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

Abstract

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.

Transferrin receptor 2 (Tfr2) genetic deletion makes transfusion‐independent a murine model of transfusion‐dependent β‐thalassemia

“…The accumulation of free α‐globin chains can be counteracted by decreasing α‐globin 54 or increasing the expression of γ‐globin and the production of fetal hemoglobin 55 . We demonstrated that BM Tfr2 deletion was associated with reduced mRNA levels of α‐globin, both as absolute value (Figure 3A), and as a ratio over the expression of the α‐hemoglobin stabilizing protein ( Ahsp , Figure S2G), a chaperone required to prevent the harmful aggregation of α‐globin 56,57 .…”

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

“…This is particularly true for a large proportion of patients living in poor and underdeveloped countries in South and Southeast Asia 22 . Most current research on therapeutics of thalassaemia have resulted in high-cost, technically demanding epigenetic and genetic-based therapies which would not be affordable to many at need 7 , 23 – 26 . Hence it is important to identify low-cost, easily accessible adjunct therapies that can either cure or ameliorate β-thalassaemia.…”

Innovative and Needs-led research on β-thalassemia treatment methods

“…The second way to control the a gene expression through gene editing techniques involves affecting gene enhancers. It is considered that gene activity is mediated by 4 enhancers (MCS-R1 to R4), but among them MCS-R2 is the key element as researches show (39,41,42). According to an in vitro study, the deletion of MCS-R2 with CRISPR/Cas9 led to significant reductions in the a chains' levels: 60% in the case of monoallelic mutation and 90% if the deletion targeted both alleles of the enhancer (41,42).…”

“…It is considered that gene activity is mediated by 4 enhancers (MCS-R1 to R4), but among them MCS-R2 is the key element as researches show (39,41,42). According to an in vitro study, the deletion of MCS-R2 with CRISPR/Cas9 led to significant reductions in the a chains' levels: 60% in the case of monoallelic mutation and 90% if the deletion targeted both alleles of the enhancer (41,42). These results demonstrate the huge potential that gene therapy techniques have in the context of treating b thalassemia by modifying HBA activity and suggest the need for further investigations to study the concrete efficacy of these techniques.…”