Note: Supplementary information is available on the Nature Genetics website. ACKNOWLEDGMENTS We thank the clinicians and families for providing samples, N. Killeen for technical assistance and the Wellcome Trust for financial support.

Mutations in mitofusin 2 (MFN2) have been reported in Charcot-Marie-Tooth type 2 (CMT2) families. To study the distribution of mutations in MFN2 we screened 323 families and isolated patients with distinct CMT phenotypes. In 29 probands, we identified 22 distinct MFN2 mutations, and 14 of these mutations have not been reported before. All mutations were located in the cytoplasmic domains of the MFN2 protein. Patients presented with a classical but rather severe CMT phenotype, since 28% of them were wheelchair-dependent. Some had additional features as optic atrophy. Most patients had an early onset and severe disease status, whereas a smaller group experienced a later onset and milder disease course. Electrophysiological data showed in the majority of patients normal to slightly reduced nerve conduction velocities with often severely reduced amplitudes of the compound motor and sensory nerve action potentials. Examination of sural nerve specimens showed loss of large myelinated fibres and degenerative mitochondrial changes. In patients with a documented family history of CMT2 the frequency of MFN2 mutations was 33% indicating that MFN2 mutations are a major cause in this population.

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Hereditary demyelinating peripheral neuropathies consist of a heterogeneous group of genetic disorders that includes hereditary neuropathy with liability to pressure palsies (HNPP), Charcot-Marie-Tooth disease (CMT), Dejerine-Sottas syndrome (DSS), and congenital hypomyelination (CH). The clinical classification of these neuropathies into discrete categories can sometimes be difficult because there can be both clinical and pathologic variation and overlap between these disorders. We have identified five novel mutations in the myelin protein zero (MPZ) gene, encoding the major structural protein (P0) of peripheral nerve myelin, in patients with either CMT1B, DSS, or CH. This finding suggests that these disorders may not be distinct pathophysiologic entities, but rather represent a spectrum of related "myelinopathies" due to an underlying defect in myelination. Furthermore, we hypothesize the differences in clinical severity seen with mutations in MPZ are related to the type of mutation and its subsequent effect on protein function (i.e., loss of function versus dominant negative).

A European collaboration on Charcot-Marie-Tooth type 1 (CMT1) disease and hereditary neuropathy with liability to pressure palsies (HNPP) was estab lished to estimate the duplication and deletion frequency, respectively, on chromosome 17pl 1.2 and to make an inventory of mutations in the myelin genes, peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ) and connexin 32 (Cx32) located on chromosomes 17pl 1.2, Iq21-q23 andXql3.1, respectively. In 70.7% of 819 unrelated CMT1 patients, the 17p11.2 duplica tion was present. In 84.0% of 156 unrelated HNPP patients, the 17p 11.2 dele tion was present. In the nonduplicated CMT1 patients, several different muta tions were identified in the myelin genes PMP22, MPZ and Cx32.

We observed a missense mutation in the peripheral myelin protein zero gene (MPZ, Thr124Met) in seven Charcot-Marie-Tooth (CMT) families and in two isolated CMT patients of Belgian ancestry. Allele-sharing analysis of markers flanking the MPZ gene indicated that all patients with the Thr124Met mutation have one common ancestor. The mutation is associated with a clinically distinct phenotype characterized by late onset, marked sensory abnormalities and, in some families, deafness and pupillary abnormalities. Nerve conduction velocities of the motor median nerve vary from <38 m/s to normal values in these patients. Clusters of remyelinating axons in a sural nerve biopsy demonstrate an axonal involvement, with axonal regeneration. Phenotype-genotype correlations in 30 patients with the Thr124Met MPZ mutation indicate that, based on nerve conduction velocity criteria, these patients are difficult to classify as CMT1 or CMT2. We therefore conclude that CMT patients with slightly reduced or nearly normal nerve conduction velocity should be screened for MPZ mutations, particularly when additional clinical features such as marked sensory disturbances, pupillary abnormalities or deafness are also present.

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.

SummaryCharcot-Marie-Tooth disease (CMT) is the most common neuromuscular disorder. It represents a group of clinically and genetically heterogeneous inherited neuropathies. Here, we review the results of molecular genetic investigations and the clinical and neurophysiological features of the different CMT subtypes. The products of genes associated with CMT phenotypes are important for the neuronal structure maintenance, axonal transport, nerve signal transduction and functions related to the cellular integrity. Identifying the molecular basis of CMT and studying the relevant genes and their functions is important to understand the pathophysiological mechanisms of these neurodegenerative disorders, and the processes involved in the normal development and function of the peripheral nervous system. The results of molecular genetic investigations have impact on the appropriate diagnosis, genetic counselling and possible new therapeutic options for CMT patients.