Study Of The Genetic Variation In Plasmodium Falciparum Strains From Malaria Endemic Regions Of Kenya

Abstract

Genetic variation has been found to exist in P. falciparum strains both within a defined geographic region and between diverse geographic areas. In Kenya, however, a genotypic comparison of P. falciparum strains between the two malaria endemic regions represented by Coast and Nyanza Province has not been carried out before. In this study, various P. falciparum genes were analysed by Polymerase Chain Reaction (PCR) to determine whether genetic variation exists between P. falciparum positive samples from the two malaria endemic regions of Kenya. The genes that were analysed in the study include genes coding for the Merozoite Surface Proteins I and II (MSP I and MSP II), and the Ring Erythrocyte Surface Antigen. Genes thought to be involved in sexual development and differentiation in P. falciparum including Pfmap-L, Pfcrk-I and the Pfg 27/25 gene were also analysed in the study. The MSP I, and MSP II genes, exhibited extensive size polymorphism within each endemic region gene, thus both endemic regions contain a large number of distinct strains. Occurrence of multiple infections was also observed, though at a low frequency. Mixed infections accounted for 28% of samples from Mombasa with respect to the MSP I gene. In-patients from Mombasa, with severe malaria, seemed to be infected by single genotype parasite populations while out-patients with less severe malaria were found to be infected with more than one parasite genotype. Pfmap-I and Pfcrk-l genes were also found to exhibit size polymorphism, with the Pfcrk-I gene revealing at least three different genotypes existing in P. falciparum field isolates. The RESA gene appeared. to be conserved among all the samples in both the endemic regions. Three fragments within the P. falciparum Pfg 27/25 gene were analysed for polymorphism. These include a 400bp open reading frame, Orf P, a 700bp fragment within the coding region of the gene and a 250bp fragment within the Dispensable Intergenic Region of the gene. The 700bp and the 400bp fragments were found to be present and appeared to be highly conserved in all P. falciparum samples analysed in the study. These two fragments also served as positive controls in peR amplification of sequences from the Pfg 27/25 genomic locus. About 68.5% of the samples from Kisumu, representing Nyanza Province, and 37.5% of the samples from the coast amplified the full 250bp fragment within the Dispensable Intergenic Region. Thus, the 250bp fragment within the Dispensable Intergenic Region of the Pfg 27/25 gene provides a useful marker for distinguishing between P. falciparum samples from the two malaria endemic regions of Kenya. Sequence comparison between the 250bp fragment from K25, a Kisumu sample and that of ~3, a Honduras laboratory isolate showed a 47.3% homology This difference in nucleotide sequence can be attributed to geographic distribution of the parasite and the high mutation rate occurring in the field with time.