Axonal and demyelinating forms of the MPZ Thr124Met mutation

OBJECTIVE: We report on a Japanese family with Charcot Marie Tooth disease (CMT) with the Thr124Met mutation in the peripheral myelin protein zero (MPZ) gene. MATERIAL AND METHODS: Based on the clinical study, we investigated MPZ gene by direct sequence analysis and polymerase chain reaction restriction fragment length polymorphism analysis. RESULTS: Genotyping of four symptomatic family members showed that one family member with severe disease symptoms was homozygous, while the other three were heterozygous. The heterozygous cases were clinicopathologically determined to be the axonal type, which is characterized by late-onset and slow progression associated with Adie's pupil and deafness. The homozygous case was the demyelinating type, which showed earlier onset, rapid progression, sural nerve demyelination, and cranial nerve demyelination at autopsy. CONCLUSIONS: We suggest that axonal and demyelinating forms of CMT are not two distinct classes, but rather parts of a spectrum of genotypically related conditions, particularly with some MPZ mutations.     

Rapid progression of late onset axonal Charcot-Marie-Tooth disease associated with a novel MPZ mutation in the extracellular domain

Myelin protein zero (MPZ) is a major component of compact myelin in peripheral nerves where it plays an essential role in myelin formation and adhesion. MPZ gene mutations are usually responsible for demyelinating neuropathies, namely Charcot-Marie-Tooth (CMT) type 1B, Déjèrine-Sottas neuropathy and congenital hypomyelinating neuropathy. Less frequently, axonal CMT (CMT2) associated with MPZ mutations has been described. We report six patients (one sporadic case and five subjects from two apparently unrelated families) with a late onset, but rapidly progressive, axonal peripheral neuropathy. In all patients, molecular analysis demonstrated a novel heterozygous missense mutation (208C>T) in MPZ exon 2, causing the Pro70Ser substitution in the extracellular domain. The diagnosis of CMT2 associated with MPZ mutations should be considered in both sporadic and familial cases of late onset, progressive polyneuropathy. The mechanism whereby compact myelin protein mutations cause axonal neuropathy remains to be elucidated.     

Charcot-Marie-Tooth disease: a novel Tyr145Ser mutation in the myelin protein zero (MPZ,P0) gene causes different phenotypes in homozygous and heterozygous carriers within one family

Charcot-Marie-Tooth disease type 1B (CMT 1B) is caused by mutations in the gene coding for peripheral myelin protein zero (MPZ, P0) that plays a fundamental role in adhesion and compaction of peripheral myelin. Here we report a Costa Rican family with a hereditary peripheral neuropathy due to a novel Tyr145Ser MPZ mutation. Four family members were heterozygously affected; two siblings of two heterozygous carriers were homozygous for this mutation. On neurological examination the heterozygous parents and their homozygous children both showed distal sensory deficits. The mother and the siblings displayed impaired deep tendon reflexes and mild sensory ataxia. The homozygous individuals were more severely affected with an earlier age of onset, distal motor weakness, and pupillary abnormalities. Electrophysiological studies revealed both signs of demyelination and axonal nerve degeneration. The sural nerve biopsy of one sibling showed thinly myelinated nerve fibers, onion bulb formation, and clusters of regenerating fibers. On electron microscopy axonal degeneration and decompaction of inner myelin layers were found. This Costa Rican family shows phenotypic variability depending on the homozygous or heterozygous state of the Tyr145Ser mutation carriers.A. Leal and C. Berghoff contributed equally to this work.     

