Trends in Molecular Medicine
ReviewHepatitis C: molecular virology and antiviral targets
Section snippets
Genetic organization
HCV contains a single-stranded RNA genome of positive polarity and ∼9600 nucleotides length. The viral genome is composed of a 5′ noncoding region (5′ NCR), a long open reading frame (ORF) encoding a polyprotein precursor of ∼3000 amino acids, and a 3′ NCR (Fig. 1). Functional (infectious) cDNA clones have been assembled for genotypes 1a [8], 1b and 2a (see Box 1) (reviewed in Ref. [12]).
The 5′ NCR is highly conserved among different HCV isolates and contains an internal ribosomal entry site
Polyprotein processing
Translation of the HCV ORF yields a polyprotein precursor that is co- and post-translationally processed by cellular and viral proteases into the mature structural and nonstructural proteins (Fig. 1). The structural proteins include the core protein and the envelope glycoproteins E1 and E2. The structural proteins are released from the polyprotein precursor by the endoplasmic reticulum (ER) signal peptidase. The nonstructural proteins NS2–NS5B include the NS2–3 autoprotease, which cleaves at
Core
The first structural protein encoded by the HCV ORF is the highly basic, RNA-binding core protein which presumably forms the viral nucleocapsid. Intriguingly, the core protein has been reported to interact with a variety of cellular proteins and to influence numerous host cell functions [16]. However, the relevance of these observations, derived mainly from heterologous overexpression, for the natural course and pathogenesis of hepatitis C is presently unknown.
Envelope glycoproteins
The envelope proteins E1 and E2
NS2–3 autoprotease
Cleavage of the polyprotein precursor at the NS2–NS3 junction is accomplished in cis by an autoprotease encoded by NS2 and the N-terminal one-third of NS3. NS2–3 protease activity is essential for the replication of full-length HCV genomes in vivo [36]. However, NS2 is dispensable for replication of subgenomic replicons in vitro (see below). Recombinant proteins lacking the N-terminal membrane domain of NS2 have been found to retain cleavage activity and may allow further examination of this
Viral dynamics
HCV infection is a highly dynamic process with a viral half-life of only a few hours and average daily virion production and clearance rates of up to more than 1012 [52]. This high replicative activity, together with the lack of a proof-reading function of the viral RdRp, provides the basis for the genetic variability of HCV. In addition, these findings are similar to the dynamics of HIV infection and provide a rationale for the development and implementation of combination antiviral therapies.
Model systems
Given the lack of a robust cell culture system allowing natural infection, replication and release of viral progeny, various in vitro and in vivo models have been used to study HCV (see Box 2) (reviewed in Ref. [12]).
Evolving therapeutic strategies and vaccine development
The presumed life cycle of HCV includes (1) binding to an as yet unidentified cell-surface receptor and internalization into the host cell, (2) cytoplasmic release and uncoating of the viral RNA genome, (3) IRES-mediated translation, (4) polyprotein processing by cellular and viral proteases, (5) RNA replication, (6) packaging and assembly, (7) virion maturation and (8) release from the host cell (Fig. 5). In principle, each of these steps represents a target for antiviral intervention.
Concluding remarks
Recent progress in the molecular virology of hepatitis C has allowed the identification of novel antiviral targets and therapeutic strategies. These may complement existing therapeutic modalities in the near future. Much work remains to be done to elucidate the HCV virion structure, the early and late steps of the viral life cycle, the mechanism and regulation of RNA replication, and the pathogenesis of HCV-induced liver disease. Ultimately, an improved understanding of the viral life cycle
Acknowledgements
Research in the authors’ laboratory is supported by the Deutsche Forschungsgemeinschaft (Mo 799/1–3), the Bundesministerium für Bildung und Forschung (01 KI 9951) and the European Commission (QLK2-CT1999–00356).
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