INACTIVATION OF THE MYOGENIC BHLH GENE MRF4 RESULTS IN UP-REGULATION OF MYOGENIN AND RIB ANOMALIES

The myogenic basic helix-loop-helix (bHLH) proteins MyoD, myf5, myogen in, and MRF4 can initiate myogenesis when expressed in nonmuscle cells . During embryogenesis, each of the myogenic bHLH genes is expressed i n a unique temporospatial pattern within the skeletal muscle lineage, suggesting that they play distinct roles in muscle development. Gene t argeting has shown that MyoD and myf5 play partially redundant roles i n the genesis of myoblasts, whereas myogenin is required for terminal differentiation. MRF4 is expressed transiently in the somite myotome d uring embryogenesis and then becomes up regulated during late fetal de velopment to eventually become the predominant myogenic bHLH factor ex pressed in adult skeletal muscle. On the basis of its expression patte rn, it has been proposed that MRF4 may regulate skeletal muscle matura tion and aspects of adult myogenesis. To determine the function of MRF 4, we generated mice carrying a homozygous germ-line mutation in the M RF4 gene. These mice showed only a subtle reduction in expression of a subset of muscle-specific genes but showed a dramatic increase in exp ression of myogenin, suggesting that it may compensate for the absence of MRF4 and demonstrating that MRF4 is required for the down-regulati on of myogenin expression that normally occurs in postnatal skeletal m uscle. paradoxically, MRF4-null mice exhibited multiple rib anomalies, including extensive bifurcations, fusions, and supernumerary processe s. These results demonstrate an unanticipated regulatory relationship between myogenin and MRF4 and suggest that MRF4 influences rib outgrow th through an indirect mechanism.