MODIFICATIONS OF GENE-EXPRESSION IN MYOTONIC MURINE SKELETAL-MUSCLE ARE ASSOCIATED WITH ABNORMAL EXPRESSION OF MYOGENIC REGULATORY FACTORS

The mouse mutants ADR (''arrested development of righting'') and the allelic CRP (''cramp'') are characterized by a myotonic phenotype resulting from a dysfunction of the skeletal muscle chloride channel which leads to myotonic trains of action potentials in response to stimuli. Compared to normal mouse muscle, numerous biochemical modifications have been found in the ADR muscle, and changes are observed in the expression of certain isoforms of contractile proteins. We have therefore measured the levels of the mRNA transcripts encoding the myosin heavy chain isoforms (MyHC) in both mutants. Transcripts for the myogenic regulatory factors were also studied since they are known to play a role in the induction of muscle-specific gene transcription, and their own expression is modified by different electrical activity patterns. In both mutants, the mRNA encoding the IIB MyHC was considerably decreased. In contrast, the mRNAs for the IIA, IIX, and beta/slow MyHCs were increased. The mRNA for the neonatal MyHC mRNA was not detectable, and therefore fiber regeneration does not appear to play a role in these phenomena. Among the myogenic regulatory factors, herculin is the most abundant in adult muscle; however, herculin mRNA undergoes a large decrease in myotonic muscle which does not seem to be related to the changing fiber type. The levels of MyoD and myogenin mRNAs are also modified with the former decreasing and the latter increasing. Qualitatively similar changes are seen in the ADR and CRP mutants; however, they are generally less pronounced in CRP. These observations suggest that specific myogenic factors may be linked to the expression of individual MyHC genes and that abnormal expression of some of the factors may be associated with myotonic muscle pathology. (C) 1993 Academic Press, Inc.