A clinical researcher's best friend is a faithful animal model of thehuman disease he or she is studying.
With the advent and refinement of sophisticated gene-targeting andreplacement technology, scientists can now create "knockout" micein which the genes held responsible for a given disease in humansare removed or inactivated. This transgenic capability has greatlyincreased the availability and potential usefulness of murine models.But their utility is often limited by the fact that the model animals diefor the same reasons human patients do.
Knockout mice made to mimic cystic fibrosis, by deleting theircystic fibrosis transmembrane conductor regulator (CFTR) gene, area case in point. In parallel to human cystic fibrosis sufferers, thesedemonstrator rodents lack chloride ion transport, which is normallystimulated in epithelial cells of lung, sweat glands, pancreas andintestines. The most serious effect of this absent transport functionoccurs in the gut, where intestinal obstruction and perforation lead todeath during the first month of life. Ergo, these knockout mice offera less-than-useful model system in which to study the symptoms ofcystic fibrosis.
In an effort to make a more viable cystic fibrosis model, molecularbiologist Jeffrey Whitsett, director of pulmonary biology atChildren's Hospital Medical Center in Cincinnati, has partiallycorrected the CFTR gene ablation, so that the animals survive withmanifestations similar to those in human cystic fibrosis patients.
Whitsett is principal author of a paper in this week's Science, datedDec. 9, titled, "Correction of lethal intestinal defect in a mousemodel of cystic fibrosis by human CFTR." It describes mice inwhich he and his co-workers have eliminated the native CFTR gene,then bred them to express the inserted human CFTR sequenceselectively in intestinal epithelial cells only. To accomplish thistargeted germline gene therapy, they introduced a vector containingthe cDNA for CFTR, linked to a rat fatty-acid binding protein(FABP) as gene promoter.
Eliminating Deadliest Symptom
This replacement resulted in progeny without those death-dealing guttraumas. These new-model murine cystic fibrosis surrogates havenow lived for over a year.
Thirty years ago, human infants born with cystic fibrosis also diedyoung of intestinal blockage and perforations brought on by thedisease. These defects are now routinely corrected surgically at birth,and nutritional problems are treated with dietary supplements. Suchmeasures of modern medical practice have significantly prolonged acystic fibrosis child's life span.
Because knockout mice modeling cystic fibrosis cannot have theirintestinal defects surgically corrected at birth, development of thesetransgenic animals, spared the associated gut disorders, wasnecessary as a model system useful for current clinical research.
Nowadays, respiratory problems _ obstruction of lungs and airways_ present the clinically important symptoms seen in cystic fibrosispatients. As Whitsett's FABP-CFTR gene construct is designed to beexpressed only in the gut, his animals faithfully reproduce the criticalrespiratory symptoms associated with the human disease.
"We wanted to correct the intestinal blockages in the mice," Whitsetttold BioWorld Today, "because they would then live, and we wouldstill have their lung troubles as a model for the respiratory symptomsseen today in humans with cystic fibrosis."
In addition to the clinical importance of these rodents, Whitsettobserved, his work sheds light on several basic questions in the areaof genetic replacement therapy. He pointed out, "This is one of thefirst demonstrations that expression of a cDNA is sufficient tocorrect a genetic defect. At the present stage in the development ofgene therapy, this is a critical piece of knowledge, because it is notusually possible to insert the entire, much larger, genomic-DNAclone."
Making Mice Closer To Human Condition
Whitsett also stated that even though "the inserted gene is notexpressed in all the right places in our mice, and we see only 20 to30 percent of the normal chloride transport, intestinal functionreturns. While many groups are using the CFTR gene to developtreatments for correcting cystic fibrosis," he added, "this is the firstwork to demonstrate a `proof of concept' for cystic fibrosis genetherapy."
He and his associates at Children's Hospital in Cincinnati, jointlywith Genetic Therapy Inc., of Gaithersburg, Maryland, initiated aPhase I/II clinical trial in September (see BioWorld Financial Watch,Oct. 17, 1994). The study is enrolling 15 cystic fibrosis patients, towhom it will deliver adenovirus-based aerosolized vectors to thelungs. n
-- Chester Bisbee Special To BioWorld Today
(c) 1997 American Health Consultants. All rights reserved.