Molecular genetics of autosomal-recessive axonal Charcot-Marie-Tooth neuropathies

Comparative genetic diagnostic evaluation of pediatric neuromuscular diseases in a consanguineous population

Neuromuscular diseases (NMD) are a group of neurological diseases that manifest with various clinical symptoms affecting different components of the peripheral nervous system, which play a role in voluntary body movements control. The primary objective of this study is to explore the diagnostic efficacy of a combined genetic and biochemical testing approach for patients with neuromuscular diseases with diverse presentations in a population with high rate of consanguinity. Genetic testing was performed using selected Next Generation Sequencing (NGS) gene panels and whole exome sequencing on the peripheral blood sample from the patients. The study results revealed that the majority of patients in our cohort had a history of consanguinity (83%). Genetic testing through gene panels and Whole Exome Sequencing yielded similar result. Out of the patients tested, 66% underwent gene panels testing, 56% had Whole Exome Sequencing, 32% received array Comparative Genomic Hybridization (CGH) assays, and 40% underwent metabolic testing. Overall, 58 patients (61%) received definitive results after following all tests. Among the remaining 36 patients, 19 exhibited variants of unknown significance (VOUS) (21%).

Genetic and clinical profile of 15 Chinese families with GDAP1-related Charcot-Marie-Tooth disease and identification of H256R as a frequent mutation

BACKGROUND AND AIMS

Mutations in ganglioside-induced differentiation-associated protein 1 (GDAP1) cause axonal or demyelinating Charcot-Marie-Tooth disease (CMT) with autosomal dominant or recessive inheritance. In this study, we aim to report the genotypic and phenotypic features of GDAP1-related CMT in a Chinese cohort.

METHODS

Clinical, neurophysiological, genetic data, and available muscle/brain imaging information of 28 CMT patients with GDAP1 variants were retrospectively collected.

RESULTS

We identified 16 GDAP1 pathogenic variants, among which two novel variants c.980dup(p.L328FfsX25) and c.480+4T>G were first reported. Most patients (16/28) presented with AR or AD CMT2K phenotype. Clinical characteristics in our cohort demonstrated that the AR patients presented earlier onset, more severe phenotype compared with the AD patients. Considerable intra-familial phenotypic variability was observed among three AD families. Muscle atrophy and fatty infiltration in the lower extremity were detected by Muscle magnetic resonance imaging (MRI) scans in four patients. MRI showed two AR patients showed more severe muscle involvement of the posterior compartment than those of the anterolateral compartment in the calf. One patient carrying Q38*/H256R variants accompanied with mild periventricular leukoaraiosis.

CONCLUSIONS

In this study, we conducted an analysis of clinical features of the GDAP1-related CMT patients, expanded the mutation spectrum in GDAP1 by reporting two novel variants, and presented the prevalent occurrence of the H256R mutation in China. The screening of GDAP1 should be particularly emphasized in Chinese patients with CMT2, given the incomplete penetrance and pathogenic inheritance patterns involving dominant and recessive modes.

Nuclear envelopathies: a complex LINC between nuclear envelope and pathology

Since the identification of the first disease causing mutation in the gene coding for emerin, a transmembrane protein of the inner nuclear membrane, hundreds of mutations and variants have been found in genes encoding for nuclear envelope components. These proteins can be part of the inner nuclear membrane (INM), such as emerin or SUN proteins, outer nuclear membrane (ONM), such as Nesprins, or the nuclear lamina, such as lamins A and C. However, they physically interact with each other to insure the nuclear envelope integrity and mediate the interactions of the nuclear envelope with both the genome, on the inner side, and the cytoskeleton, on the outer side. The core of this complex, called LINC (LInker of Nucleoskeleton to Cytoskeleton) is composed of KASH and SUN homology domain proteins. SUN proteins are INM proteins which interact with lamins by their N-terminal domain and with the KASH domain of nesprins located in the ONM by their C-terminal domain.Although most of these proteins are ubiquitously expressed, their mutations have been associated with a large number of clinically unrelated pathologies affecting specific tissues. Moreover, variants in SUN proteins have been found to modulate the severity of diseases induced by mutations in other LINC components or interactors. For these reasons, the diagnosis and the identification of the molecular explanation of "nuclear envelopathies" is currently challenging.The aim of this review is to summarize the human diseases caused by mutations in genes coding for INM proteins, nuclear lamina, and ONM proteins, and to discuss their potential physiopathological mechanisms that could explain the large spectrum of observed symptoms.

