Muscular Dystrophy Health Article

Definition

Muscular dystrophy is the name for a group of inherited disorders in which strength and muscle bulk gradually decline. Nine types of muscular dystrophies are generally recognized.

Description

The muscular dystrophies include:

• Duchenne muscular dystrophy (DMD): DMD affects young boys, causing progressive muscle weakness, usually beginning in the legs. It is a severe form of muscular dystrophy. DMD occurs in about one in 3,500 male births, and affects approximately 8,000 boys and young men in the United States. A milder form occurs in a very small number of female carriers.
• Becker muscular dystrophy (BMD): BMD affects older boys and young men, following a milder course than DMD. It occurs in about one in 30,000 male births.
• Emery-Dreifuss muscular dystrophy (EDMD): EDMD affects both males and females because it can be inherited as an autosomal dominant or recessive disorder. Symptoms include contractures and weakness in the calves, weakness in the shoulders and upper arms, and problems in the way electrical impulses travel through the heart to make it beat (heart conduction defects). Fewer than 300 cases of EDMD have been reported in the medical literature.
• Limb-girdle muscular dystrophy (LGMD): LGMD begins in late childhood to early adulthood and affects both men and women, causing weakness in the muscles around the hips and shoulders, and weakness in the limbs. It is the most variable of the muscular dystrophies, and there are several different forms of the condition now recognized. Many people with suspected LGMD have probably been misdiagnosed in the past, and therefore, the prevalence of the condition is difficult to estimate. The highest prevalence of LGMD is in a small mountainous Basque province in northern Spain, where the condition affects 69 persons per million.
• Facioscapulohumeral muscular dystrophy (FSH): FSH, also known as Landouzy-Dejerine condition, begins in late childhood to early adulthood and affects both men and women, causing weakness in the muscles of the face, shoulders, and upper arms. The hips and legs may also be affected. FSH occurs in about one out of every 20,000 people, and affects approximately 13,000 people in the United States.
• Myotonic dystrophy: Also known as Steinert's disease, it affects both men and women, causing generalized weakness first seen in the face, feet, and hands. It is accompanied by the inability to relax the affected muscles (myotonia). Symptoms may begin from birth through adulthood. It is the most common form of muscular dystrophy, affecting more than 30,000 people in the United States.
• Oculopharyngeal muscular dystrophy (OPMD): OPMD affects adults of both sexes, causing weakness in the eye muscles and throat. It is most common among French Canadian families in Quebec, and in Spanish-American families in the southwestern United States.
• Distal muscular dystrophy (DD): DD is a group of rare muscle diseases that have weakness and wasting of the distal (farthest from the center) muscles of the fore-arms, hands, lower legs, and feet in common. In general, the DDs are less severe, progress more slowly, and involve fewer muscles than the other dystrophies. DD usually begins in middle age or later, causing weakness in the muscles of the feet and hands. It is most common in Sweden, and rare in other parts of the world.
• Congenital muscular dystrophy (CMD): CMD is a rare group of muscular dystrophies that have in common the presence of muscle weakness at birth (congenital), and abnormal muscle biopsies. CMD results in generalized weakness, and usually progresses slowly. A sub-type, called Fukuyama CMD, also involves mental retardation and is more common in Japan.

Genetic profile

The muscular dystrophies are genetic conditions, meaning they are caused by alterations in genes. Genes, which are linked together on chromosomes, have two functions; they code for the production of proteins, and they are the material of inheritance. Parents pass along genes to their children, providing them with a complete set of instructions for making their own proteins.

Because both parents contribute genetic material to their offspring, each child carries two copies of almost every gene, one from each parent. For some conditions to occur, both copies must be altered. Such conditions are called autosomal recessive conditions. Some forms of LGMD and DD exhibit this pattern of inheritance, as does CMD. A person with only one altered copy, called a carrier, will not have the condition, but may pass the altered gene on to his children. When two carriers have children, the chances of having a child with the condition is one in four for each pregnancy.

Other conditions occur when only one altered gene copy is present. Such conditions are called autosomal dominant conditions. DM, FSH, and OPMD exhibit this pattern of inheritance, as do some forms of DD and LGMD. When a person affected by the condition has a child with someone not affected, the chances of having an affected child is one in two.

Because of chromosomal differences between the sexes, some genes are not present in two copies. The chromosomes that determine whether a person is male or female are called the X and Y chromosomes. A person with two X chromosomes is female, while a person with one X and one Y is male. While the X chromosome carries many genes, the Y chromosome carries almost none. Therefore, a male has only one copy of each gene on the X chromosome, and if it is altered, he will have the condition that alteration causes. Such conditions are said to be X-linked. X-linked conditions include DMD, BMD, and EDMD. Women are not usually affected by X-linked conditions, since they will likely have one unaltered copy between the two chromosomes. Some female carriers of DMD have a mild form of the condition, probably because their one unaltered gene copy is shut down in some of their cells.

Women carriers of X-linked conditions have a one in two chance of passing the altered gene on to each child born. Daughters who inherit the altered gene will be carriers. A son born without the altered gene will be free of the condition and cannot pass it on to his children. A son born with the altered gene will have the condition. He will pass the altered gene on to each of his daughters, who will then be carriers, but to none of his sons (because they inherit his Y chromosome).

