MFN2 Mutations Research Articles

45. Evidence-based guidelines regarding the role of laboratory and genetic test for the assessment of DSP. What is the perspective in 2016?

Distal symmetric polyneuropathy (DSP) is a relatively common variety of neuropathy. The AAN, AANEM and AAPMR the recommendations where: (1) Screening laboratory tests may be considered for all patients with polyneuropathy (Level C). The highest yield of abnormality were found in blood glucose, serum B12 with metabolites and serum protein immunofixation electrophoresis (Level C). If there is no definite evidence of diabetes mellitus on routine testing of blood glucose, testing for impaired glucose tolerance may be considered in DSP (Level C). (2) Genetic testing should be conducted for the accurate diagnosis and classification of hereditary neuropathies (Level A). Also, in patients with cryptogenic polyneuropathy who exhibit an hereditary neuropathy phenotype (Level C). Initial genetic testing should be guided by the clinical phenotype, inheritance pattern, and electrodiagnostic features and should focus on the most common abnormalities which are CMT1A duplication/HNPP deletion, Cx32 (GJB1), and MFN2 mutation screening. There is insufficient evidence to determine the usefulness of routine genetic testing in patients with cryptogenic polyneuropathy who do not exhibit a hereditary neuropathy phenotype (Level U).

Assessment of upper motor neuron dysfunction by triple stimulation technique in patients with Charcot-Marie-Tooth disease

To investigate the presence of upper motor neuron dysfunction in patients with Charcot-Marie-Tooth disease (CMT) by triple stimulation technique (TST) to provide evidence for gene diagnosis. A total of 65 CMT patients confirmed by genetic testing from Peking University Third Hospital between August 2013 and August 2015, underwent physical examination and routine electrophysiological tests and triple stimulation technique. The TST combined transcranial magnetic stimulation (TMS) of the motor cortex with peripheral collision studies. The results were expressed by the TST amplitude ratio (TST test/ TST control). Based on the result of physical examination and the ratio of TST, the function of upper motor neuron was assessed. All of the CMT patients had typical presentations and were confirmed genetically. Hyperreflexia, Babinski sign and muscular hypertonia were discovered in lower limbs in 7 CMT patients, while 2 patients complicated with hyperreflexia and Hoffmann sign in upper limbs. TST amplitude ratio was significantly altered in 10 patients, including 5 patients with pyramid sign: hyperreflexia, Babinski sign and muscular hypertonia was discovered in lower limbs while 2 patients complicated with hyperreflexia and Hoffmann sign in upper limbs. The disease-causing mutations were: MFN2 mutations in 5 patients, 1 patient with BSCL2 mutation, 3 patients with GJB1 mutations and 1 patient with GDAP mutation. In summary, 18.5% (12 patients) of the 65 CMT patients had upper motor neuron dysfunction based on the result of physical examination and the ratio of TST. Upper motor neuron dysfunction may be found in CMT patients, which may provide clues for the testing of disease-causing mutations.

Charcot-Marie-Tooth type 2 and distal hereditary motor neuropathy: Clinical, neurophysiological and genetic findings from a single-centre experience

ObjectivesCMT is a group of heterogeneous motor and sensory neuropathies divided into demyelinating (CMT1) and axonal forms (CMT2). Distal Hereditary Motor Neuropathy (dHMN) is a motor neuropathy/neuronopathy which resembles CMT. Final genetic diagnosis is poor in CMT2 and in dHMN when compared with CMT1. Our aim is to report clinical, neurophysiological and genetic findings in a cohort of patients with axonal inherited neuropathies. Patients and methodsWe report clinical, neurophysiological and genetic findings from 45 patients with CMT2 or dHMN, coming from 39 unrelated families, observed in our Institute of Neurology over a 20-year period. ResultsClinical and electrophysiological examinations showed that 38 patients had CMT2 and 7 patients presented dHMN. Extensive genetic evaluation showed 6 mutations in MFN2, 4 mutations in HSPB1, 2 mutations in BSCL2, 3 mutations in GJB1, 1 mutation in MPZ. ConclusionSince next-generation sequencing will not be easily accessible, epidemiological data and clinical “phenotyping” remain the best strategy for clinicians to reach a correct genetic diagnosis in CMT2 and dHMN patients.

