Understanding α-globin gene regulation and implications for the treatment of β-thalassemia

Use of HSC-targeted LNP to generate a mouse model of lethal α-thalassemia and treatment via lentiviral gene therapy

ABSTRACT

α-Thalassemia (AT) is one of the most commonly occurring inherited hematological diseases. However, few treatments are available, and allogeneic bone marrow transplantation is the only available therapeutic option for patients with severe AT. Research into AT has remained limited because of a lack of adult mouse models, with severe AT typically resulting in in utero lethality. By using a lipid nanoparticle (LNP) targeting the receptor CD117 and delivering a Cre messenger RNA (mRNACreLNPCD117), we were able to delete floxed α-globin genes at high efficiency in hematopoietic stem cells (HSC) ex vivo. These cells were then engrafted in the absence or presence of a novel α-globin-expressing lentiviral vector (ALS20αI). Myeloablated mice infused with mRNACreLNPCD117-treated HSC showed a complete knock out (KO) of α-globin genes. They showed a phenotype characterized by the synthesis of hemoglobin H (HbH; also known as β-tetramers or β4), aberrant erythropoiesis, and abnormal organ morphology, culminating in lethality ∼8 weeks after engraftment. Mice infused with mRNACreLNPCD117-treated HSC with at least 1 copy of ALS20αI survived long term with normalization of erythropoiesis, decreased production of HbH, and amelioration of the abnormal organ morphology. Furthermore, we tested ALS20αI in erythroid progenitors derived from α-globin-KO CD34+ cells and cells isolated from patients with both deletional and nondeletional HbH disease, demonstrating improvement in α-globin/β-globin mRNA ratio and reduction in the formation of HbH by high-performance liquid chromatography. Our results demonstrate the broad applicability of LNP for disease modeling, characterization of a novel mouse model of severe AT, and the efficacy of ALS20αI for treating AT.

Therapeutic Relevance of Inducing Autophagy in β-Thalassemia

The β-thalassemias are inherited genetic disorders affecting the hematopoietic system. In β-thalassemias, more than 350 mutations of the adult β-globin gene cause the low or absent production of adult hemoglobin (HbA). A clinical parameter affecting the physiology of erythroid cells is the excess of free α-globin. Possible experimental strategies for a reduction in excess free α-globin chains in β-thalassemia are CRISPR-Cas9-based genome editing of the β-globin gene, forcing "de novo" HbA production and fetal hemoglobin (HbF) induction. In addition, a reduction in excess free α-globin chains in β-thalassemia can be achieved by induction of the autophagic process. This process is regulated by the Unc-51-like kinase 1 (Ulk1) gene. The interplay with the PI3K/Akt/TOR pathway, with the activity of the α-globin stabilizing protein (AHSP) and the involvement of microRNAs in autophagy and Ulk1 gene expression, is presented and discussed in the context of identifying novel biomarkers and potential therapeutic targets for β-thalassemia.

Interplay between α-thalassemia and β-hemoglobinopathies: Translating genotype-phenotype relationships into therapies

α-Thalassemia represents one of the most important genetic modulators of β-hemoglobinopathies. During this last decade, the ongoing interest in characterizing genotype-phenotype relationships has yielded incredible insights into α-globin gene regulation and its impact on β-hemoglobinopathies. In this review, we provide a holistic update on α-globin gene expression stemming from DNA to RNA to protein, as well as epigenetic mechanisms that can impact gene expression and potentially influence phenotypic outcomes. Here, we highlight defined α-globin targeted strategies and rationalize the use of distinct molecular targets based on the restoration of balanced α/β-like globin chain synthesis. Considering the therapies that either increase β-globin synthesis or reactivate γ-globin gene expression, the modulation of α-globin chains as a disease modifier for β-hemoglobinopathies still remains largely uncharted in clinical studies.

CRISPR/Cas-based gene editing in therapeutic strategies for beta-thalassemia

Beta-thalassemia (β-thalassemia) is an autosomal recessive disorder caused by point mutations, insertions, and deletions in the HBB gene cluster, resulting in the underproduction of β-globin chains. The most severe type may demonstrate complications including massive hepatosplenomegaly, bone deformities, and severe growth retardation in children. Treatments for β-thalassemia include blood transfusion, splenectomy, and allogeneic hematopoietic stem cell transplantation (HSCT). However, long-term blood transfusions require regular iron removal therapy. For allogeneic HSCT, human lymphocyte antigen (HLA)-matched donors are rarely available, and acute graft-versus-host disease (GVHD) may occur after the transplantation. Thus, these conventional treatments are facing significant challenges. In recent years, with the advent and advancement of CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 (CRISPR-associated protein 9) gene editing technology, precise genome editing has achieved encouraging successes in basic and clinical studies for treating various genetic disorders, including β-thalassemia. Target gene-edited autogeneic HSCT helps patients avoid graft rejection and GVHD, making it a promising curative therapy for transfusion-dependent β-thalassemia (TDT). In this review, we introduce the development and mechanisms of CRISPR/Cas9. Recent advances on feasible strategies of CRISPR/Cas9 targeting three globin genes (HBB, HBG, and HBA) and targeting cell selections for β-thalassemia therapy are highlighted. Current CRISPR-based clinical trials in the treatment of β-thalassemia are summarized, which are focused on γ-globin reactivation and fetal hemoglobin reproduction in hematopoietic stem cells. Lastly, the applications of other promising CRISPR-based technologies, such as base editing and prime editing, in treating β-thalassemia and the limitations of the CRISPR/Cas system in therapeutic applications are discussed.

RNA polymerase II pausing temporally coordinates cell cycle progression and erythroid differentiation

Controlled release of promoter-proximal paused RNA polymerase II (RNA Pol II) is crucial for gene regulation. However, studying RNA Pol II pausing is challenging, as pause-release factors are almost all essential. In this study, we identified heterozygous loss-of-function mutations in SUPT5H, which encodes SPT5, in individuals with β-thalassemia. During erythropoiesis in healthy human cells, cell cycle genes were highly paused as cells transition from progenitors to precursors. When the pathogenic mutations were recapitulated by SUPT5H editing, RNA Pol II pause release was globally disrupted, and as cells began transitioning from progenitors to precursors, differentiation was delayed, accompanied by a transient lag in erythroid-specific gene expression and cell cycle kinetics. Despite this delay, cells terminally differentiate, and cell cycle phase distributions normalize. Therefore, hindering pause release perturbs proliferation and differentiation dynamics at a key transition during erythropoiesis, identifying a role for RNA Pol II pausing in temporally coordinating the cell cycle and erythroid differentiation.

