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.     

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 17pl1.2 (CMT la). Micromutations were found in the gene for peripheral myelin protein 22 (PMp22) located in the duplicated region of CMT la, and in the peripheral myelin protein zero (PO) located on chromosome lq21-23 (CMT Ib). 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. © 1995 Wiley-Liss, Inc.     

MPZ mutation in an early-onset Charcot-Marie-Tooth disease type 1B family by genome-wide linkage analysis

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous peripheral neuropathy. The objective of this study was to find the causative mutation(s) in a demyelinating autosomal dominant CMT family. A high density SNP-based genome-wide linkage scan was performed, and causative mutations were determined by sequencing of candidate genes in the linkage disequilibrium region. Linkage analysis mapped the underlying gene to a region on chromosome 1q22-q23 with a maximum two-point LOD score of 2.036. Sequencing analysis revealed a novel c.243C>G (His81Gln) mutation in the MPZ gene, which encodes the major integral membrane protein of the peripheral nerve system. MPZ is well known as a CMT-causative gene with wide phenotypic spectrum. The clinical symptoms were more similar to those of patients with the His81Arg than patients with the His81Tyr mutation. The novel mutation completely co-segregated with affected members, and was not found in controls. Therefore, we suggest that the identified mutation in MPZ is the underlying cause of CMT in the family. In addition, this study demonstrated that the clinical phenotypes may be variable with different mutations at the same site in the MPZ gene.     

A new locus for autosomal dominant Charcot-Marie-Tooth disease type 2 (CMT2F) maps to chromosome 7q11-q21

Charcot-Marie-Tooth disease (CMT) constitutes a genetically heterogeneous group of inherited motor and sensory peripheral neuropathies. The axonal type of CMT is designated CMT type 2 (CMT2). Four loci for autosomal dominant CMT2 have been reported so far. Only in CMT2E, linked to chromosome 8p21, disease-causing mutations in the gene for neurofilament light chain (NEFL) were identified. In this study we report a multigenerational Russian family with autosomal dominant CMT2 and assign the locus to chromosome 7q11-q21. The CMT2 neuropathy in this family represents a novel genetic entity designated CMT2F.     

The kinesin superfamily protein Rab6KIFL is not involved in the pathophysiology of Charcot-Marie-Tooth disease type 4C

Charcot-Marie-Tooth disease type 4C (CMT4C) is an autosomal recessive peripheral neuropathy reported in several Algerian families. The gene locus of this disease has been narrowed to 5q31-33. Recently, a missense mutation in the gene for the kinesin superfamily KIF1B was reported as the cause of Charcot Marie Tooth disease type 2A (CMT2A). We suspected that Rab6KIFL, one of the kinesin superfamily proteins, might be involved in the pathophysiology of CMT4C, because Rab6KIFL gene is located in 5q31. The coding regions of the Rab6KIFL gene of genomic DNA derived from one Algerian family with CMT4C were analyzed by direct sequencing. No mutation in Rab6KIFL gene was found in this family. Further investigation is necessary to identify the causative gene for CMT4C.     

Myelin protein zero (MPZ) gene mutations in nonduplication type 1 Charcot-Marie-Tooth disease

The myelin protein zero gene (MPZ) maps to chromosome 1q22-q23 and encodes the most abundant peripheral nerve myelin protein. The P_o protein functions as a homophilic adhesion molecule in myelin compaction. Mutations in the MPZ gene are associated with the demyelinating peripheral neuropathies Charcot-Marie-Tooth disease type 1B (CMT1B), and the more severe Dejerine-Sottas syndrome (DSS). We have surveyed a cohort of 70 unrelated patients with demyelinating polyneuropathy for additional mutations in the MPZ gene. The 1.5-Mb DNA duplication on chromosome 17p11.2-p12 associated with CMT type 1A (CMT1A) was not present. By DNA heteroduplex analysis, four base mismatches were detected in three exons of MPZ. Nucleotide sequence analysis identified a de novo mutation in MPZ exon 3 that predicts an Ile(135)Thr substitution in a family with clinically severe early-onset CMT1, and an exon 3 mutation encoding a Gly(137)Ser substitution was identified in a second CMT1 family. Each predicted amino acid substitution resides in the extracellular domain of the P_o protein. Heteroduplex analysis did not detect either base change in 104 unrelated controls, indicating that these substitutions are disease-associated mutations rather than common polymorphisms. In addition, two polymorphic mutations were identified in MPZ exon 5 and exon 6, which do not alter the codons for Gly(200) and Ser(228), respectively. These observations provide further confirmation of the role of MPZ in CMT1B and suggest that MPZ coding region mutations may account for a limited percentage of disease-causing mutations in nonduplication CMT1 patients. © 1996 Wiley-Liss, Inc.     

Significant evidence for linkage of febrile seizures to chromosome 5q14-q15

Febrile seizures (FSs) represent the most common form of childhood seizure. In the Japanese population, the incidence rate is as high as 7%. It has been recognized that there is a significant genetic component for susceptibility to this type of seizure. Two putative FS loci, FEB1 (chromosome 8q13-q21) and FEB2 (chromosome 19p), have been mapped. Furthermore, a mutation in the voltage-gated sodium (Na(+))-channel beta1 subunit gene ( SCN1B ) at chromosome 19q13.1 was identified in a family with a clinical subset, termed generalized epilepsy with febrile seizures plus (GEFS(+)). These loci are linked to some large families. In this study, we conducted a genome-wide linkage search for FS in one large family with subsequent linkage confirmation in 39 nuclear families. Significant linkage was found at D5S644 by multipoint non-parametric analysis using GENEHUNTER ( P = 5.4 x 10(-6)). Estimated lambda(s)at D5S644 was 2.5 according to maximum likelihood analysis. Significant linkage disequilibria with FS were observed at the markers D5S644, D5S652 and D5S2079 in 47 families by transmission disequilibrium tests. These findings indicate that there is a gene on chromosome 5q14-q15 that confers susceptibility to FSs and we call this gene FEB4.     

The p.R1109X mutation in SH3TC2 gene is predominant in Spanish Gypsies with Charcot-Marie-Tooth disease type 4

Charcot-Marie-Tooth (CMT) disease type 4 (CMT4) is the name given to autosomal recessive forms of hereditary motor and sensory neuropathy (HMSN). When we began this study, three genes or loci associated with inherited peripheral neuropathies had already been identified in the European Gypsy population: HMSN-Lom (MIM 601455), HMSN-Russe (MIM 605285) and the congenital cataracts facial dysmorphism neuropathy syndrome (MIM 604168). We have carried out genetic analyses in a series of 20 Spanish Gypsy families diagnosed with a demyelinating CMT disease compatible with an autosomal recessive trait. We found the p.R148X mutation in the N-myc downstream-regulated gene 1 gene to be responsible for the HMSN-Lom in four families and also possible linkage to the HMSN-Russe locus in three others. We have also studied the CMT4C locus because of the clinical similarities and showed that in 10 families, the disease is caused by mutations located on the SH3 domain and tetratricopeptide repeats 2 (SH3TC2) gene: p.R1109X in 20 out of 21 chromosomes and p.C737_P738delinsX in only one chromosome. Moreover, the SH3TC2 p.R1109X mutation is associated with a conserved haplotype and, therefore, may be a private founder mutation for the Gypsy population. Estimation of the allelic age revealed that the SH3TC2 p.R1109X mutation may have arisen about 225 years ago, probably as the consequence of a bottleneck.     

