Duchenne muscular dystrophy (DMD) is the most common of the muscular dystrophies, affecting approximately one in every 3,500 newborn boys. It is caused by a fault in a gene called the DMD or dystrophin gene. A fault in this gene stops the body making a protein called dystrophin. This protein is important in muscle fibres, and its absence results in muscle weakness that gets worse over time because muscle cells break down and are gradually lost.
Becker muscular dystrophy (BMD) is a related but less severe condition where a fault in the same gene causes an internally shortened version of the dystrophin protein to be produced which is still partly to largely functional.
DMD and BMD are both characterized by progressive symmetrical muscular weakness. DMD symptoms typically present before age five whereas the milder BMD may not be diagnosed until later.
Because the dystrophin gene is on the X chromosome, DMD and BMD only affect boys. Girls have two X chromosomes, so if one of these is unaffected it can usually compensate for the faulty one. Boys have one X and one Y chromosome, so if their single copy of the dystrophin gene is faulty, they have the symptoms of DMD or BMD, while girls with one affected gene and one normal one usually won’t show symptoms but can be “carriers”. This means that the disease can be passed on in families – a mother who is a carrier has a 50:50 chance of having a son who is affected. In up to about a third of cases, the mutation arises spontaneously in the boy.
Facioscapulohumeral muscular dystrophy (FSHD) is the third most common type of muscular dystrophy worldwide. The prevalence of the disease is estimated at about one in 20,000. Symptoms often present in the teenage years although it can be much earlier in what is described as ‘infantile FSHD’. The symptoms tend to present as muscle weakness in the face (facio), the shoulder blade (scapula) and the upper arm (humeral). As the disease progresses, it can also affect the legs. Symptoms increase in severity over time and FSHD can be a painful and debilitating disease leading to the loss of ambulation; however, in the mildest cases people can remain unaware of the symptoms even into late adulthood.
In most cases FSHD is an autosomal dominant disease, meaning that only one copy of the genetic mutation is enough for the disease to show symptoms and there is 50% likelihood that the mutation will be inherited. Most cases of FSHD are caused by a partial deletion of DNA near the end of Chromosome 4. While there is no cure for FSHD great strides are being made in understanding the genetic abnormalities that result in muscle weakness.
Limb girdle muscular dystrophy (LGMD) describes a number of different inherited muscular dystrophies that are grouped under the label “limb girdle” because they generally affect the pelvic and shoulder girdles, causing progressive weakness in the muscles in the top part of the arms and shoulder, the hips and thighs. These muscles are often called the “proximal” muscles because they are nearest to the body. The weakness usually affects the legs first, followed by the arms. Other groups of muscles can also be affected depending on the type of LGMD.
To date, over 20 different types of LGMD have been identified and are caused by different genetic faults. The genes that may be affected in LGMD are responsible for various aspects of muscle function, and when there is a fault in one of these genes, the muscles cannot work properly and weakness occurs.
The types of LGMD are split into two groups: those that are inherited in an autosomal recessive fashion (type 2) and those that are inherited in an autosomal dominant fashion (type 1), which are much rarer. About 90% of the LGMDs are autosomal recessive. This means that two copies of the faulty gene need to be inherited, one from each parent, both of whom are carriers. Those LGMDs which are inherited in a dominant pattern need only one copy of the genetic mutation for the disease to show symptoms and there is 50% likelihood that the mutation will be inherited.
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