Recent sequencing of Plasmodium vivax field isolates and monkey-adapted strains enabled characterization of SNPs throughout the genome. These analyses relied on mapping short reads onto the P. vivax reference genome that was generated using DNA from the monkey-adapted strain Salvador I. Any genomic locus deleted in this strain would be lacking in the reference genome sequence and missed in previous analyses. Here, we report de novo assembly of a P. vivax field isolate genome. Out of 2,857 assembled contigs, we identify 362 contigs, each containing more than 5 kb of contiguous DNA sequences absent from the reference genome sequence. These novel P. vivax DNA sequences account for 3.8 million nucleotides and contain 792 predicted genes. Most of these contigs contain members of multigene families and likely originate from telomeric regions. Interestingly, we identify two contigs containing predicted protein coding genes similar to known Plasmodium red blood cell invasion proteins. One gene encodes the reticulocyte-binding protein gene orthologous to P. cynomolgi RBP2e and P. knowlesi NBPXb. The second gene harbors all the hallmarks of a Plasmodium erythrocyte-binding protein, including conserved Duffy-binding like and C-terminus cysteine-rich domains. Phylogenetic analysis shows that this novel gene clusters separately from all known Plasmodium Duffy-binding protein genes. Additional analyses showing that this gene is present in most P. vivax genomes and transcribed in blood-stage parasites suggest that P. vivax red blood cell invasion mechanisms may be more complex than currently understood. The strategy employed here complements previous genomic analyses and takes full advantage of next-generation sequencing data to provide a comprehensive characterization of genetic variations in this important malaria parasite. Further analyses of the novel protein coding genes discovered through de novo assembly have the potential to identify genes that influence key aspects of P. vivax biology, including alternative mechanisms of human erythrocyte invasion.
Plasmodium vivax is responsible for most malaria cases outside Africa, but is poorly understood, as the parasite is difficult to study in vitro. Genome sequencing studies offer a novel and exciting opportunity to better understand this parasite but, so far, have directly mapped reads onto the reference genome sequence generated from a single P. vivax strain. Here, we use sequence data generated from a field isolate to reconstruct long DNA sequences without relying on the reference genome. Our analyses reveal many P. vivax DNA sequences that are absent from the reference genome and contain 792 predicted genes. One of these novel genes encodes a predicted protein similar to known Plasmodium proteins involved in red blood cell invasion. This new gene is present in all P. vivax strains sequenced so far, except for the strain used to generate the reference genome, and is transcribed in blood-stage parasites. Overall, our analyses show that the catalogue of P. vivax genes was incomplete and that potentially important genes have been missed. We notably identified one putative invasion gene that seems functional and could dramatically change our understanding of the mechanisms determining red blood cell invasion by this important malaria parasite.
Citation: Hester J, Chan ER, Menard D, Mercereau-Puijalon O, Barnwell J, Zimmerman PA, et al. (2013) De Novo Assembly of a Field Isolate Genome Reveals Novel Plasmodium vivax Erythrocyte Invasion Genes. PLoS Negl Trop Dis 7(12): e2569. doi:10.1371/journal.pntd.0002569
Editor: Hernando A. del Portillo, Barcelona Centre for International Health Research (CRESIB) and Institució Catalana de Recerca i Estudis Avançats (ICREA), Spain
Received: May 8, 2013; Accepted: October 21, 2013; Published: December 5, 2013
This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.
Funding: This work was funded by a Cleveland CTSC Pilot award and a NIAID award (R01 AI103228) to DS and a NIAID award (R21 AI093922) to PAZ. DM was supported by the French Ministry of Foreign Affairs during this work. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Despite being responsible for several million cases of clinical malaria every year, we still know very little about the biology of Plasmodium vivax. An important limitation of P. vivax research is the lack of continuous in vitro propagation that hampers development of functional assays and collection of sufficient amounts of biological material for studying this parasite. To circumvent these constraints, researchers often rely on materials derived from P. vivax strains that have been adapted to non-human primates (typically Saimiri and Aotus monkeys). These monkey-adapted strains are an essential resource for the P. vivax research community as they can provide large amount of parasites for protein and nucleic acid studies and ex vivo assays. It is therefore not surprising that monkey-adapted strains have played a prominent role in the identification of P. vivax proteins responsible for red blood cell (RBC) invasion –. While analyses of monkey-adapted strains have yielded invaluable insights, only a small number of strains are available  which limits their use for studying P. vivax biological diversity. In addition, development of monkey-adapted strains requires the parasite to switch hosts and adapt to a novel in vivo environment, including differences in immune systems and RBC surface proteins. It is therefore not clear whether monkey-adapted strains provide an unbiased perspective on the biology of P. vivax in vivo.
