New perspective on cystic fibrosis.

New Perspective on Cystic Fibrosis 

Molecular biologists have overturned a fundamental assumption about the mechanism behind cystic fibrosis, pointing the way to new strategies for diagnosis and treatment.

Until now, scientists believed that most cases of the disease stemmed from defective activity of a mutant protein at the surfaces of cells in respiratory passages and certain organs. But the latest evidence indicates that the key to cystic fibrosis may be the protein's absence in the cell membrane.

The discovery offers "a new perspective on the disease" and may "explain a lot of what we see [in cystic fibrosis]," comments Douglas M. Jefferson of Tufts University School of Medicine in Boston.

At the heart of cystic fibrosis lies a protein called CFTR, which normally helps pump chloride ions across cell membranes. Without CFTR, an imbalance in ion and fluid transport ensues, leading to the thick mucus buildup and frequent respiratory infections that characterize the disease. Earlier this fall, scientists demonstrated that inserting the normal CFTR gene into cultured airway cells from cystic fibrosis patients corrected the chloride transport defect in vitro (SN: 9/22/90, p.181). Scientists at Tufts University, Genzyme Corp. in Framingham, Mass., and the University of Iowa in Iowa City carried out the research.

Now, some of the Genzyme scientists involved in that work have found that an error in the processing of CFTR appears to be a primary cause of the disease. The slip-up prevents the protein from maturing and reaching its cell-surface destination, the researchers report in the Nov. 16 CELL.

Normally, newly synthesized CFTR undergoes final adjustments in two cellular compartments -- the endoplasmic reticulum and the Golgi apparatus -- where carbohydrates get tacked onto the protein. The fully processed CFTR then moves from the Golgi to the cell surface.

But in cystic fibrosis, "the [mutated] protein never makes it to the cell surface," says study leader Alan E. Smith. He and his colleagues chemically synthesized a DNA sequence coding for CFTR but missing a specific amino acid, creating a mutation known as delta F508. After inserting the altered gene into monkey kidney cells in vitro, the scientists observed that the CFTR made by these cells was only partially processed.

"The protein doesn't mature," Smith says. "It gets stuck, we think, in the endoplasmic reticulum." Other cystic-fibrosis-causing mutations in the same region yielded similarly incomplete CFTR, the researchers found.

Since the delta F508 defect shows up in 70 percent of cystic fibrosis patients, the investigators conclude that this and other mutations interfering with CFTR maturation probably underlie most cases of the disorder. They suggest that the mutant CFTR may have an abnormal shape, which the endoplasmic reticulum detects through a "quality control system" that prevents fine-tuning and transport of defective proteins. This system could rely on a "molecular chaperone" that must bind the CFTR in order for processing to occur, Smith speculates. If misshapen CFTR goes unrecognized by the chaperone, it may never exit the endoplasmic reticulum and may instead undergo degradation.

Jefferson says the new findings may explain why parents of affected children remain healthy even though each carries one defective copy of the CFTR gene in addition to one "good" copy. If the mutant CFTR is absent from the cell surface, it can't interfere with the functional activity of CFTR encoded by the normal gene, and thus cannot induce the disease, he says.

The Genzyme team speculates that measuring children's levels of mature CFTR might offer a simpler diagnostic test than the current searches for chromosomal mutations. And if studies show that mutant CFTR becomes at least partially functional when mature, drugs that complete its processing and transport may provide a new treatment approach, they say. Smith told SCIENCE NEWS his group is attempting to develop a protein-replacement aerosol that, when inhaled, would deliver normal CFTR to cell surfaces.