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

New evidence for secondary axonal degeneration in demyelinating neuropathies

Development of peripheral nervous system (PNS) myelin involves a coordinated series of events between growing axons and the Schwann cell (SC) progenitors that will eventually ensheath them. Myelin sheaths have evolved out of necessity to maintain rapid impulse propagation while accounting for body space constraints. However, myelinating SCs perform additional critical functions that are required to preserve axonal integrity including mitigating energy consumption by establishing the nodal architecture, regulating axon caliber by organizing axonal cytoskeleton networks, providing trophic and potentially metabolic support, possibly supplying genetic translation materials and protecting axons from toxic insults. The intermediate steps between the loss of these functions and the initiation of axon degeneration are unknown but the importance of these processes provides insightful clues. Prevalent demyelinating diseases of the PNS include the inherited neuropathies Charcot-Marie-Tooth Disease, Type 1 (CMT1) and Hereditary Neuropathy with Liability to Pressure Palsies (HNPP) and the inflammatory diseases Acute Inflammatory Demyelinating Polyneuropathy (AIDP) and Chronic Inflammatory Demyelinating Polyneuropathy (CIDP). Secondary axon degeneration is a common feature of demyelinating neuropathies and this process is often correlated with clinical deficits and long-lasting disability in patients. There is abundant electrophysiological and histological evidence for secondary axon degeneration in patients and rodent models of PNS demyelinating diseases. Fully understanding the involvement of secondary axon degeneration in these diseases is essential for expanding our knowledge of disease pathogenesis and prognosis, which will be essential for developing novel therapeutic strategies.

Recent Advances in Drosophila Models of Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth disease (CMT) is one of the most common inherited peripheral neuropathies. CMT patients typically show slowly progressive muscle weakness and sensory loss in a distal dominant pattern in childhood. The diagnosis of CMT is based on clinical symptoms, electrophysiological examinations, and genetic testing. Advances in genetic testing technology have revealed the genetic heterogeneity of CMT; more than 100 genes containing the disease causative mutations have been identified. Because a single genetic alteration in CMT leads to progressive neurodegeneration, studies of CMT patients and their respective models revealed the genotype-phenotype relationships of targeted genes. Conventionally, rodents and cell lines have often been used to study the pathogenesis of CMT. Recently, Drosophila has also attracted attention as a CMT model. In this review, we outline the clinical characteristics of CMT, describe the advantages and disadvantages of using Drosophila in CMT studies, and introduce recent advances in CMT research that successfully applied the use of Drosophila, in areas such as molecules associated with mitochondria, endosomes/lysosomes, transfer RNA, axonal transport, and glucose metabolism.

Gene replacement therapy after neuropathy onset provides therapeutic benefit in a model of CMT1X

X-linked Charcot-Marie-Tooth disease (CMT1X), one of the commonest forms of inherited demyelinating neuropathy, results from GJB1 gene mutations causing loss of function of the gap junction protein connexin32 (Cx32). The aim of this study was to examine whether delayed gene replacement therapy after the onset of peripheral neuropathy can provide a therapeutic benefit in the Gjb1-null/Cx32 knockout model of CMT1X. After delivery of the LV-Mpz.GJB1 lentiviral vector by a single lumbar intrathecal injection into 6-month-old Gjb1-null mice, we confirmed expression of Cx32 in lumbar roots and sciatic nerves correctly localized at the paranodal myelin areas. Gjb1-null mice treated with LV-Mpz.GJB1 compared with LV-Mpz.Egfp (mock) vector at the age of 6 months showed improved motor performance at 8 and 10 months. Furthermore, treated mice showed increased sciatic nerve conduction velocities, improvement of myelination and reduced inflammation in lumbar roots and peripheral nerves at 10 months of age, along with enhanced quadriceps muscle innervation. Plasma neurofilament light (NEFL) levels, a clinically relevant biomarker, were also ameliorated in fully treated mice. Intrathecal gene delivery after the onset of peripheral neuropathy offers a significant therapeutic benefit in this disease model, providing a proof of principle for treating patients with CMT1X at different ages.

Novel gap junction protein beta-1 gene mutation associated with a stroke-like syndrome and central nervous system involvement in patients with X-linked Charcot-Marie-Tooth Type 1: A case report and literature review

Gap junction protein beta-1 (GJB1) gene mutations lead to X-linked Charcot-Marie-Tooth Type 1 (CMTX1). We studied a Chinese family with CMTX1 and identified a novel GJB1 point mutation. Our patient had a transient stroke-like clinical manifestations and magnetic resonance imaging (MRI) changes. An analysis of the genomic DNA of the proband showed a T to C hemizygous mutation in the GJB1 gene at nucleotide position 380, causing a predicted amino acid change from isoleucine to threonine at codon 127, which predicted structural alterations disrupting the function of the GJB1 protein. This novel point mutation expanded the spectrum of GJB1 mutations known to be associated with CMTX1. We performed a PubMed review of CMTX cases with central nervous system involvement in the English-language literature from the past 20 years, and summarized the demographic data, nucleotide and amino acid changes, clinical characteristics, clinical manifestations, and neuroimaging features.

Towards understanding the schizophrenia code: an expanded convergent functional genomics approach

Identifying genes for schizophrenia through classical genetic approaches has proven arduous. Here, we present a comprehensive convergent analysis that translationally integrates brain gene expression data from a relevant pharmacogenomic mouse model (involving treatments with a psychomimetic agent - phencyclidine (PCP), and an anti-psychotic - clozapine), with human genetic linkage data and human postmortem brain data, as a Bayesian strategy of cross validating findings. Topping the list of candidate genes, we have three genes involved in GABA neurotransmission (GABRA1, GABBR1, and GAD2), one gene involved in glutamate neurotransmission (GRIA2), one gene involved in neuropeptide signaling (TAC1), two genes involved in synaptic function (SYN2 and KCNJ4), six genes involved in myelin/glial function (CNP, MAL, MBP, PLP1, MOBP and GFAP), and one gene involved in lipid metabolism (LPL). These data suggest that schizophrenia is primarily a disorder of brain functional and structural connectivity, with GABA neurotransmission playing a prominent role. These findings may explain the EEG gamma band abnormalities detected in schizophrenia. The analysis also revealed other high probability candidates genes (neurotransmitter signaling, other structural proteins, ion channels, signal transduction, regulatory enzymes, neuronal migration/neurite outgrowth, clock genes, transcription factors, RNA regulatory genes), pathways and mechanisms of likely importance in pathophysiology. Some of the pathways identified suggest possible avenues for augmentation pharmacotherapy of schizophrenia with other existing agents, such as benzodiazepines, anticonvulsants and lipid modulating agents. Other pathways are new potential targets for drug development. Lastly, a comparison with our earlier work on bipolar disorder illuminates the significant molecular overlap between schizophrenia and bipolar disorder.

X-linked Charcot-Marie-Tooth disease (CMTX) in a severely affected female patient with scattered lesions in cerebral white matter

Charcot-Marie-Tooth neuropathy (CMT) is an inherited degenerative disorder of the peripheral nervous system that results in slowly progressive distal muscle weakness, atrophy and loss of proprioception in the affected areas. X-linked CMT (CMTX) has been localized to the pericentric region of the X chromosome. CMTX neuropathy is usually associated with mutations in exon 2 of the gap junction protein beta1 (GJB1) gene. GJB1 is a gap junction protein expressed in various cells including oligodendrocytes, astrocytes and myelinating schwann cells. Here, we report a female case of CMTX with a GJB1 mutation. The patient was severely clinically affected and exhibited both the features of demyelination and axonopathy. This is the first female patient with CMTX who showed permanent atypical scattered lesions in cerebral white matter of the brain on T2-weighted magnetic resonance images (MRI), which is very rare. The existence of a female patient with severe clinical symptoms may show that gain of function mechanism also leads to the disorders seen in these patients.

Clinical and molecular analysis of X-linked Charcot-Marie-Tooth disease type 1 in Spanish population

From 1995 to 2004, 979 families with hereditary peripheral neuropathy were referred to the Genetic Diagnosis Center. Using single-strand conformation analysis (SSCA), the connexin 32 gene was analysed in all the patients from 498 families with sporadic or dominant inheritance with no male-to-male transmission and absence of the 17p2 duplication or deletion. Affected males had pes cavus, distal leg weakness, muscular distal atrophy, areflexia and distal sensory loss. The 106 families in which SSCA revealed abnormal migration electrophoresis were directly sequenced. We found 34 families (59 patients) with mutations in connexin 32 gene. In electrophysiological studies, 58.8% families presented slow and 14.7% intermediate nerve conduction velocities. Molecular findings revealed that codon 164 (29.4 +/- 15.3%) and the second extracellular (EC2) domain (44.1 +/- 16.6%) were the most frequently affected codon and domain of the connexin 32. Six novel mutations, Leu39fs, Glu47Gly, His153fs, Cys179Tyr, Cys201Phe and Ser211fs, were found in our study.

Molecular genetics of X-linked Charcot-Marie-Tooth disease

The X-linked form of Charcot-Marie-Tooth disease (CMT1X) is the second most common molecularly designated form of hereditary motor and sensory neuropathy. The clinical phenotype is characterized by progressive distal muscle atrophy and weakness, areflexia, and variable sensory abnormalities. Affected males have moderate-to-severe symptoms, whereas heterozygous females are usually mildly affected or even asymptomatic. Several patients also have manifestations of central nervous system involvement or hearing impairment. Electrophysiological and pathological studies of peripheral nerves show evidence of demyelinating neuropathy with prominent axonal degeneration. A large number of mutations in the GJB1 gene encoding the gap junction (GJ) protein connexin32 (Cx32) cause CMT1X. Cx32 is expressed by Schwann cells and oligodendrocytes, as well as by other tissues, and the GJ formed by Cx32 play an important role in the homeostasis of myelinated axons. The reported CMT1X mutations are diverse and affect both the promoter region as well as the coding region of GJB1. Many Cx32 mutants fail to form functional GJ, or form GJ with abnormal biophysical properties. Furthermore, Cx32 mutants are often retained intracellularly either in the endoplasmic reticulum or Golgi in which they could potentially have additional dominant-negative effects. Animal models of CMT1X demonstrate that loss of Cx32 in myelinating Schwann cells causes a demyelinating neuropathy. No definite phenotype-genotype correlation has yet been established for CMT1X and effective molecular based therapeutics for this disease, remain to be developed.