Globin gene switching in transgenic mice carrying HS2-globin gene constructs.

We have examined the pattern of human globin gene switching in transgenic mice containing three different gamma and beta gene constructs (HS2G gamma A gamma delta beta, HS2A gamma beta neo, and HS2A gamma en beta) and compared the results with previously described transgenics (HS2A gamma beta, HS2G gamma A gamma-117 delta beta, and LCR epsilon G gamma A gamma delta beta). Developmental regulation was observed in all cases with identical patterns in lines bearing the same construct. Three different patterns of switching were observed: LCR epsilon G gamma A gamma delta beta and HS2A gamma beta neo mice switched rapidly, HS2G gamma A gamma delta beta and HS2G gamma A gamma-117 delta beta at an intermediate rate, and HS2A gamma beta and HS2A gamma en beta mice showed delayed switching, with a plateau in late fetal-early neonatal life and readily detectable levels of gamma mRNA in adults. No difference was observed in the time of switching of the HS2G gamma A gamma delta beta mice compared with those with the A gamma-117 hereditary persistence of fetal hemoglobin mutation, but adult levels of gamma mRNA were significantly higher (approximately 5%) in lines carrying the mutation than in those without (approximately 1%). Reversion to the rapid switch of the LCR epsilon G gamma A gamma delta beta mice was observed in three lines with the HS2A gamma beta neo construct in which expression of the tk-neo gene was approximately equal to that of the globin genes. The inclusion of the A gamma enhancer in HS2A gamma beta mice did not alter the pattern of switching, or reduce the relatively high levels of gamma mRNA in these lines. However, unlike other HS2 mice, the combination of HS2 and the A gamma enhancer resulted in copy number-dependent expression in HS2A gamma en beta lines, with intrauterine death at approximately 12.5 days gestation at high copy numbers. These results demonstrate that numerous elements throughout the beta globin gene cluster interact to produce the correct pattern of developmental regulation of these genes. Furthermore, extinction of gamma gene expression in adult life is not completely autonomous and is incomplete when HS2 is the only LCR element present.