Genomic studies leveraging developments in sequencing technology have the potential to complement in vitro studies and to fill some of the gaps in our understanding of P. vivax biology. Recent whole genome sequencing studies of monkey-adapted strains  and field isolates ,  have enabled genome-wide characterization of single nucleotide polymorphisms (SNPs). However, these studies have so far relied on mapping short reads generated by massively parallel sequencing onto the P. vivax reference genome sequence that was generated using DNA from a single strain, the monkey-adapted Salvador I strain . These previous studies were therefore only able to analyze variations at genomic loci present in the Salvador I strain and would have overlooked any loci deleted in this genome. We have shown that polymorphic DNA sequence rearrangements exist among P. vivax strains and include large deletions containing annotated protein coding genes . Thus, it is possible that the Salvador I strain lacks genomic loci present in other P. vivax parasites. In this regard, it is interesting to note that recent genome sequencing of the closely related P. cynomolgi parasite highlighted several invasion protein genes without known orthologous genes in the P. vivax reference genome .
Whole genome sequencing data also provide an opportunity to circumvent these shortcomings. Instead of directly mapping massively parallel sequencing reads onto a reference genome sequence, one can de novo assemble them into long contiguous DNA sequences or “contigs” . These contigs can then be compared to the reference genome sequence to identify sequence rearrangements and novel DNA sequences. Here, we apply this approach to P. vivax and report the de novo genome assembly of a P. vivax field isolate from Cambodia. Comparisons with the Salvador I genome sequence reveal many DNA sequences absent from the reference genome sequence and numerous previously uncharacterized protein coding genes.
De novo genome assembly of a Plasmodium vivax field isolate
We selected a Cambodian P. vivax field isolate previously sequenced in our laboratory  to perform de novo assembly of large genomic regions. De novo assemblies are complicated when high levels of heterozygosity are present in the sequencing dataset. While P. vivax is haploid in humans, we have shown that most patients are infected by several genetically distinct parasites leading to many apparently heterozygous positions (with the additional caveat that the two alleles are often present in unequal proportions ). We therefore selected for our analysis massively parallel sequencing reads generated from a Cambodian patient (C127) in which more than 95% of all P. vivax DNA derived from a single strain. We hypothesized that this lower infection complexity would reduce apparent heterozygosity and facilitate de novo genome assembly. Despite residual contamination with human DNA, we have sequenced this sample at high coverage: on average, any given position of the P. vivax reference genome sequence was covered by more than 400 sequence reads (see  for details). Note that our previous genomic analyses  did not reveal any atypical features (for example the C127 fell within the range of P. vivax diversity) suggesting that this sample was not a particularly divergent strain, nor more closely related to a monkey malaria parasite (e.g., P. cynomolgi or P. simiovale).
We first identified and removed DNA sequences originating from the host genome by mapping all reads to the human genome reference sequence. This process is unlikely to accidentally remove many P. vivax DNA sequences: less than 0.01% of the assembled P. vivax reference genome sequence would be discarded by this procedure (2,220 DNA sequences of 100 bp can be mapped on the human genome out of 22,577,222 possible sequences). We then applied an error-correction algorithm to the filtered dataset and de novo assembled the short reads into large contiguous DNA sequences (contigs). After removing highly homologous contigs (n = 99), the final assembly contained 2,857 contigs larger than 1,000 bp, with a median size of 4,255 bp and a total assembly length of ~28.4 million nucleotides (Table 1).
We mapped back all massively parallel sequencing reads generated from the C127 sample onto the de novo assembly and calculated the average sequence coverage of each contig. Consistent with our initial mapping on the P. vivax reference genome, we observed that most contigs were covered at ~500× and therefore likely originated from the P. vivax genome (Figure 1). A minority of contigs, all shorter than 1 kb, showed significantly higher or lower coverage and possibly represented a combination of unfiltered host DNA sequences, P. vivax sequences from one of the minor strains present in C127, or repeated DNA sequences of the P. vivax genome.
The figure shows the average sequence coverage (y-axis, in read per base) of each de novo assembled C127 contig according to the contig length (x-axis, in bp). Six short (<200 bp) contigs displayed coverage greater than 2,000 reads by base and are not shown on this figure.
It is important to note that the assembly described here is too fragmented to provide an alternative to the Salvador I reference genome sequence. However, the length of the DNA sequences assembled is sufficient to enable genomic analyses and identify DNA sequences absent from the Salvador I reference genome.
Identification and characterization of novel P. vivax genomic sequences
We mapped all de novo assembled C127 contigs greater than 1 kb back to the P. vivax Salvador I reference genome sequence. 1,477 out of 2,857 assembled contigs mapped more than 95% of their length to the Salvador I reference genome sequence accounting for 21.6 Mb (or 95.6% of the assembled reference genome). For the rest of our analyses we focused on the 362 contigs that each contained more than 5 kb of contiguous DNA sequences absent from the Salvador I reference genome sequence (Table S1). In total, these contigs accounted for 3.8 Mb of novel P. vivax DNA sequence.
