Charcot-Marie-Tooth disease with giant axons: a clinicopathological and genetic entity

Neurofilaments: Novel findings and future challenges

Neurofilaments (NFs) are abundant cytoskeletal proteins that emerge as a critical hub for cell signalling within neurons. As we start to uncover essential roles of NFs in regulating microtubule and organelle dynamics, nerve conduction and neurotransmission, novel discoveries are expected to arise in genetics, with NFs identified as causal genes for various neurodegenerative diseases. This review will discuss how the latest advances in fundamental and translational research illuminate our understanding of NF biology, particularly their assembly, organisation, transport and degradation. We will emphasise the notion that filaments are not one entity and that future challenges will be to apprehend their diverse composition and structural heterogeneity and to scrutinize how this regulates signalling, sustains neuronal physiology and drives pathophysiology in disease.

Neurofilaments in health and Charcot-Marie-Tooth disease

Neurofilaments (NFs) are the most abundant component of mature neurons, that interconnect with actin and microtubules to form the cytoskeleton. Specifically expressed in the nervous system, NFs present the particularity within the Intermediate Filament family of being formed by four subunits, the neurofilament light (NF-L), medium (NF-M), heavy (NF-H) proteins and α-internexin or peripherin. Here, we review the current knowledge on NF proteins and neurofilaments, from their domain structures and their model of assembly to the dynamics of their transport and degradation along the axon. The formation of the filament and its behaviour are regulated by various determinants, including post-transcriptional (miRNA and RBP proteins) and post-translational (phosphorylation and ubiquitination) modifiers. Altogether, the complex set of modifications enable the neuron to establish a stable but elastic NF array constituting the structural scaffold of the axon, while permitting the local expression of NF proteins and providing the dynamics necessary to fulfil local demands and respond to stimuli and injury. Thus, in addition to their roles in mechano-resistance, radial axonal outgrowth and nerve conduction, NFs control microtubule dynamics, organelle distribution and neurotransmission at the synapse. We discuss how the studies of neurodegenerative diseases with NF aggregation shed light on the biology of NFs. In particular, the NEFL and NEFH genes are mutated in Charcot-Marie-Tooth (CMT) disease, the most common inherited neurological disorder of the peripheral nervous system. The clinical features of the CMT forms (axonal CMT2E, CMT2CC; demyelinating CMT1F; intermediate I-CMT) with symptoms affecting the central nervous system (CNS) will allow us to further investigate the physiological roles of NFs in the brain. Thus, NF-CMT mouse models exhibit various degrees of sensory-motor deficits associated with CNS symptoms. Cellular systems brought findings regarding the dominant effect of NF-L mutants on NF aggregation and transport, although these have been recently challenged. Neurofilament detection without NF-L in recessive CMT is puzzling, calling for a re-examination of the current model in which NF-L is indispensable for NF assembly. Overall, we discuss how the fundamental and translational fields are feeding each-other to increase but also challenge our knowledge of NF biology, and to develop therapeutic avenues for CMT and neurodegenerative diseases with NF aggregation.

New evidence for secondary axonal degeneration in demyelinating neuropathies

Development of peripheral nervous system (PNS) myelin involves a coordinated series of events between growing axons and the Schwann cell (SC) progenitors that will eventually ensheath them. Myelin sheaths have evolved out of necessity to maintain rapid impulse propagation while accounting for body space constraints. However, myelinating SCs perform additional critical functions that are required to preserve axonal integrity including mitigating energy consumption by establishing the nodal architecture, regulating axon caliber by organizing axonal cytoskeleton networks, providing trophic and potentially metabolic support, possibly supplying genetic translation materials and protecting axons from toxic insults. The intermediate steps between the loss of these functions and the initiation of axon degeneration are unknown but the importance of these processes provides insightful clues. Prevalent demyelinating diseases of the PNS include the inherited neuropathies Charcot-Marie-Tooth Disease, Type 1 (CMT1) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and the inflammatory diseases Acute Inflammatory Demyelinating Polyneuropathy (AIDP) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP). Secondary axon degeneration is a common feature of demyelinating neuropathies and this process is often correlated with clinical deficits and long-lasting disability in patients. There is abundant electrophysiological and histological evidence for secondary axon degeneration in patients and rodent models of PNS demyelinating diseases. Fully understanding the involvement of secondary axon degeneration in these diseases is essential for expanding our knowledge of disease pathogenesis and prognosis, which will be essential for developing novel therapeutic strategies.

