Analysis of kappa and omega repeats of the cg2 gene and chloroquine susceptibility in isolates of Plasmodium falciparum from sub-Saharan Africa.

The correlation between the structure of two short sequences from the Plasmodium falciparum cg2 gene and parasite chloroquine susceptibility was evaluated in unselected clinical isolates obtained from travellers returning mainly from Africa to France in 1995 and 1996. As determined by an isotopic semi-microtest, 74 isolates were susceptible to chloroquine (50% inhibitory concentration < 80 nM), 13 were intermediate (80 nM < 50% inhibitory concentration < 110 nM) and 53 were resistant (50% inhibitory concentration > 110 nM). Two polymerase chain reaction assays were developed, one for the kappa and one for the omega repeat domains of cg2 gene. The kappa and the omega repeat domains of 99 isolates were sequenced. A variation in the unit number of kappa and omega repeats was observed. Variations in repetitive sequences, which were not previously described, were found: three for the kappa repeat region: kappa9; kappa10 and kappa11 and three for the omega repeat region: omega8; omega9 and omega22. A polymorphism was observed inside the repeat units of kappa and omega regions. There were six possible kappa repeat units and seven possible omega repeat units. The presence of a particular pattern, containing kappa14 and omega16 repeat units, was associated with a lack of chloroquine susceptibility in 44 out of 46 cases. However, not all resistant isolates had this 'resistant' genotype. Among 43 resistant isolates, 36 (84%) had the kappa14 repeats sequence and 36 had the omega16 repeats sequence. These results lend further support to linkage between cg2 polymorphisms and chloroquine resistance without excluding the existence of other resistance component(s).