A therapeutic strategy to overcome intractable blood diseases by restoring fetal hemoglobin using CRISPR gene editing
Background
Hemoglobin abnormalities have long imposed a significant disease burden. Representative examples include sickle cell disease (SCD) and beta-thalassemia, which are caused by mutations in the beta-globin (HBB) gene. Mutated sickle hemoglobin (HbS) aggregates during oxygen deficiency, distorting red blood cells into a sickle shape. These stiffened red blood cells block microvessels, leading to vaso-occlusive crises (VOC) characterized by pain and organ necrosis. Beta-thalassemia also causes severe chronic anemia due to a deficiency in adult hemoglobin.
Existing treatments remain limited to regular blood transfusions and iron chelator administration. Frequent transfusions lead to long-term organ toxicity, such as heart failure, and pose infection risks. The only curative option, allogeneic hematopoietic stem cell transplantation (HSCT), faced a significant barrier in finding compatible donors. Graft-versus-host disease (GvHD) and the burden of lifelong immunosuppressant use are also among the challenges.
In response, researchers focused on the biological switch during development. Fetuses breathe using fetal hemoglobin (HbF), which has high oxygen affinity, and then undergo a transition to adult hemoglobin after birth. Patients with hereditary persistence of fetal hemoglobin (HPFH), where fetal hemoglobin production continues into adulthood, remain nearly asymptomatic despite HBB defects. This provided the background for the rise of CRISPR-Cas9 technology to awaken dormant gamma-globin genes in adult cells.
Key Findings
Researchers focused on the BCL11A transcription factor, which suppresses gamma-globin expression. Since systemic inhibition of BCL11A causes immunodeficiency, they precisely targeted the erythroid-specific enhancer of BCL11A that operates only in the erythroid lineage.
Using an ex vivo gene editing approach, they extracted a patient's autologous hematopoietic stem and progenitor cells (HSPCs) and disrupted the enhancer using a guide RNA and Cas9 complex. Once the target site was cleaved, the transcription of the suppressed gamma-globin genes (HBG1, HBG2) was strongly induced. In preclinical stages, allele editing efficiency exceeded 80%, and high levels of HbF expression were achieved in over 85% of differentiated red blood cells.
Efficacy was clearly demonstrated in global clinical trials (CLIMB SCD-121 and CLIMB THAL-111). More than 95% of the SCD patient group treated with exagamglogene autotemcel (exa-cel) did not experience vaso-occlusive crises for over a year. The transfusion-dependent beta-thalassemia (TDT) patient group also completely discontinued transfusions for over 12 months, maintaining total hemoglobin at or above the normal range of 11 g/dL.
Significance and Outlook
The autologous cell editing strategy addressed the issues of donor shortage and rejection. It established a paradigm for managing genetic diseases with a single dose. The accumulated safety data is considered the driving force behind the FDA's approval of the world's first CRISPR therapeutic.
Despite near-curative efficacy, practical obstacles remain. High-dose busulfan chemotherapy is essential to facilitate the engraftment of edited cells into the bone marrow. Patients are exposed to risks of bone marrow suppression and infertility. There is an urgent need to develop non-chemotherapy-based targeted regulation technologies to prevent loss of reproductive capacity.
The complexity of manufacturing and high drug prices are also challenges to overcome. The customized ex vivo process requires large-scale facilities and specialized personnel. Costs reaching billions of won hinder treatment access for patients in developing countries. In vivo technology, which directly delivers editing tools to hematopoietic stem cells within the body, is considered the next-generation alternative.
Ξ²-Hemoglobinopathies, including sickle cell disease (SCD) and Ξ²-thalassemia, are inherited disorders caused by mutations in the Ξ²-globin gene (
Ex vivo gene editing therapy is a turning point that makes the possibility of curing rare genetic diseases a reality. Patients who had spent their lives undergoing repeated blood transfusions and visiting emergency rooms now have the opportunity to return to education and the workforce with a single treatment. In clinical settings, this achieves the effect of significantly saving medical resources previously consumed by managing acute vaso-occlusive crises and chronic complications. For the biopharmaceutical industry, it has pioneered the standard pathway for commercial manufacturing processes of complex cell and gene therapies and for navigating global regulatory requirements. If combined with immune-modulating technologies that replace busulfan conditioning, an outpatient-centered treatment model that significantly reduces the burden of hospitalization is expected to become established in clinical practice.