Early onset Charcot‐Marie‐Tooth neuropathy type 2A and severe developmental delay: expanding the clinical phenotype of MFN2‐related neuropathy

Charcot‐Marie‐Tooth (CMT) syndromes are a group of clinically heterogeneous disorders of the peripheral nervous system. Mutations of mitofusin 2 (MFN2) have been recognized to be associated with CMT type 2A (CMT2A). CMT2A is primarily an axonal disorder resulting in motor and sensory neuropathy. We report a male child with psychomotor delay, dysmorphic features, and weakness of lower limbs associated with electrophysiological features of severe, sensory‐motor, axonal neuropathy. The patient was diagnosed with early onset CMT2A and severe psychomotor retardation associated with c.310C>T mutation (p.R104W) in MFN2 gene. CMT2A should be considered in patients with both axonal sensory‐motor neuropathy and developmental delay.     

Late‐onset hereditary sensory and autonomic neuropathy expands the phenotypic spectrum of MFN2‐related diseases

Mutations in the Mitofusin 2 (MFN2) gene have been identified in patients with autosomal dominant axonal motor and sensory neuropathy or Charcot–Marie‐Tooth 2A (CMT2A). Here we describe clinical and pathological changes in an adult patient with sporadic hereditary sensory and autonomic neuropathy (HSAN) due to an MFN2 mutation. The patient was a 53‐year‐old man who had sensory involvement and anhidrosis in all limbs without motor features. The electrophysiological assessment documented severe axonal sensory neuropathy. The sural nerve biopsy confirmed the electrophysiological findings, revealing severe loss of myelinated and unmyelinated fibers with regeneration clusters. Genetic analysis revealed the previously identified mutation c.776 G > A in MFN2. Our report expands the phenotypic spectrum of MFN2‐related diseases. Sequencing of MFN2 should be considered in all patients presenting with late‐onset HSAN.     

Mutational mechanisms in MFN2‐related neuropathy: compound heterozygosity for recessive and semidominant mutations

Mitofusin‐2 (MFN2) mutations are the most common cause of autosomal dominant axonal Charcot‐Marie‐Tooth disease (CMT, type 2A), sometimes complicated by additional features such as optic atrophy (CMT6) and upper motor neuron involvement (CMT5). Several pathogenic mutations are reported, mainly acting in a dominant fashion, although few sequence variants behaved as recessive or semidominant in rare homozygous or compound heterozygous patients. We describe a 49‐year‐old woman with CMT5 associated with compound heterozygosity for two MFN2 variants, one already reported missense mutation (c.748C>T, p.R250W) and a novel nonsense sequence change (c.1426C>T, p.R476*). Her mother, carrying the p.R250W variant, had very late‐onset minimal axonal neuropathy, whilst the father harboring the nonsense sequence change had neither clinical nor electrophysiological neuropathy. The missense mutation is likely pathogenic according to in silico analyses and a previous report, while the nonsense variant is predicted to behave as a null allele. The p.R250W variant behaves as semidominant by causing only a mild, almost subclinical, neuropathy when heterozygous; the nonsense mutation in the father was phenotypically silent, suggesting that haploinsufficiency for MFN2 is not disease causative, but was deleterious in the daughter who had only one active mutated MFN2 allele.     

Dominant GDAP1 founder mutation is a common cause of axonal Charcot-Marie-Tooth disease in Finland

We describe a founder mutation in the gene encoding ganglioside-induced differentiation associated-protein 1 (GDAP1), leading to amino acid change p.H123R, as a common cause of autosomal dominant axonal Charcot-Marie-Tooth (CMT2) neuropathy in Finland. The mutation explains up to 14 % of CMT2 in Finland, where most patients with axonal neuropathy have remained without molecular diagnosis. Only three families out of 28 were found to carry putative disease mutations in the MFN2 gene encoding mitofusin 2. In addition, the MFN2 variant p.V705I was commonly found in our patients, but we provide evidence that this previously described mutation is a common polymorphism and not pathogenic. GDAP1-associated polyneuropathy caused predominantly a mild and slowly progressive phenotype. Besides distal leg muscle weakness, most patients showed mild proximal weakness, often with asymmetry and pes cavus. Our findings broaden the understanding of GDAP1 mutations in CMT2 phenotypes and provide support for the use of whole-exome sequencing in CMT gene diagnostics.     

