Rapid screening of myelin genes in CMT1 patients by SSCP analysis: identification of new mutations and polymorphisms in the P0 gene

Charcot-Marie-Tooth type (CMT1) disease or hereditary motor and sensory neuropathy type I (HMSNI) is an autosomal dominant peripheral neuropathy. In most CMT1 families, the disease cosegregates with a 1.5-Mb duplication on chromosome 17p11.2 (CMT1A). A few patients have been found with mutations in the peripheral myelin protein 22 (PMP-22) gene located in the CMT1A region. In other families mutations have been identified in the major peripheral myelin protein p_o gene localized on chromosome Iq21-q23 (CMT1B). We performed a rapid mutation screening of the PMP-22 and P₀ genes in non-duplicated CMT1 patients by single-strand conformation polymorphism analysis followed by direct polymerase chain reaction sequencing of genomic DNA. Six new single base changes in the P₀ gene were observed: two missense mutations in, respectively, exons 2 and 3, two nonsense mutations in exon 4, and two silent mutations or polymorphisms in, respectively, exons 3 and 6.     

Mutational analysis of the MPZ, PMP22 and Cx32 genes in patients of Spanish ancestry with Charcot-Marie-Tooth disease and hereditary neuropathy with liability to pressure palsies

Charcot-Marie-Tooth disease (CMT) and hereditary neuropathy with liability to pressure palsies (HNPP) are two inherited peripheral neuropathies. The most prevalent mutations are a reciprocal 1.5-Mb duplication and 1.5-Mb deletion, respectively, at the CMT1A/HNPP locus on chromosome 17p11.2. Point mutations in the coding region of the myelin genes, peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) or connexin 32 (Cx32) have been reported in CMT patients, including CMT type 1 (CMT1), CMT type 2 (CMT2) and Déjérine-Sottas neuropathy (DS) patients, and only in the coding region of PMP22 in HNPP families lacking a deletion. We have investigated point and small mutations in the MPZ, PMP22 and Cx32 genes in a series of patients of Spanish ancestry: 47 CMT patients without duplications, and 5 HNPP patients without deletions. We found 15 different mutations in 16 CMT patients (34%). Nine different mutations in ten patients were detected in the Cx32 gene, this being the most frequently involved gene in this series, whereas five mutations involved the MPZ gene and only one the PMP22 gene. Six out of nine nucleotide substitutions in the Cx32 gene involved two codons encoding arginine at positions 164 and 183, suggesting that these two codons may constitute two Cx32 regions prone to mutate in the Spanish population. Analysis of HNPP patients revealed a 5′ splicing mutation in intron 1 of the PMP22 gene in a family with autosomal dominance, which confirms allelic heterogeneity in HNPP. Ectopic mRNA analysis on leukocytes suggests that this mutation might behave as a null allele. Received: 25 July 1996 / Revised: 15 November 1996     

Screening for connexin 32 mutations in Charcot-Marie-Tooth disease families with possible X-linked inheritance

The X-linked dominant form of Charcot-Marie-Tooth disease (CMTX) is associated with mutations in a gene coding for the gap-junction protein connexin 32 (Cx32). We screened 32 CMT families with a pedigree pattern suggestive of X-linked inheritance for the presence of mutations in the coding region of Cx32 by direct sequencing. Five of the families had a CMT1 diagnosis, 24 had a CMT2 diagnosis and 3 patients had an unspecified CMT. Eight families with a Cx32 point mutation were detected. Five different mutations (four of them published previously) were found in six CMT2 families and one mutation was found in a sporadic CMT1 male patient. One of the mutations, Met194Val, is among the first described in the fourth transmembrane domain of Cx32. Two CMT2 families and the sporadic CMT1 patient had the same mutation, Arg22Gln. An additional, previously unpublished mutation, Arg75Trp, was found in a male patient with unspecified CMT, who subsequently was verified to have a variant Klinefelter syndrome with 48,XXYY karyotype. Our findings show the difficulty in distinguishing CMTX patients from CMT1 and CMT2 patients, and they emphasize the need for Cx32 mutation screening in families previously diagnosed with CMT2. Received: 29 November 1996 / Revised: 17 February 1997     

