Genetic: Cystic Fibrosis
Genetic: Cystic Fibrosis
Cystic fibrosis, or CF, is a fatal inherited disease caused by a mutation in a gene that leads to the buildup of thick, sticky mucus in the lungs and pancreas. The mucus makes breathing difficult and prevents proper nutrition by blocking the flow of digestive enzymes from the pancreas. Most patients eventually die in late adolescence or their early adult years from damage to the lungs caused by the buildup of scar tissue (fibrosis).
CF is a disease that affects many organ systems in the body. The defective gene involved in cystic fibrosis results in lower levels of chloride, a
chemical compound that is important in the production of sweat, digestive juices, and mucus of normal consistency. Chloride regulates the flow of water in and out of cells, which is needed to produce mucus that is thin enough to act as a lubricant and sweat that contains normal amounts of salt. When there is not enough chloride, the mucus that the tissues produce is thicker and stickier; it interferes with the proper functioning of the lungs, pancreas, sweat glands, and digestive tract. The thick mucus in the lungs provides a breeding ground for bacteria and fungi, which cause inflammation and gradual destruction of lung tissue.
In the digestive system, the mucus blocks the ducts that convey digestive enzymes from the pancreas, a large gland located just below the stomach. Without these enzymes, the patient's body cannot absorb nutrients from food as it passes through the digestive tract, a condition known as malabsorption. The thick mucus may also cause irreversible damage to the pancreas itself and cause inflammation of the liver as well. In men, it may block the tubes that carry sperm from the testicles, thus leading to infertility.
Cystic fibrosis is primarily a disease of people of northern European ancestry; in fact it is the most common fatal inherited disease among Caucasians in the United States, affecting one child in every 3,200. CF
affects one in every 9,200 Hispanic children, one in every 15–17,000 African American children, and one in every 31,000 Asian American children. As of 2008, there were about 30,000 people with CF in the United States and about 70,000 in the world.
Males and females are equally likely to be born with CF; however, females are more severely affected and tend to die at younger ages.
Cystic fibrosis is caused by a mutation in the CFTR gene located on chromosome 7. The gene itself was first identified in 1989. A child must inherit the defective gene from both parents in order to develop CF. If either parent has a normal CFTR gene, the child may be a carrier for the disease but will not have any symptoms of cystic fibrosis. There are over 900 different known mutations of the CFTR gene. There are an estimated 10 million carriers of the defective gene in the United States, or almost one in every thirty-one people. If two carriers marry, they have a 25 percent chance of having a child with cystic fibrosis, and a 50 percent chance of having a child who will be a carrier of the defective CFTR gene.
Some mutations of the CFTR gene cause more severe symptoms than others or may affect different body systems differently. In addition, the symptoms of CF vary somewhat with age. In newborns, the intestines may be blocked by meconium (a blackish-green stool that most babies pass in the first few days after birth). These children may also have such symptoms as failure to grow; bulky and greasy stools; and frequent respiratory infections.
Tay-Sachs Disease (TSD)
TSD is another fatal inherited disease. It is caused by a genetic mutation in the HEXA gene on chromosome 15. It occurs in one in every 320,000 newborns in the United States. A normal HEXA gene helps to produce an enzyme that prevents a fatty substance called GM2 ganglioside from building up in the nerve cells of the brain. Without the enzyme, the child’s brain cells begin to deteriorate and die. Children with TSD seem to develop normally until they are about six months old. They then begin to lose their mental functions. By age two, most children with TSD develop seizures, become blind, paralyzed, and unable to respond to their parents. They usually die around age five. As with cystic fibrosis, there is no cure for TSD.
TSD is inherited in the same pattern as cystic fibrosis; that is, both parents must have an abnormal HEXA gene for their child to be born with Tay-Sachs. About one person in every 250 in the general American population is a carrier of TSD. However, the defective gene is much more common among Jews of Eastern European descent (one in every twenty-seven) and French Canadians (Cajuns) living in Louisiana (one in every twenty-five).
