Understanding the hepatitis C virus life cycle paves the way for highly effective therapies

Abstract

More than two decades of intense research has provided a detailed understanding of hepatitis C virus (HCV), which chronically infects 2% of the world's population. This effort has paved the way for the development of antiviral compounds to spare patients from life-threatening liver disease. An exciting new era in HCV therapy dawned with the recent approval of two viral protease inhibitors, used in combination with pegylated interferon-α and ribavirin; however, this is just the beginning. Multiple classes of antivirals with distinct targets promise highly efficient combinations, and interferon-free regimens with short treatment duration and fewer side effects are the future of HCV therapy. Ongoing and future trials will determine the best antiviral combinations and whether the current seemingly rich pipeline is sufficient for successful treatment of all patients in the face of major challenges, such as HCV diversity, viral resistance, the influence of host genetics, advanced liver disease and other co-morbidities.

Figure 1

Timeline of the milestones in HCV functional and antiviral research. Major developments in natural history, virology and model systems, direct-acting antivirals (DAA) development, host-targeting agents (HTA) development and clinical implementations are indicated. Immediate implications of breakthroughs in basic research for inhibitor development and patient therapy are indicated with horizontal arrows.

Figure 2

The HCV genome and polyprotein processing. The HCV RNA genome (top) contains one long ORF (blue) flanked by 5′ and 3′ UTRs (red). The binding of two copies of miR-122 (green) to the 5′ UTR is highlighted in the inset. IRES-mediated translation of the ORF leads to a polyprotein (bottom) that is co- and post-translationally processed into ten viral proteins. The maturation process of the core protein involves a cellular signa peptide peptidase cleavage of a C-terminal signal peptide (white triangle) and cleavage from E1 by the cellular signal peptidase, which also cleaves E1, E2 and p7 from the polyprotein (gray triangles). In an autocleavage mechanism requiring two identical molecules to make up the composite active site, the NS2-NS3 protease cleaves itself (red triangle). The NS3 protease located in the first one-third of NS3 (ref. 165), assisted by its membrane-bound cofactor, NS4A, cleaves the remaining proteins NS3, NS4A, NS4B, NS5A and NS5B (green triangles). Glycosylation of the envelope proteins (black dots) and the functions of the individual HCV proteins are indicated.

Figure 3

The HCV life cycle and points of intervention. Points of intervention in the HCV life cycle are marked with numbered circles, and types of inhibitors of the individual steps are indicated in the legend. Interaction of extracellular HCV LVPs (1) with cellular surface receptors initiates the entry process (2), which can also occur from direct cell-to-cell transmission. After pH-dependent fusion and uncoating, the incoming HCV genome is translated and the resulting polyprotein processed (bottom inset and Fig. 2, (3)). Replication takes place in ER-derived membrane spherules (membranous web, bottom right inset, (4)), the architecture of which remains to be fully defined. The spatiotemporal contribution of miR-122 binding to the HCV genome is not yet fully understood, and miR-122 presence is indicated with ‘?’. In the assembly and release process (top right inset, (5)), core protein is transferred from cLDs to form nucleocapsids that, assisted by NS5A, are loaded with RNA. Replicase proteins supposedly bind HCV RNA during transfer from replication to packaging, the intracellular sites of which might converge. It is not clear whether the RNA is transiently located on the cLD. The p7, NS2 and NS3-NS4A proteins are also involved in coordination of assembly. HCV virion morphogenesis is coupled to the VLDL pathway, and particles are produced as LVPs. Particles released from cell culture have less ApoB association and resemble the ApoB-deficient particle illustrated. LVPs also associate with ApoC, which for simplicity is not shown here. EphA2, ephrin receptor type A2; GAG, glycosaminoglycans; PL, phospholipids; TG, triglycerides. In the translation inset, NS5A is shown as a dimer, though several other HCV proteins also form homo- or heterodimers or oligomers. For comprehensive discussion and illustration of HCV assembly and release and of membrane-associated HCV protein structures, see refs. 25,, respectively.