一个表皮松解性掌跖角化病维吾尔家系致病基因研究

目的 对一个维吾尔族表皮松解性掌跖角化病(epidermolytic palmoplantar keratoderma,EPPK)家系角蛋白9基因(keratin 9 gene,KRT9)进行测序,以检测其是否为该病的致病基因.方法 提取新疆地区一个维吾尔族EPPK家系外周血基因组DNA,针对已知候选基因KRT9和KRT1,分别对其所在染色体位置17q12-q21和12q13选取遗传标记进行连锁分析研究,确定连锁区域后,对区域内KRT9基因所有外显子进行测序分析.结果 分别得到48个家庭成员遗传标记的基因型和单倍型,经Linkage软件计算分析,发现标记D17S1787在=0时Lod值达到8.65,并最终将该病候选区域定位于遗传标记17/TG/36620115-D17S846之间约1 Mb范围内.排除该病与位于染色体12q13上的遗传标记DI2S96(θ=0时Lod=-∞)连锁.未发现KRT9基因存在致病性突变.结论 提示该表皮松解性掌跖角化病家系的致病基因位于染色体17q21.2上(chr17:36620083-37146934)约1 Mb区域内,且突变位点不位于KRT9基因编码区.     

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, Déjérine-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. Hum Mutat 12:59–68, 1998. © 1998 Wiley-Liss, Inc.     

Genetic basis of inherited peripheral neuropathies

Progress in the elucidation of the genetic basis for inherited peripheral neuropathies has been remarkable over the last years. In particular, the molecular mechanisms underlying the autosomal dominantly inherited disorders Charcot–Marie–Tooth disease type 1A (CMT1 A), Charcot–Marie–Tooth disease type 1B (CMT1B), and hereditary neuropathy with liability to pressure palsies (HNPP) have been determined. While mutation in the gene encoding the major myelin protein, P_o has been associated with CMT1B, CMT1A and HNPP have been shown to be associated with reciprocal recombination events leading either to a large submicroscopic duplication in CMT1 A, or the corresponding DNA deletion in HNPP. Available evidence is consistent with the hypothesis that one or more genes within the relevant rearranged segment of 1.5 Mb on chromosome 17 is sensitive to gene dosage providing a novel mechanism for inherited human disorders. It is likely that the gene encoding the peripheral myelin protein PMP22 is at least one of the genes involved since the PMP22 gene maps within the CMT1A duplication (or HNPP deletion), and point mutations within it have been shown to cause a CMT phenotype in humans and comparable neuropathies in rodents (trembler and trembler^J). The mechanism(s) by which gene dosage and point mutations affecting the same gene might lead to a similar phenotype are currently unknown but recent transgenic mouse experiments suggest that similar mechanisms may also underlie other genetic diseases. © 1994 Wiley-Liss, Inc.     

Refinement of the locus for hereditary congenital facial palsy on chromosome 3q21 in two unrelated families and screening of positional candidate genes

Hereditary congenital facial palsy (HCFP) is an autosomal-dominant disorder consisting of paresis or paralysis of the VIIth (facial) cranial nerve. Genetic heterogeneity for this disorder has been suggested based on linkage analysis in two large Dutch families. Two loci have been identified, one on chromosome 3q21.2-q22.1 (HCFP1) and another on chromosome 10q21.3-q22.1 (HCFP2). Here, we report linkage analysis in a large Pakistani family with dominant congenital facial palsy. A region cosegregating with the disorder was identified on the long arm of chromosome 3, which overlaps with the previously identified HCFP1 locus on chromosome 3q21-q22, thus confirming the involvement of this locus in HCFP. The critical region could be reduced from 5.7 to 3.0 cM between the markers D3S3607 and GDB ID:11524500. In addition, mutation analysis on seven candidate genes: KLF15, FLJ40083, PODXL2, TMCC1, PLEXIN-A1, PLEXIN-D1, and GATA-2, was performed. All genes are located within the critical interval of the Dutch HCFP1 family. The genes PODXL2, PLEXIN-D1, GATA-2, and TMCC1 are also located within the smaller critical interval of the Pakistani HCFP family. Based on the results obtained, all seven genes could be excluded as causative genes in HCFP.     