Loss of miR-144/451 alleviates β-thalassemia by stimulating ULK1-mediated autophagy of free α-globin

Most cells can eliminate unstable or misfolded proteins through quality control mechanisms. In the inherited red blood cell disorder β-thalassemia, mutations in the β-globin gene (HBB) lead to a reduction in the corresponding protein and the accumulation of cytotoxic free α-globin, which causes maturation arrest and apoptosis of erythroid precursors and reductions in the lifespan of circulating red blood cells. We showed previously that excess α-globin is eliminated by Unc-51-like autophagy activating kinase 1 (ULK1)-dependent autophagy and that stimulating this pathway by systemic mammalian target of rapamycin complex 1 (mTORC1) inhibition alleviates β-thalassemia pathologies. We show here that disrupting the bicistronic microRNA gene miR-144/451 alleviates β-thalassemia by reducing mTORC1 activity and stimulating ULK1-mediated autophagy of free α-globin through 2 mechanisms. Loss of miR-451 upregulated its target messenger RNA, Cab39, which encodes a cofactor for LKB1, a serine-threonine kinase that phosphorylates and activates the central metabolic sensor adenosine monophosphate-activated protein kinase (AMPK). The resultant enhancement of LKB1 activity stimulated AMPK and its downstream effects, including repression of mTORC1 and direct activation of ULK1. In addition, loss of miR-144/451 inhibited the expression of erythroblast transferrin receptor 1, causing intracellular iron restriction, which has been shown to inhibit mTORC1, reduce free α-globin precipitates, and improve hematological indices in β-thalassemia. The beneficial effects of miR-144/451 loss in β-thalassemia were inhibited by the disruption of Cab39 or Ulk1 genes. Together, our findings link the severity of β-thalassemia to a highly expressed erythroid microRNA locus and a fundamental, metabolically regulated protein quality control pathway that is amenable to therapeutic manipulation.

Clinical significance of mutational variants in beta and alpha genes in patients with hemoglobinopathies from two large Greek centers: a complex interplay between genotype and phenotype

Hemoglobinopathies affect patients in the wider Mediterranean area consisting of 4 distinct subgroups: beta thalassemia major (TM), beta thalassemia intermedia (TI), sickle cell disease (SCD) and hemoglobin H disease (alpha thalassemia). The clinical spectrum varies from mild to severe. Complex interactions between genes and environmental factors form the clinical manifestations. There is an unmet need to clarify these multifactorial mechanisms. This is the first Greek study describing mutational alleles (HBB and HBA1/HBA2 gene variants) in 217 patients with hemoglobinopathies of two large centers in Greece (Larissa and Athens) and associating particular genotypes or gene variants with clinical manifestations (transfusion frequency, complications). Thus, the complex interplay between corresponding genotypes and phenotypes was investigated. Our results are in accordance with previous national studies with limited variations, due to regional prevalence of specific gene variants, as expected. It is also a description of the prevalence of hemoglobinopathies in the Greek population. The type and prevalence of beta and alpha globin gene variants differ significantly among countries. We also confirm the well-known observation of many studies that in our beta thalassemic or SCD patients, co-inheritance of variants in the alpha globin genes, leading to absence or reduction of alpha globin synthesis were associated with milder clinical course, whereas the inheritance of additional alpha genes (triplication) led to a more severe clinical phenotype. In cases in whom the genotype and phenotype did not correlate, factors like the function or modification of possible regulatory genes or additional nutritional-environmental effects should be investigated. KEY MESSAGES: • This is the first Greek study, fully molecularly defining the beta and alpha mutational alleles in 217 patients with hemoglobinopathies of two large centers in Greece and correlating particular genotypes or gene variants with clinical manifestations (transfusion frequency, complications). • In the beta thalassemic or SCD patients of our cohort, co-inheritance of variants in the alpha globin genes, leading to absence or reduction of alpha globin synthesis were associated with milder clinical course (confirmation of a well-known previous observation). • The inheritance of additional alpha genes (triplication) led to a more severe clinical phenotype (confirmation of a well known previous observation). • The function or modification of possible regulatory genes should be investigated in cases in whom the genotype and phenotype did not correlate.

Latent generative landscapes as maps of functional diversity in protein sequence space

Variational autoencoders are unsupervised learning models with generative capabilities, when applied to protein data, they classify sequences by phylogeny and generate de novo sequences which preserve statistical properties of protein composition. While previous studies focus on clustering and generative features, here, we evaluate the underlying latent manifold in which sequence information is embedded. To investigate properties of the latent manifold, we utilize direct coupling analysis and a Potts Hamiltonian model to construct a latent generative landscape. We showcase how this landscape captures phylogenetic groupings, functional and fitness properties of several systems including Globins, β-lactamases, ion channels, and transcription factors. We provide support on how the landscape helps us understand the effects of sequence variability observed in experimental data and provides insights on directed and natural protein evolution. We propose that combining generative properties and functional predictive power of variational autoencoders and coevolutionary analysis could be beneficial in applications for protein engineering and design.

Molecular Basis and Genetic Modifiers of Thalassemia

Thalassemia syndromes are common monogenic disorders and represent a significant health issue worldwide. In this review, the authors elaborate on fundamental genetic knowledge about thalassemias, including the structure and location of globin genes, the production of hemoglobin during development, the molecular lesions causing α-, β-, and other thalassemia syndromes, the genotype-phenotype correlation, and the genetic modifiers of these conditions. In addition, they briefly discuss the molecular techniques applied for diagnosis and innovative cell and gene therapy strategies to cure these conditions.

RNA Polymerase II pausing temporally coordinates cell cycle progression and erythroid differentiation

The controlled release of promoter-proximal paused RNA polymerase II (Pol II) into productive elongation is a major step in gene regulation. However, functional analysis of Pol II pausing is difficult because factors that regulate pause release are almost all essential. In this study, we identified heterozygous loss-of-function mutations in SUPT5H , which encodes SPT5, in individuals with β-thalassemia unlinked to HBB mutations. During erythropoiesis in healthy human cells, cell cycle genes were highly paused at the transition from progenitors to precursors. When the pathogenic mutations were recapitulated by SUPT5H editing, Pol II pause release was globally disrupted, and the transition from progenitors to precursors was delayed, marked by a transient lag in erythroid-specific gene expression and cell cycle kinetics. Despite this delay, cells terminally differentiate, and cell cycle phase distributions normalize. Therefore, hindering pause release perturbs proliferation and differentiation dynamics at a key transition during erythropoiesis, revealing a role for Pol II pausing in the temporal coordination between the cell cycle and differentiation.

Epigenetic Regulation of β-Globin Genes and the Potential to Treat Hemoglobinopathies through Epigenome Editing

Beta-like globin gene expression is developmentally regulated during life by transcription factors, chromatin looping and epigenome modifications of the β-globin locus. Epigenome modifications, such as histone methylation/demethylation and acetylation/deacetylation and DNA methylation, are associated with up- or down-regulation of gene expression. The understanding of these mechanisms and their outcome in gene expression has paved the way to the development of new therapeutic strategies for treating various diseases, such as β-hemoglobinopathies. Histone deacetylase and DNA methyl-transferase inhibitors are currently being tested in clinical trials for hemoglobinopathies patients. However, these approaches are often uncertain, non-specific and their global effect poses serious safety concerns. Epigenome editing is a recently developed and promising tool that consists of a DNA recognition domain (zinc finger, transcription activator-like effector or dead clustered regularly interspaced short palindromic repeats Cas9) fused to the catalytic domain of a chromatin-modifying enzyme. It offers a more specific targeting of disease-related genes (e.g., the ability to reactivate the fetal γ-globin genes and improve the hemoglobinopathy phenotype) and it facilitates the development of scarless gene therapy approaches. Here, we summarize the mechanisms of epigenome regulation of the β-globin locus, and we discuss the application of epigenome editing for the treatment of hemoglobinopathies.