A French family with Charcot-Marie-Tooth disease related to simultaneous heterozygous MFN2 and GDAP1 mutations

Either dominantly inherited mutations in MFN2 encoding mitofusin 2 or GDAP1 encoding ganglioside-induced differentiation associated protein 1 may be associated with mild neuropathy. The proband, a 41-year-old woman, and her daughter present a severe axonal form of Charcot-Marie-Tooth (CMT) disease. Both are heterozygous for the well-described mild variant p.R120W in GDAP1, which was transmitted by the pauci symptomatic proband's mother. Given that they had an early onset in the first decade and delayed walking acquisition, the other genes implicated in axonal forms of CMT disease were analyzed. A second mutation truncating MFN2 (p.Val160fsX26) was found in the proband and her daughter. This mutation was transmitted by the proband's father who has normal neurological examination. The proband underwent two nerve biopsies which showed an axonal degeneration, myelin modifications, and intra-axonal mitochondria with distorted cristae. Such abnormal mitochondria have been reported in cases with autosomal dominant MFN2 mutations and in one patient with an autosomal recessive GDAP1 mutation. Our two cases show that heterozygous truncation of MFN2, which is silent at least until the sixth decade, when combined with the mild p.R120W GDAP1 variant, leads to a severe neuropathy. This supports the emerging hypothesis of cumulative effects of MFN2 and GDAP1 mutation.     

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.     

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.     

夏科-马里-图斯病2A2型一家系的临床病理特点

目的 报道一个早发型夏科-马里-图斯病(CMT)2A2家系,探讨其临床和病理特点.方法 该家系共有5例患者,呈常染色体显性遗传,先证者为36岁女性,6岁开始出现下肢进行性无力,8岁出现双足内翻.家族中另有2例男性和2例女性发病,发病年龄3～7岁,主要表现为缓慢进展的四肢远端肌肉无力、萎缩,伴随四肢远端感觉减退、腱反射减退及关节挛缩.先证者和其儿子的上肢感觉神经、下肢感觉和运动神经诱发电位波幅不能引出.对先证者左侧腓肠神经进行活体组织病理检查.对先证者和其他4例家系患者、3名无症状家系成员行MFN2基因测序.结果 病理检查可见腓肠神经有髓纤维数目重度减少,以大有髓神经纤维减少为主,伴随个别有髓神经纤维再生簇结构以及不典型的洋葱球样结构.电镜下可见轴索中线粒体聚集,未发现线粒体结构异常.5例患者存在MFN2基因R94W突变,无症状家系成员无此突变.结论 我国存在早发型CMT2A2家系,患者周围神经缺乏有髓神经纤维再生改变,提示MFN2基因突变对神经元的损害更大.     

Effect of an R69C mutation in the myelin protein zero gene on myelination and ion channel subtypes

BACKGROUND: Most mutations in the myelin protein zero gene (MPZ) typically cause a severe demyelinating/dysmyelinating neuropathy that begins in infancy or an adult-onset axonal neuropathy. Axonal degeneration in the late-onset H10P mutation may be caused by the disruption of axoglial interaction. OBJECTIVE: To evaluate sural nerve biopsy samples from a patient with early-onset Charcot-Marie-Tooth disease type 1B caused by an arg69-to-cys (R69C) mutation. Design and PARTICIPANTS: Biopsies of sural nerves were performed 20 years apart in a patient with an R69C mutation (early onset). In addition, peripheral nerves were obtained from autopsy material from a patient with a T95M mutation (late onset). These nerves were analyzed using light microscopy of semithin sections, teased nerve fiber immunohistochemical analysis, electron microscopy, and immunologic electron microscopy. MAIN OUTCOME MEASURES: Pathological changes in sural nerve. RESULTS: Both R69C biopsy samples showed prominent demyelination and onion bulb formation, unlike the late-onset T95M mutation, which showed primarily axonal degeneration with no onion bulbs. The sural biopsy sample obtained 20 years earlier from the R69C patient showed minimal difference from the present sample, consistent with the lack of clinical progression during the 2 decades. Teased fiber immunohistochemical analysis of R69C revealed voltage-gated sodium channel subtype 1.8 expressions at the nodes of Ranvier around the areas of segmental demyelination. Internodal length in all R69C nerve fibers was invariably short (>94% of all internodes are <150 mum). CONCLUSIONS: Morphologic abnormalities in this early-onset R69C neuropathy were severe in childhood but progressed very slowly after adolescence. The switch to voltage-gated sodium channel subtype 1.8 expression at the nodes may provide clues into the pathogenesis of this case of early-onset neuropathy, and the short internodes may contribute to the extremely slowed conduction velocities in this case (<10 m/s).     

