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In total, 2836 HFMD patients were enrolled in our previous study. Using PCR methodologies, EVs that were eligible for genetic characterization in this study were detected in 2517 (89%) patients (Gao et al. 2018).
Among the 18 EVs serotypes identified, CVA16, CVA6 and EV-A71 were the most frequently detected, accounting for 32.5% (n = 819/2517), 31.2% (n = 785/2517) and 20.4% (n = 514/2517), respectively (Fig. 1A). Additionally, the serotypes were identified as follows: 5.9% (n = 149/2517) CVA10, 3.0% (n = 55/2517) CVA4, 1.6% (n = 40/2517) CVA8, 1.2% (n = 31/2517) CVA2, 0.5% (n = 12/2517) E18, 0.3% (n = 8/2517) CVA5, 0.3% (n = 8/2517) CVB5, 0.2% (n = 6/2517) CVB2, and 0.2% (5/2517) E9. Other EVs serotypes were detected in 2 or fewer patients.
Figure 1. Distribution of EV serotypes found in Anhua County, China, October 2013–September 2016. A Distribution of EV-A71, CVA16 and CVA6 HFMD in Anhua County, China, October 2013–September 2016. Total monthly numbers, samples planned for VP1 sequencing and successfully sequenced of B EV-A71, C CVA16, and D CVA6.
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According to the selection criteria, we selected 17% of the CVA16 isolates (n = 136/819), 23% of the CVA6 isolates (n = 181/785) and 28% of the EV-A71 isolates (n = 144/514) for complete VP1 sequencing. Then, we successfully obtained complete VP1 sequences from 76% of the CVA16 isolates (n = 103/136), 97% of the CVA6 isolates (n = 176/181) and 50% of the EV-A71 isolates (n = 72/144).
After the removal of identical sequences (where variation was < 1%), 24 EV-A71, 43 CVA16 and 65 CVA6 VP1 sequences remained for analysis (Fig. 1B–D, Table 1). All sequences fell within 3 subgenogroups: C4a for EV-A71, B1b for CVA16 and D3a for CVA6 (Figs. 2, 3 and 4).
Serotype Region Total samples in study, N No. for VP1 sequencing, n1 (n1/N, %) No. successfully sequenced, n2 (n2/n1, %) After removing sequences that were < 1% divergent, n3 (n3/n2, %) EV-A71 > 90% complete in the VP1 region 514 144 (28) 72 (50) 24 (33) CVA16 > 90% complete in the VP1 region 819 136 (17) 103 (76) 43 (42) CVA6 > 90% complete in the VP1 region 785 181 (23) 176 (97) 65 (37) CVA10 Partial VP1 149 119 (80) 119 (100) 66 (55) CVA2 Partial VP1 31 16 (52) 16 (100) 11 (69) CVA4 Partial VP1 75 55 (73) 55 (100) 29 (53) CVA5 Partial VP1 8 7 (88) 7 (100) 6 (86) CVA8 Partial VP1 40 38 (95) 38 (100) 28 (74) CVB2 Partial VP1 6 6 (100) 6 (100) 5 (83) CVB5 Partial VP1 8 8 (100) 8 (100) 8 (100) E18 Partial VP1 12 10 (83) 10 (100) 8 (80) Table 1. The Pretreatments of VP1 sequences obtained in this study.
Figure 2. Phylogenetic analyses of the entire VP1 sequences (891 bp) of EV-A71 from Anhua County based on ML methods. The tree was rooted on genotype A of the EV-A71 strain. The branches of sequences are color-coded according to the EV-A71 lineage. The names of the study sequences are colored in green; the reference strains are uncolored. Bootstrap values > 70% are shown on the branches. The phylogenetic tree indicates that evolutionary branch C4a was responsible for infections in Anhua County during 2013–2016.
