Practical Genetics - CMT disease FULL TEXT

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Practical Genetics - Charcot–Marie–Tooth disease FULL TEXT

European Journal of Human Genetics advance online publication 11 March 2009;

doi: 10.1038/ejhg.2009.31

Charcot–Marie–Tooth disease FULL TEXT

http://www.nature.com/ejhg/journal/vaop/ncurrent/full/ejhg200931a.html

Kinga Szigeti1,2 and R Lupski2,3

1Department of Neurology, Baylor College of Medicine, Houston, TX, USA

2Department of Molecular and Human Genetics, Baylor College of Medicine, Texas

Children Hospital, Houston, TX, USA

3Department of Pediatrics, Baylor College of Medicine, Houston, TX, USA

Abstract

Charcot-Marie-Tooth (CMT) disease is a heterogeneous group of genetic disorders

presenting with the phenotype of a chronic progressive neuropathy affecting both

the motor and sensory nerves. During the last decade over two dozen genes have

been identified in which mutations cause CMT. The disease illustrates a

multitude of genetic principles, including diverse mutational mechanisms from

point mutations to copy number variation (CNV), allelic heterogeneity,

age-dependent penetrance and variable expressivity. Population based studies

have determined the contributions of the various genes to disease burden

enabling evidence-based approaches to genetic testing.

In brief

The cardinal clinical feature is peripheral neuropathy: lower motor neuron-type

motor deficits and sensory signs and symptoms. As neuropathy can be associated

with many multisytemic disorders, it is required to be the predominant

manifestation.

Clinical phenotypes are established by age of onset, neurophysiological findings

and in some cases by neuropathology.

Clinical phenotypes include Charcot–Marie–Tooth disease type 1 (CMT1),

Charcot–Marie–Tooth disease type 2 (CMT2), Dejerine–Sottas neuropathy (DSN),

congenital hypomyelinating neuropathy (CHN) and Roussy–Levy syndrome (RLS).

Genetic heterogeneity, age-dependent penetrance and variable expressivity are

key characteristics of CMT and related peripheral neuropathies.

Thirty-six loci and more than two dozen genes are involved in CMT, implicating

pathways in myelination, radial and axonal transport, Schwann cell

differentiation, signal transduction, mitochondrial function, endosome, protein

translation and single-stranded DNA break repair.

The plethora of genetic information necessitates a rational approach to genetic

testing. CMT is one of the genetic conditions in which molecular-based therapies

are progressing to the clinical trial phase.

Introduction

The prevalence of Charcot–Marie–Tooth disease (CMT) is 1 per 2500 population,

which results in 125 000 patients in the United States alone, making it the most

common inherited neurological disease. Over the last 15 years, molecular

genetics research identified over two dozen genes in which mutations cause the

CMT phenotype. In vitro functional assays and experiments in animal models of

specific genetic alterations elucidated the pathomechanisms by which mutations

in certain genes cause disease and delineated pathways involved in peripheral

nerve biology. Some of these genes/mutations contribute to a significant

fraction of inherited peripheral neuropathy cases and thus molecular analysis

can have a substantial function in establishing a precise molecular diagnosis.

Population-based cohorts established the contribution of the individual genes to

disease burden, allowing evidence-based prioritization of genetic testing.

Compounds have been shown to ameliorate symptoms in animal models progressing

the field toward clinical trials. This in turn stimulated clinical research to

establish the natural history of the disease and to develop tools to assess

outcome in prospective clinical trials.

Clinical overview

Cardinal features of CMT

The main features of CMT are a combination of lower motor neuron-type motor

deficits and sensory signs and symptoms, reflecting the sensory-motor

neuropathy. Length-dependent paresis and muscle atrophy develops, with

areflexia, although a subset of patients will retain deep tendon reflexes,

especially in the axonal forms. The chronic nature of the motor neuropathy will

result in foot deformity (eg, pes cavus), hammertoes and high-arched feet.

