastric
carcinomas and HNPCC
carcinomas (P=0.31). Methylation of RASSF1A was present in 16 of 31 (52%) sporadic MSI colorectal and 11 of 25 (44%) MSI gastric
carcinomas, and in six of 20 (30%) HNPCC
carcinomas. Nearly 36% of MSI sporadic
colorectal carcinomas (
CRCs) had RASSF1A methylation and activating mutations at KRAS and/or BRAF. In contrast, only 10 and 8% of HNPCC and sporadic gastric
carcinomas, respectively, had concomitant KRAS mutations and RASSF1A methylation. The MSI sporadic gastric and
CRCs with RASSF1A methylation were preferentially poorly differentiated (P=0.03, 0.05, respectively). We show that the profile of alterations RASSF1A, KRAS/BRAF is different among the three groups of MSI gastrointestinal tumours. Further, we demonstrate that MSI sporadic
CRCs accumulated significantly more epigenetic/genetic alterations in RASSF1A, KRAS/BRAF than MSI sporadic gastric or HNPCC
carcinomas (P=0.016). These results are likely to have therapeutic implications in the near future, due to the possibilities of using specific
kinase inhibitors alone or in association with demethylating agents in MSI tumour types harbouring KRAS or BRAF mutations and RASSF1A methylation.