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Journal of Clinical Microbiology, January 2005, p. 36-40, Vol. 43, No. 1
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.1.36-40.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Division of Public Health, Department of Infectious Control and International Medicine,1 Division of Clinical Nephrology, Rheumatology, Respiratory Medicine and Infection Control and Prevention, Niigata University Graduate School of Medical and Dental Sciences,4 Sano Clinic,2 Niigata Prefectural Institute of Public Health and Environmental Sciences, Niigata, Japan3
Received 13 May 2004/ Returned for modification 6 July 2004/ Accepted 2 September 2004
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RSV strains have been classified into antigenic groups A and B (RSV-A and RSV-B, respectively) on the basis of the reactivities of the viruses with monoclonal antibodies directed against the attachment glycoprotein (G protein) (1, 7, 12, 19) and also by genetic analyses (9, 25, 26). G protein is the most variable RSV protein (10, 14, 19), and its C-terminal region (the second hypervariable region) accounts for strain-specific epitopes (3, 4, 6, 9, 14, 22, 23). The molecular epidemiology and evolutionary patterns of G protein have provided important information about the clinical and epidemiological features of RSV: several different genotypes cocirculate, and the one that predominates in a community every year may change (5, 21, 22, 28). However, the importance of strain diversity to the clinical and epidemiological features of RSV has yet to be elucidated in detail.
Our objectives in the present study were to clarify the molecular epidemiology of RSV in a community over three seasons, determine the relationship between genotypes and circulation patterns, and assess clinical features and repeated infections.
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Nasopharyngeal aspirates were obtained from the patients and were stored at 4°C in the clinic. The specimens were transported to the Department of Public Health, Niigata University School of Medical and Dental Sciences, within 5 days of sampling and were kept frozen at 80°C until further examination.
RT-PCR and nucleotide sequencing. Viral RNA was extracted from 100-µl samples of the nasopharyngeal aspirates with 500 µl of Trizol (Invitrogen Corp., Carlsbad, Calif.) in 100 µl of chloroform and centrifuged. RNA from the upper aqueous phase was precipitated with 100% isopropanol and purified with ethanol. Reverse transcription (RT) was then performed for cDNA synthesis by using 3 µg of random primers (Invitrogen Corp.) and 200 U of Moloney murine leukemia virus (Invitrogen Corp.) by incubation at 37°C for 1 h.
First and heminested PCRs targeting the second hypervariable region of G protein were performed. The subgroup A-specific forward primer used for the first PCR was primer GPA (nucleotide positions 511 to 530), based on the primer described by Peret et al. (22), and that used for the heminested PCR was primer nRSAG (5'-TATGCAGCAACAATCCAACC-3'; nucleotide positions 539 to 558). Subgroup B-specific forward primer GPB was used for the first PCR (nucleotide positions 494 to 515) (22), and primer nRSBG (5'-GTGGCAACAATCAACTCTGC-3'; nucleotide positions 512 to 531) was used for the heminested PCR. The reverse primer specific for both subgroups A and B was primer F1 (nucleotide positions 3 to 22) (22). The nucleotide positions were based on the sequences of prototype strains A2 and 18537 of subgroups A and B, respectively (14). We modified the heminested forward primers reported by Peret et al. (22), since both RSV-A and -B became positive by the heminested primer for RSV-A and the same misannealing happened to RSV-B due to the similar nucleic acid alignment of our strains of RSV-A and -B. Similar sequences between subtypes in the region of G protein were not found in prototype RSV strains. cDNA (1 to 3 µl) was added to 20 µl of the reaction mixtures, which contained optimized buffers, each deoxynucleoside triphosphate at a final concentration of 200 µM, 3.0 mM MgCl2, 0.5 µM forward and reverse primers, and 0.5 U of Taq DNA polymerase (Promega, Madison, Wis.). Amplification was conducted for 2 min at 95°C, followed by 30 cycles of 94°C for 1 min, 50°C for 1 min, and 72°C for 2 min, with a final 7 min of extension at 72°C. Finally, the amplified product was analyzed by electrophoresis on a 3% agarose gel containing ethidium bromide, and the sizes of the amplicons were compared with those of standard molecular size markers. To validate the amplification process to exclude the presence of carryover contamination, positive and negative controls were run in each PCR.
