Evolutionary mechanisms involved in the virulence of infectious salmon anaemia virus (ISAV), a piscine orthomyxovirus

Abstract

Infectious salmon anaemia virus (ISAV) is an orthomyxovirus causing a multisystemic, emerging disease in Atlantic salmon. Here we present, for the first time, detailed sequence analyses of the full-genome sequence of a presumed avirulent isolate displaying a full-length hemagglutinin-esterase (HE) gene (HPR0), and compare this with full-genome sequences of 11 Norwegian ISAV isolates from clinically diseased fish. These analyses revealed the presence of a virulence marker right upstream of the putative cleavage site R267 in the fusion (F) protein, suggesting a Q266-->L266 substitution to be a prerequisite for virulence. To gain virulence in isolates lacking this substitution, a sequence insertion near the cleavage site seems to be required. This strongly suggests the involvement of a protease recognition pattern at the cleavage site of the fusion protein as a determinant of virulence, as seen in highly pathogenic influenza A virus H5 or H7 and the paramyxovirus Newcastle disease virus.

First field evidence of the evolution from a non-virulent HPR0 to a virulent HPR-deleted infectious salmon anaemia virus

“…Putatively uncultivable non-virulent ISAV variants (ISAV-HPR0) are characterized by an HE protein carrying a full-length highly polymorphic region (HPR) composed of at least 35 amino acids just upstream of the transmembrane domain [15][16][17][18][19]. All currently described pathogenic ISAV isolates associated with ISA disease have deletions in the HPR with over 30 different HPR-deleted subtypes identified in Europe, North America and Chile [16][17][18][19][20][21][22]. Sequencing and functional characterization further suggest both HPR-deletion and a Q 266 L substitution, or insertion adjacent to the cleavage site in the F protein influences ISAV virulence [22], by promoting viral fusion and the activation of proteolytic cleavage [23,24].…”

“…All currently described pathogenic ISAV isolates associated with ISA disease have deletions in the HPR with over 30 different HPR-deleted subtypes identified in Europe, North America and Chile [16][17][18][19][20][21][22]. Sequencing and functional characterization further suggest both HPR-deletion and a Q 266 L substitution, or insertion adjacent to the cleavage site in the F protein influences ISAV virulence [22], by promoting viral fusion and the activation of proteolytic cleavage [23,24]. However, other viral functions, for example virus receptor binding [25], virus uptake, replication rate, shedding of new virions [26,27], modulation of the host immune response [28,29] and the ability to spread to new hosts [30], can also influence virulence.…”

Comparative aspects of infectious salmon anemia virus, an orthomyxovirus of fish, to influenza viruses

“…The genomic segments and their corresponding encoded proteins of the different orthomyxo genera are listed in Table 1. Genetic re-assortment of ISAV has been indicated by molecular and phylogenetic sequence analysis [18]. The evolutionary distance to infl uenza viruses of birds and mammals is vast, as can be displayed by phylogenic relationships based upon nucleotide or amino acid sequences [4].…”

“…1 THOV = Thogoto virus. 2 Some F proteins contain insertions near their putative cleavage site, probably acquired through a sequence based non-homologues recombination event [18].…”

“…The coiled-coil domains, trimerization, the hydrophobic N-terminal sequence of F2, and the requirement for a low pH to induce fusion, are other criteria of type I fusion proteins [3,13,36]. Analysis of the gene encoding the ISAV fusion protein has revealed insertions in several virus isolates, in proximity to the protein's putative cleavage site [18], suggesting the presence of a recombinational hot spot. It could be speculated that this could lead to alterations in the cleavage specifi city of the fusion protein, however, no specifi c motifs similar to those seen in the FluA HA, which exerts the fusion activity, of HPAIV have been found.…”