Methylation of <i>RASSF1A, RASSF2A</i> , and <i>HIN-1</i> Is Associated with Poor Outcome after Radiotherapy, but not Surgery, in Oral Squamous Cell Carcinoma

Abstract

PURPOSE: Radiotherapy is the standard adjuvant treatment for oral squamous cell carcinoma (OSCC). The Ras/PI3K/AKT pathway is the major mechanism associated with radioresistance. To evaluate the potential significance on the outcome of radiotherapy in OSCC of the Ras/PI3K/AKT pathway with respect to methylation of negative regulators, we examined the methylation status of genes known to be involved in Ras/PI3K/AKT pathway and aberrantly methylated in human cancers together with the mutation status of K-ras/H-ras. EXPERIMENTAL DESIGN: PCR--denaturing high-performance liquid chromatography was used to examine the methylation status of the RASSF1A, RASSF2A, PTEN, and HIN-1 genes, and PCR-RFLP was used to determine the mutation status of K-ras/H-ras in 482 OSCCs. Associations between mutation, methylation, clinicopathologic parameters, and outcome were evaluated. RESULTS: The frequencies of K-ras/H-ras mutation and promoter methylation of the RASSF1A, RASSF2A, PTEN, and HIN-1 genes were 6.6%, 22.4%, 27.8%, 1.2%, and 7.3%, respectively. A combination of RASSF1A and RASSF2A methylation was found to be significantly associated with poor disease-free survival (DFS). Furthermore, a gene dosage effect of the activated Ras/PI3K/AKT signal on DFS was observed in patients treated with radiotherapy after surgery but not in patients treated with surgery alone. The Ras/PI3K/AKT pathway was activated in 140 primary OSCCs among 286 patients treated with radiotherapy after surgery and methylation of RASSF1A/RASSF2A (75.7%) was the most common mechanism. CONCLUSION: Our study indicates that epigenetic silencing of tumor suppressor genes involved in the Ras/PI3K/AKT pathway plays an important role in OSCC radioresistance and this provides a rationale for exploring novel treatment strategies.