Non-coding RNA fragments account for the majority of annotated piRNAs expressed in somatic non-gonadal tissues

PIWI-interacting RNAs (piRNAs) are regarded as the guardians of the genome because they tackle genome stability-threatening transposable elements in the germline. Recently, piRNAs were 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 be broader than previously expected. Here, we show that different piRNA databases contain a subset of sequences that correspond to piRNA-sized fragments of ncRNAs (rRNAs, tRNAs, YRNAs, snRNAs, and snoRNAs) and intermediates of miRNA biogenesis. We discuss that the biogenesis of these sequences is probably independent of the PIWI pathway, and can therefore be considered contaminants in piRNA databases. Although a minority of annotated piRNAs falls in this category, they account for the vast majority of piRNA expression in somatic non-gonadal tissues. Since ncRNA fragments are ubiquitous and abundant, their confusion with piRNAs strongly impacts the estimation of piRNA expression outside of mammalian gonads.