Charcot-Marie-Tooth disease: a clinico-genetic confrontation

Charcot-Marie-Tooth disease (CMT) is the most common neuromuscular disorder. It represents a group of clinically and genetically heterogeneous inherited neuropathies. Here, we review the results of molecular genetic investigations and the clinical and neurophysiological features of the different CMT subtypes. The products of genes associated with CMT phenotypes are important for the neuronal structure maintenance, axonal transport, nerve signal transduction and functions related to the cellular integrity. Identifying the molecular basis of CMT and studying the relevant genes and their functions is important to understand the pathophysiological mechanisms of these neurodegenerative disorders, and the processes involved in the normal development and function of the peripheral nervous system. The results of molecular genetic investigations have impact on the appropriate diagnosis, genetic counselling and possible new therapeutic options for CMT patients.

Charcot-Marie-Tooth disease: a clinico-genetic confrontation

Charcot-Marie-Tooth disease (CMT) is the most common neuromuscular disorder. It represents a group of clinically and genetically heterogeneous inherited neuropathies. Here, we review the results of molecular genetic investigations and the clinical and neurophysiological features of the different CMT subtypes. The products of genes associated with CMT phenotypes are important for the neuronal structure maintenance, axonal transport, nerve signal transduction and functions related to the cellular integrity. Identifying the molecular basis of CMT and studying the relevant genes and their functions is important to understand the pathophysiological mechanisms of these neurodegenerative disorders, and the processes involved in the normal development and function of the peripheral nervous system. The results of molecular genetic investigations have impact on the appropriate diagnosis, genetic counselling and possible new therapeutic options for CMT patients.

GDAP1 mutations in Czech families with early-onset CMT

Mutations in the ganglioside-induced differentiation associated protein-1 gene (GDAP1) cause autosomal recessive (AR) demyelinating or axonal Charcot-Marie-Tooth neuropathy (CMT). In order to establish the spectrum and frequency of GDAP1 mutations in Czech population, we sequenced GDAP1 in 74 Czech patients from 69 unrelated families with early-onset demyelinating or axonal CMT compatible with AR inheritance. We identified three isolated patients with GDAP1 mutations in both alleles. In one additional sporadic and one familial case, the second pathogenic mutation remained unknown. Overall, we detected two different mutations, a novel R191X nonsense and a L239F missense mutation. L239F previously described in a German-Italian family is a prevalent mutation in Czech population and we give evidence for its common ancestral origin. All Czech GDAP1 patients developed involvement of all four limbs evident by the end of second decade, except for one isolated patient showing very slow disease progression. All patients displayed axonal type of neuropathy.

Novel mutations in the HSN2 gene causing hereditary sensory and autonomic neuropathy type II

Hereditary sensory and autonomic neuropathy type II (HSAN-II) is caused by recessive mutations in the HSN2 gene assigned to chromosome 12p13.33. The authors report three unrelated HSAN-II families with homozygous or compound heterozygous mutations resulting in the truncation of the HSN2 protein. Genotype-phenotype correlations indicated that HSN2 mutations are associated with an early childhood onset of a predominantly sensory neuropathy, complicated by acromutilations in both upper and lower limbs.

Absence of KIF1B mutation in a large Turkish CMT2A family suggests involvement of a second gene

BACKGROUND

Charcot-Marie-Tooth disease type 2A (CMT2A) was assigned to a 19.3-cM region on chromosome 1p35-36. A missense mutation in the kinesin family member 1B gene (KIF1B) was reported in a single CMT2A family.

OBJECTIVE

To report the clinical and genetic data of a Turkish family with CMT2A.

METHODS

Linkage to CMT2 loci was investigated in the family. Haplotype analysis of the CMT2A region was completed using additional single-nucleotide polymorphism and short tandem repeat markers. The KIF1B gene was sequenced on genomic DNA and cDNA in two patients.

RESULTS

A recombination event narrowed the CMT2A locus to a 9.3-cM region flanked by D1S160 and D1S434. No mutation in KIF1B was found.

CONCLUSION

The exclusion of KIF1B gene mutations in this family suggests the involvement of another CMT2A gene in the linked region.