Myelinated axons fail to develop properly in a genetically authentic mouse model of Charcot-Marie-Tooth disease type 2E

We have analyzed a mouse model of Charcot-Marie-Tooth disease 2E (CMT2E) harboring a heterozygous p.Asn98Ser (p.N98S) Nefl mutation, whose human counterpart results in a severe, early-onset neuropathy. Behavioral, electrophysiological, and pathological analyses were done on separate cohorts of Nefl^N98S/+ mutant mice and their wild type Nefl^+/+ littermates between 8 and 48 weeks of age. The motor performance of Nefl^N98S/+ mice, as evidenced by altered balance and gait measures, was impaired at every age examined (from 6 to 25 weeks of age). At all times examined, myelinated axons were smaller and contained markedly fewer neurofilaments in Nefl^N98S/+ mice, in all examined aspects of the PNS, from the nerve roots to the distal ends of the sciatic and caudal nerves. Similarly, the myelinated axons in the various tracts of the spinal cord and in the optic nerves were smaller and contained fewer neurofilaments in mutant mice. The myelinated axons in both the PNS and the CNS of mutant mice had relatively thicker myelin sheaths. The amplitude and the nerve conduction velocity of the caudal nerves were reduced in proportion with the diminished sizes of myelinated axons. Conspicuous aggregations of neurofilaments were only seen in primary sensory and motor neurons, and were largely confined to the cell bodies and proximal axons. There was evidence of axonal degeneration and regeneration of myelinated axons, mostly in distal nerves. In summary, the p.N98S mutation causes a profound reduction of neurofilaments in the myelinated axons of the PNS and CNS, resulting in substantially reduced axonal diameters, particularly of large myelinated axons, and distal axon loss in the PNS.

Nano-delivery systems for encapsulation of dietary polyphenols: An experimental approach for neurodegenerative diseases and brain tumors

Neurodegenerative diseases (NDs) and brain tumors are severe, disabling, and incurable disorders that represent a critical problem regarding human suffering and the economic burden on the healthcare system. Because of the lack of effective therapies to treat NDs and brain tumors, the challenge for physicians is to discover new drugs to improve their patients' quality of life. In addition to risk factors such as genetics and environmental influences, increased cellular oxidative stress has been reported as one of the potential common etiologies in both disorders. Given their antioxidant and anti-inflammatory potential, dietary polyphenols are considered to be one of the most bioactive natural agents in chronic disease prevention and treatment. Despite the protective activity of polyphenols, their inefficient delivery systems and poor bioavailability strongly limit their use in medicine and functional food. A potential solution lies in polymeric nanoparticle-based polyphenol delivery systems that are able to enhance their absorption across the gastrointestinal tract, improve their bioavailability, and transport them to target organs. In the present manuscript, we provide an overview of the primary polyphenols used for ND and brain tumor prevention and treatment by focusing on recent findings, the principal factors limiting their application in clinical practice, and a promising delivery strategy to improve their bioavailability.

Neurofibromatous neuropathy: An ultrastructural study

Plexiform neurofibroma is pathognomonic of neurofibromatosis 1 (NF1). An NF1-associated peripheral neuropathy has been described in a small minority of NF1 patients but its histopathological features are poorly characterized. We report the case of a 46-year-old woman presenting with bilateral supraclavicular painful masses without other stigmata of NF1. MRI showed bilateral plexiform lesions extending from cervical roots to the elbows. Nerve conduction studies documented a sensory motor polyneuropathy. Morphometric analysis of sural nerve biopsy showed a preferential loss of large-caliber myelinated fibers with a g ratio of 0.515, and the presence of regeneration clusters. By electron microscopy, marked and diffuse endoneurial fibrosis with an altered relationship between Schwann cells (SC) and collagen fibrils was observed. Moreover both myelinating and non-myelinating SC were characterized by the presence of various cell degradation products. These changes suggest that, in neurofibromatous neuropathy, a widespread axonal atrophy and degeneration take place independently on the presence of tumoral infiltration, possibly due to an impairment in SC-axon cross talk. In this case, the coexistence of plexiform neurofibromas with a peripheral neuropathy strongly suggests a diagnosis of NF1 even without fulfillment of clinical criteria. We propose that in the presence of plexiform neurofibromas, electrophysiological studies should be performed also in asymptomatic patients, in order to detect the existence of a subclinical neuropathy.

