The Prognostic Value of MicroRNAs in Thyroid Cancers—A Systematic Review and Meta-Analysis

SIMPLE SUMMARY: Current prognostication systems have inherent limitations associated with the prediction of recurrence risk from thyroid cancer (TC). Recent studies identified associations between specific levels of microRNAs and aggressive TC clinicopathological features. The objective of this research was to uncover existing knowledge regarding dysregulated microRNAs in conjunction with the long-term prognosis in TC patients. We also set out to identify specific microRNAs that could serve as prognostic biomarkers in different subtypes of TC. The findings emerging from the meta-analysis revealed that elevated expression levels of miR-146b, miR-221, and miR-222 were significantly associated with recurrence and suggested their possible prognostic ability, especially in the subgroup of papillary thyroid cancer patients. All these results could aid decision-making for clinicians and optimize the surgical management of patients with TC. Also, they could help to refine the complex prognostication system and implement microRNA-targeted therapy of TC in the future. ABSTRACT: Thyroid cancer (TC) includes various phenotypes, from indolent to highly aggressive cancer. The limitations of the current prognostication systems to predict the recurrence risk and the variability in expression of the genes involved in the thyroid carcinogenesis uncover the need for new prognostic biomarkers by taking into account potential epigenetic differences. We aimed to summarize the current knowledge regarding the prognostic impact of microRNAs (miRNAs) in TC. A literature search was conducted in PubMed, Embase, Scopus, and Web of Science databases. Both upregulated and downregulated miRNAs are significantly correlated with worse overall survival (hazard ratio (HR) = 5.94, 95% CI: 2.73–12.90, p < 0.001; HR = 0.51, 95% CI: 0.26–0.96, p = 0.048) disease/recurrence-free survival (HR = 1.58, 95% CI: 1.08–2.32, p = 0.003; HR = 0.37, 95%, CI: 0.24–0.60, p < 0.001). Sensitivity analysis revealed a significant association between the higher expression of miR-146b, miR-221, and miR-222 and the recurrence of papillary TC (OR = 9.11, 95% CI 3.00 to 27.52; p < 0.001; OR = 3.88, 95% CI 1.34 to 11.19, p < 0.001; OR = 6.56, 95% CI 2.75 to 15.64, p < 0.001). This research identified that miR-146b, miR-221, and miR-222 could serve as potential prognostic biomarkers in TC, particularly in PTC. Further studies are needed to strengthen these findings and sustain its clinical applicability.