Genotype–phenotype correlation of Charcot-Marie-Tooth type 1E patients with <Emphasis Type="Italic">PMP22</Emphasis> mutations

Charcot-Marie-Tooth disease type 1E (CMT1E) is a demyelinating motor and sensory neuropathy with peripheral myelin protein 22 (PMP22) point mutations. The objective of this study was to identify genetic causes and determine genotype–phenotype correlation in two Korean demyelinating CMT patients based on whole exome sequencing (WES), histological examination of distal sural nerve, and magnetic resonance imaging (MRI) of leg. WES revealed two de novo PMP22 mutations in the two demyelinating CMT patients, including one novel p.Leu82Pro (c.245T>A) mutation in one patient and one previously reported p.Ser72Leu (c.215C>T) mutation in the other patient. Both mutation sites were located in the well conserved second transmembrane domain. No control had the same mutations. The affected individual with the novel p.Leu82Pro mutation showed early onset, scoliosis, and sensory ataxia with ability to walk without assistance. Histopathological examination showed severe damage of myelin and axons. No compound muscle action potentials (CMAPs) were evoked in the upper or lower limb nerves. Leg MRIs revealed mild fatty infiltration of the bilateral peronei muscles consistent with clinical manifestations. The patient with the p.Ser72Leu mutation showed developmental delay in infancy. No CMAPs were elicited. However, she was also able to walk without assistance. In spite of markedly severe electrophysiological defects, leg MRIs showed almost normal findings except slight muscle atrophies of the lower legs. Both patients presented similar clinical features including no CMAPs in electrophysiological tests and mild fatty replacement in the lower leg MRI. Therefore, there was a good genotype–phenotype correlation in both cases.     

Charcot-Marie-Tooth type 1A disease caused by a novel Ser112Arg mutation in thePMP22 gene, coexisting with a slowly progressive hearing impairment

Among 57 mutations in the peripheral myelin protein 22 gene (PMP22) identified so far in patients affected by Charcot-Marie-Tooth disease (CMT), only 8 have been shown to segregate with a mixed phenotype of CMT and hearing impairment. In this study, we report a new Ser1 12Arg mutation in thePMP22 gene, identified in a patient with early-onset CMT and slowly progressive hearing impairment beginning in the second decade of life. We suggest that the Ser1 12Arg mutation in thePMP22 gene might have a causative role in the early-onset CMT with hearing impairment. Thus, our study extends the spectrum of CMT phenotypes putatively associated withPMP22 gene mutations.     

A new variant of Charcot-Marie-Tooth disease type 2 is probably the result of a mutation in the neurofilament-light gene

Charcot-Marie-Tooth (CMT) disease is the most common inherited motor and sensory neuropathy. The axonal form of the disease is designated as "CMT type 2" (CMT2). Although four loci known to be implicated in autosomal dominant CMT2 have been mapped thus far (on 1p35-p36, 3q13. 1, 3q13-q22, and 7p14), no one causative gene is yet known. A large Russian family with CMT2 was found in the Mordovian Republic (Russia). Affected members had the typical CMT2 phenotype. Additionally, several patients suffered from hyperkeratosis, although the association, if any, between the two disorders is not clear. Linkage with the CMT loci already known (CMT1A, CMT1B, CMT2A, CMT2B, CMT2D, and a number of other CMT-related loci) was excluded. Genomewide screening pinpointed the disease locus in this family to chromosome 8p21, within a 16-cM interval between markers D8S136 and D8S1769. A maximum two-point LOD score of 5.93 was yielded by a microsatellite from the 5' region of the neurofilament-light gene (NF-L). Neurofilament proteins play an important role in axonal structure and are implicated in several neuronal disorders. Screening of affected family members for mutations in the NF-L gene and in the tightly linked neurofilament-medium gene (NF-M) revealed the only DNA alteration linked with the disease: a A998C transversion in the first exon of NF-L, which converts a conserved Gln333 amino acid to proline. This alteration was not found in 180 normal chromosomes. Twenty unrelated CMT2 patients, as well as 26 others with an undetermined form of CMT, also were screened for mutations in NF-L, but no additional mutations were found. It is suggested that Gln333Pro represents a rare disease-causing mutation, which results in the CMT2 phenotype.     

