What is genetic testing?
Genetic testing consists of analyzing the cells in a blood, tissue, or saliva sample, looking for specific mutations in the DNA that are known to be associated with a given disease or condition. Each of the more than 30 different types of muscular dystrophy (MD) is believed to be associated with a unique genetic mutation, but not all of those mutations are known at this time.
Mutations can take the form of a repeated piece of DNA, a missing sequence, or an alteration or replacement of the normal genetic code with an irregular code.
Commercial genetic tests that can be used to diagnose a particular type of MD, known as direct genetic testing, are only available for some of the more common types of the disease, including Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), facioscapulohumeral muscular dystrophy (FSHD), and myotonic dystrophy (DM). Multiple genetic tests for different mutations may be needed to find the genetic defect causing symptoms, and it can take weeks or months to receive the results of these tests.
When is genetic testing used?
Clinicians will recommend diagnostic genetic testing, which may include direct genetic tests and family studies, when attempting to identify the cause of symptoms that may be related to a neuromuscular condition. Direct genetic testing refers to the method of looking for a specific mutation in the DNA. Typically, a blood sample is used for this type of testing.
If an obvious gene alteration cannot be detected through genetic testing, markers, or chunks of DNA located near an irregular genetic sequence, can be used to recognize genetic similarities among family members. This method of genetic analysis is referred to as a family study. If a person inherits the same markers as an affected relative, he or she likely also inherited the same MD type as that family member. The accuracy of this method is highly dependent on how close the markers are to the irregular gene sequence, and it requires blood samples from as many family members as possible.
Prenatal genetic testing is genetic testing carried out during a pregnancy, and used to determine whether the unborn child has inherited a dystrophy-causing gene. Such tests can be performed though either chorionic villus sampling (CVS) or amniocentesis. These two testing methods differ in the way a cell sample is collected. CVS is typically done approximately 11 weeks into the pregnancy, and involves collecting tissue from the placenta for analysis. Amniocentesis is usually done around week 15 of the pregnancy. In this method, a long needle is inserted into the mother’s abdomen and a sample of the amniotic fluid surrounding the fetus — containing cells from the unborn baby — is collected.
A prenatal genetic test, similar to a direct genetic test, can only be completed if a particular type of MD is suspected, based on family history. Because of this, a test whose results come back negative for a dystrophy does not guarantee that the child will not have inherited a given form of MD.
Carrier detection tests determine whether a person who shows no symptoms (asymptomatic) may have the irregular gene sequence associated with that disease. If someone is a carrier of a particular form of MD, there is a risk that he or she will pass the mutation to a child. Should this happen, the child could either develop the disease or be a carrier. A carrier detection test may be recommended if a person has a family history of a type of MD and is considering having children.
Are risks associated with genetic testing?
There are no major risks associated with diagnostic genetic or carrier detection testing. Prenatal testing is associated with a small risk of miscarriage, affecting approximately 1 in 200 to 1 in 400 pregnancies.
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