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Genotypic and phenotypic analysis of muscles from dystrophic - normal mouse chimeras Peterson, Alan Clarke

Abstract

1. Artificial chimeric mice, mosaic for muscular dystrophic (dy[sup 2J] /dy[sup 2J]) and normal (SWV +/+) genotypes were successfully produced. 2. These chimeric mice demonstrated no clinical features of the dystrophic condition. 3. A maximum stimulated twitch analysis of the anterior tibialis muscles of these chimeras also revealed no functional deficiences characteristic of the dystrophic condition. 4. The electrophoretic phenotype of Mod-1 (malic enzyme) was resolved with a high voltage micro starch gel system and a quantitative analysis was developed to detect and estimate the relative muscle fiber nuclear mosaicism in single muscles from the chimeras. The measured activity of the 5 bands of the Mod-1 hetero-zygote phenotype was observed to be different from the expected ratio of 1:4:6:4:1 previously reported. 5. The presence of genetically dystrophic nuclei was detected in the majority of chimera muscles examined. The overall content of (dy[sup 2J] /dy[sup 2J]) nuclei from all the muscle samples examined was estimated to be 58%. This indicated that genetically dystrophic cells contribute normally to muscle morphogenesis and that genetically dystrophic muscle fiber nuclei suffer no specific degeneration in this artificial mosaic environment. 6. Within single chimeras the composition of muscles varied extensively. Individual muscles of both normal and primarily dystrophic genetic composition were detected. 7. Histological analysis of samples of anterior tibialis muscles, prepared in 1 u thick plastic sections, was undertaken to determine if any preclinical muscle degeneration had taken place. These muscles had essentially normal phenotypes. 8. Examples of muscles with primarily dystrophic genotypic composition and remarkably normal histological phenotype were revealed. Conversely, muscles with no detectable dystrophic composition were observed to have foci of degeneration characteristic of muscular dystrophy. These results, although not extensive, are suggestive that the muscle degeneration observed in dystrophic (dy[sup 2J] /dy[sup 2J]) mice is secondary to a primary lesion residing outside the muscle fiber proper. 9. The possible similarities between these artificial cellular mosaics and a human X-linked genetic mosaic, the heterozygous female for Duchenne dystrophy, are presented. 10. The results of these studies were discussed in terms of the application of chimeric mice in the delineation of the primary lesion in mouse muscular dystrophy.

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