Genetic variants of herpes simplex virus type 1

Abstract

Herpes simplex virus type 1 (HSV-1) virus causes oral as well as genital lesions, and, on rare occasions, focal encephalitis and severe neonatal infections. Due to the partly overlapping clinical manifestations of infection with HSV-1 and its closest relative among herpesviruses, HSV-2, diagnostic methods are required to discriminate between infections with these two viruses. To improve our understanding of the genetic basis for such diagnostic procedures, we have employed DNA sequencing to investigate the genetic variability of HSV-1 viruses derived from clinical isolates.The viral envelope glycoprotein G (gG-1) has been suggested as a prototype antigen for HSV-1-specific serodiagnosis, but the interstrain variability of this gene has not been investigated. In a large material of clinical HSV-1 isolates, we found a high degree of sequence variation, especially in the middle part of the gene encoding immunodominant antibody epitopes. By phylogenetic analysis, two main groups of this gene were discovered among the clinical isolates, one represented by laboratory strains Syn 17+ and F being reactive with an anti-gG-1 monoclonal antibody (MAb), and the other variant being unreactive to the same MAb and showing sequence similarity to strain KOS. Five isolates appeared to be recombinants of these two gG-1 variants. Furthermore, one isolate displayed a gG-1-negative phenotype due to a frameshift mutation in the form of an insertion of one cytosine nucleotide. The existence of two main genetic groups of gG-1 did not markedly affect the antibody response to this protein when IgG titers in hosts carrying either of the two variant viruses were compared in a gG-1-based ELISA.To investigate whether a genetic diversity in form of dichotomy into two genogroups also existed in other HSV-1 envelope genes, we sequenced parts or complete genes coding for gB, gI, and gE in a large proportion of the clinical isolates. Surprisingly, the gE/gI genes exhibited sequence diversity that clustered into three main groups as supported by high bootstrap values. Intra- and intergenic recombinants were demonstrated in the gE/gI gene complex. In contrast, phylogenetic analysis did not separate the gB sequences into main genogroups, although a relatively large sequence variability was documented. Our approach of sequencing selected genes or fragments thereof provided insight as regards evolution of investigated genes, while phylogeny of whole-virus genomes remains to be elucidated. Despite that only short regions of the genome were sequenced, the frequent finding of recombinants among our isolates indicate that, besides point and indel mutations, recombination stands out as a major source of HSV genetic variability.Lastly, we investigated the degree of heterogeneity within individual HSV-1 and HSV-2 isolates. The frequencies of negative phenotypes of genes encoding for gG-1, gG-2 and gC-1 were estimated by antibody-staining of viral plaques. DNA-sequencing defined the responsible genetic mechanisms either as frameshift mutations due to insertion or deletion of nt G or C in homopolymer runs, or point mutations within MAb epitopes. Finally, exploiting our previous finding of a dichotomy of the gG-1 gene, we developed a KOS-genovariant-specific PCR. In a large proportion of clinical isolates showing a Syn 17+-like sequence, KOS-like sequences were detected as minority populations. Our results demonstrate a hitherto unknown degree of inter- and intrastrain genetic diversity among HSV strains.