Charcot-Marie-Tooth disease: study of a large kinship with an intermediate form

PMP22 related neuropathies: Charcot-Marie-Tooth disease type 1A and Hereditary Neuropathy with liability to Pressure Palsies

PMP22 related neuropathies comprise (1) PMP22 duplications leading to Charcot-Marie-Tooth disease type 1A (CMT1A), (2) PMP22 deletions, leading to Hereditary Neuropathy with liability to Pressure Palsies (HNPP), and (3) PMP22 point mutations, causing both phenotypes. Overall prevalence of CMT is usually reported as 1:2,500, epidemiological studies show that 20-64% of CMT patients carry the PMP22 duplication. The prevalence of HNPP is not well known. CMT1A usually presents in the first two decades with difficulty walking or running. Distal symmetrical muscle weakness and wasting and sensory loss is present, legs more frequently and more severely affected than arms. HNPP typically leads to episodic, painless, recurrent, focal motor and sensory peripheral neuropathy, preceded by minor compression on the affected nerve. Electrophysiological evaluation is needed to determine whether the polyneuropathy is demyelinating. Sonography of the nerves can be useful. Diagnosis is confirmed by finding respectively a PMP22 duplication, deletion or point mutation. Differential diagnosis includes other inherited neuropathies, and acquired polyneuropathies. The mode of inheritance is autosomal dominant and de novo mutations occur. Offspring of patients have a chance of 50% to inherit the mutation from their affected parent. Prenatal testing is possible; requests for prenatal testing are not common. Treatment is currently symptomatic and may include management by a rehabilitation physician, physiotherapist, occupational therapist and orthopaedic surgeon. Adult CMT1A patients show slow clinical progression of disease, which seems to reflect a process of normal ageing. Life expectancy is normal.

PLEKHG5 deficiency leads to an intermediate form of autosomal-recessive Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) comprises a clinically and genetically heterogeneous group of peripheral neuropathies characterized by progressive distal muscle weakness and atrophy, foot deformities and distal sensory loss. Following the analysis of two consanguineous families affected by a medium to late-onset recessive form of intermediate CMT, we identified overlapping regions of homozygosity on chromosome 1p36 with a combined maximum LOD score of 5.4. Molecular investigation of the genes from this region allowed identification of two homozygous mutations in PLEKHG5 that produce premature stop codons and are predicted to result in functional null alleles. Analysis of Plekhg5 in the mouse revealed that this gene is expressed in neurons and glial cells of the peripheral nervous system, and that knockout mice display reduced nerve conduction velocities that are comparable with those of affected individuals from both families. Interestingly, a homozygous PLEKHG5 missense mutation was previously reported in a recessive form of severe childhood onset lower motor neuron disease (LMND) leading to loss of the ability to walk and need for respiratory assistance. Together, these observations indicate that different mutations in PLEKHG5 lead to clinically diverse outcomes (intermediate CMT or LMND) affecting the function of neurons and glial cells.

DNA testing in hereditary neuropathies

The inherited neuropathies are a clinically and genetically heterogeneous group of disorders in which there have been rapid advances in the last two decades. Molecular genetic testing is now an integral part of the evaluation of patients with inherited neuropathies. In this chapter we describe the genes responsible for the primary inherited neuropathies. We briefly discuss the clinical phenotype of each of the known inherited neuropathy subgroups, describe algorithms for molecular genetic testing of affected patients and discuss genetic counseling. The basic principles of careful phenotyping, documenting an accurate family history, and testing the available genes in an appropriate manner should identify the vast majority of individuals with CMT1 and many of those with CMT2. In this chapter we also describe the current methods of genetic testing. As advances are made in molecular genetic technologies and improvements are made in bioinformatics, it is likely that the current time-consuming methods of DNA sequencing will give way to quicker and more efficient high-throughput methods, which are briefly discussed here.

Molecular genetics of charcot-marie-tooth disease: from genes to genomes

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of disorders of the peripheral nervous system, mainly characterized by distal muscle weakness and atrophy leading to motor handicap. With an estimated prevalence of 1 in 2,500, this condition is one of the most commonly inherited neurological disorders. Mutations in more than 30 genes affecting glial and/or neuronal functions have been associated with different forms of CMT leading to a substantial improvement in diagnostics of the disease and in the understanding of implicated pathophysiological mechanisms. However, recent data from systematic genetic screening performed in large cohorts of CMT patients indicated that molecular diagnosis could be established only in ∼50-70% of them, suggesting that additional genes are involved in this disease. In addition to providing an overview of genetic and functional data concerning various CMT forms, this review focuses on recent data generated through the use of highly parallel genetic technologies (SNP chips, sequence capture and next-generation DNA sequencing) in CMT families, and the current and future impact of these technologies on gene discovery and diagnostics of CMTs.

Molecular genetics of charcot-marie-tooth disease: from genes to genomes

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of disorders of the peripheral nervous system, mainly characterized by distal muscle weakness and atrophy leading to motor handicap. With an estimated prevalence of 1 in 2,500, this condition is one of the most commonly inherited neurological disorders. Mutations in more than 30 genes affecting glial and/or neuronal functions have been associated with different forms of CMT leading to a substantial improvement in diagnostics of the disease and in the understanding of implicated pathophysiological mechanisms. However, recent data from systematic genetic screening performed in large cohorts of CMT patients indicated that molecular diagnosis could be established only in ∼50-70% of them, suggesting that additional genes are involved in this disease. In addition to providing an overview of genetic and functional data concerning various CMT forms, this review focuses on recent data generated through the use of highly parallel genetic technologies (SNP chips, sequence capture and next-generation DNA sequencing) in CMT families, and the current and future impact of these technologies on gene discovery and diagnostics of CMTs.

Dominant Intermediate Charcot-Marie-Tooth disorder is not due to a catalytic defect in tyrosyl-tRNA synthetase

Charcot-Marie-Tooth disorder (CMT) is the most common inherited peripheral neuropathy, afflicting 1 in every 2500 Americans. One form of this disease, Dominant Intermediate Charcot-Marie-Tooth disorder type C (DI-CMTC), is due to mutation of the gene encoding the cytoplasmic tyrosyl-tRNA synthetase (TyrRS). Three different TyrRS variants have been found to give rise to DI-CMTC: replacing glycine at position 41 by arginine (G41R), replacing glutamic acid at position 196 by lysine (E196K), and deleting amino acids 153-156 (Δ(153-156)). To test the hypothesis that DI-CMTC is due to a defect in the ability of tyrosyl-tRNA synthetase to catalyze the aminoacylation of tRNA(Tyr), we have expressed each of these variants as recombinant proteins and used single turnover kinetics to characterize their abilities to catalyze the activation of tyrosine and its subsequent transfer to the 3' end of tRNA(Tyr). Two of the variants, G41R and Δ(153-156), display a substantial decrease in their ability to bind tyrosine (>100-fold). In contrast, the E196K substitution does not significantly affect the kinetics for formation of the tyrosyl-adenylate intermediate and actually increases the rate at which the tyrosyl moiety is transferred to tRNA(Tyr). The observation that the E196K substitution does not decrease the rate of catalysis indicates that DI-CMTC is not due to a catalytic defect in tyrosyl-tRNA synthetase.

