Medical technology now can provide information about the future health of individuals. Using genetic information, tests can be offered to determine if a person might have a child that may have certain diseases or health care needs, such as cystic fibrosis (CF).
Many expectant mothers and fathers can get a genetic test to see if they carry one of the CF mutations. A genetic test can also be used to help make the diagnosis of CF. However, the test to diagnose CF is the sweat test, also called the “gold standard” in diagnosing CF. For someone to have the disease, two copies of the defective CF gene must be inherited — one from each parent. A carrier has only one copy of the CF gene mutation and, thus, does not have the disease or symptoms.
What is genetic carrier testing for CF?
Genetic carrier testing can be used to tell if a person carries one or more mutations of the CF gene and how many copies of each mutation. The test looks at a person’s DNA (genetic material), which is taken from cells in a blood sample or from cells that are gently scraped from inside the mouth.
Although only about one of every 3,000 Caucasian newborns has CF, there are more than 1,000 known mutations of the gene that causes CF. Current tests look for the most common mutations.
The mutations screened by the test vary according to a person's race or ethnic group, or by the occurrence of CF already in the family. More than 10 million Americans are carriers of one mutation of the CF gene, including one in 29 Caucasian Americans. In other races or ethnicities, one in 46 Hispanic Americans, one in 65 African Americans and one in 90 Asian Americans carry a mutation of the CF gene.
If you have a relative who has CF or is known to carry a mutation of the CF gene, your chances of carrying a mutation are greater because of your family's history. If you are pregnant or planning to have a child, you should discuss this test and the results with a health care professional who is knowledgeable about genetic testing, such as a genetic counselor.
What is CF?
Cystic fibrosis is a genetic disease that causes the body to make a thick, sticky mucus that clogs the lungs and other organs, such as the pancreas. In the lungs, this mucus leads to chronic infections and progressive loss of lung function.
The mucus blocks the pancreas, preventing digestive enzymes from reaching the intestines to help break down and absorb food, and it can also block the bile duct in the liver, eventually causing permanent damage in approximately 6 percent of people with CF. The disease also affects the ability of most men with CF to have children.
Symptoms of CF include: salty-tasting skin; persistent coughing; frequent pneumonia, wheezing or shortness of breath; a failure to gain weight at the normal rate, perhaps with excessive appetite; and difficulty in having a bowel movement or frequent, abnormal bowel movements.
Today, because of improved medical treatments and care, more than 45 percent of patients with the disease are age 18 or older and many people with CF can expect to live into their 30s, 40s and beyond. Although the outlook for people with CF is improving, there is no cure.
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Should I get genetic carrier testing for CF?
Your decision to be genetically tested to learn if you carry a mutation or mutations of the CF gene may be difficult and is a personal choice. You may wish to talk with your medical or religious advisers to help you decide. The American College of Obstetricians and Gynecologists (ACOG) suggests that all couples who are considering having a child — or those who are already pregnant — should be offered genetic carrier testing for CF.
Although Caucasians have a higher risk of carrying a mutation of the CF gene, ACOG favors testing for any woman who is thinking about getting pregnant. To learn more about genetic carrier testing in general, visit ACOG's website at www.acog.org.
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How does someone inherit a mutation of the CF gene?
People inherit genes from their parents. To have CF, a child must inherit one copy of a mutation of the CF gene from each parent. In other words, the child must have two copies of the defective gene to have CF. They could have two copies of one type of CF mutation, or have a copy of two different CF mutations. Both males and females can have the disease.
A person who has only one copy of one mutation does not have CF and is considered a "carrier" of the CF gene.
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What does a positive genetic carrier test for CF mean?
A “positive” genetic carrier test for CF means that a person has a mutation of the CF gene; this result is more than 99 percent accurate. A "negative" carrier test result is not as accurate. With more than 1,000 different mutations of the CF gene, there are some rare ones that the test does not identify.
Thus, if your test is negative for a mutation of the CF gene, there is still a small chance you could be a carrier of one of the rare mutations. The degree of chance depends on your race or ethnic group and the type of genetic carrier test you receive.
