Genetic diversity of enterovirus subgroups

Parechoviruses, a novel group of human picornaviruses

Parechoviruses are a recently established group of human viral pathogens. At the time of their first isolation, parechoviruses were classified among the enterovirus genus in the picornavirus family, but based on their different biological properties they were separated into their own genus. The type member is human parechovirus 1 (HPEV1), which frequently infects humans, in particular small children. The parechovirus genus also includes HPEV2 and the Ljungan virus, which was recently isolated from rodents, is a candidate for the group. Seroepidemiological studies have shown that the prevalence of HPEV1 antibodies is surprisingly high, exceeding 95% in adult populations. According to present data, HPEV1 causes mainly gastrointestinal and respiratory infections; however, severe disease conditions, such as myocarditis and encephalitis, have also been reported. HPEV2 infections appear to be rare, and it is currently not known whether the Ljungan virus can infect humans.

Molecular detection and identification of an enterovirus during an outbreak of aseptic meningitis

Stool samples from sixteen cases of children with meningitis originating from four different and geographically isolated parts of Greece were investigated for enteroviruses. The conventional method of cell culture in four different cell lines was initially used for the isolation of enteroviruses. The results showed a cytopathic effect (CPE) in all cases after two, or even more successive passages in only one cell line (RD), although a less-than-satisfactory CPE was obtained in many cases. Seroneutralization with RIVM mixed hyperimmune antisera followed and the isolates were typed as Coxsackie B viruses. The method of RT-PCR with enterovirus-specific primers targeted to the highly conserved 5'-UTR of the genome was initially used for the detection of enteroviruses from the inoculated cell cultures. A positive RT-PCR result was obtained for all of the clinical samples rapidly and accurately and the isolates were further characterized with the aid of Restriction Fragment Length Polymorphism (RFLP) analysis and Single Strand Conformation Polymorphism analysis (SSCP) of the amplicons. The RFLP analysis showed first of all that the isolates had an identical restriction pattern with Coxsackie B5 Faulkner reference strain with 4 out of 5 restriction enzymes and secondly, both RFLP and SSCP analysis indicated the epidemiological association of the isolates. The speed of the molecular methodology that was used in comparison with the conventional methods and its possible significance for the description of virus evolution and circulation in the populations is discussed.

Molecular determinants for virulence in coxsackievirus B1 infection

Studies demonstrated that a strain derived from an infectious clone of coxsackievirus B1 (CVB1N) (N. Iizuka, H. Yonekawa, and A. Nomoto, J. Virol. 65:4867-4873, 1991) was 3 to 4 log10 less virulent than the myotropic Tucson strain of CVB1 (CVB1T) following intraperitoneal inoculation of newborn mice. Replacement of nucleotides (nt) 69 to 804 from the 5' untranslated region (5' UTR) and 1A coding region of CVB1N or nt 117 to 161 from the 5' UTR with the corresponding part from CVB1T restored greater than 90% of the virulence. Sequencing of the 5' UTR of CVB1T demonstrated areas with a greater similarity to particular echoviruses than to CVB1N, suggesting that recombination events might have occurred, perhaps influencing the virulence phenotype.

Kissing of the two predominant hairpin loops in the coxsackie B virus 3' untranslated region is the essential structural feature of the origin of replication required for negative-strand RNA synthesis

Higher-order RNA structures in the 3' untranslated region (3'UTR) of enteroviruses are thought to play a pivotal role in viral negative-strand RNA synthesis. The structure of the 3'UTR was predicted by thermodynamic calculations using the STAR (structural analysis of RNA) computer program and experimentally verified using chemical and enzymatic probing of in vitro-synthesized RNA. A possible pseudoknot interaction between the 3D polymerase coding sequence and domain Y and a "kissing" interaction between domains X and Y was further studied by mutational analysis, using an infectious coxsackie B3 virus cDNA clone (domain designation as proposed by E. V. Pilipenko, S. V. Maslova, A. N. Sinyakov, and V.I. Agol (Nucleic Acids Res. 20:1739-1745, 1992). The higher-order RNA structure of the 3'UTR appeared to be maintained by an intramolecular kissing interaction between the loops of the two predominant hairpin structures (X and Y) within the 3'UTR. Disturbing this interaction had no effect on viral translation and processing of the polyprotein but exerted a primary effect on viral replication, as was demonstrated in a subgenomic coxsackie B3 viral replicon, in which the capsid P1 region was replaced by the luciferase gene. Mutational analysis did not support the existence of the pseudoknot interaction between hairpin loop Y and the 3D polymerase coding sequence. Based on these experiments, we constructed a three-dimensional model of the 3'UTR of coxsackie B virus that shows the kissing interaction as the essential structural feature of the origin of replication required for its functional competence.