Fourteen of these 362 contigs contained at least 1 kb of DNA sequence mapped to the Salvador I reference sequence allowing us to position them on the genome, almost always in telomeric regions (Table S2). Telomeric regions are typically difficult to assemble into a final genome sequence as they contain low complexity- and repeated DNA sequences. To test whether the unmapped contigs may have been originally sequenced from the Salvador I strain but had not been successfully assembled in the published genome sequence, we compared their DNA sequences to the NCBI nucleotide collection database. Indeed, 85 contigs mapped to telomeric YAC sequences generated for sequencing the Salvador I genome and likely represented DNA sequences present in the Salvador I strain but unassembled in the final reference genome sequence (Table S1). To further investigate the genomic locations of the unmapped contigs, we mapped them to the P. knowlesi genome sequence , which is, for the most part, syntenic with the P. vivax genome . Consistent with our hypothesis, we observed that the 59 of the unmapped C127 contigs mapped to the telomeric ends of P. knowlesi chromosomes (Figure S1).
We then applied a gene prediction algorithm  to identify putative protein coding sequences in the 362 unmapped contigs and compared the predicted protein sequences with sequences of well-annotated protein coding genes. Of the 362 unmapped contigs, 235 contained predicted protein coding genes most similar to a member of the vir multigene family. These genes are often located in the telomeric regions of the P. vivax genome and have been shown to exhibit tremendous DNA sequence diversity among samples . This analysis confirmed that most DNA sequences assembled from C127 that did not map to the Salvador I reference genome originated from telomeric or sub-telomeric regions. Overall, we identified 792 novel P. vivax genes, including 366 vir genes, 35 members of other multigene families, 218 genes most similar to an annotated Plasmodium hypothetical protein and 13 predicted protein coding genes most similar to well-characterized P. vivax or P. cynomolgi genes (Table 2).
Analysis of vir multigene family in the C127 genome
Most of the de novo assembled contigs that did not map to the Salvador I reference genome contained one or more genes most similar to an annotated vir gene (Table 2). vir genes are thought to be involved in evasion of host adaptive immune response, cytoadherence and virulence; and have been previously shown to be extremely diverse among P. vivax parasites , –. vir genes predicted in the C127 contigs harbored the same structure as well-annotated vir genes, regardless of whether they are located in contigs mapping the reference genome or in novel sequences absent from the Salvador I genome (Figure S2). This observation suggests that most of the novel vir genes might be functional. vir genes are typically classified, based on sequence homology, into subfamilies ,  that are regulated differently and might have different functions , . We used our vir gene predictions and assigned each C127 vir gene into a given subfamily using a phylogenetic approach (Figure 2A) to test whether the composition of the vir gene repertoire differed between the Salvador I strain and C127. Our analyses revealed that the proportion of genes assigned to different vir subfamilies was indeed quite different between Salvador I and C127 (Figure 2B) and that these differences were not solely due to vir genes located in DNA sequences absent from the Salvador I genome (Figure S3). This suggests that, even at a given chromosomal locus, vir genes from different subfamilies may be present in different samples, perhaps due to rapid accumulation of mutations at these loci or to non-homologous recombination events. Overall, our analysis reinforces the notion that vir genes are extremely diverse, within and between P. vivax strains, and that our current catalogue of vir genes is likely to be far from complete.
(A) Phylogenetic tree showing the relationships between the protein coding sequences of vir genes from the Salvador I reference genome (solid dots) and those predicted from the C127 contigs (branches without dots at the tips). Annotated vir genes (solid dots) are colored according to their subfamilies. Nodes used to assign predicted C127 vir genes into subfamilies are shown by the colored branches derived from them. (B) Proportion of genes assigned to each major vir subfamily for Salvador I (empty bars) and C127 (black bars).
Identification of two novel P. vivax invasion protein genes
We identified two contigs without homologous DNA sequences in the Salvador I reference genome that contained predicted genes similar to P. cynomolgi invasion protein genes (Table 2).
A 31 kb contig (Figure 3A) contained 10 predicted protein coding genes including one single-exon gene of 3,003 coding nucleotides (encoding 1,001 amino acids) that displayed 74% identity over 99% of its amino acid sequence to the P. cynomolgi RBP2e gene. An 80 kb contig (Figure 4A) contained 20 predicted genes including eight vir genes and six genes most similar to P. cynomolgi hypothetical proteins. One of the remaining predicted genes contained 2,409 coding nucleotides (encoding 803 amino acids) in two exons. When compared to all annotated protein sequences present in the NCBI database, this predicted protein coding sequence was most similar to the P. cynomolgi DBP2 gene but with only 37% identity along 67% of its amino acid sequence.
