Charcot-Marie-Tooth disease and hereditary motor neuropathies – Update 2020

Clinical investigations

Neurophysiologic and nerve biopsy studies are used to distinguish two main types, i. e. the demyelinating form CMT1 (primarily affecting Schwann cells, the myelin-forming glia cells in the peripheral nerves) and the axonal type CMT2 (affecting the axons of peripheral neurons directly). According to a widely accepted classification, patients are classified as demyelinating CMT1 if the motor nerve conduction velocity (mCV) of the median or ulnar nerve is below 38 m/s, while axonal CMT patients have a median mCV of at least 38 m/s [3]. Axonal degeneration is indicated by reduced or absent compound motor or sensory nerve action potentials.

There are several entities designated as dominant or recessive intermediate CMT (DI-CMT, RI-CMT) [4]. The notion of intermediate CMT emerged as a particular pattern of median mCV slowing, different from that of CMT1 (usually <25 m/s) and CMT2 (usually >45 m/s). The mCV limits for intermediate CMT vary in the literature between 25–45 m/s and 30–40 m/s. Moreover, there is considerable overlap of axonal and demyelinating features in many genetic subtypes which complicates classification attempts (Fig. 1). Since sural nerve biopsy is invasive, bears a considerable burden to the patient, and is only rarely indicative of a specific gene defect [5], it is generally considered obsolete when a hereditary neuropathy is suspected. However, nerve biopsies are still useful to rule out or confirm non-genetic causes of peripheral neuropathy. This is relevant for treatable inflammatory neuropathies, particularly to demonstrate non-systemic vasculitic neuropathy.

Figure 1

Venn diagram of disease genes for Charcot-Marie-Tooth disease (subdivided into demyelinating and axonal CMT) and distal hereditary motor neuropathy (May 2020). Shaded areas represent overlap phenotypes between subtypes. AD = autosomal dominant, AR = autosomal recessive, XL = X linked.

Laboratory investigations are generally normal apart from mildly or moderately increased creatine kinase (CK) activity in a small fraction (3 %) of demyelinating CMT and 11 % of axonal CMT patients [2]. In particular, patients with NEFL and MME mutations can show markedly increased CK levels.

Differential diagnosis

Generally, the proportion of genetically determined neuropathies is higher in childhood or youth than in adulthood. Most of the non-genetic causes can be attributed to para- or postinfectious and autoimmune neuritis, toxins, and nutritional deficiencies [6].

An important differential diagnosis is chronic inflammatory demyelinating polyneuropathy (CIDP). Although clinical features can overlap, CIDP is regularly associated with a subacute or fluctuating course, multifocal demyelinating features on electrophysiology, high protein levels in cerebrospinal fluid, and a negative family history. Since immunosuppressive treatment is effective in CIDP and not indicated in CMT, establishing the diagnosis is important. Moreover, neuropathies can also develop as a part of systemic inflammatory diseases such as vasculitis.

Axonal neuropathies can be caused or triggered by a broad variety of neurotoxic drugs (e. g. antiretroviral treatment and cytostatic agents) or substances. Taking a careful clinical history is important to document potential acquired causes of neuropathy.

Demyelinating polyneuropathy can also occur in different rare metabolic and neurodegenerative diseases, such as autosomal recessive metachromatic leukodystrophy, Refsum’s disease, Krabbe’s disease, X-linked adrenomyeloneuropathy, Pelizaeus–Merzbacher syndrome, the neurologic variant of Waardenburg–Shah syndrome and congenital cataracts with facial dysmorphism and neuropathy syndrome. Peripheral neuropathy may even be the most obvious clinical presentation of some of these conditions making distinction from non-syndromic hereditary neuropathies difficult.

X-linked Fabry disease often presents in childhood with neuropathic pain due to small fibre neuropathy, and early diagnosis is critical for enzyme replacement therapy to prevent renal, cardiac, and cerebrovascular complications. Another group of hereditary disorders sometimes confused with distal motor neuropathies are distal myopathies. The distinction can usually be made by sensory examination and electrophysiological studies.

Figure 2

Proposed pathomechanisms of gene products in hereditary neuropathies. Genes causing axonal CMT or dHMN are marked in blue, genes causing demyelinating CMT are marked in red.

