Genetic diversity in hereditary axonal neuropathy: Analyzing 53 Brazilian children.

BACKGROUND AND AIMS: The genetic epidemiology of inherited neuropathies in children remains largely unknown. In this study, we specifically investigated the genetic profile of a Brazilian cohort of pediatric patients with pure or complex axonal neuropathies, a crucial knowledge in the near future for establishing treatment priorities and perspectives for this group of patients. METHODS: Fifty-three pediatric patients who were assessed prior to reaching the age of 20, and who had clinical diagnoses of axonal hereditary neuropathy or presented with axonal neuropathy as the primary clinical feature, were included in the study. The recruitment of these cases took place from January 1, 2018, to December 31, 2020. The diagnosis was based on clinical and electrophysiological data. A molecular assessment was made using target-gene panel or whole-exome sequencing. Subsequently, segregation analysis was performed on available family members, and all candidate variants found were confirmed through Sanger. RESULTS: A molecular diagnosis was reached in 68% of the patients (n = 36/53), considering only pathogenic and probably pathogenic variants. Variants in MFN2 (n = 15) and GJB1 (n = 3) accounted for half of the genetically confirmed patients (50%; n = 18/36). The other 18 genetically diagnosed patients had variants in several less common genes. INTERPRETATION: Apart from MFN2 and GJB1 genes, universally recognized as a frequent cause of axonal neuropathies in most studied population, our Brazilian cohort of children with axonal neuropathies showed an important genetic heterogeneity, probably reflecting the multi ethnicity of the Brazilian population. Diagnostic, counseling, and future interventions should consider this characteristic.

Charcot-Marie-Tooth disease: Genetic profile of patients from a large Brazilian neuromuscular reference center.

This study aimed to describe the clinical, genetic, and epidemiological features of Charcot-Marie-Tooth disease (CMT) in Brazilian patients from a tertiary center, and to compare our data with previously published findings. This retrospective observational study conducted between February 2015 and July 2020 evaluated 503 patients (94 families and 192 unrelated individuals), diagnosed with CMT. Clinical and neurophysiological data were obtained from electronic medical records and blood samples were used for genetic analyses. Multiplex ligation-dependent probe amplification was used to assess duplications/deletions in PMP22. Sanger sequencing of GJB1 was performed in cases of suspected demyelinating CMT. Targeted gene panel sequencing was used for the remaining negative demyelinating cases and all axonal CMT cases. The first decade of life was the most common period of disease onset. In all, 353 patients had demyelinating CMT, 39 had intermediate CMT, and 111 had axonal CMT. Pathogenic or likely pathogenic variants were identified in 197 index cases. The most common causative genes among probands were PMP22 (duplication) (n = 116, 58.88%), GJB1 (n = 23, 11.67%), MFN2 (n = 12, 6.09%), GDAP1 (n = 7, 3.55%), MPZ (n = 6, 3.05%), PMP22 (point mutation) (n = 6, 3.05%), NEFL (n = 3, 1.52%), SBF2 (n = 3, 1.52%), and SH3TC2 (n = 3, 1.52%). Other identified variants were ≤1% of index cases. This study provides further data on the frequency of CMT subtypes in a Brazilian clinical-based population and highlights the importance of rarer and previously undiagnosed variants in clinical practice.

Ischemic Monomelic Neuropathy: The Case for Reintroducing a Little-Known Term.

Ischemic monomelic neuropathy (IMN) is a little-known, painful axonal neuropathy, secondary to vascular occlusion or steal phenomenon. It typically occurs after vascular bypass, hemodialysis fistulization, or diabetic microvascular disease in the absence of significant clinical features of ischemia. There is limited literature to assist in the characterization and diagnosis of this condition. We describe three patients with IMN with no surgical or peripheral vascular disease history who exhibited spontaneous, persistent foot pain, edema numbness, and weakness with denervation on needle electromyogram in a distal lower leg peripheral nerve distribution. Occlusive disease was found in all patients on angiogram, requiring vascular bypass surgery.

Late-Onset Friedreich's Ataxia (LOFA) Mimicking Charcot-Marie-Tooth Disease Type 2: What Is Similar and What Is Different?

Herein, we report a patient that presented with late-onset progressive steppage gait, neuropathy and pes cavus, suggesting Charcot-Marie-Tooth (CMT) disease. Subsequent genetic investigation confirmed Friedreich's ataxia (FRDA). We demonstrate that late-onset Friedreich's ataxia (LOFA) may be a CMT mimicker. This case reinforces that other genetic conditions may clinically resemble CMT. The clinical similarities between CMT and FRDA include a symmetrical neuropathy (axonal in FRDA), steppage gait, and eventually scoliosis. We suggest that late-onset forms of hereditary neuropathies should be carefully evaluated, since LOFA may be a CMT mimicker.

Nerve conduction velocity in CMT1A: what else can we tell?

BACKGROUND AND PURPOSE: Charcot-Marie-Tooth disease (CMT) type 1A is characterized by uniformly reduced nerve conduction velocity (NCV) that is fully penetrant since the first years of life, remains fairly stable through the life and does not correlate with disability whereas compound muscular action potential (CMAP) amplitude does. The aim of the present study was to analyze the large amount of electrophysiological data collected in the ascorbic acid trial in Italy and the UK (CMT-TRIAAL/CMT-TRAUK) and to use these data to gain insights into the pathophysiology of NCV in CMT1A. METHODS: Baseline electrophysiological data from 271 patients were analysed. Electrophysiological recordings were taken from the motor ulnar, median and peroneal nerves and the sensory ulnar nerve. Distal motor latency (DML), motor (MNCV) and sensory (SNCV) nerve conduction velocity, and amplitudes of CMAPs and sensory action potentials were assessed. Electrophysiological findings were correlated with age of patients at examination and the Charcot-Marie-Tooth Examination Score (CMTES). RESULTS: NCV was markedly and uniformly reduced. CMAP amplitudes were overall reduced but more severely in lower limbs. DML decreased and MNCV and SNCV increased with age of the patients, whereas CMAP amplitudes worsened with age and also correlated with CMTES. CONCLUSIONS: This is the largest sample of electrophysiological data obtained so far from CMT1A patients. Axonal degeneration as assessed by means of CMAP amplitude reflected clinical impairment and was consistent with a slowly progressive length-dependent neuropathy. All patients typically had markedly slowed NCV that did, however, slightly increase with age of the patients. The improvement of NCV might depend on myelin thickness remodelling that occurs during the adult life of CMT1A patients.

Phenotypic convergence in Charcot-Marie-Tooth 2Y with novel VCP mutation.

Charcot-Marie-Tooth (CMT) disease, a hereditary motor and sensory neuropathy, has subtypes with varied inheritance patterns and phenotypic presentation. Subtypes additionally vary by genetic variants in a number of genes. Pathogenic variants in the VCP gene have newly been associated with CMT type 2. We present a family with CMT type 2 with a novel heterozygous VCP variant and phenotypic variability between the proband, his brother, and father.

Disease Modeling and Therapeutic Strategies in CMT2A: State of the Art.

Mitofusin 2 (MFN2) is a protein of the mitochondrial outer membrane that belongs to a family of highly conserved dynamin-related GTPases. It is implicated in several intracellular pathways; however, its main role is the regulation of mitochondrial dynamics, in particular mitochondrial fusion. Mutations in MFN2 are associated with Charcot-Marie-Tooth disease type 2A (CMT2A), a neurological disorder characterized by a wide spectrum of clinical features, primarily a motor sensory neuropathy. The cellular and molecular mechanisms by which MFN2 mutations lead to neuronal degeneration are largely unknown, and there is currently no cure for patients. Here, we present the most recent in vitro and in vivo models of CMT2A and the more promising therapeutic approaches under development. These models and therapies may represent relevant tools for the study and recovery of defective mitochondrial dynamics that seem to play a significant role in the pathogenesis of other more common neurodegenerative diseases.

Peripheral neuropathies and aging.

Neuropathies are very common in the population over 65 years old and their prevalence increases with age. The prevalence of polyneuropathies is about 7% in the elderly. However, other possible neuropathies should not be overlooked. Diabetes mellitus is the first cause of neuropathy in the world. Beyond 80 years-old, the risk of finding no etiology is about 40% but it implies a minimum initial explorations. A vitamin deficiency, a dysimmune or even hereditary cause should be eliminated in the elderly subject, keeping in mind the characteristics of age. We propose a review of the different forms of neuropathies by emphasizing the clinical or electrophysiological particularities of the elderly.

Autophagy as an Emerging Common Pathomechanism in Inherited Peripheral Neuropathies.

The inherited peripheral neuropathies (IPNs) comprise a growing list of genetically heterogeneous diseases. With mutations in more than 80 genes being reported to cause IPNs, a wide spectrum of functional consequences is expected to follow this genotypic diversity. Hence, the search for a common pathomechanism among the different phenotypes has become the holy grail of functional research into IPNs. During the last decade, studies on several affected genes have shown a direct and/or indirect correlation with autophagy. Autophagy, a cellular homeostatic process, is required for the removal of cell aggregates, long-lived proteins and dead organelles from the cell in double-membraned vesicles destined for the lysosomes. As an evolutionarily highly conserved process, autophagy is essential for the survival and proper functioning of the cell. Recently, neuronal cells have been shown to be particularly vulnerable to disruption of the autophagic pathway. Furthermore, autophagy has been shown to be affected in various common neurodegenerative diseases of both the central and the peripheral nervous system including Alzheimer's, Parkinson's, and Huntington's diseases. In this review we provide an overview of the genes involved in hereditary neuropathies which are linked to autophagy and we propose the disruption of the autophagic flux as an emerging common pathomechanism. We also shed light on the different steps of the autophagy pathway linked to these genes. Finally, we review the concept of autophagy being a therapeutic target in IPNs, and the possibilities and challenges of this pathway-specific targeting.

Early onset (childhood) monogenic neuropathies.

Hereditary neuropathies (HN) with onset in childhood are categorized according to clinical presentation, pathogenic mechanism based on electrophysiology, genetic transmission and, in selected cases, pathological findings. Especially relevant to pediatrics are the items "secondary" versus "primary" neuropathy, "syndromic versus nonsyndromic," and "period of life." Different combinations of these parameters frequently point toward specific monogenic disorders. Ruling out a neuropathy secondary to a generalized metabolic disorder remains the first concern in pediatrics. As a rule, metabolic diseases include additional, orienting symptoms or signs, and their biochemical diagnosis is based on logical algorithms. Primary, motor sensory are the most frequent HN and are dominated by demyelinating autosomal dominant (AD) forms (CMT1). Other forms include demyelinating autosomal recessive (AR) forms, axonal AD/AR forms, and forms with "intermediate" electrophysiological phenotype. Peripheral motor neuron disorders are dominated by AR SMN-linked spinal muscular atrophies. (Distal) hereditary motor neuropathies represent <10% of HN but exhibit large clinical and genetic heterogeneity. Sensory/dysautonomic HN involves five classic subtypes, each one related to specific genes. However, genetic heterogeneity is larger than initially suspected. Syndromic HN distinguish "purely neurological syndromes", which are multisystemic, such as spinocerebellar atrophies +, spastic paraplegias +, etc. Peripheral neuropathy is possibly the presenting feature, including in childhood. Autosomal recessive forms, on average, start more frequently in childhood. "Multiorgan syndromes", on the other hand, are more specific to Pediatrics. AR forms, which are clearly degenerative, prompt the investigation of a large set of pleiotropic genes. Other syndromes expressed in the perinatal period are mainly developmental disorders, and can sometimes be related to specific transcription factors. Systematic malformative workup and ethical considerations are necessary. Altogether, >40 genes with various biological functions have been found to be responsible for primary HN. Many are responsible for various phenotypes, including some without the polyneuropathic trait, and some for various types of transmission.