Functional, histopathologic and natural history study of neuropathy associated with EGR2 mutations

Mutations in the EGR2 gene cause a spectrum of Charcot–Marie–Tooth disease and related inherited peripheral neuropathies. We ascertained ten consecutive patients with various EGR2 mutations, report a novel de novo mutation, and provide longitudinal clinical data to characterize the natural history of the peripheral neuropathy. We confirmed that respiratory compromise and cranial nerve dysfunction are commonly associated with EGR2 mutations and can be useful in guiding molecular diagnosis. We also contrast morphological studies in the context of the I268N homozygous recessive mutation affecting the NAB repressor binding site and the R359W dominant-negative mutation in the zinc-finger domain.     

A novel ACTA1 mutation resulting in a severe congenital myopathy with nemaline bodies, intranuclear rods and type I fibre predominance

We describe a severe congenital myopathy patient of Xhosa native African origin with a novel de novo p.Gly152Ala skeletal muscle α-actin gene (ACTA1) mutation, who died at 6 months of age. The muscle pathology demonstrated abundant cytoplasmic and intranuclear rods, core-like areas and the unusual feature of larger type I than type II fibres. Our results further expand the phenotypes associated with ACTA1 mutations and provide support for the hypothesis that the structural abnormalities seen are a pathological continuum dependent on the precise mutation and biopsy location. Our results also demonstrate the likely world-wide distribution of de novo mutations in this gene.     

线粒体融合蛋白2基因新突变导致的遗传性运动感觉性神经病6型一家系

目的 报道1个遗传性运动感觉性神经病6型家系的临床表现、病理改变以及基因突变特点。方法 先证者男性,15岁。患者5岁出现双下肢无力,症状进行性加重,伴随出现双足跟腱挛缩;11岁开始出现慢性进行性视力下降;12岁出现双手肌肉萎缩,无肢体麻木。周围神经传导速度检查显示诱发电位未能引出或波幅显著下降,感觉神经较运动神经改变更明显。视诱发电位提示双眼P100潜伏期均延长,波幅正常。眼底照相提示视神经萎缩,视网膜电图正常。患者母亲7岁时开始出现走路费力,10岁出现视力下降。对先证者进行腓肠神经活体组织检查。对先证者及其母亲进行线粒体融合蛋白2( MFN2)基因测序,100名健康人作为正常对照。结果 腓肠神经病理改变主要为有髓神经纤维显著减少,电镜检查发现个别有髓神经纤维出现洋葱球样结构和再生簇结构,个别神经纤维的轴索内可见线粒体聚集和空泡化。先证者和母亲的MFN2基因第19号外显子存在c.2218T＞C杂合突变,导致MFN2第740位的色氨酸由精氨酸替代(W740R)。100名健康对照没有发现该突变。结论 MFN2基因c.2218T＞C突变导致了遗传性运动感觉性神经病6型,其视力下降多出现在脊神经损害之后,周围神经可以存在髓鞘损害。     

