Abstract
BackgroundAn effective malaria vaccine is an urgently needed tool to fight against human malaria, the most deadly parasitic disease of humans. One promising candidate is the merozoite surface protein-3 (MSP-3) of Plasmodium falciparum. This antigenic protein, encoded by the merozoite surface protein (msp-3) gene, is polymorphic and classified according to size into the two allelic types of K1 and 3D7. A recent study revealed that both the K1 and 3D7 alleles co-circulated within P. falciparum populations in Thailand, but the extent of the sequence diversity and variation within each allelic type remains largely unknown.MethodsThe msp-3 gene was sequenced from 59 P. falciparum samples collected from five endemic areas (Mae Hong Son, Kanchanaburi, Ranong, Trat and Ubon Ratchathani) in Thailand and analysed for nucleotide sequence diversity, haplotype diversity and deduced amino acid sequence diversity. The gene was also subject to population genetic analysis (F st) and neutrality tests (Tajima’s D, Fu and Li D* and Fu and Li’ F* tests) to determine any signature of selection.ResultsThe sequence analyses revealed eight unique DNA haplotypes and seven amino acid sequence variants, with a haplotype and nucleotide diversity of 0.828 and 0.049, respectively. Neutrality tests indicated that the polymorphism detected in the alanine heptad repeat region of MSP-3 was maintained by positive diversifying selection, suggesting its role as a potential target of protective immune responses and supporting its role as a vaccine candidate. Comparison of MSP-3 variants among parasite populations in Thailand, India and Nigeria also inferred a close genetic relationship between P. falciparum populations in Asia.ConclusionThis study revealed the extent of the msp-3 gene diversity in P. falciparum in Thailand, providing the fundamental basis for the better design of future blood stage malaria vaccines against P. falciparum.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-016-1566-1) contains supplementary material, which is available to authorized users.
Highlights
An effective malaria vaccine is an urgently needed tool to fight against human malaria, the most deadly parasitic disease of humans
Nucleotide sequence analysis of the msp‐3 gene of Plasmodium falciparum in Thailand Fragments of the P. falciparum msp-3 gene were successfully amplified from 59 human blood samples collected from five endemic areas
The derived polymerase chain reaction (PCR) products were analysed by standard agarose gel electrophoresis, as described in Methods, revealing that 17 and 42 samples had the 3D7 and K1 allelic types of msp-3 (Additional file 1), respectively, giving frequencies of the 3D7 and K1 allelic types of 28.8 and 71.2 %, respectively
Summary
An effective malaria vaccine is an urgently needed tool to fight against human malaria, the most deadly parasitic disease of humans. The erythrocyte invasion is a critical point in the life cycle of malaria, and at this point the merozoites can be targeted by antibodies and subsequently eliminated by white blood cells. This type of vaccine generally incorporates antigenic proteins that are highly expressed on, or associated with, the surface of merozoites, such as the merozoite surface proteins (MSPs). One of the key difficulties for malaria vaccine development is the structural diversity of the malaria antigens that is caused by the extensive genetic polymorphism of the antigen-encoding genes. A complete understanding of the malaria parasite’s biology and the extent of genetic diversity in natural parasite populations will be necessary for improving vaccine design and efficacy
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