Viral determinants of resistance to treatment in patients with hepatitis C

Abstract

Chronic hepatitis C virus (HCV) infection affects more than 170 million persons worldwide and is responsible for the development of liver cirrhosis in many cases. Standard treatment with pegylated alpha interferon (IFN-alpha) in combination with the nucleoside analogue ribavirin leads to a sustained virologic response in approximately half of the patients. IFN-alpha is classified as an indirect treatment, as it interacts with the host's immune response. The mechanism of action of ribavirin is still unknown. The benefit of triple therapy by adding other antiviral agents, e.g., amantadine, is controversial. Currently, new direct antiviral drugs (HCV protease/polymerase inhibitors) are being evaluated in phase 1/phase 2 trials. Phenotypic resistance to antiviral therapy has been attributed to amino acid variations within distinct regions of the HCV polyprotein. While sensitivity to IFN-alpha-based antiviral therapy in vivo is clearly correlated with the number of mutations within the HCV NS5A protein, the underlying functional mechanisms for this association are unknown. In turn, in vitro, several mechanisms to circumvent the host immune defense or to block treatment-induced antiviral activities have been described for different HCV proteins. By the introduction of direct antiviral drugs, hepatitis C therapy now is entering a new era in which the development of resistance may become the most important parameter for treatment success or failure.

FIG. 1.

Potential strategies of the different HCV proteins to antagonize IFN therapy. (A) IFN resistance through inhibition of RIG-I and TLR3 by the HCV NS3/4A protein. Viral RNA binding to RIG-I and TLR3 leads to the activation of IRF3 and NF-κB. Both transcription factors enter the nucleus and bind to target genes, resulting in the expression of antiviral defense proteins. The NS3/4A protein disrupts the activation of IRF3 and NF-κB by the inactivation of Cardif, which is an adaptor protein in the RIG-I antiviral pathway, and through inhibition of Trif, an adaptor molecule in the TLR3 signaling pathway. (B) Disruption of the IFN-mediated Jak/STAT pathway. IFN binding to its receptor triggers the autophosphorylation of tyrosine residues through the Tyk2 and Jak1 kinases, which function as docking sites for STAT1 and STAT2. The subsequent phosphorylation of STAT1 and STAT2 promotes the formation of heterodimers between STAT1 and STAT2 and the assembly of the trimeric ISGF3 complex. After translocation of the ISGF3 complex to the cell nucleus, it binds to the IFN-stimulated response element (ISRE) in ISG promoters, leading to the expression of IFN effector proteins. The HCV core protein induces the expression of SOCs, which block Jak/STAT signaling through the IFN-α/β receptor. The HCV NS5A protein induces the expression of IL-8, which blocks the assembly and function of the ISGF3 complex. The expression of the whole HCV polyprotein inhibits STAT1 function through elevated levels of PP2A, which is associated with an increased binding of STAT1 to PIAS and a reduced activation of ISGs. (C) IFN resistance through inhibition of PKR by the HCV E2 and HCV NS5A proteins. After IFN-α binding, autophosphorylation of PKR takes place, leading to the inhibition of the phosphorylation of eIF-2α, which is responsible for the initiation of translation of cellular and viral proteins by facilitating Met-tRNA binding to the 40S ribosomal subunit. HCV E2 and HCV NS5A antagonize the effects of IFN-α by the inhibition of PKR.

FIG. 2.

Different proposed antiviral mechanisms of ribavirin. (A) Enhancement of the host adaptive antiviral immune response. Ribavirin modulates the balance of T-helper 1 (T_H1) and T_H2 responses by enhancing T_H1 and inhibiting T_H2 cytokine production. (B) Inhibition of host IMPDH. Intracellular ribavirin is converted by cellular kinases to ribavirin monophosphate (RMP), ribavirin diphosphate (RDP), and ribavirin triphosphate (RTP). Ribavirin monophosphate is a competitive inhibitor of IMPDH, which leads to the depletion of the intracellular GTP pool necessary for viral RNA synthesis. (C) Direct inhibition of HCV NS5B RdRp. The phosphorylated form of ribavirin, ribavirin triphosphate, binds the nucleotide-binding site of polymerases, thereby competitively inhibiting viral replication. Moreover, ribavirin is utilized and incorporated by viral RNA polymerases including the HCV RdRp opposite cytosine or uridine, which can result in a significant block to RNA elongation. (D) RNA virus mutagenesis and error catastrophe. Ribavirin acts as an RNA virus mutagen, thereby leading to an increased mutational frequency that exceeds the mutational threshold of viral fitness and drives RNA viruses into lethal mutagenesis and error catastrophe. CTL, cytotoxic T lymphocyte. TNF-α, tumor necrosis factor alpha.