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.

Hereditary motor-sensory, motor, and sensory neuropathies in childhood

Hereditary neuropathies (HN) are categorized according to clinical presentation, pathogenic mechanism based on electrophysiology, genetic transmission, age of occurrence, and, in selected cases, pathological findings. The combination of these parameters frequently orients towards specific genetic disorders. Ruling out a neuropathy secondary to a generalized metabolic disorder remains the first pediatric concern. Primary, motor-sensory are the most frequent HN and are dominated by demyelinating AD forms (CMT1). Others are demyelinating AR forms, axonal AD/AR forms, and forms with "intermediate" electrophysiological phenotype. Pure motor HN represent<10% of HN but exhibit large clinical and genetic heterogeneity. Sensory/dysautonomic HN cover five classical subtypes, each one related to specific genes. However, genetic heterogeneity is largly greater than initially suspected. Syndromic HN distinguish: "purely neurological syndromes", which are multisystemic, usually AD disorders, such as spinocerebellar atrophies +, spastic paraplegias +, etc. Peripheral Neuropathy may be the presenting feature, including in childhood. Clearly degenerative, AR forms prompt to investigate a large set of pleiotropic genes. Other syndromes, expressed in the perinatal period and comprising malformative features, are mainly developmental disorders, sometimes related to specific transcription factors. Altogether, >40 genes with various biological functions have been found responsible for HN. Many are responsible for various phenotypes, including some without the polyneuropathic trait: for the pediatric neurologist, phenotype/genotype correlations constitute a permanent bidirectional exercise.

Clinical and histopathological study of Charcot-Marie-Tooth neuropathy with a novel S90W mutation in BSCL2

The objective of the study was to investigate the disease-causing mutation in an autosomal dominant Charcot-Marie-Tooth disease type 2 family and examine the clinical and histopathological evaluation. We enrolled a family of Korean origin with axonal Charcot-Marie-Tooth disease neuropathy (FC305; 13 males, six females) and applied genome-wide linkage analysis. Whole exome sequencing was performed for two patients. In addition, sural nerve biopsies were obtained from two patients. Through whole exome sequencing, we identified an average of 20,336 coding variants from two patients. We also found evidence of linkage mapped to chromosome 11p11-11q13.3 (LOD score of 3.6). Among these variants in the linkage region, we detected a novel p.S90W mutation in the Berardinelli-Seip congenital lipodystrophy 2 (BSCL2) gene, after filtering 31 Korean control exomes. Our p.S90W patients had frequent sensory disturbances, pyramidal tract signs, and predominant right thenar muscle atrophy in comparison with reported p.S90L patients. The phenotypic spectra were wide and demonstrated intrafamilial variability. Two patients with different clinical features underwent sural nerve biopsies; the myelinated fiber densities were increased slightly in both patients, which differed from two previous case reports of BSCL2 mutations (p.S90L and p.N88S). This report expands the variability of the clinical spectrum associated with the BSCL2 gene and describes the first family with the p.S90W mutation.

Inherited peripheral neuropathies

Mutations in genes expressed in Schwann cells and the axons they ensheathe cause the hereditary motor and sensory neuropathies, also known as Charcot-Marie-Tooth disease (CMT). More than 40 different genes have been shown to cause inherited neuropathies; chromosomal localizations of many other distinct inherited neuropathies have been mapped, and new genetic causes for inherited neuropathies continue to be discovered. How to keep track of all of these disorders, when to pursue genetic testing, and what tests to order for specific patients are difficult challenges for any neurologist. This review addresses these issues and provides illustrative cases to help in dealing with them. CMT serves as a living system to identify molecules necessary for normal peripheral nervous system (PNS) function. Understanding how these various molecules interact will provide a better understanding of the pathogenesis of peripheral neuropathies in general as well as other neurodegenerative disorders involving the PNS.