SEIPIN S90L Mutation in an Italian family with CMT2/dHMN and pyramidal signs

Heterozygous mutations in the Berardinelli–Seip congenital lipodystrophy (BSCL2) gene have been associated with different clinical phenotypes including Silver syndrome/spastic paraplegia 17, distal hereditary motor neuropathy type V, and Charcot–Marie–Tooth disease type 2 (CMT2) with predominant hand involvement. We studied an Italian family with a CMT2 phenotype with pyramidal signs that had subclinical sensory involvement on sural nerve biopsy. Direct sequencing analysis of the BSCL2 gene in the three affected siblings revealed an S90L mutation. This report confirms the variability of clinical phenotypes associated with a BSCL2 Ser90Leu mutation and describes the first Italian family with this mutation. Muscle Nerve, 2010     

Peripheral neuropathy and 46XY gonadal dysgenesis: A heterogeneous entity

Gonadal dysgenesis with normal male karyotype (46XY) is a sexual differentiation disorder. So far three patients have been reported presenting the association of 46XY gonadal dysgenesis with peripheral neuropathy. Examination of sural nerves revealed minifascicle formation in two of them. In one patient, a mutation was found in desert hedgehog homolog (Drosophila), a gene important in gonadal differentiation and peripheral nerve development.We studied neuropathological and molecular genetic aspects of a patient with 46XY gonadal dysgenesis and peripheral neuropathy.Examination of a sural nerve biopsy specimen revealed an axonal neuropathy with pronounced axonal loss, limited signs of axonal regeneration and no minifascicle formation. A normal male karyotype was found (46XY) without micro-deletions in the Y chromosome. No mutations were found in the sex determining region Y gene, peripheral myelin protein 22, Myelin Protein Zero, Gap-Junction protein Beta 1, Mitofusin 2 or desert hedgehog homolog.The absence of minifascicle formation and the absence of a mutation in desert hedgehog homolog in this patient with gonadal dysgenesis and peripheral neuropathy expand the clinical and genetic heterogeneity of this rare entity.     

Clinical and genetic diversities of Charcot‐Marie‐Tooth disease with MFN2 mutations in a large case study

Charcot‐Marie‐Tooth disease (CMT) constitutes a heterogeneous group affecting motor and sensory neurons in the peripheral nervous system. MFN2 mutations are the most common cause of axonal CMT. We describe the clinical and mutational spectra of CMT patients harboring MFN2 mutations in Japan. We analyzed 1,334 unrelated patients with clinically suspected CMT referred by neurological and neuropediatric departments throughout Japan. We conducted mutation screening using a DNA microarray, targeted resequencing, and whole‐exome sequencing. We identified pathogenic or likely pathogenic MFN2 variants from 79 CMT patients, comprising 44 heterozygous and 1 compound heterozygous variants. A total of 15 novel variants were detected. An autosomal dominant family history was determined in 43 cases, and the remaining 36 cases were reported as sporadic with no family history. The mean onset age of CMT in these patients was 12 ± 14 (range 0–59) years. We observed neuropathic symptoms in all patients. Some had optic atrophy, vocal cord paralysis, or spasticity. We detected a compound heterozygous MFN2 mutation in a patient with a severe phenotype and the co‐occurrence of MFN2 and PMP22 mutations in a patient with an uncommon phenotype. MFN2 is the most frequent causative gene of CMT2 in Japan. We present 15 novel variants and broad clinical and mutational spectra of Japanese MFN2‐related CMT patients. Regardless of the onset age and inheritance pattern, MFN2 gene analysis should be performed. Combinations of causative genes should be considered to explain the phenotypic diversity.     

Mutations in the mitofusin 2 gene are the most common cause of Charcot-Marie-Tooth type 2 disease

In contrast to Charcot-Marie-Tooth type 1 disease (CMT1), which is most commonly caused by 17p11.2-p12 duplication (in 70% of CMT1 cases), the axonal form of hereditary motor and sensory neuropathy (CMT2) seemed to be a genetically heterogeneous disease group, with no single gene playing a major pathogenetic role. In 2004, 10 mutations were identified in CMT2A families in the MFN2 gene coding for the mitochondrial protein mitofusin-2, previously mapped to the 1p35-36 locus. In the last two years, MFN2 gene mutations were shown to be the most common cause of autosomal dominant hereditary axonopathy. In addition, MFN2 gene mutations were also identified in CMT type 6 (axonal neuropathy with optic nerve atrophy). Recent reports indicate that some MFN2 gene mutations may by inherited as autosomal recessive traits. As MFN2 gene mutations are the most common cause of autosomal dominant CMT2 disease (33% of cases), MFN2 gene testing may be considered a diagnostic test for CMT2.     

