Rapid screening of myelin genes in CMT1 patients by SSCP analysis: identification of new mutations and polymorphisms in the P0 gene

Myelin protein zero mutation-related hereditary neuropathies: Neuropathological insight from a new nerve biopsy cohort

Myelin protein zero (MPZ/P0) is a major structural protein of peripheral nerve myelin. Disease-associated variants in the MPZ gene cause a wide phenotypic spectrum of inherited peripheral neuropathies. Previous nerve biopsy studies showed evidence for subtype-specific morphological features. Here, we aimed at enhancing the understanding of these subtype-specific features and pathophysiological aspects of MPZ neuropathies. We examined archival material from two Central European centers and systematically determined genetic, clinical, and neuropathological features of 21 patients with MPZ mutations compared to 16 controls. Cases were grouped based on nerve conduction data into congenital hypomyelinating neuropathy (CHN; n = 2), demyelinating Charcot-Marie-Tooth (CMT type 1; n = 11), intermediate (CMTi; n = 3), and axonal CMT (type 2; n = 5). Six cases had combined muscle and nerve biopsies and one underwent autopsy. We detected four MPZ gene variants not previously described in patients with neuropathy. Light and electron microscopy of nerve biopsies confirmed fewer myelinated fibers, more onion bulbs and reduced regeneration in demyelinating CMT1 compared to CMT2/CMTi. In addition, we observed significantly more denervated Schwann cells, more collagen pockets, fewer unmyelinated axons per Schwann cell unit and a higher density of Schwann cell nuclei in CMT1 compared to CMT2/CMTi. CHN was characterized by basal lamina onion bulb formation, a further increase in Schwann cell density and hypomyelination. Most late onset axonal neuropathy patients showed microangiopathy. In the autopsy case, we observed prominent neuromatous hyperinnervation of the spinal meninges. In four of the six muscle biopsies, we found marked structural mitochondrial abnormalities. These results show that MPZ alterations not only affect myelinated nerve fibers, leading to either primarily demyelinating or axonal changes, but also affect non-myelinated nerve fibers. The autopsy case offers insight into spinal nerve root pathology in MPZ neuropathy. Finally, our data suggest a peculiar association of MPZ mutations with mitochondrial alterations in muscle.

Novel mutation in the MPZ gene causes early-onset but slow-progressive Charcot-Marie-Tooth disease in a Russian family: a case report

Charcot-Marie-Tooth disease (CMT) is a genetically heterogeneous group of peripheral neuropathies most of which are associated with mutations in four genes including peripheral myelin protein-22 (PMP22), myelin protein zero (MPZ), gap junction protein beta1 (GJB1) and mitofusin2 (MFN2). This current case report describes the clinical and genetic characteristics of a 6-year-old male proband. A physical examination revealed muscular hypotonia. He started walking on his own at 18 months. A nerve conduction study with needle electromyography revealed conduction block. A novel MPZ mutation (c.398C > T, p.Pro133Leu) was revealed in the proband. This mutation was also found in the 32-year-old father of the proband. The father had had deformity of the feet and distal muscle weakness since childhood. The novel p.Pro133Leu pathogenic mutation was responsible for early onset but slowly progressive CMT1B. We assume that this site is an intolerant to change region in the MPZ gene. This variant in the MPZ gene is an important contributor to hereditary neuropathy with reduced nerve conduction velocity in the Russian population. This case highlights the importance of whole exome sequencing for a proper clinical diagnosis of CMT associated with a mutation in the MPZ gene.

Genetic and clinical spectrums in Korean Charcot-Marie-Tooth disease patients with myelin protein zero mutations

Background

Charcot‐Marie‐Tooth disease (CMT) is the most common disorder of inherited peripheral neuropathies characterized by distal muscle weakness and sensory loss. CMT is usually classified into three types, demyelinating, axonal, and intermediate neuropathies. Mutations in myelin protein zero (MPZ) gene which encodes a transmembrane protein of the Schwann cells as a major component of peripheral myelin have been reported to cause various type of CMT.

Methods

This study screened MPZ mutations in Korean CMT patients (1,121 families) by whole exome sequencing and targeted sequencing.

Results

We identified 22 pathogenic or likely pathogenic MPZ mutations in 36 families as the underlying cause of the CMT1B, CMTDID, or CMT2I subtypes. Among them, five mutations were novel. The frequency of CMT patients with the MPZ mutations was similar or slightly lower compared to other ethnic groups.

Conclusions

We showed that the median onset ages and clinical phenotypes varied by subtypes: the most severe in the CMT1B group, and the mildest in the CMT2I group. This study also observed a clear correlation that earlier onsets cause more severe symptoms. We believe that this study will provide useful reference data for genetic and clinical information on CMT patients with MPZ mutations in Korea.

Impaired Mitochondrial Mobility in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is a progressive, peripheral neuropathy and the most commonly inherited neurological disorder. Clinical manifestations of CMT mutations are typically limited to peripheral neurons, the longest cells in the body. Currently, mutations in at least 80 different genes are associated with CMT and new mutations are regularly being discovered. A large portion of the proteins mutated in axonal CMT have documented roles in mitochondrial mobility, suggesting that organelle trafficking defects may be a common underlying disease mechanism. This review will focus on the potential role of altered mitochondrial mobility in the pathogenesis of axonal CMT, highlighting the conceptional challenges and potential experimental and therapeutic opportunities presented by this "impaired mobility" model of the disease.