(A) Next-generation sequencing read coverage along the ~30 kb contig assembled from the C127 sample. The samples displayed are, from top to bottom: C127, C08, M08, M15, Brazil I, Mauritania I, North Korea, Belem, Chesson and Salvador I. The bottom track shows, in grey, predicted protein coding genes and in black, the position of the predicted reticulocyte-binding protein gene (also highlighted by the red box). Note that there is no coverage of the contig in Salvador I. (B) Phylogenetic tree showing the relationships among protein sequences of P. vivax, P. cynomolgi, P. simiovale and P. knowlesi RBP genes. The position of the predicted P. vivax RBP2e gene is highlighted by the red arrow.
(A) Sequence coverage along the 80 kb contig containing a novel predicted P. vivax Erythrocyte-Binding Protein gene. The bottom track shows, in grey, predicted protein coding genes and, in black, the position of the predicted EBP gene (also highlighted by the red box). The upper panels display next-generation sequencing read coverage along the contig sequence. The samples are from top to bottom: C127, C08, M08, M15, Brazil I, Mauritania I, North Korea, Belem, Chesson and Salvador I. Note that in the Salvador I sample, there are essentially no reads mapping to this contig. (B) Phylogenetic tree showing the relationships among EBP protein sequences from P. vivax, P. cynomolgi, P. simiovale and P. knowlesi. The position of the novel predicted P. vivax EBP gene is highlighted by the red arrow. (C) Comparison of the protein domain annotations for the novel predicted EBP gene (top) and the known P. vivax DBP gene. The red box indicates the signal sequence; the blue box the Duffy-binding like domain; the yellow box the C-terminus cysteine-rich domain and the green box the transmembrane domain. (D) Amino acid alignment of the Duffy-binding like domain for different Plasmodium DBP genes and the novel P. vivax and P. cynomolgi EBP genes. The alignment shows the amino acid positions 215–509 of PvDBP (the section of the alignment not displayed corresponds to amino acid 303–371). The grey boxes indicate conserved cysteine positions. The red boxes indicate the positions of two additional cysteines present in the novel EBP genes. (E) The novel P. vivax EBP gene is expressed in blood-stage parasites. The gel picture shows the PCR products for the novel EBP gene (left) and known DBP gene (right) amplified from cDNA of the Belem strain. Note that, for both genes, the primers are located on either side of an intron and that the product size is consistent with amplification of cDNA molecules and excludes DNA contamination.
To test whether these DNA sequences were present in P. vivax parasites other than C127, we mapped on these contigs reads generated from samples that we  and others  have previously sequenced. Interestingly, while we observed no read coverage for these contigs in the Salvador I sample that we re-sequenced, all other samples tested showed relatively uniform coverage along these entire contig sequences (Figure 3A and Figure 4A). These results indicated that these novel DNA sequences were indeed absent from the Salvador I genome but present in all other P. vivax genomes sequenced so far. We validated these results by locus-specific PCRs that failed to amplify Salvador I DNA but, under the same conditions, successfully amplified DNA from other P. vivax samples (Figure S4). Note that we detected these contig sequences in several monkey-adapted strains suggesting that these genes were not systematically deleted from the P. vivax genome during host switch. In fact, mapping next-generation sequencing reads generated from various monkey-adapted strains on all contigs assembled from the C127 sample revealed that none of the novel DNA sequences were systematically absent from monkey-adapted stains. This observation suggests that genomic adaptations to the novel host (if any) may not involve deletions of large chromosomal segments.
PvRBP2e is deleted in the Salvador I strain but is present in other P. vivax samples
To further examine the putative P. vivax RBP gene, we compared its predicted amino acid sequence with sequences of known P. vivax , P. cynomolgi  and P. knowlesi  RBP genes as well as homologous sequences predicted from the genome assembly of the closely related species P. simiovale. The novel P. vivax RBP gene clustered within the RBP2 gene family and showed highest sequence similarity to the P. cynomolgi RBP2e gene and to a predicted P. simiovale gene (Figure 3B). In this regard, it is interesting to note that sequencing and annotation of the P. cynomolgi genome  showed the presence of the RBP2e gene in three different P. cynomolgi strains as well as in the P. knowlesi genome but failed to identify a P. vivax orthologous gene in the Salvador I genome. Our analyses suggested that RBP2e was actually present in most P. vivax parasites but absent in the Salvador I strain.
The similarity between the C127, the P. cynomolgi and P. simiovale DNA sequences extended far beyond the RBP2e protein coding gene sequence and could be detected for several kb outside the gene boundaries (Figure S5). This synteny confirmed that the RBP2e genes of the different Plasmodium species were indeed orthologous genes. In addition, using DNA sequence similarity to the P. cynomolgi DNA sequence, we were able to determine that the P. vivax RBP2e gene was likely located at the telomeric end of chromosome 7 (similar to its position in the P. cynomolgi genome), approximately 25 kb upstream of the annotated start of the Salvador I chromosome 7 (Figure S5).
