Evidence of structural genomic region recombination in Hepatitis C virus
© Cristina and Colina; licensee BioMed Central Ltd. 2006
Received: 14 April 2006
Accepted: 30 June 2006
Published: 30 June 2006
Hepatitis C virus (HCV) has been the subject of intense research and clinical investigation as its major role in human disease has emerged. Although homologous recombination has been demonstrated in many members of the family Flaviviridae, to which HCV belongs, there have been few studies reporting recombination on natural populations of HCV. Recombination break-points have been identified in non structural proteins of the HCV genome. Given the implications that recombination has for RNA virus evolution, it is clearly important to determine the extent to which recombination plays a role in HCV evolution. In order to gain insight into these matters, we have performed a phylogenetic analysis of 89 full-length HCV strains from all types and sub-types, isolated all over the world, in order to detect possible recombination events.
Putative recombinant sequences were identified with the use of SimPlot program. Recombination events were confirmed by bootscaning, using putative recombinant sequence as a query.
Two crossing over events were identified in the E1/E2 structural region of an intra-typic (1a/1c) recombinant strain.
Only one of 89 full-length strains studied resulted to be a recombinant HCV strain, revealing that homologous recombination does not play an extensive roll in HCV evolution. Nevertheless, this mechanism can not be denied as a source for generating genetic diversity in natural populations of HCV, since a new intra-typic recombinant strain was found. Moreover, the recombination break-points were found in the structural region of the HCV genome.
Hepatitis C virus (HCV) is estimated to infect 170 million people worldwide and creates a huge disease burden from chronic, progressive liver disease . HCV has become a major cause of liver cancer and one of the commonest indications of liver transplantation [2, 3]. HCV has been classified in the family Flaviviridae, although it differs from other members of the family in many details of its genome organization from the original (vector-borne) members of the family . Like most RNA viruses, HCV circulates in vivo as a complex population of different but closely related viral variants, commonly referred to as a quasispecies [4–7].
HCV is an enveloped virus with an RNA genome of approximately 9400 bp in length. Most of the genome forms a single open reading frame (ORF) that encodes three structural (core, E1, E2) and seven non-structural (p7, NS2-NS5B) proteins. Short unstranslated regions at each end of the genome (5'NCR and 3'NCR) are required for replication of the genome. This process also requires a cis-acting replication element in the coding sequence of NS5B recently described . Translation of the single ORF is dependent on an internal ribosomal entry site (IRES) in the 5'NCR, which interacts directly with the 40S ribosomal subunit during translation initiation .
Comparison of nucleotide sequences of variants recovered from different individuals and geographical regions has revealed the existence of at least six major genetic groups [1, 10–12]. On the average over the complete genome, these differ in 30–35% of nucleotide sites. Each of the six major genetic groups of HCV contains a series of more closely related sub-types that typically differ from each other by 20–25 % in nucleotide sequences .
Different genotypes and sub-types seem to correlate differently for susceptibility to treatment with interferon (IFN) monotherapy or IFN/ribavirin (RBV) combination therapy. Only 10–20 % and 40–50 % of individuals infected chronically with genotype 1 HCV on monotherapy and combination therapy, respectively, exhibit complete and permanent clearance of virus infection. These rates are much lower than the rates of 50 and 70–80 % that are observed on treatment of HCV genotype 2 or 3 infections [3, 13].
Until 1999, there was no evidence for recombination in members of the family Flaviviridae, although the possibility was considered [14–16]. Accordingly, the vast majority of work on members of this family, including vaccine studies and phylogenetic analyses in which genotypes were identified and sometimes correlated with disease severity, has rested on the implicit assumption that evolution in the family Flaviviridae is clonal, with diversity generated through the accumulation of mutational changes [17–19].
This assumption have shown to be invalid, as homologous recombination has been demonstrated in pestiviruses,(bovine viral diarrhoea virus) , flaviviruses (all four serotypes of dengue virus) [21–24], hepaciviruses (GB virus C/hepatitis G virus) , Japanese encephalitis or St Louis encephalitis virus .
Recombination plays a significant role in the evolution of RNA viruses by creating genetic variation. For example, the frequent recovery of poliovirus that result from recombination has the potential to produce "escape mutants" in nature as well as in experiments .
Recombination has also been detected in other RNA viruses for which multivalent vaccines are in use or in trials [21, 24, 28]. The potential for recombination to produce new pathogenic hybrid strains needs to be carefully considered whenever vaccines are used or planned to control RNA viruses. Assumptions that recombination either does not take place or is unimportant in RNA viruses have a history of being proved wrong .
Recently, a natural intergenotypic recombinant (2k/1b) of HCV has been identified in Saint Petersburg (Russia) [29, 30]. Phylogenetic analyses of HCV strains circulating in Peru, demonstrated the existence of natural intra-genotypic HCV recombinant strains (1a/1b) circulating in the Peruvian population .
Given the implications that recombination has for RNA virus evolution , it is clearly important to determine the extent to which recombination plays a role in HCV evolution.
