Inclusion body myopathy 2 is a condition that primarily affects skeletal muscles, which are muscles that the body uses for movement. This disorder causes muscle weakness that appears in late adolescence or early adulthood and worsens over time.

The first sign of inclusion body myopathy 2 is weakness of a muscle in the lower leg called the tibialis anterior. This muscle helps control up-and-down movement of the foot. Weakness in the tibialis anterior alters the way a person walks and makes it difficult to run and climb stairs. As the disorder progresses, weakness also develops in muscles of the upper legs, hips, shoulders, and hands. Unlike most forms of myopathy, inclusion body myopathy 2 usually does not affect the quadriceps, which are a group of large muscles at the front of the thigh. This condition also does not affect muscles of the eye or heart, and it does not cause neurological problems. Weakness in leg muscles makes walking increasingly difficult, and most people with inclusion body myopathy 2 require wheelchair assistance within 20 years after signs and symptoms appear.

People with the characteristic features of inclusion body myopathy 2 have been described in several different populations. When the condition was first reported in Japanese families, researchers called it distal myopathy with rimmed vacuoles (DMRV) or Nonaka myopathy. When a similar disorder was discovered in Iranian Jewish families, researchers called it rimmed vacuole myopathy or hereditary inclusion body myopathy (HIBM). It has since become clear that these conditions are variations of a single disorder caused by mutations in the same gene.

GNE Myopathy is a rare ( autosomal recessive ) genetic disorder that causes progressive skeletal muscle atrophy and weakness. Previous names include hereditary inclusion body myopathy (HIBM), inclusion body myopathy type 2 (IBM2) or Nonaka myopathy.

Symptoms of the disease usually appear between 20 and 40 years of age and include foot drop and difficulty walking. The disease slowly affects other muscles of the arms and legs. Patients typically start using a wheelchair one or two decades later and eventually some patients may need assistance with activities of daily living.

GNE Myopathy results from variants in a gene called GNE, which is responsible for a step in the production of a sugar called sialic acid. The diagnosis may be suggested by the presence of red-rimmed vacuoles (inclusions) in muscle biopsy tissue. Identification of GNE gene disease-causing variants by DNA sequencing can confirm the diagnosis.

GNE Myopathy patients may go undiagnosed or misdiagnosed with a different disease for many years. There are ~2,000 patients known worldwide and ~200 in the United States. However, we estimate (based on examining DNA sequencing databases) that there are at least 40,000 GNE Myopathy patients worldwide, including ~13,000 in Asia (~750 in Japan), ~4,000 in Europe and ~3,000 in North America, meaning that many patients are undiagnosed.

No therapies are currently approved for GNE Myopathy.

Mutations in the GNE gene cause inclusion body myopathy 2. The GNE gene provides instructions for making an enzyme found in cells and tissues throughout the body. This enzyme is involved in a chemical pathway that produces sialic acid, which is a simple sugar that attaches to the ends of more complex molecules on the surface of cells. By modifying these molecules, sialic acid influences a wide variety of cellular functions including cell movement (migration), attaching cells to one another (adhesion), signaling between cells, and inflammation.

The mutations responsible for inclusion body myopathy 2 reduce the activity of the enzyme produced from the GNE gene, which decreases the production of sialic acid. As a result, less of this simple sugar is available to attach to cell surface molecules. Researchers are working to determine how a shortage of sialic acid leads to progressive muscle weakness in people with inclusion body myopathy 2. Sialic acid is important for the normal function of many different cells and tissues, so it is unclear why the signs and symptoms of this disorder appear to be limited to the skeletal muscles.

Normal Function

The GNE gene provides instructions for making an enzyme that is found in cells and tissues throughout the body. This enzyme plays a key role in a chemical pathway that produces sialic acid, which is a simple sugar that attaches to the ends of more complex molecules on the surface of cells. By modifying these molecules, sialic acid influences a wide variety of cellular functions including cell movement (migration), attaching cells to one another (adhesion), signaling between cells, and inflammation.

