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

Abstract

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.

Figure 1

Schematic presentation of the human α‐globin gene cluster depicting functional and topological regulators of α‐globin gene expression. (A) The human α‐globin gene cluster located proximal to the telomeric end of chromosome 16p (16p13.3), and encodes the embryonic ζ‐globin gene, and two fetal/adult (α2‐ and α1)‐globin genes. Upstream of the α‐globin genes are four highly conserved multispecies conserved sequences (MCS), called MCS‐R1‐R4, involved in the regulation of the α‐like globin genes. The scale is in kilobases (kb). (B) Representative ChIP‐seq analysis of transcription factor occupancy and epigenetic landscape of the α‐globin locus in human erythroid cells. ChIP‐seq profiles for GATA1, KLF1, and TAL1, including ATRX and CTCF, are enriched either within or around the MCS‐R2 site. Also shown are H3K27ac, H3K4me3, and H3K4me1 binding sites representing epigenetic marks associated with transcriptionally active chromatin. All data sets used in the analysis for ATACseq, CTCF, H3K27ac, H3K4me1, and H3K4me3 were obtained from King et al., GATA1, KLF1, and TAL1 were obtained from Ulirsch et al., and ATRX from Truch et al. (C) Below the α‐gene cluster is shown the most common α‐thalassemia deletions indicated as horizontal bars and subdivided into common α⁺‐ and α⁰‐thalassemia deletions (adapted from Farash et al.)

Figure 2

Schematic diagram of the human α‐globin gene cluster depicting relative CTCF‐binding sites during erythroid differentiation. (A) Delineated chromatin architecture depicting accessibility and location of CTCF internal and external boundary elements. (B) CTCF‐binding influencing the formation of a dynamic hairpin conformation and enhancer/promoter interactions at the α‐globin gene cluster. Diagram adapted from the murine α‐globin gene cluster reported by Chiariello et al.

Figure 3

Model depicting the regulation of free α‐globin by UPS and autophagy pathways. The accumulation of toxic free α‐globin is a major determinant of β‐thalassemia pathophysiology. (A) During normal erythropoiesis, the molecular chaperone, AHSP, binds to α‐globin and prevents misfolding and protease digestion prior to HbA assembly. (B) Excess or misfolded α‐globin is recognized by UBE2O and selectively eliminated by the ubiquitin‐proteasome system. (C) Under oxidative stress, the entire ubiquitin–proteasome system can malfunction by both increasing ubiquitination activity and inhibiting the 26S proteasome system, resulting in the accumulation of polyubiquitinated proteins., ,  (D) Reduced autophagic clearance of aggregated α‐globin in erythroblasts induces ineffective erythropoiesis and apoptosis, whereas the induction of ULK1‐mediated autophagy by rapamycin is associated with reduced hemolysis and enhanced cell survival.

Figure 4

Pharmacological and genetic‐based approaches targeting α‐globin expression. (A) Rapamycin has been demonstrated to clear excess α‐globin accumulation via the induction of ULK1 and autophagy, whereas epigenetic drugs such as IOX1 and vorinostat down‐regulate α‐globin gene expression via the inhibition of histone lysine demethylation and histone deacetylation, respectively. (B) Possible applications of CRISPR‐Cas9‐based gene editing strategies include (i) targeting MCS‐R2, the major regulator of α‐globin gene expression, (ii) targeting individual α‐globin genes, (iii) combined targeting of individual α‐globin genes with β‐globin gene addition, and (iv) using a modified lentiviral vector expressing the β‐globin gene, while concurrently selectively silencing α₂‐globin gene expression by RNAi.