Phylogenetic profile analysis of full-length HCV strains
Full-length HCV sequences.
Profiles of synonymous and non-synonymous substitutions among parental-like and recombinant HCV strains
Interestingly, the rates of synonymous substitutions in AY651016–D10749 comparison are significantly lower in the region spanned by the recombination break-points, while significantly higher rates are obtained when AF511949–D10749 comparison is performed (see Fig. 3). The results of these studies show that even though recombination took place in the structural region of HCV genome, is has not produced a drastic change in the mode of evolution of the E1/E2 region, since the nonsynonymous substitution rate was maintained at very low rate (see Fig. 3). Thus, at least on this basis, the E1/E2 genomic region does not appear to have been perturbed by the recombination event.
In the present study, analysis of full-length sequences from HCV strains of all types and sub-types provided the opportunity to test the roll that recombination may play in HCV genetic diversity.
The results of this study revealed that recombination may not be extensive in HCV, since from 89 strains studied, recombination was observed in only one case. This is in agreement with the current methodology for HCV genotyping for the vast majority of the cases . Nevertheless, the true frequency of recombination may be underestimated because although there is comparative important number of complete genomes sequences from common genotypes, such as 1b, most studies of HCV variability in high diversity areas are based on analysis of single sub-genomic regions, making detection of potential recombination events unlikely .
On the other hand, this study reveals that recombination can not be denied as an evolutionary mechanism for generating diversity in HCV (see Figs. 1 and 2). Moreover, an infectious HCV chimera comprising the complete open reading frame of sub-type 1b strain and the 5'- and 3' non translated regions of a sub-type 1a strain has been constructed and is infectious in vivo . A natural inter-genotype recombinant (2k/1b) has been identified in St. Petersburg, Russia [29, 30] and a natural intra-typic recombinant (1a/1b) has been identified in Peru .
The recombination break-points for non-segmented positive-strand RNA viruses, such as polioviruses and other picornaviruses [35–37] as well as members of the family Flaviviridae, are often located in the part of the genome encoding non structural proteins. More recently, recombination break-points have been found in genes encoding structural proteins [38, 39]. In the present study, we report recombination events in structural genes (E1/E2 region) between two different sub-types (1a/1c, see Figs. 1 and 2). Recombination may serve two opposite purposes: exploration of a new combination of genomic region from different origins or rescuing of viable genomes from debilitated parental genomes .
The recognition of recombination is important not only for unraveling the phylogenetic history of genes, but also for molecular phylogenetic inference. By ignoring the presence of recombination, phylogenetic analysis may be severely compromised [41, 42]. For that reason, although recombination may be not appeared to be extensive in natural populations of HCV, this possibility should be taken into account as a mechanism of genetic variation for HCV.
The results of this study, as well as previous ones [29–31] provide evidence that not only does recombination occurs in HCV, but that it occurs in natural populations. In the case of the recombinant described in this study, the distribution of non-synonymous substitutions showed very low rates, revealing that the E1/E2 region of this isolate might have not been perturbed by the recombination events (see Fig. 3). This may also be related to the fact that the differences in this region of the genome among sub-genotypes 1A and 1C, at least in the case of the isolates involved in these studies, are not particularly significant at the amino acid level in the genomic region where the recombination events have occurred.
Only one of 89 full-length strains studied resulted to be a recombinant HCV strain, revealing that homologous recombination does not play an extensive roll in HCV evolution. A new intra-typic (1a/1c) recombinant strain was found. The recombination break-points were found in the structural (E1/E2) region of the HCV genome. Whether new HCV variants may appear, as a result of recombination events, remains to be established as well as if their fitness permits them to be selected in an HCV population.
Full-length genome sequences from 89 HCV isolates where obtained by means of the use of the HCV LANL database . For names, genotypes and accession numbers see Table 1. Sequences were aligned using the CLUSTAL W program .
Putative recombinant sequences were identified with the SimPlot program . This program is based on a sliding window method and constitutes a way of graphically displaying the coherence of the sequence relationship over the entire length of a set of aligned homologous sequences. The window width and the step size were set to 500 bp and 20 bp, respectively.
Bootscaning  was carried out employing software from the SimPlot program , using putative recombinant sequence as a query. Mosaicism is suggested when high levels of phylogenetic relatedness between the query sequence and more than one reference sequence in different genomic regions is obtained.
Substitution rate analysis
The substitution rate along the open reading frame of the HCV genome, from position 1 to 2490 (relative to the first coding position of strain D10749), was measured using a sliding window method according to the procedure implemented by Alvarez-Valin . Pairwise nucleotide distances (synonymous and nonsynonymous) within each window were estimated by the method of Comeron  as implemented in the computer program k-estimator . The window had a size of 150 codons and a movement of 10.
This work was supported by ICGEB, PAHO, and RELAB through Project CRP.LA/URU03-032 and International Atomic Energy Agency through project ARCAL 6050.
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