Background
Molecular mechanisms of invasion by
Plasmodium vivax merozoites are mediated by the Duffy-binding-like (DBL) family of homologous erythrocyte binding protein (EBPs) located within the micronemes of merozoites that recognize specific receptors on red blood cell [
1].
PvDBP is a 140-kDa protein belonging to a family of erythrocyte binding proteins characterized by a functionally conserved cysteine-rich region. The similarity among DBL families of EBPs is the greatest within two adjacent cysteine-rich domains, designated as the amino cysteine-rich domain (N-cys) and carboxyl cysteine-rich domain (C-cys) [
2]. The amino cysteine-rich domain has been identified as the principle adhesion region binding to erythrocyte receptors, while the function of the carboxyl cysteine-rich domain remains unclear. This amino-cysteine-rich domain is present within the 330 amino acid region II, which has been shown to contain the binding motifs necessary for adherence to the Duffy antigen receptor for chemokines (DARC), required for erythrocyte invasion [
3]. Critical binding motifs in
PvDBPII have been mapped to a 170 amino acid spanning region between cysteines 4 and 7 (amino acids 291–460) [
4]. Cysteine residues are conserved within the identified binding motif, whereas other amino acids are highly polymorphic, having a high ratio of nonsynonymous to synonymous mutations [
5].
The polymorphic nature of the
PvDBP, particularly its region II (
DBPII) is a major impediment to the development of a broadly protective vaccine against vivax malaria. A study in Papua New Guinea showed 43 and 18 unique nonsynonymous mutations in 10 Madang [
6] and 24 Wosera vivax isolates [
7], respectively, whereas 19 unique nonsynonymous mutations were described in 10 Colombian vivax isolates [
8]. Often the mutations were found in corresponding locations in the protein sequence of isolates from different geographical areas. Amino acids substitutions at D384G, K386(N/Q), N417K, L424I, W437R, and I503K were found in both Papua New Guinea isolates and Colombia isolates [
5‐
8]. Such results demonstrate that many similar alleles are widely distributed among
P. vivax from different geographical areas. Polymorphic patterns in the
PvDBP cluster within the critical erythrocyte-binding segment imply the existence of a selection pressure on
PvDBP [
9]. Under the influence of immune pressure, the distribution of existing antigenic polymorphisms affects the population structure in which newly selected mutants spread and may provide insight to the evolution and selection of parasite populations over time [
10]. Polymorphisms of
PvDBPII require further exploration among vivax isolates from different geographic areas, particularly south-east Asia where a large proportion of vivax infections take place. Therefore, the gene polymorphisms within the region II domain of the
P. vivax DBP among Thai
P. vivax isolates were investigated.
Methods
P. vivax samples and DNA preparation
Plasmodium vivax isolates were obtained from malaria patients with informed consent at the outpatient clinic, Hospital for Tropical Diseases, Faculty of Tropical Medicine, Mahidol University, Bangkok, from May, 2002 to June, 2003 under a protocol approval by the Ethics Committee, Faculty of Tropical Medicine, Mahidol University. The confirmation of P. vivax infection was done by microscopic examination of the parasites in thick and thin blood smears. Approximately 1 ml of venous blood was collected from each individual in 0.5 M ethylene-diamine tetra-acetic acid (EDTA) tubes and kept at -20°C until use. Genomic DNA was extracted from 200 μl of blood using a High Pure PCR Template Preparation Kit (Roche, Mannhein, Germany) according to the manufacturer's protocol.
Genotyping of P. vivax isolates at the PvDBPII locus
P. vivax DBPII gene was amplified by PCR using the oligonucleotide primers, DBPII-F; 5'-CACCACGATCTCTAGTGCTATTATA-3' and DBPII-R; 5'-TGTCACAACTTCCTGAGTATT-3' [
11]. PCR cycling conditions were 94°C for 3 min, 55°C for 1 min, and 72°C for 1 min for 35 cycles followed by a 10 min extension at 72°C. PCR products were stained with 0.1 μg/ml ethidium bromide and visualized by digital photography under ultraviolet light. The PCR products were run adjacently to 0.1 μg/lane of a 2-log DNA ladder (NewEngland BioLabs, Inc. MA, USA) as a standard DNA marker.
Purification of PCR products, DNA cloning and DNA sequencing of PvDBPII gene
PCR products were purified by QIAquick PCR purification Kit (Qiagen Inc., Valencia, CA) following the manufacturer's instructions. The PCR products were cloned directly into pCR-Blunt II-TOPO cloning vector using a Zero Blunt TOPO PCR cloning kit (Invitrogen, La Jolla, CA). For DNA sequencing of the species-specific amplicons, excess dNTPs and unincorporated primers were eliminated using the QIAquick PCR purification kit (Qiagen Inc., Valencia, CA). DNA sequencing was performed by fluorescence based methodologies using MegaBACE DNA Analysis Systems (Amersham Biosciences, Piscataway, NJ). The sequences have been submitted to the GenBank under the accession number EF219451, EF368159–368180, EF379127–379135. Due to the possibility of multiple genotypes within parasites taken from one individual, direct sequencing was not performed. Instead, at least five clones were sequenced from each corresponding PCR product and sequences of these had to be identical to meet our quality control standards.
