Novel approaches to the treatment of sickle cell disease: the potential of histone deacetylase inhibitors

The histone deacetylase inhibitor CT-101 flips the switch to fetal hemoglobin expression in sickle cell disease mice

The most common hemoglobin disorder worldwide is sickle cell disease (SCD) caused by a point mutation in the adult β-globin gene. As a result, hemoglobin S production occurs leading to clinical symptoms including vaso-occlusive pain, organ damage, and a shortened lifespan. Hydroxyurea is the only FDA-approved fetal hemoglobin (HbF) inducer in the United States that ameliorates the clinical severity of SCD. Due to challenges with hydroxyurea, our study aimed to address the unmet need for the development of non-chemotherapeutic HbF inducers. We investigated the ability of CT-101, a Class 1 histone deacetylase inhibitor, to flip the γ-globin to β-globin switch in a humanized SCD mouse model. Pharmacokinetic parameters were assessed in CD-1 and Townes SCD mice after a single intraperitoneal drug dose. Similar drug uptake and half-life were observed in both animals. Subsequent studies in β-YAC mice expressing human γ-globin and β-globin genes established the optimal dose of CT-101 that induces HbF without peripheral blood toxicity. Subsequent confirmatory studies were conducted in the SCD mouse treated with intraperitoneal CT-101, demonstrating increases in F-cells, HbF, and γ-globin gene mRNA levels. Hydroxyurea combined with CT-101 significantly decreased spleen size and hemorrhagic infarcts and improved splenic extramedullary hematopoiesis. Our novel agent, CT-101, flipped the switch by activating γ-globin gene transcription and HbF protein synthesis in the preclinical SCD mouse model without significant toxicity in the peripheral blood. These findings support the development of an oral CT-101 formulation for clinical testing in SCD.

A critical review of therapeutic interventions in sickle cell disease: Progress and challenges

Sickle cell disease (SCD) is an autosomal recessive genetic disorder that occurs due to the point mutation in the β-globin gene, which results in the formation of sickle hemoglobin (HbS) in the red blood cells (RBCs). When HbS is exposed to an oxygen-depleted environment, it polymerizes, resulting in hemolysis, vaso-occlusion pain, and impaired blood flow. Still, there is no affordable cure for this inherited disease. Approved medications held promise but were met with challenges due to limited patient tolerance and undesired side effects, thereby inhibiting their ability to enhance the quality of life across various individuals with SCD. Progress has been made in understanding the pathophysiology of SCD during the past few decades, leading to the discovery of novel targets and therapies. However, there is a compelling need for research to discover medications with improved efficacy and reduced side effects. Also, more clinical investigations on various drug combinations with different mechanisms of action are needed. This review comprehensively presents therapeutic approaches for SCD, including those currently available or under investigation. It covers fundamental aspects of the disease, such as epidemiology and pathophysiology, and provides detailed discussions on various disease-modifying agents. Additionally, expert insights are offered on the future development of pharmacotherapy for SCD.

Cell and Gene Therapy for Anemia: Hematopoietic Stem Cells and Gene Editing

Hereditary anemia has various manifestations, such as sickle cell disease (SCD), Fanconi anemia, glucose-6-phosphate dehydrogenase deficiency (G6PDD), and thalassemia. The available management strategies for these disorders are still unsatisfactory and do not eliminate the main causes. As genetic aberrations are the main causes of all forms of hereditary anemia, the optimal approach involves repairing the defective gene, possibly through the transplantation of normal hematopoietic stem cells (HSCs) from a normal matching donor or through gene therapy approaches (either in vivo or ex vivo) to correct the patient’s HSCs. To clearly illustrate the importance of cell and gene therapy in hereditary anemia, this paper provides a review of the genetic aberration, epidemiology, clinical features, current management, and cell and gene therapy endeavors related to SCD, thalassemia, Fanconi anemia, and G6PDD. Moreover, we expound the future research direction of HSC derivation from induced pluripotent stem cells (iPSCs), strategies to edit HSCs, gene therapy risk mitigation, and their clinical perspectives. In conclusion, gene-corrected hematopoietic stem cell transplantation has promising outcomes for SCD, Fanconi anemia, and thalassemia, and it may overcome the limitation of the source of allogenic bone marrow transplantation.

