Selections, frameshift mutations, and copy number variation detected on the surf _4.1 gene in the western Kenyan Plasmodium falciparum population

Plasmodium falciparum SURFIN_4.1 is a putative ligand expressed on the merozoite and likely on the infected red blood cell, whose gene was suggested to be under directional selection in the eastern Kenyan population, but under balancing selection in the Thai population. To understand this difference, surf _4.1 sequences of western Kenyan P. falciparum isolates were analysed. Frameshift mutations and copy number variation (CNV) were also examined for the parasites from western Kenya and Thailand.

Positively significant departures from neutral expectations were detected on the surf _4.1 region encoding C-terminus of the variable region 2 (Var2) by 3 population-based tests in the western Kenyan population as similar in the Thai population, which was not covered by the previous analysis for eastern Kenyan population. Significant excess of non-synonymous substitutions per nonsynonymous site over synonymous substitutions per synonymous site was also detected in the Var2 region. Negatively significant departures from neutral expectations was detected on the region encoding Var1 C-terminus consistent to the previous observation in the eastern Kenyan population. Parasites possessing a frameshift mutation resulting a product without intracellular Trp-rich (WR) domains were 22/23 in western Kenya and 22/36 in Thailand. More than one copy of surf _4.1 gene was detected in western Kenya (4/24), but no CNV was found in Thailand (0/36).

The authors infer that the high polymorphism of SURFIN_4.1 Var2 C-terminus in both Kenyan and Thai populations were shaped-up by diversifying selection and maintained by balancing selection. These phenomena were most likely driven by immunological pressure. Whereas the SURFIN_4.1 Var1 C-terminus is suggested to be under directional selection consistent to the previous report for the eastern Kenyan population. Most western Kenyan isolates possess a frameshift mutation that would limit the expression of SURFIN_4.1 on the merozoite, but only 60% of Thai isolates possess this frameshift, which would affect the level and type of the selection pressure against this protein as seen in the two extremities of Tajima’s D values for Var1 C-terminus between Kenyan and Thai populations. CNV observed in Kenyan isolates may be a consequence of this frameshift mutation to increase benefits on the merozoite surface.