The spectrum of Charcot-Marie-Tooth disease due to myelin protein zero: An electrodiagnostic, nerve ultrasound and histological study

OBJECTIVE

Nerve ultrasound (US) data on myelin protein zero (MPZ)-related Charcot-Marie-Tooth disease (CMT) are lacking. To offer a comprehensive perspective on MPZ-related CMTs, we combined nerve US with clinics, electrodiagnosis and histopathology.

METHODS

We recruited 36 patients (12 MPZ mutations), and correlated nerve US to clinical, electrodiagnostic measures, and sural nerve biopsy.

RESULTS

According to motor nerve conduction velocity (MNCV) criteria, nine patients were categorized as "demyelinating" CMT1B, 17 as "axonal" CMT2I/J, and 10 as dominant "intermediate" CMTDID. Sural nerve biopsy showed hypertrophic de-remyelinating neuropathy with numerous complex onion bulbs in one patient, de-remyelinating neuropathy with scanty/absent onion bulbs in three, axonal neuropathy in two, mixed demyelinating-axonal neuropathy in five. Electrodiagnosis significantly differed in CMT1B vs. CMT2I/J and CMTDID subgroups. CMT1B had slightly enlarged nerve cross sectional area (CSA) especially at proximal upper-limb (UL) sites. CSA was negatively correlated to UL MNCV and not increased at entrapment sites. Major sural nerve pathological patterns were uncorrelated to UL nerve US and MNCV.

CONCLUSIONS

Sural nerve biopsy confirmed the wide pathological spectrum of MPZ-CMT. UL nerve US identified two major patterns corresponding to the CMT1B and CMT2I/J-CMTDID subgroups.

SIGNIFICANCE

Nerve US phenotype of MPZ-CMT diverged from those in other demyelinating peripheral neuropathies and may have diagnostic value.

Clinical and in silico evidence for and against pathogenicity of 11 new mutations in the MPZ gene

Mutations in the myelin protein zero (MPZ) gene are one of the frequent causes of Charcot-Marie-Tooth (CMT) hereditary neuropathies. Because the mutation rate of MPZ gene is rather high and some mutations are reported as polymorphisms, the proper clinical, electrophysiological examination and the segregation of the new mutation in larger families are crucial for the correct interpretation of the pathogenic or non-pathogenic character of each novel mutation. We examined 11 families with novel MPZ mutations. Eight of the mutations (L48Q, T65N, E97fs, G103W, P132T, T143R, V146G, c.645+1G>T) seem to be pathogenic on the basis of perfect segregation with the CMT phenotype and two (G213R and D246N), on the contrary, seem to be non-pathogenic/rare polymorphisms because they are present in healthy relatives. The character of the V46M mutation is difficult to interpret definitely; it may cause a sensory neuropathy or may also be a rare polymorphism. Phenotypes associated with each of the new mutations include severe hereditary motor and sensory neuropathy type III (HMSN III), and mild phenotype CMT1B presented mostly with only decreased or absent reflexes, foot deformities and mild or even absent atrophies in the lower limbs. Our report and careful family investigations with genotype-phenotype correlations should help to improve genetic counselling and correct interpretation of DNA testing results in further isolated patients or smaller families worldwide where these novel mutations might be found.

Mutational analysis of GJB1, MPZ, PMP22, EGR2, and LITAF/SIMPLE in Serbian Charcot-Marie-Tooth patients

We report the results of mutational analysis in the following genes: GJB1, MPZ, PMP22, EGR2, and LITAF/SIMPLE in 57 Charcot-Marie-Tooth (CMT) patients of Serbian origin without the PMP22 duplication. We found 10 different mutations in 14 CMT patients: 6 mutations in GJB1, 3 in MPZ, and 1 in PMP22. Five of six GJB1 mutations are reported for the first time, and the most frequent one appears to be a founder mutation in the Serbian population. No mutations were found in EGR2 or LITAF. Thus, GJB1 mutation analysis should be done in patients without the PMP22 duplication and male-to-male transmission of CMT.

Clinical features and molecular modelling of novel MPZ mutations in demyelinating and axonal neuropathies

Mutations in the myelin protein zero (MPZ) gene have been associated with different Charcot-Marie-Tooth disease (CMT) phenotypes, including classical demyelinating CMT1B and the axonal form of the disease (CMT2). The MPZ role in the pathogenesis of both demyelinating and axonal inherited neuropathies was evaluated in the Italian population by screening a cohort of 214 patients with CMT1 or CMT2. A MPZ mutation frequency of 7.9% in demyelinating cases and of 4.8% in axonal cases was observed. In the total cohort (264 patients), including those with mutations in other genes, a mutation frequency of 5.8% (7/121) in demyelinating cases and 4.2% (6/143) in axonal cases was found. Three novel MPZ mutations, two missense (p.Ser111Cys, p.Thr124Ala) and one frameshift (p.Tyr145fs) were found, and a molecular modelling approach was used to test the effects of these mutations on the protein structure. Electrostatic distribution changes within the protein, caused by the amino acid substitution, fit in with phenotypes presented by patients herein described. Our findings suggest that the clinical features associated with MPZ mutations depend partly on the nature of amino acid change and that molecular modelling may provide useful support, based on effects on secondary and tertiary protein structure, to predict the phenotype associated with MPZ mutations.

