Hereditaryinclusion bodymyopathies are a group of rare genetic disorders which have different symptoms. Generally, they are neuromuscular disorders characterized by muscle weakness developing in young adults. Hereditary inclusion body myopathies comprise both autosomal recessive and autosomal dominant muscle disorders that have a variable expression in individuals, but all share similar structural features in the muscles. HIBMs are a group of muscle wasting disorders, which are uncommon in the general world population. One autosomal recessive form of HIBM is known as IBM2 or GNE myopathy, which is a common genetic disorder amongst people of Iranian Jewish descent. IBM2 has also been identified in other minorities throughout the world, including people of Asian, European, and South American origin, as well as Muslim people in the Middle Eastern, Palestinian, and Iranian origin. In Japan and many East Asian countries, this disorder is known as Distal Myopathy with Rimmed Vacuoles. IBM2 causes progressive muscle weakness and wasting. Muscle wasting usually starts around the age of 20 – 30 years, although young onset at 17 and old onset at 52 has been recorded. As such, it affects the most productive times of our lives. It can progress to marked disability within 10 to 15 years, confining many people with IBM2 to a wheelchair. The weakness and severity can vary from person to person. In some, weakness in the legs is noticed first. In few others, the hands are weakened more rapidly than the legs. Weakness is progressive, which means the muscle becomes weaker over time. IBM2 does not seem to affect the brain, internal organs or sensation. The quadriceps are relatively spared, and remain strong until the late stages of disease, which is the reason IBM2 is often referred to as Quadriceps Sparing Myopathy.
Signs and symptoms
Some early signs of HIBMs includes:
Difficulty walking on heels, and difficulty running;
The different forms have different mutations and inheritance patterns. See the detailed descriptions for details.
Diagnosis
The most useful information for accurate diagnosis is the symptoms and weakness pattern. If the quadriceps are spared but the hamstrings and iliopsoas are severely affected in a person between ages of 20 - 40, it is very likely HIBM will be at the top of the differential diagnosis. The doctor may order any or all of the following tests to ascertain if a person has IBM2:
IBM2 is the most common form, and is an autosomal recessive form, caused by mutations in the GNE gene; this form mainly affects leg muscles, but with an unusual distribution that spares the quadriceps. The incidence of this form is about 10 per million per year. There are two forms: a quadriceps sparing myopathy and Nonaka type distal myopathy. While occurring worldside it is most common in Jews of Persian origin. The GNE gene ecodes the rate-limiting, bifunctional enzyme of sialic acid biosynthesis, uridine diphosphate-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase. Disease occurs when both copies in the genome are non functional. The pathophysiology is still unclear. Cardiac involvement may occur.
IBM3 is a sometimes autosomal dominant and sometimes autosomal recessive form caused by mutations in MYHC2A; it is relatively mild muscle disorder.
The condition now called Desmin-related myofibrillar myopathy was formerly called inclusion body myopathy 1. More types of HIMBs, linked to other genes, may be identified in the future.
Mechanisms
The exact mechanisms of these diseases are not well understood. GNE/MNK a key enzyme in the sialic acid biosynthetic pathway, and loss-of-function mutations in GNE/MNK may lead to a lack of sialic acid, which in turn could affect sialoglycoproteins. GNE knockout mice show problems similar to people with IBM and in people with IBM dystroglycan has been found to lack sialic acid. However, the part of the dystroglycan that is important in muscle function does not seem to be affected. Another protein, neural cell adhesion molecule is under-sialyated in people with IBM, but as of 2016 it had no known role in muscle function.
Treatment
Treatment is palliative, not curative. Treatment options for lower limb weakness such as foot drop can be through the use of Ankle Foot Orthoses which can be designed or selected by an Orthotist based upon clinical need of the individual. Sometimes tuning of rigid AFOs can enhance knee stability.
Prognosis
A 2009 review noted that muscle weakness usually begins after age 20 and after 20–30 years, the person usually requires a wheel chair for mobility. There was no mention of increased mortality.
Research
Because lack of sialic acid appears to be part of the pathology of IBM caused by GNE mutations, clinical trials with sialic acid supplements, and with a precursor of sialic acid, N-Acetylmannosamine, have been conducted, and as of 2016 further trials were planned.
History
Hereditary inclusion body myopathy constitutes a unique group of neuromuscular disorders characterized by adult-onset slowly progressive distal and proximal weakness, and a typical muscle pathology including rimmed vacuoles and filamentous inclusions. Autosomal dominant and autosomal recessive forms have been described. The autosomal recessive form, first characterized in Jews of Persian descent, is a myopathy that affects mainly leg muscles, but with an unusual distribution that spares the quadriceps, so-called quadriceps-sparing myopathy. This disorder was subsequently found in other Middle Eastern families, the gene was mapped to 9p13-p12, and in 104 affected persons from 47 Middle Eastern families the same mutation in homozygous state was found in the GNE gene. Affected individuals in families of other ethnic origins were found to be compound heterozygotes for other distinct mutations in the GNE gene. From OMIM 603824.
