To have cystic fibrosis, a child must inherit one copy of the cystic fibrosis transmembrane conductance regulator (CFTR) gene mutation from each parent. People who have only one copy of a CFTR gene mutation do not have CF. They are called "CF carriers."
Each time two CF carriers have a child, the chances are:
- 25 percent (1 in 4) the child will have CF
- 50 percent (1 in 2) the child will be a carrier but will not have CF
- 25 percent (1 in 4) the child will not be a carrier of the gene and will not have CF
People with CF can also pass copies of their CF gene mutations to their children. If someone with CF has a child with a CF carrier, the chances are:
- 50 percent (1 in 2) the child will be a carrier but will not have CF
- 50 percent (1 in 2) the child will have CF
The Genetic Test: How It Works
Genetic testing can be used to tell if a person carries a mutation of the CFTR gene. 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.
More than 10 million Americans are carriers of one mutation of the CFTR gene. The chances of being a carrier of one CFTR mutation or having CF (with two CFTR disease-causing mutations) depend on race and ethnicity. CFTR gene mutations are most common
in Caucasian Americans (white people whose ancestors or family are from Europe).
About one in every 31 Americans is a symptomless carrier of the defective CFTR gene.
Only about one of every 3,000 Caucasian newborns has CF. We know there are more than 1,700 mutations of the CFTR gene. However, not all mutations cause CF. The ones that do cause CF are called "disease-causing" mutations. Current tests look for the most
common disease-causing mutations.
Who Should Get Tested?
The decision to get a genetic test is a personal one. Medical professionals or other advisers can help guide your decision. 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 testing to find out if they are CF carriers.
A standard test recommended by ACOG looks for 23 of the most common disease-causing mutations. But, the exact mutations screened by the genetic test can vary according to a person's race or ethnic group, or if someone in the family has CF. If you have
a relative who has CF or is known to carry a mutation of the CFTR gene, your chances of carrying a mutation are greater because of your family's history.
Although Caucasians have a higher risk of carrying a mutation of the CFTR gene,
guidelines from the American College of Obstetricians and Gynecologists favor
testing for any woman who is thinking about getting pregnant. If you are pregnant or planning to have a child, you should discuss the genetic test and the results with a health care professional who is knowledgeable about genetic testing, such as
a genetic counselor.
In the United States, the number of people who carry a CFTR gene mutation is about:
- 1 in 29 Caucasian-Americans
- 1 in 46 Hispanic-Americans
- 1 in 65 African-Americans
- 1 in 90 Asian-Americans
In the United States, 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
- 1 in 100,000 Asian-Americans1
Results From a Carrier Test
A positive carrier test result for CF means that a person has one mutation of the CFTR gene. A positive result is more than 99 percent accurate. A negative carrier test result is not as accurate. With more than 1,700 different mutations of the CFTR gene,
there are some rare disease-causing mutations that the test may not find.
If your test is negative for a mutation of the CFTR gene, there is still a small chance you could carry one CFTR mutation. The chance depends on your race or ethnic group and how many mutations the test looks for.
Because having one CFTR gene mutation does not cause symptoms, this mutation can be passed down to family members without anyone having the disease. Unless they have a child with CF, many people who are carriers do not know it.
If parents have had a child
diagnosed with CF, all of their children should be tested for the disease, whether they have symptoms or not. Other relatives also have a higher than average chance
of being carriers of a CFTR mutation. They may wish to get a genetic carrier test for CF or a
sweat test if they have symptoms of CF, based on their doctor's recommendations.
How CFTR Mutations Affect Symptoms of the Disease
Certain types of CFTR mutations are associated with different symptoms of the disease. Researchers continue to study the relationship between CFTR mutations and how mild or severe an individual's CF symptoms are likely to be. For example, some mutations
are more likely to affect the gastrointestinal (GI) tract than the lungs. However, knowing an individual's CFTR mutations cannot fully tell you how severe that person's CF symptoms will be.
The most common CFTR mutation is F508del (sometimes called delta F508). F508del causes the most common CF symptoms. But the effects of this mutation can vary in different people, and some people with the F508del mutation may have more severe CF symptoms
than others.
Likewise, there are less common mutations that produce milder symptoms than F508del, while other rare mutations cause symptoms just as severe. Your doctor or genetic counselor can provide you with more detailed information about the specific mutation
that you carry, or provide you with more information about genetic testing.