ESTABLISHED EPIGENETIC MODIFICATIONS DETERMINE THE EXPRESSION OF DEVELOPMENTALLY-REGULATED GLOBIN GENES IN SOMATIC-CELL HYBRIDS

Somatic cell hybrids generated from transgenic mouse cells have been u sed to examine the developmental regulation of human gamma-to beta-glo bin gene switching. In hybrids between mouse erythroleukemia (MEL) cel ls and transgenic erythroblasts taken at various stages of development , there was regulated expression of the human fetal gamma and adult be ta genes, reproducing the in vivo pattern prior to fusion. Hybrids for med from embryonic blood cells produced predominantly gamma mRNA, wher eas beta gene expression was observed in adult hybrids and a complete range of intermediate patterns was found in fetal liver hybrids. The a dult environment of the MEL cells, therefore, did not appear to influe nce selective transcription from this gene complex. Irradiation of the embryonic erythroid cells prior to fusion resulted in hybrids contain ing only small fragments of donor chromosomes, but the pattern of gene expression did not differ from that of unirradiated hybrids. This fin ding suggests that continued expression of trans-acting factors from t he donor erythroblasts is not necessary for continued expression of th e human gamma gene in MEL cells. These results contrast with the lack of developmental regulation of the cluster after transfection of naked DNA into MEL cells and suggest that epigenetic processes established during normal development result in the gene cluster adopting a develo pmental stage-specific, stable conformation which is maintained throug h multiple rounds of replication and transcription in the MEL cell hyb rids. On prolonged culture, hybrids that initially expressed only the gamma transgene switched to beta gene expression. The time period of s witching, from similar to 10 to >40 weeks, was similar to that seen pr eviously in human fetal erythroblast x MEL cell hybrids but in this ca se bore no relationship to the time of in vivo switching. It seems unl ikely, therefore, that switching in these hybrids is regulated by a de velopmental clock.