Genetic and clinical characteristics of NEFL-related Charcot-Marie-Tooth disease

OBJECTIVES

To analyse and describe the clinical and genetic spectrum of Charcot-Marie-Tooth disease (CMT) caused by mutations in the neurofilament light polypeptide gene (NEFL).

METHODS

Combined analysis of newly identified patients with NEFL-related CMT and all previously reported cases from the literature.

RESULTS

Five new unrelated patients with CMT carrying the NEFL mutations P8R and N98S and the novel variant L311P were identified. Combined data from these cases and 62 kindreds from the literature revealed four common mutations (P8R, P22S, N98S and E396K) and three mutational hotspots accounting for 37 (55%) and 50 (75%) kindreds, respectively. Eight patients had de novo mutations. Loss of large-myelinated fibres was a uniform feature in a total of 21 sural nerve biopsies and 'onion bulb' formations and/or thin myelin sheaths were observed in 14 (67%) of them. The neurophysiological phenotype was broad but most patients with E90K and N98S had upper limb motor conduction velocities <38 m/s. Age of onset was ≤3 years in 25 cases. Pyramidal tract signs were described in 13 patients and 7 patients were initially diagnosed with or tested for inherited ataxia. Patients with E90K and N98S frequently presented before age 3 years and developed hearing loss or other neurological features including ataxia and/or cerebellar atrophy on brain MRI.

CONCLUSIONS

NEFL-related CMT is clinically and genetically heterogeneous. Based on this study, however, we propose mutational hotspots and relevant clinical-genetic associations that may be helpful in the evaluation of NEFL sequence variants and the differential diagnosis with other forms of CMT.

Charcot Marie Tooth 2B Peripheral Sensory Neuropathy: How Rab7 Mutations Impact NGF Signaling?

Charcot-Marie-Tooth 2B peripheral sensory neuropathy (CMT2B) is a debilitating autosomal dominant hereditary sensory neuropathy. Patients with this disease lose pain sensation and frequently need amputation. Axonal dysfunction and degeneration of peripheral sensory neurons is a major clinical manifestation of CMT2B. However, the cellular and molecular pathogenic mechanisms remain undefined. CMT2B is caused by missense point mutations (L129F, K157N, N161T/I, V162M) in Rab7 GTPase. Strong evidence suggests that the Rab7 mutation(s) enhances the cellular levels of activated Rab7 proteins, thus resulting in increased lysosomal activity and autophagy. As a consequence, trafficking and signaling of neurotrophic factors such as nerve growth factor (NGF) in the long axons of peripheral sensory neurons are particularly vulnerable to premature degradation. A “gain of toxicity” model has, thus, been proposed based on these observations. However, studies of fly photo-sensory neurons indicate that the Rab7 mutation(s) causes a “loss of function”, resulting in haploinsufficiency. In the review, we summarize experimental evidence for both hypotheses. We argue that better models (rodent animals and human neurons) of CMT2B are needed to precisely define the disease mechanisms.

The instability of the BTB-KELCH protein Gigaxonin causes Giant Axonal Neuropathy and constitutes a new penetrant and specific diagnostic test

Background

The BTB-KELCH protein Gigaxonin plays key roles in sustaining neuron survival and cytoskeleton architecture. Indeed, recessive mutations in the Gigaxonin-encoding gene cause Giant Axonal Neuropathy (GAN), a severe neurodegenerative disorder characterized by a wide disorganization of the Intermediate Filament network. Growing evidences suggest that GAN is a continuum with the peripheral neuropathy Charcot-Marie-Tooth diseases type 2 (CMT2). Sharing similar sensory-motor alterations and aggregation of Neurofilaments, few reports have revealed that GAN and some CMT2 forms can be misdiagnosed on clinical and histopathological examination. The goal of this study is to propose a new differential diagnostic test for GAN/CMT2. Moreover, we aim at identifying the mechanisms causing the loss-of-function of Gigaxonin, which has been proposed to bind CUL3 and substrates as part of an E3 ligase complex.