Effect of an R69C mutation in the myelin protein zero gene on myelination and ion channel subtypes

BACKGROUND: Most mutations in the myelin protein zero gene (MPZ) typically cause a severe demyelinating/dysmyelinating neuropathy that begins in infancy or an adult-onset axonal neuropathy. Axonal degeneration in the late-onset H10P mutation may be caused by the disruption of axoglial interaction. OBJECTIVE: To evaluate sural nerve biopsy samples from a patient with early-onset Charcot-Marie-Tooth disease type 1B caused by an arg69-to-cys (R69C) mutation. Design and PARTICIPANTS: Biopsies of sural nerves were performed 20 years apart in a patient with an R69C mutation (early onset). In addition, peripheral nerves were obtained from autopsy material from a patient with a T95M mutation (late onset). These nerves were analyzed using light microscopy of semithin sections, teased nerve fiber immunohistochemical analysis, electron microscopy, and immunologic electron microscopy. MAIN OUTCOME MEASURES: Pathological changes in sural nerve. RESULTS: Both R69C biopsy samples showed prominent demyelination and onion bulb formation, unlike the late-onset T95M mutation, which showed primarily axonal degeneration with no onion bulbs. The sural biopsy sample obtained 20 years earlier from the R69C patient showed minimal difference from the present sample, consistent with the lack of clinical progression during the 2 decades. Teased fiber immunohistochemical analysis of R69C revealed voltage-gated sodium channel subtype 1.8 expressions at the nodes of Ranvier around the areas of segmental demyelination. Internodal length in all R69C nerve fibers was invariably short (>94% of all internodes are <150 mum). CONCLUSIONS: Morphologic abnormalities in this early-onset R69C neuropathy were severe in childhood but progressed very slowly after adolescence. The switch to voltage-gated sodium channel subtype 1.8 expression at the nodes may provide clues into the pathogenesis of this case of early-onset neuropathy, and the short internodes may contribute to the extremely slowed conduction velocities in this case (<10 m/s).     

Two families of Charcot-Marie-Tooth disease with Adie's pupil, axonal neuropahy and the Thr124Met mutation in the peripheral myelin protein zero gene]

We reported two families of Charcot-Marie-Tooth disease (CMT) with Thr124Met mutation in the peripheral myelin protein zero (MPZ). The clinical features of the proband patients of both families showed Adie's pupil, severe sensory dominant neuropathy in lower extremities, and axonal changes in sural nerve biopsies and nerve conduction studies. Muscle atrophy and weakness was mild in the lower legs, while sensory impairment was marked. The proband patient of family 1 had four symptomatic siblings and one of them showed Adie's pupil. The elderly daughter of the proband of family 2 showed Adie's pupil and younger daughter showed photophobia. The biopsied sural nerves of both proband patients revealed prominent axonal sprouting, and sub-perineurial edema and mild fascicular enlargement. Segmental demyelination was not frequent in teased fiber assessment. The present two family cases strongly suggest that this MPZ gene mutation (Thr124Met) could be present among the patients with CMT type 2, axonal form. Furthermore, the patients showing sensory neuropathy and Adie's pupil may need to be reexamined with this mutation. It is also necessary to reassess genotype-phenotype correlation in CMT patients particularly in reference to type 1 and type 2.     

Four novel mutations of the myelin protein zero gene presenting as a mild and late-onset polyneuropathy

Inherited neuropathies caused by mutations of the major structural protein of peripheral myelin, myelin protein zero (MPZ), contribute to 5% of all cases of Charcot-Marie-Tooth disease (CMT). They can be divided into an early-onset neuropathy with symptoms prior to the stage of walking, and a late-onset neuropathy with symptoms at the age of 40 and older. In this study, five patients with four novel MPZ mutations were identified by molecular genetic testing which presented as mild and late-onset neuropathies. We recommend testing for MPZ mutations in patients with a late-onset neuropathy, as late-onset inherited neuropathies might be more frequent than previously thought.     

