Nemaline myopathy is a disorder that primarily affects skeletal muscles, which are muscles that the body uses for movement. People with nemaline myopathy have muscle weakness (myopathy) throughout the body, but it is typically most severe in the muscles of the face; neck; trunk; and other muscles close to the center of the body (proximal muscles), such as those of the upper arms and legs. This weakness can worsen over time. Affected individuals may have feeding and swallowing difficulties, foot deformities, abnormal curvature of the spine (scoliosis), and joint deformities (contractures). Most people with nemaline myopathy are able to walk, although some affected children may begin walking later than usual. As the condition progresses, some people may require wheelchair assistance. In severe cases, the muscles used for breathing are affected and life-threatening breathing difficulties can occur.

Nemaline myopathy is divided into six types. In order of decreasing severity, the types are: severe congenital, Amish, intermediate congenital, typical congenital, childhood-onset, and adult-onset. The types are distinguished by the age when symptoms first appear and the severity of symptoms; however, there is overlap among the various types. The severe congenital type is the most life-threatening. Most individuals with this type do not survive past early childhood due to respiratory failure. The Amish type solely affects the Old Order Amish population of Pennsylvania and is typically fatal in early childhood. The most common type of nemaline myopathy is the typical congenital type, which is characterized by muscle weakness and feeding problems beginning in infancy. Most of these individuals do not have severe breathing problems and can walk unassisted. People with the childhood-onset type usually develop muscle weakness in adolescence. The adult-onset type is the mildest of all the various types. People with this type usually develop muscle weakness between ages 20 and 50.

Mutations in one of many genes can cause nemaline myopathy. These genes provide instructions for producing proteins that play important roles in skeletal muscles. Within skeletal muscle cells, these proteins are found in structures called sarcomeres. Sarcomeres are necessary for muscles to tense (contract). Many of the proteins associated with nemaline myopathy interact within the sarcomere to facilitate muscle contraction. When the skeletal muscle cells of people with nemaline myopathy are stained and viewed under a microscope, these cells usually appear abnormal. These abnormal muscle cells contain rod-like structures called nemaline bodies.

Most cases of nemaline myopathy with a known genetic cause result from mutations in one of two genes, NEB or ACTA1. NEB gene mutations account for about 50 percent of all cases of nemaline myopathy and ACTA1 gene mutations account for 15 to 25 percent of all cases. When nemaline myopathy is caused by NEB gene mutations, signs and symptoms are typically present at birth or beginning in early childhood. When nemaline myopathy is caused by ACTA1 gene mutations, the condition's severity and age of onset vary widely. Mutations in the other genes associated with nemaline myopathy each account for only a small percentage of cases.

Mutations in any of the genes associated with nemaline myopathy lead to disorganization of the proteins found in the sarcomeres of skeletal muscles. The disorganized proteins cannot interact normally, which disrupts muscle contraction. Inefficient muscle contraction leads to muscle weakness and the other features of nemaline myopathy.

Some individuals with nemaline myopathy do not have an identified mutation. The genetic cause of the disorder is unknown in these individuals.

Normal Function

The NEB gene provides instructions for making a protein called nebulin. This protein plays an important role in skeletal muscles, which are muscles used for movement. Within skeletal muscle cells, nebulin is found in structures called sarcomeres. Sarcomeres are necessary for muscles to tense (contract). Nebulin is one of several proteins that interact to generate the mechanical force needed for muscle contraction.

Health Conditions Related to Genetic Changes

Nemaline myopathy

More than 60 mutations in the NEB gene have been found to cause nemaline myopathy. Some of these mutations change single protein building blocks (amino acids) in the nebulin protein or result in the production of an abnormally short protein. Other mutations insert or delete genetic material in the NEB gene. Most NEB gene mutations alter the structure or reduce the production of nebulin. A lack of functional nebulin impairs the muscle cells' ability to contract. Inefficient muscle contraction leads to muscle weakness and the other features of nemaline myopathy.

Other Names for This Gene

• NEBU_HUMAN
• NEM2

Genomic Location

Normal Function

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). These proteins also help maintain the cytoskeleton, which is the structural framework that determines cell shape and organizes cell contents.

