H-Ras mutation modulates the expression of major cell cycle regulatory proteins and disease prognosis in oral carcinoma

Activating mutations of the Ras is a moderately frequent event in oral carcinogenesis in Indian patients. Ras pathway has essential roles in regulation of various phases of the cell cycle, especially at G(1) phase. Despite a large body of in vitro evidence, the multidimensional interaction between mutated Ras pathway and G1 cell cycle regulatory proteins in tumours in vivo is poorly determined. In the present study, DNA samples were screened for mutations in hot spot exons of B-Raf and hot spot codons 12, 13 and 61 of H-, K- and N-Ras by PCR-SSCP. Mutations were confirmed by direct sequencing. Expression of G1 cell cycle regulatory proteins cyclin D1, CDK4, Rb, p53, p16 and p21 and proliferation marker PCNA was analysed immunohistochemically. The results revealed the absence of B-Raf mutations in oral carcinoma in spite of 12.5% of the samples showing H- Ras mutation. The H- Ras mutant cases showed significantly low cyclin D1 ( P 0.027) and CDK4 ( P 0.046) expression and overexpression of Rb ( P 0.011) and p16 ( P 0.026). H- Ras mutant carriers also had significantly high recurrence-free survival ( P 0.033). In summary the present study demonstrated an epistatic interaction between H- Ras mutation and G1 cell cycle regulatory proteins in vivo. H- Ras mutation, thus, defines a molecular subtype of oral carcinoma with favourable outcome and unique biology.