Non-coding RNA fragments account for themajority of annotated piRNAs expressed in somaticnon-gonadal tissues

Abstract

PIWI-interacting RNAs (piRNAs) are regarded as the guardians of the genome because theytackle genome stability-threatening transposable elements in the germline. Recently, piRNAswere also reported in other types of cells, including mouse brain, malignant and non-malignant somatic tissues, and human plasma. This suggests that piRNA function might bebroader than previously expected. Here, we show that different piRNA databases contain asubset of sequences that correspond to piRNA-sized fragments of ncRNAs (rRNAs, tRNAs,YRNAs, snRNAs, and snoRNAs) and intermediates of miRNA biogenesis. We discuss that thebiogenesis of these sequences is probably independent of the PIWI pathway, and cantherefore be considered contaminants in piRNA databases. Although a minority of annotatedpiRNAs falls in this category, they account for the vast majority of piRNA expression insomatic non-gonadal tissues. Since ncRNA fragments are ubiquitous and abundant, theirconfusion with piRNAs strongly impacts the estimation of piRNA expression outside ofmammalian gonads.