Charcot-Marie-Tooth (CMT) disease is the most common type of the hereditary neuro-pathies, with a prevalence of approximately 1 in 2,500 people.1 The CMTs are a relatively heterogenous group of inherited diseases, with variable manifestations, from mild sensorimotor neuropathy and foot deformities to more debilitating phenotypes with proximal weakness and other systemic manifestations.2 Most people with CMT present with a long history of slowly progressive distal lower and upper extremity weakness and length-dependent sensory symptoms. Foot deformities, steppage gait, and sensory ataxia are the clinical hallmarks of most hereditary neuropathies. Upper extremities are often involved, and hand symptoms may cause significant disability. The CMTs are associated with other systemic manifestations, such as vocal cord paralysis,3,4 diaphragm palsy,5 hearing loss,6 and optic neuropathy.7
Over the past decade, better understanding of the underlying pathophysiology of the more common types of hereditary neuropathies has developed.8 Based on electrodiagnostic criteria and pathophysiologic correlations, CMTs are broadly divided into demyelinating (type 1) and axonal (type 2) types.9 Demyelinating subtypes are characterized pathologicaly by myelin and Schwann cell dysfunction and electrophysiologically by slow nerve conduction velocities (< 38 m/s in the arm). Axonal types are characterized by primary axonal degeneration. Genetic testing can be a useful tool in identifying the cause of a neuropathy. In this review, we summarize current practice for genetic testing of hereditary neuropathies.
Genetic Testing—When and Why?
Genetics related to hereditary peripheral neuropathies, in general, and CMT in particular, is a rapidly evolving field.2,10,11 In 2013, approximately 63% of cases of CMT were linked to 1 of 69 genes.12 In addition to discovering new autosomal and mitochondrial genetic mutations, we are learning about more complex heterogenous disease models.11,13 Through the Inherited Neuropathy Consortium (INC) collaboration, a National Institute of Health (NIH)-funded project, we are identifying new novel genetic modifiers in more common types of CMT.14 More than 100 mutations have been found that cause hereditary neuropathy. As treatments evolve, it is important to establish a molecular diagnosis when possible. A more definite diagnosis may also help avoid unnecessary testing or treatments (eg, immunotherapy for a presumed acquired inflammatory neuropathy). Genetic testing can facilitate family and genetic counseling, provide prognostic information, alert providers to the potential for other organ involvement, and promote participation in clinical trials.
Genetic testing does not always lead to a definitive molecular diagnosis. It is not uncommon to encounter variants of unknown significance (VUS), pathogenic variants not associated with the individual’s phenotype, or benign variants. There are guidelines to help determine relevance of genetic test findings, such as testing family members or molecular prediction models that determine the possible pathogenicity of a mutation. However, confirmatory testing and tissue sequencing may be necessary to establish causality.15
Not everyone goes through more in-depth genome sequencing and even when people do so, there may not be a final diagnosis. The INC has valuable resources to help collect information about newly discovered mutations through an online variant browser linked to other databases.10 Despite all advances in this field, we fail to establish definitive molecular diagnoses in almost half the people tested for suspected hereditary neuropathies.16
The decision to pursue genetic testing is made on an individual basis, incorporating clinical, social, and psychologic factors. Valuable input from genetic counsellors can be helpful. As with all conditions known to have a genetic inheritance, recording a detailed family history is important. Assessing whether a person has family members with weakness, difficulty with walking or balance, tremors, temperature insensitivity, foot deformities, difficulty fitting shoes, hearing problems, or vision loss can provide insight into modes of inheritance, penetrance, and variability of symptoms. Furthermore, these details can narrow the differential diagnosis and help inform our understanding of secondary findings and VUS.
Individuals with CMT, however, may have a negative family history for many reasons, including mild subclinical expression in other family members, autosomal recessive inheritance, or a de novo heterozygous pathogenic variant in a gene with autosomal dominant or X-linked inheritance. In many cases, no specific features or genotype–phenotype correlations distinguish different genetic forms of CMT within the same subtype. Even among genetically homogeneous groups, variability in presentation and course are consistent features of the CMTs.14 When new genes are discovered, phenotypes may not be clearly defined or can overlap. This can make choosing the correct genetic test problematic. There are a growing number of genetic testing laboratories that offer ever-changing panels at widely ranging prices. Because a positive result may reveal information about the course, prognosis, or severity of disease and help guide medical treatment, developing a genetic testing strategy is important.