Knowledge and practices on childhood anaemia, thalassaemia and iron deficiency among mothers of children aged between 6 and 59 months in a suburban area of Sri Lanka

BACKGROUND

Childhood anaemia is one of the most common public health problems worldwide. Here, we aim to describe the knowledge and practices on childhood anaemia, thalassaemia and iron deficiency among mothers of children aged between 6 and 59 months in a suburban district of Sri Lanka.

METHODS

We performed a cross-sectional survey in the Gampaha District of Sri Lanka from December 2020 to February 2021. One well-baby clinic each from four Medical Officer of Health areas in the district was selected using stratified random sampling. Mothers of all children aged between 6 and 59 months attending well-baby clinics were recruited until the sample size was achieved. Data were collected using a self-administered questionnaire and analysed using logistic regression.

RESULTS

A total of 392 mothers were recruited; 53% of their children were males. Only 33% of mothers had an accurate understanding of anaemia, while 71% and 28%, respectively, could name at least one symptom and two causes of anaemia; 12% could not name a single food rich in iron. Only 13% of mothers knew that thalassaemia is a cause of anaemia, and 14% had been screened for thalassaemia. Logistic regression analysis that examined for factors associated with higher knowledge of anaemia revealed that an accurate understanding of anaemia was associated with maternal age over 30 years (p < 0.05) and maternal education level beyond grade ten (p < 0.001). In contrast, higher knowledge of symptoms of anaemia was associated with maternal employment (p < 0.01).

CONCLUSIONS

The knowledge of anaemia and awareness of thalassaemia among mothers was poor. Very few mothers were aware of iron-rich food and feed it to their children. Despite being located in a thalassaemia-endemic region, very few knew that thalassaemia is a cause of anaemia and have got themselves screened for thalassaemia.

Segregation of α- and β-Globin Gene Cluster in Vertebrate Evolution: Chance or Necessity?

The review is devoted to the patterns of evolution of α- and β-globin gene domains. A hypothesis is presented according to which segregation of the ancestral cluster of α/β-globin genes in Amniota occurred due to the performance by α-globins and β-globins of non-canonical functions not related to oxygen transport.

A systematic review on hydroxyurea therapy for sickle cell disease in India

Background & objectives

Sickle cell disease (SCD) constitutes frequently inherited haemoglobin disorders and poses a significant health burden in India. Hydroxyurea (HU), the most commonly used drug, has shown promising results in the clinical management of SCD. The present systematic review was undertaken to assess the efficacy and toxicity of HU in Indian sickle cell patients.

Methods

A systematic review of studies on HU therapy was conducted to identify the application of HU and its outcome(s) across India. PubMed, Scopus and Cochrane Library was used as data sources for various studies on the efficacy and toxicity of HU therapy for treatment for SCD in India published between January 2001 and October 2021. Two authors independently extracted the data on study design, patient characteristics and therapeutic outcomes of HU in order to determine the study quality of the present review.

Results

Overall, 14 studies were included for a systematic analysis. Of these 11 were prospective, two cross-sectional and one double-blind randomized controlled trial. Low-dose HU (10 mg/kg/day) was found to reduce the rates of vaso-occlusive crisis and hospitalization as well as decreased the requirement of blood transfusion in SCD patients. The foetal haemoglobin (HbF) level was recorded in 13 (80%) studies all of whom reported an elevation in the HbF levels, with a mean increase in per cent HbF from 15.8 to 21.4 per cent across studies. The common adverse events were reversible, mild-to-moderate cytopenia and anaemia.

Interpretation & conclusions

The findings of the present review suggest that there is still insufficient information presently to determine the long-term or major adverse effects on organ damage, fertility as well as pregnancy on the use of HU therapy for SCD. Long-term multi-centric studies are thus required to address these problems.

A randomised double-blind placebo-controlled clinical trial of oral hydroxyurea for transfusion-dependent β-thalassaemia

Hydroxyurea is an antimetabolite drug that induces fetal haemoglobin in sickle cell disease. However, its clinical usefulness in β-thalassaemia is unproven. We conducted a randomised, double-blind, placebo-controlled clinical trial to evaluate the efficacy and safety of hydroxyurea in transfusion-dependent β-thalassaemia. Sixty patients were assigned 1:1 to oral hydroxyurea 10–20 mg/kg/day or placebo for 6 months by stratified block randomisation. Hydroxyurea treatment did not alter the blood transfusion volume overall. However, a significantly higher proportion of patients on hydroxyurea showed increases in fetal haemoglobin percentage (89% vs. 59%; p < 0.05) and reductions in erythropoietic stress as measured by soluble transferrin receptor concentration (79% vs. 40%; p < 0.05). Based on fetal haemoglobin induction (> 1.5%), 44% of patients were identified as hydroxyurea-responders. Hydroxyurea-responders, required significantly lower blood volume (77 ± SD27ml/kg) compared to hydroxyurea-non-responders (108 ± SD24ml/kg; p < 0.01) and placebo-receivers (102 ± 28ml/kg; p < 0.05). Response to hydroxyurea was significantly higher in patients with HbE β-thalassaemia genotype (50% vs. 0%; p < 0.01) and Xmn1 polymorphism of the γ-globin gene (67% vs. 27%; p < 0.05). We conclude that oral hydroxyurea increased fetal haemoglobin percentage and reduced erythropoietic stress of ineffective erythropoiesis in patients with transfusion-dependent β-thalassaemia. Hydroxyurea reduced the transfusion burden in approximately 40% of patients. Response to hydroxyurea was higher in patients with HbE β-thalassaemia genotype and Xmn1 polymorphism of the γ-globin gene.

Recent Approaches for Manipulating Globin Gene Expression in Treating Hemoglobinopathies

Tissue oxygenation throughout life depends on the activity of hemoglobin (Hb) one of the hemeproteins that binds oxygen in the lungs and secures its delivery throughout the body. Hb is composed of four monomers encoded by eight different genes the expression of which is tightly regulated during development, resulting in the formation of distinct hemoglobin tetramers in each developmental stage. Mutations that alter hemoglobin structure or its regulated expression result in a large group of diseases typically referred to as hemoglobinopathies that are amongst the most common genetic defects worldwide. Unprecedented efforts in the last decades have partially unraveled the complex mechanisms that control globin gene expression throughout development. In addition, genome wide association studies have revealed protective genetic traits capable of ameliorating the clinical manifestations of severe hemoglobinopathies. This knowledge has fueled the exploration of innovative therapeutic approaches aimed at modifying the genome or the epigenome of the affected cells to either restore hemoglobin function or to mimic the effect of protective traits. Here we describe the key steps that control the switch in gene expression that concerns the different globin genes during development and highlight the latest efforts in altering globin regulation for therapeutic purposes.