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

The authors report an Italian family with autosomal-dominant Charcot-Marie-Tooth disease (CMT) in which there were giant axons in the sural nerve biopsy. Linkage to the known CMT2 loci (CMT2A, CMT2B, CMT2D, CMT2F) and mutations in the known CMT2 genes (Cx32, MPZ, NEFL), GAN, NEFM, and CMT1A duplication/HNPP deletion were excluded. This family with CMT and giant axons has a pathologic and genetic entity distinct from classic CMT.

Mutations in GDAP1: autosomal recessive CMT with demyelination and axonopathy

BACKGROUND

Mutations in the ganglioside-induced differentiation-associated protein 1 gene (GDAP1) were recently shown to be responsible for autosomal recessive (AR) demyelinating Charcot-Marie-Tooth disease (CMT) type 4A (CMT4A) as well as AR axonal CMT with vocal cord paralysis.

METHODS

The coding region of GDAP1 was screened for the presence of mutations in seven families with AR CMT in which the patients were homozygous for markers of the CMT4A locus at chromosome 8q21.1.

RESULTS

A nonsense mutation was detected in exon 5 (c.581C>G, S194X), a 1-bp deletion in exon 6 (c.786delG, G262fsX284), and a missense mutation in exon 6 (c.844C>T, R282C).

CONCLUSIONS

Mutations in GDAP1 are a frequent cause of AR CMT. They result in an early-onset, severe clinical phenotype. The range of nerve conduction velocities (NCV) is variable. Some patients have normal or near normal NCV, suggesting an axonal neuropathy, whereas others have severely slowed NCV compatible with demyelination. The peripheral nerve biopsy findings are equally variable and show features of demyelination and axonal degeneration.

Exclusion of 5 functional candidate genes for distal hereditary motor neuropathy type II (distal HMN II) linked to 12q24.3

Distal hereditary motor neuropathies (distal HMNs) are characterised by degeneration of anterior horn cells of the spinal cord resulting in muscle weakness and atrophy. Distal HMN type II is genetically linked to chromosome 12q24.3 and located within a 13 cM region flanked by markers D12S86 and D12S340. We previously excluded the human phospholipase A2 group 1B gene (PLA2G1B) as the disease causing gene. Here, we report the mutation analysis of five other candidate genes localised within the distal HMN II region: the cytoskeletal proteins paxillin (PXN) and restin (RSN); the acidic ribosomal phosphoprotein, large P0 subunit (RPLP0); a nucleoside diphosphate kinase (NME2B); and the beta 3 subunit of the voltage-gated calcium channel (CACNB3). DNA sequencing of the coding regions was performed but no disease causing mutations could be identified, hence excluding these five genes for distal HMN type II.

Further evidence that neurofilament light chain gene mutations can cause Charcot-Marie-Tooth disease type 2E

A missense mutation in the neurofilament light chain gene (NEFL, NF-L) at chromosome 8p21 was recently reported in a single Charcot-Marie-Tooth type 2 family (CMT2). This new CMT2 variant is designated CMT2E. The NEFL gene mutation showed co-segregation with the disease phenotype and is thus most likely the disease-causing mutation. However, the possibility that it is a closely linked rare polymorphism can not be ruled out with certainty. We observed a novel NEFL missense mutation in a second CMT family, providing supporting evidence that CMT2E is caused by NEFL gene mutations.

Charcot-Marie-Tooth disease and related peripheral neuropathies

Soon after the description of Charcot-Marie-Tooth disease (CMT) in 1886, it became apparent that this syndrome is clinically and genetically heterogeneous. Neuropathological and electrophysiological studies have further dissected this syndrome into distinct categories that are now classified in a complex nosology of the inherited peripheral neuropathies. The recent advent of molecular genetics has dramatically increased our understanding of the underlying disease mechanisms. Genetic linkage studies have identified at least 17 genetic loci for different types of inherited neuropathies although most genes involved still remain to be found. The application of molecular genetics has already had an important impact on clinical practice and genetic counselling. Three genes responsible for hereditary motor and sensory neuropathy type I (HMSNI) or CMT1 have been identified: peripheral myelin protein 22 (PMP22) and myelin protein zero (MPZ) for the autosomal dominant form and connexin 32 (Cx32) for the X-linked dominant variant. The PMP22 gene is also involved in the majority of families with hereditary neuropathy with liability to pressure palsies (HNPP). The observation of a 1.5 Mb tandem duplication in chromosome 17p11.2, containing the PMP22 gene, in CMT1 and the reciprocal deletion in the same region in HNPP has provided a novel disease paradigm for autosomal dominant disorders, i.e. the gene dosage mechanism. The study of phenotype-genotype correlations in transgenic animal models for PMP22, MPZ and Cx32 mutations will help elucidate the underlying disease mechanisms and will provide a basis for gene therapy and/or other therapeutic approaches such as treatment with neurotrophic growth factors.

