microorganisms Review Evolution and Adaptation of the Avian H7N9 Virus into the Human Host Andrew T. Bisset 1,* and Gerard F. Hoyne 1,2,3,4 1 School of Health Sciences, University of Notre Dame Australia, Fremantle WA 6160, Australia; [email protected] 2 Institute for Health Research, University of Notre Dame Australia, Fremantle WA 6160, Australia 3 Centre for Cell Therapy and Regenerative Medicine, School of Biomedical Sciences, The University of Western Australia, Nedlands WA 6009, Australia 4 School of Medical and Health Sciences, Edith Cowan University, Joondalup WA 6027, Australia * Correspondence: [email protected] Received: 19 April 2020; Accepted: 19 May 2020; Published: 21 May 2020 Abstract: Influenza viruses arise from animal reservoirs, and have the potential to cause pandemics. In 2013, low pathogenic novel avian influenza A(H7N9) viruses emerged in China, resulting from the reassortment of avian-origin viruses. Following evolutionary changes, highly pathogenic strains of avian influenza A(H7N9) viruses emerged in late 2016. Changes in pathogenicity and virulence of H7N9 viruses have been linked to potential mutations in the viral glycoproteins hemagglutinin (HA) and neuraminidase (NA), as well as the viral polymerase basic protein 2 (PB2). Recognizing that effective viral transmission of the influenza A virus (IAV) between humans requires efficient attachment to the upper respiratory tract and replication through the viral polymerase complex, experimental evidence demonstrates the potential H7N9 has for increased binding affinity and replication, following specific amino acid substitutions in HA and PB2. Additionally, the deletion of extended amino acid sequences in the NA stalk length was shown to produce a significant increase in pathogenicity in mice. Research shows that significant changes in transmissibility, pathogenicity and virulence are possible after one or a few amino acid substitutions. This review aims to summarise key findings from that research. To date, all strains of H7N9 viruses remain restricted to avian reservoirs, with no evidence of sustained human-to-human transmission, although mutations in specific viral proteins reveal the efficacy with which these viruses could evolve into a highly virulent and infectious, human-to-human transmitted virus. Keywords: H7N9; avian influenza virus; hemagglutinin; neuraminidase; polymerase basic protein 2; evolution; mutation; reassortment 1. Introduction The pandemic potential of the influenza A virus (IAV) is well known, with the most significant impact occurring during the 1918 Spanish Flu, where mortality was estimated between 21.5 million and 100 million [1]. In the one hundred years since this initial event, evolutionary adaptations in human and animal influenza viruses have resulted in another three IAV pandemic events; the 1957 Asian flu (H2N2), the 1968 Hong Kong flu (H3N2) and the 2009 swine flu (H1N1) [2]. While pandemic events remain limited in number, recurring seasonal influenza virus epidemics result in approximately three to five million cases of severe illness annually, with between 290,000 and 650,000 deaths linked to virally associated respiratory diseases [3]. The morbidity rate for influenza epidemics underscores the constant molecular changes taking place within the viral genome, which in turn facilitates the evasion of host immunity. In response to selective evolutionary pressures, the IAV is adapting, resulting in Microorganisms 2020, 8, 778; doi:10.3390/microorganisms8050778 www.mdpi.com/journal/microorganisms Microorganisms 2020, 8, 778 2 of 13 Microorganisms 2020, 8, x FOR PEER REVIEW 2 of 14 viraladapting, diversity resulting and the in creationviral diversity of novel and genotypes. the creation The of emergencenovel genotypes. of the The novel emergence IAV H7N9 of inthe 2013 novel and theIAV resulting H7N9 in morbidity 2013 and andthe resulting mortality morbidity signalled and an evolutionary mortality signalled adaptation an evolutionary of unknown adaptation consequence. of Theunknown purpose consequence. of this review The is topurpose document of this the review emergence is to document of the H7N9 the virus,emergence how itof adaptedthe H7N9 to virus, human hosts,how andit adapted also highlight to human the hosts, molecular and also changes highlight that the could molecular bring about changes a human-to-human that could bring pandemic.about a human-to-human pandemic. 