November 29, 1997
by J. Travis
Ending a decade-long quest, scientists have now identified a gene that enables the malaria-causing parasite Plasmodium falciparum to mount resistance to chloroquine, a major antimalarial drug that has become increasingly ineffective around the world.
Armed with the new information, investigators will try to develop versions of chloroquine that sidestep the parasite's resistance but still safely kill the blood-thirsty organism, says Thomas E. Wellems of the National Institute of Allergy and Infectious Diseases in Bethesda, Md.
A number of complex variations in the newly discovered gene cg2 appear to bestow chloroquine resistance upon P. falciparum, Wellems and his colleagues report in the Nov. 28 Cell.
Resistance to the drug emerged 40 years ago, notes Wellems, and appeared almost simultaneously in Southeast Asia and South America. Today, the problem has spread worldwide, fueling a resurgence of malaria that kills millions of people annually.
Several years ago, Wellems and his colleagues took a crucial step toward unraveling chloroquine resistance when they bred a drug-resistant strain of P. falciparum from Indochina with a chloroquine-sensitive strain from Central America.
By examining the resulting progeny, some of which were resistant and others of which were not, the researchers narrowed the search for a resistance-conferring gene to a small portion of one of the parasite's chromosomes.
The region harbors many genes, but Wellems' team eventually found that cg2's DNA sequence consistently shows more than a dozen differences between chloroquine-resistant and chloroquine-sensitive P. falciparum strains .
The elaborate changes observed in cg2 contrast with the simple gene mutations through which many microorganisms have thwarted other drugs and may therefore explain why chloroquine worked so well for so long. "It took over a decade for resistance to arise," says Wellems.
Investigators believe that chloroquine functions by accumulating within the malaria parasite and that the drug prevents the parasite from sequestering toxic components created as it digests the hemoglobin it steals from blood cells.
Some investigators have suggested that chloroquine-resistant strains have an increased ability to pump the drug from their bodies; others contend that resistance stems from changes that prevent chloroquine from entering the parasites in the first place. Also, there's evidence that resistant parasites specifically reduce the concentration of chloroquine in the internal compartments where they digest hemoglobin.
Using antibodies that bind to the protein encoded by cg2, Wellems and his coworkers observed the protein in the complex of membranes that separates the parasite from its host blood cell and in the vicinity of the organism's hemoglobin-digesting compartments.
Both locations bolster the hypothesis that the protein plays a role in transporting chloroquine. "It's exactly where it should be, and it fits all the theories," says Wellems.
His group plans to add the resistant version of cg2 to chloroquine-sensitive P. falciparum to confirm the gene's role in protecting the parasites from the drug.
The investigators have found that in South American strains of chloroquine-resistant P. falciparum, cg2 has a significantly different DNA sequence than it does in either drug-sensitive or drug-resistant Asian strains.
That result "lends support to the hypothesis that there was a separate origin of resistance in the New World," says Daniel E. Goldberg of Washington University in St. Louis.
While the identification of cg2 may help scientists search for new chloroquinelike drugs, Goldberg suggests it will also enable investigators to develop compounds that specifically block the resistance mechanism.
When used with chloroquine, such agents could restore the drug's effectiveness, he says. First, however, Wellems and his colleagues must determine the function of cg2's protein.
spread of chloroquine-resistant
Plasmodium falciparum strains
Su, X. . . . T.E. Wellems. 1997. Complex polymorphisms in an ~330 kDa protein are linked to chloroquine-resistant P. falciparum in Southeast Asia and Africa. Cell 91(Nov. 28).
Shell, E.R. 1997. Resurgence of a deadly disease. Atlantic Monthly August. Available at http://www.theatlantic.com/atlantic/issues/97aug/malaria.htm.
Thomas E. Wellems
National Institute of Allergy and Infectious Diseases
National Institutes of Health
Bethesda, MD 20892-0425
copyright 1997 ScienceService