Laboratory of Malariology, Research Institute for Microbial Diseases, Osaka University, Osaka 565-0871, Japan.
Vaccine. 2012 Feb 21;30(9):1583-93. doi: 10.1016/j.vaccine.2011.12.124. Epub 2012 Jan 9.
SERA5 is regarded as a promising malaria vaccine candidate of the most virulent human malaria parasite Plasmodium falciparum. SERA5 is a 120 kDa abundantly expressed blood-stage protein containing a papain-like protease. Since substantial polymorphism in blood-stage vaccine candidates may potentially limit their efficacy, it is imperative to fully investigate polymorphism of the SERA5 gene (sera5). In this study, we performed evolutionary and population genetic analysis of sera5. The level of inter-species divergence (kS=0.076) between P. falciparum and Plasmodium reichenowi, a closely related chimpanzee malaria parasite is comparable to that of housekeeping protein genes. A signature of purifying selection was detected in the proenzyme and enzyme domains. Analysis of 445 near full-length P. falciparum sera5 sequences from nine countries in Africa, Southeast Asia, Oceania and South America revealed extensive variations in the number of octamer repeat (OR) and serine repeat (SR) regions as well as substantial level of single nucleotide polymorphism (SNP) in non-repeat regions (2562 bp). Remarkably, a 14 amino acid sequence of SERA5 (amino acids 59-72) that is known to be the in vitro target of parasite growth inhibitory antibodies was found to be perfectly conserved in all 445 worldwide isolates of P. falciparum evaluated. Unlike other major vaccine target antigen genes such as merozoite surface protein-1, apical membrane antigen-1 or circumsporozoite protein, no strong evidence for positive selection was detected for SNPs in the non-repeat regions of sera5. A biased geographical distribution was observed in SNPs as well as in the haplotypes of the sera5 OR and SR regions. In Africa, OR- and SR-haplotypes with low frequency (<5%) and SNPs with minor allele frequency (<5%) were abundant and were mostly continent-specific. Consistently, significant genetic differentiation, assessed by the Wright's fixation index (Fst) of inter-population variance in allele frequencies, was detected for SNPs and both OR- and SR-haplotypes among almost all parasite populations. The exception was parasite populations between Tanzania and Ghana, suggesting frequent gene flow in Africa. The present study points to the importance of investigating whether biased geographical distribution for SNPs and repeat variants in the OR and SR regions affect the reactivity of human serum antibodies to variants.
SERA5 被认为是最具毒力的人类疟原虫恶性疟原虫的有前途的疟疾疫苗候选物。SERA5 是一种 120 kDa 的丰富表达的血期蛋白，含有木瓜蛋白酶样蛋白酶。由于血期疫苗候选物的大量多态性可能潜在地限制其功效，因此必须充分研究 SERA5 基因（sera5）的多态性。在这项研究中，我们对 sera5 进行了进化和群体遗传学分析。恶性疟原虫和与其密切相关的黑猩猩疟原虫 Plasmodium reichenowi 之间的种间差异水平（kS=0.076）与管家蛋白基因相当。在酶原和酶结构域中检测到了纯化选择的特征。对来自非洲、东南亚、大洋洲和南美洲的 9 个国家的 445 个近全长 P. falciparum sera5 序列的分析显示，在八聚体重复（OR）和丝氨酸重复（SR）区域的数量以及非重复区域（2562 bp）的单核苷酸多态性（SNP）水平存在广泛的变化。值得注意的是，在所有评估的 445 个来自世界各地的 P. falciparum 分离株中，SERA5 的 14 个氨基酸序列（氨基酸 59-72）被发现是寄生虫生长抑制抗体的体外靶标，被发现是完全保守的。与其他主要疫苗靶抗原基因（如裂殖子表面蛋白-1、顶膜抗原-1 或环子孢子蛋白）不同，在 sera5 的非重复区域中没有发现 SNP 的强烈正选择证据。在非重复区域的 SNP 以及 sera5 的 OR 和 SR 区域的单倍型中观察到偏态地理分布。在非洲，低频率（<5%）的 OR-和 SR-单倍型以及小等位基因频率（<5%）的 SNP 很丰富，并且大多是特定于大陆的。一致地，在几乎所有寄生虫种群中，通过等位基因频率的种群间方差 Wright 的固定指数（Fst）来评估，SNP 以及 OR-和 SR-单倍型都存在显著的遗传分化。例外是坦桑尼亚和加纳之间的寄生虫种群，表明非洲频繁的基因流动。本研究表明，有必要研究 OR 和 SR 区域的 SNP 和重复变异的偏态地理分布是否会影响人类血清抗体对变异的反应性。
