Charcot-Marie-Tooth Neuropathy Type 4A

[CMT4A, Charcot-Marie-Tooth Disease Type 4A]

Clinical Diagnosis

The following findings suggest the clinical diagnosis of CMT4A, findings that overlap those in other CMT forms:

• Early onset of peripheral neuropathy, presenting especially with foot deformities, muscle wasting, and involvement of the sensory nerves

• Disability within the first and second decade of life

• Vocal cord paresis (hoarse voice)

• Diaphragm paralysis

• Facultative involvement of cranial and enteric nerves

• Proximal muscle involvement later in the disease course

Electrophysiology. Motor nerve conduction velocities (NCVs) are variable. Most affected individuals exhibit an axonal neuropathy with normal NCVs and reduced amplitudes [Sevilla et al 2003]. Some families have a demyelinating neuropathy with slowed NCVs [Baxter et al 2002, Nelis et al 2002, Ammar et al 2003, Sandre-Giovannoli et al 2003] and others have NCVs that fall in the intermediate range [Senderek et al 2003].

The axonal phenotype is probably more often associated with vocal cord paresis than the demyelinating phenotype [Cuesta et al 2002], but the converse has also been observed [Boerkoel et al 2003].

Sensory NCVs are moderately reduced.

Neuropathology. Both demyelinating and axonal peripheral nerve lesions have been observed. Prominent loss of medium-sized and large myelinated fibers has been described [Nelis et al 2002, Ammar et al 2003, Boerkoel et al 2003, Sevilla et al 2003]. Onion bulb formations as well as thinly myelinated and unmyelinated axons have been observed [Nelis et al 2002, Sandre-Giovannoli et al 2003]. In one study, findings were interpreted as an intermediate type of neuropathy [Senderek et al 2003]. Focally folded myelin is not a feature.

Molecular Genetic Testing

GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.—ED.

Gene. GDAP1 is the only gene known to be associated with CMT4A [Baxter et al 2002, Cuesta et al 2002].

Clinical uses

• Diagnostic testing

• Carrier testing

• Prenatal diagnosis

Clinical testing

• Sequence analysis. Mutations are identified in nearly 100% of individuals with autosomal recessive CMT whose disease has been mapped to 8q13-q21.1.

Table 1 summarizes molecular genetic testing for this disorder.

Table 1. Molecular Genetic Testing Used in Charcot-Marie-Tooth Neuropathy Type 4A

Test Method	Mutations Detected	Mutation Detection Frequency 1	Test Availability
Sequence analysis	GDAP1 sequence alterations	~100%	Clinical

1. Proportion of affected individuals with a mutation(s) as classified by gene/locus, phenotype, population group, and/or test method

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Genetically Related (Allelic) Disorders

In addition to CMT4A, mutations in GDAP1 cause autosomal dominant CMT2K [Claramunt et al 2005]. (See GeneReview on CMT2).

Natural History

CMT4A is an aggressive form of hereditary motor and sensory neuropathy (HMSN) with early onset, increased severity, and unusual symptoms. The disease is confined to the peripheral nervous system. Intelligence is normal. Variability in disease progression has been reported within one family [Azzedine et al 2003].

Onset is in infancy, often before two years of age. At birth, children might be hypotonic (the so-called "floppy infant").

Affected children can show delayed achievement of motor milestones, including walking. Typically, initial symptoms are in the distal lower extremities, including foot deformities such as high arch, hammertoe, and pes cavus or equinovarus; muscle wasting; areflexia; and sensory loss.

Most authors describe early involvement of the upper extremities with muscle wasting and finger contractions (claw hands), weakness of proximal muscles, and a hoarse voice caused by vocal cord paresis, which occurs during the disease progression [Sevilla et al 2003, Stojkovic et al 2004].

Progression leads to disability of the lower and upper extremities. At the end of the second decade, most individuals are wheelchair bound. Respiratory dysfunction has not been described in CMT4A.

Rare symptoms are spinal deformities [Birouk et al 2003, De Sandre-Giovannoli et al 2003, Sevilla et al 2003], facial weakness [Boerkoel et al 2003], and painless lower leg ulcers [Nelis et al 2002].

Life expectancy is usually not affected, but on occasion may be reduced because of secondary complications.

Genotype-Phenotype Correlations

Genotype-phenotype correlations have not been reported.

Penetrance

All published pedigrees demonstrating CMT4A with the expected autosomal recessive inheritance show complete penetrance of the phenotype. In these families, heterozygotes for one disease-causing allele appeared to be free of symptoms.

Anticipation

Anticipation has not been observed.

Prevalence

Currently, CMT4A is considered the most frequent of all autosomal recessive forms of CMT.

