Myotonic dystrophy type 2 (DM2) is an autosomal dominant disorder caused by an untranslated CCTG repeat expansion in intron 1 of the zinc finger 9 gene (ZNF9) on chromosome 3. A predominant and early symptom of myotonic dystrophy is myotonia, manifested as delayed skeletal muscle relaxation following voluntary contraction. Myotonia can be caused by a loss of function of the muscle specific chloride channel (ClC-1). In muscle from patients with DM2 we identified a shorter variant of ClC-1-mRNA with exons 6 and 7 spliced out. This splicing variant contains a premature termination codon at position 236 that prevents expression of full length ClC-1 protein. Heterologous ClC1236X expression did not yield functional channels. Co-expression with ClC1 did not show a dominant negative effect, but a slightly suppressive effect. In C2C12 cells, the clc1 splice variants generated by 18xCCUG)-RNA resembled those in DM2 muscle and differed from those generated by 24xCUG and 24x(AAG).

ClC1 splice variants contribute to myotonia in DM2. Aberrant clcn1 pre-mRNA splicing can be produced in C2C12 cells by non-pathologic 72bp long repeats. The splice variants depend on the types of repeats expressed. These data suggest repeat-dependent pathogenetic mechanisms in myotonic dystrophies.