Genetic polymorphism of the extracellular region in surface associated interspersed 1.1 gene of Plasmodium falciparum field isolates from Thailand.

BACKGROUND: A novel variable surface antigens (VSAs), Surface-associated interspersed proteins (SUFRINs), is a protein that is modified on the surface of infected red blood cell (iRBC). Modified proteins on the iRBC surface cause severe malaria, which can lead to death throughout the life cycle of a malaria parasite. Previous study suggested that SURFIN1.1 is an immunogenic membrane-associated protein which was encoded by using the surf1.1 gene expressed during the trophozoite and schizont stages. This study aimed to identify the regions of SURFIN1.1 and investigate the genetic diversity of the extracellular region of the surf1.1 gene.

METHODS: A total of 32 blood samples from falciparum malaria cases that were diagnosed in Si Sa Ket Province, Thailand were collected. Plasmodium genomic DNA was extracted, and the extracellular region of surf1.1 gene was amplified using the polymerase chain reaction (PCR). A sequence analysis was then performed to obtain the number of haplotypes (H), the haplotype diversity (Hd), and the segregating sites (S), while the average number of nucleotide differences between two sequences (Pi); in addition, neutrality testing, Tajima's D test, Fu and Li's D* and F* statistics was also performed.

RESULTS: From a total of 32 patient-isolated samples, 31 DNA sequences were obtained and analysed for surf1.1 gene extracellular region polymorphism. Researchers observed six distinct haplotypes in the current research area. Haplotype frequencies were 61.3%, 16.2%, and 12.9% for H1, H2, and H3, respectively. The remaining haplotype (H4-H6) frequency was 3.2% for each haplotype. Hd was 0.598 ± 0.089 with the Pi of 0.00381, and S was 15. The most common amino acid polymorphic site was E251Q; other sites included N48D, I49V, E228D, E235S, L265F, K267T, E276Q, and S288F. Fu and Li's D* test value was - 1.24255, Fu and Li's F* test value was - 1.10175, indicating a tendency toward negative balancing selection acting on the surf1.1 N-terminal region. The most polymorphic region was variable 2 (Var2) while cysteine-rich domain (CRD) was conserved in both the amino acid and nucleotide extracellular region of surf1.1 gene.

CONCLUSIONS: The Thai surf1.1 N-terminal region was well-conserved with only a few polymorphic sites remaining. In this study, the data regarding current bearing on the polymorphism of extracellular region of surf1.1 gene were reported, which might impact the biological roles of P. falciparum. In addition, may possibly serve as a suitable candidate for future development of SURFIN-based vaccines regarding malaria control.