Zebrafish Tg(hb9:MTS-Kaede): a new in vivo tool for studying the axonal movement of mitochondria

ObjectivesDeregulation of axonal transport in neurons is emerging as the major cause of many neurodegenerative diseases in human, such as Charcot-Marie-Tooth (CMT) neuropathy. However, little is known about how mitochondria move in vivo and whether cell culture systems truly represent what happens in living animals. Here we describe the generation of a new zebrafish transgenic line that specifically allows to study mitochondrial dynamics in motor neurons and its application to analyse mitochondrial movement in zebrafish models expressing CMT2A causing mutations. MethodsThe Tol2 transposon system was used to generate a transgenic zebrafish line expressing the photoconvertible fluorescent protein Kaede in mitochondria of motor neurons. Mitochondrial shape and movement were monitored by time-lapse confocal live imaging and measured by kymograph analysis. The effects of two well-known CMT causing mutations, L76P and R94Q substitutions in MFN2, were then investigated with the same methods. ResultsWe generated the transgenic zebrafish Tg(hb9:MTS-Kaede) line with genetically labelled mitochondria in motor neurons. Kaede protein was correctly and stably targeted to mitochondrial matrix while retaining its photoconvertibility, thus qualifying this model for in vivo studies. Expression of the L76P and R94Q mutations reduced mitochondrial movement in axons and altered mitochondrial distribution in distinct ways. Conclusions and general significanceThese findings confirm previously published data obtained in cell cultures and strengthen the hypothesis of different mechanism of action of the two MFN2 mutations. Considering the number of neurodegenerative diseases associated to mitochondrial dynamics, the Tg(hb9:MTS-Kaede) zebrafish line is a promising model to study in vivo alterations of mitochondrial transport underlying human diseases.

MFN2-related genetic and clinical features in a cohort of Chinese CMT2 patients.

Charcot-Marie-Tooth disease 2A (CMT2A), caused by mutations in the mitofusin 2 gene (MFN2), is the most common CMT2 subtype. The aim of our study is to assess the frequency and summarize the genetic and clinical characteristics of Chinese CMT2A patients. A total of 17 coding exons of MFN2 were detected by direct sequencing in 82 unrelated Chinese families diagnosed as CMT2. Clinical evaluations were analyzed among CMT2A patients. We identified 14 missense variants in 9 sporadic and 6 familial cases, including four novel mutations (T129A, S249F, Q367P, and Q674L), 4 known mutations (R94W, R94Q, T105M, C132Y, M376V and Q751X), and 4 rare missense variants (K120E, C217F, K307E and T356S). A total of 23 patients had early-onset phenotype. Two patients had a CMTNS score of 0 to 10; 16 had a score of 11 to 20; and 7 had a score greater than 20. Five patients were confirmed a de novo origin. Six of 14 variants were located or closed to the GTPase domain. We report four novel mutations and four rare missense variants. MFN2 mutations account for 18% of CMT2 families in mainland China. The common characteristics of Chinese pedigree are early disease onset and moderate phenotypes.

Pathology of Charcot-Marie-Tooth Disease

Although genetic testing is available, nerve biopsy is useful in selected patients for the diagnosis of Charcot-Marie-Tooth disease (CMT). These are sporadic cases of hereditary neuropathy, or familial cases in which genetic testing is negative. CMT is caused by mutations of various genes. The pathological features of CMT have mostly been investigated using nerve biopsy, which may shed light on the presumed functions of mutated gene products. PMP22 duplication in CMT1A induces numerous large onion bulb lesions (OB). Compared to chronic inflammatory demyelinating polyradiculoneuropathy, the differential features of CMT1A are patchy distribution of OB and non-inflammatory lesions. CMT1B also manifests as OB, but presents abnormal compaction of myelin sheaths caused by uncompacted myelin or excessive myelin folding. CMT2 includes axonal neuropathies and many causative genes have been found. CMT2A (MFN2 mutation) shows abnormal mitochondria with a spherical morphology instead of tubular in the longitudinal direction. CMT4 consists of autosomal recessive forms with demyelinating pathology. Most subtypes have mutations of genes relating to myelin maintenance, and pathologically, they show abnormal folding of the myelin structure.

