Duchenne muscular dystrophy
Mutations in the dystrophin gene can lead to different conditions:
- Duchenne muscular dystrophy: the most severe form of muscular dystrophy in children
- Becker Muscular Dystrophy: a milder variant of muscular dystrophy.
- DMD-associated dilated cardiomyopathy (DCM): a heart disease not associated with skeletal muscle weakness.
Affected individuals have a mutation on both copies (alleles) of the dystrophin gene. Carriers have one affected copy of the gene and one healthy copy. Carriers can show symptoms or not. This depends on the severity of the mutation in the affected allele.
Patients with DMD have a complete (or almost complete) absence of dystrophin in the muscle fibres. This is due to so-called out-of-frame mutations in the dystrophin gene.
Patients with Becker muscular dystrophy carry in-frame mutations. They allow the production of shorter but still functional dystrophin protein. Or they may have a lower amount of normal dystrophin protein. In both cases, they present with milder symptoms. Becker muscular dystrophy also presents with progressive muscle weakness and wasting.
The age of onset of symptoms, the rate of progression and severity of muscle weakness can vary. They can range from those with severe disease to those with very mild Becker disease. Individuals with the mild Becker form of the disease may not show muscle weakness or heart problems until their fifties or even sixties.
Females are not usually affected by Duchenne or Becker muscular dystrophy. Female carriers do have an increased risk of dilated cardiomyopathy (DCM). Approximately 8% of female carriers of DMD show some muscle symptoms. They can range from muscle aches, cramps and fatigability to severe muscle weakness. Female carriers who show some muscle symptoms are 'manifesting carriers'.