Genetic and molecular investigation of the CMTX1 locus
X-linked Charcot-Marie-Tooth disease (CMTX1), a peripheral neuropathy, is clinically characterised by slow progressive weakness and wasting of the distal muscles with associated sensory loss. The CMTX1 locus had previously been localised to the pericentromeric region of the X chromosome. Our initial linkage analysis utilising Restriction Fragment Length Polymorphisms (RFLPs) confirmed that CMTX1 mapped proximally to the DXYS1X locus (Xq21.31). Subsequent linkage analysis, carried out as part of an international consortium, utilising microsatellite polymorphisms further delineated the CMTX1 locus to a 2cM region around DXS453 (Xq31.1). To help with this analysis new microsatellites are generated at the loci DXS106 and DXS227. In parallel with the linkage analysis a physical map of the region was under construction simultaneously with candidate gene evaluation. The physical map was constructed by Pulsed Field Gel Electrophoresis (PFGE) used in conjunction with partial digestion of Yeast Artificial Chromosomes (YACs). The physical map of nine YACs had been obtained in which seven potential CpG islands were identified. Candidate genes were investigated by sequencing Polymerase Chain Reaction (PCR) amplified gene fragments from DNA isolated from patients with CMTX1. This led to the identification of missense, nonsense and base pair deletion mutations within the previously described GJβ1 gene. This gene encodes, connexin 32, a gap junction subunit. Gap junctions are channels which allow direct transfer of cellular components, which are below 900D in diameter, between coupled cells. It is proposed that mutations affecting the GJβ1 gene are the underlying biological defect which results in the CMTX1 phenotype.