The major cause of cardiomyopathy in DMD and BMD is probably a lack in the necessary protein dystrophin, the same necessary protein deficiency that exists in the skeletal muscle tissues and qualified prospects to generalized weakness, wasting and respiratory complications. Dystrophin is also required by cardiac muscle, and its lack (complete in DMD and partial in BMD) probably qualified prospects to the loss of cardiac muscle tissue below the pressure of constant contraction.
When heart difficulties do occur in other forms of MD, such as LGMD or congenital, it’s likely that the underlying cause is also a lack of the muscle protein. In four forms of LGMD, for example, the sarcoglycan proteins are known to lie close to dystrophin in the muscle cell membrane, and their absence could possibly be a factor in any heart issue in all those disorders.
But the story is a little far more complicated. For instance, most individuals with DMD and BMD have heart involvement, but some don’t. And, in the sarcoglycan-deficient forms of LGMD, heart involvement is unusual, despite the probable part in the sarcoglycans in heart muscle.
The solutions to these puzzles may lie in the heart muscle’s differences from skeletal muscle. heart muscle has a slightly different structure, and its proteins often arrive in the slightly different form from their “cousins” in skeletal muscle. These slightly differing proteins, known as isoforms, arrive from your similar gene. (Each gene is a “recipe” for a protein.) Some genetic mutations, such as all those in Duchenne, Becker and limb-girdle MD, probably possess a preference for the skeletal or cardiac form in the protein, leading to far more or less heart involvement.
Then, too, necessary protein deficiencies are probably not the whole story in cardiac dysfunction in neuromuscular disease. For example, blood flow through the tissues and the return of blood to the heart are compromised in a person who can’t exercise, so different exercise capabilities may be also a factor in the degree of cardiac involvement.