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Journal of Virology, August 2005, p. 10561-10570, Vol. 79, No. 16
0022-538X/05/$08.00+0 doi:10.1128/JVI.79.16.10561-10570.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Departamento de Arbovirologia e Febres Hemorrágicas, Instituto Evandro Chagas (IEC), Ministério da Saúde, Belem, Para, Brazil,1 Department of Pathology and Center for Tropical Diseases, University of Texas Medical Branch (UTMB), Galveston, Texas2
Received 4 February 2005/ Accepted 21 May 2005
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Group C viruses were first described in the Brazilian Amazon region during the 1950s (4, 5, 14, 27, 33). A total of 13 distinct group C viruses have been isolated from humans, wild animals (principally rodents, marsupials, and bats), and mosquitoes (16, 27). The current classification of group C is based on antigenic relationships determined using complement fixation (CF), neutralization (NT), and hemagglutination inhibition (HI) tests. According to this serological classification, the group C viruses can be divided into four antigenic complexes: the Caraparu complex, which includes Caraparu virus (CARV), Ossa virus (OSSAV), Apeu virus (APEUV), Vinces virus (VINV), and Bruconha virus (BRCV); the Madrid complex, which includes Madrid virus (MADV); the Marituba complex, which includes Marituba virus (MTBV), Murutucu virus (MURV), Restan virus (RESV), Nepuyo virus (NEPV), and Gumbo limbo virus (GLV); and the Oriboca complex, which includes Oriboca virus (ORIV) and Itaqui virus (ITQV) (5, 7, 14, 26, 28-30).
Geographically, group C viruses occur in tropical and subtropical areas of the Americas, including the United States, Mexico, Panama, Honduras, Guatemala, Trinidad, Brazil, Peru, Ecuador, Venezuela, and French Guiana (9, 14, 16, 27). Ten of the 13 registered viruses (CARV, ORIV, ITQV, NEPV, APEUV, MTBV, MURV, RESV, OSSAV, and MADV) have been associated with human disease, which generally presents as a self-limited, dengue-like illness consisting of fever, headache, myalgia, nausea, vomiting, weakness, etc., of 2 to 5 days in duration (17, 18, 32). Given the public and veterinary health importance of other viruses included in the genus Orthobunyavirus, surprisingly little attention has been paid to the molecular biology and genetics of the group C viruses.
To address this deficiency, we determined the complete SRNA sequence for group C viruses, as well as their SRNA genetic organization. We used nucleotide sequences for the nucleocapsid gene and partial nucleotide sequences (345 nucleotides [nt]) of the Gn glycoprotein of group C viruses, as well as homologous sequences for California encephalitis, Simbu, and Bunyamwera virus groups, to determine phylogenetic relationships among group C viruses and other orthobunyaviruses. We also attempted to correlate these data with serological relationships previously provided by Shope and Causey (28).
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TABLE 1. Group C virus strains used in the current study
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SRNA and partial MRNA amplification. A one-step reverse-transcription PCR (RT-PCR) protocol was used for amplification of the full-length SRNA, using primers that corresponded to the highly conserved termini of the SRNA segment of viruses included in the genus Orthobunyavirus (8). RT-PCRs were carried out in a 50-µl reaction mixture containing 10 µl (1 to 5 ng) of viral RNA, 10 pmol of a forward primer (AGTAGTGTGCTCCAC), 10 pmol of a reverse primer (AGTAGTGTGCTCCAC), 1x PCR buffer (50 mM Tris-HCl, pH 8.3, 75 mM KCl), 2.5 mM MgCl2, 2.5 mM dithiothreitol (DTT), 20 U of RNAsin RNase inhibitor (Invitrogen, Carlsbad, CA), 200 µM of deoxynucleoside triphosphates (dNTPs) (Invitrogen, Carlsbad, CA), 1.125 U of Platinum Taq DNA polymerase (Invitrogen, Carlsbad, CA), and 1 unit of Superscript II reverse transcriptase (Invitrogen). The RT reaction was first performed for 60 min at 42°C, followed by 35 PCR cycles, each consisting of 94°C for 40 s, 54°C for 40 s, and 72°C for 1 min.
