Molecular epidemiology of malaria in Yaounde, Cameroon V. Analysis of the omega repetitive region of the Plasmodium falciparum CG2 gene and chloroquine resistance

A novel Plasmodium falciparum gene, denoted cg2 gene, has been recently dis covered, and a distinct genotype, characterized by 12 point mutations and 3 size polymorphisms, has been shown to be associated with chloroquine resis tance in laboratory-adapted parasite strains. One of the polymorphic region s, denoted the omega region, consists of 16 tandem repeat units in chloroqu ine-resistant strains, while the chloroquine-sensitive strains have either less than or equal to 15 or greater than or equal to 17 repeat units. In th is study, the in vivo and in vitro responses were compared with the number of repeat units in the omega region of the cg2 gene for 75 Cameroonian isol ates determined either by DNA sequencing or agarose gel electrophoresis. Th e 16-repeat units that characterize the resistant strains were found in 10 chloroquine-sensitive isolates (50% inhibitory concentration [IC50] < 100 n M) and 30 chloroquine-resistant isolates (IC50 greater than or equal to 100 nM). Thirty-five isolates (28 chloroquine-sensitive isolates and 7 chloroq uine-resistant isolates) displayed less than or equal to 15 or greater than or equal to 17 repeat units. Of the 18 patients responding with treatment failure, 15 were infected with parasites carrying 16 repeat units. Twenty-e ight patients (11 with isolates carrying 16 repeat units and 17 with isolat es carrying less than or equal to 15 or greater than or equal to 17 repeat units) showed an adequate clinical response. The sensitivity, specificity, and predictive value were 81% (83%), 74% (61%), and 75% (58%), respectively compared with in vitro (or in vivo) responses. Neither the level of IC50 n or the key P. falciparum multidrug resistance gene 1 (pfmdr I) allele at po sition 86 was associated with the number of omega repeat units. Although in vitro and in vivo resistance to chloroquine was statistically associated w ith the presence of 16 repeat units in the omega region (P < 0.05), the num ber of omega repeat units did not adequately discriminate patients infected with chloroquine-resistant parasites from those infected with chloroquine- sensitive parasites. Further studies on the cg2 gene are needed to determin e whether cg2 gene is a reliable genetic marker for chloroquine resistance.