Molecular characterization of the alloherpesvirus anguillid herpesvirus 1

Abstract

All herpesviruses belong to the order Herpesvirales, which consists of the families Herpesviridae, Alloherpesviridae and Malacoherpesviridae. Although herpesviruses share unique morphological characteristics, only the gene encoding the ATPase subunit of terminase is detectably conserved throughout the order. The family Herpesviridae, which comprises mammalian, avian and reptilian herpesviruses, has been studied extensively, but much less knowledge is available for members of the families Alloherpesviridae and Malacoherpesviridae, which respectively comprise amphibian and fish, and invertebrate herpesviruses. Anguillid herpesvirus 1 (AngHV1) frequently causes disease in wild and cultured European eel, a traditionally important fish species in the Netherlands. Hence, in this study AngHV1 was chosen as a model for the family Alloherpesviridae. The aim of the study was to characterize AngHV1 at the molecular level, and to determine its similarities and differences as compared with other herpesviruses. AngHV1 has a genome of close to 250 kbp, including an 11 kbp terminal direct repeat. The genome contains a total of 129 protein-coding genes, five of which are duplicated in the terminal repeat. Since only a dozen genes are detectably conserved among fish and amphibian herpesviruses, the family Alloherpesviridae appears to be more divergent than the family Herpesviridae, among which more than 40 genes are conserved. Taxonomically, AngHV1 is most closely related to the cyprinid herpesviruses. High-resolution transcriptome analysis based on deep sequencing revealed that RNA splicing is much more abundant than had been assumed. A total of 58 functional splice junctions were identified. Eleven genes contain integral, spliced protein-coding exons, and nine contain 5’-untranslated exons or, in instances of alternative splicing, 5’-untranslated or -translated exons. In contrast to mammalian herpesviruses, overall levels of antisense transcription in AngHV1 were low, and no abundant, non-overlapping non-coding RNAs were identified. A genome-wide expression study using qPCR showed that gene expression is regulated in a temporal fashion, similar to mammalian herpesviruses. The putative regulatory immediate-early genes of AngHV1 were identified, and appeared to be located within and near the terminal repeats. The remaining open reading frames were classified into early, early-late and late genes. Most early genes encode enzymes and proteins involved in DNA replication, and most late genes encode structural proteins. The structural proteins of AngHV1 were identified using a combination of classical virus purification and fractionation techniques and modern mass spectrometry analyses. A total of 40 different structural proteins were identified, of which 7 could be assigned to the capsid, 11 to the envelope, and 22 to the tegument. Although no convincing sequence homology is apparent between the herpesvirus families for any of the structural proteins, virion composition shows many similarities. AngHV1 encodes several viral homologs of components of the host immune system, including an interleukin-10-like open reading frame. Although amino acid sequence homology between the European eel interleukin-10 and the AngHV1 interleukin-10 homolog is low, the three-dimensional structures as predicted by modelling are highly similar, suggesting functionality. Overall, despite the virtual absence of detectable genetic similarities, AngHV1 and the other alloherpesviruses resemble members of the family Herpesviridae in many fundamental aspects.