Sequence comparison of echovirus type 30 isolates to other enteroviruses in the 5'noncoding region

The genetic relationship between echovirus type 30 (E30) isolates were characterised by means of amplification of a part of the 5'noncoding region by polymerase chain reaction and direct sequencing. Later, the E30 sequences were compared with other enteroviruses. Homology between recent clinical E30 isolates from different years exceeds 98% at the nucleotide level. Comparing the E30 sequence with other enteroviruses, homology varied between 68% (coxsackievirus A24) and 93% (coxsackievirus B3). E30 appeared to have coxsackie B-like characteristics in this genomic part. It is considered that E30 and coxsackie B viruses belong to the same enterovirus subgroup.

Detection of adenoviruses and enteroviruses in polluted waters by nested PCR amplification

A procedure has been developed for the rapid detection of enteroviruses and adenoviruses in environmental samples. Several systems for virus concentration and extraction of nucleic acid were tested by adding adenovirus type 2 and poliovirus type 1 to different sewage samples. The most promising method for virus recovery involved the concentration of viruses by centrifugation and elution of the virus pellets by treatment with 0.25 N glycine buffer, pH 9.5. Nucleic acid extraction by adsorption of RNA and DNA to silica particles was the most efficient. One aliquot of the extracted nucleic acids was used for a nested two-step PCR, with specific primers for all adenoviruses; and another aliquot was used to synthesize cDNA for a nested two-step PCR with specific primers for further detection of seeded polioviruses or all enteroviruses in the river water and sewage samples. The specificity and sensitivity were evaluated, and 24 different enterovirus strains and the 47 human adenovirus serotypes were recognized by the primers used. The sensitivity was estimated to be between 1 and 10 virus particles for each of the species tested. Twenty-five samples of sewage and polluted river water were analyzed and showed a much higher number of positive isolates by nested PCR than by tissue culture analysis. The PCR-based detection of enteroviruses and adenoviruses shows good results as an indicator of possible viral contamination in environmental wastewater.

A distinct picornavirus group identified by sequence analysis

Although echovirus 22 is presently classified as a member of the enterovirus group in the family of picornaviruses, it has been reported to have exceptional biological properties when compared with other representatives of the group. We have determined the complete nucleotide sequence of the echovirus 22 (Harris strain) genome, which appears to be significantly different from all the other studied picornaviruses. However, the organization of the genome [7339 nucleotides, excluding the poly(A) tract] is similar to that of previously sequenced picornaviruses. This genome includes a 5' untranslated region, relatively well-conserved when compared with aphtho- and cardioviruses, followed by an open reading frame coding for a 2180-amino acid-long polyprotein. The amino termini of capsid polypeptides VP1 and VP3 were determined by direct sequencing, and the other proteolytic cleavage sites in the polyprotein were predicted by comparison with other picornavirus proteins. The amino acid identities of echovirus 22 polypeptides with the corresponding proteins of other picornaviruses are in the 14-35% range, similar to those percentages seen when representatives of the five picornavirus groups (entero-, rhino-, cardio-, aphtho-, and hepatoviruses) are compared. Our results suggest that echovirus 22 belongs to an independent group of picornaviruses.

The 3' terminal sequence of a human astrovirus

We have determined the sequence for 1,000 bases from the 3' terminus of a human astrovirus serotype 1 isolated in Newcastle. This is the first sequence reported for a representative of this virus family. We find one open reading frame which terminates 83 bases from a poly A tail. The 3' non-coding-region has similarities to some picornavirus termini. However the amino acids specified by the coding region have no significant homology to the picornavirus protein 3D, encoded at the 3' terminus of these viruses. Northern blot analysis of intracellular virus-specific RNAs revealed one size of transcript which corresponded to full-length virus RNA. Available data thus indicate that astroviruses may resemble picornaviruses in replication strategy.

Towards identification of cis-acting elements involved in the replication of enterovirus and rhinovirus RNAs: a proposal for the existence of tRNA-like terminal structures

On the basis of a comparative analysis of published sequences, models for the secondary structure of the 3'-terminal [poly(A)-preceding] untranslated region of the entero- and rhinovirus RNAs were worked out. The models for all these viruses share a common core element, but there are an extra enterovirus-specific element and still an additional element characteristic of a subset of enterovirus RNAs. The two latter models were verified for poliovirus and coxsackievirus B genomes by testing with single-strand and double-strand specific enzymatic and chemical probes. A tRNA-like tertiary structure model for the 3'-terminal folding of enterovirus RNAs was proposed. A similar folding was proposed for the 3' termini of the negative RNA strands as well as for the 5' termini of the positive strand of all entero- and rhinovirus RNAs. Implications of these data for template recognition during negative and positive RNA strands synthesis and for the evolution of the picornavirus genomes are discussed.

Competition binding studies with biotinylated echovirus 11 in cytofluorimetry analysis

Competition binding studies between viruses are usually performed with radiolabelled probes. In this report, a cytofluorimetric method using biotinylated echovirus (EV) 11 is described for the study of competition of enteroviruses for a common cell receptor site. An N-hydroxysuccinimide ester biotin spacer arm was used for biotinylation of CsSO4-purified EV 11. Biotinylation did not change the infectivity of the virus (attachment to and replication in susceptible cells). With the exception of EV 22 and EV 23, all the echovirus serotypes and also coxsackievirus A9 (CA 9) were able to inhibit the absorption of biotinylated EV 11 onto cells. The taxonomic implications of these findings are discussed.

