Two novel mutations of GARS in Korean families with distal hereditary motor neuropathy type V

Glycyl‐tRNA synthetase (GARS), which encodes the enzyme responsible for charging tRNA(Gly) with glycine in both the cytoplasm and mitochondria, is implicated to Charcot‐Marie‐Tooth disease 2D (CMT2D) and distal hereditary motor neuropathy type V (dHMN‐V). We performed whole exome sequencing (WES) to identify the genetic defects in the two dHMN families. WES revealed several decades of non‐synonymous variants in the CMT and aminoacyl‐tRNA synthetase genes. The subsequent capillary sequencing for family members and controls revealed two novel causative mutations, c.598G>A (D200N) and c.794C>T (S265F), in the GARS gene in each dHMN family. Both mutations were cosegregated with affected individuals in each family, and were not found in the 200 controls. The mutation sites were well conserved between the different species and in silico analysis predicted that both mutations may affect protein function. Therefore, we believe that these two novel GARS mutations are the underlying causes of the dHMN phenotype.     

A novel autosomal dominant GDAP1 mutation in an Italian CMT2 family

We report the clinical, electrophysiological, and skin biopsy findings of an Italian Charcot-Marie-Tooth disease type 2 (CMT2) family with a novel heterozygous GDAP1 mutation. We observed a marked intra-familial phenotypic variability, in age at onset and disease severity which ranged from a typical CMT phenotype to an asymptomatic status. Electrophysiological study, consistent with an axonal sensory-motor neuropathy, confirmed a different degree of severity and disclosed minimal electrophysiological abnormalities also in the asymptomatic subjects. Skin biopsy findings showed a variable loss of large and small somatic nerve fibers. Molecular analysis identified a novel heterozygous missense mutation (Arg120Gly) in the GDAP1 gene which co-segregated with the disease within the pedigree. In conclusion, our findings confirm that the GDAP1 autosomal dominant mutations underlie a pronounced phenotypic variability, mimicking the effects of reduced penetrance. Notably, electrophysiological study in this family allowed to reveal hidden positive family history and assess a dominant inheritance pattern, revealing subclinical neuropathy in asymptomatic mutation carriers.     

Characteristics of clinical and electrophysiological pattern of Charcot-Marie-Tooth 4C

To describe the clinical and electrophysiological features evoking CMT4C, an autosomal recessive (AR) form of Charcot-Marie-Tooth disease (CMT) due to mutations in the SH3TC2 gene, we screened the coding sequence of SH3TC2 gene in 102 unrelated patients with a demyelinating or intermediate CMT and a family history compatible with an AR transmission. We identified among this cohort 16 patients carrying two mutations in the SH3TC2 gene, but medical records finally analyzed 14 patients. We report clinical, electrophysiological, and molecular data of 14 patients (9 men, 5 women) with CMT4C. Mean age at examination was 43.6 years (median = 42.5). Among the 14 studied cases 6 had scoliosis as the presenting sign. Cranial nerve involvement affecting either the VIIIth, VIIth, XIIth or a combination of the IXth and Xth nerves was noted in 10 patients. Remarkably, 50% of the patients had proximal limb involvement at the time of examination. The hallmark of the electrophysiological study was the presence of probable conduction block and temporal dispersion. Thus the clinical and paraclinical spectrum of CMT4C can guide the clinician to perform analysis of the SH3TC2 gene.     

Demyelinating prenatal and infantile developmental neuropathies

The prenatal and infantile neuropathies are an uncommon and complex group of conditions, most of which are genetic. Despite advances in diagnostic techniques, approximately half of children presenting in infancy remain without a specific diagnosis. This review focuses on inherited demyelinating neuropathies presenting in the first year of life. We clarify the nomenclature used in these disorders, review the clinical features of demyelinating forms of Charcot-Marie-Tooth disease with early onset, and discuss the demyelinating infantile neuropathies associated with central nervous system involvement. Useful clinical, neurophysiologic, and neuropathologic features in the diagnostic work-up of these conditions are also presented.     

Insights into the pathogenesis of ATP1A1‐related CMT disease using patient‐specific iPSCs

The development of patient‐specific induced pluripotent stem cells (iPSCs) offered interesting insights in modeling the pathogenesis of Charcot‐Marie‐Tooth (CMT) disease and thus we decided to explore the phenotypes of iPSCs derived from a single CMT patient carrying a mutant ATP1A1 allele (p.Pro600Ala). iPSCs clones generated from CMT and control fibroblasts, were induced to differentiate into neural precursors and then into post‐mitotic neurons. Control iPSCs differentiated into neuronal precursors and then into post‐mitotic neurons within 6‐8 days. On the contrary, the differentiation of CMT iPSCs was clearly defective. Electrophysiological properties confirmed that post‐mitotic neurons were less mature compared to the normal counterpart. The impairment of in vitro differentiation of CMT iPSCs only concerned with the neuronal pathway, because they were able to differentiate into mesendodermal cells and other ectodermal derivatives. ATP1A1 was undetectable in the few neuronal cells derived from CMT iPSCs. ATP1A1 gene mutation (p.Pro600Ala), responsible for a form of axonal CMT disease, is associated in vitro with a dramatic alteration of the differentiation of patient‐derived iPSCs into post‐mitotic neurons. Thus, the defect in neuronal cell development might lead in vivo to a decreased number of mature neurons in ATP1A1‐CMT disease.     

Inherited peripheral neuropathies: what does a paediatrician need to know?

Collectively, inherited peripheral neuropathies (IPN) are relatively common in children. They have a combined estimated incidence of 1 in 2500 live births. Therefore, it is likely that most paediatricians and many healthcare professionals working in primary care settings will encounter children with IPN during their careers. This article outlines when to suspect an IPN, the common manner of presentation and how to diagnose and classify these conditions in children. It offers advice on the differential diagnosis, the genetic aspects of childhood IPN and the importance of genetic counselling. The modern aspects of diagnosis, which often use next generation sequencing to rapidly diagnose both common and rare IPNs, are considered. The common surveillance measures and treatment modalities together with the basic science developments that underpin future treatments for IPNs are discussed.     

DGAT2 Mutation in a Family with Autosomal‐Dominant Early‐Onset Axonal Charcot‐Marie‐Tooth Disease

Charcot‐Marie‐Tooth disease (CMT) is the most common inherited peripheral neuropathy and is a genetically and clinically heterogeneous disorder. We examined a Korean family in which two individuals had an autosomal‐dominant axonal CMT with early‐onset, sensory ataxia, tremor, and slow disease progression. Pedigree analysis and exome sequencing identified a de novo missense mutation (p.Y223H) in the diacylglycerol O‐acyltransferase 2 (DGAT2) gene. DGAT2 encodes an endoplasmic reticulum‐mitochondrial‐associated membrane protein, acyl‐CoA:diacylglycerol acyltransferase, which catalyzes the final step of the triglyceride (TG) biosynthesis pathway. The patient showed consistently decreased serum TG levels, and overexpression of the mutant DGAT2 significantly inhibited the proliferation of mouse motor neuron cells. Moreover, the variant form of human DGAT2 inhibited the axonal branching in the peripheral nervous system of zebrafish. We suggest that mutation of DGAT2 is the novel underlying cause of an autosomal‐dominant axonal CMT2 neuropathy. This study will help provide a better understanding of the pathophysiology of axonal CMT and contribute to the molecular diagnostics of peripheral neuropathies.