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Journal of Clinical Microbiology, April 2005, p. 1869-1878, Vol. 43, No. 4
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.4.1869-1878.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Jerome L. and Dawn Greene Infectious Disease Laboratory, Mailman School of Public Health, Columbia University, New York, New York,1 Neurovirosis Division, Virology Department, Instituto Nacional de Enfermedades InfecciosasANLIS "Dr. Carlos G. Malbrán," Buenos Aires, Argentina,2 Services of Diagnostic Microbiology and Virology, Centro Nacional de Microbiología, Instituto de Salud Carlos III, Majadahonda, Madrid, Spain,3 Cátedra de Virología, Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Buenos Aires, Argentina4
Received 10 September 2004/ Returned for modification 9 November 2004/ Accepted 15 December 2004
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The single-stranded mumps virus genomic RNA contains seven genes encoding the nucleocapsid (N), phospho (P), membrane (M), fusion (F), small hydrophobic (SH), hemagglutinin-neurominidase (HN), and large (L) proteins (7, 8). The SH gene encodes a protein of 57 amino acids (24). Reduced or absent expression of the SH gene, with a concomitant reduction of SH protein, has been described for certain mumps virus strains (3, 24, 25). Lack of expression of the SH protein does not affect virus replication in vitro, but may modify virus pathogenesis in vivo (24). Phylogenetic comparison of the SH gene revealed the existence of 10 genotypes, designated A to L (2, 10-14, 20, 27-30, 33, 34). Confirmation of mumps viral infection is generally carried out by direct culture of virus from cerebrospinal fluid (CSF). This method is time-consuming, expensive, and unable to detect low titers of virus.
The most promising development in direct detection of virus in the central nervous system has been the application of PCR. In a retrospective study in Argentina, mumps virus RNA was detected by a mumps reverse transcription (RT)-nested PCR targeting the NP gene in 25% of 236 CSF samples from patients with a clinical diagnosis of aseptic meningitis or acute encephalitis (22). Mumps virus PCR is more sensitive than viral culture and may detect mumps virus in CSF when viral culture does not (22). Here we describe a consensus RT-nested PCR assay, targeting the SH gene, for characterization of mumps virus. A simple sequence analysis of amplified products readily distinguishes vaccine and wild-type infections and can be used to characterize circulating genotypes for epidemiological research.
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Clinical specimens. Clinical specimens were obtained from the specimen collections at the Diagnostic Microbiology Service (Centro Nacional de Microbiología, ISCIII, Spain) and the Neurovirosis Division (INEI, Instituto C. Malbrán, Argentina). Specimens consisted of two virus isolates and 23 clinical samples (16 CSF, 7 sera, and 1 saliva). Clinical data for the samples and patients is shown in Table 1.
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TABLE 1. Clinical data for the specimens presented in this studya
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Synthetic standards. To test the reactivity of the selected primer pairs against mumps virus genotypes that were not available as clinical or laboratory isolates, we synthesized the SH genomic region using overlapping PCR. Eighty nucleotides of the target sequences were designed to overlap each other by 25 bp. Two artificial 25-nucleotide sequences were added to the 5' and 3' ends of the region selected. Equimolar amounts of these oligonucleotides were mixed, and the full-length product was generated in an extension reaction. Following amplification by PCR using primers complementary to the artificial 25-nucleotide flanking sequences, the product was cloned into vector pGEM-T Easy (Invitrogen, Carlsbad, CA). Serial dilutions of linearized plasmid were used to optimize the assay. Thereafter, RNA standards were generated by in vitro transcription of linearized plasmid DNA (mMESSAGE mMACHINE T7; Ambion, Austin, TX) and used to determine assay sensitivity.
RNA extraction and nested RT-PCR from clinical samples. Nucleic acids from virus isolates and clinical samples were precipitated as previously described (5a). After processing, the dried pellet was dissolved in 10 µl of water for immediate use. Five µl of RNA was added to 45 µl of RT-PCR mixture comprising components for reverse transcription and PCR steps (Access RT-PCR System; Promega Co., Madison, WI). The first reaction mixtures contain avian myeloblastosis virus (AMV)/Tfl 5x reaction buffer, 400 mM each deoxynucleoside triphosphate (dNTP), 2 mM Mg2SO4, 5 units of AMV reverse transcriptase, 5 units of Tfl DNA polymerase, and 10 pmol of each of the mumps SH sense (Mumps-SH-1S) and mumps antisense (Mumps-SH-1AS) primers. Samples, isolated virus, and controls were subjected to an initial cycle for reverse transcription at 42°C for 45 min, forty cycles (94°C for 1 min, 50°C for 1 min and 68°C for 1 min), and a final incubation at 68°C for 5 min.