Results

We establish that determining Gigaxonin level constitutes a very valuable diagnostic test in discriminating new GAN cases from clinically related inherited neuropathies. Indeed, in a set of seven new families presenting a neuropathy resembling GAN/CMT2, only five exhibiting a reduced Gigaxonin abundance have been subsequently genetically linked to GAN. Generating the homology modeling of Gigaxonin, we suggest that disease mutations would lead to a range of defects in Gigaxonin stability, impairing its homodimerization, BTB or KELCH domain folding, or CUL3 and substrate binding. We further demonstrate that regardless of the mutations or the severity of the disease, Gigaxonin abundance is severely reduced in all GAN patients due to both mRNA and protein instability mechanisms.

Conclusions

In this study, we developed a new penetrant and specific test to diagnose GAN among a set of individuals exhibiting CMT2 of unknown etiology to suggest that the prevalence of GAN is probably under-evaluated among peripheral neuropathies. We propose to use this new test in concert with the clinical examination and prior to the systematic screening of GAN mutations that has shown strong limitations for large deletions. Combining the generation of the structural modeling of Gigaxonin to an analysis of Gigaxonin transcripts and proteins in patients, we provide the first evidences of the instability of this E3 ligase adaptor in disease.

Ubiquitin ligase defect by DCAF8 mutation causes HMSN2 with giant axons

OBJECTIVE

To identify the genetic cause of axonal hereditary motor and sensory neuropathy (HMSN2) with infrequent giant axons.

METHODS

We studied 11 members of a previously described HMSN2 family with infrequent giant axons and variable cardiomyopathy. Whole-exome sequencing (WES) was performed on 2 affected persons and 1 unaffected person. Sanger sequencing was utilized to confirm the identified novel variant tracking with the affected status. Linkage analysis and haplotype mapping were obtained to confirm the causal nature of the identified variant. Cotransfection of HEK293 cells and co-immunoprecipitation assay were performed to assess the impact of the identified mutant protein in the implicated ubiquitin ligase pathway.

RESULTS

Giant axons with neurofilament accumulations were found in 3 affected persons who had undergone nerve biopsy evaluations. Six novel variants were identified by WES, but only DCAF8 p.R317C tracked with affected status within the family. Linkage and haplotype analysis using microsatellite markers supported this variant as causal. The mutation is within the DCAF8 WD repeat region critical for its binding to DDB1. Functional analysis shows DCAF8 p.R317C reduces the association of DCAF8 and DDB1, which is important in Cul4-ubiquitin E3 function.

CONCLUSIONS

Our results indicate that DCAF8 p.R317C mutation is responsible for this specific variety of HMSN2 with infrequent giant axons and mild cardiomyopathy. This mutation results in decreased DDB1-DCAF8 association, leading to an E3 ubiquitin ligase defect that is likely associated with neurofilament degradation.

Giant axonal neuropathy caused by a novel compound heterozygous mutation in the gigaxonin gene

Giant axonal neuropathy is a rare autosomal recessive disorder, which typically involves both central and peripheral nervous system. Yet the phenotypic-genotypic correlation remains obscure. We report a novel compound heterozygous mutation with the c. 805C>T in exon 4(Arg545His missense mutation) and the c. 1634G>A in exon 11(Arg269Trp missense mutation) in an 11-year-old Chinese giant axonal neuropathy case. This patient had an atypical giant axonal neuropathy phenotype rather similar to Charcot-Marie-Tooth disease, without tightly curled hair and mental retardation. The patient had a slowly progressive sensory motor neuropathy since age 3 years, and she also had nystagmus, feet deformities, scoliosis, and cerebellar tonsillar protrusion. Electrophysiological studies indicated a predominantly axonal sensory-motor neuropathy. The diagnosis was confirmed by sural nerve biopsy and direct sequencing of all the 11 gigaxonin exons. The proband's parents are heterozygotes of the disease without symptoms. Our findings extend the number of gigaxonin mutations that cause giant axonal neuropathy.

The absence of curly hair is associated with a milder phenotype in Giant Axonal Neuropathy

