Developmentally distinct effects on human epsilon-, gamma- and delta-globin levels caused by the absence or altered position of the human beta-globin gene in YAC transgenic mice.

The human beta-globin locus has been an important model system in the study of developmentally regulated transcription in multigene chromosomal domains. In this study, primer extension and sensitive real-time RT-PCR assays were used to quantify the effects of beta-globin sequence modifications on epsilon-, gamma- and delta-globin levels in transgenic mice. E11.5 primitive erythroid cells showed a surprisingly large increase in epsilon-globin in the absence of the beta-globin gene (beta- locus), which is weakly expressed at that stage of development. E17.5 fetal liver and adult erythroid cells, in which beta-globin expression approaches its maximum, showed an unexpectedly small, statistically insignificant stimulation of gamma- and delta-globin levels in the absence of beta-globin sequence. Analysis of erythroid colonies produced by in vitro differentiation of embryonic stem cells indicated that the absence of the human beta-globin gene had no effect on gamma-globin expression. These results suggest that competitive influences need not be linked directly to transcription level or distance from the locus control region (LCR), and that the large increases in gamma-globin levels seen in some human deletional beta-thalassemias and hereditary persistence of fetal hemoglobin conditions are most likely to be due to effects other than loss of beta-globin competition. In transgenic mice with beta-globin sequences inserted between epsilon and the LCR in a beta- locus (betaup), the expression of epsilon-, gamma- and delta-globins suggested that stage-specific sensitivity to loss of LCR activity may be a more important parameter than position relative to the LCR. The relationship of these measurements of transgenic globin expression to a possible binary model of globin LCR action and to mimicry from red blood cell loss due to transgenic globin imbalances are discussed.