A Canadian-American research team said Thursday that it has isolated and cloned the defective gene that causes cystic fibrosis, the most common lethal genetic defect in the United States.
The researchers have also identified the defective protein produced by the gene and believe they know how it functions.
The isolation of the gene and the protein was widely hailed by other scientists as a major step toward conducting prenatal screening for the disease and toward a potential cure. They hope to use their knowledge of the biochemical defect to design new drugs and ultimately to develop genetic therapy to insert a healthy gene into victims.
"This is the most significant breakthrough in the Cystic Fibrosis Foundation's 35-year history," President Robert K. Dresing said. "It's the piece of the puzzle we had to get in place before we can focus our attention on why we are really in this business--developing new therapies and a cure for the disease."
One in every 20 whites and a far smaller number of minorities--about 12 million Americans in all--carry the defective gene and one in every 1,800 children suffers from the disease. Cystic fibrosis is marked by a buildup of mucus in the lungs that impairs breathing and leaves the victims susceptible to respiratory infections. The disease also affects the pancreas in about 75% of victims, blocking secretion of enzymes necessary for digesting and absorbing fats in the diet.
Earlier in this century, cystic fibrosis was uniformly fatal, killing most affected children during their first year of life. The development of new antibiotics during World War II extended their life spans to the mid-teens by fighting the respiratory infections that are the normal cause of death.
More recently, better antibiotics and newly devised forms of nutritional therapy have expanded the victims' expected life span to a median of about 26 years, and many males live into their 30s and 40s, Dr. Henry Levison of the Hospital for Sick Children in Toronto said. A crucial advance in cystic fibrosis research came in October, 1985, when a team headed by molecular geneticist Lap-Chee Tsui of the Hospital for Sick Children narrowed the gene's location to a small region of chromosome 7, one of the 23 pairs of chromosomes in the human cell.
Tsui ultimately discovered the defective gene, working with Jack Riordan of the Hospital for Sick Children and geneticist Francis Collins of the University of Michigan.
The cystic fibrosis gene has been difficult to isolate in large part because the change in the defective gene is subtle. Many of the disease genes that have recently been identified using new molecular biology techniques--such as those for muscular dystrophy and retinitis pigmentosa--are missing rather large segments of DNA (deoxyribonucleic acid, the cell's genetic blueprint), allowing researchers to readily distinguish them from their healthy counterpart.
But the cystic fibrosis gene, Tsui and Collins have found, is missing just three of the 250,000 individual chemicals, called bases, of which it is composed. The researchers thus had to adopt the "brute force" method of identifying every single base on the chromosome in the area of the suspected gene, an approach made doubly difficult by the gene's large size, biochemist Ron Davis of Stanford University said. "It's a magnificent achievement," he said.
Collins and Tsui have also identified the protein for which the defective gene codes. Like the gene, it is huge, containing 1,480 individual amino acids. Compared to the healthy protein, it is missing just one amino acid. It is probably located on the membrane of cells in the lungs, pancreas and elsewhere in the body.
Many researchers have previously shown that the cause of cystic fibrosis is a defect in the transport of charged atoms called ions, particularly chloride ions, into and out of cells. This defect reduces the amount of water found inside the cells, and thus leads to the production of thicker-than-normal mucus, which is involved in cleaning the interior of the lung.
Tsui said he and Collins believe that the newly identified protein is the actual agent that pumps chloride ions through the cellular membrane. Because of its defect, they suggest, the protein is not able to get enough energy from other molecules in the cell to perform the pumping successfully.
The researchers will publish their results in the Sept. 8 issue of Science.
Researchers have a "wealth of experiments" to carry out now, Collins said. The defective gene they identified is found in only about 70% of cystic fibrosis victims, so they must find other defects, presumably in the same gene, that account for the rest. But that should be "relatively simple" now that they know which gene is involved, he added.
Discovery of all the genetic defects will make possible prenatal screening for cystic fibrosis. Currently, such screening can be performed only in families that already have one child with cystic fibrosis, and it is only 80% to 90% effective. When the other defects are identified, the test should be "100% effective for everybody," Tsui said.
Dresing said the Cystic Fibrosis Foundation will call a meeting within the next month to bring together scientists who are already working on cystic fibrosis and many who are not. "We are going to be extremely aggressive in identifying and getting into the loop those people who can help us with the development of new therapies," he said.