The parents of a 4 year old boy noticed that their son seemed uncoordinated and

Question

The parents of a 4 year old boy noticed that their son seemed uncoordinated and frequently unsteady on his feet. When given the choice, the toddler would rather crawl to get a toy instead of walking. Neither parent had a history of any relevant diseases but the child had been adopted at 6 months of age and there are no known histories of his biological parents.

On physical exam, the child displayed a weakness of most muscle groups. Laboratory tests were ordered and it was discovered that there were elevated levels of creatine kinase. As this pattern of findings is consistent with Duchenne muscular dystrophy, a muscle biopsy was taken and confirmed this diagnosis.

1. What is the underlying cause of Duchenne muscular dystrophy? How is it acquired?

2. Describe the sliding filament model for muscle contraction. How does the protein associated with Duchenne muscular dystrophy alter this?

3. Why is there a progressive muscle weakness?

4. How is this muscle weakness similar or different to muscle weakness of somebody who has been bed-ridden for a few months?

5. What is the prognosis and cure for Duchenne muscular dystrophy?

6. Why is creatine kinase elevated in the blood?

7. What are the genetics behind transmission of this disease from mother to son? Mother to daughter?

8. How is smooth muscle affected in someone with this disease?

9. How is cardiac muscle affected in someone with this disease ?

Explanation / Answer

1. DMD is caused due to absence of protein called as dystrophin which enables muscle cells to maintain integration. It is an X linked recessive disorder common in young boys due to defective or absence of synthesis of dystrophin which causes muscular damage.

2. In sliding filament model, during active contraction - calcium ions from sarcoplasmic reticulum enables activation of myosin heads & sliding of actin filaments over myosin due to myosin head ATPase activity. This results in thick bands causing muscle contraction. Muscle cells are intact & kept strong by dystrophin. In DMD, dystrophin deficiency leads to fragility of muscles & destruction of myofibrils.

3. Progressive muscle weakness is due to the pattern of muscles affected in DMD. Initially pelvic girdle, thighs, shoulder muscles are affected. In later stages, skeletal muscles of trunk, hands & legs are also affected.

4. DMD muscular weakness results in difficulty in squatting, getting up from sitting position, difficulty in climbing stairs, equilibrium maintenance etc.

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