Genetic diagnosis

Autosomal dominant CMT (AD-CMT) is the most common genetic subtype, followed by X-linked CMT, while autosomal recessive (AR) forms are rare in Middle European populations but are increasingly identified in populations with high consanguinity rates. Mutations in more than 90 genes related to CMT have been reported (Fig. 1), and many of these genes were identified in the past 10 years. Some neuropathy genes are directly linked to development, function, or maintenance of Schwann cells, myelin sheaths, neurons, and their axons while others are involved in more general biological processes (Fig. 2, modified from Weis and Senderek [7]). Molecular genetic diagnosis has become available for the majority (50–80 %) of CMT1 patients, especially for those with AD disease, as seen in larger studies based on conventional genetic analysis, i. e. MLPA and Sanger sequencing [2], [8], [9], [10], [11], [12]. This high detection rate is still maintained by three of the ‘The Big Four’ genes (Fig. 3), while the majority of mutations in rare genes affect only few families. CMT2 has no major gene accounting for a considerable proportion of cases and, thus, molecular genetic diagnosis is available only for about 20–30 % patients [2], [8], [9], [10], [11], [12]. One obvious limitation of previous studies based on step-wise diagnostic algorithms is the lack of systematic screening of less frequent CMT genes (see below). In an international cross-sectional study summarising data from 13 centres (Inherited Neuropathies Consortium, 10 sites in the USA and one each in the UK, Italy, and Australia), 997 of 1652 patients (60.4 %) received a genetic diagnosis, 91.2 % of demyelinating CMT and 43 % of axonal CMT [13], confirming that the diagnostic yield is higher in demyelinating CMT than in axonal CMT.

Figure 3

Mutation detection rates of the ‘Big Four’ genes (%) in demyelinating CMT1 and axonal CMT2 in various studies based on similar diagnostic algorithms (all patients included, not restricted to index patients).

Three genetic defects (PMP22 duplication, GJB1 or MPZ point mutations) are responsible for about 90 % of detectable mutations in patients with demyelinating CMT1. Similarly, the contribution of the three main genetic defects in axonal CMT2 (GJB1, MFN2, and MPZ mutations) to all CMT2 cases with a genetic diagnosis is about 80–85 %.

Autosomal recessive CMT and dHMN

In patients with autosomal recessive inheritance, the clinical picture is often more severe and starts in early childhood or even infancy. Autosomal recessive inheritance can be assumed if there are at least two affected offspring of healthy parents or in isolated patients with parental consanguinity. Linkage analysis and homozygosity mapping were in the past used to reduce the number of potential disease genes for subsequent mutation analysis. Demyelinating autosomal recessive CMT is classified in OMIM as CMT4 and axonal autosomal recessive CMT as axonal AR-CMT. In an average Caucasian population autosomal recessive CMT is rare, as reflected by the small proportion of mutation-positive cases (0.9 % GDAP1 and 0.2 % SH3TC2) among 17,000 CMT patients [14].

In a systematic study of 174 families with autosomal recessive CMT, Zimón et al. [15] obtained a mutation detection rate of 41.3 % with mutations identified in 10 different genes. The contribution to the overall diagnostic yield was 10.9 % for GDAP1 and HINT1, 7.5 % for SH3TC2, and only 1.1 % for other genes. HINT1 neuropathy was discovered in 2012 and is among the most frequent causes in the Czech population. It is autosomal recessively inherited and caused by a single mutation in 90 % of Czech patients [16]. The clinical features are axonal neuropathy starting in the first two or three decades, often combined with action neuromyotonia or myokymic discharges.

Recently, biallelic SORD mutations were discovered in 38 unrelated families as the most frequent autosomal recessive form of axonal or intermediate CMT or dHMN [17]. All patients shared the mutation p.Ala253GlnfsTer27 in homozygous or compound heterozygous state. Since SORD is an enzyme that converts sorbitol into fructose and the absence of SORD protein results in high intracellular sorbitol accumulation, this might be a target for medical intervention, e. g. with aldose reductase inhibitors.

Whole exome sequencing (WES) applied to 15 autosomal recessive CMT2/dHMN families negative for 94 known neuropathy genes showed novel or very rare variants in genes not previously associated with peripheral neuropathy (ARHGEF28, KBTBD13, AGRN, GNE) or associated with other phenotypes (VRK1, PNKP), underlining the complexity of variant interpretation in WES analysis [18].