Impairment of human terminal erythroid differentiation by histone deacetylase 5 deficiency

Histone deacetylases (HDACs) are a group of enzymes catalyzing the removal of acetyl groups from histone and non-histone proteins. HDACs have been shown to play diverse functions in a wide range of biological processes. However, their roles in mammalian erythropoiesis remain to be fully defined. We show here that of the eleven classic HDAC family members, six of them (HDAC 1,2,3 and HDAC 5,6,7) are expressed in human erythroid cells with HDAC5 most significantly up regulated during terminal erythroid differentiation. Knockdown of HDAC5 by either shRNA or siRNA in human CD34+ cells followed by erythroid cell culture led to increased apoptosis, decreased chromatin condensation, and impaired enucleation of erythroblasts. Biochemical analyses revealed that HDAC5 deficiency resulted in activation of p53 in association with increased acetylation of p53. Furthermore, while acetylation of histone 4 (H4) is decreased during normal terminal erythroid differentiation, HDAC5 deficiency led to increased acetylation of H4 (K12) in late stage erythroblasts. This increased acetylation was accompanied by decreased chromatin condensation, implying a role for H4 (K12) deacetylation in chromatin condensation. ATAC-seq and RNA-seq analyses revealed that HDAC5 knockdown leads to increased chromatin accessibility genome wide and global changes in gene expression. Moreover, pharmacological inhibition of HDAC5 by the inhibitor LMK235 also led to increased H4 acetylation, impaired chromatin condensation and enucleation. Taken together, our findings have uncovered previously unrecognized roles and molecular mechanisms of action for HDAC5 in human erythropoiesis. These results may provide insights into understanding the anemia associated with HDAC inhibitor treatment.

Regulating the Regulators: The Role of Histone Deacetylase 1 (HDAC1) in Erythropoiesis

Histone deacetylases (HDACs) play important roles in transcriptional regulation in eukaryotic cells. Class I deacetylase HDAC1/2 often associates with repressor complexes, such as Sin3 (Switch Independent 3), NuRD (Nucleosome remodeling and deacetylase) and CoREST (Corepressor of RE1 silencing transcription factor) complexes. It has been shown that HDAC1 interacts with and modulates all essential transcription factors for erythropoiesis. During erythropoiesis, histone deacetylase activity is dramatically reduced. Consistently, inhibition of HDAC activity promotes erythroid differentiation. The reduction of HDAC activity not only results in the activation of transcription activators such as GATA-1 (GATA-binding factor 1), TAL1 (TAL BHLH Transcription Factor 1) and KLF1 (Krüpple-like factor 1), but also represses transcription repressors such as PU.1 (Putative oncogene Spi-1). The reduction of histone deacetylase activity is mainly through HDAC1 acetylation that attenuates HDAC1 activity and trans-repress HDAC2 activity through dimerization with HDAC1. Therefore, the acetylation of HDAC1 can convert the corepressor complex to an activator complex for gene activation. HDAC1 also can deacetylate non-histone proteins that play a role on erythropoiesis, therefore adds another layer of gene regulation through HDAC1. Clinically, it has been shown HDACi can reactivate fetal globin in adult erythroid cells. This review will cover the up to date research on the role of HDAC1 in modulating key transcription factors for erythropoiesis and its clinical relevance.

New and emerging treatments for vaso-occlusive pain in sickle cell disease

Introduction: Acute pain from episodic vaso-occlusion (VOC) spans the lifespan of almost everyone with sickle cell disease (SCD), while additional chronic pain develops in susceptible individuals in early adolescences. Frequent acute pain with chronic pain causes significant physical and psychological morbidity, and frequent health-care utilization. Available pharmacologic therapies reduce acute pain frequency but few evidence-based therapies are available for chronic pain. Areas covered: An extensive PubMed literature search was performed with appropriate search criteria. The pathophysiology of acute pain from VOC in SCD is very complex with many events subsequent to sickle polymer formation. Sensitization of pain pathways and alterations of brain networks contributes to the experience of chronic pain. Numerous therapies targeting putative VOC mechanisms are in clinical trials, and show considerable promise. Alternative analgesic treatments for acute and chronic pain have been examined in small patient cohorts, but formal clinical trials are lacking. Expert opinion: Childhood is likely a critical window for prevention of acute and later chronic pain. New multimodal analgesic therapies are needed, particularly for chronic pain, and should be examined in clinical trials. Given the multifactorial nature of both pain and VOC, simultaneously targeting multiple mechanisms may be the optimal approach for effective preventive therapies.

