Muscular dystrophy is one of the important hereditary muscle diseases. It is characterized by the progressive weakness of the skeletal muscles. It is caused by several factors, like death of muscle cells or tissues, and muscular protein defect.
There are about nine diseases are classified under muscular dystrophy. These are :-
- Duchenne muscular dystrophy,
- Becker muscular dystrophy,
- limb girdle muscular dystrophy,
- congenital muscular dystrophy,
- facioscapulohumeral muscular dystrophy,
- myotonic muscular dystrophy,
- oculopharyngea muscular dystrophy,
- distal muscular dystrophy and
- Emer-Dreifuss muscular dystrophy.
Here we will only talk about Duchenne muscular dystrophy and its genetic basis.
Duchenne Muscular Dystrophy
It is also genetically based X-linked recessive form of disease. The gene is present on the short arm of X-chromosome. It is called DMD gene ( Duchenne muscular dystrophy). It is largest known human gene so far. It is characterized by muscular degeneration, difficult breathing, problem in walking and sometime leading to death. Its occurrence ratio in new born babies is 1/ (3500-5000).
It affects males mostly, in female it is only act as carrier. Because I said earlier it is X-linked recessive disease, so it is necessary to the presence of both the alleles on the chromosome of the females (XX) that is rare. So if one allele is present on the chromosome of females it cannot cause disease, but remain carrier for the disease in its offspring especially males. It is due to the presence of only one X chromosome in male (XY), so it does not require having second allele to be expressed. That’s why males are mostly affected then females and male never act as carrier. The occurrence of disease from carrier females in next generation is given below;
- ½ chances to affect the male offspring.
- ½ chances to make carrier to his female offspring.
- No chance to affect the female offspring.
In males one third (1/3) have previous relative with Duchenne muscular dystrophy. Remaining two third (2/3) cases are infrequent or sporadic. There are two further groups that make up the sporadic cases. Half of the sporadic cases are due to genetic mutation in the mother’s egg or early mutation in the embryo development that cause the condition. In other half of sporadic cases mother is carrier, but there is new mutation carried by mother that occurred in her mother’s egg, or may be in her father’s sperm or early stages of her development. This explains the reason of disease in those kids, whose previous family was absolutely free from this disease condition.
Actually mutation occurs in dystrophin gene present on X chromosome. This gene is responsible for the production of a protein named as dystrophin. This protein is main structural component of muscle tissues.
In affected males symptoms usually begins to appear in 2-3 year of age. Delayed development is a milestone. Young male kid cannot sit up and crawls like other normal kids. When he begins to walk, it is usually seen that it is more difficult as time goes on. Walking on their toes is the characteristic gait shown by such kids. Muscles begin to replace with fat and connective tissue. Finally muscle become weak and male kids may appear awkward and fall frequently.