Genetic Testing for Cystic Fibrosis

Looking For Cystic FIbrosis Mutations With Genetic Testing

DNA sequence on a computer monitor screen
DNA sequence on a computer monitor screen. Getty Images/TEK IMAGE

How can you find out if you have the genetic mutations which may lead to cystic fibrosis in your child? Why can't we always know if someone has a mutation that could lead to cystic fibrosis and why is testing so expensive?

Diagnosing Cystic Fibrosis vs Diagnosing the Carrier State

It's important to clarify exactly what genetic testing for cystic fibrosis means. We have tests which can diagnose cystic fibrosis in children, even at birth before any symptoms are present.

Testing for the carrier state, however, is different. Genetic testing for cystic fibrosis mutations involves testing a person who does not have (and will not develop cystic fibrosis) for the gene mutations which he or she could pass on to offspring, which in turn could lead to cystic fibrosis.

How Common are the Mutations Which Cause Cystic Fibrosis?

It is estimated that in the United States, more than 30,000 people have cystic fibrosis (CF) and more than 10 million people carry the cystic fibrosis trait. A carrier does not have the disease or any of the symptoms, but is capable of passing CF on to the child.

What Exactly is a Gene Mutation?

Genes are our genetic blueprint, and are in charge of carrying the information for everything from eye color, to the design of the enzymes that digest food in our digestive tract. Our DNA is made up of 46 chromosomes, 23 which come from our mothers and 23 from our fathers.

A gene is a distinct section of a chromosome. Our genes, in turn, are made up of combinations of four amino acids, which act as "letters" that, when combined, spell out "words." A mutation refers to any damage to the gene in which the normal order of these letters is mixed up. There can be letters that are missing, letters that are added, or they can be rearranged in various ways.

Simplistically, you may think of a mutation as a word in a letter. When a letter is missing or rearranged, a different word is spelled out. In the body, our DNA is "translated" in the formation of proteins. When a mutation results in the production of an abnormal protein, diseases such as cystic fibrosis may result.

The Genetics of Cystic Fibrosis

Genetic testing for cystic fibrosis is done to determine if a person has, or is a carrier of, cystic fibrosis. The test is usually done on a blood sample, but sometimes can be done on other DNA-containing cells such as a swab from the inside of the cheek. The sample is examined for a specific panel of mutations of the cystic fibrosis transmembrane regulator (CFTR) gene. this gene

Everyone has two copies of the CFTR gene in every one of their cells. If one copy, also known as an allele is found to have a mutation, the person is a carrier. If both copies of the gene are mutated, the person has cystic fibrosis disease.

For a child to develop cystic fibrosis, he needs to inherit two genes that are mutated, one from his mother and one from his father.

If only his mother or father has the mutated gene, he may become a carrier of the CF trait, but will not develop cystic fibrosis.

Cystic Fibrosis is Caused by Many Different CFTR Mutations

Mutations in a gene are not usually of only one type. Rather, there are many different ways in which a gene may be abnormal. There are more than 1,500 different CFTR mutations known to cause CF, and new mutations are being discovered as research on the gene continues.

No test checks for all of the possible CF-causing mutations, but several tests are available that check for the most common CFTR mutations. The cost of genetic testing ranges from a few hundred to several thousand dollars, and may or may not be covered by insurance.

Basic Cystic Fibrosis Test: ACMG/ACOG Mutation Panel

The basic genetic test for cystic fibrosis, sometimes referred to as the ACMG/ACOG Mutation Panel or the 23-mutation panel, looks for the most commonly occurring CFTR mutations. This test is about 90 percent effective in detecting CF mutations in the Caucasian population, but only about 70 percent effective in African Americans and 60 percent effective in the Hispanic population.

The test is widely available at local and regional labs, but the specific mutations tested for will vary according to lab protocols. Some labs check for more mutations, but the basic test should at least include the 23 mutations recommended by the American College of Medical Genetics (ACMG) and the American College of Obstetricians and Gynecologists (ACOG). Results are usually available within a few days, but may take longer if the specimen must be sent to a larger lab for testing.

Advanced Testing for Cystic Fibrosis

Some companies have developed tests that can identify less common mutations in addition to those in the ACMG/ACOG recommended panel. These tests can be especially helpful in detecting rare cystic fibrosis mutations when done as follow-up tests if the basic panel produced normal results. Some people opt to have this more thorough testing for their initial test, especially if they are likely to have a less common CFTR mutation. The advanced tests are highly specialized and must be sent to the labs that offer them. These tests may cost more and take longer to receive results than the basic testing panel done at a local lab.

Impact of Testing

If you are considering carrier testing for cystic fibrosis, it is extremely important to talk to a genetic counselor. Overall, testing has been found to have a positive impact in many ways, ranging from allowing parents to be aggressive with prenatal testing so that their child can be treated starting at birth, to providing reassurance to those who find they are not carriers of the trait. For those who are not pregnant at the time of testing, yet find they are carriers, there are many heart-wrenching ethical and personal choices which will require careful consideration and much support.

Sources:

Brennan, M., and I. Schrijver. Cystic Fibrosis: A Review of Associated Phenotypes, Use of Molecular Diagnostic Approaches, Genetic Characteristics, Progress, and Dilemmas. The Journal of Molecular Diagnostics. 2016. 18(1):3-14.

Dugueperoux, I., L’Hostis, C., Audrezet, M et al. Highlighting the Impact of Cascade Carrier Testing in Cystic Fibrosis Families. Journal of Cystic Fibrosis. 2016. 15(4):452-9.

Straniero, L., Solda, G., Costantino, L. et al. Whole-Gene CFTR Sequencing Combined with Digital RT-PCR Improves Genetic Diagnosis of Cystic Fibrosis. Journal of Human Genetics. 2016. 61(12):977-984.

Continue Reading