NONSENSE AND MISSENSE MUTATIONS IN THE MUSCULAR CHLORIDE CHANNEL GENECLC-1 OF MYOTONIC MICE

In mature vertebrate muscle, the chloride channel Clc-1 is necessary f or the stabilization of the resting potential. Its functional defect l eads to the disease myotonia. The ADR mouse (phenotype ADR, genotype a dr/adr) is an animal model for human myotonias. The adr gene is a memb er of a family of non-complementing recessive autosomal mutations (''a lleles'' of adr) that cause myotonia in the mouse. The standard allele adr has arisen by the insertion of a retroposon into the chloride cha nnel gene Clc-1 (Steinmeyer, K., Klocke, R., Ortland, C., Gronemeier, M., Jockusch, H., Grunder, S., and Jentsch, T. J. (1991) Nature 354, 3 04-308). In order to study the nature of two other alleles, adr(mto) a nd adr(K), we have analyzed overlapping Clc-1 cDNA amplification produ cts by the hydroxylamine and osmium tetroxide modification technique a nd direct sequencing. A comparison between ADRMTO and C57BL/6 wild ty pe showed six base pair substitutions, one of which resulted in a stop codon in position 47, whereas the five others are either silent or le ad to amino acid substitutions in non-conserved regions of the Clc-1 s equence and were already present in the wild type inbred SWR/J strain from which adr(mto) was derived. The detection of the stop codon in th e adr(mto) allele is further indication of the identity of the Clc-1 c hloride channel with the adr myotonia gene in the mouse, because a cha in termination close to the N terminus would necessarily destroy gene function. For the ethylnitrosourea-induced mutation adr(K), an Ile --> Thr exchange in codon 553 was identified. As this affects a conserved residue within a highly conserved region of the Clc-1 gene, a functio nal significance of this residue is suggested.