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.     

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.     

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.     

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)     

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.     

Late onset Charcot-Marie-Tooth 2 syndrome caused by two novel mutations in the MPZ gene

MPZ gene mutations cause demyelinating and axonal Charcot-Marie-Tooth (CMT) disease. Two novel MPZ mutations are reported in very late onset and progressive CMT syndrome. The N60H caused axonal CMT in a large family, whereas the I62M occurred in a single patient presenting with a primary axonal neuropathy. Previously, chronic polyradiculoneuritis was assumed in two patients. Molecular genetic testing and particularly screening for MPZ mutations in late onset neuropathies are important to differentiate acquired and inherited neuropathies.     

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.     

A case of CMT 1B due to Val 102/fs null mutation of the MPZ gene presenting as hyperCKemia

Charcot-Marie-Tooth disease (CMT) is a group of clinically and genetically heterogeneous neuropathies classically divided into demyelinating (CMT1) and axonal forms (CMT2). The most common demyelinating form is CMT1A, due to a duplication in the gene encoding the peripheral myelin protein 22 (PMP22). Less frequently, mutations in the myelin protein zero gene (MPZ/P0) account for demyelinating CMT1B. Herein, we report a patient presenting with an isolated hyperCKemia in whom electrophysiological and pathological findings revealed a demyelinating and axonal neuropathy. Sequencing of the MPZ gene revealed a 306delA at codon 102 in the proband and in two relatives. This mutation has been already described in association with paucisymptomatic CMT without hyperCKemia.     

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.     

Major myelin protein gene (P0) mutation causes a novel form of axonal degeneration

Mutations in the major peripheral nervous system (PNS) myelin protein, myelin protein zero (MPZ), cause Charcot-Marie-Tooth Disease type 1B (CMT1B), typically thought of as a demyelinating peripheral neuropathy. Certain MPZ mutations, however, cause adult onset neuropathy with minimal demyelination but pronounced axonal degeneration. Mechanism(s) for this phenotype are unknown. We performed an autopsy of a 73-year-old woman with a late-onset neuropathy caused by an H10P MPZ mutation whose nerve conduction studies suggested severe axonal loss but no demyelination. The autopsy demonstrated axonal loss and reorganization of the molecular architecture of the axolemma. Segmental demyelination was negligible. In addition, we identified focal nerve enlargements containing MPZ and ubiquitin either in the inner myelin intralaminar and/or periaxonal space that separates axons from myelinating Schwann cells. Taken together, these data confirmed that a mutation in MPZ can cause axonal neuropathy, in the absence of segmental demyelination, thus uncoupling the two pathological processes. More important, it also provided potential molecular mechanisms as to how the axonal degeneration occurred: either by disruption of glial-axon interaction by protein aggregates or by alterations in the molecular architecture of internodes and paranodes. This report represents the first study in which the molecular basis of axonal degeneration in the late-onset CMT1B has been explored in human tissue.     

Autonomic and respiratory dysfunction in Charcot-Marie-Tooth disease due to Thr124Met mutation in the myelin protein zero gene.

OBJECTIVE: To report the clinical and electrophysiological characteristics of a family presenting Charcot-Marie-Tooth disease (CMT) associated with autonomic nervous system disturbances. METHODS: We studied nerve conduction values, postural adaptation, sympathetic skin reflex, the variation in heart rate by the Valsalva ratio and pupillometry in 7 members of a French family in which CMT due to a Thr124Met mutation in the myelin protein zero (MPZ) gene was diagnosed. RESULTS: Clinical and laboratory evidence of autonomic nervous system disturbances were found in the affected individuals. The clinical phenotype was characterized by sensorimotor peripheral neuropathy, defined as axonal type by electrophysiological studies, and was associated with severe pain, bladder dysfunction, sudorimotor disturbances and abolished pupillary reflex to light. Moreover, two patients had severe restrictive respiratory insufficiency requiring noninvasive mechanical ventilation. CONCLUSIONS: Our study demonstrates that autonomic disturbances may be one of the major clinical signs associated with CMT secondary to MPZ gene mutation in codon 124. Testing of pupillary reflex allows the discrimination of affected and unaffected subjects in our family. However, involvement of the autonomic nervous system in this type of neuropathy is unclear and further studies are required to elucidate the role of the MPZ gene in the autonomic nervous system.     

Phenotypic variation of a novel nonsense mutation in the P0 intracellular domain.

