Motor end-plate disease is expressed in congenitally athymic mice

Mutual interference of myotonia and muscular dystrophy in the mouse: a study on ADR-MDX double mutants

For Duchenne muscular dystrophy (DMD, dystrophin deficiency) and Thomsen/Becker myotonia (muscular chloride channel deficiency) genetically homologous mouse models are available, the dystrophin-deficient MDX mouse and the myotonic ADR mouse. Whereas the latter shows more severe symptoms than human myotonia patients, the MDX mouse, in contrast to DMD patients, is only mildly affected. We have introduced, by appropriate breeding, the defect leading to myotonia (Clc1 null mutation, adr allele) into MDX mice, thus creating ADR-MDX double mutants. The expectation was that, due to mechanical stress during myotonic cramps, the ADR status should symptomatically aggravate the muscle fibre necrosis caused by the dystrophin deficiency. The overall symptoms of the double mutants were dominated by myotonia. Weight reduction and premature death rate were higher in ADR-MDX than in ADR mice. Sarcolemmal ruptures as indicated by influx into muscle fibres of serum globulins and injected Evans blue were found with great inter-individual variation in MDX and in ADR-MDX muscles. Affected fibres were found mainly in large groups in MDX but single or in small clusters in ADR-MDX leg muscles. The symptoms of myotonia (aftercontractions, shift towards oxidative fibres) were less pronounced in ADR-MDX than in ADR muscles. Conversely, numbers of damaged fibres as well as the percentage of central nuclei (an indicator of fibre regeneration) were significantly lower in ADR-MDX than in MDX skeletal muscles. Thus it appears that, at the level of the muscle fibre, myotonia and muscular dystrophy attenuate each other.

The genes for two neuromuscular diseases of the mouse, 'arrested development of righting response', adr, and 'myotonia', mto, are allelic

Two hitherto unmapped recessive autosomal mutations, ‘arrested development of righting response’ (adr) and ‘myotonia’ (mto, now adrmto), were found to be allelic, so that preliminary linkage data on adr and adrmto can now be combined. No linkage was found to the glucose phosphate isomerase (Gpi-1) locus on Chr 7 that marks a region of homology between mouse Chr 7 and human Chr 19, and in man is closely linked to the myotonic dystrophy (DM) gene. The adr gene was expressed on diverse genetic backgrounds including the athymic nude mouse.

Nerve transplantation shows that motor end-plate disease is not a primary Schwann cell defect

Motor end-plate diseased (MED) mice have altered nerve impulse conduction velocities and refractory periods. To test whether these pathological properties are caused by a primary Schwann cell defect, nerves were transplanted from MED and wildtype (WT) animals onto WT recipients. The donor origin of cells in the regenerated nerve was assessed by prelabeling with [3H]thymidine and by electrophoretic analysis of glucose phosphate isomerase allotypes. Nerve fiber regeneration through MED and WT implants was equally efficient. No difference was found in nerve conductivities of MED and WT grafts. Therefore a primary defect in the Schwann cells of the MED mouse is unlikely.