Abstract
With total dependence on the host cell, several viruses have adopted strategies to modulate the host cellular environment, including the modulation of microRNA (miRNA) pathway through virus-encoded miRNAs. Several avian viruses, mostly herpesviruses, have been shown to encode a number of novel miRNAs. These include the highly oncogenic Marek’s disease virus-1 (26 miRNAs), avirulent Marek’s disease virus-2 (36 miRNAs), herpesvirus of turkeys (28 miRNAs), infectious laryngotracheitis virus (10 miRNAs), duck enteritis virus (33 miRNAs) and avian leukosis virus (2 miRNAs). Despite the closer antigenic and phylogenetic relationship among some of the herpesviruses, miRNAs encoded by different viruses showed no sequence conservation, although locations of some of the miRNAs were conserved within the repeat regions of the genomes. However, some of the virus-encoded miRNAs showed significant sequence homology with host miRNAs demonstrating their ability to serve as functional orthologs. For example, mdv1-miR-M4-5p, a functional ortholog of gga-miR-155, is critical for the oncogenicity of Marek’s disease virus. Additionally, we also describe the potential association of the recently described avian leukosis virus subgroup J encoded E (XSR) miRNA in the induction of myeloid tumors in certain genetically-distinct chicken lines. In this review, we describe the advances in our understanding on the role of virus-encoded miRNAs in avian diseases.
Highlights
MicroRNAs are ~22-nucleotide small RNA molecules that profoundly affect gene expression by directing repressive protein complexes to the untranslated region (UTR) of target messenger RNA transcripts
All of the pathogenic members of avian herpesviruses belong to the same subfamily Alphaherpesvirinae. These include the infectious laryngotracheitis virus (ILTV, Gallid herpesvirus) in the Iltovirus genus that induces laryngotracheitis, a contagious viral respiratory tract infection that results in high mortality and severe losses in egg production in infected poultry flocks
Since it was first described in the UK in the late 1980s [22], avian leukosis virus subgroup J (ALV-J) has been primarily associated with myeloid leukosis in meat-type chickens and caused more serious damage than any other subgroup worldwide
Summary
MicroRNAs (miRNAs) are ~22-nucleotide small RNA molecules that profoundly affect gene expression by directing repressive protein complexes to the untranslated region (UTR) of target messenger RNA (mRNA) transcripts. Out of over 24,000 miRNAs identified until now, 295 are encoded by viruses (miRBase v.20). DNA viruses encode most of the virus-encoded miRNAs, with members of the family Herpesviridae accounting for the vast majority demonstrating the significance of miRNA-mediated gene regulation in the biology of herpesvirus infections. Virus-host interactions in herpesviruses are characterized by long-term survival as latent infections in different cell types. This demands sophisticated methods of survival without being detected by the innate and adaptive immune mechanisms of the host. The small size of the miRNAs, combined with their ability for specific repression of the expression of multiple transcript targets, make them ideal tools for herpesviruses to reshape the gene expression in an infected cell to favor viral replication. MiRNA encoded by avian leukosis virus subgroup J (ALV-J) using the canonical miRNA biogenesis pathway [13] showed that retroviruses exploit the miRNA pathway
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