A NOVEL MYELIN PROTEIN ZERO MUTATION ASSOCIATED WITH CHARCOT-MARIE-TOOTH TYPE II DISEASE

Charcot-Marie-Tooth type II disease (CMT2) is a typical peroneal muscular atrophy syndrome and is characterised by normal or slightly reduced nerve conduction velocities with signs of axonal degeneration. CMT2 is genetically heterogeneous: linkage to 1p35–p36 (CMT2A; KIF1B gene), 3q13–q22 (CMT2B), 7p14 (CMT2D) and 8p21 (CMT2E; NF-L gene) loci has been reported for the autosomal dominant disease; however, the majority of CMT2 families do not link to any of the reported loci. Mutations of the myelin protein zero (MPZ) gene were found associated with demyelinating forms of hereditary neuropathies such as CMT1B, Dejerine-Sottas syndrome and congenital hypomyelination. So far, few CMT2 cases (CMT2F) were found to be caused by point mutations in the MPZ (see CMT Mutation Database, http://molgen-www.uia.ac.be/CMTMutations/ ) in 1q22 region.
We report a family in which three members are affected with a late-onset peripheral neuropathy. The index patient is a 68-year-old male who presents with pronounced distal muscle weakness of inferior limbs, bilateral pes cavus and absence of deep tendon reflexes. Electrophysiological findings were suggestive of an axonal form of peripheral neuropathy, thus allowing the diagnosis of CMT type 2. At the clinical and electrophysiological examination, two other family members (first cousins of the proband) resulted to be affected. MPZ gene direct sequencing revealed a heterozygous T/A transversion in the exon 3 of the gene, predicting an Asp103Glu aminoacid substitution in the extracellular domain of the protein. This variant was not found in unaffected relatives and in 100 normal chromosomes. This finding confirms the role of protein zero in axonal neuropathies and the phenotypic heterogeneity associated with MPZ mutations.
(The laboratory is a member of the European CMT Consortium; partially granted by Ministero della Sanitá to PM, MURST to FA)     

Inherited peripheral neuropathy

Hereditary disorders of the peripheral nerves constitute a group of frequently encountered neurological diseases. Charcot-Marie-Tooth neuropathy type 1 (CMT1) is genetically heterogeneous and characterized by demyelination with moderately to severely reduced nerve conduction velocities, absent muscle stretch reflexes and onion bulb formation. Genetic loci for CMT1 map to chromosome 17 (CMT1A), chromosome 1 (CMT1B), and another unknown autosome (CMT1C). CMT1A is most often associated with a tandem 1.5-megabase (Mb) duplication in chromosome 17p11.2-12, or in rare patients may result from a point mutation in the peripheral myelin protein-22 (PMP22) gene. CMT1 B result from point mutations in the myelin protein zero (Po or MPZ) gene. The molecular defect in CMT1 C is unknown. Mutations in the early growth response 2 gene (EGR2) are also associated with demyelinating neuropathy. Other rare forms of demyelinating peripheral neuropathies map to chromosome 8q, 10q, and 11q. X-linked Charcot-Marie-Tooth neuropathy (CMTX), which has clinical features similar to CMT1, is associated with mutations in the connexin32 gene. Charcot-Marie-Tooth neuropathy type 2 (CMT2) is characterized by normal or mildly reduced nerve conduction velocity with decreased amplitude and axonal loss without hypertrophic features. One form of CMT2 maps to chromosome 1 p36 (CMT2A), another to chromosome 3p (CMT2B) and another to 7p (CMT2D). Dejerine-Sottas disease (DSD), also called hereditary motor and sensory neuropathy type III (HMSNIII), is a severe, infantile-onset demyelinating polyneuropathy that may be associated with point mutations in either the PMP22 gene or the Po gene and shares considerable clinical and pathological features with CMT1. Hereditary neuropathy with liability to pressure palsies (HNPP) is an autosomal dominant disorder that results in a recurrent, episodic demyelinating neuropathy. HNPP is associated with a 1.5-Mb deletion in chromosome 17p11.2-12 and results from reduced expression of the PMP22 gene. CMT1A and HNPP are reciprocal duplication/deletion syndromes originating from unequal crossover during germ cell meiosis.     

Normal expression of myelin protein zero with frame-shift mutation correlates with mild phenotype

Mutations in the gene encoding for myelin protein zero (MPZ) cause inherited demyelinating peripheral neuropathies of different severity. The molecular and cellular mechanisms by which the MPZ mutations cause neuropathy are incompletely understood. We investigated MPZ, myelin basic protein, and peripheral myelin protein 22 (PMP22) protein expression levels in a nerve biopsy of a Charcot-Marie-Tooth type 1B patient heterozygous for the Val 102 frame-shift mutation. We demonstrate by quantitative immunohistochemical as well as by Western blot analyses that MPZ expression levels were not reduced in myelin membranes, a finding that is in accordance with the mild phenotype of this patient. Our data show that heterozygous 'loss-of-function' of MPZ may not necessarily lead to reduced protein levels. In conclusion, we demonstrate that careful analysis of protein expression levels in peripheral nerve tissues provides important information with respect to the understanding of the molecular basis of these neuropathies.     

