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J Clin Microbiol. Aug 2007; 45(8): 2641–2648.
Published online Jun 13, 2007. doi:  10.1128/JCM.00736-07
PMCID: PMC1951270
Development and Validation of DNA Microarray for Genotyping Group A Rotavirus VP4 (P[4], P[6], P[8], P[9], and P[14]) and VP7 (G1 to G6, G8 to G10, and G12) Genes[down-pointing small open triangle]
Shinjiro Honma,1 Vladimir Chizhikov,2 Norma Santos,1,3 Masatoshi Tatsumi,1 Maria do Carmo S. T. Timenetsky,4 Alexandre C. Linhares,5 Joana D'Arc P. Mascarenhas,5 Hiroshi Ushijima,6 George E. Armah,7 Jon R. Gentsch,8 and Yasutaka Hoshino1*
Epidemiology Section, Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland,1 Laboratory of Method Development, Center for Biologics Evaluation and Research, Food and Drug Administration, Rockville, Maryland,2 Instituto de Microbiologia, Universidade Federal do Rio de Janeiro, Rio de Janerio, Brazil,3 Instituto Adolfo Lutz, São Paulo, Brazil,4 Instituto Evandro Chagas, Secretaria de Vigilância em Saúde, Belém, Brazil,5 University of Tokyo, Tokyo, Japan,6 University of Ghana, Legon, Ghana,7 Gastroenteritis and Respiratory Viruses Laboratory Branch, Centers for Disease Control and Prevention, Atlanta, Georgia8
*Corresponding author. Mailing address: National Institutes of Health, Building 50, Room 6308, 50 South Drive, MSC 8026, Bethesda, MD 20892-8026. Phone: (301) 594-1851. Fax: (301) 480-1387. E-mail: thoshino/at/niaid.nih.gov
Supplemental material for this article may be found at http://jcm.asm.org/.
Present address: Department of Pediatrics, Sapporo University School of Medicine, Sapporo, Japan.
Received April 4, 2007; Revised May 14, 2007; Accepted June 4, 2007.
Previously, we reported the development of a microarray-based method for the identification of five clinically relevant G genotypes (G1 to G4 and G9) (V. Chizhikov et al., J. Clin. Microbiol. 40:2398-2407, 2002). The expanded version of the rotavirus microarray assay presented herein is capable of identifying (i) five clinically relevant human rotavirus VP4 genotypes (P[4], P[6], P[8], P[9], and P[14]) and (ii) five additional human rotavirus VP7 genotypes (G5, G6, G8, G10, and G12) on one chip. Initially, a total of 80 cell culture-adapted human and animal reference rotavirus strains of known P (P[1] to P[12], P[14], P[16], and P[20]) and G (G1-6, G8 to G12, and G14) genotypes isolated in various parts of the world were employed to evaluate the new microarray assay. All rotavirus strains bearing P[4], P[6], P[8], P[9], or P[14] and/or G1 to G6, G8 to G10, or G12 specificity were identified correctly. In addition, cross-reactivity to viruses of genotype G11, G13, or G14 or P[1] to P[3], P[5], P[7], P[10] to P[12], P[16], or P[20] was not observed. Next, we analyzed a total of 128 rotavirus-positive human stool samples collected in three countries (Brazil, Ghana, and the United States) by this assay and validated its usefulness. The results of this study showed that the assay was sensitive and specific and capable of unambiguously discriminating mixed rotavirus infections from nonspecific cross-reactivity; the inability to discriminate mixed infections from nonspecific cross-reactivity is one of the inherent shortcomings of traditional multiplex reverse transcription-PCR genotyping. Moreover, because the hybridization patterns exhibited by rotavirus strains of different genotypes can vary, this method may be ideal for analyzing the genetic polymorphisms of the VP7 or VP4 genes of rotaviruses.
Group A rotaviruses remain the single most important etiologic agents of severe diarrhea in infants and young children worldwide. Rotavirus diarrhea has been estimated to be responsible for a median of 611,000 deaths annually in children under 5 years of age, predominantly in developing countries (43). In the United States, approximately 2.7 million children are affected by rotavirus illness each year, resulting in about 20 deaths, 50,000 hospitalizations, 500,000 physician visits, and more than 1 billion U.S. dollars in societal costs (5, 19, 20, 30, 43). Because of the significant morbidity and mortality associated with rotavirus diarrhea, the development and implementation of a safe and effective rotavirus vaccine has been an important public health priority. Recently, two rotavirus vaccines have been licensed in many countries; the effectiveness of such vaccines in poor populations in certain sub-Saharan African as well as Southeast Asian countries remains to be determined (20).
