An altered lipid metabolism characterizes Charcot-Marie-Tooth type 2B peripheral neuropathy

Charcot-Marie Tooth type 2B (CMT2B) is a rare inherited peripheral neuropathy caused by five missense mutations in the RAB7A gene, which encodes a small GTPase of the RAB family. Currently, no cure is available for this disease. In this study, we approached the disease by comparing the lipid metabolism of CMT2B-derived fibroblasts to that of healthy controls. We found that CMT2B cells showed increased monounsaturated fatty acid level and increased expression of key enzymes of monounsaturated and polyunsaturated fatty acid synthesis. Moreover, in CMT2B cells a higher expression of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS), key enzymes of de novo fatty acid synthesis, with a concomitantly increased [1-¹⁴C]acetate incorporation into fatty acids, was observed. The expression of diacylglycerol acyltransferase 2, a rate-limiting enzyme in triacylglycerol synthesis, as well as triacylglycerol levels were increased in CMT2B compared to control cells. In addition, as RAB7A controls lipid droplet breakdown and lipid droplet dynamics have been linked to diseases, we analyzed these organelles and showed that in CMT2B cells there is a strong accumulation of lipid droplets compared to control cells, thus reinforcing our data on abnormal lipid metabolism in CMT2B. Furthermore, we demonstrated that ACC and FAS expression levels changed upon RAB7 silencing or overexpression in HeLa cells, thus suggesting that metabolic modifications observed in CMT2B-derived fibroblasts can be, at least in part, related to RAB7 mutations.     

Alterations of autophagy in the peripheral neuropathy Charcot-Marie-Tooth type 2B

Charcot-Marie-Tooth type 2B (CMT2B) disease is a dominant axonal peripheral neuropathy caused by 5 mutations in the RAB7A gene, a ubiquitously expressed GTPase controlling late endocytic trafficking. In neurons, RAB7A also controls neuronal-specific processes such as NTF (neurotrophin) trafficking and signaling, neurite outgrowth and neuronal migration. Given the involvement of macroautophagy/autophagy in several neurodegenerative diseases and considering that RAB7A is fundamental for autophagosome maturation, we investigated whether CMT2B-causing mutants affect the ability of this gene to regulate autophagy. In HeLa cells, we observed a reduced localization of all CMT2B-causing RAB7A mutants on autophagic compartments. Furthermore, compared to expression of RAB7A^WT, expression of these mutants caused a reduced autophagic flux, similar to what happens in cells expressing the dominant negative RAB7A^T22N mutant. Consistently, both basal and starvation-induced autophagy were strongly inhibited in skin fibroblasts from a CMT2B patient carrying the RAB7A^V162M mutation, suggesting that alteration of the autophagic flux could be responsible for neurodegeneration.     

A novel RAB7 mutation in a Chinese family with Charcot–Marie–Tooth type 2B disease

Charcot–Marie–Tooth type 2B (CMT2B) disease is a hereditary motor and sensory neuropathy subtype characterized by prominent loss of sensation, distal muscle weakness and wasting skin ulcers. Recurrent ulcers often require amputation of lower limbs. To date, only four mutations of the RAB7 gene, which encodes the small GTPase, have been associated with CMT2B. A Chinese family with CMT2B was identified. Direct DNA sequencing performed on the affected individuals in this family revealed a novel mutation (p.Asn161Ile) in RAB7. The mutation is located in a potential mutational hotspot region, implicating the importance of this region for RAB7 protein. This is the first report of RAB7 mutation in Asian population.     

Characterization of the Rab7K157N mutant protein associated with Charcot-Marie-Tooth type 2B

Four missense mutations, that target highly conserved amino acid residues in the small GTPase Rab7, have been associated with the Charcot-Marie-Tooth (CMT) type 2B phenotype. CMT2B peripheral axonal neuropathies are characterized by severe sensory loss, often complicated by infections, arthropathy, and amputations. Here, we have investigated the biochemical and functional properties of the Rab7 K157N mutated protein. Interestingly, Rab7 K157N showed altered nucleotide exchange rate and GTP hydrolysis compared to the wild type protein. Consistently, the majority of the expressed protein in HeLa cells was bound to GTP. In addition, Rab7 K157N was able to restore EGF degradation, previously inhibited by Rab7 silencing. Altogether these data indicate that Rab7 K157N, similarly to the other three mutated proteins causative of CMT2B, is predominantly in the GTP-bound form and behaves as an active mutant. Therefore, activated forms of Rab7 protein cause the CMT2B disease.     

