Facioscapulohumeral Muscular ... Health Article

Definition

The term muscular dystrophy refers to a group of conditions characterized by progressive muscle weakness and atrophy (deterioration). Many different types of muscular dystrophy have been described, each of which have unique features and usually a unique underlying genetic cause. Facioscapulohumeral (FSH) muscular dystrophy affects the muscles of the face and shoulders first. Usually the first signs of weakness appear before the age of 20 years. The symptoms of FSH muscular dystrophy are variable and are not fatal. One in five people who are affected require a wheelchair after the age of 40 years.

Description

Facio refers to the face, scapulo to the shoulder blades, and humeral to the bone of the upper arm. The symptoms of FSH muscular dystrophy are quite variable, even within the same family. Some individuals who have the altered DNA sequence never develop noticeable symptoms. Most people with the condition first notice weakness in their teenage years. Muscles of the shoulders and face are usually the first to be affected. These may remain the only parts of the body that are affected, or the weakness may progress to include the pelvic muscles, the lower limbs, and the hands. Intelligence and life expectancy are not affected.

Genetic profile

FSH muscular dystrophy has autosomal dominant inheritance. This means that an affected person has a 50% chance, with each pregnancy, to pass the altered gene on to the child. Every person has two copies of every DNA sequence, one inherited maternally and the other inherited paternally. The altered DNA sequence that causes FSH muscular dystrophy is on chromosome 4. If a person has one normal sequence and one altered sequence, he or she will probably develop FSH muscular dystrophy.

When an autosomal dominant condition is present in multiple generations of a family, usually someone from each generation is affected. If a person is the first in his or her family to have an autosomal dominant condition, doctors often assume that the gene mutated for the first time in the egg or sperm that came together to make that person. (This is called a new mutation.) However, when the physical symptoms associated with an altered gene are highly variable, the distinction between these two scenarios is less obvious.

The term nonpenetrance refers to altered genes that do not always cause a person to have the typical associated symptoms. FSH muscular dystrophy is nonpenetrant in some individuals. Therefore, an individual who appears to be the first person affected in his or her family may have actually inherited the mutated DNA sequence from his or her mother or father. If so, his or her siblings would be at a 50% risk to also have inherited the altered sequence. Similarly, a mildly affected individual may have a child who is severely affected. Occasionally, two affected siblings are born to unaffected parents because of a genetic process called germline mosaicism.

Describing the genetics of FSH muscular dystrophy is slightly complicated by an interesting phenomenon. Genes are the DNA sequences that give the body instructions for growth, development, and functioning. Usually a mutation that causes a disease occurs in the gene associated with that disease. The above description refers to the mutation in FSH muscular dystrophy as an altered DNA sequence because it does not appear that this sequence is actually part of a gene. The mutated sequence affects the gene for FSH muscular dystrophy, but probably is not part of the gene itself.