Regulation of Tissue-Specific Gene Expression in Microcell Hybrids.

Microcell-mediated chromosome transfer has been used to study genetic loci that can either activate or repress tissue-specific gene expression. We have used microcell hybrids to study activation of the muscle phenotype, as well as to identify a genetic locus responsible for inhibiting expression of the myogenic determination gene MyoD in primary fibroblasts. We have shown that microcell transfer of human fibroblast chromosome 11, which contains the MyoD locus, into the embryonic fibroblast cell line 10T1/2 results in activation of human MyoD and, consequently, activation of the entire muscle program of differentiation. In addition, chromosome segregation analysis indicates that the continued presence of human chromosome 11 is not required for maintenance of the myogenic phenotype. In contrast, whole-cell hybrids between 10T1/2 cells and primary nonmuscle cells fail to activate the muscle phenotype, suggesting the presence of MyoD inhibitory loci. Microcell hybrids retaining human fibroblast chromosome 4 fail to activate MyoD expression. Analysis of chromosome fragment-containing hybrids localizes the repressing activity to a small region on 4p.