Involvement of the hands may follow as the disease progresses. Sensory symptoms

are less frequent than in acquired chronic neuropathies, but may point to

specific gene involvement. Signs of sensory system dysfunction are common (70%)

and include loss of vibration and joint position sense followed by decreased

pain and temperature sensation in stocking and glove distribution. Clinical

features do not distinguish between the demyelinating or axonal forms.

Ancillary diagnostic tests include electrophysiological studies and sural nerve

biopsy. Recently, peripheral nerve MRI and skin biopsy have emerged as potential

diagnostic aids in certain types of hereditary neuropathies, though further

research studies are needed. EMG and nerve conduction studies (NCS) are

extremely helpful in the clinical classification of hereditary peripheral

neuropathies and in guiding genetic testing. Electrophysiological studies

distinguish two major types – the demyelinating form, which is characterized by

symmetrically slowed nerve conduction velocity (NCV; usually <38 m/s), and the

axonal form, which is associated with normal or subnormal NCV and reduced

compound muscle action potential. The term intermediate CMT is used without

consensus in the literature. It identifies the group of patients who cannot be

classified readily as either CMT1 or CMT2, as they tend to have features of both

demyelination and axonopathy. The NCV falls in the 30–45 m range, with overlap

with both the demyelinating and the axonal form.1 If this pattern is recognized,

certain genes are more likely to be involved than others (eg, GJB1 and MPZ).

Sural nerve biopsies from patients with the demyelinating type reveal segmental

demyelination and onion bulb formation, whereas the nerve biopsies from patients

with the axonal form show axonal loss, absent or few onion bulbs and no evidence

of demyelination. With the advent of genetic testing, invasive diagnostic tests

such as nerve biopsy are reserved for patients in whom genetic testing does not

yield to a molecular diagnosis, patients with atypical presentation or patients

in whom inflammatory neuropathy is suspected.

Depending on age of onset and neurophysiological findings, several clinical

phenotypes have been described historically. As molecular characterization of

phenotypes became available, genetic and clinical heterogeneity of the

hereditary motor and sensory neuropathies (HMSNs) became apparent.

Disease phenotypes

Charcot–Marie–Tooth Disease (MIM 118200, 118220) As CMT1 and CMT2 present with

similar clinical features, distinction on the basis of the neurological exam is

often impossible. The onset of clinical symptoms is in the first or second

decade of life. Weakness starts distally in the feet and progresses proximally

in an ascending pattern. Neuropathic bony deformities develop including pes

cavus (high-arched feet) and hammer toes. With further progression the hands

become weak. Muscle stretch reflexes disappear early in the ankles and later in

the patella and upper limbs. Mild sensory loss to pain, temperature or vibration

sensation in the legs is consistent with the phenotype. Patients also complain

of numbness and tingling in their feet and hands, but paresthesias are not as

common as in acquired neuropathies. Restless leg syndrome occurs in nearly 40%

of patients with the axonal form.

Hereditary neuropathy with liability to pressure palsies (MIM 162500) The

clinical phenotype is characterized by recurrent nerve dysfunction at

compression sites. Asymmetric palsies occur after relatively minor compression

or trauma. Repeated attacks result in the inability of full reversal. Thus with

ageing the patients with hereditary neuropathy with liability to pressure

palsies (HNPP) can have significant clinical overlap with CMT1.

Electrophysiological findings include mildly slowed NCV, increased distal motor

latencies and conduction blocks.3 The neuropathological hallmark is sausage-like

thickening of myelin sheaths (tomacula).

Dejerine–Sottas neuropathy (MIM 145900) Dejerine–Sottas neuropathy (DSN) is a

clinically distinct entity defined by delayed motor milestones. Signs of lower

motor neuron-type lesion accompany the delayed motor milestones.

Neurophysiological studies reveal severe slowing of NCV (<10 m/s).

Neuropathology reveals pronounced demyelination, and a greater number of onion

bulbs are present compared to CMT. Cerebrospinal fluid proteins may be elevated.

Most patients have significant disability.