One puzzling observation was that the predicted RBP2e gene was significantly shorter in P. vivax than in other Plasmodium species: the predicted P. vivax protein was only 1001 amino acid long, compared to 2732, 2745 and 2927 amino acids in, respectively, P. cynomolgi, P. simiovale and P. knowlesi. In addition, the C127 contig contained four additional predicted genes with short protein coding sequences and located immediately next to the predicted P. vivax RBP2e, which also showed high similarity with P. cynomolgi RBP2e (Table 2 and Figure S6A). Close inspection of the DNA sequence alignments between the C127 contig and P. cynomolgi, P. simiovale and P. knowlesi sequences revealed several short out-of-frame deletions in the P. vivax DNA sequence that occurred between the beginning of the protein sequence predicted in the other Plasmodium genomes and the beginning of the protein sequence predicted in P. vivax, resulting in the introduction of premature stop codons and multiple short predicted protein coding sequences (Figure S6A). To test whether the deletions in the C127 contigs were genuine or artifacts of the de novo assembly, we mapped sequencing reads from C127 and other P. vivax samples onto the assembled C127 contig sequence. This analysis confirmed the accuracy of the contig DNA sequence and indicated that other P. vivax samples also carried deletions resulting in shorter forms of the RBP2e protein coding gene. Amplification by locus-specific PCRs of P. vivax cDNA showed that the RBP2e was expressed by blood-stage P. vivax parasites. However, this analysis also revealed a single extended transcript that encompassed several (if not all) of the P. vivax short predicted protein coding genes, similar to the RBP2e gene structure predicted in P. cynomolgi, P. simiovale and P. knowlesi (Figure S6B). Sanger sequencing of the amplified cDNA products confirmed the premature stop codons observed in the C127 contig sequence. One possible explanation for these observations was that the RBP2e protein coding sequence had been disrupted by several deletions in P. vivax, resulting in a non-functional protein, while the proximal promoter and other regulatory elements were kept intact and could still induce transcription of this probable pseudogene. However it is important to note that PfRH5, a gene encoding a P. falciparum RBP, shows a similar pattern, with an early stop codon compared to other PfRH genes, but is translated into a functional protein that binds specifically to human erythrocytes . Further studies will therefore be required to rigorously determine whether RBP2e is functional in P. vivax.
A novel P. vivax Erythrocyte-Binding Protein gene sequence
We next examined the novel predicted P. vivax gene displaying similarities with the P. cynomolgi DBP2. We first compared its predicted protein coding sequence with the sequences of known P. vivax, P. cynomolgi and P. knowlesi DBP genes as well as predicted homologous protein sequences. Phylogenetic analyses revealed that the novel predicted gene fell outside the cluster of known Plasmodium DBPs which included P. vivax DBP, two P. cynomolgi genes, one P. simiovale and three P. knowlesi genes. The putative novel protein coding sequence was only similar to a previously unannotated protein coding gene predicted in the P. cynomolgi genome (with 83% identity at the amino acid level) (Figure 4B). Interestingly, this DNA sequence similarity extended far beyond the predicted protein coding sequence since the contigs containing the novel P. vivax and P. cynomolgi predicted genes could be aligned over more than 12 kb (Figure S7) suggesting that these genes were indeed orthologous. Unfortunately, the lack of overlapping DNA sequences between either of these contigs and any of the Plasmodium genomes assembled prevented us from determining the position of this predicted gene in the P. vivax genome.
While the novel predicted P. vivax gene clearly differed from known DBP genes, it displayed several interesting features. The predicted protein sequence contained a N-terminal 22 amino acid long signal peptide, a 276 amino acid Duffy-binding like domain (DBL or pfam05424), partially overlapping with a P. falciparum erythrocyte membrane protein 1 (PfEMP1) domain, an 81 amino acid erythrocyte binding antigen 175 (EBA-175) domain, also referred to as a C-terminal cysteine-rich domain , , and a final 20 amino acid long transmembrane domain (Figure 4C). These features are the hallmark of the Erythrocyte-Binding Protein (EBP) superfamily , which includes the DBPs and P. falciparum EBA proteins, and are critical for invasion of human RBC by malaria parasites. The novel P. vivax predicted EBP gene contained a single DBL domain, as in the P. vivax, P. knowlesi and P. cynomolgi DBPs, while all P. falciparum EBP genes carry two consecutive domains  (note that RBPs have a different structure and do not harbor a DBL domain). The DBL domain is classically characterized by 12 conserved cysteines, five of them (cysteines 4 to 8) being critical for binding to erythrocyte ligands , . Alignment of the novel predicted P. vivax and P. cynomolgi EBP amino acid sequences with known Plasmodium DBPs revealed that, while the rest of the protein sequence showed numerous differences, 11 out of the 12 cysteines of the DBL domain were conserved, including all cysteines of the minimal region implicated for receptor recognition (Figure 4D). This observation suggested that the novel P. vivax predicted EBP might also be able to bind to human erythrocytes. In the C-terminus cysteine-rich domain, 6 out of 8 cysteine positions were also conserved between this newly identified P. vivax EBP and known DBPs (Figure S8) indicating that this domain might, as in other Plasmodium EBPs , , regulate the export of the protein to the micronemes where it can be displayed at the apical end of the merozoites during erythrocyte invasion.
