The present study examined the genetic association of HCV infection under PEG-IFNα-RBV therapy on a responsive gene, STAT6, and responsiveness of treatment. We confirmed that this gene, located on chromosome 12, is a novel candidate susceptibility gene. The results showed that rs1059513 at the 3′ UTR of STAT6 may be associated with SVR rates under SOC treatment. Therapeutic outcome association studies showed that the presence of the GGGC haplotype was significantly associated with a higher rate of SVR (OR = 1.06–4.89) in HCV-1 infected but not HCV-2 infected populations. By using multivariate logistic regression analyses, we observed that the GGGC haplotype, age of entry, and viral load may act interactively to influence the therapeutic outcomes. This result suggests that the SVR rates of PEG-IFNα-RBV treatment could, at least partially, depend on STAT6 activation.
Thus far, the standard treatment regimen for chronic hepatitis C infection has been a combination of PEG-IFNα and RBV . About 50% of treatment-naïve populations will typically achieve SVR by using this regimen . The SVR rate is markedly higher in HCV-2/3 infected patients than in HCV-1 infected patients (~ 85% versus 40%) . In order to identify the biomarkers that distinguish the profiles of HCV therapeutic responsiveness, Younossi and colleagues  collected RNA transcripts from peripheral blood mononuclear cells (PBMCs) of 68 HCV-infected patients under PEG-IFNα-RBV treatment. After preparing expression profiles of patients under this treatment, the investigators observed that genetic expression of STAT6 and cytokine signaling 1 are putative predictive markers that correlate with SVR . In support of this finding, we also observed a correlation between STAT6 genetic polymorphisms and SVR.
During viral infections, innate immunity remains the most important first line of defense in the body. Viral nucleic acids are detected by endosomal Toll-like receptors (TLRs) and cytoplasmic RIG-I–like receptors (RLRs), which subsequently induce the production of type 1 IFN to inhibit viral replication . Consequently, many cytokines (including type 1 IFNs) trigger the IFN-stimulated gene expression cascade through the canonical JAK-STAT signaling pathway . Previous studies have shown that STAT6 activation is ubiquitously detected during viral infections, concluding that STAT6 may play a fundamental role in the defense mechanism against viral infections . Viral load, age of entry into the study, and BMI were not significantly different among the STAT6 genotypes of our enrolled populations. Thus, it is unlikely that specific STAT6 genotypes predispose individuals to infection with HCV or contribute to the process of spontaneous viral elimination.
Studies using a microarray platform to examine the differential gene expression profiles of IL-4–stimulated B cells between wild-type and Stat6−/− mice were recently performed . Seventy known genes were found to be differentially expressed with significant discrepancies between the 2 mouse groups, which were all negatively or positively regulated by STAT6 via a palindromic consensus sequence TTC(N)2-4GAA . In addition, STAT6 remains a critical determinant in the polarization and differentiation process of T-helper type 2 (Th2) cells in a Stat6 knockout mouse model . The expression levels of Th2-related cytokines (including IL-4, IL-5, and IL-13) were diminished in this mouse model . Moreover, STAT6 may trigger a Th1 response by enhancing the production of IL-12 through the inhibition of IL-10 production in dendritic cells . Therefore, STAT6 is also a critical factor in the homeostasis of inflammatory and hypersensitivity immune responses other than those associated with viral immunity.
Several reports have provided strong evidence that patients infected with HCV-1 will have approximately a 50% probability (in Caucasians) or 80% probability (in African Americans) of a poor response toward PEG-IFNα-RBV treatment . Although viral clearance rates have been strongly associated with clinical features of HCV, for example: gender, age < 40 years, low HCV RNA levels pre-treatment, in the absence of liver cirrhosis, and HCV genotypes 2/3 . Table 7 lists 11 patients with low viremia. Fried et al. concluded that HCV RNA level is one of the main factors affecting treatment outcome; therefore, we performed multivariate analysis to check whether HCV RNA level is an independent factor of treatment outcome. We first determined the HCV RNA baseline without categorizing the patients; however, in this analysis, P = 0.182 and OR = 1.00. This result led us to conclude that it is not the best model for prediction of treatment outcome. Therefore, we categorized the HCV RNA level according to the description of Fujimoto et al., with low HCV RNA level <100 KIU/ml by Amplicore-monitor assay and performed statistical analysis, and the results for this analysis are more obviously (P = 0.050, OR = 8.11).
Many investigators continue to monitor the host genetic factors that may relate to clinical outcomes to provide custom-made therapy for HCV infection. In our study, the overall achievement response rates of SVR were less than 62% and 92% in the HCV-1 and HCV-2 populations, respectively. Thus, a reliable prediction of a subject as a potential non-viral responder (NVR) at the start of treatment would allow the clinician to make appropriate, evidence-based decisions on the compatibility of future treatment, thereby reducing side effects and/or the cost of treatment.
The contributions of host genetic factors to HCV-2/3 clearance are relatively small compared with HCV-1 clearance, because the former is more likely to be eliminated by SOC therapy . In the present study, we observed an association between the STAT6 haplotype GGGC and the rate of SVR, although its statistical significance in the HCV-1 infected populations based on estimation by multivariate logistic regression was marginal. The frequency of the GGGC allele in our population was small, whether the frequency of this haplotype is significant in other ethnic groups remains unknown. We propose that this STAT6 haplotype GGGC might play a role in the outcome of PEG-IFNα-RBV therapy (adjusted r2 = 11.2%). The linkage among the STAT6 polymorphisms and SVR should be confirmed in future studies with larger sample sizes. Moreover, the significance of the genetic effects of STAT6 on other racial and ethnic groups remains to be elucidated.
The mechanisms of action of the aforementioned 4 SNPs located within the 3′ UTR and intronic regions of STAT6 gene are hampered by the lack of published functional studies on these polymorphisms and warrants further investigation at the molecular level.