Cystic fibrosis is a genetic disorder that causes thick mucus to build up in the respiratory system and in the pancreas. People with cystic fibrosis are highly susceptible to respiratory infections and are typically malnourished due to the malfunctioning of the pancreas. In the United States, the disease affects about one in every 3,900 babies born annually; currently 30,000 Americans are afflicted with this disease. The genetic defect that causes cystic fibrosis is most common in people of northern European descent. It is estimated that one in 31 Americans carries the gene for cystic fibrosis. No cure for cystic fibrosis exists, and the disease is invariably fatal. Only 50% of people with the disease survive into their 30s. Ninety-five percent of cystic fibrosis deaths are caused by lung complications; the other 5% are due to liver failure.
Cystic fibrosis is a homozygous recessive genetic disorder. In this type of disorder two defective alleles, or copies, of the gene, one from each parent, must combine to produce the defective gene. If two people who each carry the defective allele have a child, chances are one in four that the child will have cystic fibrosis.
Recently researchers have located a number of defects on a particular genes that appear to be responsible for the majority of cystic fibrosis cases. Knowing the location of these gene mutations makes it possible to test for carriers of the disease (individuals who have only one defective gene copy, and therefore have no symptoms themselves). This genetic discovery may lead to a genetic treatment in which a normal gene is inserted into the cells of cystic fibrosis patients. This kind of treatment, called gene therapy, is currently being tested in several research centers around the world.
In 1989, a team of researchers located the defective cystic fibrosis gene on the long arm of chromosome 7. Genes are segments of deoxyribonucleic acid (DNA) that code for certain proteins. If the sequence of DNA mutates in a gene the protein for which it encodes also changes. In cystic fibrosis an infinitesimal change in the DNA sequence leads to the production of a defective version of an important protein. This protein, called the CF transmembrane conductance regulator (CFTCR), apparently performs a crucial function in airway and pancreas cells. The protein works as a pump within the cell membrane regulating the movement of sodium and chloride (the components of salt) in and out of cells. In people with cystic fibrosis this pump does not work. As a result water is retained within the cells depriving the tissues of much-needed moisture. A dry, sticky mucus builds up in the airway and the pancreas obstructing breathing and interfering with digestive processes. The mucus also clogs sweat glands and salivary glands.
An estimated 10-15% of people with cystic fibrosis have a different genetic disorder other than the mutation found by the researchers in 1989. Some of these mutation cause a less severe disease; others cause a more severe disease. Scientists currently are searching for all the mutation linked to cystic fibrosis.
Researchers hope that the discovery of the gene responsible for cystic fibrosis will lead to a genetic approach to curing the disease. Gene therapy involves cells that are removed from the body and infected with a virus that has been induced to carry the normal gene. The newly infected cells are then put back into the body, where the normal gene then encodes for the correct protein, In cystic fibrosis, this method has failed. Researchers are working on an approach in which the cystic fibrosis inhales the gene-carrying virus directly.
Currently the screening test for the cystic fibrosis gene is 85% effective in detecting the gene in a person's blood, cheek scrapings, or saliva. Some researchers feel that this effectiveness rate is still too low and that testing be performed only on persons who have a familial history of cystic fibrosis. Others argue that because the test is relatively inexpensive, easy to perform, and the effectiveness rate acceptable, the test should be widely offered.
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