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).     

Unusual electrophysiological findings in X-linked dominant Charcot-Marie-Tooth disease

X-linked Charcot-Marie-Tooth disease (CMTX) is the second most common form of Charcot-Marie-Tooth disease. Variable histopathological and nerve conduction velocity (NCV) results have suggested either a primary demyelinating or axonal polyneuropathy. We identified five individuals across three generations in a family with CMTX associated with a mutation in the gene coding for connexin 32. All individuals were studied by clinical neurological examination, DNA analysis, and nerve conduction studies. The proband (1174/KD) also underwent a sural nerve biopsy. As expected, all the affected males were more clinically affected than the females. All affected males and obligate female carriers exhibited some electrophysiological characteristics of demyelination. However, striking heterogeneity of nerve conduction velocities was seen. This family shows that CMTX is a heterogeneous and distinctly nonuniform demyelinating polyneuropathy, the severity of which varies with sex and age. Such electrophysiological variability is unique among hereditary neuropathies.     

A novel mutation of gap junction protein β 1 gene in X-linked Charcot-Marie-Tooth disease

Introduction: In this study we report a novel mutation in the gap junction protein beta 1 (GJB1) gene of a Chinese X‐linked Charcot–Marie–Tooth disease (CMTX1) family, which has specific electrophysiological characteristics. Methods: Twenty members in the family were studied by clinical neurological examination and GJB1 gene mutation analysis, and 3 patients were studied electrophysiologically. The proband and his mother also underwent sural nerve biopsy. Results: All patients have the CMT phenotype, except for 2 asymptomatic carriers. Electrophysiological examinations showed non‐uniform slowing of motor conduction velocities and partial motor conduction blocks and temporal dispersion. Sural nerve biopsy confirmed a predominantly demyelinating neuropathy, and an Asn2Lys mutation in the amino‐terminal domain was found in 9 members of this family, but not in 25 normal controls in the family. Conclusions: This family represents a novel mutation in the GJB1 form of CMTX1. The mutation in the amino‐terminus has an impact on the electrophysiological characteristics of the disease. Muscle Nerve, 2011     

A micro-electrode study of peripheral neuropathy in man. Part 2. Responses to conditioning stimuli.

Surface, needle and micro-electrode recordings were obtained from sensory nerves of patients with various types of peripheral neuropathy. Changes in amplitude and conduction velocity of nerve action potentials were measured after a single conditioning stimulus and after tetanic stimulation for 2 min. In patients with hereditary forms of axonal degeneration (AD), recovery processes of nerve fibres of all conduction velocities were normal; in acquired forms of AD fibres with conduction velocity less than 30 m/sec had greater and more prolonged post-tetanic depression than control nerves of similar conduction velocity. Where neuropathy was associated with segmental demyelination (SD), fibres of all conduction velocities had prolonged recovery processes after both single and tetanic stimulation. The changes were especially marked at higher skin temperature, and were greater than the changes seen in nerves with acquired forms of AD. Finally, 2 sural nerves were studied during the process of Wallerian degeneration after a biopsy had been obtained proximally, and recovery processes did not change during the period of degeneration. Perceptual abnormalities were similar in AD and SD. It is suggested that changes in recovery processes of nerve fibres with segmental demyelination or regeneration after injury contribute to the perceptual abnormalities which occur in clinically encountered peripheral neuropathies.     

腓骨肌萎缩症X1型1个家系的临床、电生理和Connexin32基因突变分析

目的观察腓骨肌萎缩症(CMT)X1型的临床、电生理特点和Connexin32(Cx32)基因突变情况.方法对1个无基因重复的临床可疑的CMTX1家系中的3例患者进行详尽的临床和神经电生理检查,并应用变性高效液相色谱结合混和样品池法和DNA序列测定对包括先证者在内的3名成员的Cx32基因进行突变检测.结果 该家系中的病人发生了Gly12Ser,50名正常人中未发现上述改变,提示该突变为致病性突变.家系中男性病人临床症状重于女性;电生理特点为脱髓鞘改变;同一病人的不同神经间存在异质性.结论 Gly12Ser突变可能导致原发性脱髓鞘性神经病,不伴有特殊的临床表现.     