SIRT1 activates the expression of fetal hemoglobin genes

High fetal hemoglobin (HbF, α₂ γ₂ ) levels ameliorate the clinical manifestations of sickle cell disease and β thalassemia. The mechanisms that repress HbF expression and silence γ-globin genes in adults are incompletely characterized and only a single HbF inducer, hydroxyurea, is approved for treatment, and only in patients with sickle cell disease. We identified SIRT1, a protein deacetylase, as a new inducer of γ-globin. SIRT1 knockdown decreased, while SIRT1 ectopic expression upregulated γ-globin gene (HBG) expression in primary human erythroid cells and in K562 cells. The small molecule SIRT1 activators SRT2104 and SRT1720 enhanced HBG expression in cord blood human erythroblasts and reactivated silenced HBG in adult human erythroblasts. Furthermore, SIRT1 binds in the β-globin gene cluster locus control region (LCR) and HBG promoters, promotes the looping of the LCR to HBG promoter, and increases the binding of RNA polymerase II and H4K16Ac in the HBG promoter. SIRT1 suppressed the expression of the HBG suppressors BCL11A, KLF1, HDAC1 and HDAC2. Lastly, SIRT1 did not change the proliferation of human erythroid progenitor cells or the expression of differentiation marker CD235a. These data suggest that SIRT1 activates HBG expression through facilitating LCR looping to the HBG promoter, inhibiting the expression of transcriptional suppressors of HBG, and indirectly increasing histone acetylation in the HBG promoter. SIRT1 is a potential therapeutic target for γ-globin gene induction, and small molecule SIRT1 activators might serve as a lead compound for the development of new HbF inducers.

Induction of foetal haemoglobin synthesis in erythroid progenitor stem cells: mediated by water-soluble components of Terminalia catappa

Current novel therapeutic agents for the treatment of sickle cell anaemia (SCA) focus on increasing foetal haemoglobin (HbF) levels in SCA patients. Unfortunately, the only approved HbF-inducing agent, hydroxyurea, has long-term unpredictable side effects. Studies have shown the potential of plant compounds to modulate HbF synthesis in primary erythroid progenitor stem cells. We isolated a novel HbF-inducing Terminalia catappa distilled water active fraction (TCDWF) from Terminalia catappa leaves that induced the commitment of erythroid progenitor stem cells to the erythroid lineage and relatively higher HbF synthesis of 9.2- and 6.8-fold increases in both erythropoietin (EPO)-independent and EPO-dependent progenitor stem cells respectively. TCDWF was differentially cytotoxic to EPO-dependent and EPO-independent erythroid progenitor stem cell cultures as revealed by lactate dehydrogenase release from the cells. TCDWF demonstrated a protective effect on EPO-dependent and not EPO-independent progenitor cells. TCDWF induced a modest increase in caspase 3 activity in EPO-independent erythroid progenitor stem cell cultures compared with a significantly higher (P˂0.05) caspase 3 activity in EPO-dependent ones. The results demonstrate that TCDWF may hold promising HbF-inducing compounds, which work synergistically, and suggest a dual modulatory effect on erythropoiesis inherent in this active fraction.

Development of phenotypic screening assays for γ-globin induction using primary human bone marrow day 7 erythroid progenitor cells

Sickle cell anemia (SCA) is a genetic disorder of the β-globin gene. SCA results in chronic ischemia with pain and tissue injury. The extent of SCA symptoms can be ameliorated by treatment with drugs, which result in increasing the levels of γ-globin in patient red blood cells. Hydroxyurea (HU) is a Food and Drug Administration-approved drug for SCA, but it has dose-limiting toxicity, and patients exhibit highly variable treatment responses. To identify compounds that may lead to the development of better and safer medicines, we have established a method using primary human bone marrow day 7 erythroid progenitor cells (EPCs) to screen for compounds that induce γ-globin production. First, human marrow CD34(+) cells were cultured and expanded for 7 days and characterized for the expression of erythroid differentiation markers (CD71, CD36, and CD235a). Second, fresh or cryopreserved EPCs were treated with compounds for 3 days in 384-well plates followed by γ-globin quantification by an enzyme-linked immunosorbent assay (ELISA), which was validated using HU and decitabine. From the 7408 compounds screened, we identified at least one new compound with confirmed γ-globin-inducing activity. Hits are undergoing analysis in secondary assays. In this article, we describe the method of generating fit-for-purpose EPCs; the development, optimization, and validation of the ELISA and secondary assays for γ-globin detection; and screening results.