Overlapping molecular pathological themes link Charcot-Marie-Tooth neuropathies and hereditary spastic paraplegias

In this review we focus on Charcot-Marie-Tooth (CMT) neuropathies and hereditary spastic paraplegias (HSPs). Although these diseases differ in whether they primarily affect the peripheral or central nervous system, both are genetically determined, progressive, long axonopathies that affect motor and sensory pathways. This commonality suggests that there might be similarities in the molecular pathology underlying these conditions, and here we compare the molecular genetics and cellular pathology of the two groups.

SNP array-based whole genome homozygosity mapping as the first step to a molecular diagnosis in patients with Charcot-Marie-Tooth disease

Considerable non-allelic heterogeneity for autosomal recessively inherited Charcot-Marie-Tooth (ARCMT) disease has challenged molecular testing and often requires a large amount of work in terms of DNA sequencing and data interpretation or remains unpractical. This study tested the value of SNP array-based whole-genome homozygosity mapping as a first step in the molecular genetic diagnosis of sporadic or ARCMT in patients from inbred families or outbred populations with the ancestors originating from the same geographic area. Using 10 K 2.0 and 250 K Nsp Affymetrix SNP arrays, 15 (63%) of 24 CMT patients received an accurate genetic diagnosis. We used our Java-based script eHoPASA CMT—easy Homozygosity Profiling of SNP arrays for CMT patients to display the location of homozygous regions and their extent of marker count and base-pairs throughout the whole genome. CMT4C was the most common genetic subtype with mutations detected in SH3TC2, one (p.E632Kfs13X) appearing to be a novel founder mutation. A sporadic patient with severe CMT was homozygous for the c.250G > C (p.G84R) HSPB1 mutation which has previously been reported to cause autosomal dominant dHMN. Two distantly related CMT1 patients with early disease onset were found to carry a novel homozygous mutation in MFN2 (p.N131S). We conclude that SNP array-based homozygosity mapping is a fast, powerful, and economic tool to guide molecular genetic testing in ARCMT and in selected sporadic CMT patients.

Mediator and human disease

Since the identification of a metazoan counterpart to yeast Mediator nearly 15 years ago, a convergent body of biochemical and molecular genetic studies have confirmed their structural and functional relationship as an integrative hub through which regulatory information conveyed by signal activated transcription factors is transduced to RNA polymerase II. Nonetheless, metazoan Mediator complexes have been shaped during evolution by substantive diversification and expansion in both the number and sequence of their constituent subunits, with important implications for the development of multicellular organisms. The appearance of unique interaction surfaces within metazoan Mediator complexes for transcription factors of diverse species-specific origins extended the role of Mediator to include an essential function in coupling developmentally coded signals with precise gene expression output sufficient to specify cell fate and function. The biological significance of Mediator in human development, suggested by genetic studies in lower metazoans, is emphatically illustrated by an expanding list of human pathologies linked to genetic variation or aberrant expression of its individual subunits. Here, we review our current body of knowledge concerning associations between individual Mediator subunits and specific pathological disorders. When established, molecular etiologies underlying genotype-phenotype correlations are addressed, and we anticipate that future progress in this critical area will help identify therapeutic targets across a range of human pathologies.

Dominant GDAP1 mutations cause predominantly mild CMT phenotypes

OBJECTIVE

Ganglioside-induced differentiation associated-protein 1 (GDAP1) mutations are commonly associated with autosomal recessive Charcot-Marie-Tooth (ARCMT) neuropathy; however, in rare instances, they also lead to autosomal dominant Charcot-Marie-Tooth (ADCMT). We aimed to investigate the frequency of disease-causing heterozygous GDAP1 mutations in ADCMT and their associated phenotype.

METHODS

We performed mutation analysis in a large cohort of ADCMT patients by means of bidirectional sequencing of coding regions and exon-intron boundaries of GDAP1. Intragenic GDAP1 deletions were excluded using an allele quantification assay. We confirmed the pathogenic character of one sequence variant by in vitro experiments assaying mitochondrial morphology and function.

RESULTS

In 8 Charcot-Marie-Tooth disease (CMT) families we identified 4 pathogenic heterozygous GDAP1 mutations, 3 of which are novel. Three of the mutations displayed reduced disease penetrance. Disease onset in the affected individuals was variable, ranging from early childhood to adulthood. Disease progression was slow in most patients and overall severity milder than typically seen in autosomal recessive GDAP1 mutations. Electrophysiologic changes are heterogeneous but compatible with axonal neuropathy in the majority of patients.