Molecular genetics of inherited peripheral neuropathies: who are the actors?

Charcot-Marie-Tooth disease, the most common variant of the inherited peripheral neuropathies, has a prevalence of 1/2500. Clinical, electrophysiological, neuropathological and molecular genetic studies have demonstrated extensive heterogeneity. Currently, 30 genetic loci are known for distinct CMT types and related inherited peripheral neuropathies, while many other types have been excluded for linkage to these loci. Recent molecular genetic studies have demonstrated the involvement of 8 genes that encode proteins with very diverse functions. These include a structural protein confined to the compact myelin, a cytoskeletal protein, an adhesion molecule, a gap-junction protein, a transcription factor, a receptor for a neurotrophic factor, a phosphatase and a protein involved in signal transduction and cell cycle regulation.

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.

A novel type of hereditary motor and sensory neuropathy characterized by a mild phenotype

BACKGROUND

Three loci for autosomal dominant hereditary motor and sensory neuropathy type I (HMSN I) or Charcot-Marie-Tooth disease type 1 (CMT1) have been identified on chromosomes 17p11.2 (CMT1A), 1q21-q23 (CMT1B), and 10q21.1-q22.1 (designated here as CMT1D). The genes involved are peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ), and the early growth response element 2 (EGR2), respectively. Probably a fourth locus (CMT1C) exists since some autosomal dominant HMSN I families have been excluded for linkage with the CMT1A and CMT1B loci. Four loci for autosomal dominant hereditary motor and sensory neuropathy type II (HMSN II) or Charcot-Marie-Tooth disease type 2 (CMT2) have been localized on chromosomes 1p35-p36 (CMT2A), 3q13-q22 (CMT2B), 7p14 (CMT2D), and 3p (HMSN-P).

OBJECTIVE

To describe the clinical, electrophysiologic, and neuropathological features of a novel type of Charcot-Marie-Tooth disease.

PATIENTS AND METHODS

We performed linkage studies with anonymous DNA markers flanking the known CMT1 and CMT2 loci. Patients and their relatives underwent clinical neurologic examination and electrophysiologic testing. In the proband, a sural nerve biopsy specimen was examined.

RESULTS

Linkage studies excluded all known CMT1 and CMT2 loci. The clinical phenotype is mild and almost all affected individuals remain asymptomatic. Electrophysiologic and histopathological studies showed signs of a demyelinating neuropathy, but the phenotype is unusual for either autosomal dominant HMSN I or HMSN II.

CONCLUSION

Our findings indicate that the HMSN in this family represents a novel clinical and genetic entity.

Molecular diagnostic testing in Charcot-Marie-Tooth disease and related disorders. Approaches and results

The inherited neuropathies of the peripheral nervous system are clinically and genetically a heterogeneous group of disorders. Molecular genetic studies have made major breakthroughs in unraveling the underlying gene defects, and DNA diagnosis can now be offered to a large number of families with distinct forms of hereditary peripheral neuropathies. With the currently available technology, however, molecular genetic diagnosis still remains a labor-intensive and costly procedure. We have developed an algorithm for mutation screening based on clinical phenotype, electrophysiological findings, and the relative frequencies of mutations in the distinct peripheral myelin genes.