The Future of Gene Therapy for Transfusion-Dependent Beta-Thalassemia: The Power of the Lentiviral Vector for Genetically Modified Hematopoietic Stem Cells

β-thalassemia, a disease that results from defects in β-globin synthesis, leads to an imbalance of β- and α-globin chains and an excess of α chains. Defective erythroid maturation, ineffective erythropoiesis, and shortened red blood cell survival are commonly observed in most β-thalassemia patients. In severe cases, blood transfusion is considered as a mainstay therapy; however, regular blood transfusions result in chronic iron overload with life-threatening complications, e.g., endocrine dysfunction, cardiomyopathy, liver disease, and ultimately premature death. Therefore, transplantation of healthy hematopoietic stem cells (HSCs) is considered an alternative treatment. Patients with a compatible human leukocyte antigen (HLA) matched donor can be cured by allogeneic HSC transplantation. However, some recipients faced a high risk of morbidity/mortality due to graft versus host disease or graft failure, while a majority of patients do not have such HLA match-related donors. Currently, the infusion of autologous HSCs modified with a lentiviral vector expressing the β-globin gene into the erythroid progenitors of the patient is a promising approach to completely cure β-thalassemia. Here, we discuss a history of β-thalassemia treatments and limitations, in particular the development of β-globin lentiviral vectors, with emphasis on clinical applications and future perspectives in a new era of medicine.

Thalassemias: From gene to therapy

Thalassemias (α, β, γ, δ, δβ, and εγδβ) are the most common genetic disorders worldwide and constitute a heterogeneous group of hereditary diseases characterized by the deficient synthesis of one or more hemoglobin (Hb) chain(s). This leads to the accumulation of unstable non-thalassemic Hb chains, which precipitate and cause intramedullary destruction of erythroid precursors and premature lysis of red blood cells (RBC) in the peripheral blood. Non-thalassemic Hbs display high oxygen affinity and no cooperativity. Thalassemias result from many different genetic and molecular defects leading to either severe or clinically silent hematologic phenotypes. Thalassemias α and β are particularly diffused in the regions spanning from the Mediterranean basin through the Middle East, Indian subcontinent, Burma, Southeast Asia, Melanesia, and the Pacific Islands, whereas δβ-thalassemia is prevalent in some Mediterranean regions including Italy, Greece, and Turkey. Although in the world thalassemia and malaria areas overlap apparently, the RBC protection against malaria parasites is openly debated. Here, we provide an overview of the historical, geographic, genetic, structural, and molecular pathophysiological aspects of thalassemias. Moreover, attention has been paid to molecular and epigenetic pathways regulating globin gene expression and globin switching. Challenges of conventional standard treatments, including RBC transfusions and iron chelation therapy, splenectomy and hematopoietic stem cell transplantation from normal donors are reported. Finally, the progress made by rapidly evolving fields of gene therapy and gene editing strategies, already in pre-clinical and clinical evaluation, and future challenges as novel curative treatments for thalassemia are discussed.

Genetic and Epigenetic Therapies for β-Thalassaemia by Altering the Expression of α-Globin Gene

β-Thalassaemia is caused by over 300 mutations in and around the β-globin gene that lead to impaired synthesis of β-globin. The expression of α-globin continues normally, resulting in an excess of α-globin chains within red blood cells and their precursors. These unpaired α-globin chains form unstable α-hemichromes that trigger cascades of events to generate reactive oxygen species, leading to ineffective erythropoiesis and haemolysis in patients with β-thalassaemia. The clinical genetic data reported over several decades have demonstrated how the coinheritance of α-thalassaemia ameliorates the disease phenotype of β-thalassaemia. Thus, it is evident that down-regulation of the α-globin gene expression in patients with β-thalassaemia could ameliorate or even cure β-thalassaemia. Over the last few years, significant progress has been made in utilising this pathway to devise a cure for β-thalassaemia. Most research has been done to alter the epigenetic landscape of the α-globin locus or the well-characterised distant enhancers of α-globin. In vitro, pre-clinical studies on primary human erythroid cells have unveiled inhibition of histone lysine demethylation and histone deacetylation as potential targets to achieve selective downregulation of α-globin through epigenetic drug targeting. CRISPR based genome editing has been successfully used in vitro to mutate α-globin genes or enhancers of α-goblin to achieve clinically significant knockdowns of α-globin to the levels beneficial for patients with β-thalassaemia. This review summarises the current knowledge on the regulation of human α-globin genes and the clinical genetic data supporting the pathway of targeting α-globin as a treatment for β-thalassaemia. It also presents the progress of epigenetic drug and genome editing approaches currently in development to treat β-thalassaemia.

Coordinated changes in gene expression kinetics underlie both mouse and human erythroid maturation

BACKGROUND

Single-cell technologies are transforming biomedical research, including the recent demonstration that unspliced pre-mRNA present in single-cell RNA-Seq permits prediction of future expression states. Here we apply this RNA velocity concept to an extended timecourse dataset covering mouse gastrulation and early organogenesis.

RESULTS

Intriguingly, RNA velocity correctly identifies epiblast cells as the starting point, but several trajectory predictions at later stages are inconsistent with both real-time ordering and existing knowledge. The most striking discrepancy concerns red blood cell maturation, with velocity-inferred trajectories opposing the true differentiation path. Investigating the underlying causes reveals a group of genes with a coordinated step-change in transcription, thus violating the assumptions behind current velocity analysis suites, which do not accommodate time-dependent changes in expression dynamics. Using scRNA-Seq analysis of chimeric mouse embryos lacking the major erythroid regulator Gata1, we show that genes with the step-changes in expression dynamics during erythroid differentiation fail to be upregulated in the mutant cells, thus underscoring the coordination of modulating transcription rate along a differentiation trajectory. In addition to the expected block in erythroid maturation, the Gata1-chimera dataset reveals induction of PU.1 and expansion of megakaryocyte progenitors. Finally, we show that erythropoiesis in human fetal liver is similarly characterized by a coordinated step-change in gene expression.

CONCLUSIONS

By identifying a limitation of the current velocity framework coupled with in vivo analysis of mutant cells, we reveal a coordinated step-change in gene expression kinetics during erythropoiesis, with likely implications for many other differentiation processes.

