Control of fetal globin expression in man: new opportunities to challenge past discoveries

Elevating fetal hemoglobin: recently discovered regulators and mechanisms

ABSTRACT

It has been known for over half a century that throughout ontogeny, humans produce different forms of hemoglobin, a tetramer of α- and β-like hemoglobin chains. The switch from fetal to adult hemoglobin occurs around the time of birth when erythropoiesis shifts from the fetal liver to the bone marrow. Naturally, diseases caused by defective adult β-globin genes, such as sickle cell disease and β-thalassemia, manifest themselves as the production of fetal hemoglobin fades. Reversal of this developmental switch has been a major goal to treat these diseases and has been a driving force to understand its underlying molecular biology. Several review articles have illustrated the long and at times arduous paths that led to the discovery of the first transcriptional regulators involved in this process. Here, we survey recent developments spurred by the discovery of CRISPR tools that enabled for the first time high-throughput genetic screens for new molecules that impact the fetal-to-adult hemoglobin switch. Numerous opportunities for therapeutic intervention have thus come to light, offering hope for effective pharmacologic intervention for patients for whom gene therapy is out of reach.

Targeting HIF-1α in sickle cell disease and cancer: unraveling therapeutic opportunities and risks

INTRODUCTION

HIF-1α, a key player in medical science, holds immense significance in therapeutic approaches. This review delves into its complex dynamics, emphasizing the delicate balance required for its modulation. HIF-1α stands as a cornerstone in medical research, its role extending to therapeutic strategies. This review explores the intricate interplay surrounding HIF-1α, highlighting its critical involvement and the necessity for cautious modulation.

AREAS COVERED

In sickle cell disease (SCD), HIF-1α's potential to augment fetal hemoglobin (HbF) production and mitigate symptoms is underscored. Furthermore, its role in cancer is examined, particularly its influence on survival in hypoxic tumor microenvironments, angiogenesis, and metastasis. The discussion extends to the intricate relationship between HIF-1α modulation and cancer risks in SCD patients, emphasizing the importance of balancing therapeutic benefits and potential hazards.

EXPERT OPINION

Managing HIF-1α modulation in SCD patients requires a nuanced approach, considering therapeutic potential alongside associated risks, especially in exacerbating cancer risks. An evolutionary perspective adds depth, highlighting adaptations in populations adapted to low-oxygen environments and aligning cancer cell metabolism with primitive cells. The role of HIF-1α as a therapeutic target is discussed within the context of complex cancer biology and metabolism, acknowledging varied responses across diverse cancers influenced by intricate evolutionary adaptations.

Prognostic Value of Pretreatment Fetal Hemoglobin Levels in Patients with Myelodysplastic Syndromes and Acute Myeloid Leukemia Treated with Azacitidine: A Single-center Retrospective Study

Objective Azacitidine (AZA) has been the standard of care for elderly patients with high-risk myelodysplastic syndromes (MDS). However, reliable clinical predictors of outcome have yet to be identified. The prognostic value of fetal hemoglobin (HbF) levels has been reported for decitabine therapy. We evaluated pretreatment HbF levels in AZA monotherapy as a prognostic marker in MDS/acute myeloid leukemia (AML). Methods This study included chemotherapy-naïve patients who had received seven-day treatment schedules of AZA and whose HbF levels were measured at the onset of treatment between March 2011 and July 2020. Patients were grouped into HbF-normal (<1.0%) or HbF-elevated (≥1.0%) groups. Responses were classified according to the International Working Group 2006 criteria. Patients Twenty-nine patients were included and classified as having either MDS (n=21), chronic myelomonocytic leukemia (n=5), myelodysplastic/myeloproliferative neoplasm unclassifiable (n=1), or AML with <30% marrow blasts (n=2) based on the World Health Organization 2016 diagnostic criteria. According to the revised International Prognostic Scoring System classification, 20/29 patients were at intermediate, high, or very high risk. Pretreatment HbF levels were elevated in 13/29 patients. Results The median follow-up duration was 13.0 (range 1.5-93.5) months. The HbF-elevated group was associated with a significantly higher hematologic improvement rate (76.9% vs. 25%, p=0.009) and better overall survival (median, 21.0 vs. 13.0 months, p=0.048) than the HbF-normal group. Conclusion These results suggest that elevated pretreatment HbF levels can predict better outcomes in patients with MDS/AML treated with AZA.

