Association of the Plasmodium falciparuminfected erythrocyte surface molecule with ist virulence

恶性疟原虫（Plasmodium falciparum） SURFIN是一种多态抗原，定位于卵裂子的顶端末端和被感染红细胞的表面。在使用实验室系寄生虫的分析中，n端胞外区域显示出高度多态性。为了研究野生分离株的多态性水平，并评估该区域是否处于正选择状态，我们对在泰国收集的两个不同时间点的恶性疟原虫分离株的SURFIN4.2胞外结构域的2167进行了核苷酸（nt）编码测序。分析了1988/1989年采集的21个序列和2003/2005年采集的32个序列。共观察到258个多态性位点，平均核苷酸多样性为0.043。基于预测的蛋白结构和多态性水平，我们将胞外区分为三个部分；富含半胱氨酸的结构域（nt 1-585）、可变结构域1 （nt 586-1392）和可变结构域2 （nt 1393-2194）。多数多态位点聚集在可变结构域2。在所有区域都检测到非同义取代多于同义取代，表明正选择在整个细胞外区域起作用。Tajima的D检验和Fu & Li的D^*和F^*检验都在99%的置信水平上检测到变量域2上的正选择特征。因此，SURFIN_4.2的胞外结构域似乎是在正向多样化选择下进化的，可能是由宿主免疫压力介导的。毒力与特定的氨基酸取代之间没有相关性。比较1988/1989年和2003/2005年样品各氨基酸位点的氨基酸频率，除4个位点外，大部分位点的c291频率相同。需要进一步的评价来证实这一观察结果。

Plasmodium falciparum SURFIN is a polymorphic antigen localized at the apical end of the merozoite, and on the surface of infected erythrocytes. In an analysis using laboratory line parasites, the N-terminal extracellular region was shown to be highly polymorphic. In order to investigate the level of polymorphism in wild isolates, and to assess whether this region is under positive selection, we sequenced the 2167 by nucleotide (nt) encoding the extracellular domain of SURFIN4.2 from P falciparum field isolates collected in Thailand at two different time points. Twenty-one sequences were analyzed from samples collected in 1988/1989 and 32 sequences from 2003/2005. A total of 258 polymorphic sites were observed with an average nucleotide diversity of 0.043. Based on the predicted protein structure and the level of polymorphism, we divided the extracellular region into three parts; a Cysteine-rich domain (nt 1-585), variable domain 1 (nt 586-1392) and variable domain 2 (nt 1393-2194). The majority of polymorphic sites were clustered in variable domain 2. An excess of nonsynonymous substitutions over synonymous substitutions was detected in all regions, suggesting that positive selection is acting throughout the extracellular region. Tajima's D test and Fu & Li's D^* and F^* tests all detected the signature of positive selection on variable domain 2 at the 99% confidence level. Thus, the extracellular domain of SURFIN_4.2 appears to have evolved under positive diversifying selection, possibly mediated by host immune pressure. There are no correlation between virulence and particular amino acid substitution. When the amino acid frequency was compared at each amino acid position between samples in 1988/1989 and 2003/2005, most positions showed same C291frequency except 4 positions. Further evaluation is needed to confirm this observation.