Molecular genetics and biology of inherited peripheral neuropathies: a fast-moving field

The recent progress of molecular genetics has considerably increased our knowledge about the underlying disease mechanism of inherited peripheral neuropathies. Mutations in three genes coding for the myelin proteins peripheral myelin protein 22, myelin protein zero and connexin 32 and in one gene coding for the transcription factor early growth response 2 element are associated with Charcot-Marie-Tooth type 1 and 2, hereditary neuropathy with liability to pressure palsies, Dejerine-Sottas syndrome and congenital hypomyelination. This review focuses on the correlation of the different human phenotypes associated with distinct mutations with those found in cellular and animal models.

Novel missense mutation in the early growth response 2 gene associated with Dejerine-Sottas syndrome phenotype

BACKGROUND

Mutations in the early growth response 2 (EGR2) gene have recently been found in patients with congenital hypomyelinating neuropathy and Charcot-Marie-Tooth type 1 (CMT1) disease.

OBJECTIVE

To determine the frequency of EGR2 mutations in patients with a diagnosis of CMT1, Dejerine-Sottas syndrome (DSS), or unspecified peripheral neuropathies.

METHODS

Fifty patients and 70 normal control subjects were screened.

RESULTS

A de novo missense mutation (Arg359Trp) in the alpha-helix of the first zinc-finger domain of the EGR2 transcription factor was identified in a patient diagnosed with a clinical phenotype consistent with DSS. This patient had a motor median nerve conduction velocity of 8 m/s. A sural nerve biopsy showed a severe loss of myelinated and unmyelinated fibers, evidence for demyelination, numerous classic onion bulbs, and focally folded myelin sheaths. DSS is a severe, childhood-onset demyelinating peripheral neuropathy initially thought to be inherited as an autosomal recessive trait. However, several dominant heterozygous mutations in the peripheral myelin protein 22 (PMP22) gene and dominant mutations in the peripheral myelin protein zero (MPZ) gene, both in the heterozygous and homozygous state, have been reported in patients with DSS.

CONCLUSIONS

Hereditary peripheral neuropathies represent a spectrum of disorders due to underlying defects in myelin structure or formation.

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.

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.

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.

An adhesion test system based on Schneider cells to determine genotype-phenotype correlations for mutated P0 proteins

Myelin protein zero (MPZ, P0) is well known as the adhesion molecule responsible for the compaction of the myelin sheath of peripheral nerves. Mutations are linked to Charcot-Marie-Tooth syndrome type 1B (CMT1B) and the more severe Dejerine-Sottas syndrome (DSS). Three mutations leading to phenotypes of increasing severity (Ser34del/CMT1B, Ser34Cys/DSS, INS663GC/DSS) were expressed in S2 insect cells and resulted in a decreased adhesion capability in correlation with their respective phenotypes.

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.

Human meiotic recombination products revealed by sequencing a hotspot for homologous strand exchange in multiple HNPP deletion patients

The HNPP (hereditary neuropathy with liability to pressure palsies) deletion and CMT1A (Charcot-Marie-Tooth disease type 1A) duplication are the reciprocal products of homologous recombination events between misaligned flanking CMT1A-REP repeats on chromosome 17p11. 2-p12. A 1.7-kb hotspot for homologous recombination was previously identified wherein the relative risk of an exchange event is 50 times higher than in the surrounding 98.7% identical sequence shared by the CMT1A-REPs. To refine the region of exchange further, we designed a PCR strategy to amplify the recombinant CMT1A-REP from HNPP patients as well as the proximal and distal CMT1A-REPs from control individuals. By comparing the sequences across recombinant CMT1A-REPs to that of the proximal and distal CMT1A-REPs, the exchange was mapped to a 557-bp region within the previously identified 1.7-kb hotspot in 21 of 23 unrelated HNPP deletion patients. Two patients had recombined sequences suggesting an exchange event closer to the mariner-like element previously identified near the hotspot. Five individuals also had interspersed patches of proximal or distal repeat specific DNA sequence indicating potential gene conversion during the exchange of genetic material. Our studies provide a direct observation of human meiotic recombination products. These results are consistent with the hypothesis that minimum efficient processing segments, which have been characterized in Escherichia coli, yeast, and cultured mammalian cells, may be required for efficient homologous meiotic recombination in humans.