A new de novo SYT2 mutation presenting as distal weakness. Neuropathy or neuromuscular junction dysfunction?

We report the case of a patient with a clinical phenotype characterized by distal lower limb weakness and pes cavus. The electrophysiological study showed slightly reduced or normal amplitude of motor potentials, a decremental response to repetitive nerve stimulation and post‐exercise facilitation. Muscle biopsy showed only mild neurogenic features. Genetic analysis included a clinical exome sequencing, followed by Sanger analysis. Three‐dimensional (3D) models were generated with a SwissModel ( https://swissmodel.expasy.org/ ) to explain the clinical observations and reinforce the pathogenic nature of the genetic variant identified. Genetic analysis demonstrated a new de novo heterozygous in frame deletion of the SYT2 gene (NM_177402.4: c.1082_1096del), confirmed by Sanger sequencing, which removes five aminoacids in the C2B domain of synaptotagmin‐2 protein, that cause a profound effect on the structure and function of this synaptic vesicle protein. We identified a de novo genetic variant in the SYT2 gene, further supporting its association with a highly stereotyped clinical and electrophysiological phenotype. Our case showed electrophysiological features consistent with a presynaptic dysfunction in the neuromuscular junction with normal post‐exercise amplitudes, not supporting the presence of predominant axonal damage. Although the analysis of SYT2 gene should be included in genetic analysis of patients presenting with this clinical phenotype that mimics motor neuropathy, clinicians have to consider the study of neuromuscular transmission to early identify this potentially treatable condition.     

A novel Lys141Thr mutation in small heat shock protein 22 (HSPB8) gene in Charcot–Marie–Tooth disease type 2L

Charcot–Marie–Tooth disease (CMT) is a group of clinically and genetically heterogeneous peripheral neuropathies. HSPB8 gene encodes heat shock protein 22 (HSP22) which belongs to the superfamily of small stress induced proteins. Mutations in HSPB8 are implicated to CMT2L and distal hereditary motor neuropathy 2A (dHMN2A). All three reported HSPB8 mutations are interestingly located in the Lys141 residue. In the present study, we examined a Korean axonal CMT patient who presented distal limb atrophy, sensory loss, areflexia, and axonal loss of large myelinated fibers. Whole exome sequencing identified a novel missense mutation c.422A>C (p.Lys141Thr) in HSPB8 as the underlying cause of the CMT2 patient. The mutation was regarded as a de novo case because both unaffected parents have no such mutation. The patient with HSPB8 mutation is the first case in Koreans. Clinical heterogeneities have been revealed in patients with Lys141 mutation; the present patient revealed similar phenotype of CMT2L. In addition, the lower limb MRI revealed a similarity between our HSPB8 and HSPB1 patients. It seems that the Lys141 site in the alpha-crystallin domain of HSPB8 is regarded as a mutational hot spot for peripheral neuropathy development, and mutations even in the same codon can exhibit different CMT phenotypes.     

Clinical and molecular evidence of possible digenic inheritance for MFN2/GDAP1 genes in Charcot-Marie-Tooth disease

Charcot Marie Tooth disease (CMT) is a progressive motor and sensory polyneuropathy, it is characterized by a very heterogeneous molecular basis and phenotype. MFN2 and GDAP1 participate in mitochondrial energy metabolism and the rare coinheritance of its pathogenic variants has been associated with a cumulative effect in the observed phenotype. We describe a patient with a severe axonal CMT and inherited heterozygous MFN2 (p.Leu741Val) and GDAP1 (p.Gln163*) variants. In accordance with a possible digenic inheritance, none of the heterozygous carriers in his family were symptomatic or exhibited electrophysiological abnormalities. We also review all of the previously reported patients with coinheritance of variants in these two genes; similar to our patient, all exhibit a predominantly axonal severe CMT phenotype. Our findings expand the genotypic spectrum of CMT and further support that digenic inheritance should be considered for analyzing and counseling CMT patients.