Expression of the affected A gamma globin gene associated with Greek nondeletion hereditary persistence of fetal hemoglobin

The T----C substitution at -198 of the A gamma-globin gene associated with the British form of HPFH generates overlapping recognition sites for two DNA-binding proteins

Defects in the developmental changes of human hemoglobin production characterized by the continued expression of fetal globin during adult life are classified as hereditary persistence of fetal hemoglobin (HPFH). Among the various molecular lesions associated with this phenotype, the non-deletion forms with point mutations in the promoter region are thought to provide mechanistic clues for gamma-globin gene regulation. The natural occurrence of four different base substitutions mapping within six nucleotides of a homopurine.homopyrimidine motif in the upstream promoter region demarcate a potential control element. To assess its importance for transcriptional activity, we compared the -202 (C----G), -198 (T----C) and -196 (C----T) HPFH mutations with the normal sequence in binding studies with nuclear proteins from erythroid and non-erythroid cells. Wildtype DNA and HPFH mutations at -202 or -196 showed only a weak protein interaction of unclear functional significance. In contrast, -198 (T----C) generated overlapping, high-affinity binding sites for two ubiquitous nuclear proteins. One cognate protein was identified as the transcription factor Sp1. The second one was termed NF-G.C as it interacted strongly with the homopolymer poly(dG).poly(dC). The generation of additional recognition sites for trans-acting factors by the -198 HPFH mutation correlated with a modest increase in promoter activity in vitro specifically with nuclear extracts from erythroid cells. The activation appears to be mediated by binding of Sp1, but it requires interaction with an erythroid-specific factor, most likely GF-1. Templates containing the -196 HPFH mutation showed a transcriptional activity identical to wildtype. This suggests that despite the topological proximity of the mutations, the HPFH phenotype may be established by different mechanisms.

A naturally occurring gamma globin gene mutation enhances SP1 binding activity

Transcription of the human fetal globin genes in erythroid cells is tightly regulated during different stages of development and differentiation. Two naturally occurring mutations 202 base pairs upstream of the duplicated gamma globin genes are associated with incorrectly regulated gamma globin gene gene expression; elevated levels of fetal globin are synthesized during adult life. A C-to-G base substitution upstream of the G gamma-globin gene is highly correlated with a dramatic increase in gene expression. It increases the similarity of the region to the consensus Sp1 recognition site. We determined that the mutated DNA had a 5- to 10-fold-higher affinity for Sp1 than did normal gamma globin gene sequence. We also observed a reduction in normal factor-binding activity. A different substitution at -202, C to T, upstream of the A gamma-globin gene was associated with a more moderate increase in fetal globin expression. This mutation decreased the similarity of the sequence to an Sp1 recognition site. We determined that it did not result in enhanced Sp1 binding but did alter normal factor binding. We suggest that these changes in nuclear protein-binding properties detected in vitro are responsible for the enhanced gamma globin gene expression found in -202 G gamma beta + patients with hereditary persistence of fetal hemoglobin.

A naturally occurring gamma globin gene mutation enhances SP1 binding activity

Transcription of the human fetal globin genes in erythroid cells is tightly regulated during different stages of development and differentiation. Two naturally occurring mutations 202 base pairs upstream of the duplicated gamma globin genes are associated with incorrectly regulated gamma globin gene gene expression; elevated levels of fetal globin are synthesized during adult life. A C-to-G base substitution upstream of the G gamma-globin gene is highly correlated with a dramatic increase in gene expression. It increases the similarity of the region to the consensus Sp1 recognition site. We determined that the mutated DNA had a 5- to 10-fold-higher affinity for Sp1 than did normal gamma globin gene sequence. We also observed a reduction in normal factor-binding activity. A different substitution at -202, C to T, upstream of the A gamma-globin gene was associated with a more moderate increase in fetal globin expression. This mutation decreased the similarity of the sequence to an Sp1 recognition site. We determined that it did not result in enhanced Sp1 binding but did alter normal factor binding. We suggest that these changes in nuclear protein-binding properties detected in vitro are responsible for the enhanced gamma globin gene expression found in -202 G gamma beta + patients with hereditary persistence of fetal hemoglobin.

Nuclear proteins that bind the human gamma-globin gene promoter: alterations in binding produced by point mutations associated with hereditary persistence of fetal hemoglobin

The molecular mechanisms responsible for the human fetal-to-adult hemoglobin switch have not yet been elucidated. Point mutations identified in the promoter regions of gamma-globin genes from individuals with nondeletion hereditary persistence of fetal hemoglobin (HPFH) may mark cis-acting sequences important for this switch, and the trans-acting factors which interact with these sequences may be integral parts in the puzzle of gamma-globin gene regulation. We have used gel retardation and footprinting strategies to define nuclear proteins which bind to the normal gamma-globin promoter and to determine the effect of HPFH mutations on the binding of a subset of these proteins. We have identified five proteins in human erythroleukemia cells (K562 and HEL) which bind to the proximal promoter region of the normal gamma-globin gene. One factor, gamma CAAT, binds the duplicated CCAAT box sequences; the -117 HPFH mutation increases the affinity of interaction between gamma CAAT and its cognate site. Two proteins, gamma CAC1 and gamma CAC2, bind the CACCC sequence. These proteins require divalent cations for binding. The -175 HPFH mutation interferes with the binding of a fourth protein, gamma OBP, which binds an octamer sequence (ATGCAAAT) in the normal gamma-globin promoter. The HPFH phenotype of the -175 mutation indicates that the octamer-binding protein may play a negative regulatory role in this setting. A fifth protein, EF gamma a, binds to sequences which overlap the octamer-binding site. The erythroid-specific distribution of EF gamma a and its close approximation to an apparent repressor-binding site suggest that it may be important in gamma-globin regulation.