Types of Available Genetic Tests
If an individual’s symptoms, family history, and neurophysiologic test findings fit a pattern consistently associated with a specific genetic diagnosis, single-gene testing may be appropriate. The peripheral myelin protein-22 gene (PMP22) duplication is identified in 60% to 70% of people with demyelinating polyneuropathy CMT type 1A.17 In a person with uniform motor response slowing and a history of autosomal dominant inheritance, single-gene testing for PMP22 duplication might be the best initial choice. Similarly, pathogenic mutations in gap junction protein β1 (GJB1) on chromosome X is another common mutation associated with primarily demyelinating polyneuropathies.18 In addition to selected physical characteristics,18 absence of male-to-male transmission in the family, and milder phenotypes in female relatives raise clinical suspicion for CMT type 1X, in which case, single-gene testing for GJB1 may have a higher yield. If a family member has had a positive test result, single-gene testing for a known specific familial variant is the best course of action, especially with family counselling.
Gene Panel Testing
A limited panel may be adequate to establish the diagnosis for common mutations.19 Among individuals with signs of autosomal dominant demyelinating CMT, testing a panel of 6 genes (EGR2, GJB1, LITAF, MPZ, NEFL, or PMP22) would identify most of them.20 There are situations, however, in which an individual’s phenotype does not suggest an obvious specific gene. In such cases, finding a single test covering multiple genes within or across CMT subtypes can be faster and more cost effective. Some-single gene tests cost more than a prepaid panel of 83 genes. Such panels assess many genes at once while maintaining a focus on CMT, reducing off-target findings and likelihood of VUS. Gene panel testing may be more sensitive at identifying abnormal gene variants because of increased read depth. Gene panels may also include genetic testing methodologies other than NGS, which is known to miss some deletions, duplications, and repetitive sequences. With more than 8 commercial labs offering more than 30 different CMT-specific gene panels for testing and multiple published algorithms for cascade testing based on polyneuropathy defining features,21 choosing the most appropriate test can feel daunting.
Whole Exome Sequencing
Given the complexity of choosing among gene panels, whole exome sequencing (WES) is a suitable alternative that is comprehensive, cost effective, and relatively accessible. In some cases (eg, adoption or inconclusive electrodiagnostic findings), WES is more likely to yield a diagnosis. In contrast to other genetic tests, WES examines approximately 20,000 genes simultaneously, offering much broader testing. With WES, there is also the option to receive results for some adult-onset genetic conditions unrelated to the primary symptoms, but for which there are preventive measures. However, WES is not as sensitive at finding mutations in a particular gene and has up to a 10% chance of missing a mutation that a single-gene test could identify. Understanding genotype–phenotype relationships to ensure that the relevant candidate genes are assessed is vital. Microdeletion or duplication of genomic data may be missed in routine WES.
Use of WES increases the likelihood of finding VUS. As technology outstrips our ability to analyze the effects that each genetic variant has on human molecular biology, testing may identify variations that sound potentialy interesting but for which there are no data on pathogenicity. Ultimately, further research may be required to prove that an identified variation is causal.
It is beneficial for individuals who are symptomatic to meet with a genetic counselor prior to genetic testing to discuss the motivation for testing, set expectations, and review costs. Individuals are often seeking a diagnosis and a community; they want to identify a specific cause for their symptoms. Expectation setting can include likelihood of discovering a positive result and reinforcing the possibility that no diagnosis may be identified. After a diagnostic odyssey, some struggle with negative test results or may be reluctant to dismiss a report that includes a VUS. Even when a specific causative gene is identified, some may become frustrated if there is limited data on natural history or if no disease-modifying therapy exists. Recurrence risk is another issue to mention during pretest counseling. Accurately estimating risk for current or future family members may require parental genetic testing to identify whether a given individual represents a de novo mutation or possesses an inherited defect. Decision-making autonomy for minors or genetic discrimination in medical and/or life insurance may be a concern for family members who are asymptomatic or minimally affected and opt for genetic testing.
Ultimately, health care providers should look for a constellation of features that provides strong evidence to suspect an underlying genetic cause. A firm genetic diagnosis can provide information on the rate of progression, clarify potential problems that may develop, and, in some instances, allow for targeted therapies. A genetic diagnosis can be helpful to asymptomatic at-risk family members who might wish to pursue early intervention. Depending on the person’s phenotype, single-gene, gene panel testing, or WES may be the most likely and the most cost-effective way to reach a diagnosis.
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TM-B and RS have no disclosures to report.