DEVELOPMENTAL REGULATION OF HUMAN GAMMA-GLOBIN GENES IN TRANSGENIC MICE

We report results showing that several gamma gene promoter elements participate in the developmental control of gamma-globin genes. Four gamma gene constructs with 5' truncated at -141, -201, -382, and -730 of the (A)gamma gene promoter linked to a micro locus control region (muLCR) cassette were used for production of transgenic mice and analysis of gamma gene expression during development. Mice carrying a muLCR -141 (A)gamma construct displayed downregulation of gamma gene expression in the adult stage of development, indicating that the proximal promoter contains elements participating in gamma gene silencing. Mice carrying a muLCR -201 (A)gamma or a muLCR -382 (A)gamma construct displayed high gamma gene expression in the fetal stage of development and complete loss of gamma gene downregulation in the adult stage, suggesting that the - 141 to - 201 gamma gene sequence contains elements which upregulate gamma gene expression and are dominant over the negative element 3' to -141. Extension of the promoter to -730 resulted in reappearance of gamma gene downregulation, suggesting that the -382 to -730 sequences contain an adult-stage-specific silencer. Gamma gene expression in the muLCR -201 (A)gamma and the muLCR -382 (A)gamma transgenic mice was copy number dependent. All the muLCR -730 A(gamma) transgenic mice expressed gamma mRNA; however, gamma gene expression was copy number independent, indicating that levels of gamma gene expression were modulated by the surrounding chromatin. Our results suggest that multiple elements participate in gamma gene silencing. The findings in the muLCR -201 A(gamma) and muLCR -382 (A)gamma transgenic mice are interpreted to indicate that the LCR interacts not only with the minimal gamma gene promoter but also with sequences of the upstream promoter. We postulate that gamma gene downregulation is achieved when the interaction between LCR and the upstream promoter is disturbed by the silencer located in the -382 to -730 region. We propose that gamma gene silencing is achieved by the combined effect of negative elements located 3' to -141, the negative element located between -382 and -730, and the competition by the beta gene promoter during the adult stage of development.