Charcot-Marie-Tooth disease type 4C (CMT4C) is a childhood-onset demyelinating form of hereditary motor and sensory neuropathy associated with an early-onset scoliosis and a distinct Schwann cell pathology. CMT4C is inherited as an autosomal recessive trait and has been mapped to a 13-cM linkage interval on chromosome 5q23-q33. By homozygosity mapping and allele-sharing analysis, we refined the CMT4C locus to a suggestive critical region of 1.7 Mb. We subsequently identified mutations in an uncharacterized transcript, KIAA1985, in 12 families with autosomal recessive neuropathy. We observed eight distinct protein-truncating mutations and three nonconservative missense mutations affecting amino acids conserved through evolution. In all families, we identified a mutation on each disease allele, either in the homozygous or in the compound heterozygous state. The CMT4C gene is strongly expressed in neural tissues, including peripheral nerve tissue. The translated protein defines a new protein family of unknown function with putative orthologues in vertebrates. Comparative sequence alignments indicate that members of this protein family contain multiple SH3 and TPR domains that are likely involved in the formation of protein complexes.

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■ Abstract Mutations in the mitofusin 2 (MFN2) gene are a major cause of primary axonal CharcotMarie-Tooth (CMT) neuropathy. This study aims at further characterization of cerebral white matter alterations observed in patients with MFN2 mutations. Molecular genetic, magnetic resonance imaging (MRI), magnetic resonance spectroscopy (MRS), and diffusion tensor imaging (DTI) investigations were performed in four unrelated patients aged 7 to 38 years with early onset axonal CMT neuropathy. Three distinct and so far undescribed MFN2 mutations were detected. Two patients had secondary macrocephaly and mild diffuse predominantly periventricular white matter alterations on MRI. In addition, one boy had symmetrical T2-hyperintensities in both thalami. Two patients had optic atrophy, one of them with normal MRI. In three patients proton MRS revealed elevated concentrations of total N-acetyl compounds (neuronal marker), total creatine (found in all cells) and myo-inositol (astrocytic marker) in cerebral white and gray matter though with regional variation. These alterations were most pronounced in the two patients with abnormal MRI. DTI of these patients revealed mild reductions of fractional anisotropy and mild increase of mean diffusivity in white matter. The present findings indicate an enhanced cellular density in cerebral white matter of MFN2 neuropathy which is primarily due to a reactive gliosis without axonal damage and possibly accompanied by mild demyelination.

The Na(v)1.7 sodium channel is preferentially expressed in nocioceptive dorsal root ganglion and sympathetic ganglion neurons. Gain-of-function mutations in Na(v)1.7 produce the nocioceptor hyperexcitability underlying inherited erythromelalgia, characterized in most kindreds by early-age onset of severe pain. Here we describe a mutation (Na(v)1.7-G616R) in a pedigree with adult-onset of pain in some family members. The mutation shifts the voltage-dependence of channel fast-inactivation in a depolarizing direction in the adult-long, but not in the neonatal-short splicing isoform of Na(v)1.7 in dorsal root ganglion neurons. Altered inactivation does not depend on the age of the dorsal root ganglion neurons in which the mutant is expressed. Expression of the mutant adult-long, but not the mutant neonatal-short, isoform of Na(v)1.7 renders dorsal root ganglion neurons hyperexcitable, reducing the current threshold for generation of action potentials, increasing spontaneous activity and increasing the frequency of firing in response to graded suprathreshold stimuli. This study shows that a change in relative expression of splice isoforms can contribute to time-dependent manifestation of the functional phenotype of a sodium channelopathy.