Functional alterations in gap junction channels formed by mutant forms of connexin 32: evidence for loss of function as a pathogenic mechanism in theX-linked form of Charcot-Marie-Tooth disease

CMTX, the X-linked form of Charcot-Marie-Tooth disease, is an inherited per ipheral neuropathy arising in patients with mutations in the gene encoding the gap junction protein connexin 32 (Cx32). In this communication, we desc ribe the expression levels and biophysical parameters of seven mutant forms of Cx32 associated with CMTX, when expressed in paired Xenopus oocytes. Pa ired oocytes expressing the R15Q and H94Q mutants show junctional conductan ces not statistically different from that determined for Cx32WT, though bot h show a trend toward reduced levels. The S85C and G12S mutants induce redu ced levels of junctional conductance. Three other mutants (R15W. H94Y and V 139M) induce no conductance above baseline when expressed in paired oocytes . Analysis of the conductance voltage relations for these mutants shows tha t the reduced levels of conductance are entirely (H94Y and V139M) or partly (S85C and R15W) explicable by a reduced open probability of the mutant hem ichannels, The R15Q and H94Q mutations also show alterations in the conduct ance voltage relations that would be expected to minimally (H94Q) or modera tely (R15Q) reduce the available gap junction communication pathway. The re duction in G12S induced conductance cannot be explained by alterations in h emichannel open probability and are more likely due to reduced junction for mation. These results demonstrate that many CMTX mutations lead to loss of function of Cx32. For these mutations, the loss of function model is likely to explain the pathogenesis of CMTX. (C) 2001 Elsevier Science B.V. All ri ghts reserved.