Rotaviruses are nonenveloped, icosahedral viruses of the family Reoviridae with 11 genomic segments of double-stranded RNA, each encoding at least one structural or nonstructural protein. The rotavirus outer capsid proteins VP7 (which defines G types) and VP4 (which defines P types) evoke neutralizing antibodies independently (22, 25, 42). In addition, since neutralizing antibodies appear to play an important role in protection against rotavirus disease and infection in a serotype-specific manner (for reviews, see references 26 and 29), rotavirus strain surveillance (i.e., G and/or P type determination) has been conducted throughout the world. Various assays have been developed to determine G and P serotypes of rotaviruses, including type-specific-monoclonal antibody (MAb)-based enzyme-linked immunosorbent assay (ELISA) (2, 10, 55, 57) and a neutralization assay using type-specific polyclonal antibodies (18, 26, 60). However, because of the lack of appropriate and readily available reagents (e.g., G or P type-specific high-titer polyclonal antisera; MAb to G5, G6, G8, G9, or G10; and MAbs to various P types), serotyping is not an available option for processing a variety of rotavirus field samples. Genotyping by reverse transcription PCR (RT-PCR) developed in the early 1990s as a proxy method for serotyping, which employs a set of genotype-specific primers (multiplex RT-PCR), is rapid, simple, and sensitive (11, 14, 15, 21, 24, 58). Today, multiplex RT-PCR is the most widely used method for the identification of rotavirus G and P genotypes. However, there are several drawbacks in this method; for example, (i) because a majority of genotype-specific primers used in multiplex RT-PCR are designed from the sequences of strains isolated more than a decade ago, they may carry sequence mismatches in the primer binding regions of the target gene(s) of more-recent rotavirus strains, resulting in decreased sensitivity, and (ii) because a genotype is determined only by the size of PCR products in a gel, it is not uncommon to encounter a situation in which the possibility that certain spurious bands are present cannot be excluded or a mixed infection cannot be incriminated. Indeed, it has been reported that genotyping results could vary depending upon the primer pairs used in the multiplex RT-PCR due to the accumulation of point mutations in primer binding regions of the VP4 or VP7 gene (1, 4, 13, 27, 28, 38, 44, 48, 52). Not only for epidemiological rotavirus strain surveillance but also for effective rotavirus vaccine development, there is a need for sensitive and reliable diagnostic techniques which do not bear such disadvantages. Previously, we reported the development of a rapid and reliable method for the identification of clinically relevant human rotavirus G genotypes (i.e., G1 to G4 and G9) using oligonucleotide microarray hybridization (8). By using this method, which combines the sensitivity of PCR and the specificity of hybridization, we were successful in detecting and unambiguously identifying such G genotypes. Although 16 G genotypes and 28 P genotypes have been identified thus far (23, 36, 37, 46, 56), five G genotypes (G1 to G4 and G9) and two P genotypes (P[4] and P[8]) have been repeatedly shown worldwide to be of epidemiologic importance in humans (16, 17, 30, 32, 51). More recently, however, G genotypes other than G1 to G4 and G9 and P genotypes other than P[4] and P[8] have been detected in various parts of the world and include G5, G6, G8, G10, G12, P[6], P[9], and P[14] (for reviews, see references 17, 47, and 51). In this study, we modified the original microarray assay and expanded it to include the identification of (i) five clinically relevant human rotavirus VP4 genotypes (P[4], P[6], P[8], P[9], and P[14]) and (ii) five additional human rotavirus VP7 genotypes (G5, G6, G8, G10, and G12). The usefulness and validity of this assay were confirmed by analyzing a total of 128 stool rotaviruses collected between 1990 and 2004 from three countries. In addition, we evaluated whether this method could be applied to analyze the genetic polymorphism of the VP7 or VP4 genes of rotaviruses in a given G or P genotype isolated in different parts of the world.
Rotavirus strains.