Founder Effect and Estimation of the Age of the c.892C>T (p.Arg298Cys) Mutation in LMNA Associated to Charcot-Marie-Tooth Subtype CMT2B1 in Families from North Western Africa

CMT2B1, an axonal subtype (MIM 605588) of the Charcot-Marie-Tooth disease, is an autosomal recessive motor and sensory neuropathy characterized by progressive muscular and sensory loss in the distal extremities with chronic distal weakness. The genetic defect associated with the disease is, to date, a unique homozygous missense mutation, p.Arg298Cys (c.892C>T), in the LMNA gene. So far, this mutation has only been found in affected individuals originating from a restricted region of North Western Africa (northwest of Algeria and east of Morocco), strongly suggesting a founder effect. In order to address this hypothesis, genotyping of both STRs and intragenic SNPs was performed at the LMNA locus, at chromosome 1q21.2-q21.3, in 42 individuals affected with CMT2B1 from 25 Algerian families. Our results indicate that the affected individuals share a common ancestral haplotype in a region of about 1.0 Mb (1 cM) and that the most recent common ancestor would have lived about 800–900 years ago (95% confidence interval: 550 to 1300 years).     

The CMT2D Locus: Refined Genetic Position and Construction of a Bacterial Clone-Based Physical Map

Charcot-Marie-Tooth (CMT) disease is a progressive neuropathy of the peripheral nervous system, typically characterized by muscle weakness of the distal limbs. CMT is noted for its genetic heterogeneity, with four distinct loci already identified for the axonal form of the disease (CMT2). In 1996, linkage analysis of a single large family revealed the presence of a CMT2 locus on chromosome 7p14 (designated CMT2D). Additional families have been linked subsequently to the same genomic region, including one with distal spinal muscular atrophy (dSMA) and one with mixed features of dSMA and CMT2; symptoms in both of these latter families closely resemble those seen in the original CMT2D family. There is thus a distinct possibility that CMT2 and dSMA encountered in these families reflect allelic heterogeneity at a single chromosome 7 locus. In the study reported here, we have performed more detailed linkage analysis of the original CMT2D family based on new knowledge of the physical locations of various genetic markers. The region containing the CMT2D gene, as defined by the original family, overlaps with those defined by at least two other families with CMT2 and/or dSMA symptoms. Both yeast artificial chromosome (YAC) and bacterial clone-based [bacterial artificial chromosome (BAC) and P1-derived artificial chromosome (PAC)] contig maps spanning approximately 3.4 Mb have been assembled across the combined CMT2D critical region, with the latter providing suitable clones for systematic sequencing of the interval. Preliminary analyses have already revealed at least 28 candidate genes and expressed-sequence tags (ESTs). The mapping information reported here in conjunction with the evolving sequence data should expedite the identification of the CMT2D/dSMA gene or genes.     

一个遗传性乳光牙本质家系致病基因的初步定位

目的：研究遗传性乳光牙本质家系致病基因是否与染色体４ｑ２１连锁。方法：提取在天津塘沽地区发现的一个遗传性乳光牙本质家系成员的外周血ＤＮＡ,选择染色体４ｑ２１上的４仆短串联重复序列多态性标记（ｓｈｏｒｔ ｔａｎｄｅｍ ｒｅｐｅａｔ ｐｏｌｙｍｏｒｐｈｉｓｍ,ＳＴＲＰ）：ＧＡＴＡ６２Ａ１１、ＤＳＰ（Ｐ）、ＳＰＰ１的Ｄ４Ｓ１５６３做荧光标记ＰＣＲ、等位片段分析,用Ｌｏｄ连锁分析法分析该家系致病基因位点与上述４个ＳＴＲ的连锁关系。结果：分别得到１３个家庭成员上述４个位点的基因型和单体型。ＭＬＩＮＫ软件分析显示：ＧＡＴＡ６２Ａ１１、ＫＳＰ（Ｐ）与致病基因位点连锁的最大Ｌｏｄ值分别为１．６３（θ＝０）。结论：遗传性乳光牙本质家系的致病基因初步定位在４号染色体上。     

Charcot-Marie-Tooth neuropathy related to chromosome 1.

