Flow cytometric analysis of hydroxyurea effects on fetal hemoglobin production in cultures of beta-thalassemia erythroid precursors

Effects of Mithramycin on BCL11A Gene Expression and on the Interaction of the BCL11A Transcriptional Complex to γ-Globin Gene Promoter Sequences

The anticancer drug mithramycin (MTH), has been proposed for drug repurposing after the finding that it is a potent inducer of fetal hemoglobin (HbF) production in erythroid precursor cells (ErPCs) from β-thalassemia patients. In this respect, previously published studies indicate that MTH is very active in inducing increased expression of γ-globin genes in erythroid cells. This is clinically relevant, as it is firmly established that HbF induction is a valuable approach for the therapy of β-thalassemia and for ameliorating the clinical parameters of sickle-cell disease (SCD). Therefore, the identification of MTH biochemical/molecular targets is of great interest. This study is inspired by recent robust evidence indicating that the expression of γ-globin genes is controlled in adult erythroid cells by different transcriptional repressors, including Oct4, MYB, BCL11A, Sp1, KLF3 and others. Among these, BCL11A is very important. In the present paper we report evidence indicating that alterations of BCL11A gene expression and biological functions occur during MTH-mediated erythroid differentiation. Our study demonstrates that one of the mechanisms of action of MTH is a down-regulation of the transcription of the BCL11A gene, while a second mechanism of action is the inhibition of the molecular interactions between the BCL11A complex and specific sequences of the γ-globin gene promoter.

Expression of γ-globin genes in β-thalassemia patients treated with sirolimus: results from a pilot clinical trial (Sirthalaclin)

Introduction

β-thalassemia is caused by autosomal mutations in the β-globin gene, which induce the absence or low-level synthesis of β-globin in erythroid cells. It is widely accepted that a high production of fetal hemoglobin (HbF) is beneficial for patients with β-thalassemia. Sirolimus, also known as rapamycin, is a lipophilic macrolide isolated from a strain of Streptomyces hygroscopicus that serves as a strong HbF inducer in vitro and in vivo. In this study, we report biochemical, molecular, and clinical results of a sirolimus-based NCT03877809 clinical trial (a personalized medicine approach for β-thalassemia transfusion-dependent patients: testing sirolimus in a first pilot clinical trial, Sirthalaclin).

Methods

Accumulation of γ-globin mRNA was analyzed using reverse-transcription quantitative polymerase chain reaction (PCR), while the hemoglobin pattern was analyzed using high-performance liquid chromatography (HPLC). The immunophenotype was analyzed using a fluorescence-activated cell sorter (FACS), with antibodies against CD3, CD4, CD8, CD14, CD19, CD25 (for analysis of peripheral blood mononuclear cells), or CD71 and CD235a (for analysis of in vitro cultured erythroid precursors).

Results

The results were obtained in eight patients with the β+/β+ and β+/β0 genotypes, who were treated with a starting dosage of 1 mg/day sirolimus for 24-48 weeks. The first finding of this study was that the expression of γ-globin mRNA increased in the blood and erythroid precursor cells isolated from β-thalassemia patients treated with low-dose sirolimus. This trial also led to the important finding that sirolimus influences erythropoiesis and reduces biochemical markers associated with ineffective erythropoiesis (excess free α-globin chains, bilirubin, soluble transferrin receptor, and ferritin). A decrease in the transfusion demand index was observed in most (7/8) of the patients. The drug was well tolerated, with minor effects on the immunophenotype, and an only side effect of frequently occurring stomatitis.

Conclusion

The data obtained indicate that low doses of sirolimus modify hematopoiesis and induce increased expression of γ-globin genes in a subset of patients with β-thalassemia. Further clinical trials are warranted, possibly including testing of the drug in patients with less severe forms of the disease and exploring combination therapies.

Clinical experience with fetal hemoglobin induction therapy in patients with β-thalassemia

Recent molecular studies of fetal hemoglobin (HbF) regulation have reinvigorated the field and shown promise for the development of clinical HbF inducers to be used in patients with β-thalassemia and sickle cell disease. However, while numerous promising inducers of HbF have been studied in the past in β-thalassemia patient populations, with limited success in some cases, no universally effective agents have been found. Here we examine the clinical studies of such inducers in an attempt to systematically review the field. We examine trials of agents, including 5-azacytidine, hydroxyurea, and short-chain fatty acids. This review highlights the heterogeneity of clinical studies done on these agents, including both the patient populations examined and the study end points. By examining the published studies of these agents, we hope to provide a resource that will be valuable for the design of future studies of HbF inducers in β-thalassemia patient populations.

