MicroRNA Biomarkers in Chronic Neuropathic Pain

Background The pathophysiology of neuropathic pain remains incompletely understood. Small fiber neuropathy (SFN) is characterized by selective impairment of small- diameter A-delta and C-fibers. Diagnosis relies on clinical characteristics and assessment of intraepidermal nerve fiber density (IENFD). MicroRNAs (miRNAs), non-coding regulatory RNAs, have emerged as biological regulators modulating gene expression related to pain sensitization. This research investigates miRNA expression in skin samples from patients with clinically diagnosed painful SFN exhibiting different innervation patterns on skin biopsy, aiming to elucidate molecular profiles and potential biomarkers. Methods and Materials A scoping review was conducted to identify candidate microRNAs with altered expression in painful polyneuropathy patients and establish scientific rationale for investigation. Specific microRNAs have been selected for expression analysis using dd-PCR: miR-146a-5p, miR-155-5p, miR-203a-3p, and miR-21-5p. From 30 archived distal leg skin specimens, 16 quality-approved paraffin-fixed punch biopsy specimens were divided into two groups: "Norm" (normal IENFD) and "Den" (reduced IENFD), each containing 8 sex- and age-matched patients. Statistical analysis employed Kolmogorov-Smirnov testing, Mann-Whitney U tests for group comparisons, and Spearman's correlation for reference endogenous control assessment, with iii significance set at p<0.05. Results were normalized to SNORD controls and expressed as medians with quartiles for non-parametric data. Results MicroRNA expression was normalized using SNORD44 and SNORD48 as endogenous controls, which demonstrated strong correlation. Patients with denervation showed significantly elevated expression of all four investigated microRNAs compared to those with preserved IENFD: in the denervated cohort MiR-146a-5p demonstrated significant upregulation (p=0.0092), MiR-155-5p showed the most significant elevation (p=0.0030), MiR-203a-3p exhibited significant upregulation (p=0.0499), MiR-21-5p also demonstrated significant elevation (p=0.0207). Conclusions This study findings demonstrated that patients diagnosed with SFN and exhibiting signs of denervation presented significantly elevated expression levels of miR-146a- 5p, miR-155-5p, miR-203a-3p, and miR-21-5p when compared with patients without denervation, indicating a possible role as molecular biomarkers.