How Retroviruses Escape the Nonsense-Mediated mRNA Decay

Many posttranscriptional processes are known to regulate gene expression and some of them can act as an antiviral barrier. The nonsense-mediated mRNA decay (NMD) was first identified as an mRNA quality control pathway that triggers rapid decay of mRNA containing premature stop codons due to mutations. NMD is now considered as a general posttranscriptional regulation pathway controlling the expression of a large set of cellular genes. In addition to premature stop codons, many other features including alternative splicing, 5' uORF, long 3' UTR, selenocystein codons, and frameshift are able to promote NMD. Interestingly, many viral mRNAs exhibit some of these features suggesting that virus expression and replication might be sensitive to NMD. Several studies, including recent ones, have shown that this is the case for retroviruses; however, it also appears that retroviruses have developed strategies to overcome NMD in order to protect their genome and ensure a true expression of their genes. As a consequence of NMD inhibition, these viruses also affect the expression of host genes that are prone to NMD, and therefore can potentially trigger pathological effects on infected cells. Here, we review recent studies supporting this newly uncovered function of the NMD pathway as a defense barrier that viruses must overcome in order to replicate.