Epigenetic Silencing of Tumor Suppressor Genes through Promoter Hypermethylation in Oral Squamous Cell Carcinoma: From Molecular Insights to Clinical Applications

Oral squamous cell carcinoma (OSCC) continues to be a devastating disease, largely because it is often detected too late and tends to resist conventional therapies. Collectively, promoter hypermethylation of tumor suppressor genes represents an early, reversible, and clinically actionable event in OSCC. Emerging methylation-based biomarkers, particularly salivary gene panels, demonstrate promise for non-invasive early detection and longitudinal monitoring. Furthermore, the reversibility of epigenetic silencing provides a rationale for epigenetic therapies, including DNMT (DNA methyltransferase) inhibitors in combination with immunotherapy. Although most evidence remains exploratory and often extrapolated from broader head and neck cancer cohorts, these molecular insights lay the groundwork for future OSCC-specific diagnostic and therapeutic strategies.HPV-specific methylation thresholds, such as CDKN2A (Cyclin Dependent Kinase Inhibitor 2A) in HPV-negative cancers—where OSCC-specific thresholds are limited and most data are derived from OPSCC (Oropharyngeal Squamous Cell Carcinoma)—and RASSF1A (Ras Association Domain Family Member 1A) in HPV-positive cancers—primarily based on OPSCC rather than OSCC—improve diagnostic and prognostic precision. New non-invasive approaches for early detection and monitoring, such as saliva-based methylation testing HOXA9/NID2 panels (HomeoboxA9), are also highlighted. This review summarizes new evidence that promoter hypermethylation causes oral squamous cell carcinoma (OSCC) by abnormally silencing tumor suppressor genes. We also examine the molecular basis of this phenomenon, stressing how tobacco exposure and HPV infection affect these cancers' methylation landscapes. We believe that incorporating these epigenetic indicators into clinical practice may improve OSCC management.