What are Dystrophinopathies (including Duchenne/Becker Muscular Dystrophy)?
Dystrophinopathies are a group of conditions that generally cause muscle weakness. There are two main forms of dystrophinopathies related to the DMD gene: Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD). Both DMD and BMD are caused by harmful genetic changes or mutations in the DMD gene. DMD and BMD are X-linked conditions, which means that the DMD gene is on the X-chromosome. Males have one copy of the X-chromosome and the DMD gene, while females have two copies. Because of this, DMD and BMD primarily affect males, while most female carriers are unaffected. However, it is possible for some females with a mutation in the DMD gene to have symptoms.
Common presentations of DMD and BMD are described below. It is not always possible to predict, based on a genetic testing result, which of the presentations could occur in a child who has a mutation in the DMD gene. Additionally, there have been rare reports of males with a mutation in the DMD gene who do not have the characteristic symptoms described below.
Duchenne Muscular Dystrophy
The primary symptoms of DMD are weakness and breakdown of muscles, which get worse over time. Symptoms typically begin in early childhood, with weakness in the muscles of the hips, pelvic region, thighs, and shoulders. This results in an abnormal way of walking (gait) and in delays to sitting up, standing, and walking (general motor skills). A small percentage of males also develop learning difficulties early in life, though the level of intellectual disability is variable. Because DMD gets worse over time, most affected individuals will need a wheelchair by 13 years of age. By the mid-teenage years, the heart muscles will weaken (this is known as dilated cardiomyopathy or DCM), as will the respiratory muscles. This weakening causes a shortened life expectancy.
Becker Muscular Dystrophy
BMD also causes muscle weakness and DCM. However, symptoms are much more variable in presentation, may be milder, and tend to develop later than DMD. Individuals with BMD also generally have a longer life expectancy than those with DMD.
DMD-Associated Dilated Cardiomyopathy
DMD-associated dilated cardiomyopathy (DMD-associated DCM) may occur without any muscle weakness, and both males and females are at risk of developing this condition. However, its onset is generally earlier in males than in females, and progression tends to be more rapid.
Additional considerations for carriers
While most individuals affected with DMD and BMD are males, females who carry mutations in the DMD gene may rarely experience symptoms. Less than 5% of carriers will experience muscle weakness as an adult, but the severity of symptoms depends on whether the mutations in the DMD gene is associated with BMD or DMD. Up to 17% of female carriers will have heart problems, including DCM.
How common are Dystrophinopathies (including Duchenne/Becker Muscular Dystrophy)?
Together, the incidence of DMD and BMD is an estimated 1 in 3,000 males. Approximately 1/3 of individuals affected by DMD or BMD do not inherit a mutation from a carrier mother (a de novo mutation). Note: because this is an X-linked condition, estimates of frequency generally do not include affected females.
How are Dystrophinopathies (including Duchenne/Becker Muscular Dystrophy) treated?
There is no cure for DMD-related dystrophinopathy; a combination of physical therapy, medication, and regular cardiac and respiratory screenings is the current standard practice for treating the condition. Carrier females are at an increased risk for DCM and should also regularly see a cardiologist.
What is the prognosis for individuals with Dystrophinopathies (including Duchenne/Becker Muscular Dystrophy)?
The prognosis for DMD is variable, but most males will be wheelchair-dependent by age 13 and die before 30 years of age due to heart or respiratory failure. Males who have BMD have a longer life expectancy, typically reaching into their forties or fifties. The prognosis in females is generally better, but those with DCM may still have a shortened lifespan. In rare cases, males with a mutation in the DMD gene may not experience the characteristic symptoms of dystrophinopathies.