Mutation analysis of PMP22 in Slovak patients with Charcot-Marie-Tooth disease and hereditary neuropathy with liability to pressure palsies

Charcot-Marie-Tooth disease (CMT) and related peripheral neuropathies are the most commonly inherited neurological disorders in humans, characterized by clinical and genetic heterogeneity. The most prevalent clinical entities belonging to this group of disorders are CMT type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP). CMT1A and HNPP are predominantly caused by a 1.5 Mb duplication and deletion in the chromosomal region 17p11.2, respectively, and less frequently by other mutations in the peripheral myelin protein 22 (PMP22) gene. Despite being relatively common diseases, they haven't been previously studied in the Slovak population. Therefore, the aim of this study was to identify the spectrum and frequency of PMP22 mutations in the Slovak population by screening 119 families with CMT and 2 families with HNPP for causative mutations in this gene. The copy number determination of PMP22 resulted in the detection of CMT1A duplication in 40 families and the detection of HNPP deletion in 7 families, 6 of which were originally diagnosed as CMT. Consequent mutation screening of families without duplication or deletion using dHPLC and sequencing identified 6 single base changes (3 unpublished to date), from which only c.327C>A (Cys109X) present in one family was provably causative. These results confirm the leading role of PMP22 mutation analysis in the differential diagnosis of CMT and show that the spectrum and frequency of PMP22 mutations in the Slovak population is comparable to that seen in the global population.     

Founder mutations in NDRG1 and HK1 genes are common causes of inherited neuropathies among Roma/Gypsies in Slovakia

Autosomal recessive forms of Charcot–Marie–Tooth disease (CMT) account for less than 10 % of all CMT cases, but are more frequent in the populations with a high rate of consanguinity. Roma (Gypsies) are a transnational minority with an estimated population of 10 to 14 million, in which a high degree of consanguineous marriages is a generally known fact. Similar to the other genetically isolated founder populations, the Roma harbour a number of unique or rare autosomal recessive disorders, caused by “private” founder mutations. There are three subtypes of autosomal recessive CMT with mutations private to the Roma population: CMT4C, CMT4D and CMT4G. We report on the molecular examination of four families of Roma origin in Slovakia with early-onset demyelinating neuropathy and autosomal recessive inheritance. We detected mutation p.R148X (g.631C>T) in the NDRG1 (NM_006096.3) gene in two families and mutation g.9712G>C in the HK1 (NM_033498) gene in the other two families. These mutations cause CMT4D and CMT4G, respectively. The success of molecular genetic analysis in all families confirms that autosomal recessive forms of CMT caused by mutations on the NDRG1 and HK1 genes are common causes of inherited neuropathies among Slovak Roma. Providing genetic analysis of these genes for patients with Roma origin as a common part of diagnostic procedure would contribute to a better rate of diagnosed cases of demyelinating neuropathy in Slovakia and in other countries with a Roma minority.     

The mutational spectrum in a cohort of Charcot-Marie-Tooth disease type 2 among the Han Chinese in Taiwan

Background

Charcot-Marie-Tooth disease type 2 (CMT2) is a clinically and genetically heterogeneous group of inherited axonal neuropathies. The aim of this study was to extensively investigate the mutational spectrum of CMT2 in a cohort of patients of Han Chinese.

Methodology and Principal Findings

Genomic DNA from 36 unrelated Taiwanese CMT2 patients of Han Chinese descent was screened for mutations in the coding regions of the MFN2, RAB7, TRPV4, GARS, NEFL, HSPB1, MPZ, GDAP1, HSPB8, DNM2, AARS and YARS genes. Ten disparate mutations were identified in 14 patients (38.9% of the cohort), including p.N71Y in AARS (2.8%), p.T164A in HSPB1 (2.8%), and p.[H256R]+[R282H] in GDAP1 (2.8%) in one patient each, three NEFL mutations in six patients (16.7%) and four MFN2 mutations in five patients (13.9%). The following six mutations were novel: the individual AARS, HSPB1 and GDAP1 mutations and c.475-1G>T, p.L233V and p.E744M mutations in MFN2. An in vitro splicing assay revealed that the MFN2 c.475-1G>T mutation causes a 4 amino acid deletion (p.T159_Q162del). Despite an extensive survey, the genetic causes of CMT2 remained elusive in the remaining 22 CMT2 patients (61.1%).