Phenotypic expression of a Pro 87 to Leu mutation in the connexin 32 gene in a large Swiss family with Charcot-Marie-Tooth neuropathy

BACKGROUND: The clinical manifestations of CMTX have been well described but the natural history has not yet been studied in detail. We studied phenotype variability in a family with a Pro 87 to Leu mutation of the connexin 32 (Cx32) gene. METHODS: A total of 32 family members, of which 19 patients were affected, underwent clinical, electrophysiological, and genetic studies. RESULTS: Onset was in the second decade. Clinical features were similar in both sexes when quantitative scores were compared, but more males had a steppage gait and skeletal deformities. All adult patients had a predominant involvement of the thenar muscles. The median values of nerve conduction velocities (NCVs) were not statistically different in men and in women. The correlation coefficients were low between motor NCVs within the same extremities, indicating nonuniform slowing between nerves, the ulnar nerve being the least affected. When disability was rated, a strong correlation was seen in male patients between severity of motor axonal loss and duration of the disease. The main pathological features were axonal loss, clusters of regenerating fibers and paranodal demyelination, the hallmark of a Schwann cell pathology. CONCLUSIONS: Our data support the hypothesis that clinical disability in CMTX is caused by loss of large myelinated axons in men. Furthermore, this study shows that the nerves are not uniformly affected in terms of axonal loss. Preventing axonal degeneration and promoting axonal regeneration in the most affected nerves might be the best therapeutic approaches to ameliorate disability in CMTX.     

Sensorineural deafness in X-linked Charcot-Marie-Tooth disease with connexin 32 mutation (R142Q)

OBJECTIVE: To report a family with X-linked Charcot-Marie-Tooth disease (CMTX) with proven connexin 32 (Cx32) mutation associated with deafness. METHODS: Twelve members of a CMTX family were examined clinically. Electromyography and sensory and motor conduction studies were performed in three men, two women, and a 7-year-old boy. Audiometric testing was carried out in the three men, one woman, and an 8-year-old girl. Molecular genetic analysis was performed in six men and five women. RESULTS: The three men and the 7-year-old boy had the usual sensorimotor deficit and pronounced reduction of motor nerve conduction velocity. A 15-year-old boy was asymptomatic and had only areflexia. The women had impairment of vibratory sensation and slight slowing of nerve conduction velocities. Sensorineural deafness was observed in the three men and in an 8-year-old girl without any motor or sensory deficit. Molecular genetic analysis revealed a new missense mutation located in codon 142 of the Cx32 gene leading to the substitution of an arginine by a glutamine. CONCLUSION: CMTX due to Cx32 mutations often shows interfamilial and intrafamilial phenotypic variation, which is also the hallmark of this family. The sensorineural deafness observed in this family suggests that Cx32 could play an important role in the auditory pathway.     

Conduction velocity versus amplitude analysis: Evidence for demyelination in diabetic neuropathy

Motor conduction velocities (CVs) were correlated with distal compound muscle action potential (CMAP) amplitudes for tibial, peroneal, and median nerves in patients with biopsy-proven chronic inflammatory demyelinating polyneuropathy (CIDP), diabetic neuropathy, and amyotrophic lateral sclerosis. Only in the diabetic patients did CV significantly correlate with CMAP amplitude. The data show that diabetic neuropathy produces conduction velocity slowing that cannot be explained by axon loss alone, and that differentiation between diabetic neuropathy and CIDP in an individual nerve is difficult. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21: 1228–1230, 1998.     

多发性硬化周围神经损害的肌电图及病理研究

目的：探讨多发性硬化（MS）产生周围神经损害的肌电图，病理特点和影响MS累及周围神经的相关因素。方法：33例MS患者，均满足Poser的确定诊断标准，排除其他神经系统疾病，30名正常自愿受试者作为对照，排除周围神经损害的相关因素，两组分别进行运动，感觉神经传导检测，F波潜伏期及出现率，H反射潜伏期检测，腓肠神经活检，光镜及电镜观察周围神经病理变化。结果：（1）33例MS患者中，9例有根性疼痛，3例有手袜套样感觉障碍，6例不对称性肌萎缩，2例有明显的自主神经症状；（2）肌电图显示复合肌肉动作电位波幅降低，正中神经，尺神经感觉运作电位波幅增高，F波及H反射的潜伏期延长，F波出现率降低。MS周围神经损害的程度与神经功能缺损、病程及病变部位有关，神经功能缺损越重，病程越长，胫神经和腓总神经运动传导波幅降低越明显，正中神经、尺神经感觉动作电位波幅增高越明显；脊髓型MS周围神经受损明显高于脑型；（3）6例患者腓肠神经活检，光镜下可见有髓纤维呈不同程度的髓鞘脱失。电镜下以轴索变性为主，髓鞘板层解离及髓球形成。结论：MS是一种以CNS受损为主的脱髓鞘疾病，在部分患者可对同时累及周围神经系统，脱髓鞘改变主要发生在脊神经根，远端轴突可继发轴索损害，肌电图是比较理想的可全面评价MS周围神经损害的临床检测手段，对判断预后有一定的实用价值。     

