Abstract
BackgroundHuman endogenous retroviruses of the HERV-K(HML-2) group have been associated with the development of tumor diseases. Various HERV-K(HML-2) loci encode retrovirus-like proteins, and expression of such proteins is upregulated in certain tumor types. HERV-K(HML-2)-encoded Rec and Np9 proteins interact with functionally important cellular proteins and may contribute to tumor development. Though, the biological role of HERV-K(HML-2) transcription and encoded proteins in health and disease is less understood. We therefore investigated transcription specifically of HERV-K(HML-2) rec and np9 mRNAs in a panel of normal human tissues.ResultsWe obtained evidence for rec and np9 mRNA being present in all examined 16 normal tissue types. A total of 18 different HERV-K(HML-2) loci were identified as generating rec or np9 mRNA, among them loci not present in the human reference genome and several of the loci harboring open reading frames for Rec or Np9 proteins. Our analysis identified additional alternative splicing events of HERV-K(HML-2) transcripts, some of them encoding variant Rec/Np9 proteins. We also identified a second HERV-K(HML-2) locus formed by L1-mediated retrotransposition that is likewise transcribed in various human tissues.ConclusionsHERV-K(HML-2) rec and np9 transcripts from different HERV-K(HML-2) loci appear to be present in various normal human tissues. It is conceivable that Rec and Np9 proteins and variants of those proteins are part of the proteome of normal human tissues and thus various cell types. Transcription of HERV-K(HML-2) may thus also have functional relevance in normal human cell physiology.Electronic supplementary materialThe online version of this article (doi:10.1186/s13100-015-0035-7) contains supplementary material, which is available to authorized users.
Highlights
Human endogenous retroviruses of the HERV-K(HML-2) group have been associated with the development of tumor diseases
We identified a second HERV-K(HML-2) locus formed by L1-mediated retrotransposition that is likewise transcribed in various human tissues
HERV-K(HML-2) rec and np9 transcripts from different HERV-K(HML-2) loci appear to be present in various normal human tissues
Summary
Human endogenous retroviruses of the HERV-K(HML-2) group have been associated with the development of tumor diseases. Various HERV-K(HML-2) loci encode retrovirus-like proteins, and expression of such proteins is upregulated in certain tumor types. HERV-K(HML-2)-encoded Rec and Np9 proteins interact with functionally important cellular proteins and may contribute to tumor development. The biological role of HERV-K(HML-2) transcription and encoded proteins in health and disease is less understood. Human endogenous retroviruses (HERVs) stem from ancient germ line infections by exogenous retroviruses. There are about 40 phylogenetically distinct HERV groups documenting germ line integration, that is, provirus formations by different ancient exogenous retroviruses millions of years ago. HERV groups no longer encode former retroviral proteins due to long time presence in the genome and accumulation of nonsense mutations including smaller and larger indels.
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