Clinical and allelic heterogeneity in a pediatric cohort of 11 patients carrying MFN2 mutation

IntroductionThe Mitofusin 2 gene (MFN2), which encodes a mitochondrial membrane protein, is known to be the first cause of autosomal dominant Charcot–Marie–Tooth disease type 2 (CMT2) with early onset. This gene is involved in typical CMT2A and in more atypical phenotypes as optic atrophy or spastic paraplegia. CMT2 refers to inherited axonal polyneuropathy, which associates progressive peripheral motor and sensory neuropathy, a family history consistent mainly with autosomal dominant inheritance, and normal nerve conduction velocities. SubjectsBetween 1999 and 2012, the genetic diagnosis of MFN2 mutation was made in 11 children who were treated in our department for different neurological symptoms. All data including family and personal history data, results of standardized clinical and electrophysiology testing, brain magnetic resonance imaging (MRI), neuro-ophthalmic evaluation, muscle biopsy histopathology and molecular diagnosis were retrospectively analyzed. ResultsFive different mutations were found in 6 unrelated families. Three of them have previously been described; the two remaining are new mutations: one of them related a new phenotype.Clinical signs appeared before the age of 6years in more than half of the patients (54%). The motor deficit was predominant in 8 patients (72%). Two children presented an acute onset of disease that stabilized afterwards; the other children showed a more progressive deterioration that was managed symptomatically. ConclusionThis large pediatric study describes a great interfamilial and intrafamilial phenotypic variability. We recommend screening this gene in pediatric patient with chronic neurologic symptoms such as motor deficit or optic atrophy but also in acute neurologic deficiencies such as subacute polyradiculoneuritis.

Clinical and genetic spectra in a series of Chinese patients with Charcot–Marie–Tooth disease

The aim of this study was to determine the clinical features and frequencies of genetic subtypes in a series of patients with Charcot-Marie-Tooth (CMT) disease from Eastern China. Patients were divided into three subtypes, CMT1, CMT2 and hereditary neuropathy with liability to pressure palsy (HNPP), according to their electrophysiological manifestations. Multiplex ligation-dependent probe analysis (MLPA) was performed to detect duplications/deletions in the PMP22 gene. The coding regions and splice sites of the GJB1, MPZ, MFN2 and GDAP-1 genes were determined by direct sequencing. Among the 148 patients in the study, 37.2% of the cases had mutations in genes assessed. The mutation detection rate was higher in patients with family histories than in spontaneous cases. PMP22 duplication (13.5%) was predominant in this group of patients, followed by PMP22 deletion (11.5%), and point mutations in GJB1 (8.8%), MPZ (2.0%) and MFN2 (0.7%). Three novel mutations (c.151T>C and c.310 A>G in GJB1 and c.1516 C>G in MFN2) were detected. A small deletion in PMP22 exon 4 was detected in a patient with severe CMT1. Genetic tests have great value in CMT patients with family histories. The frequency of PMP22 duplications was lower in Asian patients than in others. We suggest that genetic testing strategies in CMT patients should be primarily based on electromyography data.

Homozygous mutations in MFN2 cause multiple symmetric lipomatosis associated with neuropathy.

Multiple symmetric lipomatosis (MSL) is a mitochondrial disorder with impaired brown fat metabolism that has been associated with MERRF mutations in some, but not all, patients. We studied a sibling pair and an unrelated indiviadual who presented with MSL and neuropathy to determine the genetic etiology of this disorder in patients who did not carry the MSL-associated MERRF mutation. Whole-exome sequencing was performed on the siblings, and a rare, shared homozygous mutation in MFN2 (c.2119C>T: p.R707W) was identified. The mutation was not present in their healthy siblings. In silico programs predict it to be pathogenic, and heterozygous carriers of the MFN2 p.R707W substitution are known to have Charcot-Marie-Tooth (CMT) disease. A third, unrelated patient with multiple symmetrical lipomatosis and neuropathy also harbored the same homozygous mutation and had been previously diagnosed with CMT. Functional studies in patient fibroblasts demonstrate that the p.R707W substitution impairs homotypic (MFN2-MFN2) protein interactions required for normal activity and renders mitochondria prone to perinuclear aggregation. These findings show that homozygous mutations at p.R707W in MFN2 are a novel cause of multiple symmetrical lipomatosis.

Diagnostic algorithms in Charcot-Marie-Tooth neuropathies: experiences from a German genetic laboratory on the basis of 1206 index patients.