Conclusions and Significance

This study illustrates the spectrum of CMT2 mutations in a Taiwanese CMT2 cohort and expands the number of CMT2-associated mutations. The relevance of the AARS and HSPB1 mutations in the pathogenesis of CMT2 is further highlighted. Moreover, the frequency of the NEFL mutations in this study cohort was unexpectedly high. Genetic testing for NEFL and MFN2 mutations should, therefore, be the first step in the molecular diagnosis of CMT2 in ethnic Chinese.     

Connexin32 gene mutations in X-linked dominant Charcot-Marie-Tooth disease (CMTX1)

Single-strand conformational polymorphisms (SSCP) of the connexin32 gene were analyzed in 121 patients possibly affected by Charcot-Marie-Tooth (CMT) disease. The 121 patients were selected from 443 possible CMT/HNPP (hereditary neuropathy with liability to pressure palsies) patients based on genetic linkage to Xq13.1, absence of the 17p12 duplication and deletion, and absence of point mutations in PMP22 and P0. We found five new mutations at nucleotides 105 (C-T), 316 (C-G), 321 (C-T), 328 (T-C), and 657 (G-C), and three mutations at nucleotide 126 (C-T), 249 (G-A), and 477 (G-A) previously described in other unrelated families. The nucleotide changes resulted in seven amino-acid substitutions and one premature stop codon. Received: 21 June 1996 / Revised: 29 November 1996     

Axonal Transport Defects in a Mitofusin 2 Loss of Function Model of Charcot-Marie-Tooth Disease in Zebrafish

Charcot-Marie-Tooth disease (CMT) represents a group of neurodegenerative disorders typically characterised by demyelination (CMT1) or distal axon degeneration (CMT2) of motor and sensory neurons. The majority of CMT2 cases are caused by mutations in mitofusin 2 (MFN2); an essential gene encoding a protein responsible for fusion of the mitochondrial outer membrane. The mechanism of action of MFN2 mutations is still not fully resolved. To investigate a role for loss of Mfn2 function in disease we investigated an ENU-induced nonsense mutation in zebrafish MFN2 and characterised the phenotype of these fish at the whole organism, pathological, and subcellular level. We show that unlike mice, loss of MFN2 function in zebrafish leads to an adult onset, progressive phenotype with predominant symptoms of motor dysfunction similar to CMT2. Mutant zebrafish show progressive loss of swimming associated with alterations at the neuro-muscular junction. At the cellular level, we provide direct evidence that mitochondrial transport along axons is perturbed in Mfn2 mutant zebrafish, suggesting that this is a key mechanism of disease in CMT. The progressive phenotype and pathology suggest that zebrafish will be useful for further investigating the disease mechanism and potential treatment of axonal forms of CMT. Our findings support the idea that MFN2 mutation status should be investigated in patients presenting with early-onset recessively inherited axonal CMT.     

Polymorphisms in the PMP-22 gene region (17p11.2–12) are crucial for simplified diagnosis of duplications/deletions

DNA duplications and deletions of a 1.5-Mb region in chromosome 17p11.2–12 comprising the gene encoding peripheral myelin protein 22 (PMP-22) are the common mutations in Charcot-Marie-Tooth disease type 1 (CMT1) and hereditary neuropathy with liability to pressure palsies (HNPP). A 1.7-kb recombination hotspot region has been identified within misaligned flanking repeats (CMT1-REP elements) by detection of CMT- and HNPP-specific junction fragments in Southern blot analyses. In order to simplify routine diagnosis we introduce a polymerase chain reaction-based method to identify directly specific REP junction fragments. Using this test, specific fragments were detected in ∼ 67% of both CMT duplication and HNPP deletion cases. Polymorphism within a specific restriction enzyme recognition site is crucial for both Southern blot and PCR analyses of junction fragments. Received: 25 October 1996 / Revised: 16 December 1996     

Screening of the myelin protein zero gene in patients with Charcot-Marie-Tooth disease