Congentital hypomyelinating neuropathy (MIM 605253) Congentital hypomyelinating

neuropathy (CHN) is usually present at birth, although frequently the delayed

motor development draws the first attention to the peripheral neuropathy. The

distinction between DSN and CHN is often difficult by clinical examination as

they both may present as a hypotonic infant. The differentiation of CHN and DSN

is based on pathology: the presence of onion bulbs suggest DSN whereas their

absence indicate CHN. CHN may present as arthrogryposis multiplex congenita.

Roussy–Levy syndrome (MIM 180800) Roussy–Levy syndrome (RLS) was originally

described as demyelinating CMT associated with sensory ataxia and tremor. As

molecular data became available, it was shown that these patients have the same

molecular abnormalities as observed in patients clinically classified as

demyelinating CMT. RLS represents the spectrum of CMT.

Differential diagnosis of CMT

Peripheral neuropathy has a broad differential diagnosis: it can be the only

manifestation, part of a complex neurological phenotype or part of a

multisystemic disorder. Careful search for other affected organ systems or

central nervous system (CNS) involvement during the history and physical

examination is of utmost importance. Laboratory screening for correctable causes

should always be performed, including screening for diabetes, vitamin B

deficiency and serum immunofixation electrophoresis, especially in the adult

population. Marked CNS involvement makes CMT less likely; in these cases

leukodystrophies, mitochondrial disorders, the hereditary ataxias with

neuropathy (Friedreich ataxia, abetalipoproteinemia), Refsum disease,

Pelizeaus–Merzbacher disease and amyloid neuropathies should be considered.

Hereditary sensory neuropathies lack motor symptoms and are associated with

autonomic dysfunction. The lower motor neuron-type weakness may mimic a distal

myopathy; however, electrophysiology is useful in differentiating between the

two.

CMT is predominantly a peripheral neuropathy phenotype; however, certain

features are consistent with the disease and in fact may even help guide the

molecular genetic testing. Sensorineuronal hearing loss is present in 5% of the

patients. Adie's pupil is almost pathognomic for the Thr124Met mutation in MPZ.5

Ophthalmoparesis, facial weakness, vocal cord paralysis and bulbar signs reflect

cranial nerve involvement; these are common in EGR2 mutations.6 Hyperkeratosis

and juvenile glaucoma are associated with mutations in NEFL7 and MTMR138 genes,

respectively. Scoliosis is present in 20% of the cases and is a secondary

phenomenon caused by the neuromuscular weakness.

Inheritance pattern

All forms of Mendelian inheritance – autosomal dominant (AD), autosomal

recessive (AR) and X-linked (XL) – can be seen in CMT families. The AD

demyelinating form is the most frequent pattern observed.9 Out of the 36 linked

loci, 14 are AD, 13 AR and 3 XL. HNPP and RLS show AD inheritance whereas CHN is

AR or sporadic. DSN has both AD and AR forms. Genotype–phenotype correlation

studies suggest that genetic heterogeneity, age-dependent penetrance and

variable expressivity significantly contribute to the genetics of CMT. It is

estimated that about one-third of the point mutations and 5–24% of the

duplication mutation may occur de novo;10, 11, 12 thus, the absence of family

history does not preclude genetic testing.

Classification

The classification system for CMT and related peripheral neuropathies was

initially developed on the basis of the clinical phenotype, electrophysiological

and inheritance patterns (Table 1). This classification was derived from

clinical data on large pedigrees and served as an invaluable tool in identifying

genes responsible for certain types of CMT. The molecular classification added

further refinement and introduced ambiguities. Genes identified as a specific

locus-associated and type-associated gene were found to be responsible for other

types of CMT, or with a different inheritance pattern depending on the specific

mutant allele.