An axonal form of Charcot-Marie-Tooth disease showing distinctive features in association with mutations in the peripheral myelin protein zero gene (Thr124Met or Asp75Val)

OBJECTIVES AND METHODS: Seven families were studied with an axonal form of Charcot-Marie-Tooth disease (CMT) associated with mutations in the peripheral myelin protein zero (MPZ) gene-Thr124Met or Asp75Val. RESULTS: Patients with these mutations commonly showed relatively late onset sensorimotor neuropathy predominantly involving the lower limbs. Sensory impairment typically was marked, and distal muscle atrophy and weakness were also present in the legs. Adie's pupil and deafness were often present, and serum creatine kinase concentrations were often raised irrespective of which MPZ mutation was present. Relatively well preserved motor and sensory nerve conduction velocities contrasted with reduced or absent compound muscle action potentials and sensory nerve action potentials. Axonal change with marked axonal sprouting was seen in sural nerve specimens. CONCLUSION: The similar associated clinical findings suggest that patients with axonal CMT with an MPZ gene mutation share distinctive clinical features.     

Late-onset CMT2 associated with a novel missense mutation in the cytoplasmic domain of the MPZ gene

Phenotypic variations have been reported in Charcot-Marie-Tooth disease type 2 (CMT2) including age-at-onset, disease progression and severity. Sporadic cases with CMT2 have also been demonstrated by genetic test. We here report a patient with late-onset CMT2 without family history, who developed gait disturbance at the age of 68. Sequence analysis revealed a novel heterozygous Arg198Gly mutation in the cytoplasmic domain of the major peripheral myelin protein zero (MPZ). The mutation is located in the protein kinase C (PKC) alpha substrate motif (RSTK) of MPZ, presumably leading to the loss of PKC-mediated phosphorylation in adhesion. Routine genetic test for CMT is not recommended for every patient with late-onset peripheral neuropathy without known causes, however, the genetic test may be taken into consideration if the patient shows a clinical phenotype similar to that of CMT, and the possibility of a de novo mutation cannot be excluded.     

Charcot-Marie-Tooth Neuropathy Type 2 And P0 Point Mutations: Two Novel Amino Acid Substitutions (ASP61GLY, TYR119CYS) and a Possible "Hotspot" on THR124MET

Mutations in the gene for the major protein component of peripheral nerve myelin, myelin protein zero (MPZ, PO), cause hereditary disorders of Schwann cell myelin such as Charcot-Marie-Tooth neuropathy type 1B (CMT1B), Dejerine-Sottas syndrome (DSS), and congenital hypomyelinating neuropathy (CHN). More recently, PO mutations were identified in the axonal type of CMT neuropathy, CMT2, which is different from the demyelinating variants with respect to electroneurography and nerve pathology. We screened 49 patients with a clinical and histopathological diagnosis of CMT2 for mutations in the PO gene. Three heterozygous single nucleotide changes were detected: two novel missense mutations, Asp61Gly and Tyr119Cys, and the known Thr124Met substitution, that has already been reported in several CMT patients from different European countries. Haplotype analysis for the PO locus proved that our patients with the 124Met allele were not related to a cohort of patients with the same mutation, all of Belgian descent and all found to share a common ancestor (7). Our data suggest that PO mutations account for a detectable proportion of CMT2 cases with virtually every patient harbouring a different mutation but recurrence of the Thr124Met amino acid substitution. The high frequency of this peculiar genotype in the European CMT population is presumably not only due to a founder effect but Thr124Met might constitute a mutation hotspot in the PO gene as well.     