Heterogeneity of F cells in β-thalassemia

BACKGROUND

Fetal hemoglobin (HbF), which is largely replaced after birth by the adult Hb, is concentrated in a few "F cells." Their number significantly increases in certain physiologic and clinical situations, including in β-thalassemia (β-thal). Their quantification is used to detect fetal-maternal hemorrhage (FMH), where fetal cells enter the maternal circulation. We were confronted with a pregnant woman with β-thal who was suspected to have FMH. To establish the usefulness of a flow cytometric procedure to differentiate between fetal cells and the maternal F cells, we screened adult β-thal patients.

STUDY DESIGN AND METHODS

Blood samples were simultaneously stained with fluorescent antibodies to HbF and to carbonic anhydrase (CA) isotype II, which is specific to adult red blood cells (RBCs).

RESULTS

A heterogeneous distribution of RBCs with respect to HbF and CA expression was observed: adult non-F cells (CA+HbF-) and F cells (CA+HbF+/HbF++) as well as F cells with characteristics of fetal cells (CA-HbF++).

CONCLUSIONS

The presence of CA-HbF++ RBCs in nonpregnant women, and even men, with thal indicates that the CA/HbF method is inappropriate for detection of FMH. The coexistence of F cells carrying fetal or adult markers suggests that they originate from two types of stem cell, adult and fetal, lineages. Normally, the fetal lineage is insignificant, but in β-thal, as HbF-containing RBCs have a selective advantage, the "fetal" lineage gains significance.

Therapeutic levels of fetal hemoglobin in erythroid progeny of β-thalassemic CD34+ cells after lentiviral vector-mediated gene transfer

β-Thalassemia major results from severely reduced or absent expression of the β-chain of adult hemoglobin (α₂β₂;HbA). Increased levels of fetal hemoglobin (α₂γ₂;HbF), such as occurs with hereditary persistence of HbF, ameliorate the severity of β-thalassemia, raising the potential for genetic therapy directed at enhancing HbF. We used an in vitro model of human erythropoiesis to assay for enhanced production of HbF after gene delivery into CD34(+) cells obtained from mobilized peripheral blood of normal adults or steady-state bone marrow from patients with β-thalassemia major. Lentiviral vectors encoding (1) a human γ-globin gene with or without an insulator, (2) a synthetic zinc-finger transcription factor designed to interact with the γ-globin gene promoters, or (3) a short-hairpin RNA targeting the γ-globin gene repressor, BCL11A, were tested. Erythroid progeny of normal CD34(+) cells demonstrated levels of HbF up to 21% per vector copy. For β-thalassemic CD34(+) cells, similar gene transfer efficiencies achieved HbF production ranging from 45% to 60%, resulting in up to a 3-fold increase in the total cellular Hb content. These observations suggest that both lentiviral-mediated γ-globin gene addition and genetic reactivation of endogenous γ-globin genes have potential to provide therapeutic HbF levels to patients with β-globin deficiency.

Hydroxycarbamide-induced changes in E/beta thalassemia red blood cells

In thalassemia, fetal hemoglobin (HbF) augmentation with hydroxycarbamide (also known as hydroxyurea) is not always successful. The expected parallel effects on red cell (RBC) membrane deformability, cell hydration, and membrane phospholipid organization, all important for extending RBC life span and increasing Hb, have been infrequently examined. We analyzed these characteristics in 15 nontransfused E/beta(0) thalassemia patients treated with HU (mean 10.2 months). Membrane deformability and cell hydration mildly improved in association with increased HbF levels approaching statistical significance (r = 0.51, P = 0.06). All measures improved considerably in splenctomized patients. These findings underscore the disappointing results of hydroxyurea treatment in clinical trials and the importance of examining the effect on RBC characteristics for the development and understanding of HbF-enhancing agents.

Response to hydroxyurea therapy in beta-thalassemia

Although a relatively small number of previous studies suggest a modest response to hydroxyurea (HU) therapy in beta-thalassemia, more recent investigations have revealed that some transfusion-dependent patients can become transfusion-independent following HU therapy. Patients with Ggamma XmnI polymorphism, several beta-globin mutations, and alpha-thalassemia deletions were inconsistently reported to have significant responses to HU therapy. To better predict who may respond, we retrospectively evaluated the clinical response and the molecular background of 18 beta-thalassemia patients treated with HU for a mean of 46 months. The majority of transfusion-dependent patients responded to HU therapy with 9 out of 11 (82%) becoming transfusion-independent. Five thalassemia intermedia (TI) patients receiving occasional blood transfusion did not require any additional transfusions following therapy and two TI patients who had never received transfusions had a 2 g/dl increase in their hemoglobin level. The majority of beta-thalassemia major patients who became transfusion-independent (7/9) were either homozygous (5) or heterozygous (2) for the XmnI polymorphism. No correlation was identified between response to therapy and the presence of specific beta-thalassemia mutations or alpha-globin deletions. We conclude that further analysis of the degree of response of transfusion-dependent beta-thalassemia patients to HU therapy, as well as, the impact of their genetic background on this response is required to identify patients likely to have significant response.