A total of 80 cell culture-adapted human and animal reference rotavirus strains of known G and P genotypes were tested in this study (see Table S1 in the supplemental material). These strains, which were isolated from humans and eight different animal species in 19 countries on five continents between 1958 and 1999, represented collectively 13 G genotypes (G1 to G6 and G8 to G14) and 15 P genotypes (P[1] to P[12], P[14], P[16], P[18], and P[20]) (See Table TableS1S1 in the supplemental material). Each of the 80 rotavirus strains was plaque purified three times in MA104 monkey kidney cells prior to use.
TABLE 1.
TABLE 1.
G and P genotypes of stool rotavirus samples determined by RT-PCR and microarray assays
Rotavirus-positive human stool samples.
For further validation of this assay, we analyzed a total of 128 rotavirus-positive human stool field samples collected in three countries (Brazil, Ghana, and the United States) between 1990 and 2004, which were all previously analyzed by RT-PCR.
RNA extraction, RT-PCR, and nested PCR.
Viral double-stranded RNA was extracted with TRIzol (Invitrogen) from infected cell culture lysates or stool suspensions (approximately 10%) as described previously (53). In order to increase the sensitivity of the microarray assay, gene amplifications were carried out in a first-round RT-PCR, followed by a nested PCR. RT was performed as follows: extracted double-stranded RNA (1 μl) was added to 10 μl of 1× first-strand buffer containing 10 mM Tris-HCl (pH 8.3); 40 mM KCl; 1.5 mM MgCl2; a 200 μM concentration (each) of dATP, dCTP, dGTP, and dTTP; 5 mM dithiothreitol; 3.5% dimethyl sulfoxide; and a 100 pM concentration (each) of two primers (Beg9 and End9 for VP7 and F1 and C10 for VP4) listed in Table S2 in the supplemental material. The mixture was heated at 97°C for 5 min and placed on ice for 5 min, and then 2 U of SuperScript II (Invitrogen) was added. The tubes were set in a GeneAmp PCR system thermocycler (Applied Biosystems) and incubated at 42°C for 45 min. First-round PCR was performed in 25 μl of 1× storage buffer B containing 2.5 mM MgCl2, a 200 nM concentration of each primer, a 100 μM concentration of each of the four deoxynucleoside triphosphates, 5 U of Taq DNA polymerase (Promega), and 1 μl of the RT product. Nested PCR was carried out in 25 μl of the same reaction mixture except that the different primer sets (LID3 and G922 for VP7 and F4 and C8 for VP4) and the first-round-PCR product (1 μl) were included. In both PCR steps, amplification was done under the following conditions: initial denaturation at 94°C for 2 min; 30 cycles at 94°C for 30 s, 60°C for 30 s, and 72°C for 2 min; and a final extension at 72°C 5 min. The PCR products were analyzed by electrophoresis in 1% agarose gel in 1× Tris-acetate-EDTA buffer containing 0.2 μg/ml of ethidium bromide.
Synthesis of Cy5-labeled DNA samples.
The fluorescently labeled single-stranded DNA samples for hybridization were generated by the single primer extension (PE) reaction in the presence of a single forward primer used in the nested PCR. The reaction was performed in 25 μl of a reaction mixture containing 1× storage buffer B with 2.5 mM MgCl2; a 600 nM concentration of each forward primer; a 200 nM concentration (each) of dGTP, dATP, and dTTP; 40 nM dCTP; 40 nM Cy5-dCTP; 5 U of Taq DNA polymerase (Promega); and 4 to 10 μl of a DNA template of the purified (QIAquick PCR purification kit) nested-PCR product. The following conditions were used in the PE reaction: 1 min at 94°C; 40 cycles of 30 s at 94°C, 45 s at 55°C, and 2 min at 72°C; and a final intubation for 10 min at 72°C. Cy5-labeled samples were separated from nonincorporated Cy5-dCTP by centrifugation on Centri-Sep columns (Princeton Separations).
Synthesis of a Cy3-labeled QC oligonucleotide.
The Cy3-labeled quality control (QC) oligonucleotide was prepared as described previously (8).
Design of genotype-specific oligonucleotide probes.
More than 720 VP7 gene sequences and 450 VP4/VP8* gene sequences obtained from GenBank were aligned and analyzed with MacVecor 6.0 (Accelrys). Sequences of highly conserved regions within a given G or P genotype were selected to design genotype-specific oligonucleotide probes which are listed in Table S2 in the supplemental material. The 5′ end of each probe bore an amino-link group to immobilize the probe effectively on silylated (aldehyde-coated) glass slides (CEL Associates, Inc.).
Microchip fabrication.