Emerging drug targets for sickle cell disease: shedding light on new knowledge and advances at the molecular level

INTRODUCTION

In sickle cell disease (SCD), a single amino acid substitution at β6 of the hemoglobin (Hb) chain replaces glutamate with valine, forming HbS instead of the normal adult HbA. Loss of a negative charge, and the conformational change in deoxygenated HbS molecules, enables formation of HbS polymers. These not only distort red cell morphology but also have other profound effects so that this simple etiology belies a complex pathogenesis with multiple complications. Although SCD represents a common severe inherited disorder with life-long consequences, approved treatments remain inadequate. Hydroxyurea is currently the most effective, with a handful of newer treatments, but there remains a real need for novel, efficacious therapies.

AREAS COVERED

This review summarizes important early events in pathogenesis to highlight key targets for novel treatments.

EXPERT OPINION

A thorough understanding of early events in pathogenesis closely associated with the presence of HbS is the logical starting point for identification of new targets rather than concentrating on more downstream effects. We discuss ways of reducing HbS levels, reducing the impact of HbS polymers, and of membrane events perturbing cell function, and suggest using the unique permeability of sickle cells to target drugs specifically into those more severely compromised.

Effective therapies for sickle cell disease: are we there yet?

Sickle cell disease (SCD) is a common genetic blood disorder associated with acute and chronic pain, progressive multiorgan damage, and early mortality. Recent advances in technologies to manipulate the human genome, a century of research and the development of techniques enabling the isolation, efficient genetic modification, and reimplantation of autologous patient hematopoietic stem cells (HSCs), mean that curing most patients with SCD could soon be a reality in wealthy countries. In parallel, ongoing research is pursuing more facile treatments, such as in-vivo-delivered genetic therapies and new drugs that can eventually be administered in low- and middle-income countries where most SCD patients reside.

Activation of γ-globin expression by hypoxia-inducible factor 1α

Around birth, globin expression in human red blood cells (RBCs) shifts from γ-globin to β-globin, which results in fetal haemoglobin (HbF, α2γ2) being gradually replaced by adult haemoglobin (HbA, α2β2)1. This process has motivated the development of innovative approaches to treat sickle cell disease and β-thalassaemia by increasing HbF levels in postnatal RBCs2. Here we provide therapeutically relevant insights into globin gene switching obtained through a CRISPR-Cas9 screen for ubiquitin-proteasome components that regulate HbF expression. In RBC precursors, depletion of the von Hippel-Lindau (VHL) E3 ubiquitin ligase stabilized its ubiquitination target, hypoxia-inducible factor 1α (HIF1α)3,4, to induce γ-globin gene transcription. Mechanistically, HIF1α-HIF1β heterodimers bound cognate DNA elements in BGLT3, a long noncoding RNA gene located 2.7 kb downstream of the tandem γ-globin genes HBG1 and HBG2. This was followed by the recruitment of transcriptional activators, chromatin opening and increased long-range interactions between the γ-globin genes and their upstream enhancer. Similar induction of HbF occurred with hypoxia or with inhibition of prolyl hydroxylase domain enzymes that target HIF1α for ubiquitination by the VHL E3 ubiquitin ligase. Our findings link globin gene regulation with canonical hypoxia adaptation, provide a mechanism for HbF induction during stress erythropoiesis and suggest a new therapeutic approach for β-haemoglobinopathies.

The Cas9 Hammer-and Sickle: A Challenge for Genome Editors

Genome editing using CRISPR-Cas9 has produced a functional cure for a small number of patients with sickle cell disease and beta-thalassemia. Rather than repairing the causative mutation, this striking outcome was attained by the knockout of a lineage-specific regulatory element for a gene, BCL11A, that controls fetal hemoglobin levels: a first example of clinical success in targeting a locus initially identified in a genome-wide association study, and formal proof of the "in the age of CRISPR, the entire genome is a druggable target" notion. This remarkable development, along with advancement to the clinic of several additional editing-based approaches to the hemoglobinopathies, highlights a sense of urgency in accelerating scientific, regulatory, and public health innovation that will allow broad and equitable access to editing-based cures.