Nested PCR amplifications were performed using a reaction mixture containing 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 200 µM each dNTP, 2.5 mM MgCl2, 2.5 units of Taq polymerase (Amplitaq; Perkin-Elmer Cetus, Norwalk) and 10 pmol of Mumps-SH-2A and Mumps-SH-2S degenerate primers. Nested PCR assays were subjected to an initial cycle of 94°C for 2 min and thirty cycles (94°C for 1 min, 50°C for 1 min and 72°C for 1 min) followed by a final incubation at 72°C for 5 min. Amplifications were carried out in a PTC-200 (Peltier Thermal Cycler, MJ Research, Inc., MA) utilizing thin-walled reaction tubes. Upon completion of nested PCR assays, 5 µl of each reaction mixture was analyzed by 2% agarose gel electrophoresis. Amplification products were purified by isopropanol precipitation and sequenced with standard sequencing methods.
Mumps virus sequence database. A mumps virus sequence database was constructed by extracting sequences from the National Center for Biotechnology Information GenBank. Each sequence was identified by name, place of isolation or detection, year, and genotype. Previously described genotypes were taken from their corresponding references.
A manual search was carried out for all the sequences in GenBank encompassing the targets of our selected primers. Genetic characterization was performed on a total data set of 375 mumps virus sequences. The database is accessible at http://www.greeneidlab.columbia.edu
Sequence analysis of amplified products. Original sequence data were first analyzed by the CHROMAS software (version 1.3, McCarthy 1996; School of Biomolecular and Biomedical Science, Faculty of Science and Technology, Griffith University, Brisbane, Queensland, Australia), and forward and reverse sequence data of each sample were aligned with the program EDITSEQ (DNASTAR Inc., Software, Madison, Wisconsin). The consensus sequence was compared and aligned to other samples or DNA database sequences using the program CLUSTAL X (version 1.83). The MEGA package (version 2.1) was used to produce phylogenetic trees using neighbor joining as the method to reconstruct the phylogeny and Kimura two-parameter as the nucleotide substitution calculation method. The statistical significance of a particular tree topology was evaluated by bootstrap resampling of the sequences 1,000 times.
Pairwise comparisons of the mumps virus database were done by global alignment using the Needleman Wunsch (17) algorithm, implemented by a program from EMBOSS, the European Molecular Biology Open Software Suite (23). Z-scores were calculated to test the significance of each pairwise alignment by Monte Carlo simulation on the shuffled sequences. Statistical analysis was conducted with the SPSS statistical package (SPSS Software, Chicago, Ill.). An automated program to perform the same analysis is available at http://www.greeneidlab.columbia.edu.
Nucleotide sequence accession number. The GenBank accession numbers of the nucleotide sequences determined in this study are AY735412 to AY735441.
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TABLE 2. Sequence alignment of primers and mumps virus strain sequences available on genomic databases
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Specificity of RT-nested PCR assays was examined using a panel consisting of dilutions equivalent to 10 TCID50 of prototype strains of the following paramyxoviruses: measles virus; canine distemper virus; respiratory syncytial viruses A and B; and human parainfluenza viruses 1, 2, 3, and 4. All were negative in the assay.
Analysis of MV-positive clinical samples. Isolates of MV from clinical samples obtained through years 1995 to 2001 and samples of CSF previously positive for mumps virus RNA were amplified and directly sequenced in both directions. All samples previously positive for mumps virus RNA (diagnostic assay) were positive for the SH RT-nested PCR assay.
Phylogenetic analysis of clinical samples and generation of a complete mumps virus SH database. A database was constructed comprising all published SH gene sequences and used to analyze the classification scheme and phylogenetically compare the sequences obtained by sequencing. The trees obtained by analysis of representative strains of all genotypes and our unknown sample sequences allowed quick and easy differentiation of the corresponding genotype (Fig. 1).
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FIG. 1. The set of nucleotide sequences was aligned with Clustal W. Phylogenetic analysis was performed using the Kimura two-parameter model as a model of nucleotide substitution and using the neighbor-joining method to reconstruct the phylogenetic tree (MEGA version 2.1 software package). The statistical significance of the phylogenies constructed was estimated by bootstrap analysis with 1,000 pseudoreplicate data sets. For clarity, only a subset of the database is shown.
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However, the generation of a database comprising all sequence information available for the region selected allowed us to delineate some classification problems; these problems were mainly related to the use of limited sets of available sequences for phylogenetic analysis.