For the amplification of the partial Gn glycoprotein gene, a standard two-step RT-PCR protocol was used. For the first-strand amplification, a 20-µl reaction mixture was used, consisting of 5 µl of virus RNA (1 ng to 5 µg) and 15 µl of the RT master mix including 1x first-strand buffer (250 mM Tris-HCl, pH 8.3, 375 mM KCl, 15 mM MgCl2, 0.1 M DDT), 20 U RNasin RNase inhibitor (Invitrogen), 200 µM of dNTPs, and 50 to 250 ng of random hexamer primers. The reactions were reverse transcribed for 60 min at 42°C. The PCR was performed using 2 ng of the RT products and a PCR mixture containing 1x PCR buffer, 2.5 mM MgCl2, 200 µM of dNTPs, 10 pmol of degenerate primer BUN-GnF (AC[T/A]AAG[C/T]TATA[C/T]AG[A/G]TA[T/C]AT) and 10 pmol of degenerate primer BUN-GnR (TGACATATG[C/T]TG[G/A]TT[A/G]AAGCA), with 1.125 U of Platinum Taq DNA polymerase adjusted for a final volume of 50 µl. The amplified products were visualized on a 1.2% agarose gel, purified using the GFX PCR DNA and Band purification kit (Amersham Biosciences, Piscataway, NJ), cloned, and sequenced.
cDNA cloning.
Cloning of the cDNA fragments was done with a plasmidial-bacterial system. Purified amplicons were ligated to the pGMT-Easy Vector (Invitrogen) at the lacZ
peptide gene, and plasmid DNA from at least three recombinant bacterial colonies was recovered using the GFX microplasmid Prep kit (Amersham Biosciences, Piscataway, NJ). The restriction endonuclease EcoRI (Invitrogen) was used to release the ligated cDNA from the plasmid in order to verify the cloning efficiency.
Sequencing. The nucleotide sequences of the cloned cDNAs were determined using the ABI PRISM Dye Terminator kit (Applied Biosystems, Foster City, CA) and an ABI 377 DNA sequencer. Universal T7 and SP6 primers were used to sequence the recombinant DNA in both directions. At least three plasmid clones were sequenced for each viral amplicon.
Sequence analyses. The nucleotide sequences determined were aligned with homologous sequences from the GenBank library using ClustalX software. Phylogenetic trees were constructed using the neighbor-joining (NJ) (23), maximum-parsimony (MP), and maximum-likelihood (ML) methods, implemented with the PAUP 4.0 (31) and Mega 2.1 (15) software packages. For the likelihood analyses, the general time-reversible model was used and nucleotide frequencies were estimated empirically. For NJ analysis, a distance matrix was calculated from the aligned sequences using the Kimura two-parameter formula. Bootstrap analyses using 1,000 replicates were used to place confidence values on groupings (10) and the Kishino-Hasegawa test was used to evaluate competing topologies obtained from different viral RNA segments and using different methods (12).
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TABLE 2. Sequence characteristics of the SRNA of group C virusesa
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TABLE 3. N gene nucleotide and deduced amino acid sequence identity among group C viruses
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TABLE 4. Nucleotide sequence divergence identity and among certain group C virusesa
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TABLE 5. Partial Gn nucleotide and deduced amino acid sequence identity among group C viruses
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All trees generated from the S segment depicted the members of group C as a monophyletic group sharing a common ancestor, and group C appeared to be most closely related to viruses belonging to the Simbu serogroup (Fig. 1). In addition, group C viruses were distributed in three major lineages designated as group I (CARV, BRCV, OSSAV, ITQV, and VINV), group II (MURV, ORIV, and RESV), and group III (APEUV, MTBV, NEPV, and GLV). MADV represented a distinct lineage in the trees (Fig. 1). The phylogenetic analysis of additional strains of ORIV, CARV, ITQV, and MURV, as well as of additional strains of NEPV and MTBV, is shown in Fig. 2. Except for the CARV strain BeH 5546 that grouped with strains of ORIV, all other group C virus strains showed the expected positions in the trees (clades representing a given virus).