We then investigated the level of conservation of the predicted EBP coding sequence among P. vivax samples. For ten sequenced P. vivax samples, sampled in Asia, Southwest Pacific, Africa and Central and South-America, in which most of P. vivax DNA derived from a single strain (see also ), we determined the predicted coding sequence directly from the DNA sequences of massively parallel sequencing reads. We did not observe any nonsense or frameshift mutations and the novel P. vivax EBP gene displayed only 11 polymorphic sites in 2,412 bp, including 2 synonymous- and 9 non-synonymous SNPs. For comparison, in the same samples the annotated P. vivax DBP gene displayed 32 polymorphic sites in 3,213 positions (4 synonymous and 28 non-synonymous SNPs) (Figure S9). Overall, the pattern of diversity observed among P. vivax samples showed no evidence that the novel predicted EBP gene was a pseudogene. Finally, we amplified cDNA generated from the Belem strain and showed that this novel EBP gene was expressed in blood-stage parasites (Figure 4E).
By de novo assembly of a field isolate genome, we have shown that large genomic regions of the P. vivax genome had not been previously characterized because they were missing from the Salvador I strain used for generating the P. vivax reference genome. In total, we identified 3.8 Mb of novel DNA sequences containing close to 800 novel predicted genes. These included at least two putative invasion genes that had not been previously described in P. vivax and is consistent with observations in P. falciparum and P. cynomolgi that invasion genes are frequently deleted among strains , –. In particular, we described a novel predicted protein coding gene containing a DBL domain and a C-terminus cysteine-rich domain. While clearly different from all previously characterized Plasmodium DBPs, this predicted protein harbored all the key features of EBPs, suggesting that it might be able to bind to human erythrocytes and contribute to RBC invasion. This putative EBP gene is missing from the Salvador I strain but present in all other P. vivax strains tested, representing a wide geographical range, and is expressed in blood-stage parasites. While further studies will be required to determine whether this gene is functional and characterize its role in erythrocyte invasion, it is tempting to speculate that the novel P. vivax EBP may enable interactions with erythrocyte membrane proteins other than the Duffy antigen and contribute to alternative invasion pathways. In this regard, it is important to note that, so far, in vitro studies have consistently shown that PvDBP is unable to bind Duffy-negative erythrocytes, suggesting that its only ligand is the human Duffy antigen. Recent observations of P. vivax infections of Duffy-negative individuals – therefore remain mysterious. The discovery of this second P. vivax EBP gene provides a candidate to possibly explain this alternative invasion mechanism and could perhaps constitute a first step towards understanding the molecular bases underlying P. vivax infection of Duffy-negative individuals. Overall, our study showed that genomic analyses of P. vivax parasites can complement in vitro approaches and significantly contribute to improving our understanding of P. vivax biology.
Materials and Methods
De novo assembly of a Cambodian P. vivax field isolate
For de novo genome assembly, we focused on massively parallel sequencing data generated previously from a Cambodian P. vivax field isolate (C127) . We first removed host DNA sequences by mapping all 211,061,945 paired-end reads generated from this sample on the human reference genome sequence  using Bowtie 2  and low-stringency mapping criteria (using the very-sensitive parameter). This approach removed 13,214,696 read pairs (6%). Next, we applied the algorithm implemented in Quake  to correct sequencing errors and remove low-quality reads using a k-mer of 17 nucleotides. We then generated a de novo assembly from the remaining 161,300,780 corrected read pairs using ABySS  and a k-mer of 51 bp (we determined the optimal k-mer by testing different k values on a subset of 24,730,906 paired-end reads and comparing the resulting assemblies). Finally, we mapped all contigs greater than 1 kb onto each other using Lastz  and, when two contigs were more than 95% identical over more than 95% of their length, we discarded the shorter sequence to filter out highly homologous contigs.
We then mapped all C127 paired-end reads on the final assembly using Bowtie 2 and calculated the average read coverage of each contig (excluding 100 bp at the end of each sequence to avoid mapping biases). We also mapped reads generated from other P. vivax samples that we  and others  have analyzed to the C127 final contigs to determine sequence coverage in each sample and identify DNA polymorphisms. We included in these analyses: two additional Cambodian field isolates (C08 and C15), three Malagasy field isolates (M08, M19 and M15) and six monkey-adapted strains (Salvador I, Belem, Chesson, Brazil I, North Korea and Mauritania I).
