Salivary miR-21 and miR-31 in Oral Squamous Cell Carcinoma: Molecular Pathogenesis, Diagnostic Significance, and Translation to Isothermal RPA.

Oral Squamous Cell Carcinoma (OSCC) is often diagnosed at advanced stages despite the oral cavity's anatomical accessibility, highlighting the need for molecular adjuncts that can identify disease-associated changes before overt morphological progression. This review evaluates the biological roles and salivary diagnostic potential of microRNA-21 (miR-21) and miR-31, two miRNAs repeatedly associated with OSCC pathogenesis and progression. We examine current clinical evidence supporting their use as salivary biomarkers and discuss major sources of methodological variability, including differences in saliva collection, RNA recovery, normalization, and diagnostic thresholds. We pay particular attention to the transition from laboratory-based RT-qPCR to Recombinase Polymerase Amplification (RPA) for decentralized detection. Although RPA offers rapid, low-temperature amplification with reduced hardware requirements, mature miRNAs present a key technical challenge because their short length requires prior target extension or conversion. Successful chairside translation will depend on standardized sample processing, robust target-specific amplification, contamination control, and validation in clinically relevant early-stage and comparator populations.