Figure 3. Phylogenetic analyses of the entire VP1 sequences (891 bp) of CVA16 from Anhua County based on ML methods. The tree was rooted on genotype A of CVA16 strain. The branches of sequences are color-coded according to the CVA16 lineage. The names of the study sequences are colored in green; the reference strains are uncolored. Bootstrap values > 70% were shown on the branches. The phylogenetic tree indicates that evolutionary branch B1b was responsible for infections in Anhua County during 2013–2016.
Figure 4. Phylogenetic analyses of the entire VP1 sequences (891 bp) of CVA6 from Anhua County based on ML methods. The tree was rooted on genotype A of CVA6 strain. The branches of sequences are color-coded according to the CVA6 lineage. The names of the study sequences are colored in green; the reference strains are uncolored. Bootstrap values > 70% were shown on the branches. The phylogenetic tree indicates that evolutionary branch D3a was responsible for infections in Anhua County during 2013–2016.
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Partial VP1 region sequences of CVA10 (n = 119/149), CVA4 (n = 55/75), CVA8 (n = 38/40), CVA2 (n = 16/31), E18 (n = 10/12), CVB5 (n = 8/8), CVA5 (n = 7/8) and CVB2 (n = 6/6) were obtained (Table 1). After removing sequences with < 1% variation, partial VP1 sequences of CVA10 (n = 66/119), CVA4 (n = 29/55), CVA8 (n = 28/38), CVA2 (n = 11/16), E18 (n = 8/10), CVB5 (n = 8/8), CVA5 (n = 6/7) and CVB2 (n = 5/6) were included for analysis (Table 1). Following the calculation of similarity in terms of both nucleotides and amino acids, the results showed little variation (Supplementary Table S2). Thus, based on the phylogenetic tree and similarity analysis, these isolates clustered within their 8 subgenogroups: D, I-A, II, D, C, D2, E and 5.3 (Supplementary Figure S1-S8).
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The remaining sequences were compared to reference prototype strains to identify variations at the amino acid level.
For the EV-A71 sequences, when compared to the prototype strain BrCr, 30 polymorphic amino acid sites were found. For the CVA16 sequences, when compared to the prototype strain G10, 29 polymorphic amino acid sites were found. For the CVA6, when compared to the prototype strain Gdula, 44 polymorphic amino acid sites were found (Figs. 5, 6 and 7, Supplementary Table S3).
Figure 5. Variations were found in EV-A71 (C4a) of our study. "-" indicates matching to the U22521/US/1970 (EV-A71 prototype strain, genotype A, BrCr type-strain). Variations potentially associated with major neutralizing/antigenicity epitopes are indicated in red. BC loop regions are indicated in green, EF loop regions are indicated in orange.
Figure 6. Variations were found in CVA16 (B1b) of our study. "-" indicates matching to the U05876/ZA/1951 (CVA16 prototype strain, genotype A, G10 type-strain). Variations potentially associated with major neutralizing/antigenicity epitopes are indicated in red and GH loop regions are indicated in blue.
Figure 7. Variations were found in CVA6 (D3a) of our study. "-" indicates matching to the AY421764/US/1949 (CVA6 prototype strain, genotype A, Gdula type-strain). Variations potentially associated with major neutralizing/antigenicity epitopes are indicated in red. BC loop regions are indicated in green, EF loop regions are indicated in orange and GH loop regions are indicated in blue.
Additionally, we identified amino acid residue variations associated with major neutralization/antigenic epitopes. For the EV-A71 sequences, 87.5% (n = 21/24) of samples exhibited a substitution of aspartic acid (D) to asparagine (N) at residue 31 (D31N). Other substitutions were also found in the EV-A71 sequences: N104D/H (n = 2/24, 8%), R145E (n = 24/24, 100%), G239E (1/24, 4%), E244K (n = 24/24, 100%), S283T (15/24, 62.5%) and A293S/G (16/24, 66.7%) (Table 2).