PMP22

CMT1A due to a 1.5 Mb genomic duplication on chromosome 17p11 encompassing the PMP22 gene is by far the most frequent hereditary neuropathy. Duplications, deletions, and rare point mutations of PMP22 lead to various types of neuropathy with predominant Schwann cell pathology (CMT1A, HNPP [see below], and very rare recessive CMT). The PMP22 gene encodes the membrane glycoprotein peripheral myelin protein 22 (PMP22); proposed functions include Schwann cell maturation, myelination, myelin maintenance, and adhesion of myelin lamellae. In nerve biopsies of CMT1A patients, de- and remyelinated axons and classical Schwann cell onion bulb formations are frequent.

In CMT1A, prolonged distal motor latencies may already be present in the first months of life, and slow mCV can be found by age two years in most patients. However, clinical features may not occur until the second decade or even later in life. At the severe end of the phenotypic spectrum, patients can present with delayed motor development along with muscular hypotonia, marked proximal and distal weakness, and loss of ambulation. Age at onset and severity do not correlate with mCV slowing, while muscle weakness correlates with progressive decrease of the amplitudes of compound muscle action potentials. This suggests that secondary axonal pathology is ultimately responsible for the neurological deficits (for review see Bird [19]). Despite the rather uniform genetic cause in most CMT1A patients – PMP22 gene duplication – there is a wide range of still unexplained clinical severity. In keeping with this, intrafamilial clinical variability can be remarkable in CMT1A pedigrees.

Patients with PMP22 single nucleotide substitutions and indel variants can have a highly variable clinical and electroneurographic picture depending on the type and localisation of the mutation. Specific amino acid substitutions that are expected to result in a toxic gain of function [20] correlate with severe demyelinating peripheral neuropathy, while heterozygous null alleles (truncating or splicing mutations) mostly result in an HNPP phenotype [21].

GJB1

The GJB1 gene on chromosome Xq13 encodes the gap junction protein beta 1, also known as connexin 32 (Cx32). Hemizygous or heterozygous GJB1 mutations are the second most common genetic cause of CMT disease, comprising about 3–4 % of patients in Northern Europe and up to 10–15 % of patients in other countries worldwide [22]. Cx32 is a gap junction protein expressed by Schwann cells and many other cell populations. Cx32 appears to be involved in the trafficking of small molecules through the layers of the Schwann cell plasma membranes at sites of non-compacted myelin, that is, at Schmidt–Lanterman incisures and at nodes of Ranvier. Most GJB1 mutations are thought to lead to loss of Cx32 functions. However, it was shown that mutations in the untranslated regions of GJB1 can cause CMTX1 and may be responsible for about 10 % of cases [23]. Morphologically, GJB1 mutations are associated with reduced myelin thickness, loss of predominantly large myelinated nerve fibres, and clusters of small, regenerated myelinated nerve fibres.

The disease is X-linked dominantly inherited, i. e. most female mutation carriers develop variable symptoms and signs of a neuropathy. Males with GJB1 mutations mostly have a demyelinating neuropathy and tend to be more severely affected than seen in CMT1A. Females often have mixed electroneurographic features and have mild to moderate symptoms of a peripheral neuropathy. Clinical manifestations can vary considerably, even within families. Symptoms typically develop in the first three decades, with onset commonly within the first decade in males [24]. Age at onset and severity might be correlated with specific mutations, and hearing loss, central nervous system abnormalities, and central visual, acoustic, and motor pathway involvement have been reported (for review see Bird [19]). In a large Japanese cohort comprising 112 patients, early-onset episodic stroke like episodes were observed in six patients [24].

MPZ

The MPZ gene on chromosome 1q22 encodes myelin protein zero, a major transmembrane glycoprotein of peripheral nerve myelin. Myelin protein zero acts as a homophilic adhesion protein that is thought to serve important functions in myelin development and compaction as well as myelin maintenance. Accordingly, initial studies reported demyelinating disease in patients with MPZ-related autosomal dominant CMT (CMT1B). Nerve biopsies may show uncompacted myelin lamellae, especially inner and outer lamellae [5]. As in CMT1A signs of repeated de- and remyelination including Schwann cell and basal lamina onion bulb formations as well as axonal loss and endoneurial fibrosis can be observed. MPZ mutations can also cause a mixed axonal and demyelinating (intermediate) or a predominantly axonal, often late-onset autosomal dominant neuropathy. In nerve biopsies of late-onset cases, there is a reduction predominantly of large myelinated nerve fibres, and clusters of regenerating nerve fibres are frequent. Some of these clusters are surrounded by concentrically arranged Schwann cell processes, forming so-called ‘pseudo-onion bulbs’. It is unclear so far why certain MPZ point mutations cause a primary axonal phenotype. One possibility is the interference of these mutations with Schwann cell–axon interactions [25].