LITAF、RAB7、LMNA和MTMR2基因在中国人腓骨肌萎缩症患者的突变分析

为了分析LITAF、RAB7、LMNA和MTMR2基因在中国人腓骨肌萎缩症(Charcot-Marie-Tooth disease, CMT)的突变特点, 文章分别应用PCR结合DNA序列分析方法和PCR-单链构象多态性(PCR-SSCP)结合DNA序列分析方法对6个常染色体显性遗传家系先证者和27个散发病例进行LITAF和RAB7基因突变分析; 应用PCR-SSCP结合DNA序列分析方法对14个常染色体遗传的CMT家系先证者和27个散发患者进行LMNA和MTMR2基因突变分析。结果发现: LITAF基因c.269G→A、c.274A→G序列变异和LMNA基因c.1243G→A、c.1910C→T序列变异, 未发现RAB7和MTMR2基因的序列变异。其中LITAF基因c.269G→A、LMNA基因c.1243G→A和c.1910C→T为新发现的单核苷酸多态; LITAF基因c.274A→G为已知多态。说明LITAF、RAB7、LMNA和MTMR2基因突变在中国人CMT患者中罕见。     

Molecular characterization and expression analysis of Mtmr2, mouse homologue of MTMR2, the Myotubularin-related 2 gene, mutated in CMT4B

Charcot-Marie-Tooth type 4B (CMT4B) is caused by mutations in the myotubularin-related 2 gene, MTMR2, on chromosome 11q22. To date, six loss of function mutations and one missense mutation have been demonstrated in CMT4B patients. It remains to be determined how dysfunction of a ubiquitously expressed phosphatase causes a demyelinating neuropathy. An animal model for CMT4B would provide insights into the pathogenesis of this disorder. We have therefore characterized the mouse homologue of MTMR2 by reconstructing the full-length Mtmr2 cDNA as well as the genomic structure. The 1932 nucleotide open reading frame corresponds to 15 coding exons, spanning a genomic region of approximately 55 kilobases, on mouse chromosome 9 as demonstrated by fluorescence in situ hybridization analysis. A comparison between the mouse and human genes revealed a similar genomic structure, except for the number of alternatively spliced exons in the 5'-untranslated region, two in mouse and three in man. In situ hybridization analysis of mouse embryos showed that Mtmr2 was ubiquitously expressed during organogenesis at E9.5, with some areas of enriched expression. At E14.5, Mtmr2 mRNA was more abundant in the peripheral nervous system, including in dorsal root ganglia and spinal roots.     

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.     

Mutation in Rab3 GTPase-activating protein (RAB3GAP) noncatalytic subunit in a kindred with Martsolf syndrome

We identified a homozygous missense mutation in the noncatalytic subunit (RAB3GAP2) of RAB3GAP that results in abnormal splicing in a family with congenital cataracts, hypogonadism, and mild mental retardation (Martsolf syndrome). Recently, mutations in the catalytic subunit of RAB3GAP (RAB3GAP1), a key regulator of calcium-mediated hormone and neurotransmitter exocytosis, were reported in Warburg micro syndrome, a severe neurodevelopmental condition with overlapping clinical features. RAB3GAP is a heterodimeric protein that consists of a catalytic subunit and a noncatalytic subunit encoded by RAB3GAP1 and RAB3GAP2, respectively. We performed messenger RNA-expression studies of RAB3GAP1 and RAB3GAP2 orthologues in Danio rerio embryos and demonstrated that, whereas developmental expression of rab3gap1 was generalized (similar to that reported elsewhere in mice), rab3gap2 expression was restricted to the central nervous system. These findings are consistent with RAB3GAP2 having a key role in neurodevelopment and may indicate that Warburg micro and Martsolf syndromes represent a spectrum of disorders. However, we did not detect RAB3GAP2 mutations in patients with Warburg micro syndrome. These findings suggest that RAB3GAP dysregulation may result in a spectrum of phenotypes that range from Warburg micro syndrome to Martsolf syndrome.     

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.     