Dysmyelinating sensory-motor neuropathy in merosin-deficient congenital muscular dystrophy

A 20-year-old man with mild myopathy, external ophthalmoparesis, epilepsy, and diffuse white matter hyperintensity in the brain on magnetic resonance imaging had partial merosin deficiency in muscle and absent merosin in the endoneurium. Motor and sensory nerve conduction velocities were slow. Nerve biopsy showed reduction of large myelinated fibers, short internodes, enlarged nodes, excessive variability of myelin thickness, tomacula, and uncompacted myelin, but no evidence of segmental demyelination, naked axons, or onion bulbs. Thus, in congenital muscular dystrophy, merosin expression may be dissociated in different tissues, and the neuropathy is sensory-motor and due to abnormal myelinogenesis.     

Sensory action potentials and biopsy of the sural nerve in neuropathy.

In 167 consecutive patients with various types of neuropathy, the amplitude of the sensory potential and the maximum conduction velocity along the sural nerve were compared with conduction in other sensory nerves, and were related to structural changes revealed by nerve biopsy. Electrophysiological findings in the sural nerve were similar to those in the superficial peroneal and the median nerve, though the distal segment of the median nerve was normal in 20 per cent of the patients when it was abnormal in the sural nerve. Quantitation of histological findings was a more sensitive method than the electrophysiological study in that two-thirds of 33 patients with normal electrophysiology in the sural nerve showed mild loss of fibres or signs of remyelination in teased fibres. The amplitude of the sensory potential was grossly related to the number of large myelinated fibres (more than 7 micrometer in diameter). Considering the 95 nerves from which teased fibres were obtained, maximum conduction velocity was abnormal in half. In 18 of these nerves, slowing in conduction was due to axonal degeneration: the velocity was as to be expected from the diameter of the largest fibres in the biopsy ("proportionate slowing"). In 9 nerves slowing was severe and more marked than to be expected from loss of the largest fibres ("disproportionate slowing"); these nerves showed paranodal or segmental demyelination in more than 30 per cent of the fibres. In 16 nerves from patients with neuropathy of different aetiology neither loss of fibres nor demyelination could explain the moderate slowing. The cause of slowing in these nerves is unknown; other conditions are referred to in which slowing in conduction cannot be attributed to morphological changes. Finally, electrophysiological and histological findings are reported in some patients with neuropathy associated with malignant neoplasm, with rheumatoid arthritis, with polyarteritis nodosa, with acute intermittent porphyria and with cirrhosis of the liver.     

Activity-dependent conduction block in chronic inflammatory demyelinating polyneuropathy

Previous studies suggested that activity-dependent conduction block (CB) contributes to weakness in chronic inflammatory demyelinating polyneuropathy (CIDP). These studies, however, investigated only one nerve segment per patient, employed cervical magnetic stimulation which may be submaximal, included nerves with extremely low compound muscle action potentials (CMAPs) which precludes assessment of CB, and lacked predefined criteria for activity-dependent CB. Obtaining more robust evidence for activity-dependent CB is important because it may be treated pharmacologically. We investigated 22 nerve segments in each of 18 CIDP patients, employed supramaximal electrical stimulation, excluded nerves with markedly reduced CMAPs, and adopted criteria for activity-dependent CB. Each nerve was tested before and immediately after 60 s of maximal voluntary contraction (MVC) of the relevant muscle. Per nerve segment we calculated segmental area ratio: (area proximal CMAP)/(area distal CMAP). Per nerve we calculated total area ratio: (area CMAP evoked at Erb's point)/(area most distally evoked CMAP). MVC induced no change in mean area ratios and no activity-dependent CB according to our criteria, except for one segment. MVC induced increases in distal and proximal CMAP area and duration. In segments with demyelinative slowing, MVC induced an increase in CMAP duration prolongation. Thus, in CIDP, muscle activity induces virtually no CB, but it may increase temporal dispersion of nerve action potentials.     