The myelin protein zero gene (MPZ) coding for the most abundant protein of the peripheral myelin was shown to be mutated in Charcot-Marie-Tooth type 1B disease (CMT1B). Later on MPZ mutations have been shown in axonal type of CMT (CMT2). Recently three novel MPZ gene mutations were reported in congenital hypomyelinating neuropathy (CHN). In contrast to the previously reported studies, focused on CMT1B disease, we aimed to analyze the coding and promoter sequences of the MPZ gene in a group of patients with three CMT phenotypes i.e.: CMT1, CMT2 and CHN. Over 500 PCR products were screened by single strand conformation polymorphism analysis (SSCP) and heteroduplex analysis (HA). In one CMT2 family we founded the E56K mutation in the MPZ gene and in one CHN patient the T124K substitution was detected. In agreement with previously reported studies we conclude that MPZ gene screening should be performed for wide phenotype spectrum of CMT.     

Current biomarkers of canine mammary tumors

Mammary tumors are the second most common neoplasia in dogs. Due to the high similarity of canine mammary tumors (CMT) to human breast cancers (HBC), human biomarkers of HBC are also detectable in cases of CMT. The evaluation of biomarkers enables clinical diagnoses, treatment options and prognosis for bitches suffering from this disease. The aim of this article is to give a short summary of the biomarkers of CMT based on current literature. Very promising biomarkers are miRNAs, cancer stem cells, and circulating tumor cells, as well as mutations of the breast cancer 1 gene (BRCA1) and breast cancer 2 gene (BRCA2). Until now, the most studied and reliable biomarkers of CMT have remained antigen Ki-67 (Ki-67), endothelial growth factor receptor, human epidermal growth factor receptor 2 (HER-2), estrogen receptor, progesterone receptor and cyclooxygenase 1 (COX-2), which can be detected in both serum and tissue samples using different molecular methods. However, carcinoembryonic antigen and cancer antigen 15-3 (CA 15-3), while poorly studied, seem to be good biomarkers, especially for the early detection and prognosis of CMT. We will also mention the following: proliferative cell nuclear antigen, tumor protein p53 (p53), E-cadherin, vascular endothelial growth factor, microRNAs, cancer stem cells and circulating tumor cells, which can also be useful biomarkers. Although many studies have been conducted so far, the estimation of biomarkers in cases of CMT is still not a common practice, and more detailed research should be done.     

Clinical and genetic heterogeneity of Charcot-Marie-Tooth disease.

The autosomal dominant forms of hereditary motor and sensory neuropathies include the hypertrophic form (CMT1) and the neuronal form of Charcot-Marie-Tooth disease (CMT2). While at least two distinct loci have been shown to be linked to the CMT1 phenotype (CMT1A and CMT1B, on chromosomes 17 and 1, respectively), whether the CMT2 phenotype results from mutations allelic to either of the CMT1 genes remains unknown. Studying one CMT1 and two CMT2 pedigrees, we were able to exclude the CMT2 disease locus from the region of chromosome 17 (Z = -2.80 at theta = 0.05 for D17S58) where the CMT1A gene maps (Z = +3.67 at theta = 0.00). Similarly, negative lod score values were obtained in CMT2 for the region of chromosome 1 where the CMT1B gene has been located (Z = -3.09 at theta = 0.05 for D1S61). The present study therefore provides evidence for genetic heterogeneity between the hypertrophic and the neuronal forms of Charcot-Marie-Tooth disease and demonstrates that the CMT2 gene is not allelic to either of the CMT1 genes mapped to date.     

Small heat-shock protein 22 mutated in autosomal dominant Charcot-Marie-Tooth disease type 2L

Charcot-Marie-Tooth (CMT) disease is the most common inherited motor and sensory neuropathy. We have previously described a large Chinese CMT family and assigned the locus underlying the disease (CMT2L; OMIM 608673) to chromosome 12q24. Here, we report a novel c.423GT (Lys141Asn) missense mutation of small heat-shock protein 22-kDa protein 8 (encoded by HSPB8), which is also responsible for distal hereditary motor neuropathy type (dHMN) II. No disease-causing mutations have been identified in another 114 CMT families.     