Microchip fabrication was performed as previously described (8), with a slight modification. Microarray chips were printed using a contact microspotting robot (Cartesian Technologies, Inc.) and a ChipMaker microspotting device (TeleChem International, Inc.). The average spot diameter was 250 μm. The final spotting solution contained a 100 μM concentration of a genotype-specific oligonucleotide probe and a 20 μM concentration of a QC oligonucleotide in 0.25 M acetic acid. After the chips were printed, slides were dehydrated for 15 min at 80°C and incubated for 10 min in a freshly prepared 0.25% solution of NaBH4. Then the slides were washed once for 5 min with 0.2% sodium dodecyl sulfate in water and five times for 1 min each with distilled water.
Hybridization.
Hybridization was performed at 45°C for 30 min in an incubation chamber (ArrayIt). Immediately before hybridization, 1.8 μl of the Cy5-labeled sample was mixed with an equal volume of 2× hybridization buffer containing the QC oligonucleotide labeled with 0.15 μM Cy3. The mixture was heated at 99°C for 1 min to denature double-stranded DNA, followed by chilling on ice. The hybridization aliquot was applied to the array chip and covered with a plastic coverslip (4 by 7 mm) to prevent evaporation of the sample during incubation. After hybridization, the slide was washed once for 1 min with 6× SSC (1× SSC is 0.15 M NaCl plus 0.015 sodium citrate) containing 0.2% Tween 20, once for 2 min with 6× SSC, and once for 2 min with 2× SSC and then dried by an air stream.
Microarray scanning and data analysis.
Fluorescent images of the microchip were generated by scanning the slides by using a confocal GenePix 4100A personal fluorescence scanner (Axon Instruments). The fluorescent signals from each spot obtained at 570 nm (Cy3) and 694 nm (Cy5) were analyzed using GenePix Pro 5.0 software (Axon). Background signals obtained from the region surrounding each spot were subtracted, and the resulting absolute value of the Cy5 fluorescent signal from each probe was divided by the Cy3 signal from the QC probe of the same spot. Fluorescent signals with a statistically significant difference (P < 0.01) from the average background level were considered to be positive.
RT-PCR.
A total of 80 rotavirus samples each of which bore one of 13 G genotypes (G1 to G6 and G8 to G14) and one of 15 P genotypes (P[1] to P[12], P[14], P[16], and P[20]) were analyzed by first-round and nested PCR. For first-round PCR, we used a conventional primer pair, Beg9 and End 9 (21), to synthesize full-length VP7 genes (1,062 bp). Seventy-five of the 80 (93.8%) samples were positive by first-round PCR. In order to increase the sensitivity, we carried out nested PCR on the first-round-PCR products using the primer pair LID3 and 922 (see Table S2 in the supplemental material) and successfully amplified all 80 samples. The size of amplicons generated by nested PCR was 894 bp. For the amplification of the VP4 genes, we designed four new PCR primers (F1, F4, C8, and C10) from conserved regions of the VP4 gene. Two conserved regions, from nucleotides 1 to 20 and 4 to 23, were used to design the forward primers F1 and F4, respectively, and two other conserved regions, from nucleotides 1446 to 1467 and 1595 to 1618, were selected to make the reverse primers C8 and C10, respectively (Table S2 in the supplemental material). The sensitivity of the first-round PCR using the F1 and C10 primer pair was 91.3% (73 of 80 samples). All 80 strains used in this study were amplified by nested PCR using the F4 and C8 primer pair. The sizes of amplicons generated by the first-round and nested PCRs were 1,618 bp and 1,464 bp, respectively.
Sensitivity of the oligonucleotide microarray assay.
To determine the sensitivity of the oligonucleotide microarray assay, the nested-PCR products of the VP4 or VP7 gene of human rotavirus strain D (P[8]G1) were analyzed. Figure Figure1A1A shows a gel electrophoresis analysis of serially diluted (fivefold) cDNA amplicons of the VP7 (lanes 2 to 6) and VP4 (lanes 7 to 11) genes of strain D. The amount of cDNA in the initial reaction was 1 μg/μl (lanes 2 and 7); this was followed by a fivefold dilution series (lanes 2 to 6 and 8 to 11). Each diluted sample was purified and used as a template for the PE reaction under the conditions described in Materials and Methods. The scanned fluorescent images and fluorescence intensities of these samples are shown in Fig. 1B and C, respectively. The minimum detectable concentration of the nested-PCR product was 8 ng/μl, which corresponded to the minimum concentration that produced a visible band in a 1% agarose gel after electrophoresis.