One family with documented male-to-male transmission of Charcot-Marie-Tooth (CMT) neuropathy was studied clinically and by genetic linkage. Patients had progressive distal weakness and atrophy, areflexia, and distal sensory loss, but early onset (before age 3 years) in all 5 cases, and phrenic nerve involvement in the propositus (a 39-year-old woman) requiring CPAP ventilator support during the night. Motor-nerve conduction velocities (MNCVs) were significantly slow, consistent with severe demyelinating neuropathy. Electromyography (EMG) data were normal. Two-point and multipoint linkage analyses strongly suggested the presence of a CMT gene on chromosome 1q. A maximum multipoint lod score of 2.70 was obtained at MUC1 (theta = 0), with the locus order centromere-MUC1-SPTA1-Fc gamma RII-AT3-telomere. Multipoint linkage analysis excluded the CMT locus from chromosome 17 markers in this family.     

Two frequent missense mutations in Pendred syndrome

Pendred syndrome is an autosomal recessive disorder characterized by early childhood deafness and goiter. A century after its recognition as a syndrome by Vaughan Pendred, the disease gene ( PDS ) was mapped to chromosome 7q22-q31.1 and, recently, found to encode a putative sulfate transporter. We performed mutation analysis of the PDS gene in patients from 14 Pendred families originating from seven countries and identified all mutations. The mutations include three single base deletions, one splice site mutation and 10 missense mutations. One missense mutation (L236P) was found in a homozygous state in two consanguineous families and in a heterozygous state in five additional non-consanguineous families. Another missense mutation (T416P) was found in a homozygous state in one family and in a heterozygous state in four families. Pendred patients in three non-consanguineous families were shown to be compound heterozygotes for L236P and T416P. In total, one or both of these mutations were found in nine of the 14 families analyzed. The identification of two frequent PDS mutations will facilitate the molecular diagnosis of Pendred syndrome.      

Charcot-marie-tooth neuropathy related to chromosome 1

One family with documented male-to-male transmission of Charcot-Marie-Tooth (CMT) neuropathy was studied clinically and by genetic linkage. Patients had progressive distal weakness and atrophy, areflexia, and distal sensory loss, but early onset (before age 3 years) in all 5 cases, and phrenic nerve involvement in the propositus (a 39-year-old woman) requiring CPAP ventilator support during the night. Motor-nerve conduction velocities (MNCVs) were significantly slow, consistent with severe demyelinating neuropathy. Electromyography (EMG) data were normal. Two-point and multipoint linkage analyses strongly suggested the presence of a CMT gene on chromosome 1q. A maximum multipoint lod score of 2.70 was obtained at MUCI (θ = 0), with the locus order centromere-MUC1-SPTA1-FcγRII-AT3-telomere. Multipoint linkage analysis excluded the CMT locus from chromosome 17 markers in this family.     

Dyschromatosis universalis hereditaria: evidence for autosomal recessive inheritance and identification of a new locus on chromosome 12q21-q23

Dyschromatosis universalis hereditaria (DUH) and dyschromatosis symmetrica hereditaria (DSH) are pigmentary dermatoses most commonly seen in Japan. Both disorders usually show autosomal dominant inheritance, although in some cases autosomal recessive inheritance was reported. DSH was mapped to chromosome 1q21.3, and mutations in the gene ADAR ( DSRAD ) were identified in Japanese, Chinese and Taiwanese families with autosomal dominant DSH. A second locus for dyschromatosis was mapped on chromosome 6q24.2-q25.2 in two Chinese families initially reported to be affected with DSH, but later suggested to have autosomal dominant DUH. The aim of this study was to investigate whether one of these two loci is involved in the development of DUH in a consanguineous Bedouin family from Saudi Arabia with four affected and three unaffected sibs, clearly pointing to autosomal recessive inheritance. After excluding mutations in ADAR and linkage to the candidate regions on chromosomes 1 and 6, we performed an single nucleotide polymorphism-based genome-wide scan for linkage with other loci. Under the assumption of autosomal recessive inheritance, we have identified a new locus for dyschromatosis on chromosome 12q21-q23 in this Arab family with a maximum logarithm of the odds (LOD) score of 3.4, spanning a distance of 18.9 cM. Our study revealed the first locus for autosomal recessive DUH and supports recent evidence that DSH and DUH are genetically distinct disorders.     

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.