Gne Myopathy

Clinical characteristics.

GNE myopathy is a slowly progressive muscle disease that typically presents between age 20 and 40 years with bilateral foot drop caused by anterior tibialis weakness. Lower-extremity muscle involvement progresses from the anterior to the posterior compartment of the lower leg, followed by hamstrings, then hip girdle muscles, with relative sparing of the quadriceps. A wheelchair may be needed about ten to 20 years after the onset of manifestations. The upper extremities, which may be affected within five to ten years of disease onset, do not necessarily follow a distal-to-proximal progression. In advanced stages, neck and core muscles can become affected.

Diagnosis/testing.

The diagnosis of GNE myopathy is suspected in a proband with suggestive clinical findings and muscle histopathology (rimmed vacuoles, no inflammation) and is established by the presence of biallelic pathogenic variants in GNE identified by molecular genetic testing.

Management.

Treatment of manifestations: Evaluation and management are often by a multidisciplinary team that includes neuromuscular specialists, physiatrists, and physical and occupational therapists to address issues secondary to muscle weakness, including the use of assistive ambulatory devices (e.g., ankle-foot orthoses, cane, walker, wheelchair, or powerchair). Adaptive devices to support fine motor function and activities of daily living are needed in advanced stages of the disease. Recommended evaluations also include baseline echocardiogram and pulmonary function tests in nonambulatory individuals, with management by pulmonologists as clinically indicated.

Surveillance: Follow up at least annually by neuromuscular specialists, physiatrists, and physical and occupational therapists to evaluate disease progression and address muscle strength, mobility, function, and activities of daily living; by pulmonologists to monitor respiratory muscle function in patients with advanced disease.

Agents/circumstances to avoid: Cautious use of medications/drugs with potential myotoxicity (e.g., colchicine and statins); avoidance of weight-lifting and repetitive activities that cause muscle pain.

Genetic counseling.

GNE myopathy is inherited in an autosomal recessive manner. If both parents are known to be heterozygous for a GNE pathogenic variant, each sib of an affected individual has at conception a 25% chance of inheriting both pathogenic variants and being affected, a 50% chance of inheriting one pathogenic variant and being an unaffected carrier, and a 25% chance of inheriting both normal alleles. When the GNE pathogenic variants have been identified in an affected family member, molecular genetic carrier testing of at-risk relatives, prenatal testing for pregnancies at increased risk, and preimplantation genetic testing are possible.

Suggestive Findings

GNE myopathy should be suspected in individuals with the following findings.

Clinical findings

• Myopathy presenting in young adults with bilateral foot drop caused by anterior tibialis weakness, followed by slowly progressive skeletal muscle weakness. Although there is relative sparing of the quadriceps, they may become affected at late stages of the disease. The clinical picture varies depending on the stage of disease progression at which individuals are evaluated (see Clinical Description).
• Serum CK may be normal or up to four times the upper limit of normal.

Muscle histopathology

• Cryosections of affected muscles show atrophy, variation of fiber size, rimmed vacuoles, and no inflammation. The most prominent finding, the presence of rimmed vacuoles, is best identified in cryosections using modified Gomori trichrome stain and may be missed in paraffin-embedded tissue or hematoxylin and eosin staining. The "rimmed vacuoles" observed on electron microscopy that correspond to autophagic vacuoles are seen in a variety of myopathies with other etiologies that lead to autophagic degeneration (see Differential Diagnosis).
• Note: (1) Because histopathologic findings may be difficult to identify in biopsies of muscles that are unaffected or that have been replaced by fibro-fatty tissue, muscle strength or muscle MRI may aid in the identification of suitable muscles to biopsy. (2) Muscle biopsy and histopathologic examination may not be required to suspect or establish the diagnosis of GNE myopathy but remain necessary when variants of unknown significance are identified on molecular genetic testing.

Establishing the Diagnosis

The diagnosis of GNE myopathy is established in a proband with suggestive clinical findings, muscle histopathology (if performed), and biallelic pathogenic variants in GNE identified by molecular genetic testing (see Table 1).

Note: Identification of biallelic GNE variants of uncertain significance (or identification of one known GNE pathogenic variant and one GNE variant of uncertain significance) does not establish or rule out a diagnosis of GNE myopathy.

Molecular genetic testing approaches can include gene-targeted testing (single-gene testing in probands with suggestive findings or affected sibs; multigene neuromuscular panel in probands with myopathy but unspecific findings) (see Option 1). In some instances, comprehensive genomic testing (exome sequencing or genome sequencing) is performed (see Option 2).

Clinical Description

GNE myopathy is characterized by adult-onset slowly progressive myopathy typically presenting with bilateral foot drop, followed by distal-to-proximal lower-extremity weakness. The upper extremities, which are affected within five to ten years of disease onset, do not necessarily follow a distal-to-proximal progression, in contrast to the lower extremities. In advanced stages, neck and core muscles can also become affected.

Onset. GNE myopathy typically presents in individuals age 20-40 years with foot drop caused by anterior tibialis weakness. Rarely, in case of muscle overuse, other muscles may be affected first [de Dios et al 2014].

Progression. In the lower extremities, the disease progresses to involve muscles from the anterior compartment of the lower leg, followed by calf muscles and hamstrings, followed by hip girdle muscles, with relative sparing of the quadriceps [Argov & Yarom 1984]. The involvement of the quadriceps muscles may become evident in late stages of the disease with the rectus femoris affected first and the vastus lateralis affected last [Huizing et al 2001, Tasca et al 2012, Carrillo et al 2018].

