Are MGMT promoter methylation and EGFR mutations early markers of tumor progression in colorectal cancer?

Abstract

3584 Background: The colorectal adenoma-carcinoma sequence summarizes the epigenetic and genetic modifications arising during cancer progression. These modifications notably affect the methylation status of different gene's promoters and mutation in the K-ras gene. In the present study, in order to determine if MGMT (O6-methylguanine-DNA methyltransferase) promoter methylation and EGFR mutation may be an early event in the progression of colorectal cancer, we determine the methylation status of H-cadherin and E-cadherin promoters as early and late markers of the disease in more than 200 patients. Methods: Tumour samples were obtained from CRC patients at the adenocarcinoma stage. After resection, they were rapidly frozen in liquid nitrogen and stored at -80°C. Methylation statuses were analyzed by methylation-specific PCR (MSP) and mutation determined by direct sequencing. Results: We screened 236 clinical tumours to detect mutations in the EGFR gene exons 18 to 21, mainly covering the EGFR TK catalytic domain and found a frequency of mutation of 0.5% to 3.8%, except for a silent point mutation in exon 20 found at a very high frequency of about 83%. 24% of the same tumours showed a mutation in K-ras gene and 78%, 63% and 49% an aberrant methylation of H-cadherin, E-cadherin and MGMT genes promoters respectively. When combining the distribution of these methylation statuses according to the mutation in K-ras using statistical analysis we showed that occurrence of G to A mutations in K-ras may be dependent on the methylation of MGMT and H-cadherin, the E-cadherin gene promoter remaining unmethylated. Conclusions: We showed that MGMT promoter methylation may occur between the methylation of H-cadherin gene promoter and the G to A mutation in K-ras gene. The status of MGMT in CRC with known G to A mutations in several genes is now under investigation. In this disease, if the mutation frequency of EGFR gene is very low, we showed the occurrence of a silent point mutation at very high frequency (83%). As this polymorphism is also present in healthy volunteers, correlation with patient's grade and outcome will be crucial to determine if this polymorphism might be used as a predictive marker of the evolution of the disease. No significant financial relationships to disclose.