Institute of Tropical Medicine, University of Tuebingen, Wilhelmstr. 27, 72074, Tuebingen, Germany.
Malaria Programme, Wellcome Trust Sanger Institute, Hinxton, CB10 1SA, UK.
Malar J. 2020 May 29;19(1):194. doi: 10.1186/s12936-020-03257-x.
The multicopy var gene family of Plasmodium falciparum is of crucial importance for pathogenesis and antigenic variation. So far only var2csa, the var gene responsible for placental malaria, was found to be highly conserved among all P. falciparum strains. Here, a new conserved 3D7 var gene (PF3D7_0617400) is identified in several field isolates.
DNA sequencing, transcriptional analysis, Cluster of Differentiation (CD) 36-receptor binding, indirect immunofluorescence with PF3D7_0617400-antibodies and quantification of surface reactivity against semi-immune sera were used to characterize an NF54 clone and a Gabonese field isolate clone (MOA C3) transcribing the gene. A population of 714 whole genome sequenced parasites was analysed to characterize the conservation of the locus in African and Asian isolates. The genetic diversity of two var2csa fragments was compared with the genetic diversity of 57 microsatellites fragments in field isolates.
PFGA01_060022400 was identified in a Gabonese parasite isolate (MOA) from a chronic infection and found to be 99% identical with PF3D7_0617400 of the 3D7 genome strain. Transcriptional analysis and immunofluorescence showed expression of the gene in an NF54 and a MOA clone but CD36 binding assays and surface reactivity to semi-immune sera differed markedly in the two clones. Long-read Pacific bioscience whole genome sequencing showed that PFGA01_060022400 is located in the internal cluster of chromosome 6. The full length PFGA01_060022400 was detected in 36 of 714 P. falciparum isolates and 500 bp fragments were identified in more than 100 isolates. var2csa was in parts highly conserved (H = 0) but in other parts as variable (H = 0.86) as the 57 microsatellites markers (H = 0.8).
Individual var gene sequences exhibit conservation in the global parasite population suggesting that purifying selection may limit overall genetic diversity of some var genes. Notably, field and laboratory isolates expressing the same var gene exhibit markedly different phenotypes.
恶性疟原虫（Plasmodium falciparum）的多拷贝 var 基因家族对发病机制和抗原变异至关重要。到目前为止，只有导致胎盘疟疾的 var2csa 基因被发现存在于所有恶性疟原虫株中高度保守。在此，在几个现场分离株中鉴定到一个新的保守 3D7var 基因（PF3D7_0617400）。
使用 DNA 测序、转录分析、CD36 受体结合、用 PF3D7_0617400 抗体进行间接免疫荧光和定量分析针对半免疫血清的表面反应性，以对转录该基因的 NF54 克隆和加蓬现场分离株克隆（MOA C3）进行特征描述。对 714 个全基因组测序寄生虫进行分析，以描述该基因座在非洲和亚洲分离株中的保守性。将两个 var2csa 片段的遗传多样性与现场分离株中 57 个微卫星片段的遗传多样性进行比较。
在来自慢性感染的加蓬寄生虫分离株（MOA）中鉴定到 PFGA01_060022400，与 3D7 基因组株的 PF3D7_0617400 高度同源（99%）。转录分析和免疫荧光显示该基因在 NF54 和 MOA 克隆中表达，但 CD36 结合测定和对半免疫血清的表面反应性在两个克隆中差异显著。长读长太平洋生物科学全基因组测序显示，PFGA01_060022400 位于染色体 6 的内部簇中。在 714 个恶性疟原虫分离株中检测到全长的 PFGA01_060022400，在 100 多个分离株中鉴定到 500bp 片段。var2csa 部分高度保守（H=0），但在其他部分可变（H=0.86），与 57 个微卫星标记（H=0.8）一样。
个体 var 基因序列在全球寄生虫群体中表现出保守性，表明纯化选择可能限制了某些 var 基因的整体遗传多样性。值得注意的是，表达相同 var 基因的现场和实验室分离株表现出明显不同的表型。