Mutation analysis in Charcot-Marie Tooth disease type 1: point mutations in the MPZ gene and the GJB1 gene cause comparable phenotypic heterogeneity

Charcot-Marie-Tooth disease type 1 (CMT1) is a demyelinating peripheral neuropathy most commonly caused by a DNA duplication on chromosome 17p11.2 including the peripheral myelin protein 22 (PMP22). Point mutations in the myelin protein zero gene (MPZ) and gap junction protein, beta-1 gene (GJB1) are also found in association with CMT1 or the subclass of CMT type X (CMTX), respectively. Recently point mutations in these genes have been found in patients showing the axonal variant of CMT, CMT type 2 (CMT2). We here describe the clinical and electro-physiological findings caused by two novel and two recently described MPZ mutations and six GJB1 mutations. Different MPZ and GJB1 mutations were associated with different grades of severity in CMT1 and CMTX. The novel MPZ Glu141st op mutation was associated with the axonal CMT2. We conclude that the clinical and electrophysiological heterogeneity among CMT patients carrying point mutations in MPZ and GJB1 is similar. Thus for clinical purposes CMT1 and CMT2 patients should be screened for mutations in these two genes after duplication on chromosome 17p11.2 has been excluded as the disease causing mutation.     

Axonal phenotype of Charcot-Marie-Tooth disease associated with a mutation in the myelin protein zero gene

A French family had Charcot-Marie-Tooth disease type 2 (CMT2) which was characterised by late onset of peripheral neuropathy involvement, Argyll Robertson-like pupils, dysphagia, and deafness. Electrophysiological studies and nerve biopsy defined the neuropathy as axonal type. Genetic analysis of myelin protein zero (MPZ) found a mutation in codon 124 resulting in substitution of threonine by methionine. One of the patients, presently 30 years old, showed only Argyll Robertson-like pupils as an objective sign but no clinical or electrophysiological signs of peripheral neuropathy.     

Novel C59T leader peptide mutation in the MPZ gene associated with late-onset, axonal, sensorimotor polyneuropathy

The objective of this study was to report a novel exon-1 mutation in the myelin protein zero (MPZ) gene, resulting in axonal Charcot–Marie–Tooth neuropathy with recurrent hyper-CK-emia. In a 64-year-old woman slowly progressive distal lower limb weakness, muscle cramps in the lower limb muscles, and stocking-type numbness had developed from the age of 61. Neurologic examination revealed discrete hip flexor weakness, weakness for foot extension, diffuse wasting of the distal lower limb muscles, reduced patella tendon reflexes, and absent Achilles tendon reflexes. There was recurrently elevated creatine kinase with a maximum of 607 U/l ( n , <145 U/l). Stimulation of the peroneal and tibial nerves did not evoke a muscular response. Electromyography was neurogenic. Biopsy of the right sural nerve showed diffuse axonal degeneration and loss of axons of all diameters. Muscle biopsy showed increased fiber-size variability, angulated fibers, internalized nuclei, accumulations of nuclei, grouped atrophic muscle fibers, and fiber splitting. Molecular genetic analysis by PCR and direct nucleotide sequencing revealed the heterozygous C59T exon-1 MPZ gene mutation, resulting in the amino acid exchange S20F of the MPZ signal protein domain (leader peptide). The novel C59T mutation in the leader peptide of the MPZ gene is pathogenic and manifests as severe, late-onset, axonal, symmetric sensorimotor polyneuropathy (CMT2) and hyper-CK-emia.     

A novel mutation of myelin protein zero associated with an axonal form of Charcot-Marie-Tooth disease

OBJECTIVE: To report a new mutation in the MPZ gene which encodes myelin protein zero (P0), associated with an axonal form of Charcot-Marie-Tooth disease (CMT). METHODS: Three patients from an Italian family with a mild, late onset axonal peripheral neuropathy are described clinically and electrophysiologically. To detect point mutation in MPZ gene the whole coding sequence was examined. The structure of the mutated protein was investigated using the three dimensional model of P0. RESULTS: All patients showed a relatively mild CMT phenotype characterised by late onset and heterogeneity of the clinical and electrophysiological features. Molecular analysis demonstrated a novel heterozygous T/A transversion in the exon 3 of MPZ gene that predicts an Asp109Glu amino acid substitution in the extracellular domain of the P0. Asp109 is found at the protein surface, on beta strand E, in the interior of the P0 tetramer. CONCLUSIONS: The identification of Asp109Glu mutation confirms the pivotal role of P0 in axonal neuropathies and stresses the phenotypic heterogeneity associated with MPZ mutations. This study suggests the value of screening for MPZ mutations in CMT family members with minor clinical and electrophysiological signs of peripheral neuropathy.     

