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Authors: Franziska Hoffmann, Stefan Endres, Simon Rothenfusser
Journal: Hepatology (2015)

Potential conflict of interest: Nothing to report. Author names in bold designate shared co‐first authorship. We thank Pedergnana et al. and Vergara et al. for their interest in our study1 and for sharing genetic data on the described melanoma differentiation‐associated gene 5 (MDA5) single‐nucleotide polymorphisms (SNPs) in their Egyptian and European/American hepatitis C cohorts. The combination of genetic and functional data we presented in our article provided evidence that MDA5 is involved in the innate immune response to hepatitis C viral (HCV) infection and that this involvement is important to restrict HCV infection. The importance of MDA5 for sensing HCV and its ability to restrict HCV replication have been confirmed meanwhile by two further studies that showed that loss of function of MDA5 in HUH7 and HepG2 cells leads to increased replication of HCV.2 Pedergnana et al., studying an Egyptian cohort, confirm our association of the T allele at rs3747517 coding for histidine at 843 with increased clearance of HCV. In addition, they found a significant association with the C allele of rs1990760 coding for alanine at 946 that we had only seen as a trend. In contrast, no such association is seen in the larger cohort of Vergara et al. A higher homogeneity concerning the infecting agent (99% genotype 4) and the genetic background (all from the same village in the Nile delta) of the Egyptian cohort might account for its higher sensitivity compared to the American/European and our European cohorts that are likely to be more heterogeneous in both aspects (e.g., infected by HCV genotypes 1b/a and 3). Both cohorts confirm the strong linkage disequilibrium of rs3747517 and rs1990760 and an underrepresentation of the haplotype coding for histidine at position 843 and threonine at 946. As neither the American/European nor the Egyptian cohort contains this haplotype at all and the 1000 Genome database just contains one heterozygous individual for this haplotype,4 we have to acknowledge that our data, due to confounder effects, might have overestimated the presence of this haplotype. The strong activity of this variant that we could demonstrate in our functional assays, even though possibly beneficial in the case of HCV infection, seems too dangerous in other aspects like autoimmunity; and therefore, this haplotype seems to have been evolutionarily purged from the population to a large extent. Our functional assays make clear that the context of surrounding amino acids influences the outcome of the rs1990760 SNP coding for amino acid 946 in MDA5: looking at the overactive H843/T946 variant, we found that in the context of histidine at 843, having threonine at 946 results in a considerably more active MDA5 molecule than having alanine at 946. However, Vergara et al., in their genetic data, compare threonine versus alanine at 946 in the context of arginine at 843 (TC or CC versus CT). In doing so they find a significant difference and conclude that alanine at 946 leads to the more protective (or active) molecule. In our cohort the genetic haplotype comparison—TC or CC versus CT for rs3747517 and rs1990760—had not shown a significant difference. However, our experimental data in Figs. 2B and 3C,D1 show a similar, even though small, tendency of H843/A946 and R843/A946 being more active than R843/T946, thereby backing the genetic result of Vergara et al. We conclude that MDA5 indeed plays an important role in HCV infection. The common SNP haplotypes at rs3747517 and rs1990760, however, confer only minor functional differences to the MDA5 molecule. Therefore, SNP association studies will only show an association with the outcome of HCV in cohorts with strong homogeneity in both genetic background and infecting virus. The haplotype coding for the overactive H843/T946 variant of MDA‐5 appears to be evolutionarily under pressure and less frequent than expected from our own results.