Activation of the human delta-globin gene promoter in primary adult erythroid cells

Kruppel-like factor 1-GATA1 fusion protein improves the sickle cell disease phenotype in mice both in vitro and in vivo

Sickle cell disease (SCD) and β-thalassemia are among the most common genetic disorders worldwide, affecting global health and mortality. Hemoglobin A2 (HbA2, α2δ2) is expressed at a low level in adult blood due to the lack of the Kruppel-like factor 1 (KLF1) binding motif in the δ-globin promoter region. However, HbA2 is fully functional as an oxygen transporter, and could be a valid antisickling agent in SCD, as well as a substitute for hemoglobin A in β-thalassemia. We have previously demonstrated that KLF1-GATA1 fusion protein could interact with the δ-globin promoter and increase δ-globin expression in human primary CD34+ cells. We report the effects of 2 KLF1-GATA1 fusion proteins on hemoglobin expression, as well as SCD phenotypic correction in vitro and in vivo. Forced expression of KLF1-GATA1 fusion protein enhanced δ-globin gene and HbA2 expression, as well as reduced hypoxia-related sickling, in erythroid cells cultured from both human sickle CD34+ cells and SCD mouse hematopoietic stem cells (HSCs). The fusion proteins had no impact on erythroid cell differentiation, proliferation, and enucleation. Transplantation of highly purified SCD mouse HSCs expressing KLF1-GATA1 fusion protein into SCD mice lessened the severity of the anemia, reduced the sickling of red blood cells, improved SCD-related pathological alterations in spleen, kidney, and liver, and restored urine-concentrating ability in recipient mice. Taken together, these results indicate that the use of KLF1-GATA1 fusion constructs may represent a new gene therapy approach for hemoglobinopathies.

HBB mutations and HbA2 level: Escaping the carrier screening programs

HbA2 level alone for beta thalassemia trait may not be accurate and reliable even without iron deficiency so molecular genetic testing is important and should be considered for some individuals.

Distinctive patterns of evolution of the δ-globin gene (HBD) in primates

In most vertebrates, hemoglobin (Hb) is a heterotetramer composed of two dissimilar globin chains, which change during development according to the patterns of expression of α- and β-globin family members. In placental mammals, the β-globin cluster includes three early-expressed genes, ε(HBE)-γ(HBG)-ψβ(HBBP1), and the late expressed genes, δ (HBD) and β (HBB). While HBB encodes the major adult β-globin chain, HBD is weakly expressed or totally silent. Paradoxically, in human populations HBD shows high levels of conservation typical of genes under strong evolutionary constraints, possibly due to a regulatory role in the fetal-to-adult switch unique of Anthropoid primates. In this study, we have performed a comprehensive phylogenetic and comparative analysis of the two adult β-like globin genes in a set of diverse mammalian taxa, focusing on the evolution and functional divergence of HBD in primates. Our analysis revealed that anthropoids are an exception to a general pattern of concerted evolution in placental mammals, showing a high level of sequence conservation at HBD, less frequent and shorter gene conversion events. Moreover, this lineage is unique in the retention of a functional GATA-1 motif, known to be involved in the control of the developmental expression of the β-like globin genes. We further show that not only the mode but also the rate of evolution of the δ-globin gene in higher primates are strictly associated with the fetal/adult β-cluster developmental switch. To gain further insight into the possible functional constraints that have been shaping the evolutionary history of HBD in primates, we calculated dN/dS (ω) ratios under alternative models of gene evolution. Although our results indicate that HBD might have experienced different selective pressures throughout primate evolution, as shown by different ω values between apes and Old World Monkeys + New World Monkeys (0.06 versus 0.43, respectively), these estimates corroborated a constrained evolution for HBD in Anthropoid lineages, which is unlikely to be related to protein function. Collectively, these findings suggest that sequence change at the δ-globin gene has been under strong selective constraints over 65 Myr of primate evolution, likely due to a regulatory role in ontogenic switches of gene expression.

The genetics of hemoglobin A2 regulation in sickle cell anemia

Hemoglobin A2 , a tetramer of α- and δ-globin chains, comprises less than 3% of total hemoglobin in normal adults. In northern Europeans, single nucleotide polymorphisms (SNPs) in the HBS1L-MYB locus on chromosome 6q and the HBB cluster on chromosome 11p were associated with HbA2 levels. We examined the genetic basis of HbA2 variability in sickle cell anemia using genome-wide association studies. HbA2 levels were associated with SNPs in the HBS1L-MYB interval and SNPs in BCL11A. These effects are mediated by the association of these loci with γ-globin gene expression and fetal hemoglobin (HbF) levels. The association of polymorphisms downstream of the β-globin gene (HBB) cluster on chromosome 11 with HbA2 was not mediated by HbF. In sickle cell anemia, levels of HbA2 appear to be modulated by trans-acting genes that affect HBG expression and perhaps also elements within the β-globin gene cluster. HbA2 is expressed pancellularly and can inhibit HbS polymerization. It remains to be seen if genetic regulators of HbA2 can be exploited for therapeutic purposes.

