REGULATION AND FUNCTION OF A HUMAN EMBRYONIC GLOBIN

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Human epsilon-globin is a beta-like globin whose expression is developmentally restricted to primitive erythroblasts in the blood islands of the embryonic yolk sac. In contrast to fetal and adult globins, the mechanistic bases for epsilon-globin gene regulation and the function of its encoded protein are poorly understood. Although transcriptional downregulation is a major effector of embryonic globin gene silencing, recent studies indicate that other, post-transcriptional events also play an important and previously unanticipated role in this process. Neither the specific post-transcriptional mechanisms involved, nor their ultimate contribution to epsilon-globin regulation have been established. Likewise, the physiologic properties of hemoglobins assembling from epsilon-globin subunits are incompletely described. The importance of fully defining the molecular controls and function of epsilon globin is magnified by the possibility that its reactivated expression might be therapeutically beneficial to adults with genetic defects in beta-globin expression. The feasibility and clinical potential of this approach cannot be judged without a comprehensive understanding of epsilon-globin regulation and function, which the current proposal will provide. First, key physiologically-important properties will be determined for hemoglobins that will assemble in definitive erythrocytes expressing epsilon globin. These studies, which will be done both in vitro and in transgenic mice, will include determinations of the O2 affinity and the anti-sickling characteristics of Hb alpha2epsilon2, of particular importance to individuals with beta thalassemia and sickle cell anemia. Second, the effect of specific post-transcriptional mechanisms on the expression of epsilon globin in definitive erythrocytes will be established. The specific processes that will be studied (mRNA stability, mRNA translational efficiency, and globin subunit stability, among others) are known to affect the expression of other human globins. As a group, these studies will begin to bring what is known about embryonic epsilon globin into parity with what is known about other fetal and adult globins. Moreover, the information provided by these studies will permit a reasoned approach to the design of molecular therapies aimed at epsilon-globin reactivation as well as an informed expectation of the likelihood that such an approach will be therapeutically beneficial.