Absence of mutations in peripheral myelin protein-22, myelin protein zero, and connexin 32 in autosomal recessive Dejerine-Sottas syndrome

Motor and sensory neuropathies with the clinical features of HMSN III (Dejerine-Sottas syndrome, DSS) are etiologically related to heterozygous mutations in either peripheral myelin protein-22 (PMP22) or myelin protein zero (MPZ). Heterozygous mutations in either of these two genes are also responsible for other hereditary peripheral neuropathies (HNPP, CMT1A, CMT1B or CH). In two families DSS was related to the homozygous presence of a MPZ mutation while heterozygosity showed a much milder phenotype. It has therefore been suggested that the clinical phenotype in peripheral neuropathies is related to the mutated gene, the type of mutation and confounding effects from other sources. In this study we describe a family with recessive DSS in which mutations were absent from the PMP22, MPZ, and connexin 32 (Cx32) genes. We conclude that DSS also exists as a distinct genetic entity with autosomal recessive inheritance as originally defined by Dejerine and Sottas in 1893.

Advances in Charcot-Marie-Tooth disease research: cellular function of CMT-related proteins, transgenic animal models, and pathomechanisms. The European CMT Consortium

The First Workshop of the European Consortium on Charcot-Marie-Tooth (CMT) disease brought together neuroscientists, molecular and cell biologists, neuropathologists, neurologists, and geneticists with a common interest in the understanding of the fundamental mechanisms that underlie the pathogenesis of CMT. The interdisciplinary group of 25 expert scientists discussed recent advances in (i) molecular genetics and histopathology of CMT, (ii) development of suitable animal models, (iii) understanding of the cellular function of CMT-related proteins, and (iv) studies using nerve biopsies from CMT patients. In this minireview, we summarize the key findings presented and discuss their impact on CMT research.

Mutational analysis of the MPZ, PMP22 and Cx32 genes in patients of Spanish ancestry with Charcot-Marie-Tooth disease and hereditary neuropathy with liability to pressure palsies

Charcot-Marie-Tooth disease (CMT) and hereditary neuropathy with liability to pressure palsies (HNPP) are two inherited peripheral neuropathies. The most prevalent mutations are a reciprocal 1.5-Mb duplication and 1.5-Mb deletion, respectively, at the CMT1A/HNPP locus on chromosome 17p11.2. Point mutations in the coding region of the myelin genes, peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) or connexin 32 (Cx32) have been reported in CMT patients, including CMT type 1 (CMT1), CMT type 2 (CMT2) and Déjérine-Sottas neuropathy (DS) patients, and only in the coding region of PMP22 in HNPP families lacking a deletion. We have investigated point and small mutations in the MPZ, PMP22 and Cx32 genes in a series of patients of Spanish ancestry: 47 CMT patients without duplications, and 5 HNPP patients without deletions. We found 15 different mutations in 16 CMT patients (34%). Nine different mutations in ten patients were detected in the Cx32 gene, this being the most frequently involved gene in this series, whereas five mutations involved the MPZ gene and only one the PMP22 gene. Six out of nine nucleotide substitutions in the Cx32 gene involved two codons encoding arginine at positions 164 and 183, suggesting that these two codons may constitute two Cx32 regions prone to mutate in the Spanish population. Analysis of HNPP patients revealed a 5' splicing mutation in intron 1 of the PMP22 gene in a family with autosomal dominance, which confirms allelic heterogeneity in HNPP. Ectopic mRNA analysis on leukocytes suggests that this mutation might behave as a null allele.

Further evidence supporting linkage of hereditary neuralgic amyotrophy to chromosome 17q

Hereditary neuralgic amyotrophy is a rare autosomal dominant disorder of the peripheral nervous system. Previous segregation analysis in two large pedigrees suggested linkage to distal 17q. Linkage data obtained in the present study investigating a three generation pedigree confirm linkage to 17q24-q25.