CONCLUSIONS

With this study, we broaden the phenotypic and genetic spectrum of autosomal dominant GDAP1-associated neuropathies. We show that patients with dominant GDAP1 mutations may display clear axonal CMT, but may also have only minimal clinical and electrophysiologic abnormalities. We demonstrate that cell-based functional assays can be reliably used to test the pathogenicity of unknown variants. We discuss the implications of phenotypic variability and the reduced penetrance of autosomal dominant GDAP1 mutations for CMT diagnostic testing and counseling.

The Arabidopsis thaliana Med25 mediator subunit integrates environmental cues to control plant development

Development in plants is controlled by abiotic environmental cues such as day length, light quality, temperature, drought, and salinity. These signals are sensed by a variety of systems and transmitted by different signal transduction pathways. Ultimately, these pathways are integrated to control expression of specific target genes, which encode proteins that regulate development and differentiation. The molecular mechanisms for such integration have remained elusive. We here show that a linear 130-amino-acids-long sequence in the Med25 subunit of the Arabidopsis thaliana Mediator is a common target for the drought response element binding protein 2A, zinc finger homeodomain 1, and Myb-like transcription factors which are involved in different stress response pathways. In addition, our results show that Med25 together with drought response element binding protein 2A also function in repression of PhyB-mediated light signaling and thus integrate signals from different regulatory pathways.

Update on Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) disease encompasses a genetically heterogeneous group of inherited neuropathies, also known as hereditary motor and sensory neuropathies. CMT results from mutations in more than 40 genes expressed in Schwann cells and neurons causing overlapping phenotypes. The classic CMT phenotype reflects length-dependent axonal degeneration characterized by distal sensory loss and weakness, deep tendon reflex abnormalities, and skeletal deformities. Recent articles have provided insight into the molecular pathogenesis of CMT, which, for the first time, suggest potential therapeutic targets. Although there are currently no effective medications for CMT, multiple clinical trials are ongoing or being planned. This review will focus on the underlying pathomechanisms and diagnostic approaches of CMT and discuss the emerging therapeutic strategies.

Exome sequencing allows for rapid gene identification in a Charcot-Marie-Tooth family

OBJECTIVE

Charcot-Marie-Tooth (CMT) disease comprises a large number of genetically distinct forms of inherited peripheral neuropathies. The relative uniform phenotypes in many patients with CMT make it difficult to decide which of the over 35 known CMT genes are affected in a given patient. Genetic testing decision trees are therefore broadly based on a small number of major subtypes (eg, CMT1, CMT2) and the observed mutation frequency for CMT genes. Since conventional genetic testing is expensive many rare genes are not being tested for at all.

METHODS

Whole-exome sequencing has recently been introduced as a novel and alternative approach. This method is capable of resequencing a nearly complete set of coding exons in an individual. We performed whole-exome sequencing in an undiagnosed family with CMT.

RESULTS

Within over 24,000 variants detected in 2 exomes of a CMT family, we identified a nonsynonymous GJB1 (Cx32) mutation. This variant had been reported previously as pathogenic in X-linked CMT families. Sanger sequencing confirmed complete cosegregation in the family. Affected individuals had a marked early involvement of the upper distal extremities and displayed a mild reduction of nerve conduction velocities.

INTERPRETATION

We have shown for the first time in a genetically highly heterogeneous dominant disease that exome sequencing is a valuable method for comprehensive medical diagnosis. Further improvements of exon capture design, next-generation sequencing accuracy, and a constant price decline will soon lead to the adoption of genomic approaches in gene testing of Mendelian disease.

The homozygous ganglioside-induced differentiation-associated protein 1 mutation c.373C > T causes a very early-onset neuropathy: case report and literature review

Mutations in the ganglioside-induced differentiation-associated protein 1 (GDAP1) gene may cause severe early-onset inherited neuropathies. Here, the authors report a clinical and neurophysiological follow-up of a Pakistani child with a very early-onset neuropathy carrying a novel homozygous mutation in the GDAP1gene. They discuss the relationship between the several forms of Charcot-Marie-Tooth disease presenting in the first months of life and focus on the literature of GDAP1-associated early-onset neuropathy. This case further expands on the clinical spectrum and the genetic heterogeneity of early-onset inherited neuropathy due to GDAP1 gene mutations.