FIG. 1.
FIG. 1.
Sensitivity of the oligonucleotide microarray hybridization assay. (A) Gel electrophoresis analysis of serially fivefold-diluted nested-PCR products of the VP7 gene (lanes 2 to 6) and VP4 gene (lanes 7 to 11) of human rotavirus D strain (G1,P[8]). A 10-μl (more ...)
Design and fabrication of G and P genotyping array chip.
We previously reported the design and fabrication of an oligonucleotide microarray chip for the identification of five clinically relevant G genotypes (G1 to G4 and G9) of human rotaviruses (8). We recently designed genotype-specific oligonucleotide probes for five additional human rotavirus G genotypes (G5, G6, G8, G10, and G12). After eliminating cross-reactive probes, we selected seven probes for each G genotype. For P genotyping, we designed 9 to 12 genotype-specific oligonucleotide probes for each of five clinically relevant human rotavirus P genotypes (i.e., P[4], P[6], P[8], P[9], and P[14]) (see Table TableS1S1 in the supplemental material). Oligonucleotide probes complementary to the primers used for nested PCR served as positive controls. The sequences of anti-LID3 and anti-F4 were complementary to those of LID3 and F4, respectively (Table S2 in the supplemental material). Each genotype-specific probe and positive-control probe were mixed with QCprb (an oligonucleotide complementary to the Cy3-labeled QC probe) before the chips were printed onto glass slides. Thus, each spot on the glass slide contained QCprb not only for an evaluation of spotting reproducibility and hybridization efficiency but also for normalization of data. Figure Figure2A2A shows the newly designed rotavirus G and P genotyping microarray chip.
TABLE 2.
TABLE 2.
Concordance and discordance in genotyping results determined by RT-PCR and microarray assays
FIG. 2.
FIG. 2.
(A) Layout of the rotavirus G (yellow) and P (blue) genotype-specific probes on the microarray chip. “G” and “P” in orange circles represent VP7 (anti-LID3) and VP4 (anti-F4) positive-control probes, respectively. (B) Scanned (more ...)
Analysis of fluorescent images.
By analyzing scanned fluorescent images of microchips, we could detect and discriminate successfully all 78 samples belonging to one of 10 G genotypes (G1 to G6, G8 to G10, and G12) and all 52 samples bearing one of 5 P genotypes (P[4], P[6], P[8], P[9], and P[14]) (see Table S1 in the supplemental material). In addition, no significant cross-reactivity was detected from 3 strains belonging to the G11, G13, or G14 genotype and 28 strains belonging to the P[1] to P[3], P[5], P[7], P[10], P[11], P[12], P[16], or P[20] genotype, against which no specific probes were available on the chip. Figure Figure2B2B shows fluorescent images of microchips obtained from six rotavirus strains bearing one of five G (G2, G5, G6, G10, and G12) and one of five P (P[4], P[8], P[9], P[11], and P[14]) genotypes. In general, the specificities of all genotype-specific probes were high; however, there were a few cross-reactive probes that hybridized with samples of heterologous genotypes. For example, (i) probe G2-8 hybridized with two G10 strains (B223 [not shown] and KC-1 [Fig. [Fig.2B])2B]) and (ii) probes G5-5, G6-4, and G9-5 hybridized with G3 strains Cat2, HCR3, and RRV, respectively. Among the P genotype-specific probes, only two probes, P4-1 and P14-6, cross-hybridized with a P[6] strain, US1205, and a P[8] strain, BD524, respectively. Of note was the finding that each cross-reactive probe hybridized with some but not all strains within a given genotype. Furthermore, there were no samples that hybridized with more than one heterologous probe, and in addition, the intensities of all cross-reactive signals were low.
Fluorescence intensity profiles.
Figure Figure33 shows the quantitative fluorescence profiles of selected rotavirus strains bearing P[4], P[6], P[8], P[9], or P[14] specificity after analysis with GenePix Pro 5.0 software. The normalized maximum of quantitative fluorescence (Cy5) signals (percentage) of each oligonucleotide probe is shown on the y axis. In such profiles, the unique hybridization pattern of each rotavirus strain within a given G or P genotype can be recognized more clearly than in scanned fluorescent images (Fig. (Fig.2B).2B). Because of spontaneous mutations in the probe binding regions of the VP4 or VP7 gene, different strains within the same genotype tended to display diverse patterns of hybridization.