In vivo activation of the human δ-globin gene: the therapeutic potential in β-thalassemic mice

β-thalassemia and sickle cell disease are widespread fatal genetic diseases. None of the existing clinical treatments provides a solution for all patients. Two main strategies for treatment are currently being investigated: (i) gene transfer of a normal β-globin gene; (ii) reactivation of the endogenous γ-globin gene. To date, neither approach has led to a satisfactory, commonly accepted standard of care. The δ-globin gene produces the δ-globin of hemoglobin A2. Although expressed at a low level, hemoglobin A2 is fully functional and could be a valid substitute of hemoglobin A in β-thalassemia, as well as an anti-sickling agent in sickle cell disease. Previous in vitro results suggested the feasibility of transcriptional activation of the human δ-globin gene promoter by inserting a Kruppel-like factor 1 binding site. We evaluated the activation of the Kruppel-like factor 1 containing δ-globin gene in vivo in transgenic mice. To evaluate the therapeutic potential we crossed the transgenic mice carrying a single copy activated δ-globin gene with a mouse model of β-thalassemia intermedia. We show that the human δ-globin gene can be activated in vivo in a stage- and tissue-specific fashion simply by the insertion of a Kruppel-like factor 1 binding site into the promoter. In addition the activated δ-globin gene gives rise to a robust increase of the hemoglobin level in β-thalassemic mice, effectively improving the thalassemia phenotype. These results demonstrate, for the first time, the therapeutic potential of the δ-globin gene for treating severe hemoglobin disorders which could lead to novel approaches, not involving gene addition or reactivation, to the cure of β-hemoglobinopathies.

Recombinant erythroid Kruppel-like factor fused to GATA1 up-regulates delta- and gamma-globin expression in erythroid cells

The β-hemoglobinopathies sickle cell disease and β-thalassemia are among the most common human genetic disorders worldwide. Hemoglobin A2 (HbA2, α₂δ₂) and fetal hemoglobin (HbF, α₂γ₂) both inhibit the polymerization of hemoglobin S, which results in erythrocyte sickling. Expression of erythroid Kruppel-like factor (EKLF) and GATA1 is critical for transitioning hemoglobin from HbF to hemoglobin A (HbA, α₂β₂) and HbA2. The lower levels of δ-globin expression compared with β-globin expression seen in adulthood are likely due to the absence of an EKLF-binding motif in the δ-globin proximal promoter. In an effort to up-regulate δ-globin to increase HbA2 expression, we created a series of EKLF-GATA1 fusion constructs composed of the transactivation domain of EKLF and the DNA-binding domain of GATA1, and then tested their effects on hemoglobin expression. EKLF-GATA1 fusion proteins activated δ-, γ-, and β-globin promoters in K562 cells, and significantly up-regulated δ- and γ-globin RNA transcript and protein expression in K562 and/or CD34(+) cells. The binding of EKLF-GATA1 fusion proteins at the GATA1 consensus site in the δ-globin promoter was confirmed by chromatin immunoprecipitation assay. Our studies demonstrate that EKLF-GATA1 fusion proteins can enhance δ-globin expression through interaction with the δ-globin promoter, and may represent a new genetic therapeutic approach to β-hemoglobinopathies.

Activation of the delta-globin gene by the beta-globin gene CACCC motif

The promoter region of adult beta globin genes in humans and other mammals contains conserved regions of pivotal importance for their regulated tissue specific expression. These include the CACCC and CAAT motifs. The CACCC motif is duplicated in humans and other mammals. The human delta-globin gene lacks these conserved regions and its expression in normal individuals is about 3% that of the beta globin gene. Previous studies have shown that the introduction of the beta-globin CACCC or CAAT can activate the delta-globin gene promoter, but the effect of the distal CACCC element has not yet been tested. In the present study, using site-specific mutagenesis, we have introduced the consensus sequence for the distal and proximal CACCC motif and the CAAT box alone or in combination in the wild-type delta-globin gene promoter. The resulting mutants, as well as the wild type (wt) delta- and beta-globin gene promoters, have been analyzed in a transient expression assay in Cos7, K562, and MEL cell lines. The results show that the CACCC boxes can increase the transcription efficiency of the delta-globin gene promoter in both erythroid and non-erythroid cell systems. The contribution of the two CACCC elements is almost equal in the non-erythroid (Cos7) and erythroid embryonic-fetal cell lines (K562), while the proximal CACCC element is more active in adult erythroid cells (MEL). Nonetheless, duplication of this element does not appear to affect the efficiency of the promoter synergistically. Furthermore, to assess the competitive ability of the delta globin promoter containing the proximal or distal CACCC consensus sequences over the wt beta globin gene promoter, we have carried out transient expression experiments using DNA constructs in which the delta and beta globin gene promoters are linked in cis and are sharing a single enhancer (competitive transient expression). The results show that both CACCC elements are able to activate the delta globin gene promoter in Cos7 and K562 cells, although to a different extent, whereas only the proximal CACCC element is effective in increasing the transcription efficiency in MEL cells. These findings are in agreement with the more severe clinical phenotype produced by the beta-thalassemia mutations affecting the proximal CACCC box as compared with those within the distal CACCC box. The Erythroid Kruppel Like Factor (EKLF) is a nuclear protein restricted to erythroid cells which specifically bind the CACCC box sequence and activate the beta-globin gene. In the present study we carried out transactivation experiments of the mutagenized delta-globin gene promoter by introducing an EKLF expressing construct in erythroid cells. Constructs containing the proximal but not those bearing the distal CACCC element are transactivated. Our results indicate that the proximal CACCC box and, to a lesser extent, also the distal box have a role in the regulated stage specific expression of a beta-like globin gene, and show that the insertion of a single CACCC motif in the delta-globin gene promoter is sufficient to increase its activity. Nevertheless only the delta globin gene promoter containing the proximal CACCC element is able to compete with the wt beta globin gene promoter in the adult erythroid environment. These findings have potential relevance for the future prospective treatment of inherited hemoglobinopathies based on the conversion of the low functioning delta-globin gene into a high functioning beta-like globin gene.