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Myostatin-related muscle hypertrophy is characterized by reduced subcutaneous fat pad thickness and increased muscle size in individuals with normal or increased muscle strength. Both heterozygotes and homozygotes for a mutation in MSTN encoding the protein growth differentiation factor 8 (myostatin) can exhibit muscle hypertrophy. Clinical manifestations depend on the amount of myostatin protein present. An infant homozygous for a MSTN mutation had muscle mass twice that of sex- and age-matched controls; intellect and cardiac function were normal. He displayed stimulus-induced myoclonus that subsided after two months. Heterozygotes may have increased muscle bulk and strength, but to a lesser degree.
Skeletal muscle size in an individual with myostatin-related muscle hypertrophy is measured by ultrasound examination, DEXA, or MRI. Subcutaneous fat pad thickness is measured by ultrasound or with a caliper. Molecular genetic testing for MSTN, the only gene known to be associated with myostatin-related muscle hypertrophy, is available on a research basis.
The phenotypes associated with myostatin-related muscle hypertrophy are inherited in an incomplete autosomal dominant manner. At conception, the sibs of a child with homozygous myostatin-related muscle hypertrophy have a 25% chance of having homozygous myostatin-related muscle hypertrophy, a 50% chance of having one MSTN mutation with or without increased muscle mass, and a 25% chance of having normal muscle mass and no MSTN mutations. Heterozygotes may have increased muscle mass. Individuals diagnosed with heterozygous myostatin-related muscle hypertrophy may have a parent with the MSTN mutation who may have increased muscle mass, or the proband may have the condition as the result of a new gene mutation. The proportion of cases caused by a de novo mutation is unknown. The chance that sibs of a proband with heterozygous myostatin-related muscle hypertrophy will inherit the MSTN mutation is 50% if a parent has increased muscle mass or has a MSTN mutation. Each child of an individual with heterozygous myostatin-related muscle hypertrophy has a 50% chance of inheriting the MSTN mutation.