Giant Axonal Neuropathy is a pediatric neurodegenerative disorder caused by autosomal recessive mutations in the GAN gene on chromosome 16q24.1. Mutations in the GAN gene lead to functional impairment of the cytoskeletal protein gigaxonin and a generalized disorder of intermediate filaments, including neurofilaments in axons. Tightly curled hair is a common but not universal feature of Giant Axonal Neuropathy. The pathogenesis of curly hair is unknown, although disruption of keratin architecture is thought to play a role. As part of a broader natural history study of Giant Axonal Neuropathy, we found that the absence of curly hair is correlated with superior motor function (p=0.013) when controlling for age, as measured by the Gross Motor Function Measure. Theoretically, higher levels of functional gigaxonin protein or compensatory mechanisms could produce fewer abnormalities of neurofilaments and keratin, accounting for this phenotype. We suggest that straight-haired patients with Giant Axonal Neuropathy are potentially underdiagnosed due to their divergence from the classic phenotype of the disease. Due to their non-specific features of an axonal neuropathy, these patients may be misdiagnosed with Charcot-Marie-Tooth Disease type 2. Genetic testing for Giant Axonal Neuropathy should be considered in relevant cases of Charcot-Marie-Tooth Disease type 2.

Defective axonal transport of Rab7 GTPase results in dysregulated trophic signaling

Retrograde trophic signaling of nerve growth factor (NGF) supports neuronal survival and differentiation. Dysregulated trophic signaling could lead to various neurological disorders. Charcot-Marie-Tooth type 2B (CMT2B) is one of the most common inherited peripheral neuropathies characterized by severe terminal axonal loss. Genetic analysis of human CMT2B patients has revealed four missense point mutations in Rab7, a small GTPase that regulates late endosomal/lysosomal pathways, but the exact pathological mechanism remains poorly understood. Here, we show that these Rab7 mutants dysregulated axonal transport and diminished the retrograde signaling of NGF and its TrkA receptor. We found that all CMT2B Rab7 mutants were transported significantly faster than Rab7(wt) in the anterograde direction, accompanied with an increased percentile of anterograde Rab7-vesicles within axons of rat E15.5 dorsal root ganglion (DRG) neurons. In PC12M cells, the CMT2B Rab7 mutants drastically reduced the level of surface TrkA and NGF binding, presumably by premature degradation of TrkA. On the other hand, siRNA knock-down of endogenous Rab7 led to the appearance of large TrkA puncta in enlarged Rab5-early endosomes within the cytoplasm, suggesting delayed TrkA degradation. We also show that CMT2B Rab7 mutants markedly impaired NGF-induced Erk1/2 activation and differentiation in PC12M cells. Further analysis revealed that CMT2B Rab7 mutants caused axonal degeneration in rat E15.5 DRG neurons. We propose that Rab7 mutants induce premature degradation of retrograde NGF-TrkA trophic signaling, which may potentially contribute to the CMT2B disease.

Dominant Charcot-Marie-Tooth syndrome and cognate disorders

Charcot-Marie-Tooth neuropathy (CMT) is a group of genetically heterogeneous disorders sharing a similar phenotype, characterized by wasting and weakness mainly involving the distal muscles of lower and upper limbs, variably associated with distal sensory loss and skeletal deformities. This chapter deals with dominantly transmitted CMT and related disorders, namely hereditary neuropathy with liability to pressure palsies (HNPP) and hereditary neuralgic amyotrophy (HNA). During the last 20 years, several genes have been uncovered associated with CMT and our understanding of the underlying molecular mechanisms has greatly improved. Consequently, a precise genetic diagnosis is now possible in the majority of cases, thus allowing proper genetic counseling. Although, unfortunately, treatment is still unavailable for all types of CMT, several cellular and animal models have been developed and some compounds have proved effective in these models. The first trials with ascorbic acid in CMT type 1A have been completed and, although negative, are providing relevant information on disease course and on how to prepare for future trials.

A novel recessive Nefl mutation causes a severe, early-onset axonal neuropathy

OBJECTIVE

To report the first cases of a homozygous recessive mutation in NEFL, the gene that encodes the light subunit of neurofilaments.

METHODS

Clinical and electrophysiologic data were evaluated, and a sural nerve biopsy from one affected child was examined by immunohistochemistry and electron microscopy. The ability of the mutant protein to form filaments was characterized in an established cell culture system.

RESULTS

Four of five siblings developed of a severe, progressive neuropathy beginning in early childhood. Serial nerve conduction studies showed progressively reduced amplitudes with age and pronounced slowing at all ages. Visual-evoked responses were slowed in three children, indicating that central nervous system axons were subclinically involved. All four affected children were homozygous for a nonsense mutation at glutamate 210 (E210X) in the NEFL gene; both parents were heterozygous carriers. A sural nerve biopsy from an affected patient showed markedly reduced numbers of myelinated axons; the remaining myelinated axons were small and lacked intermediate filaments. The E210X mutant protein did not form an intermediate filament network and did not interfere with the filament formation by wild-type human light subunit of neurofilaments in SW-13 vim(-) cells.

