The peripheral myelin protein gene PMP-22 is contained within the Charcot-Marie-Tooth disease type 1A duplication.

Charcot-Marie-Tooth disease (CMT1) is the most common form of inherited peripheral neuropathy. Although the disease is genetically heterogeneous, it has been demonstrated that the gene defect is the most frequent type (CMT1A) is the result of a partial duplication of band 17p11.2. Recent studies suggested that the peripheral hypomyelination syndrome in the trembler (Tr) mouse, a possible animal model for CMT1 disease, is associated with a point mutation in the peripheral myelin protein-22 gene (pmp-22). Expression of pmp-22 is particularly high in Schwann cells, and the protein is found in peripheral myelin. We now report that the human PMP-22 gene is contained within the CMT1A duplication. We therefore, suggest that increased dosage of the PMP-22 gene may be the cause of CMT1A neuropathy.

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).

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.

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 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.

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.

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.

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.

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.

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.

Duplication in chromosome 17p11.2 in Charcot-Marie-Tooth neuropathy type 1a (CMT 1a). The HMSN Collaborative Research Group.

Hereditary motor and sensory neuropathy type I (HMSN I) or Charcot-Marie-Tooth disease type 1 (CMT 1) is an autosomal dominant disorder of the peripheral nervous system characterized by progressive weakness and atrophy of distal limb muscles. In the majority of HMSN I families, linkage studies localized the gene (CMT 1a) to the pericentromeric region of chromosome 17. We have detected with probe pVAW409R3 (D17S122) localized in 17p11.2 a duplication, co-segregating with the disease in 12 HMSN I families. In these families the duplication was present in all 128 patients but absent in the 84 unaffected and 44 married-in individuals (lod score of 58.44 at zero recombination). Further, on one HMSN I family the disease newly appeared simultaneously with a de novo duplication originating from an unequal crossing-over event at meiosis. Since different allelic combinations were found segregating with the duplication in different families linkage disequilibrium was not a significant factor. These findings led us to propose that the duplication in 17p11.2 itself is the disease causing mutation in all the HMSN I families analyzed.

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.

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.

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.

Linkage analysis of distal hereditary motor neuropathy type II (distal HMN II) in a single pedigree.

We describe a six generation family affected with the autosomal dominant form of distal hereditary motor neuropathy type II (distal HMN II). The distal HMN shows similarities with the hereditary motor and sensory neuropathies type I and II (HMSN I and HMSN II) or Charcot-Marie-Tooth disease type 1 and 2 (CMT 1 and CMT 2) and with some proximal HMN or spinal muscular atrophies (SMA). Gene loci have been assigned to chromosomes 1q, 17p, and 19q for CMT 1 and to chromosome 5q for recessive SMA. In this study we excluded all four regions for the presence of distal HMN II, indicating that this neuropathy is genetically different from CMT 1 and recessive SMA.

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.