The PMP22 Gene and Its Related Diseases

Peripheral myelin protein-22 (PMP22) is primarily expressed in the compact myelin of the peripheral nervous system. Levels of PMP22 have to be tightly regulated since alterations of PMP22 levels by mutations of the PMP22 gene are responsible for >50 % of all patients with inherited peripheral neuropathies, including Charcot–Marie–Tooth type-1A (CMT1A) with trisomy of PMP22, hereditary neuropathy with liability to pressure palsies (HNPP) with heterozygous deletion of PMP22, and CMT1E with point mutations of PMP22. While overexpression and point-mutations of the PMP22 gene may produce gain-of-function phenotypes, deletion of PMP22 results in a loss-of-function phenotype that reveals the normal physiological functions of the PMP22 protein. In this article, we will review the basic genetics, biochemistry and molecular structure of PMP22, followed by discussion of the current understanding of pathogenic mechanisms involving in the inherited neuropathies with mutations in PMP22 gene.     

Mutations in the small GTP-ase late endosomal protein RAB7 cause Charcot-Marie-Tooth type 2B neuropathy

Charcot-Marie-Tooth type 2B (CMT2B) is clinically characterized by marked distal muscle weakness and wasting and a high frequency of foot ulcers, infections, and amputations of the toes because of recurrent infections. CMT2B maps to chromosome 3q13-q22. We refined the CMT2B locus to a 2.5-cM region and report two missense mutations (Leu129Phe and Val162Met) in the small GTP-ase late endosomal protein RAB7 which causes the CMT2B phenotype in three extended families and in three patients with a positive family history. The alignment of RAB7 orthologs shows that both missense mutations target highly conserved amino acid residues. RAB7 is ubiquitously expressed, and we found expression in sensory and motor neurons.     

A Mutation Associated with CMT2A Neuropathy Causes Defects in Fzo1 GTP Hydrolysis,Ubiquitylation, and Protein Turnover

Charcot-Marie-Tooth disease type 2A (CMT2A) is caused by mutations in the gene MFN2 and is one of the most common inherited peripheral neuropathies. Mfn2 is one of two mammalian mitofusin GTPases that promote mitochondrial fusion and maintain organelle integrity. It is not known how mitofusin mutations cause axonal degeneration and CMT2A disease. We used the conserved yeast mitofusin FZO1 to study the molecular consequences of CMT2A mutations on Fzo1 function in vivo and in vitro. One mutation (analogous to the CMT2A I213T substitution in the GTPase domain of Mfn2) not only abolishes GTP hydrolysis and mitochondrial membrane fusion but also reduces Mdm30-mediated ubiquitylation and degradation of the mutant protein. Importantly, complexes of wild type and the mutant Fzo1 protein are GTPase active and restore ubiquitylation and degradation of the latter. These studies identify diverse and unexpected effects of CMT2A mutations, including a possible role for mitofusin ubiquitylation and degradation in CMT2A pathogenesis, and provide evidence for a novel link between Fzo1 GTP hydrolysis, ubiquitylation, and mitochondrial fusion.     

Molecular analyses of unrelated Charcot-Marie-Tooth (CMT) disease patients suggest a high frequency of the CMTIA duplication.

Charcot-Marie-Tooth disease (CMT) is the most common inherited peripheral neuropathy. One form of CMT, CMT type 1A, is characterized by uniformly decreased nerve conduction velocities, usually shows autosomal dominant inheritance, and is associated with a large submicroscopic duplication of the p11.2-p12 region of chromosome 17. A cohort of 75 unrelated patients diagnosed clinically with CMT and evaluated by electrophysiological methods were analyzed molecularly for the presence of the CMT1A DNA duplication. Three methodologies were used to assess the duplication: measurement of dosage differences between RFLP alleles, analysis of polymorphic (GT)n repeats, and detection of a junction fragment by pulsed-field gel electrophoresis. The CMT1A duplication was found in 68% of the 63 unrelated CMT patients with electrophysiological studies consistent with CMT type 1 (CMT1). The CMT1A duplication was detected as a de novo event in two CMT1 families. Twelve CMT patients who did not have decreased nerve conduction velocities consistent with a diagnosis of CMT type 2 (CMT2) were found not to have the CMT1A duplication. The most informative molecular method was the detection of the CMT1A duplication-specific junction fragment. Given the high frequency of the CMT1A duplication in CMT patients and the high frequency of new mutations, we conclude that a molecular test for the CMT1A DNA duplication is very useful in the differential diagnosis of patients with peripheral neuropathies.