In our analysis, group G and the strains that belong to this group, reported by Inou et al. (10) and Jin et al. (11), could be further differentiated as members of two different subgenotypes, G1 and G2, at the nucleotide (Fig. 2a) and amino acid (Table 3) levels. The subgrouping for selected sequences of genotype H by Utz et al. (30) could be applied to all remaining known sequences of the genotype allowing the subclassification of the strains into two groups, namely H1 and H2, and characterizing the Spanish isolates in the H2 subgenotype (Fig. 2b). The subdivision is also supported at the amino acid level (Table 3).
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FIG. 2. (a) Phylogenetic tree of a genotype G strain; genotype B sequences were included as the outgroup. (b) Phylogenetic tree of genotype H strains; genotype C sequences were included as the outgroup. (c) Phylogenetic tree of genotype D and K strains, genotype F sequences were included as the outgroup. All available sequences of the corresponding genotype were included in the trees. The analysis method was the same as that reported for Fig. 1.
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TABLE 3. Alignment of deduced amino acid sequences of clusters of mumps virus SH protein sequencesa
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Genotype D described in 1997 was a highly heterogeneous group that included strains of the actually recognized genotypes D, K, and H. A subset of those sequences reported by Orvell et al. (20) were only included for analysis later by Wu et al. (33). They were not included in any posterior analysis. We observed that they form a separate group to the actual genotype D (group D1 in the tree) (Fig. 2d), forming the group D2. The new proposed group seems to be an ancestor of the sequences of genotype K described originally by Tecle et al. (26) and also sequences of genotype L. Indeed, one of the old sequences (SE V28) belongs to genotype K. Genotypes D1, D2, K, L, and H are different at the amino acid level (Table 3).
Finally, although the topology of the tree is the same when all published MV sequences are included in the analysis, bootstrap values are consistently lower and sometimes not significant for some groups (data not shown). Some strain sequences confounded the analysis due to sequence dissimilarity and were classified as outliers. The strains RW (X63708), ge2-87 (reconstructed from the publication) (2), Tay-UK50 (AF142774), MP93-N (AB003415), and UK02-19 (AY380077) are in this list. The strains Tay-UK50 and UK02-19 have been reported previously as outliers (10, 12, 14) or proposed as a new genotype (12), respectively. The strain RW has been initially presented in group B5 in the initial classification of mumps virus SH genotypes (2) and after that was reported as genotype D (20, 26) or lately as an outlier (10, 30). The strain MP93-N was reported as a member of genotype D (10, 26, 33). However, in our analysis when we include all other strains of mumps virus, it is clearly separated at the base of the group D. In the previous publications, the strain also appears as a distant member of the group. We proposed this strain as a new outlier. Finally, the postvaccinal strain ge2-87, originally reported as the unique member of group B4 (2) and then reported in group D (33) and not included in any other analysis, thereafter is proposed here as being another outlier. Interestingly, most of the outliers seem to be related to genotype D. We found that their exclusion from the phylogenetic analysis resulted in improved bootstrapping values for other groups.
Sequence analysis results. Pairwise sequence analysis using Needleman Wunsch global alignment was carried out on the 200-bp SH fragment. An all-against-all sequence comparison was made to evaluate the possibility of using sequence similarity to classify genotypes. Significant sequence similarity was observed when comparing sequences within the same genotype. This was evaluated by analysis of variance between groups, comparing the scores of sequence comparisons within genotypes to comparisons between genotypes (Fig. 3) Each group was significant to the P < 0.001 level. Genotypes with only one member sequence were excluded from the analysis. Previously untyped sequences were assigned to the group with the greatest similarity. All unknowns were classified correctly into their genotypic group (Fig. 4).
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FIG. 3. Pairwise analysis of all available sequences of mumps virus SH against themselves is represented in the graph. Pairwise nucleotide alignment between all members of the database was done. The average score between members of each genotype is plotted in the graph. The different colors represent the Needleman Wunch scores obtained for each genotype comparison.
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FIG. 4. Pairwise analysis of four mumps virus strains randomly chosen from the database. Samples are 570LCR-Argentina, genotype A; SF47-Argentina, genotype D2; V0012-Spain-1996, genotype H1; and IS98-58-KOREA-1997 (GenBank accession number AF180384). A pairwise Needleman Wunch score for the test sequence against all other members of the database was determined. The average score (with the corresponding standard deviation) against members of each genotype is plotted in the graph.
