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Two genetic diseases characterized by progressive muscle weakness and loss of muscle mass

Muscular dystrophies are a group of inherited genetic diseases that are mainly characterized by progressive muscle weakness and loss of muscle mass.

There are several types of muscular dystrophies: myotonic muscular dystrophy, Duchenne muscular dystrophy, Becker muscular dystrophy, limb-girdle muscular dystrophy, facioscapulohumeral muscular dystrophy, congenital muscular dystrophy, oculopharyngeal muscular dystrophy, distal muscular dystrophy, Emery-Dreifuss muscular dystrophy. Duchenne and Becker muscular dystrophies are the most common types.

Duchenne and Becker muscular dystrophies mainly affect males and also share similar symptoms, but differ in disease onset, severity, and progression. Of the two, Duchenne muscular dystrophy is the more common one. Symptoms usually appear in childhood (between the ages of two and six). The disease progresses rapidly, affecting first the muscles of the limbs and trunk. Calf hypertrophy is often seen in patients with Duchenne muscular hypertrophy. As muscle weakness worsens and motor functions are progressively affected, children with this condition often need a wheelchair in adolescence. They often develop scoliosis and contractures in the joints. As the disease progresses, even the muscles that control breathing may become affected, and life expectancy is greatly reduced, to about 20-30 years.

The symptoms of Becker muscular dystrophy are very similar to those seen in Duchenne muscular dystrophy, but they are usually less severe, appear later (in late childhood or even in adulthood) and progress more slowly. In general, this type of dystrophy is much more variable than Duchenne, but it can also cause breathing and joint problems. Patients can often lead a more or less active life and without having to rely on a wheelchair. Life expectancy varies greatly and largely depends on the severity of symptoms, especially respiratory and/or cardiac problems.

In both dystrophies, the heart muscle can be affected, causing what is known as dilated cardiomyopathy. This is a condition resulting from lengthening and weakening of the heart muscle that causes that blood pumping is not done properly. This condition can progress rapidly and in many cases be fatal.

Unfortunately, there is currently no definitive cure or treatment for Duchenne and Becker muscular dystrophies. Palliative treatments such as physical, occupational, and respiratory therapy are often used to improve and relieve some of the symptoms. Physical therapy is especially effective in maintaining healthy muscles and delaying the onset of contractures.

These two dystrophy types are caused by different mutations in the same gene, the DMD gene, which is responsible for the production of a protein called dystrophin. This protein plays an important role in stabilizing and protecting muscle fibers. Mutations in the DMD gene alter the structure or function of dystrophin. In the most severe cases, it can cause this protein not to be produced at all, so muscle cells become–to a greater or lesser extent–unprotected and are progressively damaged. This change causes that part of the muscle fibers is replaced by scar and fat tissue, leading to the characteristic symptoms of these dystrophies. Patients with Duchenne muscular dystrophy have virtually no dystrophin, and patients with Becker muscular dystrophy produce a larger-than-normal dystrophin that does not work properly.

Duchenne muscular dystrophy affects between 20 and 30 out of every 100,000 males born. In contrast, Becker muscular dystrophy affects 3 out of every 100,000 males born. Females are rarely affected by these types of dystrophies because the gene follows an X-linked recessive inheritance pattern.

As stated above, these disorders follow an X-linked recessive inheritance pattern. This means that the gene that causes these diseases is located on the X chromosome, also known as the sex chromosome. In males, who only have one X chromosome, one altered copy of the gene is sufficient to cause the disease, while in females, who have two X chromosomes, it is necessary that the mutation is present in both copies of the gene on each X chromosome; therefore, it is a much more common occurrence in males.

In nearly two-thirds of cases, males inherit the mutation from their carrier mother, while in the remaining on-third of cases, the disease occurs due to new mutations that occur sporadically. Therefore, it is not necessary for someone in the family to have this disease.

Sometimes, women carrying a mutation in the DMD gene may have symptoms of the disease such as muscle weakness and/or cramps and are at a greater risk of developing cardiac abnormalities. However, these symptoms are generally less severe than in males.

For individuals with this health condition, physical, occupational, and respiratory therapies are highly recommended. These treatments help keep muscles healthy and in good condition while also limiting or delaying the onset of contractures.

It is highly recommended that future parents who either suffer from this genetic disease or have a family history of it seek genetic counseling. Getting tested enables them to know the risk of the disease being transmitted their children, undergo all necessary genetic tests, and choose the most appropriate preventive measures.

Asem – Spanish Federation of Neuromuscular Diseases. 

Fundación Isabel Gemio – Research on muscular dystrophies and other rare diseases