Polymorphisms in melanoma differentiation-associated gene 5 link protein function to clearance of hepatitis C virus.

UNLABELLED: Among patients newly infected with hepatitis C virus (HCV), only 20-30% clear the infection spontaneously. In the remaining 70% the infection persists, causing chronic liver inflammation and disease. It is well established that polymorphisms in host genes, especially in components of the innate immune response, contribute to the phenomenon of spontaneous HCV clearance. Retinoic acid inducible gene-I (RIG-I)-like helicases such as melanoma differentiation-associated gene 5 (MDA-5) are cytoplasmic sensors of viral RNA that are critical for triggering innate immune responses after infection with RNA viruses. We analyzed 14 nonsynonymous single-nucleotide polymorphisms in RIG-I-like helicase-pathway-genes comparing European patients who spontaneously cleared HCV (n = 285) or had persistent infection (n = 509). We found that polymorphic haplotypes in the MDA-5 gene IFIH1 encoding histidine at position 843 and threonine at position 946 strongly correlate with the resolution of HCV infection (odds ratio [OR]: 16.23; 95% confidence interval [CI]: 3.67-71.87; P = 1.1 × 10(-6) ). Overexpression of MDA-5 genetic variants in HEK 293 cells and in a tissue culture model of HCV infection revealed that the histidine 843/threonine 946 variant leads to increased baseline and ligand-induced expression of interferon-induced genes and confers an increased ability to suppress HCV replication. CONCLUSION: These data suggest that MDA-5 plays a significant role in the defense against HCV and that polymorphisms in MDA-5 can influence the outcome of HCV infection.

The cellular RNA helicase UAP56 is required for prevention of double-stranded RNA formation during influenza A virus infection.

The cellular DEAD box RNA helicase UAP56 plays a pivotal role in the efficient transcription/replication of influenza A virus. UAP56 is recruited by the nucleoprotein (NP) of influenza A viruses, and recent data revealed that the RNA helicase is required for the nuclear export of a subset of spliced and unspliced viral mRNAs. The fact that influenza viruses do not produce detectable amounts of double-stranded RNA (dsRNA) intermediates during transcription/replication suggests the involvement of cellular RNA helicases. Hence, we examined whether the RNA-unwinding activity of UAP56 or its paralog URH49 plays a role in preventing the accumulation of dsRNA during infection. First, our data showed that not only UAP56 but also its paralog URH49 can interact with NPs of avian and human influenza A viruses. The small interfering RNA (siRNA)-mediated depletion of either RNA helicase reduced the transport of M1 and hemagglutinin (HA) mRNAs and, to a lesser extent, NP and NS1 mRNAs into the cytoplasm. Moreover, we found that virus infection of UAP56-depleted cells leads to the rapid accumulation of dsRNA in the perinuclear region. In parallel, we observed a robust virus-mediated activation of dsRNA-dependent protein kinase R (PKR), indicating that the cellular RNA helicase UAP56 may be recruited by influenza virus to prevent dsRNA formation. The accumulation of dsRNA was blocked when actinomycin D or cycloheximide was used to inhibit viral transcription/replication or translation, respectively. In summary, we demonstrate that UAP56 is utilized by influenza A viruses to prevent the formation of dsRNA and, hence, the activation of the innate immune response.