X-linked dominant Charcot–Marie–Tooth neuropathy: clinical, electrophysiological, and morphological phenotype in four families with different connexin32 mutations1
Introduction
Charcot–Marie–Tooth neuropathy (CMT) or hereditary motor and sensory neuropathy (HMSN) comprises hereditary disorders of the peripheral nervous system with autosomal as well as X-linked patterns of inheritance. Patients have progressive distal muscle weakness and atrophy, foot deformities, steppage gait, distal sensory loss, and decreased or absent tendon reflexes. Type 1, the demyelinating form of CMT, can be distinguished from type 2, the axonal form, by measurement of nerve conduction velocities (NCV) and histopathological examination of a nerve biopsy. The first presents with markedly decreased NCV and segmental degeneration of peripheral nerve myelin, the second with not or only slightly slowed NCV and loss of nerve fibres [1]. A heterozygous duplication of the locus for the peripheral myelin protein 22 (PMP22) on chromosome 17p11.2 is found in about 70% of CMT1 patients (CMT1A) [2], [3]. Occasionally CMT1A is also associated with point mutations in PMP22 [4] while CMT1B is caused by mutations in the myelin protein zero gene (P0, MPZ, locus: 1q22–q23) [5]. Recently, mutations in the early growth response 2 gene (EGR2, locus: 10q21–q22) were found in patients with severe, early onset peripheral neuropathies [6].
X-linked dominant CMT (CMTX) shows both demyelinating and axonal aspects of peripheral nerve pathology [7], [8], [9], [10], thus causing difficulties in categorising CMTX as a subtype of CMT1 or CMT2. Due to the X-linked pattern of inheritance, males tend to be earlier and more severely affected than women and show more severely decreased NCV. Female carriers may even appear asymptomatic, however, they can usually be identified on electrophysiological investigation [11], [12]. The causative role of mutations in the gap junction protein connexin32 (Cx32, locus: Xq13.1) in CMTX has been established by linkage and mutation analysis [13], [14]. Connexin32 mutations are now considered to be the second most common molecular cause of hereditary sensorimotor neuropathy accounting for at least 10% of families with CMT. Mutations reported so far are spread throughout the protein with scarcity in the fourth transmembrane domain ( see [15], [16] and references therein). Connexin32 has been localised in non-compact peripheral myelin probably forming gap junctions between adjacent myelin wraps of the same Schwann cell [13], [17], [18]. While various other tissues including central nervous system (CNS) glia express connexin32 [18], the only clinically affected one seems to be peripheral nerve. However, slowed CNS conduction of brain stem auditory evoked responses (BAER) in CMTX patients [19] suggest the involvement of central pathways.
CMTX can be confused with other types of CMT on electrophysiological and morphological investigation, or with sporadic male cases when the mutation is inherited by an asymptomatic female heterozygote. Hence, genetic testing is required for establishing a firm diagnosis which is of peculiar interest considering family planning and prognosis, especially of female patients. We have begun a regular screening of suspected CMT patients for connexin32 mutations when X-linked inheritance of the neuropathy seems likely. We have so far detected a total of four different mutations two of which are potentially novel. In this work, we analysed segregation of these mutations and phenotypic expression of the disease in the respective pedigrees. Some of the findings in the index cases of the families with the known mutations Arg15Trp and Arg22Gln have already been reported elsewhere documenting that archival, paraffin-embedded nerve biopsies allow the identification of connexin32 mutations [20].
Section snippets
Patients
We studied the segregation of connexin32 mutations in four unrelated German families with X-linked dominant CMT. For identification of index cases, DNA was extracted from archival paraffin-embedded sural nerve biopsy specimens of suspected CMT patients and screened to search for the presence of mutations within the connexin32 gene. Extending our previous study on 30 patients with a CMT2 phenotype [20], we performed genetic testing in 15 cases diagnosed with unspecified CMT, however, with a
Results
We were able to study four families with CMTX, each with a different mutation in the connexin32 gene. The families showed complete cosegregation between the mutation and the clinical phenotype, with the exception of eight female carriers who had no clinical symptoms of CMT. On electrophysiological investigation, however, five of these patients revealed signs of peripheral nerve lesion. The mutations could neither be detected in healthy relatives nor in any of the 78 normal chromosomes derived
Discussion
We were able to study four unrelated CMT families with mutations in the coding region of the connexin32 gene. Two of these mutations are potentially novel, one of which causing frameshift and premature stop of translation. Molecular analysis and clinical investigation displayed complete segregation of the mutated alleles with the CMT phenotype or transmission of the disease. Nerve biopsy samples from the index cases were indicative of a predominating axonal type of neuropathy with only minor
Acknowledgements
The technical assistance of A. Knischewski and H. Mader is gratefully acknowledged. The authors wish to thank Professor H. Buchner for reviewing the manuscript and offering helpful suggestions. This work was partially supported by grants from the Deutsche Forschungsgemeinschaft (SCHR 195/17-1), from the Wilhelm-Sander-Stiftung to Dr S. Quasthoff, and from the START programme of the Medical Faculty, RWTH Aachen.
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Presented in part at the 7th Annual Symposium of the European Charcot–Marie–Tooth Consortium, 3–5 July 1998, in Antwerp, Belgium.