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How common is it to carry a CF gene or to have CF?
The chances of being a CF carrier (with one CF mutation) or having CF (with two CF mutations) depend on your race and ethnicity. CF gene mutations are most common in Caucasian Americans (white people whose ancestors or family are from Europe).
In the U.S., the number of people who carry a CF gene is about:
- 1 in 29 Caucasian Americans;
- 1 in 46 Hispanic Americans;
- 1 in 65 African Americans; and
- 1 in 90 Asian Americans.
In the U.S., the number of people who have CF is about:
- 1 in 2,500–3,500 Caucasian Americans;
- 1 in 4,000–10,000 Hispanic Americans;
- 1 in 15,000–20,000 African Americans; and
- 1 in 100,000 Asian Americans.
Unfortunately, there is very little information about the chances among Native Americans of having CF or being a CF carrier.
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If I have one copy of the defective CF gene, can my children have CF?
Yes. While researchers continue to study the different mutations, some information is available. If you have one copy of a CF gene mutation, your child may have CF. However, your child will only have CF if your child’s other parent also has one copy of a CF gene mutation. This means that both of you are carriers.
Each child you have together has a 25 percent, or one in four, chance of having CF. If one parent is a carrier and the other parent’s genetic test is negative for a CF gene mutation (or if there is no information on whether or not he or she is a carrier), there is still a slight chance the child could have CF. Your doctor or a genetic counselor can tell you about your individual chances of having a child with CF.
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How does CF “appear” when no one in my family has ever had the disease?
Since having one copy of the defective CF gene does not cause symptoms, this copy can be passed down to family members without any impact on their health. Unless they have a child with CF, many people who are carriers of a CF gene do not know it.
Once parents have had a child diagnosed with CF, all of their children should be tested for the disease, whether they are showing symptoms or not. Other relatives also have a chance of being CF carriers and may wish to get a genetic carrier test for CF, or they may wish to be given the sweat test if they are showing symptoms of CF.
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Do certain types of the defective CF gene make the disease more mild or severe?
Yes, for some mutations. Researchers continue to study the relationship between a specific mutation and how mild or severe the CF symptoms are. For example, some mutations are more likely to affect the gastrointestinal tract (GI) than the lungs. However, knowing the CF mutation cannot fully tell you how severe a person's CF will be.
The most common CF gene mutation, Delta F508 (sometimes called F508del), causes the most common CF symptoms, but this mutation can vary in how severely it affects someone with CF. Some less common mutations of the CF gene may cause milder symptoms.
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Can genetic testing diagnose CF?
Genetic tests can help confirm a diagnosis of CF, but these tests are more often used to determine if a person is a carrier. The standard test to diagnose CF is the sweat test, which measures the concentration of salt in the sweat. People with CF have saltier sweat than most people. Also, if someone is diagnosed with CF through a sweat test, a genetic test can show the mutation or mutations that the person is carrying.
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Can a fetus be tested for CF?
Yes. If the mother and father are both carriers, or if one parent is known to be a carrier, genetic testing for CF may be possible for the fetus before birth. To learn more about prenatal genetic testing, contact your doctor, obstetrician, midwife or genetic counselor.
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Is there hope for a healthy future for people with CF?
In 1989, when CF Foundation-supported scientists discovered the CF gene, they gained a great tool for discovering and developing new CF treatments and, someday, a cure. Today, there are new treatments that help many people with CF live full, active lives, and more treatments are coming. Click here to see what potential drugs are now in development for cystic fibrosis.
The Cystic Fibrosis Foundation is focused on finding a cure and control for CF, improving the quality of life for people with the disease and sharing educational information. The Foundation does not make specific recommendations about who should have genetic tests. If you would like more information about genetic testing, you should speak with your doctor or a genetic counselor.You can find a genetic counselor familiar with CF by contacting a CF Foundation-accredited care center, which provides comprehensive diagnosis and specialized care for people with CF.
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Other Resources
The American College of Obstetricians and Gynecologists: (800) 762-2264; www.acog.org
National Society of Genetic Counselors: (610) 872-7608; www.nsgc.org
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