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

Understanding the Hidden Complexity of Latin American Population Isolates

Most population isolates examined to date were founded from a single ancestral population. Consequently, there is limited knowledge about the demographic history of admixed population isolates. Here we investigate genomic diversity of recently admixed population isolates from Costa Rica and Colombia and compare their diversity to a benchmark population isolate, the Finnish. These Latin American isolates originated during the 16th century from admixture between a few hundred European males and Amerindian females, with a limited contribution from African founders. We examine whole-genome sequence data from 449 individuals, ascertained as families to build mutigenerational pedigrees, with a mean sequencing depth of coverage of approximately 36×. We find that Latin American isolates have increased genetic diversity relative to the Finnish. However, there is an increase in the amount of identity by descent (IBD) segments in the Latin American isolates relative to the Finnish. The increase in IBD segments is likely a consequence of a very recent and severe population bottleneck during the founding of the admixed population isolates. Furthermore, the proportion of the genome that falls within a long run of homozygosity (ROH) in Costa Rican and Colombian individuals is significantly greater than that in the Finnish, suggesting more recent consanguinity in the Latin American isolates relative to that seen in the Finnish. Lastly, we find that recent consanguinity increased the number of deleterious variants found in the homozygous state, which is relevant if deleterious variants are recessive. Our study suggests that there is no single genetic signature of a population isolate.

Intermediate Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) disease is a common neurogenetic disorder and its heterogeneity is a challenge for genetic diagnostics. The genetic diagnostic procedures for a CMT patient can be explored according to the electrophysiological criteria: very slow motor nerve conduction velocity (MNCV) (<15 m/s), slow MNCV (15-25 m/s), intermediate MNCV (25-45 m/s), and normal MNCV (>45 m/s). Based on the inheritance pattern, intermediate CMT can be divided into dominant (DI-CMT) and recessive types (RI-CMT). GJB1 is currently considered to be associated with X-linked DI-CMT, and MPZ, INF2, DNM2, YARS, GNB4, NEFL, and MFN2 are associated with autosomal DI-CMT. Moreover, GDAP1, KARS, and PLEKHG5 are associated with RI-CMT. Identification of these genes is not only important for patients and families but also provides new information about pathogenesis. It is hoped that this review will lead to a better understanding of intermediate CMT and provide a detailed diagnostic procedure for intermediate CMT.

Advances in the molecular diagnosis of Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) disease or hereditary motor and sensory neuropathy is the most common inherited neuromuscular disorder affecting at least 1 in 2500. CMT disease is pathologically and genetically heterogeneous and is characterized by a variable age of onset, slowly progressive weakness and muscle atrophy, starting in the lower limbs and subsequently affecting the upper extremities. Symptoms are usually slowly progressive, especially for the classic and late-onset phenotypes, but can be rather severe in early-onset forms. CMT is grouped into demyelinating, axonal and intermediate forms, based on electrophysiological and pathological findings. The demyelinating types are characterized by severely reduced motor nerve conduction velocities (MNCVs) and mainly by myelin abnormalities. The axonal types are characterized by normal or slightly reduced MNCVs and mainly axonal abnormalities. The intermediate types are characterized by MNCVs between 25 m/s and 45 m/s and they have features of both demyelination and axonopathy. Inheritance can be autosomal dominant, X-linked, or autosomal recessive. Mutations in more than 30 genes have been associated with the different forms of CMT, leading to major advancements in molecular diagnostics of the disease, as well as in the understanding of pathogenetic mechanisms. This editorial aims to provide an account that is practicable and efficient on the current molecular diagnostic procedures for CMT, in correlation with the clinical, pathological and electrophysiological findings. The most frequent causative mutations of CMT will also be outlined.

