Cap myopathy

Alternate names

Cap disease; Congenital myopathy with caps

Definition

Cap myopathy is a disorder that primarily affects skeletal muscles, the muscles that the body uses for movement.

Summary

• The name cap myopathy comes from characteristic abnormal cap-like structures that can be seen in muscle cells when muscle tissue is viewed under a microscope.
• The severity of cap myopathy is related to the percentage of muscle cells that have these caps.
• Individuals in whom 70 to 75 percent of muscle cells have caps typically have severe breathing problems and may not survive childhood, while those in whom 10 to 30 percent of muscle cells have caps have milder symptoms and can live into adulthood.

Epidemiology

Cap myopathy is a rare disorder that has been identified in only a small number of individuals. Its exact prevalence is unknown.

Cause

• Mutations in the ACTA1, TPM2, or TPM3 genes can cause cap myopathy.
• These genes provide instructions for producing proteins that play important roles in skeletal muscles.
• The ACTA1 gene provides instructions for making a protein called skeletal alpha (α)-actin, which is part of the actin protein family.
• Actin proteins are important for cell movement and the tensing of muscle fibers (muscle contraction).
• Thin filaments made up of actin molecules and thick filaments made up of another protein called myosin are the primary components of muscle fibers and are important for muscle contraction.
• Attachment (binding) and release of the overlapping thick and thin filaments allows them to move relative to each other so that the muscles can contract.
• The mutation in the ACTA1 gene that causes cap myopathy results in an abnormal protein that may interfere with the proper assembly of thin filaments. The cap structures in muscle cells characteristic of this disorder are composed of disorganized thin filaments.
• The TPM2 and TPM3 genes provide instructions for making proteins that are members of the tropomyosin protein family.
• Tropomyosin proteins regulate muscle contraction by attaching to actin and controlling its binding to myosin.
• The specific effects of TPM2 and TPM3 gene mutations are unclear, but researchers suggest they may interfere with normal actin-myosin binding between the thin and thick filaments, impairing muscle contraction and resulting in the muscle weakness that occurs in cap myopathy.

Inheritance

Autosomal dominant pattern, a 50/50 chance.

This condition follows an autosomal dominant manner of inheritance, however, most cases are not inherited; they result from new mutations in the gene and occur in people with no history of the disorder in their family.

Signs and symptoms

People with cap myopathy have muscle weakness (myopathy) and poor muscle tone (hypotonia) throughout the body, but they are most severely affected in the muscles of the face, neck, and limbs. The muscle weakness, which begins at birth or during childhood, can worsen over time.

Clinical presentation

For most diseases, symptoms will vary from person to person. People with the same disease may not have all the symptoms listed.

30%-79% of people have these symptoms

• High palate(Elevated palate)
• Long face(Elongation of face)
• Motor delay
• Reduced tendon reflexes

5%-29% of people have these symptoms

• Aortic root aneurysm(Bulge in wall of root of large artery that carries blood away from heart)
• Central hypoventilation
• Difficulty climbing stairs(Difficulty walking up stairs)
• Difficulty running
• Easy fatigability
• Facial palsy(Bell's palsy)
• Fatiguable weakness of proximal limb muscles
• Frequent falls
• Generalized amyotrophy(Diffuse skeletal muscle wasting)
• Generalized hypotonia(Decreased muscle tone)
• Gowers sign
• Increased variability in muscle fiber diameter
• Lower limb amyotrophy
• Lower limb muscle weakness(Lower extremity weakness)
• Lumbar hyperlordosis(Excessive inward curvature of lower spine)
• Mitral valve prolapse
• Nasal speech(Nasal voice)
• Pectus excavatum(Funnel chest)
• Pes planus(Flat feet)
• Pes valgus
• Poor head control
• Reduced systolic function
• Sinus tachycardia
• Thoracic scoliosis
• Toe walking(Toe-walking)

NIH genetic and rare disease info

• NIH info on Cap myopathy

Cap myopathy is a rare disease.

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