Molecular genetic testing has shown that the following proportion of individuals with CMT have two disease-causing GDAP1 alleles:

• Five of 33 (15%) individuals from Europe with autosomal recessive (AR) CMT [Nelis et al 2002]

• One of 153 (0.7%) individuals from North America with CMT

• Three of 14 (14%) Hispanics, caused by a founder effect [Boerkoel et al 2003]

• Four of 119 (3.3%) individuals from Europe with AR CMT [Ammar et al 2003]

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with Charcot-Marie-Tooth neuropathy type 4A (CMT4A):

• Physical examination to determine extent of weakness and atrophy, pes cavus, gait stability, and sensory loss

• NCV to help distinguish demyelinating, axonal, and mixed forms of neuropathy

• Detailed family history

Treatment of Manifestations

Individuals with CMT4A are often evaluated and managed by a multidisciplinary team that includes neurologists, physiatrists, orthopedic surgeons, and physical and occupational therapists [Carter et al 1995, Grandis & Shy 2005]. Treatment is symptomatic and may include the following:

• Ankle/foot orthoses (AFOs) to correct foot drop and aid walking [Carter et al 1995]

• Orthopedic surgery to correct severe pes cavus deformity [Guyton & Mann 2000]

• Forearm crutches or canes for gait stability

• Wheelchairs for mobility because of gait instability

• Exercise within the affected individual's capability

• Treatment of musculoskeletal pain with acetaminophen or nonsteroidal anti-inflammatory agents [Carter et al 1998]

• Treatment of neuropathic pain with tricyclic antidepressants or drugs such as carbamazepine or gabapentin

• Career and employment counseling because of persistent weakness of hands and/or feet

Prevention of Primary Manifestations

No treatment for CMT or CMT4A that reverses or slows the natural disease process exists.

Prevention of Secondary Complications

Daily heel cord stretching exercises are recommended to prevent Achilles tendon shortening.

Surveillance

Individuals should be evaluated regularly by a team comprising physiatrists, neurologists, and physical and occupational therapists to determine neurologic status and functional disability.

Agents/Circumstances to Avoid

Drugs and medications that are known to cause nerve damage should be avoided [Graf et al 1996, Chaudhry et al 2003, Weimer & Podwall 2006]. These include:

• Vincristine

• Taxol

• Cisplatin

• Isoniazid

• Nitrofurantoin

Obesity is to be avoided because it makes walking more difficult.

Therapies Under Investigation

Donaghy et al (2000), and Ginsberg et al (2004) have described a few individuals with CMT1 and sudden deterioration in whom treatment with steroids (prednisone) or IVIg has produced variable levels of improvement. There is no similar report for individuals with CMT4A.

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Other

Genetics clinics are a source of information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

Support groups have been established for individuals and families to provide information, support, and contact with other affected individuals. The Resources section may include disease-specific and/or umbrella support organizations.

Mode of Inheritance

Charcot-Marie-Tooth neuropathy type 4A is inherited in an autosomal recessive manner.

Risk to Family Members — Autosomal Recessive CMT4A

Parents of a proband

• The parents of an affected child are obligate heterozygotes and therefore carry one mutant allele.

• Heterozygotes (carriers) are asymptomatic.

Sibs of a proband

• At conception, each sib of an affected individual have a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.

• Once an at-risk sib is known to be unaffected, the risk of his/her being a carrier is 2/3.

• Heterozygotes (carriers) are asymptomatic.

Offspring of a proband. The offspring of an individual with CMT4A are obligate heterozygotes (carriers) for a disease-causing mutation in the GDAP1 gene.

Other family members of a proband. Sibs of the proband's parents are at 50% risk of being carriers.

Carrier Detection

Carrier testing is available on a clinical basis once the mutations have been identified in the proband.

Related Genetic Counseling Issues

Family planning. The optimal time for determination of genetic risk and clarification of carrier status is before pregnancy.

DNA banking. DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals. DNA banking is particularly relevant in situations in which the sensitivity of currently available testing is less than 100%. See DNA Banking for a list of laboratories offering this service.

Prenatal Testing

Prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at about 15-18 weeks’ gestation or chorionic villus sampling (CVS) at about ten to 12 weeks’ gestation. Both disease-causing alleles must be identified before prenatal testing can be performed.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutations have been identified. For laboratories offering PGD, see .

Literature Cited

Published Statements and Policies Regarding Genetic Testing

No specific guidelines regarding genetic testing for this disorder have been developed.

Revision History

• 6 March 2007 (jv) Revision: prenatal testing available

• 23 June 2006 (ca) Comprehensive update posted to live Web site

• 11 May 2004 (me) Review posted to live Web site

• 23 February 2004 (jv,sz) Original submission