Molecular epidemiology of malaria in Cameroon. XVIII. Polymorphisms of the Plasmodium falciparum merozoite surface antigen-2 gene in isolates from symptomatic patients.

Merozoite surface antigen-2 (MSA-2) is a polymorphic genetic marker that is highly discriminatory for characterizing Plasmodium falciparum field isolates. Genetic diversity of isolates obtained from symptomatic patients residing in Yaounde, Cameroon was analyzed by an allele-specific polymerase chain reaction and direct sequencing of amplification products. Of 137 isolates, 25 (18%) had only FC27-type alleles, 40 (29%) had only 3D7-type alleles, and 72 (53%) had multiple parasite populations with both alleles. Of 295 fragments, 145 (49.2%) and 150 (50.8%) belonged to FC27 and 3D7 alleles, respectively. There were 23 different MSA-2 alleles (10 FC27-type and 13 3D7-type that yielded 44 different combinations in multiple infections). DNA sequencing showed distinct individual sequences. Sequences belonging to the FC27 allelic family were relatively conserved, with most of the polymorphism arising from differences in the number of repeat units. In contrast, the sequences within the GSA-rich region in 3D7 allelic family were less conserved, but many of the sequences in Cameroonian isolates have been identified in other isolates from geographically distant origins. Our results show an extensive diversity of the central region of MSA-2 in size, allelic family, combinations of these two features in multiple infections, and sequence variations underlying the complex population structure of P. falciparum clinical isolates in Yaounde, Cameroon.