Analysis of PvDBPII gene sequences
The alignment of complete sequences of 30
PvDBPII genes was analysed by Sequencher ver 4.2 software (Gene Code Corporation, Ann Arbor, MI) [
12,
13]. Sequences were aligned using CLUSTALX Multiple Sequence Alignment Program developed by the National Center for Biotechnology Information (NCBI, Bethesda, MD). The percent similarity was assessed using BLASTN (NCBI, Bethesda, MD) and BioEdit ver 7.0 software (Tom Hall Isis Therapeutics, Isis Pharmaceuticals, CA), DnaSP ver 4.0 [
14] and MEGA ver 4.0 Beta program [
15].
Analysis of natural selection
Evidence of positive natural selection was determined by comparing the rate of nonsynonymous and synonymous substitutions in region II of the
P. vivax DBP gene. The rates of substitutions were estimated using the Nei and Gojobori's method [
16] with the Jukes and Cantor correction [
17,
18] as implemented in the MEGA program. Standard error was determined by 1000 bootstrap replications. Negative selection acting on most coding genes can be identified typically when the rate on nonsynonymous mutations is less than the rate of synonymous mutations (
kn <
Ks). However, when positive selection is acting on a gene, the rate of nonsynonymous mutations will exceed that of synonymous mutations (
kn >
Ks). Two other tests of neutrality were performed by Tajima's D test, Fu and Li's
D- and
F-tests on DnaSP 4 software using
P. knowlesi DBP as an outgroup. Tajima's D test compares the estimation of nucleotide diversity calculated in two ways (θ calculated from the number of segregating sites and π calculated from average pairwise nucleotide diversity) in order to test for a departure from neutrality. Fu and Li's
D and
F test, departures from neutrality are identified as a deviation between estimates of θ (derived from the number of mutations in external branches of the phylogeny and from the total number of mutations giving the index
D or from the average pairwise diversity π giving the index
F) [
19].
Phylogenetic analysis
Phylogenetic analysis was used to investigate the associations of
PvDBPII gene with sequences elucidated from different geographical regions. The gene tree was constructed using regions common to all available
PvDBPII sequences. Forty five
PvDBPII genes sequences found in GenBank were compiled and compared to Thai isolates including the sequences from a reference strain, Sal I, one individual sequence from Vietnam, Indonesia, Brazil and India as well as 2, 3, 3 and 31 sequences from Bangladesh, Colombia (COL), South Korea (SK) and Papua New Guinea (PNG), respectively (Table
1). A phylogenetic tree was derived from the aligned nucleotide sequences using the Neighbour-Joining (NJ) method with 1000 bootstrap replicates, the Tamura's three-parameter distance model as implemented in the MEGA ver 4.0 Beta program [
20].
Table 1
PvDBPII sequences deposited in GenBank used in the study
P. vivax
| Papua New Guinea | AF289480–289653, AF469522–469602, AY970848–970925, AF291096, AF695565, DQ156519 |
P. vivax
| Colombia | AY341907, DQ156513, AY341899 |
P. vivax
| Korea | AF215737, AF215738, DQ156523 |
P. vivax
| India | DQ156514 |
P. vivax
| Brazil | DQ153520 |
P. vivax
| Vietnam | DQ156518 |
P. vivax
| Indonesia | DQ156521 |
P. vivax
| Sal I | M37514, DQ156512 |
Discussion
The process invasion into erythrocyte is essential for survival of
P. vivax parasite and requires
DBP to bind the Duffy blood group antigen that act as erythrocyte receptor, Duffy antigen receptor for chemokines (DARC).
Plasmodium vivax utilizes genetic diversity of individual functional proteins to evade the host immune system and resist many anti-malarial drugs [
21]. In this study, polymorphisms and natural selection was assessed and suggested positive natural selection in
PvDBPII of Thai isolates. Although, non significant could be observed trending positive selection. As this is only a glimpse at the overall population, a wide range study including more subjects from different areas of Thailand would give a more detailed picture.
Analysis of 300 amino acids of
PvDBPII protein relative to the Sal I reference strain, resulted in 25 nonsynonymous mutations being found. In addition, 142 nonsynonymous mutations were reported in 76 Papua New Guinea isolates [
22]. The nonsynonymous mutations in
PvDBPII might have an effect on parasite binding to erythrocyte receptors. Nonsynonymous mutations often times result in amino acid substitutions that alter charge and therefore host immune/antibody recognition [
6]. Thus, excessive stable polymorphism generation within the
PvDBPII ligand domain from high rate of nonsynonymous polymorphisms may promote the parasite escape of host immune response [
9].
