DYSTROPHIN IN OTHER TISSUES
Research in the past decade has revealed that dystrophin is not confined to voluntary muscle. It's also present in another type of muscle called involuntary, or smooth, muscle, and in the heart, brain and retina (tissue at the back of the eyes).
In each of these tissues, the dystrophin protein has a slightly different form from the voluntary muscle form, although it's still clearly recognizable as dystrophin.
The different forms, many of which are shorter than voluntary muscle dystrophin, exist because of different ways the cell can process the DNA in the large dystrophin gene.
You may be able to guess by now that not all mutations affect the dystrophin gene and dystrophin protein forms in precisely the same way. All boys with DMD have something wrong with the voluntary muscle form of dystrophin -- or else they wouldn't carry a diagnosis of muscular dystrophy. But the precise way in which this protein is changed, or mutated, and the extent to which other tissue forms of dystrophin are affected, depends at least somewhat on the nature and location of the genetic mutation in each person. Other factors, such as how the protein is put together after the first steps in DNA processing, also play a role in determining which tissues are affected and to what extent.
Involuntary muscles, which are known as smooth muscles because of their microscopic structure, aren't usually affected in boys with DMD. These muscles are found throughout the digestive system, where they move food along, and in the urinary tract, as well as in the blood vessels and some other organs. We don't consciously control the action of the involuntary muscles.
Boys with DMD almost always have normal bladder and bowel control, but may have other gastrointestinal problems. Some doctors have speculated that some of the constipation and digestive problems sometimes seen in boys with DMD may be related to lack of dystrophin in the smooth muscles of the intestines, but they could just as easily have to do with a boy's immobility and lack of voluntary muscle action in the abdomen. In general, there aren't many problems reported with smooth muscles in children with DMD. The reason may be that there aren't many mutations in the dystrophin gene that affect smooth muscle dystrophin, or it may be that dystrophin's role in smooth muscle isn't nearly as crucial as it is in voluntary muscle.
Dystrophin's role in the heart is, however, a different story. The heart is also a muscle, and it has some things in common with both involuntary and voluntary muscle. It really belongs to neither category, but it does need dystrophin. Almost all boys with DMD have some heart muscle abnormalities, and many experts believe these result from mutations that affect the heart muscle form of dystrophin. Some boys are clearly more severely affected than others, which may be related to the location of the mutation and its nature, or to other aspects of DNA processing. (For more on cardiac problems in DMD, see "Cardiac Problems are Common.")
Recent reports show that there are boys with heart problems whose "muscular dystrophy" never attracts any attention because it's so mild, but who, when tested for dystrophin abnormalities on the advice of an astute cardiologist, are shown to have them. This isn't really DMD, but a dystrophin-related heart muscle disorder.
Dystrophin has been found in several areas in the brain, although research concerning its role here is in its infancy. Some experts believe its main function is to help with a subtle aspect of memory formation. Some boys with DMD seem to have no difficulties at all with learning or memory formation, while others have significant impairment in these areas. Whether or not brain dystrophin is affected and to what extent is probably related to the specific genetic mutation and other aspects of DNA processing. (For more on learning disabilities in DMD, see "Learning Disabilities".)
As with heart problems, there are now a few reports of boys with dystrophin abnormalities whose main problem is related to brain, rather than muscle, function. This isn't DMD, but rather a dystrophin-related neurological disorder.
There is also dystrophin in the retina, but no particular vision problems have been associated with DMD. Some studies have shown abnormalities in the retina if it's subjected to special kinds of testing. It's possible that retinal dystrophin isn't crucial to the eye's function.
|