Phenotypic spectrum of dynamin 2 mutations in Charcot-Marie-Tooth neuropathy

Dominant intermediate Charcot-Marie-Tooth neuropathy type B is caused by mutations in dynamin 2. We studied the clinical, haematological, electrophysiological and sural nerve biopsy findings in 34 patients belonging to six unrelated dominant intermediate Charcot-Marie-Tooth neuropathy type B families in whom a dynamin 2 mutation had been identified: Gly358Arg (Spain); Asp551_Glu553del; Lys550fs (North America); Lys558del (Belgium); Lys558Glu (Australia, the Netherlands) and Thr855_Ile856del (Belgium). The Gly358Arg and Thr855_Ile856del mutations were novel, and in contrast to the other Charcot-Marie-Tooth-related mutations in dynamin 2, which are all located in the pleckstrin homology domain, they were situated in the middle domain and proline-rich domain of dynamin 2, respectively. We report the first disease-causing mutation in the proline-rich domain of dynamin 2. Patients with a dynamin 2 mutation presented with a classical Charcot-Marie-Tooth phenotype, which was mild to moderately severe since only 3% of the patients were wheelchair-bound. The mean age at onset was 16 years with a large variability ranging from 2 to 50 years. Interestingly, in the Australian and Belgian families, which carry two different mutations affecting the same amino acid (Lys558), Charcot-Marie-Tooth cosegregated with neutropaenia. In addition, early onset cataracts were observed in one of the Charcot-Marie-Tooth families. Our electrophysiological data indicate intermediate or axonal motor median nerve conduction velocities (NCV) ranging from 26 m/s to normal values in four families, and less pronounced reduction of motor median NCV (41-46 m/s) with normal amplitudes in two families. Sural nerve biopsy in a Dutch patient with Lys558Glu mutation showed diffuse loss of large myelinated fibres, presence of many clusters of regenerating myelinated axons and fibres with focal myelin thickenings--findings very similar to those previously reported in the Australian family. We conclude that dynamin 2 mutations should be screened in the autosomal dominant Charcot-Marie-Tooth neuropathy families with intermediate or axonal NCV, and in patients with a classical mild to moderately severe Charcot-Marie-Tooth phenotype, especially when Charcot-Marie-Tooth is associated with neutropaenia or cataracts.

Intermediate forms of Charcot-Marie-Tooth neuropathy: a review

The Charcot-Marie-Tooth (CMT) neuropathies divide into two main electrophysiological groups with slow and near normal conduction velocities corresponding to Schwann cell and axonal pathology. An intermediate group also exists with nerve conduction velocities, which overlaps the two main groups. Families with intermediate CMT can be recognized in which different affected individuals in the same family have motor conduction velocities in both the CMT type 1 and 2 ranges (i.e., above and below 38 m/s). The intermediate group is caused by a limited number of distinct gene mutations in dynamin2 (DNM2), gap-junction protein 1 (GJB1), neurofilament light polypeptide (NF-L) genes, and a rare mutation and as yet unknown genes on chromosome 1 and 10 loci. Intermediate forms of CMT may be associated with unique disease mechanisms affecting both Schwann cells and axons. It is useful to recognize this unique group of neuropathies for diagnostic and management purposes.

The dominantly inherited motor and sensory neuropathies: clinical and molecular advances

The rapid advances in the molecular genetics and cell biology of hereditary neuropathy have revealed great genetic complexity. It is a challenge for physicians and laboratories to keep pace with new discoveries. Classification of hereditary neuropathies has evolved from a simple clinical to a detailed molecular classification. However, the molecular classification is not simple to use, as different mutations of the same gene produce a range of phenotypes. The logistics of testing for multiple gene mutations are considerable. This review gives a clinical overview of molecular and clinical advances in the dominant hereditary motor and sensory neuropathies [HMSNs, Charcot-Marie-Tooth (CMT) neuropathy], which account for some 60%-70% of families with CMT. The dominant forms of CMT have cellular mechanisms different from those of recessive forms and are a separate diagnostic challenge, so they are not included in this review. Diagnostic testing requires accurate clinical information and a selective approach to gene screening until the cost of multiple gene mutation screening falls. Accurate molecular diagnosis is critical to genetic counseling. This review concentrates on how molecular information can be used clinically, on how physicians can keep pace with new developments, and on the relevance of this new knowledge to patients.

