Emery-Dreifuss Muscular Dystr... Health Article

Definition

Emery-Dreifuss muscular dystrophy (EDMD) is a rare childhood-onset degenerative muscle disease seen almost exclusively in males. Emery-Dreifuss muscular dystrophy is characterized by a classic triad of symptoms. These include early-onset contractures, very slow progressive muscle weakness and degeneration involving the upper arms and lower legs, and cardiac (heart) muscle disease.

Description

Emery-Dreifuss muscular dystrophy affects the arms, legs, spine, face, neck, and heart. This disease is characterized by contractures of the elbows and the Achilles tendons at an early age, slowly progressive muscle wasting and weakness, and life potentially life-threatening heart muscle disease. Intelligence is normal, however physical problems may be severe.

Symptoms and disease severity may vary between individuals. Three modes of inheritance exist: X-linked, autosomal dominant, and autosomal recessive. The symptoms of the autosomal dominant and X-linked forms of the disease are identical, however the autosomal dominant form appears to have a later onset of symptoms.

Genetic profile

Emery Dreifuss muscular dystrophy is inherited in different ways in different families. Most commonly EDMD is inherited in an X-linked recessive manner. Autosomal dominant inheritance of EDMD is also well characterized. Only one case of autosomal recessive inheritance of EDMD has been reported.

Rarely a new mutation causing EDMD can also occur, causing disease in a person with no family history. This is called a sporadic occurrence and is the ressult of a new change in a gene (new mutation) in that individual. New mutations account for approximately 10% of cases of EDMD.

X-linked recessive form

Emery-Dreifuss muscular dystrophy is usually inherited in an X-linked recessive manner. EDMD is the third most common type of X-linked muscular dystrophy. Symptoms begin in the first decade of life. A tendency to walk on the toes is often one of the first signs of EDMD. Muscle weakness first affects the lower extremities usually at age four or five.

X-linked diseases map to the human X chromosome, a sex chromosome. Females have two X chromosomes, whereas males have one X chromosome and one Y chromosome. Because males only have one X chromosome, they only require one X-linked disease gene to display disease. Since females have two X chromosomes, the effect of one X-linked recessive disease gene is masked by the disease gene's normal counterpart on her other X chromosome.

In classic X-linked inheritance males are affected, presenting full clinical symptoms of the disease. Females are usually not affected. Affected fathers can never pass X-linked diseases to their sons. However, affected fathers always pass X-linked disease genes to their daughters. Females who inherit the faulty gene but do not show the disease are known as carriers. Female carriers of X-linked EDMD have a 50% chance to pass the disease-causing gene to each of their children.

It is unusual for female carriers of an X-linked disease to show symptoms of the disease. In X-linked EDMD, carrier females can exhibit certain symptoms of the disease. Females have two X chromosomes in each of their body cells. Very early on in fetal development, one X chromosome in each cell of a female is inactivated. The pattern of inactivation is random, so carrier females may express the disease-causing gene in some of their cells. An estimated 10–20% of female carriers of X-linked EDMD display varying symptoms of the disease. Female carriers can display the dangerous heart symptoms of EDMD. Less commonly, carrier females may show late-onset muscle weakness.

In 1994 it was recognized that the X-linked recessive form of Emery-Dreifuss muscular dystrophy is caused by changes, or mutations, in a gene now known as EMD or STA. This gene is located on the long arm of the human X chromosome at a location designated as Xq28. The STA gene is approximately 2,100 base pairs in length. This gene codes for emerin, an amino acid protein.

Emerin is an important protein normally found on the inner nuclear membrane of skeletal, cardiac, and smooth muscle cells as well as in other tissues. Emerin is missing from the nuclear membranes of males affected with X-linked EDMD. Emerin is not altered in other neuromuscular disorders.