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FIG. 1. Phylogeny of group C virus members based on their N ORF complete nucleotide sequences. Phylogenetic analyses were carried out using both NJ and MP methods, yielding identical topologies, and the NJ tree is presented. Group C virus members were distributed into tree major lineages called groups I, II, and III. Numbers adjacent to each branch represent the percentage bootstrap support calculated for 1,000 replicates. Values inside and outside parentheses indicate bootstrap values obtained by the MP and NJ methods, respectively. Members of the California, Bunyamwera, and Simbu serogroups were used as outgroups to root the tree. The scale bar represent 10% nucleotide sequence divergence.
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FIG. 2. Phylogenetic analysis of additional group C virus strains of ORIV, MURV, ITQV, CARV, MTBV, and NEPV isolated from different geographic areas, times, and hosts. Vertical bars indicate strains isolated from humans. Bootstrap values were assigned for 1,000 replicates. Numbers inside the parentheses indicate percentage bootstrap support using MP, while numbers outside parentheses represent NJ values. Isolates labeled with stars represent group C virus prototype strains. Viruses belonging to the Simbu serogroup were used as an outgroup to root the tree. Scale bars represent 10% nucleotide sequence divergence. OROV, Oropouche virus; AINOV, Aino virus; AKAV, Akabane virus.
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FIG. 3. Comparison between S and M phylogenetic tree topologies for group C viruses. (a) N gene (705 nt) tree showing the three major groups I, II, and III. (b) Gn glycoprotein gene (345 nt) tree. Analyses using NJ and MP methods yielded identical topologies. Numbers inside and outside parentheses indicate percentages of bootstrap support obtained by the NJ and MP analyses, respectively. Horizontal branches are proportional to the scale bar, which represents 5% and 10% nucleotide sequence divergence, respectively. OROV, Oropouche virus.
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Because of these observations, group C viruses were suggested as an ideal model to study reassortment as a natural evolutionary mechanism. The patterns of antigenic relationship were first established based on serological techniques reflecting different viral proteins (CF, HI, and NT). By CF, three related pairs were identified (APEUV-MTBV, CARV-ITQV, and ORIV-MURV); however, using HI and/or NT tests, different pairings were revealed (APEUV-ITQV, ORIV-CARV, and MTBV-MURV). These specific pair associations using multiple tests became a method for rapid identification of new group C viruses and for their grouping into four complexes (Caraparu, Oriboca, Marituba, and Madrid). This classification was determined by HI and/or neutralizing antibody properties that are predominantly encoded in the virus MRNA segment and that suggested natural reassortment (28).
To better understand the genetic relationships among group C viruses, we sequenced the S segment of 13 recognized members and the partial MRNA segments of eight prototype viruses and of strain BeH 5546 were determined. Significantly different phylogenetic tree topologies were obtained for APEUV, MTBV, ORIV, CARV, ITQV, MURV, VIN, and RESV and for strain BeH 5546 using S-versus-M segment sequences, with different pairings (Fig. 3).
The phylogenetic placement of group C viruses in three major groups (I, II, and III) based on nucleocapsid gene sequences is in agreement with serologic relationships determined using the CF test, which reflect antigens of the N protein, encoded in the SRNA segment. However, these groupings differ from the current classification based on the HI and NT test results. ITQV is currently included in the Oriboca complex, APEUV is antigenically related to other group C viruses of the Caraparu complex, and RESV, MURV, and strain BeH 5546 belong to the Marituba complex. However, using S segment sequences, these three viruses grouped with the Caraparu, Marituba, and Oriboca complexes, respectively (Fig. 3). These discrepancies, supported by strong bootstrap values and significantly different likelihood scores, suggest that the S and M segments have different evolutionary histories, reflecting natural reassortment.