Clinical and electrophysiological aspects of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a genetically heterogeneous group of disorders sharing the same clinical phenotype, characterized by distal limb muscle wasting and weakness, usually with skeletal deformities, distal sensory loss, and abnormalities of deep tendon reflexes. Mutations of genes involved in different functions eventually lead to a length-dependent axonal degeneration, which is the likely basis of the distal predominance of the CMT phenotype. Nerve conduction studies are important for classification, diagnosis, and understanding of pathophysiology. The subdivision into demyelinating CMT1 and axonal CMT2 types was a milestone and is still valid for the majority of patients. However, exceptions to this partition are increasing. Intermediate conduction velocities are often found in males with X-linked CMT (CMTX), and different intermediate CMT types have been identified. Moreover, for some genes, different mutations may result either in demyelinating CMT with slow conduction, or in axonal CMT. Nerve conduction slowing is uniform and diffuse in the most common CMT1A associated with the 17p12 duplication, whereas it is often asymmetric and nonhomogeneous in CMTX, sometimes rendering difficult the differential diagnosis with acquired inflammatory neuropathies. The demyelinating recessive forms, termed CMT4, usually have early onset and run a more severe course than the dominant types. Pure motor CMT types are now classified as distal hereditary motor neuronopathy. The diagnostic approach to the identification of the CMT subtype is complex and cannot be based on the clinical phenotype alone, as different forms are often clinically indistinguishable. However, there are features that may be of help in addressing molecular investigation in a single patient. Late onset, prominent or peculiar sensory manifestations, autonomic nervous system dysfunction, cranial nerve involvement, upper limb predominance, subclinical central nervous system abnormalities, severe scoliosis, early-onset glaucoma, neutropenia are findings helpful for diagnosis.

Autosomal dominant congenital fibre type disproportion: a clinicopathological and imaging study of a large family

Congenital fibre type disproportion (CFTD) is considered a non-progressive or slowly progressive muscle disease with relative smallness of type 1 fibres on pathological examination. Although generally benign, CFTD has a variable natural course and severe progression has been observed in some patients. The pathogenesis of the disorder is unknown and many authors consider CFTD a syndrome with multiple aetiologies rather than a separate clinical entity. A positive family history has been reported in about 40% of cases, but the inheritance pattern is not clear. Both autosomal recessive and dominant modes of inheritance have been suggested. The present paper describes a large, multigenerational kindred that has an inherited myopathy fulfilling the histological criteria of CFTD, with autosomal dominant transmission and high penetrance. The clinical picture, remarkably similar in all affected family members, started in early infancy with mild limb muscle weakness. There was slow progression of symptoms into adulthood, with moderate to severe, mainly proximal, muscle weakness without loss of ambulation. Muscle biopsy from two affected individuals demonstrated predominance of small type 1 muscle fibres without other significant findings. Nerve conduction studies were normal and needle electromyography showed a myopathic pattern. MRI examination performed on three patients from successive generations showed involvement of proximal limb and paraspinal muscles. The clinical and pathological homogeneity in the present family, together with the lack of additional histological abnormalities after decades of disease progression in two affected individuals, supports this being a distinct myopathy with fibre type disproportion. Whether the disease in this family can be regarded as a form of the congenital myopathy known as CFTD or rather a unique condition sharing histological features with CFTD needs further investigation. This is, to our knowledge, the largest kindred with muscle fibre type disproportion reported to date. Our data confirm autosomal dominant inheritance, and this is the first MRI document of this disorder.

Dominant intermediate Charcot-Marie-Tooth neuropathy maps to chromosome 19p12-p13.2

The hereditary disorders of peripheral nerve form one of the most common groups of human genetic diseases, collectively called Charcot-Marie-Tooth (CMT) neuropathy. Using linkage analysis we have identified a new locus for a form of CMT that we have called "dominant intermediate CMT" (DI-CMT). A genomewide screen using 383 microsatellite markers showed strong linkage to the short arm of chromosome 19 (maximum LOD score 4.3, with a recombination fraction (straight theta) of 0, at D19S221 and maximum LOD score 5.28, straight theta=0, at D19S226). Haplotype analysis performed with 14 additional markers placed the DI-CMT locus within a 16.8-cM region flanked by the markers D19S586 and D19S546. Multipoint linkage analysis suggested the most likely location at D19S226 (maximum multipoint LOD score 6.77), within a 10-cM confidence interval. This study establishes the presence of a locus for DI-CMT on chromosome 19p12-p13.2.