The largest study on MPZ mutations summarised clinical and molecular data of 103 patients from 71 different families [26]. The cohort was subdivided into infantile, childhood, and adult-onset groups. In the infantile-onset group (40 % of patients), there was delayed motor development and onset of symptoms before 5 years of age, about 30 % of patients required walking aids, and 20 % were wheelchair-bound. Electroneurographically, very slow mCV were obtained (80 % < 15 m/s). In the childhood-onset group (7 % of patients), age at onset was between 6 and 20 years and the clinical and electrophysiological picture was that of classical CMT1. In the adult-onset group (53 % of patients), mean age at onset was 40 years, progression was variable, and electrophysiological findings were mostly classified as CMT2. The authors observed enrichment of different MPZ mutations in each group, suggesting that specific MPZ mutations can determine clinical severity and the impact on peripheral nerve function.

MFN2

Autosomal dominant MFN2 mutations on chromosome 1p36 are considered to be one of the most frequent causes of axonal CMT (CMT2A). The encoded mitofusin 2 protein is located in the outer mitochondrial membrane. Mutations alter the equilibrium between fusion and fission of the mitochondrial net and are thought to affect intra-axonal mitochondrial transport and bioenergetics including the OXPHOS system. Histopathologically, the number of myelinated fibres is reduced depending on the onset and duration of the disease and larger myelinated fibres may be preferentially affected. Regenerating axons may tend to form large clusters.

Mutation detection rates vary considerably in different populations, ranging from 2.4 % in a Spanish study [12] up to 21.9 % in a study from the US [9], but mostly accounting for 3–8 % of patients [22]. Age at onset and clinical severity of patients with MFN2 mutations (CMT2A) can be highly variable and are not related to specific mutations [2]. Early studies implied that CMT2A is characterised by an early childhood onset and severe impairment, while more recent studies found a bimodal distribution of severity [27] with early-onset CMT and severe functional disability on one hand and a milder, slowly progressive late-onset phenotype on the other hand. Optic atrophy and vocal cord palsy were described in about 10 % of 43 French patients [28]. None of these features has been recorded in a series of 20 patients from Germany [2], suggesting that population-specific mutations or the ethnic and genetic background may affect the prevalence of additional clinical features in MFN2 mutation carriers.

Major genetic causes of motor neuropathies

In the past years our understanding of pathogenic pathways in motor neuropathies has improved substantially. Autosomal dominant mutations in five genes (GARS, YARS, AARS, HARS, WARS) encoding aminoacyl-tRNA synthetases (ARSs) cause rare axonal CMT or dHMN [36]. ARSs are responsible for charging amino acids to cognate tRNA molecules. Autosomal recessive mutations in these genes severely reduce the amount of charged tRNA for protein translation and cause severe, early-onset multisystem disease (GARS: mitochondriopathy and cardiomyopathy; YARS: multisystem disease with developmental delay; AARS: infantile epileptic encephalopathy; HARS: Usher syndrome). Mutations in autosomal dominant ARS-related neuropathy most likely act as a dominant negative mechanism or a toxic gain-of-function effect [36].

Mutations in chaperone proteins (most commonly HSPB1/HSPB27 and HSPB8/HSPB22) are reported in 5–8 % of dHMN patients [37]. Chaperones are activated during stress and protect cells against damaged and unstable proteins. The phenotype of HSPB-related neuropathy is characterised by distal lower limb weakness with a wide range of age at onset, mildly elevated CK activity, sometimes along with mild sensory involvement or pyramidal signs. As such there is a considerable overlap between CMT2, dHMN, and motor neuron disease. In a large study in France and Belgium including 510 unrelated patients, the diagnostic yield of HSPB1 was 5.5 % and 0.8 % in HSPB8, while no mutation was found in HSPB3, GARS, BSCL2, TRPV4, or MFN2 [37].

One striking example of clinical heterogeneity are heterozygous BSCL2 mutations, which can lead to a broad spectrum of phenotypes ranging from CMT2 over dHMN V with uni-or bilateral wasting of the thenar or dorsalis interosseus muscles to pure spastic paraplegia. The combination of spasticity of the lower limbs accompanied by amyotrophy of the hands has historically also been referred to as Silver syndrome (SPG17). BSCL2 encodes seipin, an integral endoplasmic reticulum membrane protein, which is highly expressed in nervous tissues, including the brain and spinal cord [38]. Two hot spot mutations in BSCL2 (p.N88S and p.S90L) have been repeatedly reported.