Methylation-Enhanced Binding of Sp1 to the Stage Selector Element of the Human γ-Globin Gene Promoter May Regulate Development Specificity of Expression

Abstract

The human γ-globin gene promoter contains a stage selector element (SSE) responsible for preferential interaction of the promoter with a powerful erythroid-specific enhancer in the fetal developmental stage (S.M. Jane, P.A. Ney, E.F. Vanin, D.L. Gumucio, and A.W. Nienhuis. EMBO J. 11:2691-2699, 1992). The element binds two proteins, the ubiquitous activator Sp1 and a protein previously known as -50 γ and now named the stage selector protein (SSP). Binding of the second protein correlates with SSE activity in transient-transfection assays. We now report that a de novo binding site for the SSP is created by the -202(C→G) mutation that causes hereditary persistence of fetal hemoglobin (HPFH). This site functions in an analogous manner to the SSE in hybrid β-promoter/reporter gene constructs transfected into K562 cells. In contrast, the wild-type -202 sequence, which fails to bind the SSP, is incapable of activating the β-gene promoter. Both the -50 and -202 HPFH sites for SSP binding overlap a consensus sequence for the transcriptional regulator Sp1. In addition, both sites contain CpG dinucleotides that are contact bases for SSP. Since the γ promoter is known to be hypomethylated in fetal cells but fully methylated at CpG residues in adult erythroid cells, we examined the effects of this DNA modification on protein binding to the two regions. Gel mobility shift assays with nuclear extract from K562 cells (which contain both Sp1 and SSP) demonstrate preferential binding of SSP to the SSE and HPFH sites under conditions in which probe was limiting. Methylation of the CpG residues reverses this preference only in the SSE site, with a marked increase in the binding of Sp1 at the expense of the SSP. Purified Sp1 binds with 10-fold higher affinity to the methylated than to the nonmethylated -50 probe but with the same affinity to the -202 HPFH probe. The methylation-induced preferential binding of Sp1 to the SSE at the expense of SSP may be part of the mechanism by which the γ genes are repressed in normal adult erythroid cells. In cells containing the -202 HPFH mutation, the inability of Sp1 to displace SSP in the methylated state may explain the persistence of γ-promoter activity and γ-gene expression observed in adults with this mutation.