We present clinical features and genetic results of 1206 index patients and 124 affected relatives who were referred for genetic testing of Charcot-Marie-Tooth (CMT) neuropathy at the laboratory in Aachen between 2001 and 2012. Genetic detection rates were 56% in demyelinating CMT (71% of autosomal dominant (AD) CMT1/CMTX), and 17% in axonal CMT (24% of AD CMT2/CMTX). Three genetic defects (PMP22 duplication/deletion, GJB1/Cx32 or MPZ/P0 mutation) were responsible for 89.3% of demyelinating CMT index patients in whom a genetic diagnosis was achieved, and the diagnostic yield of the three main genetic defects in axonal CMT (GJB1/Cx32, MFN2, MPZ/P0 mutations) was 84.2%. De novo mutations were detected in 1.3% of PMP22 duplication, 25% of MPZ/P0, and none in GJB1/Cx32. Motor nerve conduction velocity was uniformly <38 m/s in median or ulnar nerves in PMP22 duplication, >40 m/s in MFN2, and more variable in GJB1/Cx32, MPZ/P0 mutations. Patients with CMT2A showed a broad clinical severity regardless of the type or position of the MFN2 mutation. Out of 75 patients, 8 patients (11%) with PMP22 deletions were categorized as CMT1 or CMT2. Diagnostic algorithms are still useful for cost-efficient mutation detection and for the interpretation of large-scale genetic data made available by next generation sequencing strategies.

Mitochondrial dynamics and inherited peripheral nerve diseases

Peripheral nerves have peculiar energetic requirements because of considerable length of axons and therefore correct mitochondria functioning and distribution along nerves is fundamental. Mitochondrial dynamics refers to the continuous change in size, shape, and position of mitochondria within cells. Abnormalities of mitochondrial dynamics produced by mutations in proteins involved in mitochondrial fusion (mitofusin-2, MFN2), fission (ganglioside-induced differentiation-associated protein-1, GDAP1), and mitochondrial axonal transport usually present with a Charcot–Marie–Tooth disease (CMT) phenotype. MFN2 mutations cause CMT type 2A by altering mitochondrial fusion and trafficking along the axonal microtubule system. CMT2A is an axonal autosomal dominant CMT type which in most cases is characterized by early onset and rather severe course. GDAP1 mutations also alter fission, fusion and transport of mitochondria and are associated either with recessive demyelinating (CMT4A) and axonal CMT (AR-CMT2K) and, less commonly, with dominant, milder, axonal CMT (CMT2K). OPA1 (Optic Atrophy-1) is involved in fusion of mitochondrial inner membrane, and its heterozygous mutations lead to early-onset and progressive dominant optic atrophy which may be complicated by other neurological symptoms including peripheral neuropathy. Mutations in several proteins fundamental for the axonal transport or forming the axonal cytoskeleton result in peripheral neuropathy, i.e., CMT, distal hereditary motor neuropathy (dHMN) or hereditary sensory and autonomic neuropathy (HSAN), as well as in hereditary spastic paraplegia. Indeed, mitochondrial transport involves directly or indirectly components of the kinesin superfamily (KIF5A, KIF1A, KIF1B), responsible of anterograde transport, and of the dynein complex and related proteins (DYNC1H1, dynactin, dynamin-2), implicated in retrograde flow. Microtubules, neurofilaments, and chaperones such as heat shock proteins (HSPs) also have a fundamental role in mitochondrial transport and mutations in some of related encoding genes cause peripheral neuropathy (TUBB3, NEFL, HSPB1, HSPB8, HSPB3, DNAJB2). In this review, we address the abnormalities in mitochondrial dynamics and their role in determining CMT disease and related neuropathies.

Axonal Charcot–Marie–Tooth disease patient-derived motor neurons demonstrate disease-specific phenotypes including abnormal electrophysiological properties