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The use of pairwise comparison to classify sequences has been used for enteroviruses (19, 21) and potyvirus (31). Multiple alignment and rigorous phylogenetic methods are preferable to establish exact lineages of sequence strains, discover recombination events, and positively identify regions; however, pairwise comparison can substitute if only a high level of taxonomic classification is desired. Here, we show that mumps virus genotypes can be classified using the sequence of SH gene region amplified by the PCR. The advantages of pairwise comparison for classification are speed, simplicity, and availability. The database and classification scheme provide a repository for sequences, complementing efforts in tracking mumps virus genotype distribution. A website has been deployed wherein clinical labs can post their sequences (www.greeneidlab.columbia.edu.) and location and circumstances of isolation or detection. The laboratory would instantly receive a report detailing the genotype, date, and location of the most similar sequence isolate in the database. New genotypes can readily be identified because the classification scheme will fail to relate them to any described group.
In conclusion, we propose a sensitive RT-nested PCR-based method for the molecular identification of mumps infections. The method has been tested using a broad range of genotypes and clinical samples. Major advantages of this approach are that cell culture is not needed, only a short fragment is required for sequencing, and a "sequence similarity-based" software is established that facilitates the rapid acquisition of results. This approach constitutes a useful tool for both the diagnosis and the epidemiological research on mumps infections in endemic and nonendemic areas. We have detected vaccine-associated disease, associated the sequences obtained with known genetic groups, and characterized strains of mumps virus circulating in South America. The use of an extended database of all available sequences of the mumps virus SH gene has also allowed us to reassess some of the previous classification attempts and propose or reevaluate some groupings.
Mumps SH-1 AS5' GATCAMYCACTCTAGAAAGATCYY 3' GAAAGATCTCCAGTTTGAACACGTCC (3) GAAAGATCTCCAATTAGGACAAGTCC (2) GAAAGATCTCCAGTTAGGACAAGTCC (2) GGAAGATCTCCAGTTAGGACACGTCC (1) GGAAGATCCCCAGGTGGGACAAGTCC (1) GAAAGATCTCCAGTTAGGGCAAGTCC (1) GAAAGATCCCTAGTTAGGACAAGTCT (1) GAACGATCTCCAGTTAGGACAAGTCT (1) GATAGATCTCCATTTAGGACAGGTCC (1) GATAGATCTCTATTTAGGACAAGTCC (1) GAAAGATCTCCATCTAGGACAAATCC (1) GAAAGGTCTCCAGCTGGGAAAAGTCC (1) GAAAGATCTCCAGCTGGGAAAAGTCC (1) GAAAGATCTCCAGCGGGGAAAAGTCC (1) GAAAGATCTCCAGCCCGGACAAGTCC (1) GAGAGATCTCCAGCCAGGACAAGTCC (1) Mumps SH-1AS5' GATCAMYCACTCTAGAAAGATCYCYAR 3' internal GATCACTCACTCTAGAAAGATCTCCAG (54) antisense GATCACCCACTCTAGAAAGATCTCCAG (16) (6427-6453) GATCACTCACTCTAGAAAGATATCCAG (12) GATCACTCACCCTAGGAAGATCTCCAG (10) GATCACTCACTCTAGGAAGATCTCCAG (8) GATCACTCACTCTAGAAAGATCTCCAA (7) GATCACTCATTCTAGATAGATCTCCAT (5) GATCAATCACTCTAGAAAGATCCCCAG (5) GATCAATCACTCTAGAAAGATCCTCAG (4) GATCACTCACTCTAGAAAGATCCCCAA (4) GATCACTCACTCTAAGAAGATCCCCAG (1) GATCACTCACTCTAGAAAGATCTCTAG (1) GATCACTCACTCTAGAAAGATCTCTAG (1) GATCACCCACTCTAGAAAGATCCCTAG (1) GATAACCCACTCTAGAAAGATCTCCAG (1) GATCACCCACTCTAGAAAAATCTCCAG (1) GATCATTCACTTTAGAAAGATCTCCAG (1) GATCACCCACTTTAGAAAGATCTCCAG (1) GATCATCAACTCTAGAAAGATCTCCAG (1) GATCACTCACTCTAGAACGATCTCCAG (1) GACCACCCACTCTAGAACGATCTCCAG (1) GATCACTCACTCTAGAAAGGTCTCCAG (1) GATCACTCACTCTAGAGAGATCTCCAG (1) GATCACTCATTCTAGATAGATCTCTAT (1) GATCACTCACTCTAGAAAGATCTCCAT (1) GATCACTCACCCTAGAAAGATCTCCAG (1)
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