GNE myopathy
What is GNE myopathy?
GNE myopathy, also known as hereditary inclusion body myopathy (HIBM) type 2, is a rare disease that causes progressive muscle weakness in the muscles of the lower legs and the feet. As the disease progresses, individuals may experience difficulty walking and may require a wheelchair. Initial symptoms include an abnormal gait and tripping frequently due to “foot drop” (difficulty lifting the front of the foot). The muscle weakness eventually spreads to the hands and thigh muscles as well, and rarely the quadriceps. GNE myopathy is an inherited rare disease; and GNE is an abbreviation for the mutated gene that causes the disorder. It is an autosomal recessive disorder, which means that both parents must carry a copy of the mutated GNE gene in order for the disease to develop. The disease is mostly diagnosed in the late teens to early adult years. Symptoms typically do not appear until a person is well into their thirties or later.
GNE myopathy, also known as hereditary inclusion body myopathy (HIBM) type 2, is a rare disease that causes progressive muscle weakness in the muscles of the lower legs and the feet. As the disease progresses, individuals may experience difficulty walking and may require a wheelchair. Initial symptoms include an abnormal gait and tripping frequently due to “foot drop” (difficulty lifting the front of the foot). The muscle weakness eventually spreads to the hands and thigh muscles as well, and rarely the quadriceps. GNE myopathy is an inherited rare disease; and GNE is an abbreviation for the mutated gene that causes the disorder. It is an autosomal recessive disorder, which means that both parents must carry a copy of the mutated GNE gene in order for the disease to develop. The disease is mostly diagnosed in the late teens to early adult years. Symptoms typically do not appear until a person is well into their thirties or later.
Globally, more than 200 individuals with GNE myopathy have been identified and 160 people were of Iranian Jewish descent. Although first described in Israel and Japan, GNE myopathy occurs worldwide in all ethnic groups. While GNE myopathy affects 1 in 1 million people worldwide, the prevalence rate for Iranian Jews is between 1:500 and 1:1000.
GNE myopathy only manifests if both parents carry a copy of the mutated GNE gene, and they both must pass the mutated gene on to their child.
The GNE gene encodes for an enzyme that is active in various tissues and cells of the body. The main function of this enzyme is its involvement in the production of sialic acid, a simple sugar that attaches to complex molecules on the surface of cells. When the gene is mutated, there are deficient levels of the enzyme, which leads to diminished production of sialic acid. The exact reason that diminished sialic acid production ultimately leads to the symptoms of GNE myopathy is not fully understood.
Recent research has found a new gene called HRPC1 that may also be linked to the disease.
The most identifiable symptom of this disease is muscle weakness in the distal (lower) limb muscles. Affected individuals initially have difficulty walking because of “foot drop” (difficulty lifting the front part of the foot). This muscle weakness eventually spreads to the hands and thighs within a few years. For many affected individuals, the four muscles on the front of the thighs (quadriceps) are unaffected or spared for a long time. The eyes, pharynx, and heart muscles are most likely unaffected as well. There is no obvious signs of intellectual disability. The severity of muscle weakness and the rate of progression of the disorder can vary from one person to another.
There are certain attributes physicians look for when diagnosing GNE myopathy. The disorder is identified as a primary skeletal muscle disease. Another distinguishing characteristic of GNE myopathy is that while there is weakness in all other proximal lower-extremity muscles, the quadriceps are usually unaffected. If the affected individual is a teenager or young adult, it increases the possibility of the disorder being GNE myopathy.
Blood tests, electromyogram, muscle biopsy and molecular genetic testing can all be used to diagnose GNE myopathy. A blood test measures the levels of creatine kinase. Affected individuals typically have a serum creatine kinase level that is two to four times the normal range. An electromyogram records the electrical activity of muscles. During a muscle biopsy, a small sample of muscle tissue is surgically removed and studied under a microscope to detect characteristic changes that indicate GNE myopathy. Molecular genetic testing can confirm a diagnosis by identifying a mutation in the GNE gene that is known to cause the disorder.
There is no cure for GNE myopathy. Individuals should follow up with a multidisciplinary team including neurologists, geneticists, physiatrists, physical therapist, and occupational therapists. Affected individuals should be screened for cardiac and respiratory dysfunction. They may also be fitted with a rigid splint or similar device to help with abnormal gait.
Because of this disease, the arms and legs of an affected individual becomes progressively impaired. Many are wheelchair-bound for approximately 20 years after onset. As of now, there have not been any studies that suggest that GNE myopathy shortens a person’s lifespan.
While safety should be a main priority, experts believe that individuals with this disease should engage in exercise or physical activities they enjoy in order to maintain the muscles that are still in good condition. Genetic counseling is recommended for affected individuals and their families.
About HIBM. Neuromuscular Disease Foundation website. http://www.ndf-hibm.org/index.php/about-hibm.
Distal Myopathy. National Organization for Rare Disorders website. https://rarediseases.org/rare-diseases/distal-myopathy/.
Huizing M, Krasnewich DM. Hereditary inclusion body myopathy: a decade of progress. Biochim Biophys Acta. 2009;1792(9):881-887.
Inclusion Body Myopathy 2. Genetic Home Reference website. http://ghr.nlm.nih.gov/condition/inclusion-body-myopathy-2
Nishino I, Carrillo-Carrasco N, Argov Z. GNE myopathy: current update and future therapy. J Neurol Neurosurg Psychiatry. 2015;86(4):385-392.
O’Farrell EK, Sinnreich M. GNE-Related Myopathy. GeneReviews website. http://www.ncbi.nlm.nih.gov/books/NBK1262/
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