The 5'-untranslated region of picornaviral genomes

Picornaviruses are small naked icosahedral viruses with a single-stranded RNA genome of positive polarity. According to current taxonomy, the family includes four genera: Enterouirus (polioviruses, coxsackieviruses, echoviruses, and other enteroviruses), Rhinovirus, Curdiouirus [encephalomyocarditis virus (EMCV), mengovirus, Theiler's murine encephalomyelitis virus (TMEV)], and Aphthouirus [foot-and-mouth disease viruses (FMDV)]. There are also some, as yet, unclassified picornaviruses [e.g., hepatitis A virus (HAW] that should certainly be assessed as a separate genus. Studies on the molecular biology of picornaviruses might be divided into two periods: those before and after the first sequencing of the poliovirus genome. The 5'-untranslated region (5-UTR) of the viral genome was one of the unexpected problems. This segment proved to be immensely long: about 750 nucleotides or ∼10% of the genome length. There were also other unusual features (e.g., multiple AUG triplets preceding the single open reading frame (ORF) that encodes the viral polyprotein). This chapter shows that the picornaviral 5-UTRs are not only involved in such essential events as the synthesis of viral proteins and RNAs that could be expected to some extent, although some of the underlying mechanisms appeared to be quite a surprise, but also may determine diverse biological phenotypes from the plaque size or thermosensitivity of reproduction to attenuation of neurovirulence. Furthermore, a close inspection of the 5-UTR structure unravels certain hidden facets of the evolution of the picornaviral genome. Finally, the conclusions drawn from the experiments with the picornaviral5-UTRs provide important clues for understanding the functional capabilities of the eukaryotic ribosomes.

Common and specific sequences in picornaviruses

Enteroviruses and rhinoviruses were studied by nucleic acid hybridization using oligonucleotide probes. The reactivity of these sequences varied from serotype specificity to wide reactivity with nearly all human picornaviruses tested. Widely reactive sequences were from the 5' non-coding region while specific oligonucleotides were found from the coding region. Sequences that were common to both enteroviruses and human rhinoviruses were identified and one of the oligonucleotides reacted with a majority enterovirus serotypes.

Identification of rhinoviruses by cDNA probes

We have used nucleic acid hybridization for the detection and grouping of human rhinoviruses (HRV) according to their genetic relationships. Fifteen rhinovirus reference strains, seventy-one clinical isolates and four enteroviruses were propagated in cell cultures, spotted onto membrane filters and hybridized with radioactively labelled cDNA probes covering different parts of the genomes of HRV-1B, HRV-2, HRV-14, HRV-85 and HRV-89. When the rhinovirus and enterovirus reference strains were tested, the 5' probe of HRV-2 hybridized with thirteen of the fifteen HRV reference strains, with poliovirus type 3 and with ECHO virus 11. The HRV-14 5' probe reacted with eleven HRV reference strains and with all the enteroviruses studied. Sixty-nine of the 71 clinical isolates were recognised by the HRV-2 5' probe, whereas the HRV-14 probe from the same part of the genome hybridized with 54 field isolates. One of the two isolates that remained negative with the HRV-2 5' probe was detected with the HRV-2 probe that derived from the P2 region of the genome, and the other isolate was not detected by any of the probes. Probes from other parts than the 5' end of the genome were generally more specific, and clusters could be formed based on the reactivity of the HRV strains with these probes.

Identification of human picornaviruses by nucleic acid probes

Human picornaviruses include rhinoviruses and enteroviruses which are responsible for both common and severe clinical diseases. Rhinoviruses are a frequent cause of respiratory infections while members of enterovirus subgroups, polio, coxsackie and ECHO viruses are often responsible for infections of the central nervous system, myocarditis, myositis etc. Human picornaviruses consist of nearly two hundred serotypes and therefore their specific identification after virus isolation, or the diagnosis based on the detection of immune response in patients, is problematic and does not usually provide virological diagnosis at the acute phase of illness. New methods for detection of picornavirus genomic RNA together with increasing knowledge of the nucleotide sequences of this virus group offer interesting possibilities for diagnostic procedures. Spot hybridization, in situ hybridization and enzymatic amplification of specific sequences have successfully been used for this purpose. Probes covering the 5' non-coding part of the genome, and also sequences derived from the region coding for non-structural proteins, can be used as broadly reacting reagents in picornavirus detection. Specific sequences are mainly found in the capsid protein region of the genome. cDNA probes and synthetic oligonucleotides are useful in rapid identification of picornaviruses after amplification in cell cultures and in epidemiological analysis. The biochemical amplification methods may enable recognition of picornaviruses directly in clinical samples in the near future. In situ hybridization methods have been of special interest because they can be used to reveal the presence of enterovirus genomes in biopsy specimens from e.g. affected heart muscle in patients with myocarditis and cardiomyopathy.