Department of Immunology, College of Basic Medical Science, China Medical University, Shenyang, 110122, Liaoning Province, People's Republic of China.
Department of Laboratory Medicine, Shanghai East Hospital, Tongji School of Medicine, Shanghai, People's Republic of China.
Parasit Vectors. 2020 Apr 7;13(1):175. doi: 10.1186/s13071-020-04050-0.
Antigens expressed in sexual stages of the malaria parasites are targets of transmission-blocking vaccines (TBVs). HAP2/GCS1, a TBV candidate, is critical for fertilization in Plasmodium. Here, the genetic diversity of PvHAP2 was studied in Plasmodium vivax parasite populations from the Greater Mekong Subregion (GMS).
Plasmodium vivax clinical isolates were collected in clinics from the China-Myanmar border region (135 samples), western Thailand (41 samples) and western Myanmar (51 samples). Near full-length Pvhap2 (nucleotides 13-2574) was amplified and sequenced from these isolates. Molecular evolution studies were conducted to evaluate the genetic diversity, selection and population differentiation.
Sequencing of the pvhap2 gene for a total of 227 samples from the three P. vivax populations revealed limited genetic diversity of this gene in the GMS (π = 0.00036 ± 0.00003), with the highest π value observed in Myanmar (0.00053 ± 0.00009). Y133S was the dominant mutation in the China-Myanmar border (99.26%), Myanmar (100%) and Thailand (95.12%). Results of all neutrality tests were negative for all the three populations, suggesting the possible action of purifying selection. Codon-based tests identified specific codons which are under purifying or positive selections. Wright's fixation index showed low to moderate genetic differentiation of P. vivax populations in the GMS, with F ranging from 0.04077 to 0.24833, whereas high levels of genetic differentiation were detected between the China-Myanmar border and Iran populations (F = 0.60266), and between Thailand and Iran populations (F = 0.44161). A total of 20 haplotypes were identified, with H2 being the abundant haplotype in China-Myanmar border, Myanmar and Thailand populations. Epitope mapping prediction of Pvhap2 antigen showed that high-score B-cell epitopes are located in the S307-G324, L429-P453 and V623-D637 regions. The E317K and D637N mutations located within S307-G324 and V623-D637 epitopes slightly reduced the predicted score for potential epitopes.
The present study showed a very low level of genetic diversity of pvhap2 gene among P. vivax populations in the Greater Mekong Subregion. The relative conservation of pvhap2 supports further evaluation of a Pvhap2-based TBV.
疟原虫有性生殖阶段表达的抗原是传播阻断疫苗（TBV）的靶标。HAP2/GCS1 是一种 TBV 候选物，对疟原虫的受精至关重要。在这里，研究了大湄公河次区域（GMS）间日疟原虫寄生虫种群中的 PvHAP2 的遗传多样性。
从中缅边境地区（135 个样本）、泰国西部（41 个样本）和缅甸西部（51 个样本）的诊所收集间日疟原虫临床分离株。从这些分离株中扩增并测序了近全长 Pvhap2（核苷酸 13-2574）。进行分子进化研究以评估遗传多样性、选择和种群分化。
对来自三个间日疟原虫种群的 227 个样本的 pvhap2 基因进行测序，结果表明 GMS 中该基因的遗传多样性有限（π=0.00036±0.00003），其中缅甸的π值最高（0.00053±0.00009）。中国-缅甸边境（99.26%）、缅甸（100%）和泰国（95.12%）的主要突变是 Y133S。所有中性检验的结果均为阴性，表明可能存在纯化选择。基于密码子的检验确定了受纯化或正选择作用的特定密码子。Wright 的固定指数显示 GMS 中间日疟原虫种群的遗传分化程度较低，F 值范围为 0.04077 至 0.24833，而中国-缅甸边境和伊朗种群之间（F=0.60266）以及泰国和伊朗种群之间（F=0.44161）的遗传分化水平较高。共鉴定出 20 种单倍型，其中 H2 是中国-缅甸边境、缅甸和泰国种群中的丰富单倍型。对 Pvhap2 抗原的表位作图预测表明，高分 B 细胞表位位于 S307-G324、L429-P453 和 V623-D637 区域。位于 S307-G324 和 V623-D637 表位内的 E317K 和 D637N 突变略微降低了潜在表位的预测评分。
本研究表明，大湄公河次区域间日疟原虫种群中 pvhap2 基因的遗传多样性非常低。pvhap2 的相对保守性支持进一步评估基于 Pvhap2 的 TBV。