In older children and young adults, symptoms of CF may include:
- An extremely salty taste to the skin.
- Constipation or blockage of the bowels.
- Frequent episodes of heartburn or indigestion.
- Smelly, greasy, or clay-colored stools.
- Coughing or wheezing.
- Frequent chest infections with bronchitis or pneumonia.
- Clubbing (swelling of the tips) of the fingers and toes.
- Failure to gain weight at a normal rate. Girls may have delayed puberty.
Some complications of CF that occur in some patients include liver damage; polyps (tissue growths) in the nose caused by recurrent sinus infections; frequent headaches; pain in the abdomen; and diabetes resulting from damage to the pancreas.
It is possible to diagnose CF before birth around the eleventh week of pregnancy by testing a small piece of tissue from the placenta, the temporary organ that allows for exchange of nutrients and waste products between the mother and baby. After the sixteenth week of pregnancy, the fetus can be tested by amniocentesis, a procedure that involves withdrawing a small amount of fluid from the sac that surrounds the baby before birth.
Many hospitals have newborn screening programs to evaluate babies for CF. Although the screening methods are not diagnostic tests in the strict sense, they can be used to rule out CF in most babies. In addition, by identifying babies who should be tested further for CF, screening allows treatment for the disease to be started as early as possible.
The sweat test has been considered the best diagnostic test for cystic fibrosis for over forty years. It measures the amount of chloride in the person's sweat. To perform the test, the doctor applies a chemical that causes sweating on a small area of the person's arm or leg. An electrode is attached to the treated area and a mild electrical current is used to stimulate sweating. The patient may feel a tingling or warm sensation. After about five minutes, the sweat from the treated area is collected on a piece of filter paper and sent to a laboratory for analysis. One benefit of the sweat test is that the values are not changed as a person grows older or affected by temporary illness.
Another more recent test that can be used to diagnose CF is genetic testing of a sample of the patient's blood for mutations in the CFTR gene.
There is no cure for CF. Treatment is aimed at thinning the mucus secretions, keeping the patient's airways clear, preventing infections, and maintaining adequate nutrition.
Medications used to treat CF include:
- Mucolytics. These are drugs that thin the mucus in the lungs so that it is easier for the patient to cough it up. Pulmozyme is the most commonly used drug of this type.
- Antibiotics. These are given to treat bacterial infections in the lungs and throat. Some antibiotics can be given in aerosolized form and delivered directly into the airway.
- Bronchodilators. Bronchodilators are drugs that help to keep the bronchi (tubes leading into the lungs) open to ease the patient's breathing. They can be taken through inhalers or nebulizers.
- Oral enzymes and vitamin supplements. Patients with CF need to take pancreatic enzymes in the form of pills or capsules with their meals in order to absorb nutrients from their food. Extra vitamins are also needed to prevent malnutrition.
- Pain relievers. Some children with CF are helped by taking ibuprofen (Advil or Motrin) for headaches. This particular non-aspirin pain reliever also may help slow down loss of lung function.
Adults as well as children with CF usually need to follow a special high-calorie high-fat diet to maintain good health. A professional dietitian can work with the patient to include foods that the patient enjoys eating as well as meeting the calorie and fat requirements.
Patients with CF also need to clear the mucus from their lungs at least twice a day. This clearing can be done by having the patient lie with his or her head over the edge of a bed and thumping or clapping on the back and chest for twenty to thirty minutes to shake the mucus loose so that the patient can cough it up. As an alternative to having a physical therapist or trained family member thumping the patient's chest, the patient can wear a device called a ThAIRapy Vest, which contains a compressor that vibrates the patient's chest wall at set times to loosen the mucus. The vest costs about $16,000.
CF patients with advanced lung disease may benefit from lung transplantation.
