ATP1A1-linked diseases require a malfunctioning protein product from one allele

Heterozygous germline variants in ATP1A1, the gene encoding the α1 subunit of the Na+/K+-ATPase (NKA), have been linked to diseases including primary hyperaldosteronism and the peripheral neuropathy Charcot-Marie-Tooth disease (CMT). ATP1A1 variants that cause CMT induce loss-of-function of NKA. This heterodimeric (αβ) enzyme hydrolyzes ATP to establish transmembrane electrochemical gradients of Na+ and K+ that are essential for electrical signaling and cell survival. Of the 4 catalytic subunit isoforms, α1 is ubiquitously expressed and is the predominant paralog in peripheral axons. Human population sequencing datasets indicate strong negative selection against both missense and protein-null ATP1A1 variants. To test whether haploinsufficiency generated by heterozygous protein-null alleles are sufficient to cause disease, we tested the neuromuscular characteristics of heterozygous Atp1a1+/- knockout mice and their wildtype littermates, while also evaluating if exercise increased CMT penetrance. We found that Atp1a1+/- mice were phenotypically normal up to 18 months of age. Consistent with the observations in mice, we report clinical phenotyping of a healthy adult human who lacks any clinical features of known ATP1A1-related diseases despite carrying a plasma-membrane protein-null early truncation variant, p.Y148*. Taken together, these results suggest that a malfunctioning gene product is required for disease induction by ATP1A1 variants and that if any pathology is associated with protein-null variants, they may display low penetrance or high age of onset.

Strategy for genetic analysis in hereditary neuropathy

Inherited neuropathies are a heterogeneous group of slowly progressive disorders affecting either motor, sensory, and/or autonomic nerves. Peripheral neuropathy may be the major component of a disease such as Charcot-Marie-Tooth disease or a feature of a more complex multisystemic disease involving the central nervous system and other organs. The goal of this review is to provide the clinical clues orientating the genetic diagnosis in a patient with inherited peripheral neuropathy. This review focuses on primary inherited neuropathies, amyloidosis, inherited metabolic diseases, while detailing clinical, neurophysiological and potential treatment of these diseases.

Treatment of Charcot-Marie-Tooth neuropathies

Charcot-Marie-Tooth (CMT) is a heterogeneous group of inherited neuropathies that affect the peripheral nerves and slowly cause progressive disability. Currently, there is no effective therapy. Patients' management is based on rehabilitation and occupational therapy, fatigue, and pain treatment with regular follow-up according to the severity of the disease. In the last three decades, much progress has been made to identify mutations involved in the different types of CMT, decipher the pathophysiology of the disease, and identify key genes and pathways that could be targeted to propose new therapeutic strategies. Genetic therapy is one of the fields of interest to silence genes such as PMP22 in CMT1A or to express GJB1 in CMT1X. Among the most promising molecules, inhibitors of the NRG-1 axis and modulators of UPR or the HDACs enzyme family could be used in different types of CMT.

Hereditary neuropathy

The hereditary neuropathies, collectively referred as Charcot-Marie-Tooth disease (CMT) and related disorders, are heterogeneous genetic peripheral nerve disorders that collectively comprise the commonest inherited neurological disease with an estimated prevalence of 1:2500 individuals. The field of hereditary neuropathies has made significant progress in recent years with respect to both gene discovery and treatment as a result of next-generation sequencing (NGS) approach. These investigations which have identified over 100 causative genes and new mutations have made the classification of CMT even more challenging. Despite so many different mutated genes, the majority of CMT forms share a similar clinical phenotype, and due to this phenotypic homogeneity, genetic testing in CMT is increasingly being performed through the use of NGS panels. The majority of patients still have a mutation in one the four most common genes (PMP22 duplication-CMT1A, MPZ-CMT1B, GJB1-CMTX1, and MFN2-CMT2A). This chapter focuses primarily on these four forms and their potential therapeutic approaches.

Replicating and redesigning ankle-foot orthoses with 3D printing for children with Charcot-Marie-Tooth disease

BACKGROUND

Children with the most common inherited neuropathy, Charcot-Marie-Tooth disease (CMT), are often prescribed ankle-foot orthoses (AFOs) to improve walking ability and prevent falls by reducing foot drop, postural instability, and other gait impairments. These externally worn assistive devices are traditionally custom-made using thermoplastic vacuum forming. This labour-intensive manufacturing process often results in AFOs which are cumbersome due to limited design options, and are associated with low acceptability, discomfort, and suboptimal impact on gait. The aim of this study was to determine how 3D printing can be used to replicate and redesign AFOs in children with CMT.

METHODS

Traditional AFOs, 3D printed replica AFOs (same design as traditional AFOs), 3D printed redesigned AFOs and a shoes only control condition were compared in 12 children with CMT. 3D printed AFOs were manufactured using material extrusion in Nylon-12. 3D gait analysis (temporal-spatial, kinematic, kinetic), in-shoe pedobarography and self-reported satisfaction were used to compare conditions. The primary kinematic and kinetic outcome measures were maximum ankle dorsiflexion in swing and maximum ankle dorsiflexor moment in loading response, to capture foot drop and an absent of heel rocker.

RESULTS

The 3D printed replica AFOs were comparable to traditional AFOs for all outcomes. The 3D printed replica AFOs improved foot position at initial contact and during loading response and significantly reduced pressure beneath the whole foot, rearfoot and forefoot compared to the shoes only. The 3D printed redesigned AFOs produced a device that was significantly lighter (mean -35.2, SD 13.3%), and normalised maximum ankle dorsiflexor moment in loading response compared to shoes only and traditional AFOs.

SIGNIFICANCE

3D printing can be used to replicate traditional handmade AFOs and to redesign AFOs to produce a lighter device with improved biomechanics by incorporating novel design features.

Characterization of a Portuguese family with Charcot-Marie-Tooth disease type 1E due to a novel point mutation in the PMP22 gene

INTRODUCTION

Point mutations in the Peripheral Myelin Protein 22 (PMP22) gene comprise less than 5% of the Charcot-Marie-Tooth (CMT) type 1 cases, and individualize either the CMT 1E subtype, or Hereditary Neuropathy with Liability to Pressure Palsy. The phenotype of CMT 1E presents with a severe early-onset polyneuropathy associated with deafness, although the clinical spectrum is broad.

CASE REPORT

We describe a novel PMP22 gene point mutation (c.84G>T;p.(Trp28Cys)) in three patients of a Portuguese family with variable phenotypes, ranging from asymptomatic to mild complaints of distal limb numbness and gait difficulties, with the age of onset of symptoms ranging from mid-twenties to late-sixties, and no associated disability. In all affected patients, there was evidence of diffuse demyelinating sensorimotor polyneuropathy. Hearing loss does not seem to be associated with this variant, albeit neuropathic pain was reported.

CONCLUSIONS

These findings suggest that this particular point mutation in the PMP22 gene is associated with a mild phenotype, further emphasizing that there are still unknown mechanisms (genetic and/or epigenetic) that may play a role in the clinical spectrum of CMT1E patients. Next generation sequencing panels including commonly mutated genes in CMT should be considered in CMT1 cases negative for PMP22 gene duplication.

Genetic neuropathies presenting with CIDP-like features in childhood

Inherited neuropathies are amongst the most common neuromuscular disorders. The distinction from chronic inflammatory demyelinating polyneuropathy (CIDP) may be challenging, considering its rarity in childhood, that genetic neuropathies may show secondary inflammatory features, and that subacute CIDP presentations may closely mimic the disease course of inherited disorders. The overlap between genetic neuropathies and CIDP is increasingly recognized in adults but rarely reported in children. Here we report 4 children with a neuropathy of subacute onset, initially considered consistent with an immune-mediated neuropathy based on suggestive clinical, laboratory and neurophysiological features. None showed convincing response to intravenous immunoglobulin therapy, leading to re-evaluation and confirmation of a genetic neuropathy in each case (including PMP22, MPZ and SH3TC2 genes). A review of the few Paediatric cases reported in the literature showed similar delays in diagnosis and no significant changes to immunomodulatory treatment. Our findings emphasize the importance of considering an inherited neuropathy in children with a CIDP-like presentation. In addition to an inconclusive response to treatment, subtle details of the family and developmental history may indicate a genetic rather than an acquired background. Correct diagnostic confirmation of a genetic neuropathy in a child is crucial for appropriate management, prognostication and genetic counselling.

Identification of a rare SEPT9 variant in a family with autosomal dominant Charcot-Marie-Tooth disease

Background

Charcot-Marie-Tooth disease (CMT) is one of the most commonly inherited neurological disorders. A growing number of genes, involved in glial and neuronal functions, have been associated with different subtypes of CMT leading to improved diagnostics and understanding of pathophysiological mechanisms. However, some patients and families remain genetically unsolved.

Methods

We report on a German family including four affected members over three generations with a CMT phenotype accompanied by cognitive deficits, predominantly with regard to visual abilities and episodic memory.

Results

A comprehensive clinical characterization followed by a sequential diagnostic approach disclosed a heterozygous rare SEPT9 missense variant c.1406 T > C, p.(Val469Ala), that segregates with disease. SEPT9 has been linked to various intracellular functions, such as cytokinesis and membrane trafficking. Interestingly, SEPT9-mutations are known to cause hereditary neuralgic amyotrophy (HNA), a recurrent focal peripheral neuropathy.

Conclusion

We, for the first time, present a SEPT9 variant associated to a CMT phenotype and suggest SEPT9 as new sufficient candidate gene in CMT.

Nerve ultrasound findings differentiate Charcot-Marie-Tooth disease (CMT) 1A from other demyelinating CMTs. Clin Neurophysiol. 2018;129:2259-2267. [PMID: 30216910 DOI: 10.1016/j.clinph.2018.08.016]

OBJECTIVE

Ulnar/median motor nerve conduction velocity (MNCV) is ≤38 m/s in demyelinating Charcot-Marie-Tooth disease (CMT). Previous nerve high resolution ultrasound (HRUS) studies explored demyelinating CMT assuming it as a homogeneous genetic/pathological entity or focused on CMT1A.

METHODS

To explore the spectrum of nerve HRUS findings in demyelinating CMTs, we recruited patients with CMT1A (N = 44), CMT1B (N = 9), CMTX (N = 8) and CMT4C (N = 4). They underwent nerve conduction study (NCS) and HRUS of the median, ulnar, peroneal nerve, and the brachial plexus.

RESULTS

Median, ulnar and peroneal MNCV significantly differed across CMT subtypes. Cross sectional area (CSA) was markedly and diffusely enlarged at all sites, except entrapment ones, in CMT1A, while it was slightly enlarged or within normal range in the other CMTs. No significant right-to-left difference was found. Age had limited effect on CSA. CSAs of some CMT1A patients largely overlapped with those of other demyelinating CMTs. A combination of three median CSA measures could separate CMT1A from other demyelinating CMTs.

CONCLUSIONS

Nerve HRUS findings are heterogeneous in demyelinating CMTs.

SIGNIFICANCE

Nerve HRUS may separate CMT1A from other demyelinating CMTs. The large demyelinating CMTs HRUS spectrum may be related to its pathophysiological variability.

SBF1 mutations associated with autosomal recessive axonal neuropathy with cranial nerve involvement

Biallelic mutations in the SBF1 gene have been identified in one family with demyelinating Charcot-Marie-Tooth disease (CMT4B3) and two families with axonal neuropathy and additional neurological and skeletal features. Here we describe novel sequence variants in SBF1 (c.1168C>G and c.2209_2210del) as the potential causative mutations in two siblings with severe axonal neuropathy, hearing loss, facial weakness and bulbar features. Pathogenicity of these variants is supported by co-segregation and in silico analyses and evolutionary conservation. Our findings suggest that SBF1 mutations may cause a syndromic form of autosomal recessive axonal neuropathy (AR-CMT2) in addition to CMT4B3.

Biomechanical effects of sensorimotor orthoses in adults with Charcot-Marie-Tooth disease

BACKGROUND

Charcot-Marie-Tooth disease is an inherited neuropathy causing progressive weakness, foot deformity and difficulty walking. Clinical anecdotes suggest orthoses designed on the 'sensorimotor' paradigm are beneficial for improving gait in Charcot-Marie-Tooth disease.

OBJECTIVES

Investigate the effect of sensorimotor orthoses on in-shoe and lower limb biomechanics in adults with Charcot-Marie-Tooth disease.

STUDY DESIGN

Randomised, repeated-measures, exploratory study.

METHODS

Eight males and two females with Charcot-Marie-Tooth disease aged 31-68 years fitted with pedorthic shoes and custom-made sensorimotor orthoses were randomly tested at baseline and after 4 weeks of adaptation. In-shoe three-dimensional multi-segment foot and lower limb kinematics and kinetics were collected as were plantar pressures, electromyography and self-reported comfort, stability, cushioning and preference.

RESULTS

Compared to the shoe only condition, sensorimotor orthoses increased midfoot eversion and plantarflexion, increased ankle eversion and produced small but significant changes at the knee and hip indicating increased internal rotation. The orthoses increased medial ground reaction forces and increased pressure at the heel, midfoot and toes. There were minimal effects on electromyography. The sensorimotor orthoses were rated higher for comfort, cushioning, stability and preference.

CONCLUSION

Sensorimotor orthoses produced changes in kinematic, kinetic and pressure variables in adults with Charcot-Marie-Tooth disease and were regarded as more comfortable, cushioned and stable during walking.

CLINICAL RELEVANCE

In this study, the walking ability of patients with Charcot-Marie-Tooth disease improved with the use of foot orthoses designed according to the sensorimotor paradigm. However, the mechanism of action appears to be primarily mechanical in origin. Randomised controlled trials are necessary to evaluate the long-term patient-reported outcomes of sensorimotor orthoses.

Molecular regulators of nerve conduction - Lessons from inherited neuropathies and rodent genetic models

Myelinated nerve fibers are highly compartmentalized. Helically wrapped lipoprotein membranes of myelin are integrated with subsets of proteins specifically in each compartment to shape the physiological behavior of these nerve fibers. With the advance of molecular biology and genetics, many functions of these proteins have been revealed over the past decade. In this review, we will first discuss how action potential propagation has been understood by classical electrophysiological studies. In particular, the discussion will be concentrated on how the geometric dimensions of myelinated nerve fibers (such as internodal length and myelin thickness) may affect nerve conduction velocity. This discussion will then extend into how specific myelin proteins may shape these geometric parameters, thereby regulating action potential propagation. For instance, periaxin may specifically affect the internodal length, but not other parameters. In contrast, neuregulin-1 may affect myelin thickness, but not axon diameter or internodal length. Finally, we will discuss how these basic neurobiological observations can be applied to inherited peripheral nerve diseases.