Mutation in the gene encoding ubiquitin ligase LRSAM1 in patients with Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) represents a family of related sensorimotor neuropathies. We studied a large family from a rural eastern Canadian community, with multiple individuals suffering from a condition clinically most similar to autosomal recessive axonal CMT, or AR-CMT2. Homozygosity mapping with high-density SNP genotyping of six affected individuals from the family excluded 23 known genes for various subtypes of CMT and instead identified a single homozygous region on chromosome 9, at 122,423,730–129,841,977 Mbp, shared identical by state in all six affected individuals. A homozygous pathogenic variant was identified in the gene encoding leucine rich repeat and sterile alpha motif 1 (LRSAM1) by direct DNA sequencing of genes within the region in affected DNA samples. The single nucleotide change mutates an intronic consensus acceptor splicing site from AG to AA. Direct analysis of RNA from patient blood demonstrated aberrant splicing of the affected exon, causing an obligatory frameshift and premature truncation of the protein. Western blotting of immortalized cells from a homozygous patient showed complete absence of detectable protein, consistent with the splice site defect. LRSAM1 plays a role in membrane vesicle fusion during viral maturation and for proper adhesion of neuronal cells in culture. Other ubiquitin ligases play documented roles in neurodegenerative diseases. LRSAM1 is a strong candidate for the causal gene for the genetic disorder in our kindred.

Sensory motor neuropathies are diseases of the peripheral nervous system, involving primarily the nerves which control our muscles. These can result from either genetic or non-genetic causes, with genetic causes usually referred to as Charcot-Marie-Tooth (CMT) disease after the three clinicians who first described the key diagnostic markers. CMT patients lose muscle function, mainly in their arms and legs, with increasing severity during their lives. There are almost two dozen known genes that can mutate to cause CMT, and these fall into a wide variety of biochemical cellular pathways. We identified a group of patients with CMT from a small rural community, with good reason to suspect a genetic basis for their disease. Using high-throughput mapping and DNA sequencing technologies developed as part of the Human Genome Project, we were able to find the likely mutated gene, which was not any of the previously known CMT genes. Based on its sequence, the gene, called LRSAM1, probably plays a role in the correct metabolism of other proteins in the cell. Among the known CMT genes, some are also involved in protein metabolism, suggesting that this is a generally important pathway in the neurons that control muscle activity.

[Hereditary peripheral neuropathies]

Currently more than 30 genes are known to be responsible for genetically determined neuropathies. Charcot-Marie-Tooth (CMT) disease is the most frequent of these hereditary neuropathies, with a prevalence of 4.7 to 36 per 100 000. In its demyelinating forms (CMT1), approximately 70% of cases are associated with a duplication of the PMP22gene. In its axonal forms (CMT2), 10-20% of the cases may be associated with a mutation of the MFN2gene. For North African patients with recessive transmission, a mutation of the LMNA gene must be sought. It is essential to stress the great variability of the phenotype--clinical, electrophysiological, and histologic--between and within families. A detailed analysis of these criteria, together with consideration of ethnic origin, may guide the search for the causal mutation. Whether the case involves certainly hereditary transmission or a sporadic form, it is desirable to be able to examine the maximum number of the patient's kin, both clinically and electrophysiologically. The forms with recessive transmission usually have a very early onset and are more serious than the dominant forms. The early- and very early-onset forms of CMT are increasingly better distinguished: congenital hypomyelination neuropathy (mutations of PMP22, MPZ or EGR2), or more axonal forms, including SMARD1 (Spinal muscle atrophy with respiratory distress; mutations of IGHMBP2) and EOHMSN (Early-onset hereditary motor and sensory neuropathy; mutations of MFN2). The prevention of cutaneous (ulcerations), bone, and amputation complications is very important in patients with hereditary sensory and autonomic neuropathies, because of the severity of the sensory disorders.

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).