Cystic fibrosis is a fatal disease. The average life expectancy of patients is36.8 years, with males on average living longer than females. This is a dramatic increase over the 1950s, when few patients with CF lived long enough to start elementary school. About 80 percent of patients live to be adults; a few patients live into their forties. Most patients die of end-stage lung disease.
Most males with CF (97 percent) cannot father children. Women with CF are more likely to be fertile; however, they must watch their diet very carefully during pregnancy and may find that pregnancy worsens the symptoms of CF.
There is no way to prevent cystic fibrosis other than genetic testing of potential parents. Genetic screening of family members of a cystic fibrosis patient may detect the CF gene in between 60 and 90 percent of carriers, depending on which test is used. There are still a few rare mutations of the CFTR gene that cannot be identified by present tests.
Cystic fibrosis was the first disease that researchers attempted to treat by using gene therapy, a technique for replacing defective genes in the body's tissues with normal genes. In 1993, scientists used a common cold virus in an attempt to deliver a healthy CFTR gene to the cells in the patient's airways. Although this first attempt was not successful, other methods of gene delivery are now being tested, including nose drops, fat capsules, and solutions delivered by nebulizers.
Another avenue of research involves mapping the genome of the bacterium that causes most of the lung infections in CF patients. By cracking the bacterium's genetic code, researchers are hoping to develop better drugs for treating the infections that it causes.
WORDS TO KNOW
Bronchus (plural, bronchi): One of the two major divisions of the airway that lead into the right and left lungs.
Clubbing: Swelling of the tips of the fingers. Clubbing is one of the symptoms of CF.
Fibrosis: The medical term for the formation of scar tissue.
Gene therapy: The insertion of normal genes into a person's cells or tissues in order to treat a disease by replacing a harmful mutation or nonfunctioning gene.
Malabsorption: Inability to absorb the nutrients in food through the digestive tract.
Meconium: A dark greenish type of stool passed by a newborn during the first few days of life. Inability to pass the meconium is an early symptom of CF.
Nebulizer: A device that delivers medication in a fine spray or mist.
SEE ALSO Bronchitis; Pneumonia
Bjorklund, Ruth. Cystic Fibrosis. New York: Marshall Cavendish Benchmark,2009.
Brody, Jane. “Personal Health: Scientists Plot Tactics to Outmaneuver Cystic Fibrosis Gene.” New York Times, January 14, 2003. Available online at http://query.nytimes.com/gst/fullpage.html?res=9C00E5DA1531F937A25752C0A 9659C8B63&sec=&spon=&pagewanted=all (accessed June 28, 2008).
Lehmann, Annie Lubliner. “Cases: She Always Had Hope; She Needed a New Lung.” New York Times, January 16, 2007. Available online at http://query.nytimes.com/gst/fullpage.html?res=9C02E1DB1030F935A25752C0A9619C8B63 (accessed June 28, 2008). This is an article about the death of a woman diagnosed with CF as an adult.
Cystic Fibrosis Foundation (CFF). About Cystic Fibrosis. Available online at http://www.cff.org/AboutCF/ (accessed June 28, 2008).
Genetics Home Reference. Cystic Fibrosis. Available online at http://ghr.nlm.nih.gov/condition=cysticfibrosis (updated January 2008; accessed June 29, 2008).
National Heart, Lung, and Blood Institute (NHLBI). What Is Cystic Fibrosis? Available online at http://www.nhlbi.nih.gov/health/dci/Diseases/cf/cf_what.html (updated August 2007; accessed June 28, 2008).
National Human Genome Research Institute (NHGRI). Learning about Cystic Fibrosis. Available online at http://www.genome.gov/10001213 (updated April 10, 2008; accessed June 28, 2008).
Teens Health. Cystic Fibrosis. Available online at http://kidshealth.org/teen/diseases_conditions/digestive/cystic_fibrosis.html (updated September 2007; accessed June 29, 2008).