Sj. Stanworth et al., ESTABLISHED EPIGENETIC MODIFICATIONS DETERMINE THE EXPRESSION OF DEVELOPMENTALLY-REGULATED GLOBIN GENES IN SOMATIC-CELL HYBRIDS, Molecular and cellular biology, 15(8), 1995, pp. 3969-3978
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.