Mild recurrent neuropathy in CMT1B with a novel nonsense mutation in the extracellular domain of the MPZ gene

Clinical, electrophysiological, and neuropathological features are reported associated with a novel heterozygote point mutation in the extracellular domain of the MPZ gene, where a transversion at codon 71 in exon 3 leads to a codon stop: Glu71stop (ie GAA-->TAA). A 36 year old woman developed a mild recurrent neuropathy after intensive manual work. The motor nerve conduction velocities were slow without conduction blocks and the nerve biopsy showed signs of demyelination-remyelination, axonal loss, and regular uncompacted myelin lamellae. She inherited the mutation from her father who displayed the same mutation with a normal phenotype. This nonsense mutation may cause a dosage difference of normal P0, and is probably underrepresented in the current mutation data bases. This report further extends the phenotype of MPZ mutations and also emphasises that mild phenotype of CMT1B may be more frequent than has been appreciated.     

Focally folded myelin in Charcot–Marie–Tooth type 1B disease is associated with Asn131Lys mutation in myelin protein zero gene: short report

Charcot-Marie-Tooth disease type 1B (CMT1B) is a demyelinating neuropathy inherited as an autosomal dominant trait. The majority of CMT1B cases are caused by mutations in the myelin protein zero (P0) gene (MPZ). Only a few mutations in MPZ gene have been reported to be associated with focally folded myelin sheaths. We have studied five patients from one family with five generations, affected by CMT1B disease. The morphological studies of sural nerve biopsy performed in the proband revealed fibers with focally folded myelin. DNA sequencing analysis showed the Asn131Lys mutation in the MPZ gene in three members of the affected family.     

Morphological And Electrophysiological Signs Of Dysmyelination In Transgenic Mice Expressing CMT1B (MPZ DELSer34) or DSS (MPZ Ser34Cys) Mutations

Charcot-Marie-Tooth 1B neuropathy (CMT1B), Déjèrine-Sottas syndrome (DSS) and Congenital Hypomyelination are each associated with mutations in MPZ , encoding P0 glycoprotein, the major structural protein of peripheral nerve myelin. To explore whether new abnormal functions of mutant MPZ can explain this phenotypic diversity, we expressed either of two MPZ mutations: DELSer34 (causing CMT1B) or Ser34Cys (causing DSS) in addition to the two normal endogeneous copies of Mpz in transgenic mice. We have shown previously that overexpression of wild type Mpz causes dose-dependent dysmyelination. However, multiple lines of mice containing the MPZ mutants showed not only hypomyelination, but also abnormalities in myelin sheaths and onion bulbs that were never observed in Mpz overexpressor mice. To create a mouse model of CMT1B with no Mpz overexpression, one line of DELSer34 that expresses mutant Mpz at levels similar to one wildtype Mpz allele was crossed with heterozygous Mpz knock-out mice, to obtain the genotype Mpz ^wt/−/ Mpz ^DELSer34. These mice manifest progressive peripheral neuropathy with hypomyelination, and onion bulbs that appear around 6 months of age. As a first step towards longitudinal studies of phenotype, we performed detailed neurophysiological analyses at 12 months of age. We found signs of demyelination, with statistically significant increases of both motor latency after proximal stimulation and F-wave latencies, and decreases in both motor and mixed afferent nerve conduction velocities of sciatic nerve. The motor response was polyphasic and there was a trend towards reduced CMAP amplitudes. Thus, the clinical onset and progression, pathological features and neurophysiological findings provide a reasonable model of CMT1B. Longitudinal studies to correlate the onset and progression of morphological and electrophysiological abnormalities in these mice are ongoing.