A comprehensive review of hydroxyurea for β-haemoglobinopathies: the role revisited during COVID-19 pandemic

Background

Hydroxyurea is one of the earliest drugs that showed promise in the management of haemoglobinopathies that include β-thalassaemia and sickle cell disease. Despite this, many aspects of hydroxyurea are either unknown or understudied; specifically, its usefulness in β-thalassaemia major and haemoglobin E β-thalassaemia is unclear. However, during COVID-19 pandemic, it has become a valuable adjunct to transfusion therapy in patients with β-haemoglobinopathies. In this review, we aim to explore the available in vitro and in vivo mechanistic data and the clinical utility of hydroxyurea in β-haemoglobinopathies with a special emphasis on its usefulness during the COVID-19 pandemic.

Main body

Hydroxyurea is an S-phase-specific drug that reversibly inhibits ribonucleoside diphosphate reductase enzyme which catalyses an essential step in the DNA biosynthesis. In human erythroid cells, it induces the expression of γ-globin, a fetal globin gene that is suppressed after birth. Through several molecular pathways described in this review, hydroxyurea exerts many favourable effects on the haemoglobin content, red blood cell indices, ineffective erythropoiesis, and blood rheology in patients with β-haemoglobinopathies. Currently, it is recommended for sickle cell disease and non-transfusion dependent β-thalassaemia. A number of clinical trials are ongoing to evaluate its usefulness in transfusion dependent β-thalassaemia. During the COVID-19 pandemic, it was widely used as an adjunct to transfusion therapy due to limitations in the availability of blood and logistical disturbances. Thus, it has become clear that hydroxyurea could play a remarkable role in reducing transfusion requirements of patients with haemoglobinopathies, especially when donor blood is a limited resource.

Conclusion

Hydroxyurea is a well-tolerated oral drug which has been in use for many decades. Through its actions of reversible inhibition of ribonucleoside diphosphate reductase enzyme and fetal haemoglobin induction, it exerts many favourable effects on patients with β-haemoglobinopathies. It is currently approved for the treatment of sickle cell disease and non-transfusion dependent β-thalassaemia. Also, there are various observations to suggest that hydroxyurea is an important adjunct in the treatment of transfusion dependent β-thalassaemia which should be confirmed by randomised clinical trials.

MZF1 regulates α-globin gene transcription via long-range interactions in erythroid differentiation

Precise spatiotemporal gene expression regulation is crucial for human erythropoiesis. However, dramatic changes in the chromatin structure and transcriptome involved in α-globin gene expression during erythropoiesis still not fully understand. To identify candidate regulators for α-globin gene regulation, we carried out an integrated approach by integrating publicly available transcriptomic and epigenomic data. We computed active enhancers by overlapping enriched regions marked with H3K4me1 and H3K27ac and correlated their activity with mRNA expression. Next, we cataloged potential transcription factors via de novo motif analysis. We highlighted the discovery of potential novel transcription factor MZF1 of the α-globin gene in erythroid differentiation. To validate the role of MZF1, we quantified the expression level of MZF1 and α-globin gene in HSPCs, early erythroid progenitors and late erythroid precursors cells. Both the mRNA and protein expression patterns of MZF1 were consistent with the α-globin gene. Also, the qPCR result showed that the expression of the α-globin gene was significantly increased by the MZF1 overexpression. To further investigate the role of MZF1 regulating α-globin gene transcriptional activity during erythroid differentiation, we performed ChIP-qPCR at the α-globin locus. Our results showed that MZF1 recruitment both at 4 upstream HS sites and α-globin gene promoter in erythroid precursor cells. To determine the importance of the MZF1 to enhancer-promoter interaction at the α-globin locus, we compared interaction frequency before and after knockdown of MZF1 by chromosome conformation capture (3C) assay. Upon MZF1 depletion, both the expression of the α-globin gene and all 3C signals were significantly decreased. Taken together, MZF1 plays an important role in regulating α-globin gene expression by binding to long-region enhancers and α-globin gene promoter and facilitates the organization of specific 3D chromatin architecture in erythroid differentiation.

Cytokine and Gene Expression Profiling in Patients with HFE-Associated Hereditary Hemochromatosis according to Genetic Profile

BACKGROUND

Hemochromatosis gene (HFE)-associated hereditary hemochromatosis (HH) is characterized by downregulation of hepcidin synthesis, leading to increased intestinal iron absorption.

OBJECTIVES

The objectives were to characterize and elucidate a possible association between gene expression profile, hepcidin levels, disease severity, and markers of inflammation in HFE-associated HH patients.

METHODS

Thirty-nine HFE-associated HH patients were recruited and assigned to 2 groups according to genetic profile: C282Y homozygotes in 1 group and patients with H63D, as homozygote or in combination with C282Y, in the other group. Eleven healthy first-time blood donors were recruited as controls. Gene expression was characterized from peripheral blood cells, and inflammatory cytokines and hepcidin-25 isoform were quantified in serum. Biochemical disease characteristics were recorded.

RESULTS

Elevated levels of interleukin 8 were observed in a significant higher proportion of patients than controls. In addition, compared to controls, gene expression of ζ-globin was significantly increased among C282Y homozygote patients, while gene expression of matrix metalloproteinase 8, and other neutrophil-secreted proteins, was significantly upregulated in patients with H63D.

CONCLUSION

Different disease signatures may characterize HH patients according to their HFE genetic profile. Studies on larger populations, including analyses at protein level, are necessary to confirm these findings.

Efficacy and safety of oral hydroxyurea in transfusion-dependent β-thalassaemia: a protocol for randomised double-blind controlled clinical trial

INTRODUCTION

Despite being one of the first diseases to be genetically characterised, β-thalassaemia remains a disorder without a cure in a majority of patients. Most patients with β-thalassaemia receive only supportive treatment and therefore have a poor quality of life and shorter life spans. Hydroxyurea, which has shown to induce fetal haemoglobin synthesis in human erythroid cells, is currently recommended for the treatment of sickle cell disease. However, its clinical usefulness in transfusion-dependent β-thalassaemia is unclear. Here, we present a protocol for a randomised double-blind controlled clinical trial to evaluate the efficacy and safety of oral hydroxyurea in transfusion-dependent β-thalassaemia.

METHODS AND ANALYSIS

This single-centre randomised double-blind placebo-controlled clinical trial is conducted at the Thalassaemia Centre of Colombo North Teaching Hospital, Ragama, Sri Lanka. Adult and adolescent patients with haematologically and genetically confirmed transfusion-dependent β-thalassaemia are enrolled and randomised into the intervention or control group. The intervention group receives oral hydroxyurea 10-20 mg/kg daily for 6 months, while the control group receives a placebo which is identical in size, shape and colour to hydroxyurea without its active ingredient. Transfused blood volume, pretransfusion haemoglobin level, fetal haemoglobin percentage and adverse effects of treatment are monitored during treatment and 6 months post-treatment. Cessation or reduction of blood transfusions during the treatment period will be the primary outcome measure. The statistical analysis will be based on intention to treat.

ETHICS AND DISSEMINATION

Ethical approval has been obtained from the Ethics Committee of Faculty of Medicine, University of Kelaniya (P/116/05/2018) and the trial is approved by the National Medicinal Regulatory Authority of Sri Lanka. Results of the trial will be disseminated in scientific publications in reputed journals.