INTERPRETATION

This is the first demonstration of a recessive NEFL mutation, which appears to cause a simple loss of function, resulting in a severe, early-onset axonal neuropathy with unique features. These results confirm that neurofilaments are the main determinant of axonal caliber and conduction velocity, and demonstrate for the first time that neurofilaments are required for the maintenance of myelinated peripheral nervous system axons.

Gigaxonin controls vimentin organization through a tubulin chaperone-independent pathway

Gigaxonin mutations cause the fatal human neurodegenerative disorder giant axonal neuropathy (GAN). Broad deterioration of the nervous system in GAN patients is accompanied by massive disorganization of intermediate filaments (IFs) both in neurons and many non-neuronal cells. With newly developed antibodies, gigaxonin is now shown to be expressed at extremely low levels throughout the nervous system. In lymphoblast cell lines derived from severe and mild forms of GAN, mutations in gigaxonin are shown to yield highly unstable proteins, thereby permitting a rapid diagnostic test for the spectrum of GAN mutations as an alternative to invasive nerve biopsy or systematic sequencing of the GAN gene. Gigaxonin has been proposed as a substrate adaptor for an E3 ubiquitin ligase, which affects proteasome-dependent degradation of microtubule-related proteins including MAP1B, MAP8 and the tubulin folding chaperone TBCB. We demonstrate that, unlike its counterpart TBCE, TBCB only moderately destabilizes microtubules. Neither TBCB abundance nor microtubule organization or densities are altered in GAN mutant fibroblasts, thus demonstrating that altered TBCB levels are not primary determinants of IF disorganization in GAN. Characteristic GAN mutant-induced ovoid aggregates of vimentin are not produced in normal fibroblasts after disrupting microtubule assembly, either by TBCE overexpression or depolymerizing drugs. Thus, IF disorganization in GAN fibroblasts is independent of TBCB and microtubule loss and must be regulated by a yet unidentified mechanism.

Autosomal-Recessive Charcot-Marie-Tooth Diseases

In certain countries around the Mediterranean basin such as Algeria, which have a high prevalence of consanguineous marriages, autosomal-recessive (AR) inheritance may account for more than 50% of all forms of Charcot-Marie-Tooth (CMT) disease. Like with the dominant forms, it is usual to differentiate the demyelinating forms (CMT 4 corresponding to autosomal-recessive CMT 1 [AR-CMT 1] from the axonal forms [AR-CMT 2]). Genetic analysis of large families with recessive transmission has uncovered novel CMT genotypes (genes: GDAP 1, MTMR 2, MTMR 13, KIAA1985, NDGR1, periaxi, lamin). The clinical and especially the histologic phenotypes often indicate that a specific gene is implicated. We present and discuss microscopic lesions seen on nerve biopsies from patients in a number of consanguineous Algerian families, and we outline the characteristic lesions that would prompt a search for mutations in genes such as MTMR 2, MTMR 13, KIAA1985, periaxin for CMT 4, and lamin for AR-CMT 2. Like with the dominant forms, there are undoubtedly many more mutations of other genes to be discovered.

Clinical utility of peripheral nerve biopsy

Histopathologic evaluation of nerve biopsy specimens provides important diagnostic information in some patients with peripheral neuropathy. The role of nerve biopsy is more restricted than that of muscle biopsy. Nerve biopsy is utilized mainly for diagnosis of vasculitis and infiltrative neuropathies. It is also utilized in diagnosis of atypical inflammatory demyelinating neuropathies in which the clinical, electrodiagnostic, and laboratory features are inconclusive. In addition, the study of nerve histopathology can also enhance our understanding of disease pathogenesis.

Giant axon and neurofilament accumulation in Charcot-Marie-Tooth disease type 2E

The axonal type 2 Charcot-Marie-Tooth disease (CMT2) is phenotypically poorly characterized. Here the authors report a family with a Pro22Ser mutation in the neurofilament-light gene (NF-L; CMT2E) manifesting electrophysiologically as the demyelinating type 1 CMT (CMT1) and pathologically as an axonopathy with giant axons and accumulation of disorganized NF. NF-L should be investigated in CMT2 as well as in CMT1 not associated with the usual genes PMP22, Cx32, and P0.