Charcot-Marie-Tooth disease type 1B: marked phenotypic variation of the Ser78Leu mutation in five Italian families

OBJECTIVES

To describe clinical, electrophysiological and genetic data of five unrelated Sicilian pedigrees harbouring a heterozygous Ser78Leu mutation in the myelin protein zero (MPZ) extracellular domain.

MATERIALS AND METHODS

Clinical, electrophysiological and genetic findings of 16 patients were reported. Polymorphic markers flanking the coding sequence of MPZ gene were also analysed.

RESULTS

A wide range of age at onset was observed in families 1 and 3, with a clinical heterogeneity, in terms of severity of the disease, within the same family (families 1 and 3), and among families. A markedly unsteady gait was a distinctive feature of many members of family 1. All patients in family 2 complained of severe cramps and painful paresthesia. Molecular genetic analysis showed that all affected subjects shared a common haplotype at three microsatellite loci D1S2858, D1S2624 and D1S484.

CONCLUSIONS

Our study provides further evidence that phenotypic features of MPZ mutations can vary within and among different families. High frequency of Ser78Leu mutation in Sicily as well as the results of haplotype analyses suggest that the mutation may have been inherited from a common ancestor.

New mutation of theMPZ gene in a family with the Dejerine–Sottas disease phenotype

Charcot-Marie-Tooth disease type 1B is associated with mutations in the myelin protein zero gene. In the present study a new myelin protein zero gene mutation (c.89T>C,Ile30Thr) was detected in a family with the Dejerine-Sottas disease phenotype. The results support the hypothesis that severe, early-onset neuropathy may be related to either an alteration of a conserved amino acid or a disruption of the tertiary structure of myelin protein zero.

Mutation frequency for Charcot-Marie-Tooth disease type 1 in the Chinese population is similar to that in the global ethnic patients

PURPOSE

To investigate the genetic loci/mutations among the Chinese Charcot-Marie-Tooth disease type 1 (CMT1), which accounts for approximately 70% of Charcot-Marie-Tooth; and to study the genetic heterogeneity and mutation frequency.

METHODS

CMT1A duplication and mutations at loci of MPZ, Cx32/GJB1, EGR2, and LITAF/SIMPLE were analyzed among 32 clinically diagnosed CMT1 patients of Chinese ancestry.

RESULTS

The CMT1A duplication was detected in 62.5% (20/32) CMT1 patients. This duplication accounts for the major mutation for Chinese CMT1. Among 12 cases that have no CMT1A duplication detected, three point mutations including one (3.1%) in MPZ and two (6.3%) in Cx32 were identified. No mutation was detected in genes PMP22, EGR2 and LITAF among the remaining nine (28.1%) CMT1 patients.

CONCLUSION

The mutation frequency for the Chinese CMT1 is similar to that seen in the global ethnic population. Molecular testing of the CMT1A duplication, along with the loci of MPZ and Cx32, may detect the majority of Chinese CMT1 patients.

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, P0), 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, P0 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 P0 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 P0 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. Our data suggest that P0 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 P0 gene as well.

Clinicopathological and virological analyses of familial human T-lymphotropic virus type I–associated polyneuropathy

Human T-lymphotropic virus type I (HTLV-I) is known to be the causative agent of the chronic myelopathy, HTLV-I--associated myelopathy (HAM), and on rare occasions infection is also associated with the development of polyneuropathy. Here the authors present an HTLV-I--positive family of whom four members developed a chronic demyelinating polyneuropathy without HAM. Four female patients in a family from Hokkaido in Japan developed distal dominant paresthesia and muscle weakness in the second and third decades of their life. Neurological findings at ages ranging from 50 to 65 years included mild painful sensorimotor disturbances with atrophy of the distal parts of the extremities but without pyramidal signs or hyperactive tendon reflexes. Magnetic resonance imaging (MRI) findings of brain and spinal cord were unremarkable. Serum HTLV-I antibody levels were elevated at 1:8192 to 1:32,768, whereas those in cerebrospinal fluid were low at 1:4 to 1:8. Electrophysiological studies revealed polyphasic compound muscle action potentials with denervation potentials on nerve conduction studies and neurogenic patterns by electromyography, which were consistent with signs of chronic motor dominant demyelinating polyneuropathy. Sural nerve biopsy showed decreased myelinated fibers, occurrence of globule formation, myelin ovoid and remyelinated fibers, and an infiltration of CD68-positive macrophages with occasional CD4-positive T cells in the nerve fascicles. The polyneuropathy was responsive to steroid therapy. Analyses of serological human leukocyte antigen (HLA) types indicated that none of the patients possessed a high-risk HLA type known to be associated with adult T-cell leukemia (ATL), whereas they did have high responsive alleles to HTLV-I env similar to that observed in HAM. Nucleotide sequence analysis of the HTLV-I tax region demonstrated the B subgroup in all patients. This study suggests that HTLV-I infection can result in the development of a familial form of polyneuropathy that is associated with distinct HLA class I alleles, which might possibly involve a distinct virus subtype.

