The present study shows that a common variant in the human bile salt export pump ABCB11 is overrepresented in patients with HCV infection. The reason underlying this observation is speculative, although it might be caused by differences in spontaneous recovery from infection and sustained virological response (SVR) rates, as was observed in patients bearing HCV genotypes 2 and 3 in a previous study: Iwata et al.  were able to show that the effect might be mediated by serum bile salt concentrations, with low levels (< 8 μM) being associated with SVR in their cohort. This is not the first time that bile salts or genes involved in their metabolism have been implicated in the pathogenesis of HCV infection. Ursodeoxycholic acid has been previously proven in various trials to be beneficial in patients suffering from liver damage due to chronic infection, albeit not via modulation of SVR[27–29]. Contrastingly, increased serum levels of bile salts are associated with negative outcomes in HCV therapy . The observed association between a polymorphism that impinges bile salt transport from the hepatocyte into the canaliculus offers an explanation of this conundrum: Viral replication might be boosted inside the hepatocyte by relatively high bile salt concentrations . A variant of the bile salt export pump with lower expression level resulting in reduced transport activity, such as the common [C] allele of c.1331 T > C, might lead to transient increases in hepatocellular bile salt concentration, depending on the efficiency of the sinusoidal import.
Ursodeoxycholic acid as rather weak FXR agonist is known to increase the expression and activity of bile salt transporters by transcriptional as well as multiple post-transcriptional mechanisms and to contribute to SHP-mediated inhibition of bile salt synthesis [31, 32]. In vitro experiments using HCV replicon-harboring cells have shown that the impact of bile salts on HCV replication is mediated by the action of FXR rather than via direct effects. FXR antagonisation by guggulsterone blocked the bile salt-induced up-regulation of virus replication; furthermore, guggulsterone inhibited basal levels of HCV replication [30, 33]. Hence, it is possible that HCV "hijacks" or uses transcriptional activation via FXR. Whatever the mode of action by which HCV takes advantage of FXR signalling, it appears to be advantageous for the hepatotropic virus to use central functions of hepatocytes for its own end.
HCV replication is tightly linked to lipid metabolism, and bile salts are crucially involved in fat absorption and transport. This reinforces the key roles of bile salt responsive regulatory molecules such as FXR and TGR5 as potential host factors modulating HCV replication, not necessarily through direct effect of bile salts but via feedback regulation of lipid metabolism. In infected cells, HCV core protein accumulates on the surface of lipid droplets [34, 35]. Other functional proteins, such as RNA binding replicase NS5A, are targeted to lipid droplets, and this targeting appears to be crucial for HCV assembly . A recent combined transcriptome and proteome analysis revealed the impact of HCV infection on previously unknown metabolic pathways, particularly an increase in cellular cholesterol and free fatty acid levels . One possible reason for this "metabolic reprogramming" might be the dependency of HCV replication on lipid metabolism as outlined above. Considering the central role of ABCB11 in bile flow, it is not inconceivable to conclude that common variants of this transporter are associated with variations in plasma cholesterol levels. In fact, a meta-analysis of genome-wide association scans supports this reasoning , thus providing further evidence for another potential connection between bile salt transport and HCV replication. The results from our study are indicative of a role for the major allele of the ABCB11 c.1331 variant in conveying susceptibility to persistent HCV infection, enhancing this link.
Iwata and colleagues observed an overrepresentation of the [C] allele at ABCB11 c.1331 in HCV patients (n = 206) compared with controls (n = 110) in the Swiss Hepatitits C cohort (allelic frequency = 62.9%), although no significant difference in median bile salt serum concentration could be detected between bearers of homozygous [C] or [T] alleles . However, a comparison of fibrosis scores from 178 patients revealed a skewed distribution: Carriers of homozygous [C] allele were overrepresented among cirrhotic patients, and the [CC] genotype was an independent risk factor for cirrhosis in multivariate analysis (p = 0.010).
Our investigations into a connection between ABCB11 c.1331 T > C and liver stiffness as measured by TE (Fibroscan) did not reveal any significant associations. This may be due to methodolocial differences in assessing fibrosis/cirrhosis [25, 26]. Scatterplot analysis of TE results from the different genotypes seems to indicate a prevalence of lower values in patients bearing the [TT] genotype (Figure 1), which would be in agreement with the data from the Swiss cohort, but the numbers of homozygous carriers of the minor allele in our cohort were too low to reach statistical significance.
Cirrhosis was significantly associated with the age of patients (p = 0.006), which may reflect longer duration of chronic HCV infection. However, since the exact time point of infection was unknown in the majority of our patients, we used age as surrogate marker in our statistical analyses.
Interestingly, when comparing allelic distribution in our cohort with figures from the Swiss Hepatitis C cohort , ABCB11 c.1331 T > C is equally skewed towards an overrepresentation of the [C] allele among British patients with non-alcoholic fatty liver disease in the study by Iwata et al., pointing towards a potential role of ABCB11 via its impact on lipid metabolism rather than viral replication. The development of diet-induced obesity and hypercholesterolemia following hepatocye-specific overexpression of ABCB11 in mice is a pointer in the same direction .
Based on the assumption that populations from both the Swiss study and our cohort are from a similar ethnic background, we speculate that the major [C] allele of ABCB11 c.1331 T > C might be a high-frequency low-risk contributor towards susceptibility for various complex liver diseases triggered by external stimuli and involving lipid metabolism .