Promiscuous expression of myosin in myotonic dystrophy

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Abstract
The pathologic changes in myotonic dystrophy (DM) skeletal muscle biopsies have been analyzed at both the histochemical and molecular level. A histochemical stain for pretyping single fibers in conjunction with sodium dodecyl sulphat—polyacrylamide gel electrophoresis allowed biochemical differences to be pinpointed in specific histochemical fiber types. These biochemical differences can be related to histochemical changes in fiber type observed in cross‐section of the DM biopsies. Such changes included, specific fiber type atrophy, hypertrophy, and disproportion. The pathogenesis of DM appears to be characterized by a large increase in the number of promiscuous fibers, that is, those fibers that express both fast and slow myosins. This promiscuity, which is rare in control muscle (<2%), is also prevalent at high levels in some family members at risk for DM. The observed promiscuity, although probably not a primary effect of DM, appears to be linked to the histochemical changes in fiber type observed in the DM biopsies.