Nasjonalt influensasenter for WHO (leder); Virologi, molekylærbiologi, cellebiologi, epidemiologi
Dr. scient i molekylær cellebiologi ved Biologisk Institutt, Universitetet i Oslo 1992
Virologisk arbeid v/FHI siden 1987, arbeidet med influensavirus siden 1996, som leder for nasjonalt influensasenter siden 1999. Bred erfaring i influensaovervåkning, nasjonalt og internasjonalt folkehelsearbeid, molekylærvirologi og -epidemiologi, seroepidemiologi.
Genet for ILAV hemagglutinin (HA) inneholder et lite, polymorft område (PR). Dette området er karakterisert ved tilstedeværelse av ’gaps’ (delesjoner) og ligger rett utenfor transmembran region, i stammen av HA molekylet. I dette arbeidet har vi analysert 33 ILAV HA sekvenser fra historiske og nyere ILA utbrudd, og supplert denne informasjonen med et utvalg av tidligere publiserte HA sekvenser. PR sekvensene kunne sammenstilles slik at hvert ’gap’ representerte en delesjon i en nærmest perfekt konservert ”ur” sekvens, og hver PR sekvens som vi undersøkte, inneholdt et ’gap’ sammenstilt mot denne konsensussekvensen. Ut i fra disse dataene foreslår vi en differensiell delesjonsmodell, som kan hjelpe til å forklare den observerte polymorfi. Det er tidligere foreslått at ulike kombinasjoner av sekvensmotiver som en finner i de forskjellige PR sekvensene, er et resultat av homolog rekombinasjon. I dette arbeidet demonstrerer vi tilstedeværelsen av ko-infeksjon, som er et absolutt krav for at reassortering og rekombinasjon skal kunne finne sted. Det er imidlertid mangelfulle bevis på homolog rekombinasjon i negativt trådet RNA virus. Vår modell tilbyr en alternativ forklaring, der differensiell delesjon fra et langt ”ur” virus gen gjør det unødvendig å postulere at ulike virus må være opphav til polymorfien. Vi foreslår at HA delesjonsmutantene kan ha utviklet seg som et resultat av sterk funksjonell seleksjon, muligens pga endrete økologiske forhold i oppdrettsnæringen, eller som en konsekvens av vertsbytte. Ut i fra den gjentatte tilstedeværelse av identiske mutasjoner i ulike genetiske linjer, så vel som de dramatiske forskjellene en ser mellom virus som trolig er nært beslektet, anser vi ikke at PR regionen inneholder tilstrekkelig informasjon til å avdekke genetiske relasjoner mellom ILA virus. Våre data indikerer en relativt kompleks epidemiologi, ettersom 1) HA sekvenser som representerer flere PR varianter ble detektert i tre prøver, 2) identiske mutasjoner oppsto i ulike genetiske linjer, og 3) store genetiske forskjeller var tilstede i nært relaterte virus.
The Orthomyxoviruses have recently been expanded by addition of the newly characterised etiological agent of Infectious Salmon Anemia (ISA), an economically important disease in farmed Atlantic Salmon (Salmo salar) in both Europe and the Americas. Study of the biology and ecology of this virus may yield insights also of relevance to influenza. A highly polymorphic region (PR) has been found in the hemagglutinin (HA) gene of the ISA virus (ISAV), characterised primarily by different gaps confined to a short region just outside the putative transmembrane region. In the present study, 33 European ISAV PR sequences were compared to previously published sequences. The sequences can be aligned so that each gap represents a deletion in a nearly perfectly conserved ancestral sequence. Every PR sequence in the study contains a gap compared to this consensus. We propose that the HA stalk deletion mutants may have emerged through strong functional selection, perhaps because of different ecological conditions in the sea cage environment or as a consequence of a host species switch. Furthermore, some covariance was seen between the PR gap pattern and observed clinical pattern and in vitro replication characteristics, suggestive of a role of the PR as a determinant of virus replicative phenotype. A more proliferative virus phenotype may have increased fitness over a slowly-replicating wild-type virus in a densely populated sea cage setting. The observed polymorphism may be analogous to glycoprotein stalk deletions observed in some avian influenza strains where they may possibly be attributed to either cross-species transfer (waterfowl-to-chicken) or to a new neuraminidase-hemagglutinin constellation. A conceivable effect of such deletions may be that they alter the balance between receptor-binding and receptor-destroying functions. The receptor-destroying enzyme activity of the ISAV is an acetyl esterase and its gene has not yet been identified.