Exome sequencing identifies GNB4 mutations as a cause of dominant intermediate Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of inherited neuropathies. Mutations in approximately 45 genes have been identified as being associated with CMT. Nevertheless, the genetic etiologies of at least 30% of CMTs have yet to be elucidated. Using a genome-wide linkage study, we previously mapped a dominant intermediate CMT to chromosomal region 3q28-q29. Subsequent exome sequencing of two affected first cousins revealed heterozygous mutation c.158G>A (p.Gly53Asp) in GNB4, encoding guanine-nucleotide-binding protein subunit beta-4 (Gβ4), to cosegregate with the CMT phenotype in the family. Further analysis of GNB4 in an additional 88 unrelated CMT individuals uncovered another de novo mutation, c.265A>G (p.Lys89Glu), in this gene in one individual. Immunohistochemistry studies revealed that Gβ4 was abundant in the axons and Schwann cells of peripheral nerves and that expression of Gβ4 was significantly reduced in the sural nerve of the two individuals carrying the c.158G>A (p.Gly53Asp) mutation. In vitro studies demonstrated that both the p.Gly53Asp and p.Lys89Glu altered proteins impaired bradykinin-induced G-protein-coupled-receptor (GPCR) signaling, which was facilitated by the wild-type Gβ4. This study identifies GNB4 mutations as a cause of CMT and highlights the importance of Gβ4-related GPCR signaling in peripheral-nerve function in humans.

Early onset (childhood) monogenic neuropathies

Hereditary neuropathies (HN) with onset in childhood are categorized according to clinical presentation, pathogenic mechanism based on electrophysiology, genetic transmission and, in selected cases, pathological findings. Especially relevant to pediatrics are the items "secondary" versus "primary" neuropathy, "syndromic versus nonsyndromic," and "period of life." Different combinations of these parameters frequently point toward specific monogenic disorders. Ruling out a neuropathy secondary to a generalized metabolic disorder remains the first concern in pediatrics. As a rule, metabolic diseases include additional, orienting symptoms or signs, and their biochemical diagnosis is based on logical algorithms. Primary, motor sensory are the most frequent HN and are dominated by demyelinating autosomal dominant (AD) forms (CMT1). Other forms include demyelinating autosomal recessive (AR) forms, axonal AD/AR forms, and forms with "intermediate" electrophysiological phenotype. Peripheral motor neuron disorders are dominated by AR SMN-linked spinal muscular atrophies. (Distal) hereditary motor neuropathies represent <10% of HN but exhibit large clinical and genetic heterogeneity. Sensory/dysautonomic HN involves five classic subtypes, each one related to specific genes. However, genetic heterogeneity is larger than initially suspected. Syndromic HN distinguish "purely neurological syndromes", which are multisystemic, such as spinocerebellar atrophies +, spastic paraplegias +, etc. Peripheral neuropathy is possibly the presenting feature, including in childhood. Autosomal recessive forms, on average, start more frequently in childhood. "Multiorgan syndromes", on the other hand, are more specific to Pediatrics. AR forms, which are clearly degenerative, prompt the investigation of a large set of pleiotropic genes. Other syndromes expressed in the perinatal period are mainly developmental disorders, and can sometimes be related to specific transcription factors. Systematic malformative workup and ethical considerations are necessary. Altogether, >40 genes with various biological functions have been found to be responsible for primary HN. Many are responsible for various phenotypes, including some without the polyneuropathic trait, and some for various types of transmission.

Dominant Charcot-Marie-Tooth syndrome and cognate disorders

Charcot-Marie-Tooth neuropathy (CMT) is a group of genetically heterogeneous disorders sharing a similar phenotype, characterized by wasting and weakness mainly involving the distal muscles of lower and upper limbs, variably associated with distal sensory loss and skeletal deformities. This chapter deals with dominantly transmitted CMT and related disorders, namely hereditary neuropathy with liability to pressure palsies (HNPP) and hereditary neuralgic amyotrophy (HNA). During the last 20 years, several genes have been uncovered associated with CMT and our understanding of the underlying molecular mechanisms has greatly improved. Consequently, a precise genetic diagnosis is now possible in the majority of cases, thus allowing proper genetic counseling. Although, unfortunately, treatment is still unavailable for all types of CMT, several cellular and animal models have been developed and some compounds have proved effective in these models. The first trials with ascorbic acid in CMT type 1A have been completed and, although negative, are providing relevant information on disease course and on how to prepare for future trials.

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.

Clinical characterization and genetic analysis of a possible novel type of dominant Charcot-Marie-Tooth disease

A family of dominant Charcot-Marie-Tooth disease with eleven members, six of them symptomatic, was characterized clinically and genetically. The ages at onset ranged from 10 to 45 years, and the clinical severity varied from no symptom to being wheelchair-bound. The median motor nerve conduction velocities ranged from 16.5 to 45.7 m/s. Men were more severely affected. The sural nerve biopsies in two patients featured demyelinating changes. No mutation in PMP22, MPZ, GJB1, NEFL, LITAF, EGR2, MFN2, HSP27, HSP22, GADP1, YARS, and DNM2 genes was found in the proband. Haplotype analyzes excluded linkage to the previously reported dominant CMT loci. A genomewide screen with 400 microsatellite markers and multipoint linkage analyzes revealed that the highest LOD score was around 1.6 on chromosome 3q28-q29, suggestive of a weak but possible linkage at this locus. The results of this study implicate the existence of a novel genetic locus for this syndrome.