Nuclear proteins that bind the human gamma-globin gene promoter: alterations in binding produced by point mutations associated with hereditary persistence of fetal hemoglobin

The molecular mechanisms responsible for the human fetal-to-adult hemoglobin switch have not yet been elucidated. Point mutations identified in the promoter regions of gamma-globin genes from individuals with nondeletion hereditary persistence of fetal hemoglobin (HPFH) may mark cis-acting sequences important for this switch, and the trans-acting factors which interact with these sequences may be integral parts in the puzzle of gamma-globin gene regulation. We have used gel retardation and footprinting strategies to define nuclear proteins which bind to the normal gamma-globin promoter and to determine the effect of HPFH mutations on the binding of a subset of these proteins. We have identified five proteins in human erythroleukemia cells (K562 and HEL) which bind to the proximal promoter region of the normal gamma-globin gene. One factor, gamma CAAT, binds the duplicated CCAAT box sequences; the -117 HPFH mutation increases the affinity of interaction between gamma CAAT and its cognate site. Two proteins, gamma CAC1 and gamma CAC2, bind the CACCC sequence. These proteins require divalent cations for binding. The -175 HPFH mutation interferes with the binding of a fourth protein, gamma OBP, which binds an octamer sequence (ATGCAAAT) in the normal gamma-globin promoter. The HPFH phenotype of the -175 mutation indicates that the octamer-binding protein may play a negative regulatory role in this setting. A fifth protein, EF gamma a, binds to sequences which overlap the octamer-binding site. The erythroid-specific distribution of EF gamma a and its close approximation to an apparent repressor-binding site suggest that it may be important in gamma-globin regulation.

A fetal globin gene mutation in A gamma nondeletion hereditary persistence of fetal hemoglobin increases promoter strength in a nonerythroid cell

Single base substitutions have been identified in the promoter regions of A gamma-globin genes from individuals with certain types of nondeletion A gamma hereditary persistence of fetal hemoglobin (HPFH). The presence of these mutations is closely associated with the A gamma HPFH phenotype, but proof that they are the nondeletion HPFH determinants is lacking. To test directly whether these base substitutions can result in an increase in A gamma-globin gene transcription, we studied cosmid clones containing the G gamma- through beta-globin gene regions from individuals with Greek-type (G-to-A base substitution at -117) and Chinese-type (C-to-T base substitution at -196) A gamma HPFH in a transient expression assay. When tested as part of a cosmid clone, the Greek HPFH A gamma-globin gene consistently produced about 1.4 times as much RNA as the wild-type A gamma-globin gene when standardized against RNA transcribed from the G gamma genes in cis. The relative strengths of the normal and HPFH A gamma-globin gene promoters were also compared in transient expression assays with plasmids containing the A gamma-globin genes. Pseudo-wild-type A gamma-globin genes containing a short, transcriptionally neutral deletion were used so that two A gamma-globin genes that differed in their promoter sequences could be compared in the same transfection. The plasmid transient expression results indicated a 1.3- to 1.4-fold increase in steady-state RNA levels from the Greek-type A gamma HPFH promoter compared with the wild-type A gamma promoter, while no difference was documented between the Chinese-type A gamma HPFH promoter and the wild-type A gamma promoter.

A fetal globin gene mutation in A gamma nondeletion hereditary persistence of fetal hemoglobin increases promoter strength in a nonerythroid cell

Single base substitutions have been identified in the promoter regions of A gamma-globin genes from individuals with certain types of nondeletion A gamma hereditary persistence of fetal hemoglobin (HPFH). The presence of these mutations is closely associated with the A gamma HPFH phenotype, but proof that they are the nondeletion HPFH determinants is lacking. To test directly whether these base substitutions can result in an increase in A gamma-globin gene transcription, we studied cosmid clones containing the G gamma- through beta-globin gene regions from individuals with Greek-type (G-to-A base substitution at -117) and Chinese-type (C-to-T base substitution at -196) A gamma HPFH in a transient expression assay. When tested as part of a cosmid clone, the Greek HPFH A gamma-globin gene consistently produced about 1.4 times as much RNA as the wild-type A gamma-globin gene when standardized against RNA transcribed from the G gamma genes in cis. The relative strengths of the normal and HPFH A gamma-globin gene promoters were also compared in transient expression assays with plasmids containing the A gamma-globin genes. Pseudo-wild-type A gamma-globin genes containing a short, transcriptionally neutral deletion were used so that two A gamma-globin genes that differed in their promoter sequences could be compared in the same transfection. The plasmid transient expression results indicated a 1.3- to 1.4-fold increase in steady-state RNA levels from the Greek-type A gamma HPFH promoter compared with the wild-type A gamma promoter, while no difference was documented between the Chinese-type A gamma HPFH promoter and the wild-type A gamma promoter.