5'-YRNA fragments derived by processing of transcripts from specific YRNA genes and pseudogenes are abundant in human serum and plasma.

Small noncoding RNAs carry out a variety of functions in eukaryotic cells, and in multiple species they can travel between cells, thus serving as signaling molecules. In mammals multiple small RNAs have been found to circulate in the blood, although in most cases the targets of these RNAs, and even their functions, are not well understood. YRNAs are small (84-112 nt) RNAs with poorly characterized functions, best known because they make up part of the Ro ribonucleoprotein autoantigens in connective tissue diseases. In surveying small RNAs present in the serum of healthy adult humans, we have found YRNA fragments of lengths 27 nt and 30-33 nt, derived from the 5'-ends of specific YRNAs and generated by cleavage within a predicted internal loop. Many of the YRNAs from which these fragments are derived were previously annotated only as pseudogenes, or predicted informatically. These 5'-YRNA fragments make up a large proportion of all small RNAs (including miRNAs) present in human serum. They are also present in plasma, are not present in exosomes or microvesicles, and circulate as part of a complex with a mass between 100 and 300 kDa. Mouse serum contains far fewer 5'-YRNA fragments, possibly reflecting the much greater copy number of YRNA genes and pseudogenes in humans. The function of the 5'-YRNA fragments is at present unknown, but the processing and secretion of specific YRNAs to produce 5'-end fragments that circulate in stable complexes are consistent with a signaling function.