We used genetic analysis to investigate a novel, naturally occurring recombinant sapovirus. Two strains were used for the analysis, Mc10 strain (GenBank accession no. AY237420), isolated from an infant hospitalized with acute gastroenteritis in Chiang Mai, Thailand, in 2000 (5), and C12 strain (AY603425), isolated from an infant with gastroenteritis in Sakai, Japan, in 2001 (unpub. data). Although the original polymerase chain reaction (PCR) primer sets that detected these two strains were different, both were directed toward the conserved 5´ end of the capsid gene and have been shown to detect a broad range of sapovirus sequences in genogroup I (GI) and GII (5,10). For Mc10, primers SV5317 and SV5749 were used; for C12, primers SV-F11 and SV-R1 were used.

The complete genomes for Mc10 and C12 were determined as previously described (6). As shown in Figure 1A, the sapovirus genome has an organization slightly different from that of the norovirus genome. ORF1 encodes nonstructural proteins, polymerase, and the capsid protein, and ORF2 encodes a small protein (1).

Initially, we grouped Mc10 and C12 into two distinct GII clusters (i.e., genotypes), on the basis of their capsid sequences (Figure 2A) and the phylogenetic classification scheme of Okada et al. (10). In addition, the overall genomic nucleotide similarity between Mc10 and C12 was 84.3%, while ORF1 and ORF2 shared 85.5% and 73.3% nucleotide identity, respectively. These results corresponded with the capsid-based grouping shown in Figure 2A. By comparing sequence similarity across the length of the genomes with SimPlot with a window size of 100 (11), we discovered a potential recombination site, where the similarity analysis showed a sudden drop in nucleotide identity after the polymerase region (Figure 1B). Nucleotide sequence analysis of ORF1 less the capsid sequence and the capsid sequence indicated 90.1% and 71.3% nucleotide identity, respectively (Figure 1A). To additionally illustrate the nucleotide identities of ORF1 less the capsid sequence, a phylogenetic tree of polymerase sequences of Mc10, C12, and other available strains was developed (Figure 2B). However, for three strains (Mex14917/00, Mex340/1990, and Cruise ship/00), the polymerase and capsid sequences of ORF1 were not continuous, i.e., they may represent two different strains. Nevertheless, Mc10 and C12 were in the same cluster by polymerase-based grouping but were in distinct clusters by capsid-based grouping (Figure 2). All other strains maintained clusters by polymerase- and capsid-based groupings.

These findings showed Mc10 and C12 had high sequence identity up to the beginning of the capsid region where the sequence identity was considerably lower. These results are easily explained by a recombination event, a single point recombination event occurring at the polymerase-capsid junction. At the end of the polymerase region, there were 44 nt, which included the first 8 nt of the capsid gene and showed 100% homology. After these nucleotides, the identity decreased and was clearly different, as shown in Figure 1B. This conserved region may represent the break and rejoin site for Mc10 and C12 during viral replication, although direct evidence for this event is lacking.

A sudden drop was indicated, followed by a rise in nucleotide identity between nt 6,250 and 6,500 (Figure 1B). Although our initial hypothesis was that another recombination event occurred, closer inspection indicated that this region corresponded to amino acids 358 and 440 for the capsid protein and likely represented the hypervariable region, as described recently in the structural analysis of sapovirus capsid protein (12). For recombinant norovirus strains, we also observed a sudden decrease in nucleotide identity in the related capsid region (13), which represents the outermost protruding domain (P2) and is subject to immune pressure (14). For these reasons, a low homology, even between closely related strains, is generally seen in this region (6), although further studies by sequence analysis with other strains are needed.

In a recent study, we genetically and antigenically analyzed two recombinant norovirus strains (13). When the polymerase-based grouping was performed, these two strains clustered together; when capsid-based grouping was performed, these two strains belonged in two distinct genotypes. When we compared the cross-reactivity of these two viruslike particles (VLPs) and hyperimmune sera against the VLPs, we found distinct antigenic types for the VLPs, although a considerable level of cross-reactivity was found between them. We recently expressed C12 capsid protein that resulted in the formation of VLPs, but we were unsuccessful in expressing Mc10 VLPs (G.S. Hansman, unpub. data); therefore the antigenicity of these two strains remains unknown.

Jiang et al. (4) reported two potential parental norovirus strains that were cocirculating in the same geographic region (Mendoza, Argentina, in 1995), which provides some evidence for where and when the recombination event may have occurred. In addition, Jiang identified the progeny strain from the event, the Arg320 strain. In our study, Mc10 and C12 were isolated from Thailand and Japan, respectively, but we have no evidence for the place and time of the event. While the genetic analysis for Mc10 and C12 identified a possible recombinant sapovirus strain, the analysis does not clarify which of the two strains was the parent strain and which was the progeny strain. Further extensive studies are needed that perform sequence analysis of polymerase and capsid genes and compare results with analysis of other strains. Nevertheless, other strains with capsid sequences that closely match those of Mc10 and C12 are in the public database, which suggests the circulation of other recombinant sapovirus strains.

Recombination and evolution are important survival events for all living creatures as well as viruses. These events in viruses are not completely understood, but they can be potentially dangerous for host species, and they likely influence vaccine designs (15). From our studies, the human sapovirus and norovirus recombination appears limited to the intragenogroup because no intergenogroup or intergenus recombination has yet been identified and recombination only occurs at the polymerase-capsid junction. Finally, the results of this study have increased our awareness of recombination in the Sapovirus genus and may have an influence on the future phylogenetic classification of sapovirus strains.