A post-transcriptional process contributes to efficient gamma-globin gene silencing in definitive erythroid cells.

OBJECTIVES: The expression of human gamma globin is developmentally regulated through mechanisms that affect the transcriptional activity of its encoding gene. The current manuscript investigates whether the efficiency of this process might be enhanced though an unrecognized post-transcriptional event that defines the stability of gamma-globin mRNA.
METHODS: Experiments were conducted in vivo in transgenic mice expressing human gamma globin in their adult erythroid cells. The expression of gamma-globin protein was manipulated by breeding the transgene into animals producing different levels of endogenous mouse beta-globin. Changes in the expression of gamma globin were then correlated to measures of gamma-globin mRNA stability in vivo.
RESULTS: Human gamma globin was expressed at higher levels in thalassemic than in than non-thalassemic control transgenics, paralleling a highly significant increase in the stability of gamma-globin mRNA. Other molecular events-including possible transcriptional induction of the transgene, or an increase in the stability of the gamma-globin protein-did not appear to contribute to the observed increase in transgene expression. As anticipated, the stability of gamma-globin mRNA also fell in bitransgenic animals that co-expressed human beta-globin mRNA.
CONCLUSIONS: Our results are consistent with a model for dynamic post-transcriptional control of gamma-globin gene expression, through modulation of the stability of its encoding mRNA. Moreover, the stability of gamma-globin mRNA appears to be inversely related to ambient levels of co-expressed beta-globin mRNA. This data suggests that therapeutic gene-reactivation and/or gene-replacement therapies may be particularly effective in individuals with severe forms of beta-thalassemia.