Duchenne-like myopathy in double-mutant mdx mice expressing exaggerated mast cell activity.

Dystrophin-deficient female mdx mice were bred with male Tsk+/+ pa mice to examine the role played by mast cells in the pathophysiology of dystrophin deficiency. Resultant mdx/Tsk double-mutant mice were then examined functionally, biochemically, and histologically. While mdx mice remained as strong as their normal counterparts, mdx/Tsk double-mutant mice became progressively weak with age. Serum creatine kinase activity was significantly elevated in both mdx and mdx/Tsk double-mutant mice over normal controls. However, mast cell-derived plasma tryptase activity was consistently higher in the double-mutant than in mdx mice. In addition, histological examination of gastrocnemius muscle revealed that while necrosis was persistent in both strains of mdx mice from 2 to 8 weeks of age, regeneration was significantly reduced in the double-mutant mice. Of particular interest was the fact that necrosis in the mdx/Tsk double mutant exceeded mdx values at 8 weeks of age, corresponding approximately with a second peak in tryptase activity. Therefore, heightened mast cell activity appears to elicit in the dystrophin-deficient mdx mouse a myopathy not unlike the human Duchenne disease.