FIG. 3.
FIG. 3.
Quantitative fluorescence (Cy5) profiles of rotavirus P genotype microarray of representative rotavirus strains belonging to P[4], P[6], P[8], P[9], or P[14]. Normalized percent fluorescent signals from each oligoprobe are shown on the y axis. Locations (more ...)
Analysis of rotavirus-positive human field stool samples by microarray.
In order to validate the microarray assay, we analyzed a total of 128 rotavirus-positive stool samples; 64 were from Brazil, 31 from Ghana, and 33 from the United States. Each of the samples was analyzed previously by RT-PCR in each country. Tables Tables11 and and22 summarize the results. Several interesting features emerged from this study: (i) all the G and/or P genotypes that were previously nontypeable by RT-PCR were typed by microarray analysis, and (ii) a relatively large number of samples gave discordant genotyping results in the two assays. Because of this, we used the third assay, PCR-ELISA, and confirmed the microarray results. In the PCR-ELISA, the biotinylated VP7 or VP4 gene amplicons generated by RT-PCR were immobilized onto streptavidin-coated 96-well microplates and hybridized to digoxigenin-labeled G or P type-specific oligoprobes (three probes per type). The hybrids were then detected using antidigoxigenin Fab fragment labeled with peroxidase, and the reaction was measured spectrophotometrically (N. Santos et al., unpublished data).
Microarray-based techniques have been well established as powerful tools in various fields of molecular biology (6, 12, 45, 49). In virology, this method has been used for analyses of gene expression profiles of cells infected with various viruses. Recently, this assay has been applied for genotyping of selected viruses, including polioviruses, hepatitis B viruses, influenza viruses, hantaviruses, coxsackieviruses, papillomaviruses, measles virus, adenoviruses, flaviviruses, poxviruses, herpesviruses, and mumps viruses (3, 7-9, 31, 33, 34, 39-41, 50, 54, 59). We reported previously the successful development and evaluation of an oligonucleotide microarray hybridization methodology for the identification of five clinically relevant human rotavirus G genotypes (i.e., G1 to G4 and G9) (8). More recently, an increasing number of epidemiological studies have reported the detection in diarrheal patients of uncommon G types, including G5, G6, G8, G10, and G12, in both developing and developed countries. Thus, in this study, we modified the original microarray assay and expanded it to include the identification of such unusual G types as well as clinically relevant P genotypes (i.e., P[4], P[6], P[8], P[9], and P[14]). Hence, this microarray assay can now identify almost all G and P genotypes of human rotaviruses that have been detected thus far. Epidemiological surveillance of rotavirus G and P types before and after an introduction of a candidate rotavirus vaccine in various parts of the world is important in order to evaluate (i) temporal and geographic changes/fluctuations of rotavirus genotype distribution, (ii) relationships between vaccine efficacy and circulating rotavirus strains, and (iii) horizontal transmission of vaccine strains.
The analytical sensitivity of the microarray method reported in this study is very high. The minimum concentration of cDNA that could be detected and typed was 8 ng/μl. In general, samples that produced a visible band in an ethidium bromide-stained 1% agarose gel after the first-round or nested PCR were all successfully P or G genotyped. By applying nested PCR to the first-round-PCR products of strains that gave no visible bands (and therefore could not be typed), we could successfully amplify and genotype both the VP4 and VP7 genes of such rotavirus strains. Thus, because of the high sensitivity of primer pairs used in the nested PCR in this study, the combination of first-round and nested PCR was shown to have a potential of amplifying both the VP4 and VP7 genes of strains bearing not only common but also uncommon G or P specificity.
The specificity of this microarray method is also very high. We could correctly and unambiguously identify both P and G genotypes of all reference rotavirus strains belonging to genotypes against which the genotype-specific microarray probes were designed. In addition, no cross-reactivity with rotavirus G or P genotypes against which specific probes were unavailable on the chip was detected. Thus, strains which do not generate fluorescent signals can safely be considered to bear a G genotype other than G1 to G6, G8 to G10, and G12 and/or a P genotype other than P[4], P[6], P[8], P[9], and P[14].