Mutations in the gene for the peripheral myelin protein zero (P0, MPZ) cause type 1B of Charcot-Marie-Tooth sensorimotor neuropathy (CMT1B). Here we report a German family with a novel heterozygous P0 nonsense mutation (G206X) that supposedly removes four-fifths of the amino acid residues constituting the P0 intracellular domain. The 12-year-old propositus had childhood-onset CMT1B associated with bilateral pes cavus, moderate lower limb weakness, and mildly reduced sensory qualities in the distal legs. The electrophysiology was consistent with a demyelinating neuropathy. He inherited the mutation from his mother who had no complaints but slight pes cavus deformity and slow nerve conduction velocities (NCV). Conclusively, truncating mutations within the P0 intracellular domain do not necessarily cause a severe phenotype such as Dejerine-Sottas syndrome (DSS) or congenital hypomyelinating neuropathy (CHN), but can result in mild or moderate CMT1B with intrafamilial clinical variability.     

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.     

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.     

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.     

A new MPZ mutation associated with a mild CMT1 phenotype presenting with recurrent nerve compression

P0 is a transmembrane protein of the immunoglobulin superfamily that plays a role in myelin structure and function. Myelin protein zero gene (MPZ) mutations usually cause a demyelinating variant of Charcot-Marie-Tooth disease type 1B (CMT1B), but there is a wide spectrum of phenotypic manifestation of these mutations. We describe three patients from one family and one separate patient who presented with a demyelinating neuropathy. Some had recurrent lesions at compression sites mimicking hereditary neuropathy with liability to pressure palsies (HNPP). A heterozygous nonsense mutation (Tyr145Stop) corresponding to a T-to-A transition at nucleotide position 435 in exon 3 of the MPZ gene was identified in all patients. This mutation leads to an extracellular truncated protein, which may explain the mild phenotype. Therefore, such MPZ gene mutations should be searched for in cases of demyelinating neuropathy with acute nerve compression as well as in cases of the HNPP phenotype associated with normal the PMP22 gene.     

A novel MPZ gene mutation in exon 2 causing late-onset demyelinating Charcot-Marie-Tooth disease

The myelin protein zero gene (MPZ) encodes the major structural protein component of myelin in the peripheral nervous system. More than 120 mutations in MPZ have been detected so far. Clinical phenotypes include CMT1B, CMT2, Dejerine-Sottas syndrome, and congenital hypomyelination neuropathy. We report a new previously unreported mutation in the MPZ gene causing a demyelinating peripheral neuropathy. The initial apparent absence of a family history resulted in the patient being treated for an inflammatory neuropathy with some subjective improvement. We subsequently identified another affected member of the same family with the same genotype leading to the correct diagnosis. Both the affected individuals had an 8-base pair deletion, c.160_167delTCCCGGGT in MPZ exon 2.     

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.     

A novel HSPB1 mutation associated with a late onset CMT2 phenotype: Case presentation and systematic review of the literature

Mutations in the HSPB1 gene are associated with Charcot‐Marie‐Tooth (CMT) disease type 2F (CMT2F) and distal hereditary motor neuropathy type 2 (dHMN2). More than 18 pathogenic mutations spanning across the whole HSPB1 gene have been reported. Three family members with a novel p.P57S (c.169C>T) HSPB1 mutation resulting in a late onset axonal neuropathy with heterogeneous clinical and electrophysiological features are detailed. We systematically reviewed published case reports and case series on HSPB1 mutations. While a genotype‐phenotype correlation was not obvious, we identified a common phenotype, which included adult onset, male predominance, motor more frequently than sensory involvement, distal and symmetric distribution with preferential involvement of plantar flexors, and a motor and axonal electrophysiological picture.     

Clinical and genetic description of a family with Charcot-Marie-Tooth disease type 1B from a transmembrane MPZ mutation

Mutations in the myelin protein zero gene (MPZ) are associated with certain demyelinating neuropathies, and in particular with Charcot-Marie-Tooth disease type 1B (CMT1B), Dejerine-Sottas syndrome, and congenital hypomyelination. MPZ mutations affecting the protein's transmembrane domain are generally associated with more severe phenotypes. We describe a family with mild CMT1B associated with a transmembrane MPZ mutation. Sequence analysis identified a G-to-C transversion at nucleotide 1064, predicting a glycine-to-arginine substitution in codon 163 (G163R) of MPZ. This report furthers the understanding of the clinical and electrophysiological manifestations of MPZ mutations.