Table 1 - Genetic classification of Charcot–Marie–Tooth disease and related

peripheral neuropathies.Full table

Genetics

The more than two dozen genes (Table 1) implicated in the HMSNs belong to

various functional classes, all involved in the biology of peripheral nerve

development and function. They include structural proteins that are important in

myelination (eg, PMP22, MPZ), radial transport proteins (eg, Cx32), proteins of

axonal transport (eg, NEFL), transcription factors involved in Schwann cell

differentiation (EGR2), members of signal transduction pathways (eg, PRX, MTMR2,

SBF2, NDGR1), proteins related to mitochondrial function (eg, MFN2, GDAP1),

proteins related to the endosome (RAB7, SIMPLE) and molecular chaperones (HSP22,

HSP27), a gene involved in DNA single-stranded break repair (TDP1), and genes

involved in protein translation (GARS, YARS), in nuclear envelope function

(LMNA) and in the actin cytoskeleton (DNM2). A detailed summary of all the

contributing genes is beyond the scope of this clinical review and has been

summarized.Table 2 summarizes the genes, their functions and the associated

phenotypes.

Table 2 - Genotype–phenotype correlation.Full table

Genetic testing

The genetic complexity of CMT necessitates a rational approach for clinical

genetic testing. Factors to consider when initiating genetic testing should

include careful evaluation of (1) the availability of clinical testing, (2) the

yield of a specific molecular test, (3) the aim of establishing a molecular

diagnosis and (4) the frequency of de novo mutations.

Evidence-based data from 12 population-based studies from various ethnic

backgrounds established the contribution of 5 genes/genomic rearrangements to

disease burden: PMP22 duplication/deletion; MPZ, Cx32 and PMP22 point mutations.

Electrophysiological classification (demyelinating versus axonal neuropathy)

markedly improves the diagnostic yield (Table 3). In families, with informative

pedigrees to determine the inheritance pattern, further targeting of the

diagnostic testing can be achieved.

Table 3 - Mutation frequencies for CMT and related neuropathies.Full table

Duplication of a chromosomal segment harboring PMP22 (ie, the CMT1A

duplication)26 represents 43% of the total CMT cases, whereas the yield of

duplication detection rises to 70% in CMT1. The deletion of the same chromosomal

segment results in HNPP.27 Although the deletion has not been reported in any

other phenotype, the yield of deletion testing is over 90% in this distinctive

phenotype.

Cx32 mutations are the next most common culprits in inherited neuropathy. In

informative pedigrees a dominant inheritance pattern and lack of male-to-male

transmission points to this gene on the X chromosome. Because electrophysiology

frequently suggests the intermediate form, molecular testing for Cx32 is

appropriate in both CMT1 (after duplication testing) and CMT2. In the CMT1

group, MPZ and PMP22 mutations are the next most common, followed by the rare

genes.25 In the CMT2 group, Cx32 mutations are followed by MPZ mutations in

frequency; however, recent data, though not population based, suggest that MFN2

mutations may be one of the most common causes of CMT2.

The high frequency of de novo mutations in duplication/deletion (37–90%) and in

point mutations11 illustrates that genetic disease is commonly sporadic in

presentation. The absence of a family history does not exclude CMT and related

peripheral neuropathies. In fact, in a patient presenting with chronic

polyneuropathy in the absence of other signs and symptoms, after the most common

systemic and treatable causes, such as diabetes, uremia and nutritional

deficiency, genetic causes are more common than autoimmune or paraneoplastic

neuropathy. A rational diagnostic approach is presented in Figure 1. In

pediatric cases, which are more severe and when reproductive plans may depend on

the genetic information, complete evaluation with panel testing is warranted.

Figure 1.

Suggested testing scheme in hereditary sensory and motor polyneuropathy for

patients with and without a family history of CMT based on the

genotype–phenotype correlations and frequency data in 12 population-based

studies.

Treatment approaches to the HMSNs can be supportive or etiologic. As CMT is a

slowly progressive neurodegenerative disease, patients require periodic

assessments. Physiotherapy and occupational therapy aid in maintaining range of

motion and thus help in functioning appropriately.The application of orthotic

devices and assistive equipment can be made if safety or function requires them.

In some instances, surgical interventions for the hands and feet are necessary.

Symptomatic treatment may have a substantial impact on the quality of life.

Nonsteroidal anti-inflammatory drugs may help to relieve lower back or leg pain.