Myelin protein zero gene mutations in Taiwanese patients with Charcot-Marie-Tooth disease type 1

BACKGROUND

Charcot-Marie-Tooth disease type 1 (CMT1) is the most common inherited peripheral neuropathy and represents a genetically heterogeneous condition. In addition to the peripheral myelin protein 22 gene (PMP22) duplication (CMT1A), myelin protein zero gene (MPZ) mutations may account for a certain portion of CMT1 patients (CMT1B).

OBJECTIVES

The authors analyzed the MPZ mutations in Taiwanese patients who do not have PMP22 duplication. Specifically, their clinical and molecular features were characterized.

MATERIALS AND METHODS

Twenty-four of 57 unrelated Taiwanese patients with CMT1 were selected after excluding the CMT1A duplication. Subsequent analysis of the coding regions of the MPZ gene was performed with single-strand-conformation polymorphism (SSCP), which was then followed by nucleotide sequencing.

RESULTS

Four missense mutations and one 4-base pair (bp) deletion, respectively, were identified in five patients, of which one mutation, c.173 T>A, has never been previously reported. Three missense mutations were located in exon 2, the other one in exon 3, and the deletion in exon 6.

CONCLUSIONS

This study expands the number of CMT1 associated MPZ mutation and suggests that analysis of the coding sequence of MPZ should be performed in all CMT patients without CMT1A duplication to clarify their disease nature.

Chapter 24 Genetic evaluation of inherited motor/sensory neuropathy

Inherited disorders of peripheral nerves represent a common group of neurologic diseases. Charcot-Marie-Tooth neuropathy type 1 (CMT1) is a genetically heterogeneous group of chronic demyelinating polyneuropathies with loci mapping to chromosome 17 (CMT1A), chromosome 1 (CMT1B), chromosome 16 (CMT1C) and chromosome 10 (CMT1D). CMT1A is most often associated with a tandem 1.5-megabase (Mb) duplication in chromosome 17p11.2-p12. In rare patients it may result from a point mutation in the peripheral myelin protein-22 (PMP22) gene. CMT1B is associated with point mutations in the myelin protein zero (Po or MPZ) gene. Mutations in the SIMPLE gene cause CMT1C, and CMT1D is the result of mutations in the early response 2 (ERG2 or Krox-20) gene. An X-linked form of CMT1 (CMT1X) maps to Xq13 and is associated with mutations in the connexin32 (Cx32) gene. Charcot-Marie-Tooth neuropathy type 2 (CMT2) is an axonal neuropathy that maps to chromosome 1p35-p36 (CMT2A), chromosome 3q13-q22 (CMT2B), chromosome 7p14 (CMT2D), chromosome 8p21 (CMT2E), chromosome 1q22-q23 (CMT2F) or chromosome 3q13 (CMT2G). Two X-linked forms of CMT2 have been reported (CMT2XA and CMT2XB), but the genes remain unidentified. An area that has recently expanded is the identification of autosomal recessive forms of CMT type 1 and 2. Of the eight recessive forms of CMT1 that have been identified to date, only two have been fully characterized at the molecular level (CMT1 AR B 1 and CMT1 AR D). Point mutations were found in the myotubularin-related protein-2 (MTM2) gene for CMT1 AR B1. CMT1 AR D is the result of point mutations in the N-myc downstream-regulated gene 1 (NDRG1). Dejerine-Sottas disease (DSD), also called hereditary motor and sensory neuropathy type III (HMSNIII), is a severe, infantile-onset demyelinating polyneuropathy syndrome that may be associated with point mutations in either the PMP22 gene, PO gene, EGR2 gene or the PRX gene (for the recessive form). It 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-p12 that results in reduced expression of the PMP22 gene. CMT1A and HNPP are reciprocal duplication/deletion syndromes that originate from unequal crossover during germ cell meiosis. Other rare forms of demyelinating peripheral neuropathies map to chromosome 8q, 10q and 11q.

The floppy infant: contribution of genetic and metabolic disorders

The floppy infant syndrome is a well-recognized entity for pediatricians and neonatologists. The condition refers to an infant with generalized hypotonia presenting at birth or in early life. The diagnostic work up in many instances is often complex, and requires multidisciplinary assessment. Advances in genetics and neurosciences have lead to recognition of newer diagnostic entities (several congenital myopathies), and rapid molecular diagnosis is now possible for several conditions such as spinal muscular atrophy (SMA), congenital muscular dystrophies (CMD), several forms of congenital myopathies and congenital myotonic dystrophy. The focus of the present review is to describe the advances in our understanding in the genetic, metabolic basis of neurological disorders, as well as the investigative work up of the floppy infant. An algorithm for the systematic evaluation of infants with hypotonia is suggested for the practicing pediatrician/neonatologist.

