Hepatitis C: virological aspects and practical implications

Abstract

.Other important pathogens for humans, such as thedengue virus and the yellow fever virus, belong to this family,as do infectious agents in animals, such as the bovine viraldiarrhea virus and the classical swine flu virus.The shape of HCV is spherical (50 nm in diameter), and ithas an enveloped nucleocapsid and a single-stranded RNAgenome of positive polarity. The HCV genome, withapproximately 10,000 nucleotides, consists of a single longopen reading frame (ORF) flanked by two noncoding regions(NCRs) at the initial 5’ end, at which the internal ribosomeentry segment (IRES) is located, and at the 3’ end. Itstranslation yields a large polyprotein (with nearly 3000 aminoacids) that is processed by viral and host cell proteases intostructural proteins, including the core, envelope 1 (E1), E2,and p7 proteins, as well as the nonstructural proteins NS2,NS3, NS4, and NS5. Recently, an alternate reading frame thatcodifies an F protein with more than 160 amino acids has beenidentified. However, its expression in natural HCV infectionhas not been confirmed.Structural proteins are cleaved by enzymes of theparasitized cell. Envelope proteins are extensively glycosylatedand are involved in the binding with receptors as well as inthe entrance and fusion of the virus. The function of p7 proteinremains unknown. Nonstructural proteins, initially NS2 andsubsequently NS3, undergo self-cleavage and positionthemselves in transmembrane domains across the host cellmembrane and into the cytosol or lumen (Figure 1).Unlike that of the hepatitis B virus (HBV), the HCV genomedoes not invade the infected cell nucleus. After the bindingthrough receptors (CD81, a tetraspanin, and the low densitylipoprotein receptor), the HCV genome acts directly as anmRNA in the cytoplasm, where the translation is initiatedthrough the IRES in the 5’ NCR. The protein produced issubsequently processed by the cell enzymes and by theenzymes within the virus itself, yielding structural andnonstructural proteins. After synthesis and maturation, thesenonstructural proteins and the RNA form replication complexesthat combine with the membrane and catalyze the translationof intermediate negative strands of RNA, from which positive-strand progeny are generated. The genomic RNA and capsidproteins unite, forming the nucleocapsid, which is transportedin cytoplasmic vesicles. While passing through the Golgicomplex, these vesicles assemble with the other particles andundergo exocytosis and cell release (Figure 2).The study of the HCV genome, even in samples obtainedfrom a single individual, reveals great heterogeneity amongthe HCV genotypes. Genotypes in which multiple mutantscoexist have been designated quasispecies. The multiplemutations represent a rapid and very efficient mechanism forthe virus to evade the immune response and persist in thehost. The selection process and the process of adaptationto the host have led to the evolution to different HCVgenotypes. The classification system most commonly usedis that proposed by Simmonds et al. and is based on thesimilarity of the sequence of nucleotides using the followingcriteria: similarity lower than 72% characterizes a newgenotype; similarity between 75 and 86% characterizes anew subgenotype. There are 6 genotypes, which arenumbered from 1 to 6, with subgenotypes 1a, 1b, 1c, 2a, 2b,2c, 3a, 3b, 4a, 5a, and 6a. Although the criterion is based onmolecular biology, this classification has practical,pathogenetic, epidemiological, and treatment-relatedimplications. Therefore, subgenotype 1a is more prevalentin the USA, 1b in Japan, 3a in Scotland, and 4a in Egypt/Zaire. In Brazil, genotype 1 is found in approximately 60% ofthe patients, followed by genotype 3, which is found in 20 to30%, and genotype 2, which is found in a lower percentage.Subgenotype 1b can cause severe forms of the infectionand, similar to genotype 4, does not respond as well totreatment with IFN- α. Therefore, genotypes 1 and 4 shouldbe treated for 48 weeks.The important advances in the knowledge of viral hepatitisB and C are a consequence of some facts that will be discussedherein.The Use of Chimpanzees as a Model for the Study of ViralHepatitisAlthough chimpanzees are not natural hosts for theseviruses, they reproduce the disease, thus allowing importantdiscoveries:• Epidemiological studies: knowledge of these diseasesas communicable;• Infectivity and titers of pools of HCV and HBV,obtained from the infected animals;• Infectivity of the molecular clones of HCV andimportance of genetic elements specific to HCV;• Neutralization capacity of antibodies specific for HBVand HCV;• Protective immunity, tested through re-exposure to theviruses;• Mutants that escape to humoral and cellular immunitycould be recognized.However, the use of nonhuman primates has advantagesand disadvantages.AdvantagesThe only animal susceptible to the acute and chronic formsof the disease; non-selected population; and sequentialbiopsies.