The enzyme produced from the GNE gene is responsible for two steps in the formation of sialic acid. It first converts a molecule known as UDP-GlcNAc to a similar molecule called ManNAc. In the next step, the enzyme transfers a cluster of oxygen and phosphorus atoms (a phosphate group) to ManNAc to create ManNAc-6-phosphate. Other enzymes then convert ManNAc-6-phosphate to sialic acid.

Health Conditions Related to Genetic Changes

Inclusion body myopathy 2

More than 40 mutations in the GNE gene have been identified in people with inclusion body myopathy 2. Most of these mutations change single protein building blocks (amino acids) in several regions of the enzyme. A few mutations delete a piece of the enzyme or otherwise alter its structure.

Different GNE mutations cause inclusion body myopathy 2 in different populations. One mutation causes the disorder in people of Iranian Jewish heritage; this genetic change replaces the amino acid methionine with the amino acid threonine at position 712 in a region of the enzyme known as the kinase domain (written as Met712Thr or M712T). In the Japanese population, where the condition is called Nonaka myopathy, the most common GNE mutation replaces the amino acid valine with the amino acid leucine at position 572 in the enzyme's kinase domain (written as Val572Leu or V572L).

The mutations responsible for inclusion body myopathy 2 reduce the activity of the enzyme produced from the GNE gene, which decreases the production of sialic acid. As a result, less of this simple sugar is available to attach to cell surface molecules. Researchers are working to determine how a shortage of sialic acid leads to progressive muscle weakness in people with inclusion body myopathy 2. Sialic acid is important for the normal function of many different cells and tissues, so it is unclear why the signs and symptoms of this disorder appear to be limited to the skeletal muscles.

Sialuria

Several mutations in the GNE gene have been found to cause sialuria. Each of these mutations changes a single amino acid in a region of the enzyme known as the allosteric site. This region is critical for the normal regulation of the enzyme.

The enzyme produced from the GNE gene is carefully controlled to ensure that cells produce an appropriate amount of sialic acid. A feedback system shuts off the enzyme when no more sialic acid is needed. Mutations in the allosteric site disrupt this feedback mechanism, resulting in an overproduction of sialic acid. This simple sugar builds up within cells and is excreted in urine. Researchers are working to determine how an accumulation of sialic acid in the body interferes with normal development in people with sialuria.

Other Names for This Gene

• Bifunctional UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase
• DMRV
• GLCNE
• GLCNE_HUMAN
• IBM2
• N-acylmannosamine kinase
• Uae1
• UDP-GlcNAc-2-epimerase/ManAc kinase
• UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase

Genomic Location

This condition is 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.

Inclusion body myopathy 2 causes muscle weakness that appears in late adolescence or early adulthood and worsens over time. The first sign of inclusion body myopathy 2 is often weakness of the tibialis anterior, a muscle in the lower leg that helps control up-and-down movement of the foot. Weakness in the tibialis anterior alters the way a person walks and makes it difficult to run and climb stairs. As the disorder progresses, weakness also develops in muscles of the upper legs, hips, shoulders, and hands. Unlike most forms of myopathy, inclusion body myopathy 2 usually does not affect the quadriceps (a group of large muscles at the front of the thigh). This condition also spares muscles of the eye or heart, and does not cause neurological problems. Weakness in leg muscles makes walking increasingly difficult, and most people with inclusion body myopathy 2 require wheelchair assistance within 20 years after signs and symptoms appear.

Currently, there is no cure and no way to prevent the progression of a Inclusion body myopathy 2. Treatment is focused on managing individual symptoms. People with this condition are often evaluated and managed by a multidisciplinary team including neurologists and physiatrists, as well as physical and occupational therapists.

More than 200 people with inclusion body myopathy 2 have been reported. Most are of Iranian Jewish descent; the condition affects an estimated 1 in 1,500 people in this population. Additionally, at least 15 people in the Japanese population have been diagnosed with this disorder. Inclusion body myopathy 2 has also been found in several other ethnic groups worldwide.