Heminested PCR primers were used as the sequencing primers. Final PCR products were sequenced by using fluorescent dye-labeled terminators on an ABI 310 sequencer (Perkin-Elmer Applied Biosystems, Foster City, Calif.).
We did not isolate RSV by tissue culture, so we confirmed the presence of infection by an additional PCR with the same samples but with different primers, which targeted the N and P proteins of RSV (24), and the results were matched with those of the G protein.
Phylogenetic analysis. The nucleotide sequences of a 270-nucleotide segment of the G-protein gene second hypervariable region were aligned by using Genetyx-WIN software (version 5.1.1; Genetyx Co. Ltd., Tokyo, Japan). Unique sequences for both subgroup A and B viruses were included in the phylogenetic analysis. Phylogenetic trees were constructed by comparison of the sequences of strains from Niigata Rochester, N.Y.; Winnipeg, Manitoba, Canada; Houston, Tex.; St. Louis, Mo., with those in the GenBank database: Soweto, South Africa; Birmingham, Ala.; West Virginia; and Buenos Aires, Argentina.
Phylogenetic trees were computed and submitted to distance-based criterion analysis with ClustalW software (version 1.7; DDBJ). Trees were plotted with TreeView software (version 1.6.6). Bootstrap probabilities for 1,000 iterations were calculated to evaluate confidence estimates. Pairwise nucleotide distances within and between subgroups A and B were calculated as the numbers of pairwise nucleotide differences divided by the total number of nucleotides in the sequenced segment and were analyzed with ClustalW software (version 1.7; DDBJ).
Statistical analyses. Statistical analysis for comparison of mean values was performed by Scheffe's test. Comparison of the proportions was accomplished with 2-by-multiple tables. Statistical significance was concluded if the P value was <0.05.
Nucleotide sequence accession numbers. The GenBank accession numbers of the nucleotide sequences obtained in the present study are AB175814 to AB175823.
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TABLE 1. Distribution of RSV subgroups and genotypes and demographic details for patients infected with RSV over three epidemic seasons from November 2001 to July 2004
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FIG. 1. Phylogenetic trees for RSV subgroup A (A) and subgroup B (B) nucleotide sequences based on the second variable region of G protein (270 bp). Genotypes were assigned by Peret et al. (21, 22) (genotypes GA1 to GA7 and GB1 to GB4) and Venter et al. (28) (genotypes SAA1 and SAB1 to SAB3). The new type, named BA virus, comprises strains with a 60-nucleotide insertion. The numbers of identical strains are indicated in parentheses. Reference GenBank sequences of strains from throughout the world were compared with strains in Niigata (NG); Rochester, N.Y. (CH) (22); Winnipeg, Manitoba, Canada (CN) (21); Houston, Tex. (TX) (21), Rochester, N.Y. (NY) (21); St. Louis, Mo. (MO) (21); Soweto, South Africa (SA) (28); Birmingham, Ala. (AL) (21); West Virginia (WV) (25); and Buenos Aires, Argentina (BA) (27). The scale bars show the proportions of nucleotide substitutions, and the numbers above the horizontal lines are bootstrap values determined for 1,000 iterations with the CLUSTALW program (DDBJ). Only bootstrap values greater than 700 are shown.
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FIG. 2. Monthly distribution of 499 suspected and 185 confirmed cases of RSV infection (A), 122 cases of subgroup A RSV infection (B), and 63 cases of subgroup B RSV infection (C) from November 2001 to July 2004. Each subgroup is classified as genotype GA2, GA5, GA7, GB3, or SAB3 or the new genotype of subgroup B with a 60-nucleotide insertion.
