Confirming the Diagnosis
The diagnosis of Duchenne must be confirmed by genetic testing. This testing is typically from a blood sample, but other tests may also be performed as well.
1) GENETIC TESTING (BOX 2)
Genetic testing is always necessary and should be offered to every patient. Different types of genetic tests are able to provide specific and more detailed information about the change in the DNA known as the genetic mutation. Having genetic confirmation of the diagnosis is very important, and it may help to determine eligibility for a number of mutation-specific clinical trials.
Once the exact genetic mutation is known, mothers should be offered the opportunity for genetic testing to check whether they are carriers. This information will be important for other female family members on the mother’s side (sisters, daughters, aunts, cousins) to understand if they may be carriers as well. Having this information will help the family to gain knowledge about the risk of having more children with Duchenne and make decisions regarding prenatal diagnosis and reproductive choices. Families should be offered genetic counselling following diagnosis (Box 2).
TYPES OF GENETIC TESTING
- Multiplex ligation-dependent probe amplification (MLPA): MLPA tests for deletions and duplications, and is able to identify 70 percent of Duchenne genetic mutations
- Gene sequencing: If MLPA testing is negative, gene sequencing can pick up mutations other than deletions or duplications (i.e., point mutations [non-sense or missense] and small duplications/insertions); this test is able to identify the other 25 to 30 percent of Duchenne genetic mutations that the MLPA test does not identify.
2) MUSCLE BIOPSY
If you have a high CK level and signs of Duchenne, but no genetic mutations were found using genetic testing, you may need to have a muscle biopsy. A muscle biopsy is done by surgically taking a small sample of muscle for analysis. The genetic mutation in Duchenne means the body cannot produce the protein dystrophin, or doesn’t produce enough of it. Tests on the muscle biopsy can provide information on the amount of dystrophin present in the muscle cells (see Box 1).
Most people with Duchenne do NOT need a muscle biopsy.
There are two types of tests normally performed on a muscle biopsy: immunochemistry and western blot test. These tests are done to determine the presence or absence of dystrophin in the muscle. Immunohistochemistry involves putting a tiny piece of muscle on a slide, putting a stain on the muscle, then looking at the muscle cells under a microscope for evidence of dystrophin. Western blot test is a chemical process that tests for the chemical presence of dystrophin.
Box 1. Muscle Biopsy
Normal muscle showing dystrophin in white around the fibers.
Dystrophic muscle with absent dystrophin.
3) OTHER TESTS
In the past, the tests known as electromyography (EMG) and nerve conduction studies (needle tests) have been a traditional part of the assessment for a suspected neuromuscular disorder. The experts agree that EMG and nerve conduction tests are NOT appropriate or necessary for the evaluation of Duchenne.
Box 2. Why Genetic Confirmation is Important
GENETIC COUNSELLING AND CARRIER TESTING:
- Sometimes the genetic mutation causing Duchenne arises by chance. This is considered to be a “spontaneous mutation.” In these cases, there is no family history of Duchenne. Thirty percent of people who are born with Duchenne have a spontaneous mutation in the gene that encodes for dystrophin rather than inherited from a family member
- If the mother has a mutation in her DNA that she passes to her child, she is called a “carrier.” A carrier has a 50 percent chance with each pregnancy to pass the genetic mutation on to her children. Boys who receive the mutated gene will have Duchenne, while the girls who receive the mutated gene will be carriers themselves. If the mother is tested and is found to have the mutation, she can make informed decisions about future pregnancies, and her female relatives (sisters, aunts, and daughters) can also be tested to see if they are also carriers
- A female who is a carrier and shows some signs of Duchenne (muscle weakness, fatigue, pain, etc.) is known as a “manifesting carrier.” There is no test to show if a female carrier will be a manifesting carrier
- Even when a woman is not a carrier, there is a small risk that future pregnancies maybe effected by Duchenne. The genetic mutation causing Duchenne may occur only in her ova or egg cells, rather than in all of her cells. This is called “germline mosaicism”. There is no blood test for germline mosaicism
- A female who is a carrier also has an increased risk of developing cardiac and skeletal muscle weakness and dysfunction. Female carriers should have cardiac monitoring (ECG, cMRI or echo) by a cardiologist every 3 to 5 years, if tests are normal (or more frequently as prescribed by the cardiologist). Knowing carrier status helps to identify this risk in order to get appropriate advice and treatment
- A genetic counselor can explain all this in detail
ELIGIBILITY FOR CLINICAL TRIALS:
- There are a number of clinical trials underway in Duchenne that are targeted to specific genetic mutations causing Duchenne. Genetic testing is important to know whether you might be eligible to participate in these trials. To help clinical trial sites find youwhen you might be eligible for trials, be sure to register in your national Duchenne patient registry – a list can be found here: http://www.treat-nmd.org/resources/patient-registries/list/DMD-BMD/
- If previously performed genetic testing did not meet currently accepted standards, allowing the exact mutation to be defined, further/repeated testing might be needed. You should discuss this with your neuromuscular specialist and/or genetic counsellor. The precise genetic mutation is also needed to register in the Duchenne registries. You can find details of the kind of tests that might be done and how effective they are at detecting the detail of the mutation in the main document.
- Adults who have not had genetic testing, or who have had in the past and require testing using more current methods (ask your genetic counsellor if your testing should be repeated), should consider obtaining genetic testing in order to be considered for clinical trial participation.