Skeletal α-actin plays an important role in skeletal muscles, which are muscles that the body uses for movement. Within skeletal muscle cells, skeletal α-actin is an essential component of structures called sarcomeres. Sarcomeres are composed of thin filaments made up of actin and thick filaments made up of another protein called myosin. 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.

Health Conditions Related to Genetic Changes

Actin-accumulation myopathy

At least nine variants (also called mutations) in the ACTA1 gene have been identified in people with actin-accumulation myopathy. Most of these variants change single protein building blocks (amino acids) in the skeletal α-actin protein sequence.

Researchers suggest that ACTA1 gene variants that cause actin-accumulation myopathy may affect the way the actin binds to ATP. ATP is a molecule that supplies energy for cells' activities and is important in the formation of thin filaments from individual actin molecules. Dysfunctional actin-ATP binding may result in abnormal thin filament formation and impair muscle contraction, leading to muscle weakness and the other signs and symptoms of actin-accumulation myopathy.

Cap myopathy

At least one ACTA1 gene variant has been identified as a cause of cap myopathy. The variant replaces the amino acid methionine with the amino acid valine at position 47 in the protein sequence, written as Met47Val or M47V. The resulting abnormal protein may interfere with the proper assembly of thin filaments. Cap myopathy is characterized by the presence of cap-like structures in muscle cells, and these structures are composed of disorganized thin filaments. The abnormal filament structure likely impairs the ability of skeletal muscles to contract, resulting in muscle weakness and the other signs and symptoms of cap myopathy.

Congenital fiber-type disproportion

At least seven variants in the ACTA1 gene have been found to cause congenital fiber-type disproportion, a disorder that causes general muscle weakness that typically does not worsen over time. The variants that cause this condition change single amino acids in skeletal α-actin. These variants lead to the production of an abnormal actin protein, which interferes with the function of normal actin proteins in the sarcomere. As a result, the function of the sarcomere is impaired, which disrupts muscle contraction. Inefficient muscle contraction leads to muscle weakness in people with congenital fiber-type disproportion.

Intranuclear rod myopathy

At least 13 variants in the ACTA1 gene have been identified in people with intranuclear rod myopathy. These variants change single amino acids in the skeletal α-actin protein sequence.

ACTA1 gene variants that cause intranuclear rod myopathy result in rod-shaped accumulations of actin in the nucleus of muscle cells. Normally, most actin is found in the fluid surrounding the nucleus (the cytoplasm), with small amounts in the nucleus itself. Researchers suggest that the ACTA1 gene variants that cause intranuclear rod myopathy may interfere with the normal transport of actin between the nucleus and the cytoplasm, resulting in the accumulation of actin in the nucleus and the formation of intranuclear rods. Abnormal accumulation of actin in the nucleus of muscle cells and a corresponding reduction of available actin in muscle fibers may impair muscle contraction and lead to the muscle weakness seen in intranuclear rod myopathy.

A few ACTA1 gene variants that have been identified in people with intranuclear rod myopathy have also been found in people with actin-accumulation myopathy (described above). It is unclear how the same variants can cause two different conditions.

Nemaline myopathy

More than 170 variants in the ACTA1 gene have been found to cause nemaline myopathy. Nemaline myopathy is the most common muscle disorder associated with ACTA1 gene variants. Some of the variants that cause this disorder alter the structure or function of skeletal α-actin, causing the protein to cluster together and form clumps (aggregates). These aggregates interfere with the normal functioning of muscle cells. Other ACTA1 gene variants prevent the production of any skeletal α-actin, impairing the muscle cells' ability to contract. ACTA1 gene variants that cause nemaline myopathy impair muscle contraction, causing weakness and the other features of this condition.

Other Names for This Gene

• ACTA
• ACTS_HUMAN
• alpha skeletal muscle actin
• ASMA

Genomic Location

Nemaline myopathy is usually inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Less often, this condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. Most cases result from new mutations in the gene and occur in people with no history of the disorder in their family.

Nemaline myopathy has an estimated incidence of 1 in 50,000 individuals.