Glycyl tRNA synthetase mutations in Charcot-Marie-Tooth disease type 2D and distal spinal muscular atrophy type V

Charcot-Marie-Tooth disease type 2D (CMT2D) and distal spinal muscular atrophy type V (dSMA-V) are axonal peripheral neuropathies inherited in an autosomal dominant fashion. Our previous genetic and physical mapping efforts localized the responsible gene(s) to a well-defined region on human chromosome 7p. Here, we report the identification of four disease-associated missense mutations in the glycyl tRNA synthetase gene in families with CMT2D and dSMA-V. This is the first example of an aminoacyl tRNA synthetase being implicated in a human genetic disease, which makes genes that encode these enzymes relevant candidates for other inherited neuropathies and motor neuron diseases.     

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.     

Charcot-Marie-Tooth-linked mutant GARS is toxic to peripheral neurons independent of wild-type GARS levels

Charcot-Marie-Tooth disease type 2D (CMT2D) is a dominantly inherited peripheral neuropathy caused by missense mutations in the glycyl-tRNA synthetase gene (GARS). In addition to GARS, mutations in three other tRNA synthetase genes cause similar neuropathies, although the underlying mechanisms are not fully understood. To address this, we generated transgenic mice that ubiquitously over-express wild-type GARS and crossed them to two dominant mouse models of CMT2D to distinguish loss-of-function and gain-of-function mechanisms. Over-expression of wild-type GARS does not improve the neuropathy phenotype in heterozygous Gars mutant mice, as determined by histological, functional, and behavioral tests. Transgenic GARS is able to rescue a pathological point mutation as a homozygote or in complementation tests with a Gars null allele, demonstrating the functionality of the transgene and revealing a recessive loss-of-function component of the point mutation. Missense mutations as transgene-rescued homozygotes or compound heterozygotes have a more severe neuropathy than heterozygotes, indicating that increased dosage of the disease-causing alleles results in a more severe neurological phenotype, even in the presence of a wild-type transgene. We conclude that, although missense mutations of Gars may cause some loss of function, the dominant neuropathy phenotype observed in mice is caused by a dose-dependent gain of function that is not mitigated by over-expression of functional wild-type protein.     

RAB23 mutations in Carpenter syndrome imply an unexpected role for hedgehog signaling in cranial-suture development and obesity

Carpenter syndrome is a pleiotropic disorder with autosomal recessive inheritance, the cardinal features of which include craniosynostosis, polysyndactyly, obesity, and cardiac defects. Using homozygosity mapping, we found linkage to chromosome 6p12.1-q12 and, in 15 independent families, identified five different mutations (four truncating and one missense) in RAB23, which encodes a member of the RAB guanosine triphosphatase (GTPase) family of vesicle transport proteins and acts as a negative regulator of hedgehog (HH) signaling. In 10 patients, the disease was caused by homozygosity for the same nonsense mutation, L145X, that resides on a common haplotype, indicative of a founder effect in patients of northern European descent. Surprisingly, nonsense mutations of Rab23 in open brain mice cause recessive embryonic lethality with neural-tube defects, suggesting a species difference in the requirement for RAB23 during early development. The discovery of RAB23 mutations in patients with Carpenter syndrome implicates HH signaling in cranial-suture biogenesis--an unexpected finding, given that craniosynostosis is not usually associated with mutations of other HH-pathway components--and provides a new molecular target for studies of obesity.     

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.     

Functional characterization of missense mutations in ATP7B: Wilson disease mutation or normal variant?

Wilson disease is an autosomal recessive disorder of copper transport that causes hepatic and/or neurological disease resulting from copper accumulation in the liver and brain. The protein defective in this disorder is a putative copper-transporting P-type ATPase, ATP7B. More than 100 mutations have been identified in the ATP7B gene of patients with Wilson disease. To determine the effect of Wilson disease missense mutations on ATP7B function, we have developed a yeast complementation assay based on the ability of ATP7B to complement the high-affinity iron-uptake deficiency of the yeast mutant ccc2. We characterized missense mutations found in the predicted membrane-spanning segments of ATP7B. Ten mutations have been made in the ATP7B cDNA by site-directed mutagenesis: five Wilson disease missense mutations, two mutations originally classified as possible disease-causing mutations, two putative ATP7B normal variants, and mutation of the cysteine-proline-cysteine (CPC) motif conserved in heavy-metal-transporting P-type ATPases. All seven putative Wilson disease mutants tested were able to at least partially complement ccc2 mutant yeast, indicating that they retain some ability to transport copper. One mutation was a temperature-sensitive mutation that was able to complement ccc2 mutant yeast at 30 degreesC but was unable to complement at 37 degreesC. Mutation of the CPC motif resulted in a nonfunctional protein, which demonstrates that this motif is essential for copper transport by ATP7B. Of the two putative ATP7B normal variants tested, one resulted in a nonfunctional protein, which suggests that it is a disease-causing mutation.     