ObjectiveCharcot–Marie–Tooth (CMT) disease is a group of inherited peripheral neuropathies associated with mutations or copy number variations in over 70 genes encoding proteins with fundamental roles in the development and function of Schwann cells and peripheral axons. Here, we used iPSC-derived cells to identify common pathophysiological mechanisms in axonal CMT. MethodsiPSC lines from patients with two distinct forms of axonal CMT (CMT2A and CMT2E) were differentiated into spinal cord motor neurons and used to study axonal structure and function and electrophysiological properties in vitro. ResultsiPSC-derived motor neurons exhibited gene and protein expression, ultrastructural and electrophysiological features of mature primary spinal cord motor neurons. Cytoskeletal abnormalities were found in neurons from a CMT2E (NEFL) patient and corroborated by a mouse model of the same NEFL point mutation. Abnormalities in mitochondrial trafficking were found in neurons derived from this patient, but were only mildly present in neurons from a CMT2A (MFN2) patient. Novel electrophysiological abnormalities, including reduced action potential threshold and abnormal channel current properties were observed in motor neurons derived from both of these patients. InterpretationHuman iPSC-derived motor neurons from axonal CMT patients replicated key pathophysiological features observed in other models of MFN2 and NEFL mutations, including abnormal cytoskeletal and mitochondrial dynamics. Electrophysiological abnormalities found in axonal CMT iPSC-derived human motor neurons suggest that these cells are hyperexcitable and have altered sodium and calcium channel kinetics. These findings may provide a new therapeutic target for this group of heterogeneous inherited neuropathies.

G.P.296: Severe, early onset Charcot-Marie-Tooth disease with rare presentations

Charcot–Marie–Tooth disease (CMT) has diverse clinical presentations and molecular genetic heterogeneity. Herein we describe the clinical features, neurophysiological and morphological studies of 2 fatal cases of CMT. Case 1 was a healthy newborn who developed foot drop, daily frequent vomiting and severe constipation at 7 months of age. His arm and leg weakness rapidly progressed within a few months. He subsequently developed diaphragmatic paralysis and died at 2.5 years of age due to respiratory failure. NCS/EMG showed a severe, axonal sensory and motor neuropathy. He had a novel and de novo mutation (c.296–297dupAAG) of the MFN2 gene. Case 2 had congenital hypomyelinating neuropathy confirmed by NCS and nerve biopsy, associated with a novel and de novo missense mutation (p.His384Arg) of the EGR2 gene. The patient presented a few weeks after birth with failure to thrive and hypotonia. She had facial, severe proximal > distal arm and leg weakness, pain insensitivity, areflexia and symptoms of autonomic dysfunction. She progressed to respiratory failure requiring mechanical ventilation. Her CSF protein was elevated (136 mg/dl). She had a fluctuating clinical course that let to the initial consideration of chronic inflammatory demyelinting polyneuropathy. She did not respond to prednisone, but she responded to IVIG therapy initially with some improvement of muscle strength and decrease autonomic dysfunction. She subsequently experienced deterioration of weakness not responding to either IVIG or plasmapheresis, and died at 16.5 months of age. Our report expanded the genotype and phenotype of MFN2 and EGR2 mutations. Although early onset CMT has been well described, autonomic features and fatality due to respiratory failure are rarely described. MFN2 mutations frequently cause severe phenotype, but diaphragmatic paralysis and autonomic dysfunction have not been described. Similarly, severe autonomic dysfunction has not been reported to be associated with EGR2 mutations.

Charcot-Marie-Tooth disease type 2A: from typical to rare phenotypic and genotypic features.

Axonal Charcot-Marie-Tooth disease (CMT) is genetically heterogeneous, with 11 genes identified. Axonal CMT has most frequently been associated with mutations in the MFN2 gene (CMT2A). To describe the clinical and molecular features of CMT2A, to delineate prognostic factors, to understand connections between a certain phenotype and more serious clinical consequences, and to identify interactions among the associated genes. We describe the clinical, molecular, electrophysiological, and additional features of 43 patients with CMT2A. The degree of physical disability was determined by the CMT neuropathy score and adapted to the CMT neuropathy score gradient to evaluate the clinical course. We evaluated all data within the context of the most recent and important publications concerning this issue. Twenty-five patients had early-onset CMT2A and severe functional disability, with 9 being wheelchair bound, and 18 had late-onset disease and a milder phenotype. Optic atrophy, vocal cord palsy, and auditory impairment were observed in 5, 6, and 2 patients, respectively. Among the 24 patients who underwent magnetic resonance imaging of the spinal cord, 6 had evidence of spinal atrophy with or without hydromyelia. In 1 patient, magnetic resonance imaging revealed hydrocephalus. Twenty different MFN2 mutations were identified, and 14 were considered new variants. Their transmission was predominantly autosomal dominant, with vertical transmission in 8 and de novo occurrence in 3. However, we also identified rare types of transmission, especially a germinal mosaicism and an autosomal recessive inheritance. One patient carried a rare variant in the GDAP1 gene and another in the OPA1 gene in association with MFN2 mutation. Charcot-Marie-Tooth disease type 2A associated with MFN2 mutations is clinically very heterogeneous. Ranging from a mild to a severe form, CMT2A exhibits various types of transmission. Optic atrophy and vocal cord palsy were observed in patients with severe disability and an early-onset form and also in patients with later onset. Hydromyelia and spinal cord atrophy support central nervous system involvement in CMT2A.

