Role of the BRAF mutations in the microsatellite instability genetic pathway in sporadic colorectal cancer

Between 10 and 15% of all cases of colorectal cancer are the result of microsatellite instability (MSI) in the genetic pathway due to an alteration in the DNA repair genes. Tumors with high MSI are characterized by a better prognosis. The BRAF oncogene has been linked to the MSI pathway in tumorogenesis. The objective of this study was to determine whether alterations in BRAF are related to MSI and whether they can result in differences in survival rates.

The study cohort comprised 351 patients diagnosed with sporadic colorectal cancer. MSI was determined in accordance with the National Cancer Institute's (NCI) recommendations by means of PCR and sequence analyses. Mutational analysis of the BRAF gene was performed by real-time PCR and subsequent sequencing of the altered samples. The methylation pattern of the hMLH1 gene was determined using methylation-specific PCR analyses of bisulfite-treated DNA and the results confirmed by sequencing.

Of the patients tested, 6.9% showed high MSI and 3.7% showed a BRAF gene mutation. hMLH1 methylation was observed in 67.2% of the patients with MSI and/or the BRAF alteration. The BRAF mutation was related to the MSI genetic pathway (P < 0.0001) and with hMLH1 methylation. In the analysis of overall survival only MSI had an independent prognostic value for the risk of death. Patients with the BRAF mutation showed a higher risk of death, although this association was found not to be statistically significant.

There is a subgroup of carcinomas which develop via the MSI pathway that carry an alteration of the BRAF gene. This alteration confers a poorer outcome on these patients within the total group of patients with MSI who have a better prognosis. This hypothesis should be further investigated in a larger study population due to the low incidence of BRAF mutations in colorectal cancer.