Neuropathic pain can be treated with antiepileptic drugs (gabapentin,

pregabalin, topiramate) or tricyclic antidepressants (amitriptyline).35, 36 The

tremor may respond to -blockers or primidone.37 Caffeine and nicotine can

aggravate the fine intentional tremor, thus avoidance of these substances is

recommended. Neurotoxic drugs (http://www.charcot-marie-tooth.org/) and

excessive alcohol should be avoided. A small dose of vincristine can produce a

devastating effect in patients with CMT, thus early detection of HMSN can avoid

life-threatening vincristine neurotoxicity.

Potential therapeutic approaches aiming at normalizing dosage by small molecules

in the CMT1A duplication models include vitamin C and onapristone, a

progesterone antagonist.39, 40, 41 An alternate molecular mechanism, point

mutations in Pmp22 in the Trembler and Trembler J mouse models cause peripheral

neuropathy; the disease was modified by the administration of curcumin likely by

alleviating the unfolded protein response.42 These treatments have been shown to

be effective only in animal models thus far; however, vitamin C has progressed

to a phase 2 clinical trial.

Genetic counseling

Because CMT follows the principles of Mendelian inheritance, genetic counseling

for recurrence of CMT1 and CMT2 is relatively straightforward if the family

history for an affected individual is defined. Because of intrafamilial

variability in disease expression, definition of parental disease status

requires either testing for a mutation defined in the propositus or, if the

mutation is not identifiable, a thorough neurological exam with objective NCS.

An affected parent with AD or XL-dominant CMT1 or CMT2 has a 50% risk of having

a child with the same mutation. At what age a child with a mutation will be

clinically affected is not known because the penetrance has not been determined

prospectively for genetically well-defined patient populations. In general only

a few patients with AD CMT1 or CMT2 have substantial difficulty walking before

age 50 years, and almost all patients express some symptoms by the sixth decade

of life.43 For fathers with XL-dominant CMT, the risk of having an affected son

is negligible but the risk of having an affected daughter is 100%, whereas for

mothers with XL-dominant CMT, the risk of having an affected son or daughter is

50%.

In the absence of a molecular diagnosis in AD CMT1, NCV slowing is detectable by

age 2–5 years;44, 45 therefore, if a young adult has normal NCVs, their risk of

developing AD CMT1 is negligible, whereas if the NCVs are abnormal, the patient

has at least a 90% lifetime risk of developing symptoms. Electrophysiological

changes associated with AD CMT2 develop with disease progression, thus only

about half of patients can be identified by age 20 years.43 In one study

performed before the molecular era in 15 unrelated families, the average age of

onset was 12.2 years. The penetrance was 28% in the first decade, but almost

complete by the third decade.46

When unaffected parents have a child affected with CMT1 or CMT2, four

possibilities exist: a de novo dominant mutation in the affected child, AR

inheritance, XL inheritance or nonpaternity. Distinction between these

possibilities requires either the identification of the causative mutation(s) or

the identification of affected siblings. The identification of a de novo

heterozygous presumed dominant mutation suggests a low recurrence risk for the

parents; however, the risk is higher than that for the general population

because of the possibility of germ-line mosaicism.47 A proband with a

heterozygous presumed dominant mutation has a 50% risk of having affected

children. For AR inheritance, the parental risk of an affected child is 25%

because penetrance is nearly complete.

CMT is one of the most prevalent neurogenetic conditions, with a plethora of

accumulated knowledge of the genes and pathways implicated in peripheral nerve

function and dysfunction. Although a lot remains to be learnt, clinical research

has aided the estimation of the contribution of specific genes to disease

burden. Animal models provide the basis for preclinical treatment trials in

which small compounds modifying gene expression to normalize gene dosage and

potentially modulating protein misfolding have been identified. Clinical

research has developed tools to assess outcome in clinical trials48 and data on

disease progression are accumulating. Thus, we have all the tools to move to the

exciting translational research phase, where patients can potentially benefit

from the translation of laboratory discoveries at the bedside.