Only four G and two P genotype-specific probes were found to be cross-reactive with samples of heterologous genotypes. Of note is the finding, however, that each of the samples that exhibited cross-reactivity reacted with only one heterologous probe. Moreover, the intensity of all cross-reactive signals was low. It has been well recognized that it is necessary to distinguish mixed rotavirus infections from cross-reactivity in multiplex RT-PCRs (14) by using a confirmatory assay for the individual genotypes. In contrast to PCR, the microarray method has the inherent ability to readily identify individual genotypes for the mixed infections. In such a case, unlike with cross-reactive signals, more than one probe of suspected G and/or P genotypes would produce strong positive fluorescence signals. Moreover, in this microarray assay, such cross-reactive signals can be utilized for accurate discrimination among various strains within the same genotype. For example, bovine G10 rotavirus KC-1 and B223 strains reacted with the heterologous G2-7 probe with low intensities, while human G10 rotavirus A64 strain exhibited no reactivity with the same probe.
While this study was in progress, Lovmar et al. reported (35) a genotyping method of five G (G1 to G4 and G9) and four P (P[4], P[6], P[8], and P[9]) types of rotavirus by using PE in a microarray format. Their method appears to differ from ours in various aspects. First, because discrimination of genotypes depends on the difference in the 3′ ends between genotype-specific probes and the target genome segments of samples, this method may not work well with strains that have a mutation in this region. For example, among the strains tested in the present study, (i) two G3 strains (HCR3 and Ro1843) which have 3′ ends different from that of each of three G3-specific probes (G3-1, G3-2, and G3-3) reported by Lovmar et al. could not be typed (false-negative result), and (ii) three or four sequence mismatches found between two G4 strains (Gottfried and SB1A) and the three G4-specific probes (G4-1, G4-2, and G4-3) described by Lovmar and coworkers may lead to a wrong conclusion (false-negative result). Second, in their assay, it may be difficult in some cases to distinguish mixed rotavirus infections from cross-reactions that may cause a false-positive result. For example, because there is only one base mismatch between the G10 strains (B223 and KC-1) tested in the present study and one of their G2-specific probes (G2-6) and because, in addition, 3′ ends are complementary to each other, such G10 samples may be misdiagnosed as a mixture of G10 and G2 viruses (false-positive result). Third, this method can provide only limited information about 3′-end differences among strains within the same genotype. Previously, we reported that our microarray method was capable of “subgenotyping” viruses in a given G genotype (G1 to G4 and G9) (8). In the present study, not only the VP7 gene but also the VP4 gene of each rotavirus strain was found to display a unique hybridization pattern that correlated closely with its nucleotide sequences. Thus, one of the major advantages of our new microarray method is that it is ideal for analyzing such genetic polymorphisms of the VP7 and VP4 genes of various rotavirus strains within the same genotype. In addition, we found that the existence of even one nucleotide mismatch located near the 3′ or 5′ end of a probe changed its pattern of hybridization profile (data not shown). In general, rotavirus strains in a given genotype that are isolated (i) at different locations, (ii) in different years even at the same location, or (iii) from different animal species display different hybridization profiles. For example, two Japanese G2 strains, KUN and S2, displayed similar hybridization patterns which were distinct from that displayed by U.S. G2 strain DS-1 or Venezuelan G2 strain HN126 (data not shown). These results indicate that this assay can be applied to monitoring the evolutionary divergence and genetic drift of the selected rotavirus genes, including the VP7 and VP4 genes. Although sequencing of the target genome segment may be needed for detailed analysis, it is time-consuming, expensive, and not practical for analyses of a large number of field isolates. This microarray method is ideal for a rapid screening of a large number of rotavirus strains to detect and analyze genetic polymorphism before sequencing. Analyses by this assay of a possible genetic/antigenic drift of the VP7 and/or VP4 gene of G1P[8] viruses collected longitudinally in three countries (Brazil, Spain, and the United States) are under way in this laboratory.
Supplementary Material
[Supplemental material]
Acknowledgments
We thank Michael Wilson (Microarray Facility Section, Research Technologies Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health) for the assistance in preparation of microchips. We also thank G. N. Gerna, M. E. Thouless, K. Banyai, G. Szucs, R. A. Hesse, and D. R. Snodgrass for kindly providing us with various rotavirus stains.
This research was supported in part by the Intramural Research Program of the National Institute of Allergy and Infectious Disease, National Institutes of Health.
There is no conflict of interest to declare.
Footnotes
[down-pointing small open triangle]Published ahead of print on 13 June 2007.
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