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RSV infections were detected from November 2001 to February 2002 and were detected again starting in September 2002 (Fig. 2A). The peak month for RSV infections was December in the 2001-2002 and 2002-2003 seasons and November in the 2003-2004 season. The predominant genotype shifted from genotype GA5 to BA viruses of subgroup B in the 2002-2003 season and returned to genotype GA5 in the 2003-2004 season (Table 1; Fig. 2B and C). The average age of the patients demonstrated no significant linkage with the subgroup or genotype infecting the patients in any of the 3 years (Table 1). The numbers of hospitalized patients infected with genotype GA5 were 3 (10.3%) of 29, 2 (20.0%) of 10, and 3 (3.7%) of 82 in the 2001-2002, 2002-2003, and 2003-2004 seasons, respectively. The values for BA viruses were 5 (12.5%) of 40 and 2 (12.5%) of 16 in the 2002-2003 and 2003-2004 seasons, respectively, and that for genotype GB3 was 1 (25.0%) of 4 in the 2002-2003 season.
Eight (4.3%) of 177 patients with RSV infections were reinfected over the study period. Two patients infected with genotype GA5 were reinfected with the same genotype over 2- to 24-month periods. One patient infected with genotype GA5 virus was reinfected with BA viruses after 1 year. Four patients infected with BA viruses in the 2002-2003 season were reinfected with genotype GA5 over 4- to 14-month periods.
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Our phylogenetic analysis revealed that genotype GA5 of subgroup A was predominant in the 2001-2002 and 2003-2004 seasons, while a new genotype of subgroup B, which featured a 60-nucleotide insertion in the second variable region of G protein (BA viruses), was predominant in the 2002-2003 season. Our observations indicate that multiple genotypes cocirculate in a single epidemic and that the genotypes in each epidemic may differ, as described previously (22, 28). Genotypes of both subgroups A and B showed temporal clustering by year of detection, which supported previous findings (6). Strains detected at the end of the previous season tended to be predominant in the next season, which might be associated with antigenic evasion from host immunity.
Viruses of genotypes similar to our BA viruses were also reported in Sapporo, Japan, in 2002 (GenBank accession number AB117522). The BA virus reported by Trento et al. (27) had an exact duplication of 60 nucleotides in the C-terminal one-third of the G-protein gene and illustrated a new type of drastic change introduced in G protein during the natural propagation of RSV. Our strains of the new genotype and strains from Sapporo were demonstrated to have 1 amino acid substitution in the insert region compared to the sequence of the BA strain. It is noted that the specific strains circulated in two countries, one in South America and another in Northeast Asia, after approximately 3 years with only a minor modification of the amino acid, which could support the robustness of the virus. The C terminus of the G-protein molecule has been shown to be immunologically relevant. Therefore, it is suggested that the 60-nucleotide insertion in the C-terminal one-third of the G-protein gene and the amino acid replacement compared with the amino acid in prototype BA strains change its antigenic structure, which confers an evolutionary advantage that allows reinfection of individuals previously exposed to the ancestor virus. However, as an emerging strain, our strain of the new genotype of subgroup B was not associated with new epidemiological or clinical features compared with those of the other clades during the three seasons that we studied. Further studies are required to determine the effect of the insertion on the immune response to RSV and susceptibility to infection and disease.
It has been reported that the severity of RSV infection may vary with the specific virus genotype (16); however, in the present study, no differences in the epidemiological or clinical manifestations, such as age or an illness that required hospitalization, were detected among these genotypes. Thus, we need continued observations to determine whether the greater severity of illness is associated with specific genotypes.
The variability of RSV strains may contribute to the cause of repeated infections, and children infected with subgroup A strains appear to be more likely to be reinfected than those infected with subgroup B strains (18). Only 8 (4.3%) of our 177 RSV patients became reinfected over the study period. With such a small number of patients, it is impossible to discuss the relationship between reinfection and genetic diversity, even with the new genotype of subgroup B strains. Furthermore, small numbers of reinfections may have been detected in our study because the patients visited other medical care facilities or the patients may have had mild symptoms during the second infections.
In conclusion, our molecular analysis of RSV in Niigata, Japan, confirmed that plural genotypes cocirculate each year and that the predominant genotype may shift with the season. A new genotype of subgroup B with a 60-nucleotide insertion, named BA-like virus, was found to be a predominant genotype, but it was not associated with new epidemiological or clinical features compared with those of the other genotypes that were present during the three seasons that we studied. Finally, our results provide support for genotype designation by RT-PCR methods as an effective tool for characterization of RSV circulation patterns in communities.
We thank A. Watanabe, A. Obata, and S. Aida for technical expertise.
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