We characterised the expression, localisation and developmental regulation of the three major dystrophin isoforms in human foetal skeletal and cardiac muscles and in the corresponding cultures. Gene expression studies in foetal cardiac muscle-tissue and cultures showed that the Muscle- and the Brain- but not the Purkinje-transcripts were always co-expressed. In skeletal muscle the Muscle-isoform was already present at 11.8 weeks while the Brain-isoform was detected only after 13 weeks. Myoblast cultures showed a similar sequence of isoform transcription. The Purkinje-isoform was never detected. Localisation studies showed that in cardiac muscle dystrophin was seen discontinuously at the sarcolemma from 8.5 weeks, and evenly expressed by 15 weeks. Cardiomyocyte cultures expressed desmin but not dystrophin after 7 days. Protein studies in foetal skeletal muscle suggested that dystrophin is expressed in the cytoplasm from 8.5 weeks and at the sarcolemma only after 10.5 weeks. Similar results were obtained in cultured myoblasts. This study shows that in cardiac muscle both the Muscle- and Brain-isoforms are transcribed in parallel from the very early stages of development, while in skeletal muscle transcription of the Muscle-isoform occurs first, followed by the Brain-isoform
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