Methylated DNA in Oral Brush and Salivary Rinse as a Non-invasive Biomarker in the Diagnosis and Prognosis of Oral Cancer: A Narrative Review

Oral cancer is one of the significant health challenges globally, with late-stage diagnosis contributing substantially to poor prognosis and limited survival rates. Although tissue biopsy is the current gold standard for diagnosis, its invasive nature and limited sensitivity for early lesions restrict its effectiveness in routine screening and longitudinal monitoring. These limitations have driven growing interest in non-invasive molecular approaches that could support earlier detection and more effective clinical monitoring. Deoxyribonucleic Acid (DNA) methylation, a well-established epigenetic modification involved in gene regulation, has emerged as early and consistent events in oral carcinogenesis. Aberrant methylation of Tumour Suppressor Genes (TSG) DNA repairs genes, and other regulatory elements have been consistently reported in Oral Squamous Cell Carcinoma (OSCC). Importantly, these methylation changes can be detected not only in tumour tissue but also in non-invasive oral samples, such as oral brush biopsies and salivary or oral rinse specimens. This narrative review examines the diagnostic and prognostic potential of DNA methylation biomarkers detected through non-invasive sampling methods such as oral brush and salivary rinses. We summarise key findings from existing studies and discuss the relative strengths and limitations of these sampling approaches. Oral brush-based assays generally provide higher sensitivity due to enriched epithelial cell content and lesion-specific signals, whereas salivary and oral rinse-based assays offer greater feasibility for large-scale screening and repeated follow-up. Several multi-gene methylation panels have demonstrated encouraging performance in distinguishing malignant and potentially malignant lesions from normal oral mucosa, as well as in predicting disease recurrence and patient outcomes. Overall, methylation-based assays using oral brush and salivary rinse samples represent promising adjunctive tools for oral cancer detection and monitoring. While further validation, standardisation, and regulatory approval are required before routine clinical implementation, the integration of methylation-based assays into clinical practice holds the potential to significantly advance the early diagnosis and personalised management of oral cancer.