A novel mutation in the GDAP1 gene is associated with autosomal recessive Charcot-Marie-Tooth disease in an Amish family

Charcot-Marie-Tooth disease (CMT) constitutes a large group of genetically heterogeneous disorders of the peripheral nervous system. Autosomal recessive forms of CMT are less common in the general population but account for the vast majority of CMT phenotypes in communities with a high prevalence of consanguinity. At least 10 genetic loci cause autosomal recessive forms of CMT. Mutations in the ganglioside-induced differentiation-associated protein 1 (GDAP1) gene are among the most frequent genetic causes of autosomal recessive forms of CMT. To date, 28 mutations in GDAP1 gene have been linked with the disease. Here, we report a novel GDAP1 mutation in an Old Order Amish family with CMT. To ascertain the Amish CMT locus, we performed a genome-wide single nucleotide polymorphism (SNP) analysis on one of three patients from a consanguineous pedigree. Assuming mutation homogeneity, the analysis sought large homozygous SNP blocks that also contained known CMT loci. The largest homozygous SNP block in the patient was localized to chromosome 8q13.1-21.3 and contained the GDAP1 gene. Sequence analysis revealed a novel homozygous mutation, c.692C>T, at codon 231 (p.P231L) in exon 5 of GDAP1 in all patients. Neither the unaffected individuals in the family nor the healthy control samples were homozygous for this mutation. Our findings suggested that this novel mutation in GDAP1 gene is associated with an autosomal recessive form of CMT in Ohio Old Order Amish community.

Two novel mutations in the GDAP1 and PRX genes in early onset Charcot-Marie-Tooth syndrome

Autosomal recessive Charcot-Marie-Tooth syndrome (AR-CMT) is often characterised by an infantile disease onset and a severe phenotype. Mutations in the ganglioside-induced differentiation-associated protein 1 (GDAP1) gene are thought to be a common cause of AR-CMT. Mutations in the periaxin (PRX) gene are rare. They are associated with severe demyelination of the peripheral nerves and sometimes lead to prominent sensory disturbances. To evaluate the frequency of GDAP1 and PRX mutations in early onset CMT, we examined seven AR-CMT families and 12 sporadic CMT patients, all presenting with progressive distal muscle weakness and wasting. In one family also prominent sensory abnormalities and sensory ataxia were apparent from early childhood. In three families we detected four GDAP1 mutations (L58LfsX4, R191X, L239F and P153L), one of which is novel and is predicted to cause a loss of protein function. In one additional family with prominent sensory abnormalities a novel homozygous PRX mutation was found (A700PfsX17). No mutations were identified in 12 sporadic cases. This study suggests that mutations in the GDAP1 gene are a common cause of early-onset AR-CMT. In patients with early-onset demyelinating AR-CMT and severe sensory loss PRX is one of the genes to be tested.

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.

Cell expression of GDAP1 in the nervous system and pathogenesis of Charcot-Marie-Tooth type 4A disease

Mutations in the mitochondrial protein GDAP1 are the cause of Charcot-Marie-Tooth type 4A disease (CMT4A), a severe form of peripheral neuropathy associated with either demyelinating, axonal or intermediate pheno-types. GDAP1 is located in the outer mitochondrial membrane and it seems that may be related with the mitochondrial network dynamics. We are interested to define cell expression in the nervous system and the effect of mutations in mitochondrial morphology and pathogenesis of the disease. We investigated GDAP1 expression in the nervous system and dorsal root ganglia (DRG) neuron cultures. GDAP1 is expressed in motor and sensory neurons of the spinal cord and other large neurons such as cerebellar Purkinje neurons, hippocampal pyramidal neurons, mitral neurons of the olfactory bulb and cortical pyramidal neurons. The lack of GDAP1 staining in the white matter and nerve roots suggested that glial cells do not express GDAP1. In DRG cultures satellite cells and Schwann cells were GDAP1-negative. Overexpression of GDAP1-induced fragmentation of mitochondria suggesting a role of GDAP1 in the fission pathway of the mitochondrial dynamics. Missense mutations showed two different patterns: most of them induced mitochondrial fragmentation but the T157P mutation showed an aggregation pattern. Whereas null mutations of GDAP1 should be associated with loss of function of the protein, missense mutations may act through different pathogenic mechanisms including a dominant-negative effect, suggesting that different molecular mechanisms may underlay the pathogenesis of CMT4A.