ALLELIC DIVERSITY AT THE MEROZOITE SURFACE PROTEIN-1 LOCUS OF PLASMODIUM-FALCIPARUM IN CLINICAL ISOLATES FROM THE SOUTHWESTERN BRAZILIAN AMAZON

Nucleotide sequences of each variable block in the Plasmodium falcipar um merozoite surface protein-1 gene (PfMSP-1) may be grouped into one of two or three possible allelic types, named after the reference isol ates MAD20, K1, and RO33. Allelic diversity at this locus basically re sults from different combinations or allelic types in variable blocks. We used a polymerase chain reaction (PCR)-based strategy to type the variable blocks 2, 4a, 4b, and 10 of the PfMSP-1 gene of P. falciparum isolates from 54 symptomatic malaria patients living in Rondonia, a h ypoendemic area in the southwestern Brazilian Amazon. Ten different Pf MSP-1 gene types, defined as unique combinations of allelic types in v ariable blocks, were identified among the 54 isolates. Twenty-one isol ates (39%) harbored more than one gene type and two had at least three genetically distinct clones. Hybrid sequences, with a MAD20-type sequ ence in the 5' segment (4a) and a K1-type sequence in the 3' segment ( 4b), were quite common in block 4. Direct sequencing of block 4 PCR pr oducts revealed a new putative recombination site in four isolates. In contrast with previous studies, the observed distribution of gene typ es does not deviate significantly from that expected under the null hy pothesis of random association between allelic types detected in each variable block. These contradictory data are discussed with reference to the immunoepidemiologic features prevailing in distinct malaria-end emic areas.