The high frequency (>50%) of L424I (86%), D384G (76%), W437R (63%), R390H (56%), and I503K (56%) residues were found in Thai isolates relative to Sal I
PvDBP sequence. The finding is in contrast to previous studies showing R308S (67%), D384G (66%), and S447K (59%) in Papua New Guinea, D384G (59%) in Colombia and D384G (85%) in Brazil isolates [
23]. Of the following variants N417K, W437R and I503K, all having been shown to be involved in evasion to antibody neutralization, variant N417K was found in low frequency (36%) while W437R and I503K were high (63% and 56% respectively) in Thai isolates. These results were different from what has been currently reported in that the low frequencies of variants N417K (27.5%) and W437R (27.5%) and the high frequency of variant I503K (55%) found in Brazil and low frequencies of all three variants (N417K, W437R, I503K) found in either Colombia (47%, 18%, 12%, respectively) and/or Papua New Guinea (23%, 26%, 29%, respectively) (Table
4) [
23]. N417K, W437R and I503K have recently been shown to not be directly involved in erythrocyte binding, but possibly in inhibition of antibody binding to erythrocyte receptor [
24]. Moreover, the variant amino acid T404R was identified in
PvDBPII domains of three Thai isolates (Table
2) which is similar to the recent data in Papua New Guinea [
22]. Other Thai variants, namely R308S, K371E, D384G, E385K, N386(N/Q), R390H, T404R, N417K, L424I, W437R, S447S and I503K, were also found in isolates of other regions, demonstrating commonality with a progenitor clone (Table
4). The other 18 polymorphic residues of
PvDBPII ligand domain were recognized only among Thai isolates, indicative of divergence from a progenitor clone.
Table 4
Frequencies of the most common variant amino acids in PvDBPII, comparing to Sal I sequence (accession number M37519)
Thai | 26 | 76 | 40 | 3 | 36 | 86 | 63 | 0 | 56 |
COL* | 0 | 59 | 23 | 0 | 47 | 47 | 18 | 0 | 12 |
PNG* | 67 | 66 | 8 | 11 | 23 | 34 | 26 | 59 | 29 |
Brazil* | 12.5 | 85 | 12.5 | 0 | 27.5 | 32.5 | 27.5 | 0 | 55 |
The study showed that 25
PvDBPII haplotypes clustered into five main groups (dominant haplotypes) (Table
3) on the basis of nonsynonymous mutations is similar, but not entirely the same, with data from Papua New Guinea isolates whose 27 different
PvDBPII haplotypes clustering into three dominant
PvDBPII haplotypes. Potentially dominant
PvDBPII haplotypes observed in these populations could be less immunogenic, thereby may help promoting parasite success and escaping the host immune response [
22].
Phylogenetic analysis of
PvDBPII suggested that Thai isolates fell into 6 different alleles groups. Among these, group 1 formed a group with a subset of Papua New Guinea isolates. Groups 4 and 6 were unique among Thai
P. vivax isolates. However, the rest of the allele groups that Thai isolates fell into were more related to the isolates from Korea, India, and Colombia than from Papua New Guinea (Figure
3). The observations derived from the
PvDBPII phylogeny suggested that single Indian isolate found in group 1 appeared to share a common ancestor close to a subset of Thai and Papua New Guinea isolates, demonstrating common ancestoral origins. Thai and Papua New Guinea isolate subsets in group 7 also may have shared common ancestor heritage. Recently, it was reported that travellers' malaria among foreigners at the Hospital for Tropical Diseases, Bangkok, Thailand including India and Papua New Guinea patients were though to have acquired their infections in their countries. Malaria importations from country to country can occur by either immigration or travel, and changing malaria attack rates in the countries of exposure are likely to influence the incidence of imported disease [
25]. Additionally, these results demonstrated that there is some difference in the nucleotide or amino acid variation in
PvDBPII among isolates from Thai, Korea, India, Papua New Guinea and Colombia, but most importantly, unique variants from Thailand do exist and should be considered in future vaccine development of
PvDBPII. It should be noted that the present highly diverse phylogenetic tree was constructed relying on polymorphisms within a single gene, demonstrating the high degree of topological variation that can be found using this technique, but also highlighting the problems that can be associated with gene sampling in phylogenetic studies [
26]. Use of a combination of several genes with different rates of evolutions has been suggested as an efficient way to overcome this difficulty to prove the existence of a unique tree relating these sequences [
27]. Finally, it must also be pointed out that in many situations a single-gene phylogeny may be interesting in itself. Awareness of the problems of orthology (genes in different species that derive from a common ancestor) assignment and tree reconstruction artifacts should be considered.
Acknowledgements
We thank the patients for their kind participation in this study; staff of the Hospital for Tropical Diseases, Faculty of Tropical Medicine, Mahidol University for their assistance in blood collection and COL Douglas S. Walsh, US Army Medical Research Unit-Kenya for his valuable guidance and comments. This study was supported by the United States Army Medical Research and Materiel Command, Fort Detrick, MD and Faculty of Tropical Medicine, Mahidol University.
Disclaimer
The views of the authors are solely there own, and do not reflect official opinion of the United States Army Medical Research and Materiel Command or the Department of Defense.
Authors' contributions
SRM and SK designed the study, SK, MMF, SRM, KES, DEL and TK, were responsible for the supervision of the work, KES, TK, DEL, and PG were responsible for the sequences and phylogenetic analysis, SK, MMF, SRM, KES, DEL, TK, and PG drafted the manuscript. All authors read and approved the final manuscript.