Causes of McArdle Disease

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For exercising muscles, one of the important energy sources is anaerobic glycolysis, which makes use of blood glucose, which will be converted to glucose-6-P inside the muscle cells. The final product of anaerobic glycolysis is lactate.

The phosphorylase, glycogen, muscle (PYGM) gene provides instructions for making an enzyme called myophosphorylase (M-GP). This enzyme breaks down a complex sugar called glycogen.


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Where is the PYGM gene located?

Cytogenetic Location: 11q12-q13.2

Molecular Location on chromosome 11: base pairs 64,746,388 to 64,760,714

The PYGM gene is located on the long (q) arm of chromosome 11 between positions 12 and 13.2.

More precisely, the PYGM gene is located from base pair 64,746,388 to base pair 64,760,714 on chromosome 11.

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Myophosphorylase is one of three related enzymes called glycogen phosphorylases that break down glycogen in cells. Myophosphorylase is found only in muscle cells, where it breaks down glycogen into a simpler sugar called glucose-1-phosphate. Additional steps convert glucose-1-phosphate into glucose, a simple sugar that is the main energy source for most cells.

The most common cause of McArdle's Disease is due to a nonsense or missense mutation in the PYGM gene that is coding for myophosphorylase. As a result, muscle cells cannot produce enough energy, so muscles become easily fatigued. The most common form of mutation is the nonsense mutation at the 49th codon (R49X).

The essential reaction for converting glycogen into glucose is as shown in the image below.

As patients suffering from McArcle's Disease is unable to use the glycogen stored in his muscle to obtain energy, due to the lack of muscle glycogen phosphorylase to convert glycogen into glucose-1-P, Due to their inability to degrade glycogen, it will accumulate in the muscle tissues and hence the other name Glycogen Storage Disease Type V. The patient will present fatigue much earlier in tests and the concentration of lactate in the blood is much lower than normal. However, blood glucose levels are not affected because muscle glycogen does not buffer blood glucose.

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