MYOTONIA (stiffness and impaired relaxation of skeletal muscle) is a symptom of several diseases caused by repetitive firing of action potentials in muscle membranes. Purely myotonic human diseases are dominant myotonia congenita (Thomsen) and recessive generalized myotonia (Becker), whereas myotonic dystrophy is a systemic disease. Muscle hyperexcitability was attributed to defects in sodium channels and/or to a decrease in chloride conductance (in Becker's myotonia and in genetic animal models). Experimental blockage of Cl- conductance (normally 70-85% of resting conductance in muscle) in fact elicits myotonia. ADR mice are a realistic animal model for recessive autosomal myotonia. In addition to Cl- conductance, many other parameters are changed in muscles of homozygous animals. We have now cloned the major mammalian skeletal muscle chloride channel (ClC-1). Here we report that in ADR mice a transposon of the ETn family has inserted into the corresponding gene, destroying its coding potential for several membrane-spanning domains. Together with the lack of recombination between the Clc-1 gene and the adr locus, this strongly suggests a lack of functional chloride channels as the primary cause of mouse myotonia.

No abstract

No abstract

Myotonias are muscle diseases in which the function of the muscular chloride channel ClC-1 is impaired. Null alleles of the corresponding Clc1 gene on mouse chromosome (Chr) 6 provide animal models for human myotonias. It was shown that the allele adr (Clc1adr) is due to an insertion of an ETn type transposon that is transcribed and leads to multiple splicing events; the allele mto (Clc1adr-mto) involves a stop codon near the N-terminus. We have determined the genomic organization of the mouse Clc1 gene and the sequence requirements for the transposon insertion in the Clc1adr allele. The mouse Clc1 gene is composed of 23 exons, ranging from 39 to 372 bp, and spans approximately 23 kb of genomic DNA. The exon/intron organization is highly homologous to that of the human CLCN1 gene; the homology of the coding sequence is 97% to rat and 89% to human. In the adr allele the ETn transposon is inserted into intron 12, the largest intron. Whereas the 5' and 3' LTR sequences of the ETn transposon are homologous to those reported for other insertional mutations of the mouse, no consensus motif for an insertion target site could be defined. On the basis of flanking sequences, we provide duplex PCR diagnoses for the adr, adr-mto, and wild-type alleles of Clc1. Close to the 3' end of intron 12, a tetranucleotide repeat (AATC)n was found that is polymorphic between mouse species Mus musculus, M. molossinus, M. castaneus, and M. spretus, and can thus be used for chromosomal mapping studies.

No abstract