The analysis of additional strains of ORIV (Table 1) revealed further evidence of reassortment among group C viruses. All strains analyzed showed high nucleotide sequence identity with their respective prototype strains, except for a single Brazilian strain (BeH 5546) of CARV. This 1956 human isolate was nearly identical (mean of 95.8% nucleotide sequence identity) to the ORIV S segment sequences (Table 4). In addition, the partial Gn nucleotide sequence of strain BeH 5546 shared 98.3% sequence identity with the prototype CARV (strain BeAn 3994) yet only 71.3% identity with the prototype ORIV (strain BeAn 17) (Table 5). These results are in agreement with previous findings of aberrant conflicting antigenic relationships for strain BeH 5546, namely, that it reacts by CF like ORIV and MURV and by HI like CARV and APEUV.
The BRCV strain 77V 14814 was isolated in Sao Paulo State, in the southeast of Brazil, and was identified as a new group C member (3). In our study it showed 99.3% nucleotide sequence identity with the N gene sequence of CARV. This suggests that BRCV obtained its S segment from CARV. Further studies on the BRCV M segment are needed to confirm the reassortant history of this strain.
Genetic and geographic data for the SRNA of group C viruses isolated from different localities in the Americas (Brazil, Peru, French Guiana, Panama, Trinidad, and United States) and from different hosts (humans, mosquitoes, and sentinel animals) suggests an intense traffic of these viruses through the Americas. Phylogenetic topologies also indicate that relationships are not correlated with geographic distributions. For example, closely related sister pairs including MURV and RESV were isolated in different localities, in Brazil and Trinidad; APEUV and GLV were isolated in Brazil and the United States; and CARV and OSSA were isolated in Brazil and Panama, respectively. Virus strains isolated from humans are generally more closely related to each other than to those isolated from arthropods and sentinel animals (Fig. 2).
Evolution of segmented viruses can occur by various mechanisms, such as mutation, genetic recombination, and genetic reassortment (1, 2, 6, 13, 19, 20, 21, 22, 24). However, little is known about the relative contributions of these mechanisms to generating virus biodiversity among orthobunyaviruses and other members of the family Bunyaviridae. Antigenic, ecological, and genetic characteristics of the group C viruses indicate that several of these agents represent natural reassortants. The ecosystems where group C viruses coexist, sometimes sharing the same arthropod vectors, and vertebrate hosts, probably facilitate natural reassortment.
A hypothetic model for reassortment among group C viruses is presented in Fig. 4a. The antigenic and genetic relationships observed for group C viruses suggest a particular reassortment pattern where the reassortants rL1M1S2 and rL2M2S1 are most frequently generated (Fig. 4b). Two hypothetical origins of strain BeH 5546 are presented in Fig. 4c: (i) strain BeH 5546 received its MRNA from CARV and its S and L segments from ORIV; (ii) strain BeH 5546 received its SRNA from ORIV and its M and L segments from CARV. Nucleotide sequences for the LRNA segments of the three viruses involved (CARV, ORIV, and BeH 5546) are needed to test these hypotheses.
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FIG. 4. (a) Hypothetical model of orthobunyavirus reassortment pattern. (Part I) Coinfection of a host cell with parental viruses 1 (P1) and 2 (P2). (Part II) Uncoating of segmented virus genomes. (Part III) Replication of segmented genomes in the host cell. (Part IV) Reassortment of genome segments. (Part V) Progeny virus with independently reassorted genome segments. (b) Genetic relationships among hypothetical parental viruses (P1 and P2) and possible reassortant progeny viruses (rL1M1S2 and rL2M2S1) involved in the reassortment process of group C viruses. (c) Hypothetical reassortment patterns for strain BeH 5546. (d) Hypothetical pattern for Jatobal virus and Ngari virus.
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Our findings represent the first genetic data for group C viruses and provide a better understand of the relationships among these viruses, as well as those of other orthobunyaviruses. They also show a good correlation between antigenic, ecological, and genetic data and should improve our understanding of the molecular epidemiology and evolution of arboviruses. However, a more complete understanding of the interactions between ecosystems, host associations, and antigenic and genetic relationships and their effects on arbovirus requires further study.
This work was supported in part by NIH contract N01-AI30027, CNPq grants 302770/02-0 and 550275/01-0, and a CAPES doctoral grant (MRTN).
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