Thalassemia and its relevance to personalized medicine

Thalassemias are the most common monogenic gene disorders in the world. Patients present with a wide variability of clinical phenotypes ranging from severe phenotype (β-thalassemia major) to a very mild, almost symptomless, condition. This variability is owing to the presence of a large number of genetic modifiers affecting the disease. Patients are treated with blood transfusions and iron chelation therapy. Pharmacological therapies have varying degrees of success depending on the genetic modifiers of the disease present in the patients. Studies undertaken to identify all the modifiers that affect β-thalassemia will lead to more appropriate genetic counseling during prenatal diagnosis and enable targeted and personalized treatment regimens for patients in the future.

Hemoglobin switch in the newborn: a flow cytometry analysis

BACKGROUND

Hemoglobin (Hb) production switches upon birth from fetal Hb (HbF) to adult Hbs. HbF production is reactivated in sickle cell anemia and beta-thalassemia, where increased HbF ameliorates clinical symptoms and HbF-stimulating drugs are used for treatment. Understanding of the switch is therefore of basic interest and important for rational drug design. We studied the switch by determining changes after birth in the frequency and HbF-content of HbF-containing red blood cells (F-RBC) and reticulocytes (F-retics).

METHODS

Blood samples of neonatal cord, babies in their first year and adults were stained with both phycoerythrin-conjugated anti-human HbF antibodies and the nucleic acid dye thiazol-orange, and analyzed by flow-cytometry. RBC and retics were distinguished by their different thiazol orange-derived fluorescence and their HbF content was determined by the intensity of phycoerythrin-derived fluorescence.

RESULTS

The frequency of both F-RBC and F-retics decreases with time after birth. In most cases, the percentage of F-RBC was higher, but their HbF-content was lower than that of retics. The HbF content of the F-RBC and the F-retics showed a gradual decrease with age.

CONCLUSIONS

The results can be explained by two models: (A) A pool of homogenous stem cells that undergo gradual changes in their globin transcription pattern. (B) A pool of heterogeneous stem cells with varying HbF potentials that become active in an age-dependent manner: first, cells with a high HbF potential and then cells with a gradually lower HbF potential. In both models, these changes are induced by a biological clock and environmental factors.

Differentiation of human erythroid cells in culture

A culture procedure for growing erythroid progenitors in liquid medium is described. The procedure is divided into two phases. The first is an erythropoietin (EPO)--independent phase in which peripheral blood mononuclear cells are isolated and cultured in the presence of a combination of growth factors, but in the absence of EPO. During this phase, early erythroid-committed progenitors proliferate and differentiate into more mature progenitors. In the second phase, the latter cells, cultured in an EPO-supplemented medium, continue to proliferate and mature into hemoglobin-containing nucleated erythroid cells. The culture procedure yields populations that are large, relatively pure, and synchronized (in terms of differentiation), and which recapitulate in vivo erythropoiesis. Since the cells are grown in suspension, samples of cells can be withdrawn at any time, without disturbing the cultures, and assayed for various parameters, e.g., morphology, size, number, viability, apoptosis, cell cycle, surface antigens, or gene expression. Several procedures for analyzing the cultured cells with respect to their hemoglobin content during their differentiation are included.

Flow cytometric analysis of fetal hemoglobin in erythroid precursors of beta-thalassemia

Fetal hemoglobin (HbF), the major hemoglobin species in fetal life, drops to <1% in normal adults, where it is restricted to a few 'F-cells', which may increase in various acquired and genetic conditions, including thalassemia. Using flow cytometry, we studied the percentage of HbF-containing cells and their HbF content in RBC, reticulocytes (retics) and normoblasts (NRBC) present in the peripheral blood of patients with beta-thalassemia. Thiazol orange, a nucleic acid-specific dye, and anti-CD45 antibodies identified the various blood cells and antihuman HbF antibodies quantitated HbF. The results indicated that F-RBC were more numerous in beta-thalassemic (both transfused and nontransfused) patients than in normal donors, but, in most cases, their HbF content was comparable, suggesting that increased HbF in thalassemia is mainly due to higher %F-cells rather than an increased HbF per cell. Among the retics, the %F-cells and their HbF content were highest in immature retics and decreased with maturation to levels of RBC. This may reflect preferential maturation of F-retics into RBC in the circulation. The NRBC population contained the lowest %F-cells. This could be due to preferential maturation of F-NRBC, having more normal phenotype than non-F-NRBC, in the bone marrow into F-retics, while non-F-NRBC enter the circulation.