Molecular genetics of autosomal-dominant axonal Charcot-Marie-Tooth disease

The autosomal-dominant axonal peripheral neuropathies comprise a genetically heterogeneous group of disorders that are clinically subsumed under Charcot-Marie-Tooth disease type 2 (CMT2). A significant increase in the number of genes underlying major forms of CMT2 has improved the classification of specific CMT phenotypes. The molecular dissection of cellular functions of the related gene products has only begun and detailed pathophysiological models are still missing, but already the biological scope of genes linked to CMT2 is more diversified than CMT1. The known CMT2 genes present key players in these pathways and will likely prove as powerful tools in identifying eventual future targets for therapeutic intervention.     

Yield of next‐generation neuropathy gene panels in axonal neuropathies

The use and utility of targeted gene panels for diagnosing the type of Charcot‐Marie‐Tooth have grown rapidly because commercial gene panels that contain most of the relevant genes are available and affordable for many patients. We used a targeted gene panel to analyze 175 patients who had an unexplained axonal polyneuropathy affecting large myelinated axons, 86 of whom reported a family history of neuropathy, and 89 of whom did not. In patients reporting a family history, the panel identified a pathogenic variant causing the neuropathy in six cases (7%); in patients not reporting a family history, the gene panel identified pathogenic variants causing neuropathy in two patients (2%). Interpretation in a tertiary referral setting, current gene panels identify the genetic cause of neuropathy in a small minority of patients who have an unexplained axonal neuropathy, even in those reporting a family history.     

Autonomic involvement in inherited neuropathies

The inherited peripheral neuropathies constitute a large group of disorders, in some of which the causative metabolic defect has been identified whereas in the majority it is still unknown. Amongst the former, autonomic involvement is an important component in porphyric neuropathy, in the familial amyloid polyneuropathies, in Fabry's disease and in dopamine-hydroxylase deficiency. The latter group includes the hereditary sensory and autonomic neuropathies in some of which, such as the Riley-Day syndrome, autonomic disturbances are prominent, whereas in others they constitute only a minor component of the symptomatology. Autonomic dysfunction is an important component of the neurological manifestations in multiple endocrine neoplasia type IIB.     

Comparison of incremental with multipoint MUNE methods in transgenic ALS mice.

Several methods of motor unit number estimation (MUNE) are in current use. Uncertainty still exists about which is preferable and how results obtained from one method compare to another. We studied changes with MUNE over time in the SOD1(G93A) transgenic mouse model of amyotrophic lateral sclerosis (ALS), using both incremental and multipoint methods. This mouse model of motor neuron degeneration is highly consistent, with a monotonic decline in motor neuron number starting at approximately 60 days of life. Five mice were studied four times each, starting at day 60 of life and approximately every 20 days thereafter, using both methods. Results were quite comparable for both methods, with the incremental method yielding slightly higher estimates of motor unit size, and hence smaller MUNEs. Correlations between the two methods were 0.71 for single motor unit action potential (SMUAP) amplitude and 0.95 for MUNE. In this model, therefore, both MUNE methods yield similar estimates and are equally effective at documenting progression of a lower motor neuron disorder.     

Mitochondrial loss indicates early axonal damage in small fiber neuropathies

Evaluation of nerve fibers in the skin provides a useful tool for the diagnosis of small fiber neuropathies (SFNs). Our aim was to determine whether mitochondria are involved in SFN, indicating early axonal damage. We quantified mitochondrial respiratory chain complex IV (OXPHOS) and axonal (PGP 9.5) fluorescence on skin sections from 32 SFN patients and 14 healthy controls. Also, a group of six patients were recruited before and after 30-day treatment with the mitotoxic antibiotic linezolid. We measured the co-localization of OXPHOS within the intraepidermal and subpapillary dermal axons (PGP-immunoreactive [PGP-ir]). SFN patients with relatively preserved intraepidermal nerve fibers (SFN borderline) showed statistically significant reduction of OXPHOS (50.5 ± 33.9 μm(2) vs. 107.6 ± 81 μm(2) in controls, p < 0.02). A positive correlation was found between both PGP-ir and OXPHOS in controls (Pearson's coefficient r = 0.59, p < 0.001), whereas such correlation was absent in SFN. With respect to baseline measurements, linezolid therapy increased both PGP-ir and OXPHOS, which could be considered an initial compensatory toxic-induced response. This study set out to identify a possible marker of axonal pre-degenerative state in SFN borderline patients.     

