Noncoding RNA of U87 host gene is associated with ribosomes and is relatively resistant to nonsense-mediated decay

Non-coding RNAs are involved in many cellular processes. In particular, most of C/D box small nucleolar RNAs (snoRNAs) function as guide RNAs in site-specific 2'-O-methylation of rRNAs. While most snoRNA genes reside in introns of protein-coding genes, here we demonstrated an unusual snoRNA gene occupying an intron of a previously unknown non-protein-coding gene U87HG. We characterized this host gene in human, mouse, rat, and dog. It is a member of 5'TOP gene family, which includes many translation apparatus genes. U87HG RNA carried multiple stop-codons and was associated with ribosomes, suggesting that it may be a target for nonsense-mediated mRNA decay (NMD), a process that eliminates transcripts bearing nonsense mutations. Surprisingly, we found that U87HG RNA was hardly susceptible to NMD. Possible mechanisms (translation reinitiation, ribosomal leaky scanning, and low efficiency of translation) of this phenomenon are discussed. Unlike transcripts of four other known non-protein-coding host genes, U87HG RNA shows a relatively high degree of conservation suggesting a selective pressure and a possible functional activity of U87HG apart from producing U87 snoRNA.