Mapping of C127 contigs to the Salvador I reference genome and identification of novel DNA sequences
To identify P. vivax DNA sequences missing from the Salvador I genome , we mapped all contigs to the reference genome sequence using Lastz, allowing for up to 10% DNA sequence divergence. We then screened for contigs in which more than 5 kb of contiguous DNA sequence did not map the P. vivax reference genome sequence. We then compared these contig sequences to the NCBI nucleotide database (nt) using blast to identify loci that had been originally sequenced from the Salvador I strain but were not included in the final genome assembly.
For all contigs with more than 5 kb of contiguous DNA sequence that did not map to the Salvador I reference genome, we searched for putative protein coding gene sequences using the SNAP gene prediction algorithm . We used the NCBI gene annotations of the P. vivax Salvador I genome as the training set for the parameter estimation of the hidden Markov model. We then compared each predicted protein coding gene sequence to annotated proteins deposited in the NCBI non-redundant protein database (nr) to preliminarily characterize these putative protein coding genes.
vir gene analysis
To optimize gene predictions for the members of the vir multigene family, we re-ran the SNAP algorithm on all de novo assembled contigs but using only vir genes annotated from the Salvador I reference as a training set. We translated the predicted genes into proteins and used blastp against the NCBI nr database to identify which predicted genes were most similar to vir genes. We then used Clustal Omega to align the protein sequences of the predicted vir genes with those of all annotated vir genes from the Salvador I genome and reconstructed a phylogenetic tree using FastTree. Finally, we annotated the branch of this tree according to the known vir subfamily annotations  and determined which node best defined each major vir subfamily using the following criteria: i) branches from this node must account for as many vir genes of the subfamily as possible, ii) branches derived from the node should include less than three vir gene from other subfamilies, iii) the node must be supported by a bootstrap value greater than 90% and iv) when multiple nodes fulfilled these previous conditions, the node closest to the root was selected. We then considered that all predicted vir genes downstream of one of these defining nodes belonged to the corresponding vir sub-families.
To validate the novel invasion protein coding genes and exclude the possibility that their DNA sequences resulted from assembly artifacts, we designed locus-specific primers and directly amplified these genes by PCR from genomic DNA. A putative P. vivax RBP gene was amplified using the primers 5′- AGAAGCTCTGGAGACACAAGC-3′ and 5′- GGTTCGTCCCTTTTCACCGT-3′ and a putative P. vivax EBP gene was amplified using the primers 5′-CAGCACAAAGACGACGGGTA-3′ and 5′-TCGTCTTTCTTTTTCGTCCTGC3′. We used the following PCR conditions: 94C for 3 min, followed by 40 cycles of 94°C for 60 s, 56°C for 45 s, and 72°C for 2 min. We sequenced all PCR products using Sanger chemistry and confirmed that the DNA sequences amplified were identical to the novel contig sequences.
To test whether these putative genes were transcribed in blood-stage parasites, we extracted total RNA from 150 µl of blood from a Saimiri boliviensis monkey infected with the Belem strain (30,000 parasites per µl) using standard Trizol/Chloroform protocol. After DNase I treatment, we prepared double-stranded cDNA using random hexamers and the Invitrogen SuperScriptIII Reverse Transcriptase kit. We then amplified putative genes using primers designed to span an intron and sequenced the resulting PCR products by Sanger sequencing. The novel P. vivax EBP transcript was amplified using the primers 5′-ATGGCAGCAGAAGGACTGAG-3′ and 5′-GCGGCGAGCACAATGAATAAT-3′. The novel P. vivax RBP transcript was amplified by several primer pairs described in Figure S6. We also amplified the P. vivax DBP transcript as a positive control using the primers 5′-AGGACATGACAGGGATAGCA-3′ and 5′- CAACAGCAGTATCAGCAACGC-3′.
Identification of orthologous Duffy- and Reticulocyte-binding protein genes in other Plasmodium genomes
We used the assembled genome sequences and gene annotations generated respectively from the P. knowlesi H strain  and P. cynomolgi B strain . We also generated 213,543,894 paired-end reads of 100 bp from DNA extracted from the P. simiovale strain maintained at the Centers for Disease Control and Prevention and de novo assembled this genome using the approach described above.
We retrieved the coding sequences from the P. vivax DBP and all annotated P. vivax RBPs from plasmodb  and added them to the sequences of the two novel predicted protein coding gene sequences identified in the C127 assembly. We used blast to search for similar protein coding sequences in the P. simiovale, P. cynomolgi and P. knowlesi genome assemblies. To avoid biasing the gene annotations towards the P. vivax annotations, we then used SNAP to predict protein coding gene sequences in any DNA sequence aligning to a P. vivax RBP or DBP gene sequence and translated these predicted genes into amino acid sequences.
Protein domain annotations
We preliminarily annotated the putative P. vivax EBP gene using the NCBI conserved domains and protein classification tool  as well as the TMHMM algorithm  for identifying transmembrane domains and SignalP  for signal peptide.
Sequence alignments and reconstruction of phylogenetic trees
We aligned DNA and protein sequences using Clustal omega  and reconstructed neighbor-joining trees with MEGA  using either the total number of amino acid differences or Jones-Taylor-Thornton  distances. Both methods yielded similar results in all reconstructions.
