Geographical patterns of allelic diversity in the Plasmodium falciparum malaria-vaccine candidate, merozoite surface protein-2

The polymorphic merozoite surface protein-2 (MSP-2) of Plasmodium falciparu m is a major malaria-vaccine candidate. In the present study, PCR and hybri dization with allelic-specific probes were used to type the Msp-2 gene from isolates from hypo-endemic Brazil (N = 113), meso-endemic Vietnam (N = 208 ) and holo-endemic Tanzania (N = 67). The typing methods were designed to g roup isolates in to the dimorphic allelic families FC27 and IC1 and to dete ct possible between-family recombination events. The analysis was complemen ted by a comparison of 156 Msp-2 sequences from the GenBank database with 1 2 additional sequences obtained during the present study. Statistically sig nificant differences were detected in pair-wise comparisons of the distribu tion of Msp-2 allelic types in Brazil and Vietnam, and in Brazil and Tanzan ia, but not in Vietnam and Tanzania. The extent of allelic diversity in the Msp-2 gene, as estimated by the total number of different alleles found in a given parasite population and the mean multiplicity of infections, clear ly paralleled the levels of malaria endemicity in the study areas. However, no correlation between age and multiplicity of infections was found in the subjects. The patterns of Msp-2 diversity in Brazil appeared to be tempora lly stable, since no significant difference was observed in the distributio n of Msp-2 allelic types among isolates collected, 10-13 years apart, in th e same area of Rondonia. Despite the extensive sequence diversity found in Msp-2 alleles, especially in the central repetitive region of the molecule, several in stances of identical or nearly identical alleles were found amo ng isolates from different countries and regions, possibly as a result of e xtensive homoplasy. No recombinant allele was detected by molecular typing in any of the study sites, and the GenBank database included only 12 recomb inant sequences (representing 7% of all reported Msp-2 sequences), all of t hem with an IC1-type 5' end and an FC27-type 3' end. A single, putative, cr ossover site was characterised for all recombinant alleles. Most of the all elic diversity observed was therefore attributable to variation in the repe titive region of the gene, instead of recombination between alleles of dimo rphic families (as commonly found, for example, in the Msp-1 gene). The imp lications of these findings for studies on the genetic and antigenic divers ity of malarial parasites are discussed.