Tetrahydrobiopterin synthesis and metabolism is impaired in dystrophin‐deficient mdx mice and humans

Abstract

Aim Loss of dystrophin causes oxidative stress and affects nitric oxide synthase‐mediated vascular function in striated muscle. Because tetrahydrobiopterin is an antioxidant and co‐factor for nitric oxide synthase, we tested the hypothesis that tetrahydrobiopterin would be low in mdx mice and humans deficient for dystrophin. Methods Tetrahydrobiopterin and its metabolites were measured at rest and in response to exercise in Duchenne and Becker muscular dystrophy patients, age‐matched male controls as well as wildtype, mdx and mdx mice transgenically overexpressing skeletal muscle‐specific dystrophins. Mdx mice were also supplemented with tetrahydrobiopterin and pathophysiology was assessed. Results Duchenne muscular dystrophy patients had lower urinary dihydrobiopterin + tetrahydrobiopterin/specific gravity_1.020 compared to unaffected age‐matched males and Becker muscular dystrophy patients. Mdx mice had low urinary and skeletal muscle dihydrobiopterin + tetrahydrobiopterin compared to wildtype mice. Mdx mice overexpressing dystrophins that localize neuronal nitric oxide synthase, restored dihydrobiopterin + tetrahydrobiopterin in mdx mice to wildtype levels while utrophin overexpression did not. Mdx mice and Duchenne muscular dystrophy patients did not increase tetrahydrobiopterin during exercise and in mdx mice tetrahydrobiopterin deficiency was likely due to lower levels of sepiapterin reductase in skeletal muscle. Tetrahydrobiopterin supplementation improved skeletal muscle strength, resistance to fatiguing and injurious contractions in vivo, increased utrophin and capillary density of skeletal muscle and lowered cardiac muscle fibrosis and left ventricular wall thickness in mdx mice. Conclusion These data demonstrate that impaired tetrahydrobiopterin synthesis is associated with dystrophin loss and treatment with tetrahydrobiopterin improves striated muscle histopathology and skeletal muscle function and in mdx mice.