Implications of methylation patterns of cancer genes in salivary gland tumors

We investigated the methylation status and protein expression of four tumor suppressor genes to determine their role in salivary gland tumorigenesis.

We performed methylation-specific PCR and protein analyses of 29 normal salivary glands, 23 benign, and 79 malignant salivary gland neoplasms to determine the pattern and potential diagnostic and/or biological role of the RASSF1, RARbeta2, DAPK, and MGMT tumor suppressor gene methylation in these tumors.

No methylation was detected in the normal tissues. Methylation occurred in 9 of 23 (39.1%) benign tumors; 3 (25.0%) pleomorphic adenomas and 6 (66.7%) Warthin's tumors at the MGMT, DAPK, or RASSF1 genes. Methylation occurred in 33 of 79 (41.8%) malignant tumors; 8 (30.8%) adenoid cystic carcinomas, 6 (33.3%) mucoepidermoid carcinomas, 6 (42.9%) acinic cell carcinomas, and 13 (62.0%) salivary duct carcinomas. RASSF1 and RARbeta2 represented 75.8% of methylation events occurring most frequently in salivary duct and acinic cell carcinomas. Overall, we found no significant correlation between protein expression and methylation status of individual genes, but observed low or absent protein expression in several methylated tumors. Significant correlations were found between methylation and aggressive malignant phenotypes (P = 0.0004) and age (P = 0.05).

(a) Benign and malignant salivary tumors differed in the frequency and pattern of gene methylation; (b) high-grade carcinomas were significantly methylated compared with low-grade phenotypes; (c) RASSF1 and RARbeta2 were highly methylated in malignant tumors and can be targeted for therapy; and (d) methylation pattern may serve as a diagnostic and biological marker in assessing these tumors.