197th ENMC international workshop: Neuromuscular disorders of mitochondrial fusion and fission – OPA1 and MFN2 molecular mechanisms and therapeutic strategies: 26–28 April 2013, Naarden, The Netherlands

The 197th ENMC workshop entitled “Neuromuscular disorders of mitochondrial fusion and fission – OPA1 and MFN2 molecular mechanisms and therapeutic strategies” took place from the 26th to the 28th of April 2013 in Naarden, The Netherlands. A multidisciplinary group of 19 participants took part in this workshop, including 18 clinical and basic science researchers from six different countries (France, Germany, Italy, Spain, the United Kingdom (UK), and the United States of America), and one patient representative from CMT UK. The principal aim of this workshop was to establish an integrated research network in order to better understand the basic mechanisms responsible for the development of neuromuscular disease in patients harbouring pathogenic OPA1 and MFN2 mutations, which cause autosomal dominant optic atrophy (DOA) and axonal Charcot–Marie–Tooth disease (CMT2A), respectively [1,2].

Segregation analysis in families with Charcot-Marie-Tooth disease allows reclassification of putative disease causing mutations

BackgroundThe identification of disease causing, or putative disease causing, mutations in index patients with Charcot-Marie-Tooth disease (CMT) allows for genetic testing of family members. Relevant variants identified in index patients are of either definite, likely or uncertain pathogenicity. The main objective of this study was to make an evaluation of the family investigations performed as part of the assessment of genetic variants of unknown clinical significance (VUS).MethodsBetween 2004 and 2010 molecular genetic family investigations were requested for 87 family members from 41 families harbouring PMP22dup or genetic variants in GJB1, MPZ, MFN2 and NEFL. Relatives were tested for the family mutation and data from the requisitions were evaluated by means of statistical tools.ResultsThe results within each indication category are presented and discussed in detail. Twenty-two relatives (9 affected) from eight families were included in the segregation analyses, which invoked reclassification of three MFN2 mutations, two of which were de novo substitutions (c.2146_2148dup, c.692C > T). One MFN2 substitution was downgraded due to non-segregation (c.1709 A > G), and a MPZ substitution (c.103 G > A) upgraded due to segregation with the phenotype in the family.ConclusionsThe results allow for the evaluation of the patient phenotypes ascertained in families, as opposed to the phenotypic descriptions of index patients. They indicate that de novo MFN2 mutations are regularly found in patients with a classical CMT2 phenotype. They also demonstrate the importance of a precise clinical and neurophysiologic diagnosis of affected family members. This particularly applies for the examination of variants of uncertain clinical significance. Finally, the fact that 14,6% of affected relatives tested for (probable or certain) pathogenic mutations were mutation negative, demonstrates that clinical evaluation alone is not always sufficient in order to determine their diagnosis. We believe that the results will aid in the estimation and planning of resources required for the various aspects of family evaluations in CMT.

Mitofusin 2 expression dominates over mitofusin 1 exclusively in mouse dorsal root ganglia - a possible explanation for peripheral nervous system involvement in Charcot-Marie-Tooth 2A.

Mitofusin 2 (Mfn2), a protein of the mitochondrial outer membrane, is essential for mitochondrial fusion and contributes to the maintenance and operation of the mitochondrial network. Mutations in the mitofusin 2 gene cause axonal Charcot-Marie-Tooth type 2A (CMT2A), an inherited disease affecting peripheral nerve axons. The precise mechanism by which mutations in MFN2 selectively cause the degeneration of long peripheral axons is not known. There is a hypothesis suggesting the involvement of reduced expression of a homologous protein, mitofusin 1 (Mfn1), in the peripheral nervous system, and less effective compensation of defective mitofusin 2 by mitofusin 1. We therefore aimed to perform an analysis of the mitofusin 1 and mitofusin 2 mRNA and protein expression profiles in different mouse tissues, with special attention paid to dorsal root ganglia (DRGs), as parts of the peripheral nervous system. Quantitative measurement relating to mRNA revealed that expression of the Mfn2 gene dominates over Mfn1 mainly in mouse DRG, as opposed to other nervous system samples and other tissues studied. This result was further supported by Western blot evaluation. Both these sets of data confirm the hypothesis that the cellular consequences of mutations in the mitofusin 2 gene can mostly be manifested in the peripheral nervous system.

