By David N. Leff
Politicians, among others, are fond of pontificating, "If it ain't broke, don't fix it!"
Gene therapist Bruce Sullenger practices the corollary preachment: "If it is broke, fix it!"
What he and his colleagues at Duke University, in Durham, N.C., have fixed are mutated red blood cells isolated from individuals with sickle-cell disease (SCD). They repaired a genetic defect in the beta-globin gene that expresses the distorted hemoglobin that causes SCD.
Their kit of tools consisted of ribozymes — catalytic RNA molecules found in nature. These act like enzymes in correcting nucleic-acid sequence errors, by splicing RNA or inhibiting gene expression.
The sickle-cell gene's mistake consists of a mutational switch between two bases, A and T (adenine and thymine) in the beta-globin gene's sixth codon.
Individuals who inherit this mutation from both parents go through life harboring long, large molecules of sickle hemoglobin deforming their erythrocytes. The sickle cells, rigid and wickedly curved rather than supple and smoothly elliptical, wreak havoc among the blood capillaries they try to enter.
Painful crises, chronic anemia and permanent tissue damage mark the lifelong, incurable course of SCD sufferers. (See BioWorld Today, Oct. 31, 1997, p. 1.)
What the Duke group did to rescue these warped genes is summed up in the title of their paper in the issue of Science, dated June 5, 1998, and titled, "Ribozyme-mediated repair of sickle ß globin messenger RNAs in erythrocyte precursors."
In short, Sullenger and his co-authors deployed ribozymes in vitro to splice the mutant beta-globin RNA transcripts. These then converted the sickle beta-globin gene to express the anti-sickling protein, gamma-globin.
The Duke team is now moving from in vitro to in vivo, with ongoing experiments to demonstrate the ability of their ribozymes to fix mutant beta-globin RNA in a suitable animal model.
At the University of Colorado, in Boulder, Nobelist Thomas Cech, co-discoverer of ribozymes, acclaimed the Duke feat as "impressive progress towards a new kind of therapy for sickle cell anemia. Instead of treating the symptoms," Cech observed, "Sullenger and co-workers have devised a way to attack the cause of the disease."
The university has granted a worldwide exclusive license on the SCD approach to Boulder-based Ribozyme Pharmaceuticals Inc. That company's president and CEO, Ralph Christoffersen, observed: "This adaptation allows for the possibility of repairing defective RNA molecules, and we look forward to many new therapeutic applications in this area." *