What is muscular dystrophy and what causes it?

Muscular dystrophy is a group of inherited muscle disorders characterised by gradually increasing weakness and degeneration of the muscles that control voluntary bodily movement. The most common type, Duchenne muscular dystrophy (about 95% of cases), results from a lack of the protein dystrophin required for proper muscle function.

Muscle tissue weakens and progressively wastes away and is replaced by connective and fatty tissue. In most cases, the arms, legs and spine become deformed.

Duchenne type affects mostly boys from the age of five and up. Types of muscular dystrophy are distinguished from one another both by the symptoms and severity and by the way they’re inherited.

There are eight other known forms of muscular dystrophy:

- Becker muscular dystrophy. The onset may be as late as young adulthood and symptoms are less severe than with the Duchenne type, but similar. Life expectancy may even be normal.
- Limb-girdle muscular dystrophy may only show up in adulthood, when problems in terms of walking or lifting the arms become noticeable.
- Facioscapulohumeral muscular dystrophy. The onset may only occur in adulthood and life span may be normal. Early warning signs include lack of facial mobility.
- Distal muscular dystrophy.
- Emery-Dreifuss muscular dystrophy.
- Fukuyama congenital muscular dystrophy.
- Oculopharyngeal muscular dystrophy.
- Ophthalmoploegic muscular dystrophy.

What are the causes?
Chemically speaking, the two most common forms of muscular dystrophy, Duchenne and Becker, are caused by a genetically inherited deficiency of the dystrophin protein (in the most common type).

Other less common types for which the causes are less well understood are caused by faulty segments in the genes, which result in the lack of other vital proteins.

To understand the causes in broader terms requires an understanding of genetic inheritance and how gender- or sex-linked dominant genes or recessive genes affect our physical characteristics.

How genetic inheritance of muscular dystrophy works
Muscular dystrophy is a result of inherited faults in genes.

When a foetus is formed, he or she receives 23 chromosomes – tiny threadlike structures that carry genes – from each parent, making a total of 46. These chromosomes pair off, each carrying genes for the same purpose. A total of 22 of these pairs or genes are identical in males and females. The composition of the 23rd pair determines the sex of the unborn child. If the pair consists of one "X" chromosome from the mother and another "X" from the father, the child will be female. If the pair is made up of an "X" from the mother and a "Y" chromosome from the father, the child will be male.

The genes packed together in precise locations on these chromosomes may, for only partly understood reasons, be flawed or lost. When this is the case, a genetic disorder – one of which is muscular dystrophy – may result.

Who risks inheriting muscular dystrophy?
Because the faulty gene may be located in the X or Y chromosome, the gender of the child is relevant in assessing the risk inheritance for all members of the extended family. The way that the disorder is inherited may vary from one type of muscular dystrophy to another.

Types include:
Autosominal dominant inheritance:
It doesn’t skip generations because the genetic errors may be present in any one of the 46 chromosomes that carry the genes, except for the sex chromosomes. Only one parent needs to transmit the abnormal gene, so the child has a 50% chance of inheriting the disorder.

Types of muscular dystrophy inherited this way include faciocapulohumeral dystrophy and myotonic dystrophy.

Autosominal recessive inheritance:
Disorders inherited this way usually appear in only one generation – there may be no family history of genetic disorders as the parent who carries the gene shows no symptoms of illness. There is a 25% chance that each child of the affected couple will receive the faulty recessive gene and be affected, and a 50% chance that they will become a carrier of the disorder. Limb-girdle muscular dystrophy is inherited this way.

X-linked recessive inheritance:
The disorder is carried by the X chromosome. When the mother is the carrier, boys have a 50% chance of being affected and girls a 50% chance of being carriers. Men who are carriers of this type of disorder cannot pass it on to their sons (as they only provide the Y chromosome for a male baby), but all daughters will be carriers.

The most common forms of muscular dystrophy, Duchenne and Becker types, are inherited via the X chromosome.

Not all genetic disorders are initially inherited. Gene mutation may occur unpredictably, apparently at random, and develop an altered or faulty characteristic. This creates an inheritable form of muscular dystrophy.

Some research indicates that about a third of boys who develop Duchenne muscular dystrophy develop it as a result of an apparently spontaneous gene mutation.

Autosominal dominant inheritance:
Disorders inherited this way don’t skip generations and children have a 50% chance of inheriting the disorder.
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