Site Anhua study n (N, %) Le et al. (2019) n (N, %)Zhu et al. (2013) n (N, %)Liu et al. (2014) n (N, %)van der Sanden et al. (2010) n (N, %)Chia et al. (2014) n (N, %)31 (D31N) 21 (24, 87.5%) 42 (43, 97.7%) 14 (14, 100%) – – 15 (16, 93.8%) 104 (N104D/H) 2 (24, 8.3%) – – 0 (49, 0%) – – 145 (R145E/G/Q) 24 (24, 100%) 43 (43, 100%) 14 (14, 100%) 49 (49, 100%) – 16 (16, 100%) 239 (G239E) 1 (24, 4%) – – – 0 (50, 0%) – 244 (E244K) 24 (24, 100%) 43 (43, 100%) 14 (14, 100%) – 50 (50, 100%) – 283 (S283T) 15 (24, 62.5%) 2 (43, 4.7%) – – – – 293 (A293S/G) 16 (24, 66.7%) 4 (43, 9.3%) – – 0 (50, 0%) – "–" Indicates not reported.
aEV-A71 prototype strain (genotype A, BrCr type-strain).Table 2. Variations detected in VP1 amino acid sequences of EV-A71 strains (compared with U22521/US/1970a).
For the CVA16 sequences, the substitutions L23I/M/V and I235V were found in 88.4% (n = 38) and 9.3% (n = 4) of the investigated samples (n = 43), respectively. Other substitutions were found in only one CVA16 sequence: P27S, L146X, V147S and T289A (Table 3).
Site Anhua study n (N, %) Xu et al. (2018) n (N, %)Chan et al. (2012) n (N, %)Iwai et al. (2009) n (N, %)Sun et al. (2017) n (N, %)23 (L23I/M/V) 38 (43, 88.4%) 15 (146, 10.3%) – – 21 (35, 60%) 27 (P27S) 1 (43, 2.3%) – – – 0 (35, 0%) 146 (L146X) 1 (43, 2.3%) – – – 0 (35, 0%) 147 (V147S) 1 (43, 2.3%) – – – 0 (35, 0%) 235 (I235V) 4 (43, 9.3%) 5 (146, 3.4%) 1 (4, 25%) 1 (7, 14.3%) – 289 (T289A) 1 (43, 2.3%) – – 1 (7, 14.3%) 1 (35, 0%) "–" Indicates not reported.
aCVA16 prototype strain (genotype A, G10 type-strain).Table 3. Variations detected in VP1 amino acid sequences of CVA16 strains (compared with U05876/ZA/1951a).
For the CVA6 sequences, for which fewer major epitopes have been described in the literature, only small numbers of samples demonstrated amino acid substitutions; T96A (n = 2/65, 3%), S97N (n = 4/65, 6%), N137S/G/D (n = 16/65, 24.6%), T141A (1/65, 1.5%), V151I (1/65, 1.5%), G160N/S (n = 65/65, 100%), T205I (n = 1/65, 1.5%) and Q216H (n = 1/65, 1.5%) (Table 4).
Site Anhua study n (N, %) Kanbayashi et al. (2017) n (N, %)96 (T96A) 2 (65, 3.1%) 0 (25, 0%) 97 (S97N) 4 (65, 6.1%) 0 (25, 0%) 137 (N137S/G/D) 16 (65, 24.6%) 14 (25, 56%) 141 (T141A) 1 (65, 1.5%) 0 (25, 0%) 151 (V151I) 1 (65, 1.5%) 0 (25, 0%) 160 (G160N/S) 65 (65, 100%) 17 (25, 68%) 165 (Q165R) 1 (65, 1.5%) 0 (25, 0%) 205 (T205I) 1 (65, 1.5%) 0 (25, 0%) 216 (Q216H) 1 (65, 1.5%) 0 (25, 0%) aCVA6 prototype strain (genotype A, Gdula type-strain). Table 4. Variations detected in VP1 amino acid sequences of CVA6 strains (compared with AY421764/US/1949a).
Despite the variations described above, an analysis of selection pressure using MEME revealed that none of the protein-coding sequences showed adaptive evolution for EV-A71, CVA16 or CVA6.