Shades of gray: a comparison of linkage disequilibrium between Hutterites and Europeans

Founder or isolated populations have advantages for genetic studies due to decreased genetic and environmental heterogeneity. However, whereas longer-range linkage disequilibrium (LD) in these populations is expected to facilitate gene localization, extensive LD may actually limit the ability for gene discovery. The North American Hutterite population is one of the best characterized young founder populations and members of this isolate have been the subjects of our studies of complex traits, including fertility, asthma and cardiovascular disease, for >20 years. Here, we directly assess the patterns and extent of global LD using single nucleotide polymorphism genotypes with minor allele frequencies (MAFs) > or =5% from the Affymetrix GeneChip Mapping 500 K array in 60 relatively unrelated Hutterites and 60 unrelated Europeans (HapMap CEU). Although LD among some marker pairs extends further in the Hutterites than in Europeans, the pattern of LD and MAF are surprisingly similar. These results indicate that (1) identifying disease genes should be no more difficult in the Hutterites than in outbred European populations, (2) the same common susceptibility alleles for complex diseases should be present in the Hutterites and outbred European populations, and (3) imputation algorithms based on HapMap CEU should be applicable to the Hutterites.

Dynamic instability of microtubules requires dynamin 2 and is impaired in a Charcot-Marie-Tooth mutant

Dynamin is a fission protein that participates in endocytic vesicle formation. Although dynamin was originally identified as a microtubule-binding protein, the physiological relevance of this function was unclear. Recently, mutations in the ubiquitously expressed dynamin 2 (dyn2) protein were found in patients with Charcot-Marie-Tooth (CMT) disease, which is an inherited peripheral neuropathy. In this study, we show that one of these mutations, 551Δ3, induces prominent decoration of microtubules with the mutant dyn2. Dyn2 was required for proper dynamic instability of microtubules, and this was impaired in cells expressing the 551Δ3 mutant, which showed a remarkable increase in microtubule acetylation, a marker of stable microtubules. Depletion of endogenous dyn2 with a small interfering RNA also resulted in the accumulation of stable microtubules. Furthermore, the formation of mature Golgi complexes, which depends on microtubule-dependent membrane transport, was impaired in both dyn2 knockdown cells and cells expressing the 551Δ3 mutant. Collectively, our results suggest that dyn2 regulates dynamic instability of microtubules, which is essential for organelle motility, and that this function may be impaired in CMT disease.

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.

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.

Hereditary neuropathies

PURPOSE OF REVIEW

The purpose of this review is to help neurologists understand new concepts in hereditary neuropathies, from the clinician's point of view, in the molecular era after the burst of information regarding peripheral nerve biology.

RECENT FINDINGS

Recent studies have focused on understanding the pathomechanisms involved in hereditary neuropathies. In the past year identification of new genes has slowed down since scientists have concentrated more on the function of genes causing Charcot-Marie-Tooth disease and Schwann cell-axon interactions to reveal the molecular cell biology of the disease. Animal models for the most common subtypes of human Charcot-Marie-Tooth disease are now available.

SUMMARY

Rapid advances in the molecular genetics and cell biology of hereditary neuropathies have highlighted the great genetic complexity of Charcot-Marie-Tooth disease. The evolution from a simple clinical classification to a complex molecular one has not facilitated our understanding of the disease. Moreover, the new molecular classification is not simple to use as different mutations of the same gene produce a range of phenotypes. The clinicians have to look for specific clinical and electrophysiological clues to direct the patient to appropriate genetic testing.

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.

Molecular diagnostics of Charcot-Marie-Tooth disease and related peripheral neuropathies

DNA diagnostics plays an important role in the characterization and management of patients manifesting inherited peripheral neuropathies. We describe the clinical integration of molecular diagnostics with medical history, physical examination, and electrophysiological studies. Molecular testing can help establish a secure diagnosis, enable genetic counseling regarding recurrence risk, potentially provide prognostic information, and in the near future may be important for the choice of therapies.

Know thy neighbor: a survey of diseases and complex syndromes that map to chromosomal regions encoding TRP channels

On the basis of their ever-expanding roles, not only in sensory signaling but also in a plethora of other, often Ca(2+)-mediated actions in cell and whole body homeostasis, it is suggested that mutations in TRP channel genes not only cause disease states but also contribute in more subtle ways to simple and complex diseases. A survey is therefore presented of diseases and syndromes that map to one or multiple chromosomal loci containing TRP channel genes. A visual map of the chromosomal locations of TRP channel genes in man and mouse is also presented.

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.

Disrupted function and axonal distribution of mutant tyrosyl-tRNA synthetase in dominant intermediate Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth (CMT) neuropathies are common disorders of the peripheral nervous system caused by demyelination or axonal degeneration, or a combination of both features. We previously assigned the locus for autosomal dominant intermediate CMT neuropathy type C (DI-CMTC) to chromosome 1p34-p35. Here we identify two heterozygous missense mutations (G41R and E196K) and one de novo deletion (153-156delVKQV) in tyrosyl-tRNA synthetase (YARS) in three unrelated families affected with DI-CMTC. Biochemical experiments and genetic complementation in yeast show partial loss of aminoacylation activity of the mutant proteins, and mutations in YARS, or in its yeast ortholog TYS1, reduce yeast growth. YARS localizes to axonal termini in differentiating primary motor neuron and neuroblastoma cultures. This specific distribution is significantly reduced in cells expressing mutant YARS proteins. YARS is the second aminoacyl-tRNA synthetase found to be involved in CMT, thereby linking protein-synthesizing complexes with neurodegeneration.

Mutations in the pleckstrin homology domain of dynamin 2 cause dominant intermediate Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) disease is a clinically and genetically heterogeneous group of peripheral neuropathies. Different chromosomal loci have been linked with three autosomal dominant, 'intermediate' types of CMT: DI-CMTA, DI-CMTB and DI-CMTC. We refined the locus associated with DI-CMTB on chromosome 19p12-13.2 to 4.2 Mb in three unrelated families with CMT originating from Australia, Belgium and North America. After screening candidate genes, we identified unique mutations in dynamin 2 (DNM2) in all families. DNM2 belongs to the family of large GTPases and is part of the cellular fusion-fission apparatus. In transiently transfected cell lines, mutations of DNM2 substantially diminish binding of DNM2 to membranes by altering the conformation of the beta3/beta4 loop of the pleckstrin homology domain. Additionally, in the Australian and Belgian pedigrees, which carry two different mutations affecting the same amino acid, Lys558, CMT cosegregated with neutropenia, which has not previously been associated with CMT neuropathies.

A novel stop codon mutation in the PMP22 gene associated with a variable phenotype

The most frequent inherited peripheral neuropathy is the peripheral myelin protein 22 (PMP22) gene related disease. Duplication, deletion, and point mutations in that gene are associated with phenotypic variability. Here we report a family carrying a novel mutation in the PMP22 gene (c. 327C>A), which results in a premature stop codon (Cys109stop). The family members who carry this mutation have a Charcot-Marie-Tooth type 1 variable phenotype, ranging from asymptomatic to severely affected. These findings suggest that the fourth transmembrane domain of the PMP22 gene may play an important role, although the intrafamilial clinical variability reinforces the observation that pathogenic mutations are not always phenotype determinant and that other factors (genetic or epigenetic) modulate the severity of the clinical course.

The novel neurofilament light (NEFL) mutation Glu397Lys is associated with a clinically and morphologically heterogeneous type of Charcot-Marie-Tooth neuropathy

Charcot-Marie-Tooth disease comprises a heterogeneous group of hereditary neuropathies which fall into two main groups: demyelinating CMT1 with reduced nerve conduction velocity and axonal CMT2 with normal nerve conduction velocity. The neuropathological features correspond in most cases to this classification. Four genes were recently identified to cause autosomal dominant CMT2, including the neurofilament light gene. Thus far, only few mutations have been reported in neurofilament light involving eight amino acids of the gene. We identified a novel mutation, Glu397Lys, in a conserved motive signaling the end of the rod domain. The affected family members from three generations showed strikingly different clinical phenotypes, including weakness of the lower extremities, foot deformities, and deafness. The mutation was associated with nerve conduction velocities ranging from 27 m/s in a 25-year-old female to 43 m/s in an 82-year-old male in the lower extremity motor nerves. Sural nerve biopsies of two affected subjects were analyzed by light and electron microscopy. The pathological changes consisted of a reduction of predominantly large myelinated nerve fibers and various stages of onion bulb formation as typically seen in CMT1. This correlative study further confirms that neurofilament light gene mutations cause a wide clinical spectrum. Thus, analysis of the neurofilament light gene should not be restricted to pure axonal neuropathies.

Dominant intermediate Charcot-Marie-Tooth type C maps to chromosome 1p34-p35

Dominant intermediate Charcot-Marie-Tooth (DI-CMT) neuropathy is a genetic and phenotypic variant of classical CMT, characterized by intermediate nerve conduction velocities and histological evidence of both axonal and demyelinating features. We report two unrelated families with intermediate CMT linked to a novel locus on chromosome 1p34-p35 (DI-CMTC). The combined haplotype analysis in both families localized the DI-CMTC gene within a 6.3-cM linkage interval flanked by markers D1S2787 and D1S2830. The functional and positional candidate genes, Syndecan 3 (SDC3), and lysosomal-associated multispanning membrane protein 5 (LAPTM5) were excluded for pathogenic mutations.

Dominant intermediate Charcot-Marie-Tooth type C maps to chromosome 1p34-p35

Dominant intermediate Charcot-Marie-Tooth (DI-CMT) neuropathy is a genetic and phenotypic variant of classical CMT, characterized by intermediate nerve conduction velocities and histological evidence of both axonal and demyelinating features. We report two unrelated families with intermediate CMT linked to a novel locus on chromosome 1p34-p35 (DI-CMTC). The combined haplotype analysis in both families localized the DI-CMTC gene within a 6.3-cM linkage interval flanked by markers D1S2787 and D1S2830. The functional and positional candidate genes, Syndecan 3 (SDC3), and lysosomal-associated multispanning membrane protein 5 (LAPTM5) were excluded for pathogenic mutations.

Refined localization of dominant intermediate Charcot-Marie-Tooth neuropathy and exclusion of seven known candidate genes in the region

Charcot-Marie-Tooth (CMT) neuropathy is one of the most common hereditary disorders of the human peripheral nervous system. The CMT syndrome includes weakness and atrophy of distal muscles, high arched feet (pes cavus), depressed or absent deep tendon reflexes, and mild sensory loss. Dominant intermediate CMT (DI-CMT) neuropathy is a form of CMT with intermediate median motor nerve conduction velocities. We previously localized the DI-CMT locus to a 16.8-cM region on chromosome 19p12-p13.2. Extended haplotype analysis and clinical assessment of additional family members and a report of a second family linked to this locus has enabled us to narrow the candidate region to a 6-cM interval flanked by D19S558 and D19S432. Selection of positional candidate genes for screening was performed on the basis of neural expression and microarray analysis of Schwann cell differentiation in vivo. Seven candidate genes have been investigated. These include six genes localized in the original linkage interval and one in the newly refined region. They are excluded as a cause for DI-CMT neuropathy.

Refined physical map of the human PAX2/HOX11/NFKB2 cancer gene region at 10q24 and relocalization of the HPV6AI1 viral integration site to 14q13.3-q21.1

BACKGROUND

Chromosome band 10q24 is a gene-rich domain and host to a number of cancer, developmental, and neurological genes. Recurring translocations, deletions and mutations involving this chromosome band have been observed in different human cancers and other disease conditions, but the precise identification of breakpoint sites, and detailed characterization of the genetic basis and mechanisms which underlie many of these rearrangements has yet to be resolved. Towards this end it is vital to establish a definitive genetic map of this region, which to date has shown considerable volatility through time in published works of scientific journals, within different builds of the same international genomic database, and across the differently constructed databases.

RESULTS

Using a combination of chromosome and interphase fluorescent in situ hybridization (FISH), BAC end-sequencing and genomic database analysis we present a physical map showing that the order and chromosomal orientation of selected genes within 10q24 is CEN-CYP2C9-PAX2-HOX11-NFKB2-TEL. Our analysis has resolved the orientation of an otherwise dynamically evolving assembly of larger contigs upstream of this region, and in so doing verifies the order and orientation of a further 9 cancer-related genes and GOT1. This study further shows that the previously reported human papillomavirus type 6a DNA integration site HPV6AI1 does not map to 10q24, but that it maps at the interface of chromosome bands 14q13.3-q21.1.