TRIAL REGISTRATION NUMBER

SLCTR/2018/024; Pre-results.

Prevalence of globin gene modifiers encountered in fetuses during antenatal diagnosis of hemoglobinopathies

INTRODUCTION

The hemoglobinopathies are the commonest group of single gene disorders in the Indian subcontinent. Although genetic modifiers are known to have a remarkable effect on phenotypic expression, the effects of the possible co-inheritance of different modifiers are not taken into account during prenatal diagnosis. The present study was undertaken to look for the frequency of globin gene modifiers like the types of β-globin gene mutations, α thalassemia, α gene triplication, and the Xmn1 polymorphism in fetuses during antenatal diagnosis of hemoglobinopathies.

MATERIALS AND METHODS

A total of 580 fetuses with different diagnoses were screened for the presence of genetic modifiers.

RESULTS

Twenty-two different β-globin gene mutations were identified of which 3.5% were milder mutations. Among the affected fetuses, 29.6% of the β-thalassemia major and 52.9% of the sickle cell anemia (SCA) fetuses had one genetic modifier while 3.7% of the β-thalassemia major and 41.1% of the SCA fetuses had co-inherited two modifiers. α-gene triplication was detected in 16 (3.5%) β-thalassemia/sickle cell heterozygous and normal fetuses of which 5 babies (2 β-thalassemia heterozygous and 3 normal) could be followed up. Of the 2 β-thalassemia heterozygous babies, one had a severe clinical presentation.

CONCLUSION

Many fetuses had one or two gene modifiers. However, the impact of these on ameliorating the severity of the disease could not be evaluated as all the fetuses with β thalassemia major or sickle cell disease were terminated. Parents having heterozygous fetuses with α gene triplication should be followed up periodically after birth for better management of these babies.

Microcytic anemia in children: parallel screening for iron deficiency and thalassemia provides a useful opportunity for thalassemia prevention in low- and middle-income countries

Microcytic anemia in children is commonly attributed to iron deficiency without attempting to find the cause. Inadequate investigations to exclude hemoglobinopathies lead to missed opportunities for identification of thalassemia carriers. Here we aim to describe the relative contribution of iron deficiency and thalassemia to microcytic anemia in children. This hospital-based prospective study was conducted at the Colombo North Teaching Hospital, Ragama, Sri Lanka. All newly diagnosed patients with microcytic anemia were recruited and data were collected using an interviewer-administered questionnaire. Full blood count, blood film, serum ferritin, c-reactive protein, quantification of hemoglobin sub-types and α-globin genotype were performed using 4 ml of venous blood. A total of 104 children (Male- 60.5%) were recruited. Iron deficiency was the cause for anemia in 49% whilst 16% and 10% had α- and β-thalassemia trait respectively. Seven (6.7%) children had co-existing iron deficiency and thalassemia trait while two coinherited α- and β-thalassemia trait. Children with β-thalassemia trait had significantly higher red cell count and lower mean corpuscular volume compared to children with iron deficiency. However, none of the red cell parameters were significantly different between children with α-thalassemia trait and iron deficiency. Iron deficiency contributes only to half of children with microcytic anemia; one-fourth had thalassemia trait. Co-existence of iron deficiency and thalassemia trait or co-inheritance of α- and β-thalassemia trait were found in 9%. Parallel investigation of children with microcytic anemia to diagnose iron deficiency and thalassemia provides an opportunity to identify thalassemia carriers which is beneficial for thalassemia prevention.

βT87Q-Globin Gene Therapy Reduces Sickle Hemoglobin Production, Allowing for Ex Vivo Anti-sickling Activity in Human Erythroid Cells

Lentiviral addition of βT87Q-globin, a modified β-globin with an anti-sickling mutation, is currently being used in gene therapy trials for sickle cell disease (SCD) and β-thalassemia patients. βT87Q-globin interferes with sickle hemoglobin (HbS) polymerization. Here, we generated the SCD mutation in an immortalized human erythroid cell line (HUDEP-2) to investigate the anti-sickling activity of βT87Q-globin. Sickle HUDEP-2 (sHUDEP-2) cells produced robust HbS after differentiation and sickled under deoxygenated conditions, comparable with SCD CD34⁺ progeny. Lentiviral transduction provided 9.5–26.8 pg/cell βT87Q-globin (R² = 0.83) in a vector copy number (VCN)-dependent manner, resulting in a significant reduction of sickling ratios (R² = 0.92). Interestingly, βT87Q-globin transduction markedly reduced endogenous β^S-globin (R² = 0.84) to an undetectable level (0.4–16.8 pg/cell) in sHUDEP-2 cells, as well as endogenous β-globin in human CD34⁺ cell-derived erythroid cells. RNA sequencing (RNA-seq) analysis with βT87Q-transduced sHUDEP-2 and human CD34⁺-derived cells revealed activation of inflammation- and proliferation-related programs, suggesting minimal changes in background gene expression except for βT87Q-globin expression and endogenous β/β^S-globin suppression. In summary, using sHUDEP-2 and CD34⁺-derived cells, we demonstrated that lentiviral addition of βT87Q-globin strongly reduced endogenous β-/β^S-globin expression, resulting in an anti-sickling effect. Our findings should be helpful to understand the anti-sickling effects of therapeutic genes in SCD gene therapy.

Synergistic silencing of α-globin and induction of γ-globin by histone deacetylase inhibitor, vorinostat as a potential therapy for β-thalassaemia

β-Thalassaemia is one of the most common monogenic diseases with no effective cure in the majority of patients. Unbalanced production of α-globin in the presence of defective synthesis of β-globin is the primary mechanism for anaemia in β-thalassaemia. Clinical genetic data accumulated over three decades have clearly demonstrated that direct suppression of α-globin and induction of γ-globin are effective in reducing the globin chain imbalance in erythroid cells hence improving the clinical outcome of patients with β-thalassaemia. Here, we show that the histone deacetylase inhibitor drug, vorinostat, in addition to its beneficial effects for patients with β-thalassaemia through induction of γ-globin, has the potential to simultaneously suppress α-globin. We further show that vorinostat exhibits these synergistic beneficial effects in globin gene expression at nanomolar concentrations without perturbing erythroid expansion, viability, differentiation or the transcriptome. This new evidence will be helpful for the interpretation of existing clinical trials and future clinical studies that are directed towards finding a cure for β-thalassaemia using vorinostat.

