Role of nitric oxide in the pathogenesis of muscular dystrophies: a "two hit" hypothesis of the cause of muscle necrosis.

Although the genetic and biochemical bases of many of the muscular dystrophies have been elucidated, the pathophysiological mechanisms leading to muscle cell death and degeneration remain elusive. Among the most well studied of the dystrophies are those due to defects in proteins that make up the dystrophin-glycoprotein complex (DGC). There has been much interest in the role of nitric oxide (NO()) in the pathogenesis of these diseases because the enzyme that synthesizes NO(), nitric oxide synthase (NOS), is associated with the DGC. Recent studies of dystrophies related to DGC defects suggest that one mechanism of cellular injury is functional ischemia related to alterations in cellular NOS and disruption of a normal protective action of NO(). This protective action is the prevention of local ischemia during contraction-induced increases in sympathetic vasoconstriction. However, the loss of this protection, alone, does not explain the subsequent muscle cell death and degeneration since mice lacking neuronal NOS (the predominant isoform expressed in muscle) do not develop a muscular dystrophy. Thus, there must be additional biochemical changes conferred upon the cells by these DGC defects, and these changes are discussed in terms of a proposed "two hit" hypothesis of the pathogenetic mechanisms that underlie the muscular dystrophies. According to this hypothesis, pathogenic defects in the DGC have at least two biochemical consequences: a reduction in NO()-mediated protection against ischemia, and an increase in cellular susceptibility to metabolic stress. Either one alone may be insufficient to lead to muscle cell death. However, in combination, the biochemical consequences are sufficient to cause muscle degeneration. The role of oxidative stress as a final common pathophysiologic pathway is discussed in terms of data showing that oxidative injury precedes pathologic changes and that muscle cells with defects in the DGC have an increased susceptibility to oxidant challenges. Accordingly, this "two hit" hypothesis may explain many of the complex spatial and temporal variations in disease expression that characterize the muscular dystrophies, such as grouped necrosis, a pre-necrotic phase of the disease, and selective muscle involvement.