Detection of the CMT1A/HNPP recombination hotspot in unrelated patients of European descent

Charcot-Marie-Tooth type 1 disease (CMT1) and hereditary neuropathy with liability to pressure palsies (HNPP) are common inherited disorders of the peripheral nervous system. The majority of CMT1 patients have a 1.5Mb tandem duplication (CMT1A) in chromosome 17p11.2 while most HNPP patients have a deletion of the same 1.5 Mb region. The CMT1A duplication and HNPP deletion are the reciprocal products of an unequal crossing over event between misaligned flanking CMT1A-REP elements. We analysed 162 unrelated CMT1A duplication patients and HNPP deletion patients from 11 different countries for the presence of a recombination hotspot in the CMT1A-REP sequences. A hotspot for unequal crossing over between the misaligned flanking CMT1A-REP elements was observed through the detection of novel junction fragments in 76.9% of 130 unrelated CMT1A patients and in 71.9% of 32 unrelated HNPP patients. This recombination hotspot was also detected in eight out of 10 de novo CMT1A duplication and in two de novo HNPP deletion patients. These data indicate that the hotspot of unequal crossing over occurs in several populations independently of ethnic background and is directly involved in the pathogenesis of CMT1A and HNPP. We conclude that the detection of junction fragments from the CMT1A-REP element on Southern blot analysis is a simple and reliable DNA diagnostic tool for the identification of the CMT1A duplication and HNPP deletion in most patients.

Clinical phenotypes of different MPZ (P0) mutations may include Charcot-Marie-Tooth type 1B, Dejerine-Sottas, and congenital hypomyelination

Hereditary demyelinating peripheral neuropathies consist of a heterogeneous group of genetic disorders that includes hereditary neuropathy with liability to pressure palsies (HNPP), Charcot-Marie-Tooth disease (CMT), Dejerine-Sottas syndrome (DSS), and congenital hypomyelination (CH). The clinical classification of these neuropathies into discrete categories can sometimes be difficult because there can be both clinical and pathologic variation and overlap between these disorders. We have identified five novel mutations in the myelin protein zero (MPZ) gene, encoding the major structural protein (P0) of peripheral nerve myelin, in patients with either CMT1B, DSS, or CH. This finding suggests that these disorders may not be distinct pathophysiologic entities, but rather represent a spectrum of related "myelinopathies" due to an underlying defect in myelination. Furthermore, we hypothesize the differences in clinical severity seen with mutations in MPZ are related to the type of mutation and its subsequent effect on protein function (i.e., loss of function versus dominant negative).

Two divergent types of nerve pathology in patients with different P0 mutations in Charcot-Marie-Tooth disease

In seven unrelated patients with a demyelinating motor and sensory neuropathy, we found mutations in exons 2 and 3 of the P0 gene. Morphologic examination of sural nerve biopsy specimens showed a demyelinating process with onion bulb formation in all cases. In four patients, ultrastructural examination demonstrated uncompacted myelin in 23 to 68% of the myelinated fibers, which is in agreement with the widely accepted function of P0 as a homophilic adhesion molecule. Three patients showed normal compact myelin, but morphology was dominated by the abundant occurrence of focally folded myelin. The two divergent pathologic phenotypes exemplify that some mutations act differently on P0 protein formation or function than others, which is probably determined by site and nature of the mutation in the P0 gene.

Distal hereditary motor neuropathy type II (distal HMN II): mapping of a locus to chromosome 12q24

The distal hereditary motor neuropathy (distal HMN) or the spinal form of Charcot-Marie-Tooth (CMT) disease is an exclusively motor disorder of the peripheral nervous system. The disorder clinically resembles the hereditary motor and sensory neuropathies (HMSN) type I and type II or CMT type 1 and type 2. Distal HMN might also be related to the spinal muscular atrophies (SMA) since, in both disorders, the lower motor neurons are affected. Electrophysiological and neuropathological examinations of peripheral nerves show the absence of sensory involvement. We performed a genome search in an extended Belgian family with autosomal dominant distal HMN type II. Significant linkage was obtained with markers located at chromosome 12q24, and the gene for distal HMN II was assigned to the 13 cM interval between D12S86 and D12S340.

Linkage and mutation analysis of Charcot-Marie-Tooth neuropathy type 2 families with chromosomes 1p35-p36 and Xq13

A locus for autosomal dominant Charcot-Marie-Tooth disease type 2 (CMT2A) was assigned by linkage analysis to chromosome 1p35-p36. We examined 11 unrelated CMT2 families for linkage to CMT2A using short tandem repeat (STR) polymorphisms. Only one family showed suggestive evidence for linkage to 1p35-p36. Further, because of an overlap in electrophysiologic data between CMT2 and CMTX female patients, we screened 6 of 11 CMT2 families compatible with dominant X-linkage for mutations in the connexin 32 (Cx32) gene at Xq13. There was a Cx32 mutation in one family, whereas another family showed suggestive evidence for Xq13 linkage upon analysis with STR polymorphisms. Our results suggest that the CMT2A locus is a minor locus for CMT2, additional linkage studies are needed to localize other CMT2 loci, and Cx32 mutations may be the underlying genetic defect in some CMT2 families.

Comparison of single-strand conformation polymorphism and heteroduplex analysis for detection of mutations in Charcot-Marie-Tooth type 1 disease and related peripheral neuropathies

To compare the sensitivity of the mutation detection techniques single-strand conformation polymorphism analysis (SSCP) and heteroduplex analysis (HA), we analyzed a cohort of 73 patients with a diagnosis of a demyelinating neuropathy, but without the CMT1A duplication, for mutations in the coding region of the myelin genes PMP22, MPZ and Cx32. In total, 21 samples showed 13 distinct altered migration patterns by one or both methods. Ten altered patterns were detected by both SSCP and HA, two were false negative by HA, and one was false negative by SSCP. Our results suggest that either technique can be useful for mutation detection, but a combination of factors appears to affect the sensitivity of both techniques.

Estimation of the mutation frequencies in Charcot-Marie-Tooth disease type 1 and hereditary neuropathy with liability to pressure palsies: a European collaborative study

A European collaboration on Charcot-Marie-Tooth type 1 (CMT1) disease and hereditary neuropathy with liability to pressure palsies (HNPP) was established to estimate the duplication and deletion frequency, respectively, on chromosome 17p11.2 and to make an inventory of mutations in the myelin genes, peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) and connexin 32 (Cx32) located on chromosomes 17p11.2, 1q21-q23 and Xq13.1, respectively. In 70.7% of 819 unrelated CMT1 patients, the 17p11.2 duplication was present. In 84.0% of 156 unrelated HNPP patients, the 17p11.2 deletion was present. In the nonduplicated CMT1 patients, several different mutations were identified in the myelin genes PMP22, MPZ and Cx32.

A de novo duplication in 17p11.2 and a novel mutation in the Po gene in two Déjérine-Sottas syndrome patients

Déjérine-Sottas syndrome (DSS), or hereditary motor and sensory neuropathy (HMSN) type III, is a severe hypertrophic demyelinating neuropathy with infantile onset. The clinical symptoms are similar to those found in Charcot-Marie-Tooth disease type 1 (CMT1) or HMSN type I patients, but they are more severe. DSS is genetically heterogeneous. Dominant mutations in two major peripheral myelin protein genes, PMP22 and Po, are associated with a DSS phenotype. Mutations in the same genes are also responsible for the CMT1 phenotype. A 1.5-Mb duplication in 17p11.2 is the major mutation found in familial and sporadic CMT1 patients. We studied two genetically sporadic DSS patients. The presence of a de novo duplication in one patient was revealed by Southern blot analysis, using polymorphic markers located in the duplicated area. The 17p11.2 allele segregation in this patient and in her parents suggests that the duplication is of maternal origin. In the other patient, single-strand conformation polymorphism (SSCP) analysis of the 6 exons of the Po gene revealed two additional bands in exon 3. Sequencing of this exon identified a novel dominant mutation replacing a sequence of 8 bp by a mutated sequence of 5 bp. The mutation apparently leads to the replacement of 4 amino acids at positions 86-89 by three different amino acids, in an area that is part of a predicted beta-strand. Our findings support the suggestion that DSS and CMT1 disease should not be considered as two different clinical entities.

Molecular analysis of three cases with hereditary motor and sensory neuropathy with myelin outfolding

We describe three patients affected by a congenital motor and sensory neuropathy with excessive myelin outfoldings (MOs) [15]. Clinical and electrophysiological features supported the diagnosis of hereditary motor and sensory neuropathy. We previously reported a genetic study on these three patients, which failed to demonstrate either the duplication in chromosome 17p11.2 or the mutations at exons 1 and 2 of the peripheral myelin protein gene (PMP-22) and suggested an autosomal recessive (AR) inheritance. In this study we described the absence of the most common mutations, which characterized other forms of hereditary motor and sensory neuropathy (HMSN). In particular the absence of molecular changes in the PMP-22 gene definitively sets HMSN with MOs apart from the more common CMT1A, hereditary neuropathy with liability to pressure palsies (HNPP) and progressive sensory-motor polyneuropathy with tomaculous changes at sural nerve biopsy.

Prenatal diagnosis of Charcot-Marie-Tooth disease type 1A (CMT1A) using molecular genetic techniques

Charcot-Marie-Tooth disease type 1A (CMT1A) is a frequent hereditary motor and sensory neuropathy of the peripheral nerves. In most cases, the disease is associated with a 1.5 Mb tandem duplication at 17p11.2. A 42-year-old pregnant women requested prenatal diagnosis because of her age and since both her husband and two children were severely affected with CMT1. The CMT1A duplication was demonstrated in the father's, the two children's, and the fetus's DNA using different molecular genetic methods. Although cytogenetical analysis showed a normal female karyotype in the fetus, the parents decided to terminate the pregnancy because of the genetic risk associated with the CMT1A duplication.

Mutations in the myelin protein zero gene associated with Charcot-Marie-Tooth disease type 1B

Charcot-Marie-Tooth type 1 (CMT1) disease is an autosomal dominant neuropathy of the peripheral nerve. The majority of CMT 1 cases are due to a duplication of an 1.5-Mb DNA fragment on chromosome 17p11.2 (CMT 1a). Micromutations were found in the gene for peripheral myelin protein 22 (PMP22) located in the duplicated region of CMT 1a, and in the peripheral myelin protein zero (PO) located on chromosome 1q21-q23 (CMT 1b). We have characterized two new mutations in the PO gene in two french families presenting CMT disease. Both mutations occur in the extracellular domain of the PO protein. One mutation is a de novo mutation and is from paternal origin.

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

Charcot-Marie-Tooth type 1 (CMT1) disease or hereditary motor and sensory neuropathy type I (HMSNI) is an autosomal dominant peripheral neuropathy. In most CMT1 families, the disease cosegregates with a 1.5-Mb duplication on chromosome 17p11.2 (CMT1A). A few patients have been found with mutations in the peripheral myelin protein 22 (PMP-22) gene located in the CMT1A region. In other families mutations have been identified in the major peripheral myelin protein P0 gene localized on chromosome 1q21-q23 (CMT1B). We performed a rapid mutation screening of the PMP-22 and P0 genes in non-duplicated CMT1 patients by single-strand conformation polymorphism analysis followed by direct polymerase chain reaction sequencing of genomic DNA. Six new single base changes in the P0 gene were observed: two missense mutations in, respectively, exons 2 and 3, two nonsense mutations in exon 4, and two silent mutations or polymorphisms in, respectively, exons 3 and 6.

Linkage and mutation analysis in an extended family with Charcot-Marie-Tooth disease type 1B

Charcot-Marie-Tooth disease type 1 (CMT1) or hereditary motor and sensory neuropathy type I (HMSNI) is an autosomal dominant peripheral neuropathy. In most families the disease segregates with a 1.5 Mb duplication on chromosome 17p11.2 (CMT1A). A few patients have been found with point mutations in the PMP-22 gene. In some families linkage has been found with markers located on chromosome 1q21-q25 (CMT1B) and more recently mutations have been identified in the P0 gene. We analysed an extended CMT1 pedigree (CMT-B) without the CMT1A duplication. Significant positive linkage with chromosome 1 indicated that this family is of the CMT1B subtype. Sequencing of the candidate gene P0 located in chromosome band 1q21-q23 showed a C to A point mutation at position 446 in exon 3 resulting in an Asp134Glu substitution. Since the P0 mutation cosegregated with CMT1 disease we suggest that this mutation is the primary genetic cause of CMT1B in family CMT-B.