Genetic Variation and HCV Genotyping
Even in a single infected individual, HCV does not exist as a homogeneous
species. Heterogeneous genomes - "quasispecies" - resulting from mutations
due to high error rates in RNA replication are found within the same host.
Many important biological features of several viruses are attributable
to their quasispecies nature, including vaccination failure, persistent
infection, and resistance to antiviral drugs.38 The amount
of diversity of the quasispecies population has also been found to be
related to the progression to liver disease.
The most striking feature of HCV is its ability to persist in the host.
The mechanism(s) of viral persistence are unclear but cannot include viral
integration into the host genome as with certain other viruses due to
the lack of a DNA intermediate in its life cycle. Instead, persistence
appears to result from HCV's ability to mutate rapidly under immune pressure,
giving rise to related but immunologically distinct variants.11
Any one of these variants can become the predominant strain, and coexistence
of multiple quasispecies allows HCV to escape the host immune response.
Most mutations occur in a short, hypervariable region of the E2/NS1 domain.
The region represents only 8% of the domain but accounts for approximately
half of the nucleotide changes in the entire envelope region.39
Because of the occurrence of this hypervariable region within the envelope
where it would be most likely to be exposed to antibody, mutations in
this region may serve to evade an immune response.11,39 It
has been reported that the nucleotide substitution rate within the hypervariable
region rises during acute infection at a time when HCV RNA levels in the
serum are decreasing, possibly due to a host immune response.40
There is also evidence that HCV can escape immune clearance by down-regulating
its replication while persisting quiescently in the liver.11
Genotypic analysis and prevalence
Traditionally, viruses have been classified according to antigenic characteristics,
but with recent advances in molecular biology, genotypic classification
through the analysis of genomic variation is now possible. Variations
in the HCV genome fall into a series of specific patterns that have been
classified into genotypes.41 Among the different HCV genotypes,
the sequence of the 5' NC region is relatively conserved and is most often
applied for diagnosis of HCV infection by PCR. In contrast, the sequences
of NS3, NS5, and core regions are more variable and are therefore often
used to define and distinguish among the HCV genotypes.42
Studies indicate that there are nine major HCV types (according to the
general classification) designated 1 through 9.18
Some of these types are further divided into subtypes. The potential
significance of this becomes apparent when considering virus-host interactions,
severity of infection, and sensitivity to treatment. The clinical importance
of HCV lies in its persistence and ability to cause chronic liver disease.
The dramatic disparities in HCV disease course among infected persons
and differences in disease patterns between countries with divergent dominant
genotypes raise the possibility that the existence of various strains
of HCV may be a critical factor in this variability.
In one study, 98 patients in the United States with chronic Hepatitis
C infection were examined (Fig 1).41 Type 1 was the dominant
genotype, present in 74% of patients, almost evenly divided between subtypes
1a and 1b. Two additional patients had a dual infection with types 1 and
2 while another case had both subtypes 1a and 1b.
One patient was untypeable. There was no correlation between infecting
genotype and presumed cause, serum indices of necroinflammatory activity,
age or sex of the patients, or known duration of infection. Patients with
HCV genotype 2 had more severe liver disease histologically than did patients
with other genotypes but paradoxically had significantly lower circulating
levels of Hepatitis C viral RNA. 41 A second, independent study,
also performed in the United States, found that HCV genotype 1 represented
70% of viral isolates, confirming that it accounts for the majority of
infections in American patients. 43
Fig 1. -- Prevalence of HCV genotypes and subtypes among American
patients N=98).41
Segments in which more than one genotype or subtype appears represent
mixed infections.
Another study performed in Japan (using study-specific genotype nomenclature)
demonstrated striking differences in the distribution of HCV genotypes
in various countries.44 This group classified HCV into four
main types based on variations in the NS5 region and studied the prevalence
of these types in Japan, China, Europe, Brazil, and the United States.
Results revealed that the prevalence of one particular strain was the
highest in every country except the United States, making it the probable
major type of HCV worldwide. Another strain was rare in Japanese and Chinese
samples but common in samples from Western countries, suggesting the existence
of a Western HCV type. Similarly, a third strain was rare in Western samples
but common in Japanese and Chinese samples, suggesting the existence of
an Eastern HCV type. Because of the study-specific classification utilized,
however, comparisons to the results of other studies are not possible.
Despite reports of an interplay between genotype and disease severity,
patients with the same genotype can have very different clinical outcomes.
Thus, differences in genotype may represent only part of a complex interaction
between host and virus. In addition, the effect of genotypes may be indirect
in that they may reflect differences in viral burden rather than differences
in strain virulence.11 There is, however, evidence for genotypic
variability in the response of HCV to therapy.
Serotyping
Serotype-specific immunodominant epitopes have recently been mapped to
the NS4 region of HCV. 45 Synthetic peptides from this region
can be used as capture antigens in an ELISA assay to differentiate serologically
among infections with the various types of HCV. Blocking antibody or competing
peptide is used to overcome cross-reactivity. The results of genotypic
and serologic analyses are well correlated for genotypes and serotypes
1, 2, and 3, which are predominant in the U.S. population.46
The value of serotyping relative to genotyping remains to be determined.
References
11. Alter HJ. To C or not to C: these are the questions. Blood. 1995;85:1681-1695.
18. Berenguer M, Wright TL. Hepatitis C virus. Advances in Gastroenterology,
Hepatology & Clinical Nutrition. 1996;1:2-21.
38. Kanazawa Y, Hayashi N, Mita E, et al. Influence of viral quasispecies
on effectiveness of interferon therapy in chronic Hepatitis C patients.
Hepatology. 1994;20:1121-1130.
39. Weiner AJ, Brauer MJ, Rosenblatt J, et al. Variable and hypervariable
domains are found in the regions of HCV corresponding to the flavivirus
envelope and NS1 proteins and the pestivirus envelope glycoproteins. Virology.
1991;180:842-848.
40. Yamaguchi K, Tanaka E, Higashi K, et al. Adaptation of Hepatitis C
virus for persistent infection in patients with acute hepatitis. Gastroenterology.
1994;106:1344-1248.
41. Mahaney K, Tedeschi V, Maertens G, et al. Genotypic analysis of Hepatitis
C virus in American patients. Hepatology. 1994;20:1405-1411.
42. Qu D, Li J-S, Vitvitski L, et al. Hepatitis C virus genotypes in France:
comparison of clinical features of patients infected with HCV type I and
type II. J Hepatol. 1994;21:70-75.
43. Lau JYN, Davis GL, Ohno T, et al. Application of Hepatitis C virus
(HCV) subtyping in chronic Hepatitis C in the United States. Hepatology.
1993;18:149A. Abstract.
44. Takada N, Takase S, Takada A, et al. Differences in the Hepatitis
C virus genotypes in different countries. J Hepatol. 1993;17:277-283.
45. Chemello L, Alberti A, Rose K, et al. Hepatitis C serotype and response
to interferon therapy. N Engl J Med. 1994; 330:143-144. Correspondence.
46. Keefe E, Blatt LM, Dusheiko G, et al. Differential response to treatment
with consensus interferon (CIFN) and IFN a-2b in chronic HCV patients
infected with different HCV serotypes. Hepatology. 1996;24:275A. Abstract.
|