Mutation analysis of the MPZ and PMP22 genes in Croatian patients

We used single-strand conformation polymorphism analysis for mutational screening in two candidate genes, MPZ and PMP22, which have an important role in the pathogenesis of Charcot-Marie-Tooth disease (CMT) and related peripheral neuropathies. A novel Ser8Ser polymorphism was found in exon 1 of the MPZ gene in two heterozygous subjects, in a father with mild CMT2 phenotype and his daughter with normal clinical data. Thr118Met polymorphism was found in exon 5 of the PMP22 gene. The patient heterozygous for 118Met allele had CMT1 disease. We can conclude that the occurrence of the 118Met allele does not usually cause CMT1 and that it is not a clinically relevant disease marker.

Dejerine-Sottas syndrome grown to maturity: overview of genetic and morphological heterogeneity and follow-up of 25 patients

Dejerine-Sottas syndrome (DSS) is an early onset demyelinating motor and sensory neuropathy with motor nerve conduction velocities below 12 m s(-1). The phenotype is genetically heterogeneous, and autosomal dominant (AD) as well as autosomal recessive (AR) inheritance is described. Nerve pathology is highly variable. It is generally presumed that clinical course is severe, leading to wheelchair dependency at an early age. In this study we documented the clinical and pathological features in 25 patients with a DSS and we evaluated the clinical course. In our series 14 patients had an AD mutation and six were probably affected by an AR disorder. In three patients inheritance mode was unknown and two patients obviously suffered from an acquired disorder. The clinical course in all patients was documented. Nine of the 25 patients showed a moderate handicap in adult life; walking distance was still at least 1 km. Age at last investigation of the ambulant patients ranged from 22 to 62 years (mean 38.6 years), and ambulant patients were found in all genetic subgroups. We conclude that DSS, although in general denoting a more serious neuropathy than CMT1, does not imply a severe disability or wheelchair dependency in adult life.

Charcot-Marie-Tooth disease and related neuropathies: mutation distribution and genotype-phenotype correlation

Charcot-Marie-Tooth disease (CMT) is a genetically heterogeneous disorder that has been associated with alterations of several proteins: peripheral myelin protein 22, myelin protein zero, connexin 32, early growth response factor 2, periaxin, myotubularin related protein 2, N-myc downstream regulated gene 1 product, neurofilament light chain, and kinesin 1B. To determine the frequency of mutations in these genes among patients with CMT or a related peripheral neuropathy, we identified 153 unrelated patients who enrolled prior to the availability of clinical testing, 79 had a 17p12 duplication (CMT1A duplication), 11 a connexin 32 mutation, 5 a myelin protein zero mutation, 5 a peripheral myelin protein 22 mutation, 1 an early growth response factor 2 mutation, 1 a periaxin mutation, 0 a myotubularin related protein 2 mutation, 1 a neurofilament light chain mutation, and 50 had no identifiable mutation; the N-myc downstream regulated gene 1 and the kinesin 1B gene were not screened for mutations. In the process of screening the above cohort of patients as well as other patients for CMT-causative mutations, we identified several previously unreported mutant alleles: two for connexin 32, three for myelin protein zero, and two for peripheral myelin protein 22. The peripheral myelin protein 22 mutation W28R was associated with CMT1 and profound deafness. One patient with a CMT2 clinical phenotype had three myelin protein zero mutations (I89N+V92M+I162M). Because one-third of the mutations we report arose de novo and thereby caused chronic sporadic neuropathy, we conclude that molecular diagnosis is a necessary adjunct for clinical diagnosis and management of inherited and sporadic neuropathy.

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.

Rapid functional analysis in Xenopus oocytes of Po protein adhesive interactions

We have developed a coupled Xenopus oocyte expression system for evaluating the functional effects of mutations in known or suspected adhesion molecules, which allows for a very rapid assessment of intercellular adhesion. As a model protein, we first used Protein zero (Po), an adhesion molecule that mediates self-adhesion of the Schwann cell plasma membrane to form compact myelin in the mammalian PNS. A wide variety of mutations in Po cause certain human peripheral neuropathies, such as the Charcot-Marie-Tooth disease (CMT) type 1B and Dejerine-Sottas syndrome (DSS). After wild-type Po mRNA is injected, the protein is synthesized and correctly targeted to the oocyte cell surface. When two oocytes are paired, wild-type Po redistributes and concentrates at the cell-cell apposition region, and by electron microscopy, the oocyte pairs show close cell-cell appositions and are devoid of the microvilli that are observed in uninjected oocyte pairs. These are hallmark features of highly adhesive cell:cell interfaces. Several point mutations in Po were engineered, corresponding to the molecular defects in the CMT type 1B or DSS. The proteins encoded by these mutations reached the cell surface but failed to concentrate at the oocyte interface. Po carrying a point mutation that is found in DSS is not targeted on the plasma membrane and fail to accumulate at the cell-cell contact site.

Expression of peripheral myelin protein zero in sural nerve of patients with Charcot-Marie-Tooth disease 1B

A Charcot-Marie-Tooth disease 1B (CMT1B) family with a mutation of the Po gene is presented. A to G substitution of nucleotide 389 in exon 3 resulted in Lys 131 Arg substitution. Immunostaining for Po in biopsied sural nerve from one family member with CMT1B was expressed in a small number of myelinated fibers. Immunoblot analysis for Po revealed that it was of normal molecular weight (29 kDa) although significantly reduced in amount. This heterozygous mutation could lead to a reduction in the total amount of normal protein in peripheral nerves through a mechanism of loss of function.

Steroid responsive polyneuropathy in a family with a novel myelin protein zero mutation

OBJECTIVE

To report a novel hereditary motor and sensory neuropathy (HMSN) phenotype, with partial steroid responsiveness, caused by a novel dominant mutation in the myelin protein zero (MPZ) gene. Most MPZ mutations lead to the HMSN type I phenotype, with recent reports of Déjérine-Sottas, congenital hypomyelination, and HMSN II also ascribed to MPZ mutations. Differing phenotypes may reflect the effect of particular mutations on MPZ structure and adhesivity.

METHODS

Clinical, neurophysiological, neuropathological, and molecular genetic analysis of a family presenting with an unusual hereditary neuropathy.

RESULTS

Progressive disabling weakness, with positive sensory phenomena and areflexia, occurred in the proband with raised CSF protein and initial steroid responsiveness. Nerve biopsy in a less severely affected sibling disclosed a demyelinating process with disruption of compacted myelin. The younger generation were so far less severely affected, becoming symptomatic only after 30 years. All affected family members were heterozygous for a novel MPZ mutation (Ile99Thr), in a conserved residue.

CONCLUSIONS

This broadens the range of familial neuropathy associated with MPZ mutations to include steroid responsive neuropathy, initially diagnosed as chronic inflammatory demyelinating polyneuropathy.

Hereditary motor and sensory neuropathy with absence of large myelinated fibers due to absence of large neurons in dorsal root ganglia and anterior horns, clinically associated with deafness, mental retardation, and epilepsy (HMSN-ADM)

Hereditary motor and sensory neuropathy (HMSN) with autosomal recessive inheritance represents a genetically heterogeneous group of disorders with variable clinical, pathologic and electrophysiologic manifestations. A new variant of autosomal recessive HMSN, clinically defined by sensorimotor polyneuropathy associated with deafness and mental retardation, has recently been described. We report on the first autopsy case with this type of HMSN: a girl of non-consanguineous parents with a presumably autosomal recessive type of motor and sensory neuropathy clinically associated with deafness, mental retardation, and epilepsy. The autopsy showed complete absence of large myelinated fibers in peripheral motor and sensory nerves corresponding to a lack of large neurons in dorsal root ganglia and anterior horns of the spinal cord, moderate neurogenic muscle atrophy, and nearly complete absence of neurons in the dentate nucleus of the cerebellum. Molecular genetic analyses in our case revealed neither genetic alterations in the survival motor neuron gene nor in the PMP-22 gene.

Screening for mutations in the peripheral myelin genes PMP22, MPZ and Cx32 (GJB1) in Russian Charcot-Marie-Tooth neuropathy patients

Charcot-Marie-Tooth disease (CMT) and related inherited peripheral neuropathies, including Dejerine-Sottas syndrome, congenital hypomyelination, and hereditary neuropathy with liability to pressure palsies (HNPP), are caused by mutations in three myelin genes: PMP22, MPZ and Cx32 (GJB1). The most common mutations are the 1.5 Mb CMT1A tandem duplication on chromosome 17p11.2-p12 in CMT1 patients and the reciprocal 1.5 Mb deletion in HNPP patients. We performed a mutation screening in 174 unrelated CMT patients and three HNPP families of Russian origin. The unrelated CMT patients included 108 clinically and electrophysiologically diagnosed CMT1 cases, 32 CMT2 cases, and 34 cases with unspecified CMT. Fifty-nine CMT1A duplications were found, of which 58 belonged to the CMT1 patient group. We found twelve distinct mutations in Cx32, six mutations in MPZ, and two mutations in PMP22. Of these respectively, eight, five, and two lead to a CMT1 phenotype. Eight mutations (Cx32: Ile20Asn/Gly21Ser, Met34Lys, Leu90Val, and Phe193Leu; MPZ: Asp134Gly, Lys138Asn, and Thr139Asn; PMP22: ValSer25-26del) were not reported previously. Phenotype-genotype correlations were based on nerve conduction velocity studies and mutation type.

Peripheral myelin modification in CMT1B correlates with MPZ gene mutations

Morphological modifications were investigated in the peripheral nerve of three unrelated patients with CMT1B. In two patients, molecular genetic analysis showed an Arg98His mutation in the extracellular domain of MPZ, associated with irregularly uncompacted lamellae. This observation confirms previous studies of a well-defined correlation between mutations and morphological phenotypes. In the third patient, a de novo Asp109Asn mutation was associated with abnormally thick myelin sheaths. This adds to the known list of MPZ gene mutations associated with this morphological phenotype.

Alternative exon 3 splicing of the human major protein zero gene in white blood cells and peripheral nerve tissue

The major protein zero (MPZ) is involved in peripheral myelin folding. Using nested reverse transcription-PCR, we amplified several fragments of MPZ mRNAs in white blood cells and in peripheral nerve tissue. Cloning of PCR products revealed the existence of three alternative splicing patterns: one resulted in the complete loss of exon 3 and two others induced partial skipping of the exon 3 sequence. All three alternative splicing mechanisms produced a frame-shift and created an identical premature stop codon in exon 4. We conclude that the existence of these MPZ RNA transcript variants may be the result of deliberate splicing decisions and may have functional implications in the cell.

Novel mutation in the myelin protein zero gene in a family with intermediate hereditary motor and sensory neuropathy

OBJECTIVES

To determine the molecular basis for autosomal dominant intermediate hereditary motor and sensory neuropathy (HMSN) in a four generation family. The gene defects in families with intermediate HMSN are not known, but it has been suggested that most have X linked HMSN.

METHODS

All participating family members were examined clinically. Genomic DNA was obtained from 10 affected and seven unaffected members. Linkage analysis for the known HMSN loci was first performed. Mutations in the peripheral myelin protein zero gene (PMP0) were sought in two affected members, using one unaffected member for comparison, by amplification of the six exons of the gene followed by single strand conformation polymorphism (SSCP) analysis, dideoxy fingerprinting (ddF), and sequencing. Subsequently, the mutation was screened for in all affected and unaffected members in the family using Alu I digestion and in 100 unrelated control subjects using "snap back" SSCP analysis. Sequencing of cDNA from a sural nerve biopsy from an affected member was also performed.

RESULTS

The clinical phenotype was of variable severity, with motor nerve conduction velocities in the intermediate range. Linkage to PMP0 was demonstrated. Analysis of genomic DNA and cDNA for PMP0 identified a novel codon 35 GAC to TAC mutation. The mutation produces an inferred amino acid change of aspartate to tyrosine at codon six of the processed protein (Asp6Tyr) in the extracellular domain and was present in all affected family members but not in 100 unrelated controls.

CONCLUSIONS

The present findings further extend the range of phenotypes associated with PMP0 mutations and indicate that families with "intermediate" HMSN need not necessarily be X-linked as previously suggested.

Congenital hypomyelination due to myelin protein zero Q215X mutation

Congenital hypomyelination (CH) is a hereditary demyelinating peripheral neuropathy characterized by early infancy onset, distal muscle weakness, hypotonia, areflexia, and severe slowing of nerve conduction velocities. In the present report, the clinical, morphological, and immunohistochemical features of a CH case and the identification of a mutation in the gene (MPZ) for protein zero (P0) associated with this phenotype are described. This "de novo" mutation in a patient presenting with clinical features quite distinct from those of the more frequent Charcot-Marie-Tooth type 1B disease (CMT1B) or Dejerine-Sottas syndrome (DSS) confirms that CH is allelic with other disorders characterized by a less severe phenotype and a different clinical and neuropathological profile.

Inherited neuropathies: from gene to disease

Inherited disorders of peripheral nerves represent a common group of neurologic diseases. Charcot-Marie-Tooth neuropathy type 1 (CMT1) is a genetically heterogeneous group of chronic demyelinating polyneuropathies with loci mapping to chromosome 17 (CMT1A), chromosome 1 (CMT1B) and to 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. CMT1B is associated with point mutations in the myelin protein zero (P0 or MPZ) gene. The molecular defect in CMT1C is unknown. 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 an axonal neuropathy, also of undetermined cause. One form of CMT2 maps to chromosome 1p36 (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 syndrome that may be associated with point mutations in either the PMP22 gene or the P0 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. Other rare forms of demyelinating peripheral neuropathies map to chromosome 8q, 10q and 11q. Hereditary neuralgic amyotrophy (familial brachial plexus neuropathy) is an autosomal dominant disorder causing painful, recurrent brachial plexopathies and maps to chromosome 17q25.

The Thr124Met mutation in the peripheral myelin protein zero (MPZ) gene is associated with a clinically distinct Charcot-Marie-Tooth phenotype

We observed a missense mutation in the peripheral myelin protein zero gene (MPZ, Thr124Met) in seven Charcot-Marie-Tooth (CMT) families and in two isolated CMT patients of Belgian ancestry. Allele-sharing analysis of markers flanking the MPZ gene indicated that all patients with the Thr124Met mutation have one common ancestor. The mutation is associated with a clinically distinct phenotype characterized by late onset, marked sensory abnormalities and, in some families, deafness and pupillary abnormalities. Nerve conduction velocities of the motor median nerve vary from <38 m/s to normal values in these patients. Clusters of remyelinating axons in a sural nerve biopsy demonstrate an axonal involvement, with axonal regeneration. Phenotype-genotype correlations in 30 patients with the Thr124Met MPZ mutation indicate that, based on nerve conduction velocity criteria, these patients are difficult to classify as CMT1 or CMT2. We therefore conclude that CMT patients with slightly reduced or nearly normal nerve conduction velocity should be screened for MPZ mutations, particularly when additional clinical features such as marked sensory disturbances, pupillary abnormalities or deafness are also present.

Ultrastructural protein zero expression in Charcot-Marie-Tooth type 1B disease

Charcot-Marie-Tooth type 1B (CMT 1B) disease, an inherited demyelinating peripheral neuropathy, results from different point mutations located in the P0 gene on chromosome 1 q21-23. We have quantified, at the ultrastructural level, the immunocytochemical expression of the P0 protein in two unrelated CMT 1B patients with mutations (Ser 78 to Leu and Asn 122 to Ser) located in two different exons in the extracellular domain of the protein. A twofold decrease in P0 expression was observed in compact myelin in each case, compared with age-matched controls. The severity of the phenotypes showed no direct relationship to the levels of P0 protein expression in these 2 patients.

Mutations in the peripheral myelin genes and associated genes in inherited peripheral neuropathies

The peripheral myelin protein 22 gene (PMP22), the myelin protein zero gene (MPZ, P0), and the connexin 32 gene (Cx32, GJB1) code for membrane proteins expressed in Schwann cells of the peripheral nervous system (PNS). The early growth response 2 gene (EGR2) encodes a transcription factor that may control myelination in the PNS. Mutations in the respective genes, located on human chromosomes 17p11.2, 1q22-q23, Xq13.1, and 10q21.1-q22.1, are associated with several inherited peripheral neuropathies. To date, a genetic defect in one of these genes has been identified in over 1,000 unrelated patients manifesting a wide range of phenotypes, i.e., Charcot-Marie-Tooth disease type 1 (CMT1) and type 2 (CMT2), Dejerine-Sottas syndrome (DSS), hereditary neuropathy with liability to pressure palsies (HNPP), and congenital hypomyelination (CH). This large number of genetically defined patients provides an exceptional opportunity to examine the correlation between phenotype and genotype.

Mutation analysis of the nerve specific promoter of the peripheral myelin protein 22 gene in CMT1 disease and HNPP

We analysed the nerve specific promoter of the peripheral myelin protein 22 gene (PMP22) in a set of 15 unrelated patients with Charcot-Marie-Tooth type 1 disease (CMT1) and 16 unrelated patients with hereditary neuropathy with liability to pressure palsies (HNPP). In these patients no duplication/deletion nor a mutation in the coding region of the CMT1/ HNPP genes was detected. In one autosomal dominant CMT1 patient, we identified a base change in the non-coding exon 1A of PMP22 which, however, did not cosegregate with the disease in the family. This study indicates that mutations in the nerve specific PMP22 promoter and 5' untranslated exon will not be a common genetic cause of CMT1A and HNPP.

A small direct tandem duplication of the myelin protein zero gene in a patient with Dejerine-Sottas disease phenotype

We present a male patient with Dejerine-Sottas disease phenotype, who had a small direct tandem duplication of the Po gene. The pathology of the sural nerve showed hypomyelinated fibers with absence of active demyelination and onion-bulb formations composed of two parallel layers of basement membrane, consistent with congenital hypomyelination neuropathy (CHN). However, his clinical features were more severe than those of previously reported CHN patients. A GGCA insertion was identified at the position of nucleotide 560 in the myelin protein zero (Po) gene. This insertional mutation was located in exon 4 coding for the transmembrane domain of the Po gene and caused a shift of reading frame, creating a stop codon. The mutation of the transmembrane domain probably has the largest impact on Po function. The mutation was not identified in both parents.

Spectrum of mutations in Finnish patients with Charcot-Marie-Tooth disease and related neuropathies

Our patient material included families and sporadic patients of Finnish origin with the diagnosis of Charcot-Marie-Tooth (CMT) disease types 1 and 2, Dejerine-Sottas syndrome (DSS), and hereditary neuropathy with liability to pressure palsies (HNPP). We screened for mutations in the peripheral myelin protein genes connexin 32 (Cx32), myelin protein zero (P0) and peripheral myelin protein 22 (PMP22) by direct sequencing. All patients chosen for mutation screening were negative for the 1.5 Mb duplication/deletion at 17p11.2-p12. Eleven Cx32 mutations were found in 12 families, six with a CMT2 diagnosis, three with a CMT1 diagnosis and three with unclassified CMT. The total number of patients in these 12 CMTX families was 61, giving a minimum prevalence of 1.2/100,000 for CMTX in Finland. Four of the mutations, Pro58Arg, Pro172Leu, Asn175Asp and Leu204Phe, have not been previously reported. One male patient with an early onset CMT had a double Cx32 mutation, Arg22Gln and Val63Ile. The double de novo mutation was found to be of maternal grandpaternal origin. In the P0 gene a Ser78Leu mutation was found in one family with severe CMT1 and a de novo Tyr82Cys mutation was found in one DSS patient. Both mutations have been previously reported in other CMT1 families. A novel PMP22 mutation, deletion of Phe84, was found in one sporadic DSS patient. Our mutation screening results show the necessity of molecular diagnosis, in addition to clinical and electrophysiological evaluation, for proper subtyping of the disease and for accurate genetic counseling.

Tomaculous neuropathy in Charcot-Marie-Tooth disease with myelin protein zero gene mutation

Mutation of the myelin protein zero (MPZ) gene is associated with a small number of Charcot-Marie-Tooth (CMT) patients. We present a patient with Lys 130 Arg substitution in the extracellular domain who showed tomacula formation in biopsied sural nerve. CMT patients with mutations Ly 96 Glu, Lys 130 Arg and Ile 135 Leu showed tomaculous neuropathy. Present and previously reported investigations suggest that the pathological phenotypes of peripheral nerve are probably related to the mutations of the MPZ gene.