Duchenne Muscular Dystrophy; Becker Muscular Dystrophy; DMD-Related Dilated Cardiomyopathy

The dystrophinopathies manifest as a spectrum of muscle diseases. The mildest of the phenotypes includes an asymptomatic increase in serum concentration of creatine phosphokinase (CK or CpK), muscle cramps with myoglobinuria, and isolated quadriceps myopathy. At the opposite end of the spectrum is Duchenne muscular dystrophy (DMD), usually presenting in early childhood with delay in the motor milestones. DMD is rapidly progressive; patients are usually wheelchair-bound by 12 years of age and death usually occurs before age 30, due most frequently to respiratory complications and/or cardiomyopathy. Becker muscular dystrophy (BMD) is characterized by later-onset skeletal muscle weakness; individuals with BMD usually remain ambulatory well into their 20s. Despite the milder skeletal muscle involvement in BMD, cardiomypathy is a common cause of morbidity and the most common cause of death (on average in the mid-40s). Finally, DMD-associated dilated cardiomyopathy (DCM) is characterized by left ventricular dilation and congestive heart failure. Female carriers of DMD mutations are at increased risk for cardiomyopathy.

The dystrophinopathies are due to mutations in dystrophin (DMD), an X-linked gene encoding a membrane-associated protein that is found in muscle and a subset of neurons. The Duchenne phenotype is almost invariably caused by mutations that disrupt the reading frame including: deletions or duplications; nonsense mutations, and splice-site mutations. These produce a dystrophin protein molecule that is degraded. The milder Becker phenotype, on the other hand, results from mutations that reduce but do not completely eliminate the production of functional dystrophin protein, including deletions or duplications that maintain the open reading frame of the transcript, some splicing mutations, and most non-truncating single-base changes that result in translation of a protein product with intact N and C termini. DMD-associated dilated cardiomyopathy is caused by mutations in DMD that affect the muscle promoter (PM) and the first exon (E1), resulting in no dystrophin transcript being produced in cardiac muscle; expression (under different promoters) is retained in skeletal muscle and the central nervous system.

1. Confirmation of diagnosis:
   1. Testing of males with a suspected diagnosis of DMD.
   2. Testing of females is warranted if there is a clinical presentation consistent with the disease.
2. Carrier testing:
   1. Testing of adult females at risk to be carriers because of a family history. NB: Carriers have the potential for health problems in addition to the ability to transmit disease to offspring; genetic counselling is recommended prior to testing.
3. Prenatal testing (prenatal diagnosis requests are not normally accepted from physicians other than Medical Geneticists):
   1. Pregnancies at risk of inheriting a known DMD deletion or duplication. Prior to testing for the DMD mutation, fetal sexing is performed; if the fetus is female, further testing is not indicated.
      NB: If the mutation segregating in the mother is not known, consult the on-service Molecular Geneticist for assessment of whether linkage analysis is available for prenatal diagnosis
4. Presymptomatic testing:
   1. Requests for presymptomatic testing are only accepted following genetic counselling by a recognized genetic service.

Multiplex ligation-dependant probe amplification (MLPA) is carried out with the P034-A2 and P035-A2 probe mixes (MRC-Holland) to detect whole exon deletions and duplications; each of the 79 exons of DMD and the alternate exon 1 (DP427c) are assessed.

Pregnancy-related/Prenatal

If pregnancy management will be altered, 3 weeks; otherwise, routine TAT.

Blood: 4 mL EDTA is optimal (Minimum: 1 mL EDTA)
DNA: NOT ACCEPTED

Label each sample with three patient identifiers; preferably patient name, PHN, and date of birth and ship to the address below. Samples should be shipped at room temperature with a completed MGL Requisition to arrive Monday to Friday (not on Canadian statutory holidays). 

Prenatal Specimens
Prenatal testing REQUIRES LABORATORY CONSULTATION PRIOR TO THE PROCEDURE and can only be ordered by a Medical Geneticist. Contact the laboratory at 604-875-2852 and choose the appropriate option for the Molecular Geneticist on service.
Chorionic Villi: 20 mg.
Direct Amniotic fluid: 25 mL collected in two separate tubes of equal volume.
Cultured Amniocytes: Two (2) 100% confluent T-25 flasks.

Label each sample with three patient identifiers; preferably patient name, PHN, and date of birth. Ship samples by overnight courier with a completed MGL Requisition to arrive Monday to Friday (not on Canadian statutory holidays) as follows:

• Villi – on wet ice or in media at room temperature
• Amniocytes, Amniotic fluid, DNA – at room temperature

Testing is only available to residents of Canada, except in very specific circumstances where testing is urgent or emergent.  Payment is not required when requests are made for individuals who are insured by Health Insurance BC (administered through the BC Medical Services Plan (MSP)) AND eligible for testing according to the test utilization guidelines / policy. If the individual undergoing testing is not insured by these providers or does not meet utilization guidelines or policy, please complete a billing form; testing will only commence after receipt of billing informationTest prices can be found here.

Molecular genetic testing is limited by the current understanding of the genome and the genetics of a particular disease, as well as by the method of detection used. This method will not detect all mutations (e.g., point mutations in the coding region, promoter mutations, and regulatory element mutations). In rare cases, a point mutation could be detected.

For carrier/predictive testing due to family history, it is generally important to first document the gene mutation in an affected or carrier family member. This information should be provided to the laboratory for assessment of whether the assay is appropriate for detection of the familial mutation, and to aid in the interpretation of data.

In some cases, DNA alterations of undetermined or unclear clinical significance may be identified.

Rare single nucleotide variants or polymorphisms could lead to false-negative results. If results obtained do not match the clinical findings, consult the on-service Molecular Geneticist.

A previous bone marrow transplant from an allogenic donor will result in molecular data that reflects the donor genotype rather than the recipient (patient) genotype. Consult the on-service Molecular Geneticist for approach to testing in such individuals.

Transfusions performed with packed red blood cells will generally not affect the outcome of molecular genetic testing. However, if there is no clinical urgency, the cautious approach is to wait one week post packed red cell transfusion before collecting a sample for genetic testing. Consult the on-service Molecular Geneticist as needed.

Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete.