Researchers have come up with a new cut-and-paste technique that corrects tiny errors in a gene that makes a crucial liver enzyme in rats. The findings, presented this week at the annual meeting of the American Association for the Study of Liver Diseases in Chicago, may usher in a new breed of gene therapy that could potentially cure diseases, such as hemophilia and sickle cell anemia, that are caused by mutations in a single nucleotide base.
Traditional forms of gene therapy have relied on retrofitted viruses to shuttle entire sequences of corrective genes into patients' cells--a difficult task. To trick the cells into fixing their own genes, the researchers--hepatologist and cell biologist Clifford Steer and biochemist Betsy Kren of the University of Minnesota, Minneapolis, and their colleagues--turned to artificial dumbbell-shaped pieces of DNA and RNA called chimeric oligonucleotides. These pieces faithfully match the genetic sequence of a mutated gene except for the corrected version of the wrong nucleotide. The investigators bypassed a viral shuttle by bundling the oligonucleotides inside coats of oil-like molecules or tacking them onto polymers designed to bind to specific liver cells.
The team tested the modified oligonucleotides in rats that carry a single-nucleotide defect in an essential liver gene known as UDP-glucuronosyltransferase-1. Humans with the faulty gene are afflicted with Crigler-Najjar disease, a fatal illness in which patients cannot break down or excrete bilirubin and end up severely jaundiced. After five daily injections of the oligonucleotides into the tail veins of 20 rats, all had converted up to 25% of their defective liver genes to normal versions. (Most gene therapy experiments consider a 1% to 2% conversion rate a success.) "It's as though the cells never knew they had the mutant sequence," says Steer, the lead investigator. Bilirubin levels dropped by greater than half and have stayed there for 6 months so far.
"This is exactly what I have been waiting for," says Michael Blaese, who is so enamored with the technique that he is leaving his post as chief of the clinical gene therapy branch at the National Human Genome Research Institute in Bethesda, Maryland, to work for the company that manufactures the novel RNA/DNA oligonucleotides. "The potential, if real, is incredible," says Mario Capecchi, a longtime gene therapy researcher at the University of Utah, Salt Lake City. "But I'd be much more excited if others are able to reproduce [the finding]."