piRNA: Molecular Mechanisms from Germline Silencing to Somatic Regulation and Roles in Disease

PIWI-interacting RNAs (piRNAs) are a class of small non-coding RNAs initially identified in germline cells as genome guardians that silence transposable elements. Recent studies have expanded this view, revealing that piRNAs and PIWI proteins are broadly expressed in somatic tissues and participate in epigenetic and post-transcriptional gene regulation. This review systematically summarizes piRNA biogenesis and molecular mechanisms, with a focus on their functional diversification from germline to somatic cells. We detail piRNA dysregulation and its association with various human diseases, including cancer, cardiovascular disorders, neurodegenerative diseases, immune dysfunction, and reproductive disorders. By integrating recent findings, this review provides a comprehensive overview of piRNA-mediated regulatory networks and highlights their potential as novel biomarkers and therapeutic targets.