Does the APC/C Mark MIWI and piRNAs for a final farewell?

PIWI-Interacting RNA in Drosophila: Biogenesis, Transposon Regulation, and Beyond

PIWI-interacting RNAs (piRNAs) are germline-enriched small RNAs that control transposons to maintain genome integrity. To achieve this, upon being processed from piRNA precursors, most of which are transcripts of intergenic piRNA clusters, piRNAs bind PIWI proteins, germline-specific Argonaute proteins, to form effector complexes. The mechanism of this piRNA-mediated transposon silencing pathway is fundamentally similar to that of siRNA/miRNA-dependent gene silencing in that a small RNA guides its partner Argonaute protein to target gene transcripts for repression via RNA-RNA base pairing. However, the uniqueness of this piRNA pathway has emerged through intensive genetic, biochemical, bioinformatic, and structural investigations. Here, we review the studies that elucidated the piRNA pathway, mainly in Drosophila, by describing both historical and recent progress. Studies in other species that have made important contributions to the field are also described.

Piwi Proteins and piRNAs step onto the systems biology stage

Animal germ cells are totipotent because they maintain a highly unique and specialized epigenetic state for its genome. To accomplish this, germ cells express a rich repertoire of specialized RNA-binding protein complexes such as the Piwi proteins and Piwi-interacting RNAs (piRNAs): a germ-cell branch of the RNA interference (RNAi) phenomenon which includes microRNA and endogenous small interfering RNA pathways. Piwi proteins and piRNAs are deeply conserved in animal evolution and play essential roles in fertility and regeneration. Molecular mechanisms for how these ribonucleoproteins act upon the transcriptome and genome are only now coming to light with the application of systems-wide approaches in both invertebrates and vertebrates. Systems biology studies on invertebrates have revealed that transcriptional and heritable silencing is a main mechanism driven by Piwi proteins and piRNA complexes. In vertebrates, Piwi-targeting mechanisms and piRNA biogenesis have progressed, while the discovery that the nuclease activity of Piwi protein is essential for vertebrate germ cell development but not completely required in invertebrates highlights the many complexities of this pathway in different animals. This review recounts how recent systems-wide approaches have rapidly accelerated our appreciation for the broad reach of the Piwi pathway on germline genome regulation and what questions facing the field await to be unraveled.

Novel dimensions of piRNAs in cancer

Piwi-interacting RNAs (piRNAs), a newly identified class of small non-coding RNAs, direct the Piwi-dependent transposon silencing, heterochromatin modification and germ cell maintenance. Owing to our limited knowledge regarding their biogenesis, piRNAs are considered as the most mysterious class of small regulatory RNAs, particularly in pathogenesis such as tumorigenesis. Recently, several lines of evidence have emerged to suggest that piRNAs may be dis-regulated and play crucial roles in tumorigenesis in previously unsuspected ways. In this prospective piece, we will discuss the emerging insights into the potential novel roles of piRNAs in carcinogenesis and highlight their potential implications in cancer detection, classification and therapy.