Early decrease of IIx myosin heavy chain transcripts in Duchenne muscular dystrophy.

We studied by in situ hybridization the expression of IIx myosin heavy chain (MHC) transcripts in Duchenne Muscular Dystrophy (DMD) muscle. In normal muscle fibers IIx MHC transcripts were only expressed in type 2b or very fast fibers. Our results indicate that muscle fibers expressing IIx MHC transcripts disappear early in DMD muscle. This may be due to the transformation of type 2b fibers into type 2a fibers and/or to selective degeneration of the type 2b fibers. The very fast 2b fibers may be subjected to a greater mechanical stress during muscle contraction and dystrophin deficiency may render them more susceptible to mechanical damage. Copyright 1999 Academic Press.