CONCLUSIONS

This revised map will allow more precise localization of chromosome rearrangements involving chromosome band 10q24, and will serve as a useful baseline to better understand the molecular aetiology of chromosomal instability in this region. In particular, the relocation of HPV6AI1 is important to report because this HPV6a integration site, originally isolated from a tonsillar carcinoma, was shown to be rearranged in other HPV6a-related malignancies, including 2 of 25 genital condylomas, and 2 of 7 head and neck tumors tested. Our finding shifts the focus of this genomic interest from 10q24 to the chromosome 14 site.

Inherited neuropathies

Inherited neuropathies are common and are usually caused by mutations in genes that are expressed by myelinating Schwann cells or neurons, which is the biological basis for long-standing distinction between primary demyelinating and axonal neuropathies. Neuropathies can be isolated, the primary manifestation of a more complex syndrome, or overshadowed by other aspects of the inherited disease. Increasing knowledge of the molecular-genetic causes of inherited neuropathies facilitates faster, more accurate diagnosis, and sets the stage for development of specific therapeutic interventions.

Periaxin mutations cause a broad spectrum of demyelinating neuropathies

Previous studies have demonstrated that apparent loss-of-function mutations in the periaxin gene cause autosomal recessive Dejerine-Sottas neuropathy or severe demyelinating Charcot-Marie-Tooth disease. In this report, we extend the associated phenotypes with the identification of two additional families with novel periaxin gene mutations (C715X and R82fsX96) and provide detailed neuropathology. Each patient had marked sensory involvement; two siblings with a homozygous C715X mutation had much worse sensory impairment than motor impairment. Despite early disease onset, these siblings with the C715X mutation had relatively slow disease progression and adult motor impairment typical of classic demyelinating Charcot-Marie-Tooth neuropathy. In contrast, a patient with the homozygous R82fsX96 mutation had a disease course consistent with Dejerine-Sottas neuropathy. The neuropathology of patients in both families was remarkable for demyelination, onion bulb and occasional tomacula formation with focal myelin thickening, abnormalities of the paranodal myelin loops, and focal absence of paranodal septate-like junctions between the terminal loops and axon. Our study indicates a prominent sensory neuropathy resulting from periaxin gene mutations and suggests a role for the carboxyl terminal domain of the periaxin protein.

Genetics of population isolates

Genetic isolates, as shown empirically by the Finnish, Old Order Amish, Hutterites, Sardinian and Jewish communities among others, represent a most important and powerful tool in genetically mapping inherited disorders. The main features associated with that genetic power are the existence of multigenerational pedigrees which are mostly descended from a small number of founders a short number of generations ago, environmental and phenotypic homogeneity, restricted geographical distribution, the presence of exhaustive and detailed records correlating individuals in very well ascertained pedigrees, and inbreeding as a norm. On the other hand, the presence of a multifounder effect or admixture among divergent populations in the founder time (e.g. the Finnish and the Paisa community from Colombia) will theoretically result in increased linkage disequilibrium among adjacent loci. The present review evaluates the historical context and features of some genetic isolates with emphasis on the basic population genetic concepts of inbreeding and genetic drift, and also the state-of-the-art in mapping traits, both Mendelian and complex, on genetic isolates.

The genetic convergence of Charcot-Marie-Tooth disease types 1 and 2 and the role of genetics in sporadic neuropathy

Charcot-Marie-Tooth (CMT) disease represents a clinically and genetically heterogeneous group of inherited neuropathies caused by aberration of the intimate relationship between the myelin sheath and the axon; disorders causing demyelination are classified as CMT1 and those causing axonal loss as CMT2. The mechanisms by which mutations disturb the relationship of the myelin sheath and axon are not fully understood; however, we hypothesize that some mutations affect this relationship more profoundly than others, and thus account for the paradox that mutation of a "myelin gene" can present with electrophysiologic features of CMT2 and vice versa. Also, contrary to popular understanding, inherited neuropathies account for a substantial number of chronic peripheral neuropathies. Because of this observation, we propose that molecular diagnosis is a necessary adjunct for differentiating genetic and acquired peripheral neuropathies, even in sporadic chronic neuropathy.