Not all genetic alterations are inherited. As many as one third of the cases of DMD are due to new mutations that arise during egg formation in the mother. New mutations are less common in other forms of muscular dystrophy.

Several of the muscular dystrophies, including DMD, BMD, CMD, and most forms of LGMD, are due to alterations in the genes for a complex of muscle proteins. This complex spans the muscle cell membrane (a thin sheath that surrounds each muscle cell) to unite a fibrous network on the interior of the cell with a fibrous network on the outside. Theory holds that by linking these two networks, the complex acts as a "shock absorber," redistributing and evening out the forces generated by contraction of the muscle, thereby preventing rupture of the muscle membrane. Alterations in the proteins of the complex lead to deterioration of the muscle during normal contraction and relaxation cycles. Symptoms of these conditions set in as the muscle gradually exhausts its ability to repair itself.

Both DMD and BMD are caused by alterations in the gene for the protein called dystrophin. The alteration leading to DMD prevents the formation of any dystrophin, while that of BMD allows some protein to be made, accounting for the differences in severity and age of onset between the two conditions. Differences among the other muscular dystrophies in terms of the muscles involved and the ages of onset are less easily explained.

A number of genes have been found to cause LGMD. A majority of the more severe autosomal recessive types of LGMD with childhood-onset are caused by alterations in the genes responsible for making proteins called sarcoglycans. The sarcoglycans are a complex of proteins that are normally located in the muscle cell membrane along with dystrophin. Loss of these proteins causes the muscle cell membrane to lose some of its shock absorber qualities. The genes responsible include LGMD2D on chromosome 17, which codes for the alpha-sarcoglycan protein; LGMD2E on chromosome 4, which codes for the beta-sarcoglycan protein; LGMD2C on chromosome 13, which codes for the gamma-sarcoglycan protein; and LGMD2F on chromosome 5, which codes for the delta-sarcoglycan protein. Some cases of autosomal recessive LGMD are caused by an alteration in a gene, LGMD2A, on chromosome 15, which codes for a muscle enzyme, calpain 3. The relationship between this alteration and the symptoms of the condition is unclear. Alterations in a gene called LGMD2B on chromosome 2 that codes for the dysferlin protein, is also responsible for a minority of autosomal recessive LGMD cases. The exact role of dysferlin is not known. Finally, alterations in the LGMD2G gene on chromosome 17 which codes for a protein, telethonin, is responsible for autosomal recessive LGMD in two reported families. The exact role of telethonin is not known. Some families with autosomal recessive LGMD are not accounted for by alterations in any of the above mentioned genes, indicating that there are as yet undiscovered genes which can cause LGMD. The autosomal dominant LGMD genes have mostly been described in single families. These types of LGMD are considered quite rare.

The genes causing these types of LGMD, their chromosomal location, and the proteins they code for (when known) are listed below:

• LGMD1A (chromosome 5): myotilin
• LGMD1B (chromosome 1): laminin
• LGMD1C (chromosome 3): caveolin
• LGMD1D (chromosome 6)
• LGMD1E (chromosome 7)
• COL6A1 (chromosome 21): collagen VI alpha 1
• COL6A2 (chromosome 21): collagen VI alpha 2
• COL6A3 (chromosome 2): collagen VI alpha 3

The causes of the other muscular dystrophies are not as well understood:

• EDMD is due to a alteration in the gene for a protein called emerin, which is found in the membrane of a cell's nucleus, but whose exact function is unknown.
• Myotonic dystrophy is caused by alterations in a gene on chromosome 19 for an enzyme called myotonin protein kinase that may control the flow of charged particles within muscle cells. This gene alteration is called a triple repeat, meaning it contains extra triplets of DNA code. It is possible that this alteration affects nearby genes as well, and that the widespread symptoms of myotonic dystrophy are due to a range of genetic disruptions.
• The gene for OPMD appears to also be altered with a triple repeat. The function of the affected protein may involve translation of genetic messages in a cell's nucleus.
• The gene(s) for FSH is located on the long arm of chromosome 4 at gene location 4q35. Nearly all cases of FSH are associated with a deletion (missing piece) of genetic material in this region. Researchers are investigating the molecular connection of this deletion and FSH. It is not yet certain whether the deleted material contains an active gene or changes the regulation or activity of a nearby FSH gene. A small number of FSH cases are not linked to chromosome 4. Their linkage to any other chromosome or genetic feature is under investigation.
• The gene(s) responsible for DD have not yet been found.
• About 50% of individuals with CMD have their condition as a result of deficiency in a protein called merosin, which is made by a gene called laminin. The merosin protein usually lies outside muscle cells and links them to the surrounding tissue. When merosin is not produced, the muscle fibers degenerate soon after birth. A second gene called integrin is responsible for CMD in a few individuals but alterations in this gene are a rare cause of CMD. The gene responsible for Fukuyama CMD is FCMD and it is responsible for making a protein called fukutin whose function is not clear.