In the upper extremities, shoulder abduction may be affected early in the disease course before grip and hand muscles are affected.

Clinical findings depend on the stage of disease progression at the time of evaluation [Quintana et al 2019]:

• Disease onset. Young adults describe symptoms such as tripping and changes in gait. On exam, there is bilateral foot drop and inability to stand on the toes or walk on the heels.
• Within five years of onset. Complete loss of ankle dorsiflexion strength, decreased knee flexion and shoulder abduction strength. Manifestations include steppage gait, some difficulty climbing stairs, and decreased balance, requiring the use of ankle-foot orthoses (AFOs).
• Five to ten years after onset. Complete loss of knee flexion strength; decreased shoulder abduction, forearm, wrist, and hand strength; quadriceps are unaffected. Manifestations include worsening gait, increased risk of falls, and poor balance requiring the use of assistive walking devices; difficulty moving from a sitting position to a standing position; and significant difficulty climbing stairs. Upper extremities: difficulty performing tasks that involve raising arms above head and initial difficulty with hand function.
• Ten to 20 years after onset. Decreased strength of hip extensors; quadriceps may be affected; the use of a wheelchair may be needed; significant difficulty with shoulder abduction and fine motor (i.e., hand) tasks. Increasing dependence for assistance with activities of daily living.
• In advanced stages. The neck, core, and respiratory muscles can be affected.

Ultimately, disease progression may result in complete loss of skeletal muscle function and dependence on caregivers. Life span is not reduced.

Other

• Respiratory muscle involvement resulting in decreased forced vital capacity has been described in the late stages of the disease; however, clinically significant involvement is rare and typically limited to individuals who are wheelchair dependent [Mori-Yoshimura et al 2013].
• Cardiac muscle is typically not affected. While cardiac involvement has been reported in persons with GNE myopathy [Chai et al 2011, Chamova et al 2015], it remains unclear whether this was due to GNE myopathy or of a different etiology.

Laboratory findings

• Serum creatine kinase (CK) activity may be normal or elevated; it typically does not exceed four times the normal value.
• Creatinine values decrease over time due to decreased muscle mass; hence, cystatin C should be used instead of creatinine to evaluate renal function.
• Mild elevation of alanine aminotransferase and aspartate aminotransferase is seen in some individuals, especially those with elevated CK.

Electromyogram and nerve conduction velocity are invasive and do not help with the diagnosis.

Muscle MRI shows fibro-fatty replacement on T₁-weighted images; short tau inversion recovery hyperintensity indicates active disease.

Genotype-Phenotype Correlations

Because reports of GNE myopathy consist mainly of single individuals or relatively small series, correlations between genotype and phenotype are difficult.

Penetrance

Penetrance of biallelic GNE pathogenic variants is likely close to 100%. Only two older individuals with biallelic GNE pathogenic variants have been reported to be asymptomatic: One (age 67 years) was homozygous for the common Middle Eastern variant p.Met743Thr [Argov et al 2003] and one (age 60 years) was homozygous for the common Japanese variant p.Asp207Val [Nishino et al 2002].

Nomenclature

In order to unify the nomenclature and avoid confusion with unrelated but similarly named disorders, an international consortium proposed the term "GNE myopathy" to replace historically used terms [Huizing et al 2014].

• The phenotype was first described by Nonaka et al [1981] in Japan; at that time, the disorder was referred to as "distal myopathy with rimmed vacuoles (DMRV)" or Nonaka myopathy.
• The terms "quadriceps-sparing myopathy" and "hereditary rimmed vacuole myopathy (HIBM)" were used by Argov & Yarom [1984] and Sadeh et al [1993] to describe the GNE myopathy phenotype in affected individuals of Iranian Jewish ancestry.

With the identification of the causative gene, GNE [Eisenberg et al 2001], it became apparent that HIBM was the same disease as DMRV [Nishino et al 2002].

Prevalence

The prevalence of GNE myopathy is estimated at 1-9:1,000,000.

To date, more than 1,000 individuals with GNE myopathy and about 255 GNE variants have been reported worldwide.

The worldwide carrier rate of a pathogenic GNE variant is estimated at 1:203 individuals.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with GNE myopathy, the evaluations summarized in Table 3 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Treatment of Manifestations

There is no approved therapy for GNE myopathy. Treatment is symptomatic only (see Table 4).

Surveillance

Routine follow up with the multidisciplinary team is recommended annually, or more frequently as determined by managing physician (see Table 5).

Agents/Circumstances to Avoid

It may be prudent to use medications/drugs with potential myotoxicity (e.g., colchicine, statins) with caution.

It is strongly recommended that affected individuals have a healthy diet and exercise to avoid developing hypercholesterolemia, in an effort to reduce the risk associated with taking statins.

Individuals with GNE myopathy should avoid lifting weights and performing repetitive activities that result in muscle pain.

Evaluation of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

N-acetylmanossamine (ManNAc) is the only therapy currently in clinical development for GNE myopathy [Galeano et al 2007, Malicdan et al 2009, Niethamer et al 2012, Xu et al 2017].

Ultragenyx discontinued the clinical development of extended-release sialic acid (Ace-ER) in 2017 following a Phase III trial that failed to detect clinical efficacy [Lochmüller et al 2019].

Preclinical studies are ongoing to advance gene therapy as a potential therapy for GNE myopathy.

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions.