X-linked Charcot-Marie-Tooth disease (CMTX1) results from numerous mutations in the GJB1 gene encoding the gap junction protein connexin32 (Cx32) and is one of the commonest forms of inherited neuropathy. Owing to the expression of Cx32 not only in Schwann cells but also in oligodendrocytes, a subset of CMT1X patients develops central nervous system (CNS) clinical manifestations in addition to peripheral neuropathy. While most GJB1 mutations appear to cause peripheral neuropathy through loss of Cx32 function, the cellular mechanisms underlying the CNS manifestations remain controversial. A novel start codon GJB1 mutation (p.Met1Ile) has been found in a CMT1X patient presenting with recurrent episodes of transient encephalomyelitis without apparent signs of peripheral neuropathy. In order to clarify the functional consequences of this mutation, we examined the cellular expression of two different constructs cloned from genomic DNA including the mutated start codon. None of the cloned constructs resulted in detectable expression of Cx32 by immunocytochemistry or immunoblot, although mRNA was produced at normal levels. Furthermore, co-expression with the other major oligodendrocyte connexin, Cx47, had no negative effect on GJ formation by Cx47. Finally, lysosomal and proteasomal inhibition in cells expressing the start codon mutant constructs failed to recover any detection of Cx32 as a result of impaired protein degradation. Our results indicate that the Cx32 start codon mutation is equivalent to a complete loss of the protein with failure of translation, although transcription is not impaired. Thus, complete loss of Cx32 function is sufficient to produce CNS dysfunction with clinical manifestations.
Bibliographical noteFunding Information:
This work is supported in part by Muscular Dystrophy Association grants (MDA 277250 and MDA 218284 to KAK). Styliana Kyriakoudi was supported with a Ph.D. fellowship donated to the Cyprus School of Molecular Medicine by the Hellenic Bank. We would like to thank Prof. Steven Scherer and Dr. Orthmann-Murphy, Department of Neurology, University of Pennsylvania, for the gift of Cx47-expressing clonal HeLa cells and Cx32 antibodies. The monoclonal antibody E7 (developed by Michael Klymkowsky) was obtained from the Developmental Studies Hybridoma Bank developed under the auspices of the National Institute of Child Health and Human Development and maintained by Department of Biology, The University of Iowa, Iowa City.
© 2015, Springer-Verlag Berlin Heidelberg.
- Gap junctions
- Start codon mutation
- X-linked Charcot-Marie-Tooth disease
ASJC Scopus subject areas
- Cellular and Molecular Neuroscience