Differentiation between axonal and demyelinating neuropathies: Identical segments recorded from proximal and distal muscles

The presence of significant slowing of motor nerve conduction velocity is considered one of the electrodiagnostic hallmarks of demyelinating neuropathies; however, slowing of conduction velocity may also accompany severe axonal loss. When compound muscle action potential (CMAP) amplitudes are markedly reduced, it is frequently difficult to determine if conduction velocity slowing is due to axonal loss with dropout of the fastest conducting fibers or demyelination. To evaluate the relationship between conduction velocity and axonal dropout, we compared conduction velocities through the same segment of nerve recording from distal and proximal peroneal muscles in patients with chronic neuropathies, in patients with motor neuron disease, and in control subjects. In controls and patients with motor neuron disease, conduction velocities were normal with no significant difference between proximal and distal sites. In patients with axonal neuropathies, conduction velocities were preferentially slowed when recording from distal muscles and relatively normal when recording from proximal sites. Patients with demyelinating neuropathies showed marked slowing of conduction at both sites. We conclude that comparing conduction velocity obtained from proximal versus distal muscle recordings provides a simple, reliable aid for differentiating between chronic axonal and demyelinating polyneuropathies, especially in cases with conduction velocity slowing and low CMAP amplitudes. © 1995 John Wiley & Sons, Inc.     

X-linked Charcot-Marie-Tooth disease: phenotypic expression of a novel mutation Ile127Ser in the GJB1 (connexin 32) gene

We report a family with X-linked dominant Charcot-Marie-Tooth disease (CMTX1). Three affected family members are described, who underwent detailed clinical, electrophysiological, molecular genetic, and histopathological studies. A novel isoleucine at position 127 with serine (Ile127Ser) mutation in the gap junction protein beta 1 (GJB1) gene was detected. The electrophysiological findings were consistent with a primary demyelinating neuropathy with secondary axonal loss and support this model of disease progression. All patients having the CMT phenotype and intermediate conduction velocities who are negative for CMT1A duplication/hereditary neuropathy with liability to pressure palsies (HNPP) deletion, and whose family shows a dominant trait without male-to-male transmission, should be screened for CMTX1.     

Saltatory conduction precedes remyelination in axons demyelinated with lysophosphatidyl choline

The changing electrical and morphological properties of demyleinating and remyelinating nerve fibres have been studied in rat ventral roots after intrathecal injection of lysophosphatidyl choline (LPC). The spatial distribution of electrical excitability within the lesion has been studied in undissected single fibres using high-resolution longitudinal current analysis. The distribution of excitability has been correlated with the ultrastructure of the fibres and with the distribution of the surrounding Schwann cells. Demyelinated axolemma was initially not excited, but conduction across demyelinated internodes appeared progressively from the 4th day after LPC injection. Conduction was never continuous, but proceeded via new foci of inward membrane current as early as 4 days after LPC injection, i.e. 3 days before the onset of remyelination. It is suggested that these foci (termed ?-nodes to distinguish them from the nodes of Ranvier distributed along myelinated nerve fibres) are precursors of nodes of Ranvier, and may indicate aggregates of sodium channels which form along the demyelinated axolemma prior to remyelination.     

Correlations of nerve conduction measures in axonal and demyelinating polyneuropathies.

OBJECTIVE: In a considerable proportion of patients with polyneuropathy the electrophysiological distinction between primarily demyelinating or axonal pathology is not straightforward. This study aimed at determining whether the relation between the sensory nerve action potential (SNAP)/compound muscle action potential (CMAP) amplitude and conduction velocity (CV) or distal motor latency (DML) in demyelinating versus axonal polyneuropathy could be helpful in distinguishing these two pathophysiologies. METHODS: The relation between amplitude reduction and conduction slowing was performed using regression analysis in nerve conduction studies from 53 axonal polyneuropathies and 45 demyelinating polyneuropathies. Sensory nerve conduction studies were performed using the near-nerve needle technique. Finally, needle EMG findings in 31 muscles in axonal and in 22 muscles in demyelinating polyneuropathies were compared. RESULTS: A linear correlation between action potential amplitude and CV was seen in the majority of nerves in both axonal and demyelinating polyneuropathies. Further, an inverse linear correlation between CMAP amplitude and DML was found in most of the nerves in axonal polyneuropathies. The incidence and degree of abnormality, including decrease in action potential amplitude, was more pronounced in demyelinating than in axonal polyneuropathies, while there was no difference in EMG findings. CONCLUSIONS: Amplitude reduction and conduction slowing were correlated in axonal as well as demyelinating polyneuropathies, and a significant reduction in SNAP and CMAP amplitudes was found in demyelinating as well as axonal polyneuropathies. The correlation in axonal polyneuropathies can be attributed to a concomitant or selective loss of large, fast conducting fibers, whereas the correlation in demyelinating polyneuropathies may be explained by temporal dispersion or secondary axonal degeneration. SIGNIFICANCE: At present, relation between amplitude reduction and conduction slowing does not seem to be useful in revealing the primary pathophysiology of a polyneuropathy. Decrease in CV, increase in DML, increase in F-wave latency, conduction block and temporal dispersion should mainly be considered. Decrease in amplitude must be interpreted with caution.     

IgM deposits at nodes of Ranvier in a patient with amyotrophic lateral sclerosis, anti-GM1 antibodies, and multifocal motor conduction block

We studied a patient with amyotrophic lateral sclerosis, multifocal motor conduction block, and IgM anti-GM1 antibodies. A sural nerve biopsy demonstrated deposits of IgM at nodes of Ranvier by direct immunofluorescence. The deposits were granular and located in the nodal gap between adjacent myelin internodes, and in some instances, they extended along the surface of the paranodal myelin sheath. When injected into rat sciatic nerve, the serum IgM bound to the nodes of Ranvier, and the binding activity was removed by preincubation with GM1. These observations suggest that anti-GM1 antibodies may have caused motor dysfunction by binding to the nodal and paranodal regions of peripheral nerve.     

Peripheral neuropathy after chronic endoneurial ischemia

We have developed a method for producing chronic regional nerve ischemia in rats by creating proximal limb arteriovenous shunts. This procedure results in a 50 to 75% reduction in endoneurial blood flow within the distal sciatic nerve as measured by the iodoantipyrine method. Nerve conduction velocities in sciatic nerves ipsilateral to the shunt fell by 25 to 30% within 2 weeks after creation of the shunt and did not recover for up to 10 months after the procedure. Morphological studies of the ischemic nerves showed structural abnormalities at nodes of Ranvier and mild axonal atrophy. Neither segmental demyelination nor axonal degeneration were evident. These results indicate that reduced endoneurial blood flow, insufficient to cause infarction, may result in measurable functional and morphological abnormalities in peripheral nerves.     

Peripheral nerve extracellular matrix remodeling in Charcot-Marie-Tooth type I disease

Charcot-Marie-Tooth type 1 disease (CMT1) is a group of inherited demyelinating neuropathies caused by mutations in genes expressed by myelinating Schwann cells. Rather than demyelination per se, alterations of Schwann cell-axon interactions have been suggested as the main cause of motor-sensory impairment in CMT1 patients. In an attempt to identify molecules that may be involved in such altered interactions, the extracellular matrix (ECM) remodeling occurring in CMT1 sural nerves was studied. For comparison, both normal sural nerves and sural nerves affected by neuropathies of different origin were used. The study was performed by immunohistochemical analysis using antibodies against collagen types I, III, IV, V, and VI and the glycoproteins fibronectin, laminin, vitronectin and tenascin. Up-regulation of collagens, fibronectin and laminin was commonly found in nerve biopsy specimens from patients affected by CMT1 and control diseases, but higher levels of overexpression were usually observed in CMT1 cases. On the other hand, vitronectin and tenascin appeared preferentially induced in CMT1 compared to other pathologies investigated here. Vitronectin, whose expression in normal nerves was limited to perineurial layers and to the walls of epineurial and endoneurial vessels, became strongly and diffusely expressed in the endoneurium in most CMT1 biopsy specimens. The expression of tenascin, confined to the perineurium, to vessel walls and to the nodes of Ranvier in normal nerves, was displaced and extended along the internodes of several nerve fibers in the majority of CMT1 nerves. Thus, compared with our pathological controls CMT1 seemed to determine the most extensive remodeling of peripheral nerve ECM.