Genetic Diversity and Natural Selection of a Malaria Vaccine Candidate Gene in the Ethiopian Plasmodium Vivax Population
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Date
2017-12-03
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Addis Ababa University
Abstract
The burden of P.vivax in Ethiopia is amongst the highest in the world. However, P. vivax diversity, particularly that associated with antigens, such as Plasmodium vivax Merozoite surface protein 3α (PvMSPα), has rarely been studied in Ethiopia. These studies are fundamental for tracking important phenotypic variants of the parasite such as drug resistance genes or antigenic variants in different transmission settings, as this aid in designing future control and elimination strategies. In the present study the genetic polymorphism in the defined target was assessed by examining genes encoding two blocks of this antigen locus. Finger prick blood samples spotted onto filter papers were collected from microscopically and RDT confirmed malaria patients attending health facilities of Shewa Robit, Melka Oda, Abure, Aje Haposto and Ilala. DNA was extracted by Chelex Saponin extraction method, the genomic DNA was used for confirmation of P.vivax infection by targeting the 18S rRNA gene. Positive samples were subsequently evaluated by Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) for identification and assessment of the genetic polymorphism of the MSP3α gene. Further single clone infections were then analyzed using Sanger sequencing. Three size variants were amplified from the 50 isolates, Type A, B and C with frequencies of 82.97%, 12.7% and 4.2% respectively. Further details of diversity were attained by Hha I RFLP, with 11 alleles and 12% multiple clone infections. The sequence analysis showed that size polymorphisms were results of insertions and deletions in the block I component of the gene, which also had higher nucleotide diversity (π) (0.10565) than the block II (0.014). The relatively conserved block II was evolving under positive selection, but a select region that encodes a predicted B cell epitope in these block is under balancing selection (Tajima’s D 2.64 (P˃0.05), Fu and Li s F 1.7621 (P˃0.05); Furthermore a peak diversity was recorded at this site (π=0.65) with low inter-population FST estimates . The conserved nature of PvMSP3α block II makes it an ideal vaccine candidate. However future vaccine design strategies targeting PvMSP3α block II should put into consideration the identified antigenic polymorphism from this study, as they might constitute an immune/vaccine escape mechanism. In contrast the polymorphic nature of PvMSP3α block I make it more suited for use as a rapid genotyping tool.
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Plasmodium Vivax, Merozoite Surface Protein 3α, PCR-RFLP, Multiple Clone Infections, Sequence Polymorphism, Balancing Selection, Antigenic Polymorphism, Ethiopia