Charcot-Marie-Tooth disease

To determine the genetic distribution and the phenotypic correlation of an extensive series of patients with Charcot-Marie-Tooth disease in a geographically well-defined Mediterranean area. A thorough genetic screening, including most of the known genes involved in this disease, was performed and analyzed in this longitudinal descriptive study. Clinical data were analyzed and compared among the genetic subgroups. Molecular diagnosis was accomplished in 365 of 438 patients (83.3%), with a higher success rate in demyelinating forms of the disease. The CMT1A duplication (PMP22 gene) was the most frequent genetic diagnosis (50.4%), followed by mutations in the GJB1 gene (15.3%), and in the GDAP1 gene (11.5%). Mutations in 13 other genes were identified, but were much less frequent. Sixteen novel mutations were detected and characterized phenotypically. The relatively high frequency of GDAP1 mutations, coupled with the scarceness of MFN2 mutations (1.1%) and the high proportion of recessive inheritance (11.6%) in this series exemplify the particularity of the genetic distribution of Charcot-Marie-Tooth disease in this region.

Diagnostic laboratory testing for Charcot Marie Tooth disease (CMT): the spectrum of gene defects in Norwegian patients with CMT and its implications for future genetic test strategies

BackgroundCurrent genetic test algorithms for Charcot Marie Tooth (CMT) disease are based on family details and comprehensive clinical and neurophysiological data gathered under ideal conditions for clinical assessment. However, in a diagnostic laboratory setting relying on external test requisitions and patient samples, such conditions are not always met. Our objective was therefore to perform a retrospective evaluation of the data given in laboratory request forms and to assess their quality and applicability with regard to the recommended algorithms for CMT diagnostics. As we are the main test centre for CMT in Norway our results also provide an overview of the spectrum of gene defects in the Norwegian CMT population.MethodsGenetic testing was performed according to polyneuropathy type; demyelinating/mixed: PMP22 duplication, MPZ, EGR2, LITAF, NEFL, PMP22, GJB1, axonal: MFN2, MPZ, NEFL, and GJB1.ResultsDiagnostic testing of index patients was requested in 435 of the 549 cases. Seventy-two (16.6%) positive molecular genetic findings were made. The majority (94.6%) of mutation positive cases showed disease onset before 50 years of age. PMP22 (duplication), MPZ, GJB1 and MFN2 mutations constituted 95.8% of the positive findings. Within the nerve conduction study groups, mutation detection rates were; demyelinating 33.8%; mixed 29.0%; axonal 8.8%; unspecified 16.5%.ConclusionWe suggest a simplified algorithm intended for referral centres, dealing with DNA/blood samples, which involves the assessment of age at onset and neurophysiological data followed by testing of four genes; PMP22 (duplication), MPZ, GJB1 and MFN2. Patients negative for mutations in those four genes should be subjected to evaluation at an interdisciplinary inherited neuropathy clinic with the capacity for extended molecular genetic analysis by next generation sequencing.

Two novel MPZ mutations in Chinese CMT patients

To investigate the myelin protein zero (MPZ) gene mutation and related clinical features in Chinese Charcot-Marie-Tooth (CMT) patients, we screened the coding sequence of MPZ in 70 unrelated CMT index patients after excluding the PMP22 duplication, Cx32 and MFN2 mutations. We found four different missense mutations: c.194C>T, c.242A>T, c.371C>T, and c.419C>G. The frequency of MPZ mutation was approximately 4.35% of the total, 3.08% of CMT1, and 6% of CMT2. Mutations c.242A>T and c.419C>G are novel. The mutation c.242A>T exhibited late onset and rapidly progressive CMT2 phenotype. The mutation c.419C>G exhibited relatively late onset and slowly progressive CMT1 phenotype.