Hereditary inclusion body myopathy

Hereditary inclusion body myopathies (HIBM) are a heterogeneous group of very 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 (phenotype) 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 (Japanese and others), 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 (DMRV).

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 – 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 become 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 (QSM).

History

Hereditary inclusion body myopathy (IBM) 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 (IMB3; OMIM 605637 [1]) and autosomal recessive (IBM2; OMIM 600737 [2]) 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 (QSM). 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. [3]

Classification

Types of hereditary inclusion body myopathy:

• 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.
• IBM3 is a sometimes autosomal dominant and sometimes autosomal recessive form caused by mutations in MYHC2A; it is relatively mild muscle disorder.
• Inclusion body myopathy with early-onset Paget disease and frontotemporal dementia (IBMPFD) is an autosomal dominant and caused by mutations in VCP, HNRPA2B1 or HNRNPA1; it is a multisystem degenerative disorder that is characterized by muscle weakness that sets in during adulthood, early-onset Paget disease of bone, and premature frontotemporal dementia. It spreads to other systems and results in respiratory or cardiac failure. A condition with mutations in the same genes has been called Multisystem proteinopathy (MSP) or Inclusion body myopathy with frontotemporal dementia, Paget’s disease of bone, and amyotrophic lateral sclerosis (“IBMPFD/ALS”). 2013 review sought to explain the degenerative process in diseases like MSP, amyotrophic lateral sclerosis, and frontotemporal lobar degeneration, via the development of pathological granules containing RNA and protein.

The condition now called Desmin-related myofibrillar myopathy (also called myofibrillar myopathy-1) was formerly called inclusion body myopathy 1 (IBM1).

More types of HIMBs, linked to other genes, may be identified in the future.

Signs and symptoms

Some early signs of HIBMs includes:

• Difficulty walking on heels, and difficulty running;
• Weak index finger;
• Frequent loss of balance.
• On muscle biopsy, the typical finding includes inclusion bodies, rimmed vacuoles and accumulation of aberrant proteins similar to those found in senile plaques of Alzheimer's disease (amyloid beta, hyperphosphorylated tau, amongst others)

Genetics

The different forms have different mutations and inheritance patterns. See the detailed OMIM descriptions for details (given above).

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:

• Blood test for serum Creatine Kinase (CK or CPK);
• Nerve Conduction Study (NCS) / Electomyography (EMG);
• Muscle Biopsy;
• Magnetic Resonance Imaging (MRI) or Computer Tomography (CT) Scan to determine true sparing of quadriceps;
• Blood Test or Buccal swab for genetic testing;

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 (as of 2009).

Treatment options for lower limb weakness such as foot drop can be through the use of Ankle Foot Orthoses (AFOs) 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.

External links

• GeneReviews/NCBI/NIH/UW entry on Inclusion Body Myopathy 2
• GeneReviews/NCBI/NIH/UW entry on Inclusion Body Myopathy with Paget Disease of Bone and/or Frontotemporal Dementia
• Inclusion body myositis, a more common and non-hereditary form.