Oxidative Stress in β-Thalassemia

Cell oxidative status, which represents the balance between oxidants and antioxidants, is involved in normal functions. Under pathological conditions, there is a shift toward the oxidants, leading to oxidative stress, which is cytotoxic, causing oxidation of cellular components that result in cell death and organ damage. Thalassemia is a hereditary hemolytic anemia caused by mutations in globin genes that cause reduced or complete absence of specific globin chains (commonly, α or β). Although oxidative stress is not the primary etiology of thalassemia, it mediates several of its pathologies. The main causes of oxidative stress in thalassemia are the degradation of the unstable hemoglobin and iron overload-both stimulate the production of excess free radicals. The symptoms aggravated by oxidative stress include increased hemolysis, ineffective erythropoiesis and functional failure of vital organs such as the heart and liver. The oxidative status of each patient is affected by multiple internal and external factors, including genetic makeup, health conditions, nutrition, physical activity, age, and the environment (e.g., air pollution, radiation). In addition, oxidative stress is influenced by the clinical manifestations of the disease (unpaired globin chains, iron overload, anemia, etc.). Application of personalized (theranostics) medicine principles, including diagnostic tests for selecting targeted therapy, is therefore important for optimal treatment of the oxidative stress of these patients. We summarize the role of oxidative stress and the current and potential antioxidative therapeutics in β-thalassemia and describe some methodologies, mostly cellular, that might be helpful for application of a theranostics approach to therapy.

Blood transfusion therapy for β-thalassemia major and hemoglobin E β-thalassemia: Adequacy, trends, and determinants in Sri Lanka

BACKGROUND

Regular blood transfusion therapy still remains the cornerstone in the management of β-thalassemia. Although recommendations are clear for patients with β-thalassemia major, uniform transfusion guidelines are lacking for patients with hemoglobin E β-thalassemia. In this study, we aim to describe the adequacy, trends, and determinants of blood transfusion therapy in a large cohort of pediatric patients with β-thalassemia major and hemoglobin E β-thalassemia.

METHODS/PROCEDURE

This cross-sectional study was performed among all regularly transfused patents with β-thalassemia aged 2 to 18 years attending three large thalassemia centers in Sri Lanka. Data were collected using an interviewer-administered questionnaire, perusal of clinical records, and physical examination of patients by trained doctors.

RESULTS

A total of 328 patients (male 47%) were recruited; 83% had β-thalassemia major, whereas 16% had hemoglobin E β-thalassemia. Sixty-one percent of patients had low pretransfusion hemoglobin levels (< 9.0 g/dL) despite receiving high transfusion volumes (> 200 mL/kg/year) by a majority (56%). Median pretransfusion hemoglobin was significantly lower in patients with hemoglobin E β-thalassemia compared with β-thalassemia major (P < 0.001); however, there was no difference in requirement for high transfusion volumes over 200 mL/kg/year in both groups (P = 0.14). Hepatomegaly and splenomegaly were more common in hemoglobin E β-thalassemia and were associated with lower pretransfusion hemoglobin. Transfusion requirements were higher among patients with hepatitis C and in those who are underweight.

CONCLUSIONS

Over 60% of regularly transfused patients with β-thalassemia have low pretransfusion hemoglobin levels despite receiving large transfusion volumes. Patients with hemoglobin E β-thalassemia are undertransfused and specific recommendations should be developed to guide transfusions in these patients.

Engineering Globin Gene Expression

Hemoglobinopathies, including sickle cell disease and thalassemia, are among the most common inherited genetic diseases worldwide. Due to the relative ease of isolating and genetically modifying hematopoietic stem and progenitor cells, recent gene editing and gene therapy strategies have progressed to clinical trials with promising outcomes; however, challenges remain and necessitate the continued exploration of new gene engineering and cell transplantation protocols. Current gene engineering strategies aim at reactivating the expression of the fetal γ-globin genes in adult erythroid cells. The γ-globin proteins exhibit anti-sickling properties and can functionally replace adult β-globin. Here, we describe and compare the current genetic engineering procedures that may develop into safe and efficient therapies for hemoglobinopathies in the near future.

Anaemia among females in child-bearing age: Relative contributions, effects and interactions of α- and β-thalassaemia

Introduction

Anaemia in women during pregnancy and child bearing age is one of the most common global health problems. Reasons are numerous, but in many cases only minimal attempts are made to elucidate the underlying causes. In this study we aim to identify aetiology of anaemia in women of child bearing age and to determine the relative contributions, effects and interactions of α- and β-thalassaemia in a region of the world where thalassaemia is endemic.

Methods

A cross sectional study was conducted at the Colombo North Teaching Hospital of Sri Lanka. The patient database of deliveries between January 2015 and September 2016 at University Obstetrics Unit was screened to identify women with anaemia during pregnancy and 253 anaemic females were randomly re-called for the study. Data were collected using an interviewer-administered questionnaire and haematological investigations were done to identify aetiologies.

Results

Out of the 253 females who were anaemic during pregnancy and were re-called, 8 were excluded due to being currently pregnant. Of the remaining 245 females, 117(47.8%) remained anaemic and another 22(9.0%) had non-anaemic microcytosis. Of anaemic females, 28(24.8%) were iron deficient, 40(35.4%) had low-normal serum ferritin without fulfilling the criteria for iron deficiency,18(15.3%) had β-haemoglobinopathy trait and 20(17.0%) had α-thalassaemia trait. Of females who had non-anaemic microcytosis, 14(66.0%) had α-thalassaemia trait. In 4 females, both α- and β-thalassaemia trait coexist. These females had higher levels of haemoglobin (p = 0.06), MCV (p<0.05) and MCH (p<0.01) compared to individuals with only β-thalassaemia trait. A significantly higher proportion of premature births (p<0.01) and lower mean birth weights (p<0.05) were observed in patients with α-thalassaemia trait.

Conclusions

Nearly one third of anaemic females in child bearing age had thalassaemia trait of which α-thalassemia contributes to a majority. Both α- and β-thalassaemia trait can co-exist and have ameliorating effects on red cell indices in heterozygous states. α-Thalassaemia trait was significantly associated with premature births and low birth weight. It is of paramount importance to investigate the causes of anaemia in women of child bearing age and during pregnancy in addition to providing universal iron supplementation.

Endothelial cell α-globin and its molecular chaperone α-hemoglobin-stabilizing protein regulate arteriolar contractility

Arteriolar endothelial cell-expressed (EC-expressed) α-globin binds endothelial NOS (eNOS) and degrades its enzymatic product, NO, via dioxygenation, thereby lessening the vasodilatory effects of NO on nearby vascular smooth muscle. Although this reaction potentially affects vascular physiology, the mechanisms that regulate α-globin expression and dioxygenase activity in ECs are unknown. Without β-globin, α-globin is unstable and cytotoxic, particularly in its oxidized form, which is generated by dioxygenation and recycled via endogenous reductases. We show that the molecular chaperone α-hemoglobin-stabilizing protein (AHSP) promotes arteriolar α-globin expression in vivo and facilitates its reduction by eNOS. In Ahsp-/- mice, EC α-globin was decreased by 70%. Ahsp-/- and Hba1-/- mice exhibited similar evidence of increased vascular NO signaling, including arteriolar dilation, blunted α1-adrenergic vasoconstriction, and reduced blood pressure. Purified α-globin bound eNOS or AHSP, but not both together. In ECs in culture, eNOS or AHSP enhanced α-globin expression posttranscriptionally. However, only AHSP prevented oxidized α-globin precipitation in solution. Finally, eNOS reduced AHSP-bound α-globin approximately 6-fold faster than did the major erythrocyte hemoglobin reductases (cytochrome B5 reductase plus cytochrome B5). Our data support a model whereby redox-sensitive shuttling of EC α-globin between AHSP and eNOS regulates EC NO degradation and vascular tone.

