Amelioration in sickle cell disease seen with persistent fetal hemoglobin expression suggests that therapies capable of reactivating gamma-globin gone expression should be explored. Gene transfer of a transcription factor capable of reversing the gamma- to beta-globin switch in adult erythroid cells represents one approach to treatment of this devastating disorder. We have identified a protein complex, the stage selector protein (SSP) that is a strong candidate for this approach. The SSP consists of the ubiquitous transcription factor CP2/LBP-1a, p22NF-E4, an erythroid-restricted component, and ALY, a chaperone protein that stabilizes the interaction between the other two. We have demonstrated that p22 NF-E4 functions as an activator of gamma-gene expression, and in the setting of fetal/adult gene competition promotes a concomitant down-regulation of adult beta-gene expression. Enforced expression of p22NF-E4 in betaYAC transgenic mice delayed the fetal/adult switch but was unable to override the intrinsic mechanisms that govern gamma-gene silencing. We have shown that the function of the SSP is mediated by direct protein/protein interactions with co-activators and general transcription factors. Based on these findings, we postulate a role for SSP in recruiting the locus control region (LCR) to the gamma-promoter through changes in factor binding to these elements.
In Specific Aim 1, we will estabh'sh the pattern of transcription factor association with the LCR and gamma-promoter in cellular and animal models of fetal erythropoiesis and examine the effects of increased expression of p22 NF-E4 in these contexts. We will determine whether the LCR and gamma-promoter are in close physical proximity in fetal erythroid cells and if this is influencedby the SSP complex.
In Specific Aim 2, we will dissect the relative contribution of the individual components of the SSP to the overall function of the complex, including their role in factor binding and LCR/promoter interaction.
Specific Aim 3 will focus on LCR/gamma-promoter interactions and factor occupancy in the setting of a mutation that disrupts intrinsic gamma-gene silencing, and the effects of p22NF-E4 expression in this context. In summary, these studies will define the role of the SSP complex in modifying factor association with the LCR and gamma-promoter, and determine whether the predominant function of the SSP is to directly recruit the LCR to the promoter. Ultimately they may provide new therapeutic strategies for sickle cell disease.
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