Intermediate forms of Charcot-Marie-Tooth neuropathy

The Charcot-Marie-Tooth (CMT) neuropathies divide into two main electrophysiological groups with slow and near normal conduction velocities corresponding to Schwann cell and axonal pathology. An intermediate group also exists with nerve conduction velocities, which overlaps the two main groups. Families with intermediate CMT can be recognized in which different affected individuals in the same family have motor conduction velocities in both the CMT type 1 and 2 ranges (i.e., above and below 38 m/s). The intermediate group is caused by a limited number of distinct gene mutations in dynamin 2 (DNM2), gap-junction protein 1 (GJB1), neurofilament light polypeptide (NF-L) genes, and a rare mutation and as yet unknown genes on chromosome 1 and 10 loci. Intermediate forms of CMT may be associated with unique disease mechanisms affecting both Schwann cells and axons. It is useful to recognize this unique group of neuropathies for diagnostic and management purposes.     

Developmental defects and neuromuscular alterations due to mitofusin 2 gene (MFN2) silencing in zebrafish: a new model for Charcot-Marie-Tooth type 2A neuropathy

The development of new animal models is a crucial step in determining the pathological mechanism underlying neurodegenerative diseases and is essential for the development of effective therapies. We have investigated the zebrafish (Danio rerio) as a new model to study CMT2A, a peripheral neuropathy characterized by the selective loss of motor neurons, caused by mutations of mitofusin 2 gene. Using a knock-down approach, we provide evidence that during embryonic development, mitofusin 2 loss of function is responsible of several morphological defects and motility impairment. Immunohistochemical investigations, revealing the presence of severe alterations in both motor neurons and muscles fibres, indicated the central role played by MFN2 in axonal and neuromuscular development. Finally, we demonstrated the ability of human MFN2 to balance the downregulation of endogenous mfn2 in zebrafish, further supporting the conserved function of the MFN2 gene. These results highlight the essential role of mitofusin 2 in the motor axon development and demonstrate the potential of zebrafish as a suitable and complementary platform for dissecting pathogenetic mechanisms of MFN2 mutations in vivo.     

Assessment of motor unit loss in patients with spinal muscular atrophy

ObjectiveTo assess motor unit (MU) changes in patients with spinal muscular atrophy (SMA) using compound muscle action potential (CMAP) scans.MethodsWe performed CMAP scan recordings in median nerves of 24 treatment-naïve patients (median age 39; range 12–75 years) with SMA types 2–4. From each scan, we determined maximum CMAP amplitude (CMAP_max), a motor unit number estimate (MUNE), and D50 which quantifies the largest discontinuities within CMAP scans.ResultsMedian CMAP_max was 8.1 mV (range 0.9–14.6 mV), MUNE was 29 (range 6–131), and D50 was 25 (range 2–57). We found a reduced D50 (<25) in patients with normal CMAP_max (n = 12), indicating MU loss and enlarged MUs due to reinnervation. Lower D50 values were associated with decreased MUNE (P < 0.001, r = 0.68, n = 43). CMAP_max, MUNE and D50 values differed between SMA types (P < 0.001). Lower motor function scores were related to patients with lower CMAP_max, MUNE and D50 values (P < 0.001).ConclusionsThe CMAP scan is an easily applicable technique that is superior to routine assessment of CMAP_max in SMA.SignificanceThe detection of pathological MU changes across the spectrum of SMA may provide important biomarkers for evaluating disease course and monitoring treatment efficacy.     

Use of Bayesian MUNE to show differing rate of loss of motor units in subgroups of ALS

Objectives

To evaluate differences among patients with different clinical features of ALS, we used our Bayesian method of motor unit number estimation (MUNE).

Methods

We performed serial MUNE studies on 42 subjects who fulfilled the diagnostic criteria for ALS during the course of their illness. Subjects were classified into three subgroups according to whether they had typical ALS (with upper and lower motor neurone signs) or had predominantly upper motor neurone weakness with only minor LMN signs, or predominantly lower motor neurone weakness with only minor UMN signs. In all subjects we calculated the half life of MUs, defined as the expected time for the number of MUs to halve, in one or more of the abductor digiti minimi (ADM), abductor pollicis brevis (APB) and extensor digitorum brevis (EDB) muscles.

Results

The mean half life of MUs was less in subjects who had typical ALS with both upper and lower motor neurone signs than in those with predominantly upper motor neurone weakness or predominantly lower motor neurone weakness. In 18 subjects we analysed the estimated size of the MUs and demonstrated the appearance of large MUs in subjects with upper or lower motor neurone predominant weakness. We found that the appearance of large MUs was correlated with the half life of MUs.

Conclusions

Patients with different clinical features of ALS have different rates of loss and different sizes of MUs.

Significance

These findings could indicate differences in disease pathogenesis.     

Motor unit remodelling in multifocal motor neuropathy: The importance of axonal loss

Objective

To estimate the degree of axonal loss in patients diagnosed with multifocal motor neuropathy (MMN) using a novel assessment of motor unit numbers and size.

Molecular genetics of autosomal-dominant demyelinating Charcot-Marie-Tooth disease

Charcot-Marie-Tooth disease (CMT) is a clinically and genetically heterogeneous group of disorders and is the most common inherited neuromuscular disorder, with an estimated overall prevalence of 17–40/10.000. Although there has been major advances in the understanding of the genetic basis of CMT in recent years, the most useful classification is still a neurophysiological classification that divides CMT into type 1 (demyelinating, median motor conduction velocity <38 m/s) and type 2 (axonal; median motor conduction velocity> 38 m/s). An intermediate type is also increasingly being described. Inheritance can be autosomal-dominant (AD), X-linked, or autosomal-recessive (AR). ADCMT1 is the most common type of CMT and was the first form of CMT in which a causative gene was described. This review provides an up-to-date overview of AD CMT1 concentrating on the molecular genetics as the clinical, neurophysiological, and pathological features have been covered elsewhere. Four genes (PMP22, MPZ, LITAF, and EGR2) have been described in the last 15 yr associated with AD CMTI and a further gene (NEFL), originally described as causing AD CMT2 can also cause AD CMT1 (by neurophysiological criteria) (Table 1, Figs. 1, and 2). Studies have shown many of these genes, when mutated, can cause a wide range of CMT phenotypes from the relatively mild CMT1 to the more severe Dejerine-Sottas disease and congenital hypomyelinating neuropathy, and even in some cases axonal CMT2 (Table 1). This review discusses what is known about these genes and in particular how they cause a peripheral neuropathy, when mutated.     

MScanFit motor unit number estimation and muscle velocity recovery cycle recordings in diabetic polyneuropathy

ObjectiveMotor Unit Number Estimation (MUNE) methods may be valuable in tracking motor unit loss in diabetic polyneuropathy (DPN). Muscle Velocity Recovery Cycles (MVRCs) provide information about muscle membrane properties. This study aimed to examine the utility of the MScanFit MUNE in detecting motor unit loss and to test whether the MVRCs could improve understanding of DPN pathophysiology.MethodsSeventy-nine type-2 diabetic patients were compared to 32 control subjects. All participants were examined with MScanFit MUNE and MVRCs in anterior tibial muscle. Lower limb nerve conduction studies (NCS) in peroneal, tibial and sural nerves were applied to diagnose large fiber neuropathy.ResultsNCS confirmed DPN for 47 patients (DPN + ), with 32 not showing DPN (DPN−). MScanFit showed significantly decreased MUNE values and increased motor unit sizes, when comparing DPN + patients with controls (MUNE = 71.3 ± 4.7 vs 122.7 ± 3.8), and also when comparing DPN− patients (MUNE = 103.2 ± 5.1) with controls. MVRCs did not differ between groups.ConclusionsMScanFit is more sensitive in showing motor unit loss than NCS in type-2 diabetic patients, whereas MVRCs do not provide additional information.SignificanceThe MScanFit results suggest that motor changes are seen as early as sensory, and the role of axonal membrane properties in DPN pathophysiology should be revisited.     

Bayesian statistical MUNE method

We have developed a new method of motor unit number estimation (MUNE) for assessing diseases such as amyotrophic lateral sclerosis (ALS). We used data from the whole stimulus-response curve and then performed a Bayesian statistical analysis. The Bayesian method uses mathematical equations that express the basic elements of motor unit activation after electrical stimulation and allows for the sources of variability and uncertainty in this formulation. The Bayesian MUNE method was used to determine the most probable number of motor units in 8 normal subjects, 49 ALS subjects, and 3 subjects with progressive lower motor neuron (LMN) weakness. In normals the number of motor units was calculated to be 75-85 in hand and 40-58 in foot muscles. In ALS subjects the number of motor units per muscle was less than in normal subjects. In 17 ALS subjects and 3 subjects with LMN weakness the median, ulnar, or peroneal nerve was studied on repeated occasions over an average of 189 days (range 63-1,071) and the number of motor units progressively declined, with a half-life ranging from 62-834 days. The results of our MUNE technique were reproducible on replicate studies. A Bayesian statistical MUNE method is a new approach that can be used to study ALS patients serially for assessment and treatment trials.     

Mechanisms of axonal dysfunction in diabetic and uraemic neuropathies

The global burden imposed by metabolic diseases and associated complications continue to escalate. Neurological complications, most commonly peripheral neuropathy, represent a significant cause of morbidity and disability in patients with diabetes and chronic kidney disease. Furthermore, health care costs are substantially increased by the presence of complications making investigation into treatment a matter of high priority. Over the last decade nerve excitability techniques have entered the clinical realm and enabled in vivo assessment of biophysical properties and function of peripheral nerves in health and disease. Studies of excitability in diabetic neuropathy have demonstrated alteration in biophysical properties, including changes in Na⁺ conductances and Na⁺/K⁺ pump function, which may contribute to the development of neuropathic symptoms. Interventional studies have demonstrated that these changes are responsive to pharmacological agents. Excitability studies in patients with chronic kidney disease have demonstrated prominent changes that may contribute to the development of uraemic neuropathy. In particular, these studies have demonstrated strong correlation between hyperkalaemia and the development of nerve dysfunction. These studies have provided a basis for future work assessing the benefits of potassium restriction as a therapeutic strategy in this condition.     

Quality of life in patients with axonal polyneuropathy

“Quality of life” (QOL) measurement reflects the impact of a disease on the daily life of a patient, and this can be used as an outcome measure in clinical trials. QOL measurements are rarely used in patients with neuromuscular disease. The aim of this study was to determine whether QOL is reduced in chronic polyneuropathy, whether there is a relationship between QOL and objective measures of disease severity, and whether measuring QOL is a useful addition to the assessment of severity of polyneuropathy. We measured QOL in 90 patients with chronic axonal polyneuropathy (33 with hereditary motor and sensory neuropathy type II and 57 with chronic idiopathic axonal polyneuropathy) using the RAND 36-item Health Survey questionnaire (RAND-36). We compared the results with the QOL of a reference population, with summed motor and sensory scores, and with the Rankin scale for handicap. Patients had worse scores than the reference population on seven of eight areas of the RAND-36. Patients with both low motor and low sensory scores rated lower in physical and emotional areas than less impaired patients. A low Rankin score was related only to physical domains. We conclude that in patients with chronic axonal polyneuropathy the severity of disease can be assessed with a general QOL instrument, and that this provides additional information, particularly on areas related to emotional and social functioning. Received: 8 April 1999/Received in revised form: /28 September 1999/Accepted: 17 November 1999     

Motor unit number estimation in the quantitative assessment of severity and progression of motor unit loss in Hirayama disease

Objective

To investigate motor unit number estimation (MUNE) as a method to quantitatively evaluate severity and progression of motor unit loss in Hirayama disease (HD).

Co-occurrence of amyotrophic lateral sclerosis and Charcot-Marie-Tooth disease type 2A in a patient with a novel mutation in the mitofusin-2 gene

Mitofusin-2 gene (MFN2) mutations cause Charcot-Marie-Tooth type 2A (CMT2A), sometimes complicated by additional features such as optic atrophy, hearing loss, upper motor neuron signs and cerebral white-matter abnormalities. Here we report, for the first time, the occurrence of motor neuron disease, consistent with amyotrophic lateral sclerosis (ALS), in a 62-year-old woman affected by early-onset slowly progressive CMT2A, due to a novel MFN2 mutation. After age 60, rate of disease progression changed and she rapidly developed generalised muscle wasting, weakness, and fasciculations, together with dysarthria and dysphagia. Clinical features, EMG findings, and fast progression were consistent with ALS superimposed on CMT.     

Length dependent loss of motor axons and altered motor unit properties in human diabetic polyneuropathy

ObjectiveTo assess the number and properties of motor units in an upper and lower limb muscle (tibialis anterior [TA] and first dorsal interosseous [FDI]) in human diabetic polyneuropathy (DPN) using decomposition-based quantitative electromyography (DQEMG).MethodsDQEMG protocols were performed in the TA and FDI of 12 patients with confirmed diabetes mellitus and associated DPN, as well as 12 age-matched control participants. Maximal dorsiflexion strength was also assessed using a dynamometer.ResultsIn both muscles, patients with DPN had significantly reduced motor unit number estimates (MUNEs) (ΔTA ∼45%; ΔFDI ∼30%), compound muscle action potentials (CMAPs) (ΔTA ∼30%; ΔFDI ∼20%), and mean firing rates were reduced (ΔTA ∼15%; ΔFDI ∼15%) compared to controls (p < 0.05). For the TA, patients with DPN had larger mean surface motor unit potentials (SMUPs) (ΔTA ∼40%; p < 0.05), whereas in the FDI no differences were found (p > 0.05).ConclusionsDPN may result in motor unit loss, remodeling, and altered firing rate patterns. The magnitude of changes in the neuromuscular properties of DPN patients are muscle dependent and reflect a length-dependent disease progression.SignificanceDQEMG may be a clinically useful technique in identifying the presence and severity of neuromuscular pathophysiology and tracking disease progression in DPN.