Body iron status of children and adolescents with transfusion dependent β-thalassaemia: trends of serum ferritin and associations of optimal body iron control

Objective

This cross sectional study aims to describe the body iron status, trends of serum ferritin and associations of optimal body iron control in patients aged below 16 years with transfusion dependent β-thalassaemia attending Paediatric and Adolescent Thalassaemia Centres of the Colombo North Teaching Hospital of Sri Lanka.

Results

Out of 54 children, 51% were males and a majority were aged 11–16 years; 83% had β-thalassaemia major while 13% had HbE β-thalassaemia. Mean serum ferritin was 1778(± 1458) µg/l and 29% had optimal serum ferritin (below 1000 µg/l). Trend of mean serum ferritin over time showed gradual decline between 2011 and 2017 and longitudinal trend of individual patients at yearly intervals showed gradual rise until 5 years of age and plateauing thereafter. All except two patients were receiving iron chelator medication of which the most commonly used was oral deferasirox (92%). The most common iron-related complications were short stature (24.1%) and pubertal delay (42.8% of > 14 years). None of the patients had hypothyroidism, hypoparathyroidism or diabetes. Optimal serum ferritin levels were significantly associated with the diagnosis of thalassaemia at a later age (23.6 vs 9.0 months) and higher family income (OR-4.81;95%CI 1.17–19.67) however was not associated with the age of the patient or duration of transfusion.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3634-9) contains supplementary material, which is available to authorized users.

Molecular Basis and Genetic Modifiers of Thalassemia

Thalassemia is a disorder of hemoglobin characterized by reduced or absent production of one of the globin chains in human red blood cells with relative excess of the other. Impaired synthesis of β-globin results in β-thalassemia, whereas defective synthesis of α-globin leads to α-thalassemia. Despite being a monogenic disorder, thalassemia exhibits remarkable clinical heterogeneity that is directly related to the intracellular imbalance between α- and β-like globin chains. Novel insights into the genetic modifiers have contributed to the understanding of the correlation between genotype and phenotype and are being explored as therapeutic pathways to cure this life-limiting disease.

Pathophysiology and treatment of patients with beta-thalassemia - an update

Thalassemia (thal) is an autosomal recessive, hereditary, chronic hemolytic anemia due to a partial or complete deficiency in the synthesis of α-globin chains (α-thal) or β-globin chains (β-thal) that compose the major adult hemoglobin (α ₂β _2). It is caused by one or more mutations in the corresponding genes. The unpaired globin chains are unstable; they precipitate intracellularly, resulting in hemolysis, premature destruction of red blood cell [RBC] precursors in the bone marrow, and a short life-span of mature RBCs in the circulation. The state of anemia is treated by frequent RBC transfusions. This therapy results in the accumulation of iron (iron overload), a condition that is exacerbated by the breakdown products of hemoglobin (heme and iron) and the increased iron uptake for the chronic accelerated, but ineffective, RBC production. Iron catalyzes the generation of reactive oxygen species, which in excess are toxic, causing damage to vital organs such as the heart and liver and the endocrine system. Herein, we review recent findings regarding the pathophysiology underlying the major symptoms of β-thal and potential therapeutic modalities for the amelioration of its complications, as well as new modalities that may provide a cure for the disease.

Molecular basis of α-thalassemia

α-Thalassemia is an inherited, autosomal recessive, disorder characterized by a microcytic hypochromic anemia. It is one of the most common monogenic gene disorders in the world population. The clinical severity varies from almost asymptomatic, to mild microcytic hypochromic, and to a lethal hemolytic condition, called Hb Bart's Hydrops Foetalis Syndrome. The molecular basis are usually deletions and less frequently, point mutations affecting the expression of one or more of the duplicated α-genes. The clinical variation and increase in disease severity is directly related to the decreased expression of one, two, three or four copies of the α-globin genes. Deletions and point mutations in the α-globin genes and their regulatory elements have been studied extensively in carriers and patients and these studies have given insight into the α-globin genes are regulated. By looking at naturally occurring deletions and point mutations, our knowledge of globin-gene regulation and expression will continue to increase and will lead to new targets of therapy.

Editing an α-globin enhancer in primary human hematopoietic stem cells as a treatment for β-thalassemia

β-Thalassemia is one of the most common inherited anemias, with no effective cure for most patients. The pathophysiology reflects an imbalance between α- and β-globin chains with an excess of free α-globin chains causing ineffective erythropoiesis and hemolysis. When α-thalassemia is co-inherited with β-thalassemia, excess free α-globin chains are reduced significantly ameliorating the clinical severity. Here we demonstrate the use of CRISPR/Cas9 genome editing of primary human hematopoietic stem/progenitor (CD34+) cells to emulate a natural mutation, which deletes the MCS-R2 α-globin enhancer and causes α-thalassemia. When edited CD34+ cells are differentiated into erythroid cells, we observe the expected reduction in α-globin expression and a correction of the pathologic globin chain imbalance in cells from patients with β-thalassemia. Xenograft assays show that a proportion of the edited CD34+ cells are long-term repopulating hematopoietic stem cells, demonstrating the potential of this approach for translation into a therapy for β-thalassemia.

β-thalassemia is characterised by the presence of an excess of α-globin chains, which contribute to erythrocyte pathology. Here the authors use CRISP/Cas9 to reduce α-globin expression in hematopoietic precursors, and show effectiveness in xenograft assays in mice.

Molecular basis of β thalassemia and potential therapeutic targets

The remarkable phenotypic diversity of β thalassemia that range from severe anemia and transfusion-dependency, to a clinically asymptomatic state exemplifies how a spectrum of disease severity can be generated in single gene disorders. While the genetic basis for β thalassemia, and how severity of the anemia could be modified at different levels of its pathophysiology have been well documented, therapy remains largely supportive with bone marrow transplant being the only cure. Identification of the genetic variants modifying fetal hemoglobin (HbF) production in combination with α globin genotype provide some prediction of disease severity for β thalassemia but generation of a personalized genetic risk score to inform prognosis and guide management requires a larger panel of genetic modifiers yet to be discovered. Nonetheless, genetic studies have been successful in characterizing the key variants and pathways involved in HbF regulation, providing new therapeutic targets for HbF reactivation. BCL11A has been established as a quantitative repressor, and progress has been made in manipulating its expression using genomic and gene-editing approaches for therapeutic benefits. Recent discoveries and understanding in the mechanisms associated with ineffective and abnormal erythropoiesis have also provided additional therapeutic targets, a couple of which are currently being tested in clinical trials.