A novel mutation of the MFN2 gene in a Chinese family with Charcot-Marie-Tooth disease

Charcot-Marie-Tooth (CMT) is a group of clinically and genetically heterogeneous inherited neuromuscular disorders. At present, more than 30 loci have been reported to be associated with CMT disease; point mutations in the mitofusin 2 (MFN2) gene is one of the most common causes. We studied a Chinese family with CMT disease in which the phenotype of affected individuals varied, and the weakness condition of the distal legs in males, except the proband, was less severe than in females in this family. Linkage analysis and PCR sequencing revealed a missense mutation (NM_014874.3:c.1066 A>G) in the MFN2 gene, resulting in an animo acid substitution of threonine to alanine in condon 356 (Thr356Ala). This is a novel phenotype and mutation for CMT family.     

The functional analysis of the CHMP2B missense mutation associated with neurodegenerative diseases in the endo-lysosomal pathway

Endosomal sorting complexes required for transport (ESCRTs) regulate a key sorting step of protein trafficking between endosomal compartments in lysosomal degradation. Interestingly, mutations in charged multivesicular body protein 2B (CHMP2B), which is a core subunit of ESCRT-III, have been identified in some neurodegenerative diseases. However, the cellular pathogenesis resulting from CHMP2B missense mutations is unclear. Furthermore, little is known about their functional analysis in post-mitotic neurons. In order to examine their cellular pathogenesis, we analyzed their effects in the endo-lysosomal pathway in post-mitotic neurons. Interestingly, of the missense mutant proteins, CHMP2B(T104N) mostly accumulated in the Rab5- and Rab7-positive endosomes and caused delayed degradation of EGFR as compared to CHMP2B(WT). Furthermore, CHMP2B(T104N) showed less association with Vps4 ATPase and was avidly associated with Snf7-2, a core component of ESCRT-III, suggesting that it may cause defects in the process of dissociation from ESCRT. Of the missense variants, CHMP2B(T104N) caused prominent accumulation of autophagosomes. However, neuronal cell survival was not dramatically affected by expression of CHMP2B(T104N). These findings suggested that, from among the various missense mutants, CHMP2B(T104N) was associated with relatively mild cellular pathogenesis in post-mitotic neurons. This study provided a better understanding of the cellular pathogenesis of neurodegenerative diseases associated with various missense mutations of CHMP2B as well as endocytic defects.     

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.     

Truncating and Missense Mutations in IGHMBP2 Cause Charcot-Marie Tooth Disease Type 2

Using a combination of exome sequencing and linkage analysis, we investigated an English family with two affected siblings in their 40s with recessive Charcot-Marie Tooth disease type 2 (CMT2). Compound heterozygous mutations in the immunoglobulin-helicase-μ-binding protein 2 (IGHMBP2) gene were identified. Further sequencing revealed a total of 11 CMT2 families with recessively inherited IGHMBP2 gene mutations. IGHMBP2 mutations usually lead to spinal muscular atrophy with respiratory distress type 1 (SMARD1), where most infants die before 1 year of age. The individuals with CMT2 described here, have slowly progressive weakness, wasting and sensory loss, with an axonal neuropathy typical of CMT2, but no significant respiratory compromise. Segregating IGHMBP2 mutations in CMT2 were mainly loss-of-function nonsense in the 5′ region of the gene in combination with a truncating frameshift, missense, or homozygous frameshift mutations in the last exon. Mutations in CMT2 were predicted to be less aggressive as compared to those in SMARD1, and fibroblast and lymphoblast studies indicate that the IGHMBP2 protein levels are significantly higher in CMT2 than SMARD1, but lower than controls, suggesting that the clinical phenotype differences are related to the IGHMBP2 protein levels.