2. Viral Characterization and Origin of Avain Influenza A(H7N9) Viruses 2. Viral Characterization and Origin of Avain Influenza A(H7N9) Viruses Influenza viruses are enveloped negative-sense, single-stranded RNA (ssRNA) comprising a segmentedInfluenza genome viruses(Figure are enveloped1)[ 4–6 ].negative-sense The three largest, single-stranded RNA segments RNA (ssRNA) (1–3) encode comprising the viral a polymerasessegmented genome PB1, PB2 (Figure and PA,1) [4–6]. which The are three necessary largest for RNA RNA segments synthesis (1 and–3) encode replication the withinviral anpolymerases infected cell. PB1, Two PB2 RNA and segmentsPA, which (4 are and necessary 6) encode for the RNA viral synthesis glycoproteins and replication hemagglutinin within (HA)an andinfected neuraminidase cell. Two RNA (NA), segments respectively, (4 and covering 6) encode the the virion viral surface glycoproteins at a ratio hemagglutinin of approximately (HA) 4:1and [ 7]. neuraminidase (NA), respectively, covering the virion surface at a ratio of approximately 4:1 [7]. The The HA protein mediates binding and viral entry via specificity for host cell surface sialic acid (SA) HA protein mediates binding and viral entry via specificity for host cell surface sialic acid (SA) residues, which are common to many animal species and cell types, whilst NA acts to cleave terminal residues, which are common to many animal species and cell types, whilst NA acts to cleave terminal SA residues, facilitating viral release [7]. Nucleoprotein (NP) is encoded on Segment 5, and mainly SA residues, facilitating viral release [7]. Nucleoprotein (NP) is encoded on Segment 5, and mainly serves to bind the segmented RNA genome. The viral RNA Segment 7 encodes proteins that enclose serves to bind the segmented RNA genome. The viral RNA Segment 7 encodes proteins that enclose the virion to provide a structural scaffold (M1) and a proton ion channel required for viral entry and the virion to provide a structural scaffold (M1) and a proton ion channel required for viral entry and exitexit (M2) (M2) [ 6[6,7].,7]. TheThe non-structur non-structuralal protein protein 1 1(NS1) (NS1) and and nuclear nuclear export export protein protein (NEP) (NEP) are encoded are encoded by byRNA RNA Segment Segment 8. 8.NS1 NS1 has has a amajor major role role in in restrict restrictinging the the host host cell cell immune immune response response by by limiting limiting interferoninterferon production, production, as as well well as as modulating modulating viral viral RNA RNA replication, replication, viral viral protein protein synthesis synthesis and and host-cell host- physiologycell physiology [8]. NEP [8]. NEP mediates mediates the exportthe export of viral of vira RNAl RNA from from the the nucleus nucleus to theto the cell cell cytoplasm cytoplasm [9 ].[9]. FigureFigure 1. 1.Diagrammatic Diagrammatic representation representation of theof the influenza influenza A virus A virus (IAV)and (IAV) itsand viral its genome.viral genome. Eight internalEight ssRNAinternal segments ssRNA segments encode the encode major the viral major proteins viral of:proteins the RNA-dependent of: the RNA-dependent RNA polymerase RNA polymerase (PB2, PB1 and(PB2, PA); PB1 HA and providing PA); HA providing the structural the structural basis for basis host for binding host binding and viral and entry;viral entry; NA NA facilitating facilitating viral release,viral release, the binding the binding viral NP;viral RNA NP; RNA Segment Segment 7 (M) 7 encoding(M) encoding the matrixthe matrix sca ffscaffoldingolding protein protein (M1) (M1) and membraneand membrane protein protein (M2); RNA(M2); SegmentRNA Segment 8 (NS) encoding8 (NS) encoding a non-structural a non-structur proteinal andprotein NEP. and Reprinted NEP. byReprinted permission by from permission Springer from Nature: Springer Springer, Natu Naturere: Springer, Reviews Nature Disease Re Primersviews Disease [6], Copyright Primers (2018).[6], Copyright (2018). The segmented nature of the IAV genome enables genetic reassortment, a process by which completeThe viralsegmented segments nature are of exchanged the IAV genome within aenables cell co-infected genetic reassortment, with differing a process influenza by which viruses. Reassortmentcomplete viral of ansegments IAV genome are exchanged can then generatewithin a ancell antigenic co-infected shift, with in whichdiffering the influenza resulting virusviruses. may produceReassortment novel antigenicof an IAV proteinsgenome forcan whichthen generate there is an no antigenic pre-existing shift, immunity. in which the The resulting mixing ofvirus viral Microorganisms 2020, 8, 778 3 of 13 Microorganisms 2020, 8, x FOR PEER REVIEW 3 of 14 may produce novel antigenic proteins for which there is no pre-existing immunity. The mixing of genomes also enhances viral diversity, since strains from different animal species may mix freely within viral genomes also enhances viral diversity, since strains from different animal species may mix freely a susceptiblewithin a susceptible host. In addition, host. In selectiveaddition, environmentalselective