Distribution of the C127 contigs according to their position on the P. knowlesi genome. The figure displays, on each bottom row, the chromosomal (y-axis) and nucleotide location (x-axis, in bp) on the P. knowlesi genome of C127 contigs mapped to Salvador I (in grey) and 59 contigs that do not map to Salvador I (in black). For comparison, the top row shows, for each chromosome, the mapping of Salvador I DNA sequences onto the P. knowlesi genome.M stands for the mitochondrial genome.
Proportion of annotated (Salvador I, in red) or predicted (C127, in grey or black) vir genes (y-axis) according to their number of exons (x-axis). For C127, the vir genes are divided according to their location on contigs that map the reference genome (in black) or were absent from the reference genome (in grey).
Proportion of genes assigned to each major vir subfamily for Salvador I (red bars), C127 contig mapped on the reference genome (black bars) or vir gene located on novel DNA sequences (in grey).
Validation by locus-specific PCR. The gel shows the amplification products of the predicted EBP gene (left) and RBP gene (right) from genomic DNA extracted from three P. vivax samples (Salvador I, Chesson and C127) and a negative control (N).
Schematic DNA sequence alignment of the contigs generated from C127 (in red), P. cynomolgi (in blue) and P. simiovale (in green) and the P. vivax reference genome (in black). The grey box indicates the position of the RBP2e gene. Note that the P. vivax chromosome 7 sequence only starts several kb after the position of the predicted RBP2e gene.
(A) Schematic representation of the predicted RBP2e proteins in different Plasmodium genomes. The solid blue boxes represent the predicted protein coding sequence in, from top to bottom, P. knowlesi, P. cynomolgi, P. simiovale and P. vivax (C127). The black asterisks indicate the locations of out-of-frame deletions that, in P. vivax, introduce stop codons (vertical red bars) and lead to multiple short predicted proteins. The black arrows indicate the primers used to amplify cDNA from the Belem strain (from left to right, RBPi, RBP3–4 and RBP4–5). (B) The gel picture shows the PCR products obtained for the P. vivax RBP2e gene using cDNA generated from the Belem strain. The leftmost amplification (RBPo) targets a region upstream of the predicted RBP2e transcript and fails to yield any PCR product. The next three amplifications all yield PCR products of the correct size and identical to the contig sequence after Sanger sequencing and correspond to i) amplification spanning an intron in the first predicted PvRBP2e gene (RBPi), ii) amplification between the 3rd and 4th predicted PvRBP2e genes (RBP3–4) and iii) amplification between the 4th and the 5th predicted PvRBP2e genes (RBP4–5). The primer sequences used are as follow: RBPoF 5′-CCTCTTCTAGCTATTGAACTCACCA-3′, RBPoR 5′-AGCGTGCATGGCTAATTGTA-3′, RBPiF 5′-TGTGATCTTTTGTAACCTCTTGTTT-3′, RBPiR 5′-TTCCCAAGGAAGTGGCATGT-3′, RBP3–4F 5′-TCCTGAGACGGTGGATAACA-3′, RBP3–4R 5′-TGTTCTTTAGCTGTGTGAGACT-3′, RBP4–5F 5-GGACTACGAGCAAAGTGCAG-3′, RBP4–5R 5′-AGCGGATTCTTTGTGACTCCTT-3′.
DNA sequence identity (y-axis, in %) between the C127 contig carrying the predicted EBP gene (indicated by the red box) and its orthologous P. cynomolgi contig (x-axis, in bp).
Amino acid alignment of the C-terminus cysteine-rich like domain for DBP genes and the novel P. vivax and P. cynomolgi EBP genes. The grey boxes indicate conserved cysteine positions.
Phylogenetic tree showing the relationships between the amino-acid sequences of P. vivax samples for several RBP2 genes and the putative RBP2e.
Contigs generated from the C127 sample with their length, percentage of the contig sequence mapped to the reference genome (%Sal I), number of predicted genes and blast comparisons to the nucleotide (nt) and protein (nr) databases.
List of all contigs mapping partially to the Salvador I reference genome.
We thank Dr. Montpetit and the sequencing group of the McGill University and Genome Quebec Innovation Center for their help with HiSeq sequencing. The raw sequence data generated for this study are accessible through SRA (www.ncbi.nlm.nih.gov/sra/): SRP0157575 (P. vivax) and SRR826495 (P. simiovale). The novel P. vivax EBP gene sequence is available through GenBank (www.ncbi.nlm.nih.gov/genbank) with the accession KC987954.
Conceived and designed the experiments: JH PAZ DS. Performed the experiments: JH ERC DS. Analyzed the data: JH ERC PAZ DS. Contributed reagents/materials/analysis tools: DM OMP JB DS. Wrote the paper: DS.
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