Multiple products of the Duchenne muscular dystrophy gene

The gene which is defective in Duchenne Muscular Dystrophy (DMD) extends over 2300 kb of the X chromosome. Its product in the muscle is a 14 kb mRNA encoding a 427 kd rod-shaped protein called dystrophin. A 14 kb transcript encoding a very similar isoform of dystrophin is produced in the brain. The brain 14 kb mRNA is transcribed from the same gene but controlled by a different promoter, located at least 75 kb upstream from the muscle dystrophin promoter. The regulation of these promoters is very stringently controlled. The muscle-type but not the brain-type dystrophin mRNA is found in cloned skeletal muscle cells and its presence is correlated with the appearance of multinucleated fibers. The brain type is expressed in neurons, while in glia cells the muscle-type promoter is active. A third DMD gene transcript which is only 6.5 kb long has been identified. It contains the sequence coding for the C-terminal domain and the cysteine-rich domain of dystrophin but not the large region encoding the spectrin-like repeats and the N-terminal domain. The cell type distribution of this transcript is also very different from that of the two 14 kb mRNA isoforms. It is the major product of the DMD gene in many nonmuscle tissues including brain. Using monoclonal antibodies we have identified a 77 Kd protein which seems to be the translation product of this mRNA. As expected from the distribution of the 6.5 Kb mRNA, this protein is the major DMD gene product detectable in brain and many other nonmuscle tissues; it is undetectable in skeletal muscle but is present in the heart and stomach (as is the 6.5 Kb mRNA).