Dysregulation of piRNA biogenesis-associated proteins in cancer: mechanisms, evidence, and translational constraints.

PIWI-interacting RNAs (piRNAs), PIWI proteins, and piRNA biogenesis-associated proteins form a conserved RNA network that protects germline genome integrity and is increasingly linked to cancer biology. Most cancer reviews have emphasized altered piRNA abundance or PIWI expression, whereas the upstream machinery that generates, loads, trims, and stabilizes piRNAs has received less critical attention. For the RNA Networks and Biology section, this Review re-evaluates piRNA biogenesis proteins as RNA-network regulators rather than as isolated tumor biomarkers. We summarize the biochemical logic of precursor selection, mitochondrial processing, PIWI loading, 3-end maturation, and effector complex assembly, focusing on PIWIL1-4, PLD6/MitoPLD, MOV10L1, DDX4/VASA, GASZ, TDRKH, PNLDC1, HENMT1, Tudor-domain proteins, HSP90, and MAEL. We then assess cancer evidence by distinguishing direct perturbation studies from expression associations, animal-germline mechanisms, and computational inference. Current evidence supports tumor-context-specific roles for PIWIL1, PIWIL2, DDX4, TDRD1, MAEL, and emerging PLD6 biology in selected tumors, whereas several canonical maturation or scaffold proteins remain underexplored in human cancer. We propose that dysregulated piRNA biogenesis proteins may influence cancer through piRNA-dependent small-RNA imbalance and piRNA-independent reactivation of germline RNA-processing, epigenetic, metabolic, and immune programs. Finally, we discuss biomarker and therapeutic opportunities while emphasizing unresolved controversies, technical limitations in piRNA detection, reproductive toxicity, and the need for causal validation in human tumor models.