Duchenne Muscular Dystrophy (DMD) and Genetic Testing

Duchenne muscular dystrophy (DMD) is a disease of progressive muscle degeneration and weakness caused by genetic mutations in the dystrophin gene. Genetic mutations that cause DMD, result in an inability to make dystrophin protein. Dystrophin connects the contracting part of muscle to the membrane surrounding it and provides stability. Without dystrophin, muscles become damaged after normal use which is why people with DMD have muscle weakness. Genetic testing for Duchenne muscular dystrophy is used to find out if muscle problems are caused by a dystrophin gene mutation.

For a child showing signs of DMD, genetic testing is the gold standard for diagnosing the disease and is necessary for choosing treatments. The mother of a child with DMD and other family members can use genetic testing to find out if they are likely to pass on the disease. Genetic testing can be used for prenatal diagnosis as well.

Why DMD genetic testing is important

Genetic testing, also known as DNA testing, is used to confirm the diagnosis of DMD. Genetic testing will show that a mutation is present in the dystrophin gene that prevents the muscle from making functional dystrophin protein. Finding the dystrophin gene mutation rules out other types of muscular dystrophy which cause similar symptoms. The type of dystrophin mutation present can predict disease progression and severity and can distinguish between Duchenne muscular dystrophy and the milder disease, Becker muscular dystrophy (BMD). Individuals with DMD produce no functional dystrophin protein but those with BMD produce partially functional dystrophin.

Genetic testing shows the type of dystrophin mutation present. The type of mutation has to be known for certain mutation-specific treatments, available now or in the future. It is also necessary to know the dystrophin mutation type to participate in clinical trials.

Once the DMD mutation of a child is known, other family members can be tested to check if they carry the mutation, which is important for family planning. Females who carry the dystrophin mutation can pass it on to children but most often do not have muscular dystrophy symptoms themselves. It is helpful to know that symptoms such as cardiomyopathy may develop in female carriers. The mother of a child with DMD may not be a carrier of the mutation as it can occur as a spontaneous mutation in the child during pregnancy.

When is DMD genetic testing offered?

If a child shows signs of Duchenne, their doctor may investigate with a creatine kinase (CK) blood test before genetic testing. CK is an enzyme that is normally enclosed inside the muscle membrane and not found in the blood. Since muscle membranes are so easily damaged in DMD, CK leaks out into the blood. If a person is found to have elevated blood CK levels, DMD is one possible cause. To confirm that there is a problem with dystrophin, positive CK blood tests are followed up with genetic testing from blood or saliva. In some cases, muscle biopsy testing may be needed to complete the diagnosis.

When a child’s doctor suspects that they have DMD, they will likely be offered genetic testing or referred to a geneticist and/or genetic counselor. Once a dystrophin mutation is known, a genetic counselor should order familial mutation testing, a less expensive type of genetic test that looks for whether that one type of genetic mutation runs in the family. If your child has a DMD mutation, a genetic counselor will go through your family history and recommend other family members who should have familial mutation testing.

Females with a family history of Duchenne may order carrier testing through their healthcare provider or OB/GYN. Receiving a negative test for being a carrier of DMD means that most likely the dystrophin mutation started in her child. However, there is a small chance that she carries the mutation in some of her eggs but not all her blood cells, which were used for testing. Due to this potential situation, called germline mosaicism, the other siblings of a child with DMD should be tested even if the mother tests negative.

Female relatives of the mother of a child with DMD, such as the mother’s sisters, mother, and maternal aunts should be offered DMD genetic testing. If a mother tests positive for being a carrier of DMD, her children should receive genetic testing. This excludes any sons who are old enough to have already shown clinical signs of Duchenne. A genetic counselor can facilitate genetic testing and explain the results to you and your family.

Types of DMD genetic tests

When DMD is suspected but the exact dystrophin mutation is not known, the first genetic tests ordered will search for the most common types of mutations which are large deletions and duplications. Deletions are chunks of DNA missing from genes and duplications are regions of DNA sequence that are repeated.