A polymorphic region (PR) has been found in the hemagglutinin (HA) gene of ISAV. This region is primarily characterised by gaps confined to a short region just outside the putative transmembrane region, in the stalk of the HA molecule. In the present study we have analysed thirty-three ISAV HA sequences from historical and recent ISA outbreaks and added this information to a selection of previously published HA sequences. The PR sequences could be aligned so that each gap represented a deletion in a nearly perfectly conserved ancestral sequence, and every PR sequence in this study contained a gap compared to this consensus. According to these data we propose a differential deletion model that may help to explain the observed polymorphism. The different combinations of sequence motifs observed in the PR have by others been interpreted as a result of homologous recombination. In this work we do demonstrate the existence of co-infection; an absolute requirement for reassortment and recombination to occur. However, as there is a lack of firm evidence for homologous recombination in negative-strand RNA viruses, differential deletion from a long “ancestral“ virus gene offers an alternative explanation, making it unnecessary to postulate different viral ancestor for the multiple polymorphism seen. We propose that the HA stalk deletion mutants may have emerged through strong functional selection, perhaps caused by different ecological conditions in the sea cage environment or as a consequence of a host species switch. Due to the frequent occurrence of identical mutations in different genetic lineages, as well as dramatic differences between viruses likely to be closely related, we do not consider that the PR region contains reliable information for inferring the genetic relationships among the ISA virus. A rather complex epidemiology is suggested, as 1) HA sequences representing several PR variants were detected in three samples, 2) identical mutations occurred in different genetic lineages, and 3) large genetic differences were present in closely related viruses.
Infectious salmon anaemia virus (ISAV), a putative orthomyxovirus, is an economically important pathogen for the salmon farming industry in the northern hemisphere. During ISA outbreaks, differences in the course of the disease development and clinical signs, as well as in cell culture replication of the virus, have been observed, indicating variation in the pathogenicity of the virus. Here we describe a small polymorph region in the ISAV genomic segment encoding the hemagglutinin. This variable region occurs in the putative ectodomain of the molecule, juxtapositioned to a transmembrane-like region. Thirty-two sequences were compared from ISA outbreaks from locations along the Norwegian coast over a ten-year period. The viral sequences could be divided in two main groups, each subdivided into several separate subgroups, depending on sequence composition in the variable region. Within each main group the polymorphism primarily consisted of different gap patterns and not single-nucleotide substitutions, suggesting several independent and relatively recent deletion events. In three outbreaks, samples from affected fish contained viral sequences belonging to two or three subgroups. Comparisons of development of cytopathic effect and replication of virus in cell culture showed variation between the subgroups. There may be a relationship between the polymorphism of the hemagglutinin and the virulence of ISAV, and possible mechanisms behind this variability is discussed. ISAV has only been found to cause disease in Atlantic salmon, but the virus can replicate in other fish species, and there is mounting evidence of ISAV in various non-salmonid fish species. We propose that the observed, partly outbreak-specific, polymorphism of the hemagglutinin may have come about through independent adaptive deletions, perhaps subsequent to cross-species transfer events. This may have fundamental implications for the understanding of the ecology of ISAV and future control and surveillance of the ISA epidemic.
Infectious salmon anaemia virus (ISAV), an economically important pathogen for the salmon farming industry in the northern hemisphere, closely resembles orthomyxoviruses, which include the influenza viruses. The influenza viruses are characterized by their continuous change in genetic material driven by the hosts immune defence, and as a consequence, a great variation in the severity of the infection is seen. In ISAV infections, differences in the course of the disease and clinical signs have been observed, as well as in cell culture replication. These differences indicate variations both in the virulence and antigenic composition of the virus. Nucleotide and amino acid sequence alignments were performed on 33 ISA disease outbreaks picked from all along the Norwegian coast covering a ten year period. A polymorf region was detected, and depending on type of variation the disease outbreaks fell into eight separate groups. From some of the more diffuse disease outbreaks, where one had not been able to isolate virus, cloning of RT-PCR products from diseased fish homogenates resulted in detection of not just one, but two to three variable groups from the same outbreak. Comparisons between members of the groups with regard to development of cytopathogenic effect and virus titres in cell culture showed variation between the groups. Here we describe the ISAV hemagglutinin gene: both nucleotide / amino acid comparisons and the estimated 3D structure of this molecule.