Localization of the gene for the intermediate form of Charcot-Marie-Tooth to chromosome 10q24.1-q25.1

Intermediate Charcot-Marie-Tooth neuropathy (CMT) is an inherited sensory motor neuropathy characterized by motor median nerve conduction velocities of 25-45 m/s. We performed a genomewide search in an Italian family with autosomal dominant intermediate CMT and mapped the locus on chromosome 10q. Analysis of key recombinants maps the gene for autosomal dominant intermediate CMT to a 10.7-Mb interval on chromosome 10q24.1-q25.1, between simple tandem repeat markers D10S1709 and D10S1795.

Polyneuropathies in paediatrics

Non-acute polyneuropathies (PNPs) encountered in paediatrics are reviewed. Emphasis is placed on three main groups of conditions: the relatively rare but treatable dysimmune PNP (chronic relapsing dysimmune polyneuropathies, CRDP); the more common hereditary motor/sensory neuropathies (HMSN and HSN); and the often missed symptomatic neuropathies of some heredodegenerative and neurometabolic disorders. Diagnostic procedures are discussed. One conclusion drawn is that so far metabolic screening procedures do not give any diagnostic or aetiological information in HMSN or in HSN, nor in heredoataxias or heredoparaplegias. When a specific neurometabolic disease is suspected from the clinical symptomatology, individually structured investigations are necessary.

Absence of linkage of hereditary motor and sensory neuropathy type I to chromosome 1 markers

Although one large family with hereditary motor and sensory neuropathy (HMSN) type I that showed linkage to the Duffy blood group (FY) on chromosome 1 has previously been reported, we have failed to find evidence for such linkage after examining 14 markers from chromosome 1 in 12 pedigrees. We have excluded linkage between HMSN I and FY up to theta = 0.15 (lod = -3.01) and also between HMSN I and markers flanking FY; amylase (AMY), polymorphic urinary mucin (PUM), serum amyloid protein (APCS), and alpha-spectrin (SPTA). We have excluded HMSN I from 70 cM around this linkage group. Other markers examined were MS1, oncogene L-myc (MYCL), beta-subunit of nerve growth factor (NGFB), oncogene N-ras (NRAS), glucocerebrosidase (GBA), apolipoprotein AII (APOA2), antithrombin III (AT3), renin (REN), and MS32. These cover both the long and the short arms of chromosome 1 in addition to the centromeric region and yielded no evidence of linkage to HMSN I. Two-point lod scores between these markers are also presented. It is possible that there are two or more loci for HMSN I and it will be necessary to obtain significant lod scores from individual families to resolve this issue. This is increasingly possible now that hypervariable genetic markers such as PUM are available.

Visual evoked responses in hereditary motor and sensory neuropathies

The P2 latency of the pattern reversal visual evoked response was measured in 11 patients with hereditary motor and sensory neuropathy (HMSN). The P2 latency was inversely related to the peripheral nerve conduction velocity, but no significant relation was found between the P2 latency and the age of the patients or the duration of their symptoms. When the whole group was considered the mean P2 latency was longer but not significantly different from a control group. When the 11 patients were differentiated into HMSN I (5 males) and HMSN II (2 females, 4 males) the P2 latencies of the patients with HMSN I were significantly longer than those with HMSN II. The data support earlier findings that subclinical involvement of the optic nerve may occur in HMSN and suggest that measurement of P2 latency may be of value in the differentiation of HMSN. More than two subtypes, however, may exist.