Microarray-based comparative genomic hybridization (CGH array testing) and multiplex ligation-dependent probe amplification (MLPA) are two methods commonly used. The dystrophin gene is divided into segments called exons and introns, numbered across the gene. MLPA identifies which exons are duplicated or deleted by detecting extra or missing parts of dystrophin. CGH array testing checks exons and introns and can detect a DNA breakpoint in an intron.

If no deletion or duplication in dystrophin is found, patient samples may be tested by DNA sequencing methods, which can find smaller DNA changes including point mutations. Sanger gene sequencing or a newer method, next-generation sequencing (NGS) may be used for this purpose. Comprehensive genetic testing means testing for deletions, duplications, and small mutations and is estimated to detect 95-99% of DMD cases.

When comprehensive genetic testing fails to detect DMD and other clinical findings support this diagnosis, a muscle biopsy may be the next step. A muscle biopsy is a small sample of muscle collected with a biopsy needle. The muscle sample is tested to see how much if any, dystrophin protein is there using techniques called immunoblot or immunostaining. No dystrophin protein usually points to DMD as the diagnosis and less than normal amounts of dystrophin suggest the condition is BMD.

Further tests on muscle tissue may be ordered, such as cDNA or mRNA testing, which assess a middle step in the production of dystrophin protein from the dystrophin gene. When dystrophin is produced in muscle cells, the DNA of the dystrophin gene is first copied or transcribed into a molecule similar to DNA, called mRNA. mRNA is processed to remove introns and join exons together to make the coding sequence that is translated by machinery that builds dystrophin protein. When analyzing mRNA, some laboratory tests copy the mRNA into cDNA. Both cDNA and mRNA testing are checked to see if mRNA is transcribed and processed properly.

After comprehensive genetic testing, 1-5% of people still test negative. Since techniques for finding DMD mutations are continually improving, it is a good idea to keep in touch with your neuromuscular center or genetic counselor in case a newer method becomes available.

DMD and prenatal diagnosis

Genetic testing can be used to diagnose Duchenne before birth if a specific dystrophin mutation is known to be in the family. If a woman is pregnant and known to be a carrier of DMD, she may be offered prenatal testing. Chorionic villus sampling (CVS) is a method in which tissue is removed from the placenta, around 11 weeks or later, and used as testing material. Another method, called amniocentesis, is when a needle is inserted into the womb through the abdomen to remove a small amount of the amniotic fluid surrounding the fetus. Amniocentesis is usually carried out after 15-16 weeks of pregnancy. Amniotic fluid contains cells that have been shed by the fetus. CVS and amniocentesis carry a small risk of miscarriage. Non-invasive prenatal diagnosis (NIPD) uses a sample of blood from the mother which contains tiny bits of free fetal DNA (ffDNA). This newer diagnostic test may soon be available to test for DMD.

If samples from CVS or amniocentesis detect a dystrophin mutation that causes DMD, the baby will likely develop muscular dystrophy when they are a young child. A genetic counselor will help discuss options with you. The options are difficult and personal and may include terminating the pregnancy. Before having a prenatal diagnosis, you should discuss with the genetic counselor the limitations of genetic testing and think about what the results would mean for you. Genetic counselors are experts who can help guide you through difficult decisions.

Pre-implantation genetic diagnosis of DMD

Couples who are at risk of having a child with DMD may consider pre-implantation genetic diagnosis (PGD). For this option, the couple conceives using in vitro fertilization (IVF). Then the embryos are tested at an early stage to select embryos unaffected by DMD to be transferred into the uterus. While PCG avoids the termination of a pregnancy, the success rate of becoming pregnant after IVF is not high. The IVF and PCG processes carry social, financial, and emotional burdens which are helpful to discuss with a genetic counselor.

Muscular dystrophy genetic testing is used to identify the root cause of muscle problems. Genetic testing to diagnose DMD helps children with Duchenne begin treatment as soon as possible. Knowing which dystrophin mutation a child has can allow them access to mutation-specific therapies. Being aware of a dystrophin mutation that runs in the family is important for family planning. Genetic counselors are experts who can guide you through the process of genetic testing and understanding test results.

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