Inhibition of Monkeypox virus replication and application to RNAi control of viral diseases

The use of RNAi pathway as a new antiviral approach is promising because viruses have relatively small genomes with a limited number of targetable genes. Recent studies utilized RNAi to silence specific viral genes. This allows not only a precise identification of the function of viral genes, but also to inhibit viral replication.

http://www.virologyj.com/content/6/1/188

The following article examined how RNAi could be used to inhibit the activity of Monkeypox virus (MPV), a member of the Poxviridae family comprising several other human pathogens such as cowpox (CPXV), Vaccinia (VACV) and Variola (VARV). These pathogens have been known to cause human diseases with various severity, ranging from mild conditions to death in the most severe cases. Monkeypox virus is believed to have been circulating for a long time in host animals, but only recently has person-to-person transmission been reported. Monkeypox disease manifestation is similar that of smallpox but MPV is less lethal to the host. Treatment against poxviruses may become a priority as most of the young population is completely immune naïve and a natural or deliberate release of these viruses constitutes a serious threat to public health. Although antiviral drugs such as Cidofovir exist, their effectiveness may erode with emergence of resistant viral strain and limiting side effects. The need for novel therapeutic strategies against such viruses are paramount, and RNAi could prove to be at the forefront in solving this challenge.

In their study, the researchers selected 12 viral genes based on genome-wide expression studies and developed numerous siRNA constructs. These constructs were evaluated for their ability to inhibit viral replication in vitro. The inhibition of viral replication varied significantly based on the targeted gene and the concentration of siRNA used. The siRNA constructs targeting the respective essential genes for viral replication (A6R) and viral entry (E8L) inhibited viral replication in cell culture by up to 95% without any cytotoxic side effects. While both constructs showed increasing antiviral activity with concentration, A6R constructs showed complete inhibition at lower concentration. This emphasized the central role of the A6R gene for viral replication.
Also of importance is the time scale of their usefulness. Effectiveness of RNAi occurs for only short periods of time mainly due to the relatively short half life of siRNA and the lack of an amplification mechanism in mammalian cells. It appears that the siA6 constructs remained active and kept viral replication rate at less than half its value for over a week, indicating the unusual stability of these constructs in vitro.
Through these observations, siA6 appears to a good candidate for the treatment of Monkeypox. Successful siRNA application in the treatment of diseases depends mainly on the two parameters analyzed above, namely stability of the siRNA and efficacy against a high viral burden.

This study illustrated the great disparity in siRNA efficacy among different targeted gene pools. Because the exact mechanisms for this disparity are unknown, identifying the effectiveness of an siRNA empirically remains necessary in order to assess the therapeutic application of RNAi. The authors also observed a variation in the efficacy of siRNA targeting for the same gene pool, most likely due to intra-cellular mechanisms altering the targeted RNA sequences and preventing the RNAi complex from identifying its viral target.

The identification of stable and efficacious siRNA probes in the inhibition of viral activity is a first and decisive step in moving towards therapeutic applications of RNAi for viral diseases. The advantages over traditional, chemically synthesized antiviral drugs are numerous: not only would RNAi allow for precise and targeted inhibition of viral growth, but also the flexibility to readapt siRNA molecules to target emerging resistant viral strains. Although more research must be conducted on the in vitro efficacy and stability of siRNA constructs before they can be applied to human subjects, RNAi therapy for viral disease control shows much promise.