Non-DNA-Cutting Epigenome Editing Therapy for Sickle Cell Disease and Beta-Thalassemia Enters Clinical Development
β-thalassemia and sickle cell disease are the most common monogenic inherited blood disorders worldwide. Although currently approved CRISPR gene therapies can effectively alleviate disease symptoms, their DNA-cleaving mechanism poses potential safety risks.
Not long ago, a research team from the University of New South Wales (UNSW) in Australia and St. Jude Children's Research Hospital in the United States published their findings in Nature Communications, confirming that throughEpigenetic EditingTechnology: Targeted removal of DNA methylation modifications in the promoter region of the fetal hemoglobin gene (HBG) can reactivate gene expression without cutting DNA.
This study not only resolves a scientific controversy that has persisted for over 40 years—Is CpG Methylation the Cause or Consequence of Gene Silencing?, paving the way for safer novel therapeutic approaches to beta-hemoglobinopathies.
β-Hemoglobinopathies are a class of inherited blood disorders caused by mutations in the adult β-globin gene (HBB), including sickle cell disease (SCD) and β-thalassemia. Patients suffer from severe consequences such as chronic hemolysis, vaso-occlusion, and organ damage due to abnormal hemoglobin function.
The γ-globin genes (HBG1 and HBG2, collectively referred to as HBG) expressed during human fetal development encode fetal hemoglobin (Fetal Hemoglobin, HbF, α2γ2). After birth, the HBG genes are gradually silenced, and adults switch to expressing the HBB gene. A key clinical finding has inspired therapeutic strategies:Reactivating the HBG gene to induce fetal hemoglobin (HbF) production in adult red blood cells can compensate for defective HBB gene function.Individuals with Hereditary Persistence of Fetal Hemoglobin (HPFH) maintain high levels of HbF throughout their lives, and even when concurrently carrying β-hemoglobinopathy mutations, they exhibit extremely mild clinical symptoms.
The CRISPR gene therapy Casgevy, approved in 2023, is based on this strategy, reactivating HBG expression by knocking out the erythroid enhancer of the suppressor BCL11A. However, this therapy relies on Cas9-mediated DNA cleavage, posing risks such as off-target mutations, chromosomal rearrangements, and potential carcinogenicity.
Epigenetic CodeEpigenome editing offers another possibility: activating or silencing genes by modifying the regulatory marks of gene expression, rather than the gene sequence itself. As early as 1979, researchers discovered that CpG dinucleotide methylation in the promoter region of the HBG gene was associated with gene silencing, but whether this association was causal or merely correlative has long been debated. New research, using precise epigenetic editing tools, has for the first time confirmed thatLocal CpG methylation is the direct cause of HBG gene silencing.
The research team first conducted a CRISPR/Cas9 genome-wide screen in the adult erythroid cell line HUDEP2 and identified UHRF1—a key protein that assists DNMT1 in maintaining DNA methylation—as an important regulator of HBG silencing. Knockout of UHRF1 led to genome-wide CpG demethylation and a significant increase in HBG expression. This finding highlights the central role of DNA methylation in HBG silencing; however, global demethylation makes it difficult to rule out indirect effects.
To clarifyLocal promoter methylationTo establish the causal relationship with HBG silencing, the research team employed dCas9-fused epigenetic editors for bidirectional validation.Positive VerificationUse the TETv4 editor (a fusion of dCas9 and the TET1 catalytic domain) to target demethylation of six CpG sites in the HBG promoter region.
The results showed that the proportion of HBG expression in HUDEP2 cells surged from 2.2% to86%, whereas the control group (without sgRNA or with sgRNA targeting the irrelevant gene EPX) showed no change in expression levels. More importantly,This activation effect remained above 75% after three months of continuous cell culture.This suggests that HUDEP2 cells may lack the capacity for de novo methylation of the HBG promoter, allowing the activated state following demethylation to be sustained.
Reverse ValidationTargeted remethylation of TETv4-treated cells using the D3AL editor (a dCas9-DNMT3A-DNMT3L fusion). The results showed that HBG expression significantly decreased from 95% to23%, while the control group maintained high expression. Professor Merlin Crossley of the University of New South Wales, the corresponding author of the study, vividly likened methylation to a “molecular anchor”: “We clearly demonstrated that if you brush away these ‘spider webs,’ the gene turns on. When we add the methyl groups back, the gene turns off again. So these compounds are not spider webs—they are anchors.”
The study further inPrimary CD34⁺ Hematopoietic Stem/Progenitor CellsThis editing strategy was validated in differentiated erythroid cells. TETv4 treatment reduced the methylation level of the HBG promoter from approximately 70% to 10%, resulting in a nearly five-fold increase in HBG mRNA expression and HbF protein levels (rising from 7–8% to approximately 35%). Notably, this epigenetic editing did not affect the expression of erythroid maturation markers CD235a, CD49d, and Band 3, indicating that the cell differentiation process remained undisturbed.
The study also elucidated the molecular mechanism underlying HBG silencing. MBD2 is the subunit within the NuRD co-repressor complex that recognizes methylated CpG sites. By introducing a Y178F point mutation to weaken the binding affinity of MBD2 for methylated CpG, researchers observed a significant upregulation of HBG expression in mutant cells (log2 FC = 2.7), confirmingMBD2’s recognition of methylated CpG sites is a critical step in mediating the silencing function of the NuRD complex.This mechanistic model indicates that CpG methylation inhibits the binding of transcriptional activators GATA1 and NF-Y by stabilizing the occupancy of the MBD2-NuRD complex at the HBG promoter, thereby remodeling local nucleosome structure.
The significance of this study lies not only in resolving a scientific question that has persisted for over four decades, but also in pioneering entirely new therapeutic strategies for β-hemoglobinopathies. Compared with conventional CRISPR therapies, epigenetic editing offers significant safety advantages.: dCas9 does not cleave DNA, thereby avoiding the risks of off-target mutations and chromosomal rearrangements that may arise from DNA double-strand break repair."Every time DNA is cut, there is a risk of carcinogenesis. For genetic diseases requiring lifelong treatment, this is an undesirable risk," noted Professor Crossley. "However, if we can achieve gene therapy without breaking the DNA strands, we can avoid these potential hazards."
The research team envisioned a scenario for future clinical applications: extracting hematopoietic stem cells from patients, using epigenetic editors in vitro to remove methylation marks from the HBG promoter, reactivating the fetal hemoglobin gene, and then infusing the edited cells back into the patient. These cells will engraft in the bone marrow and continuously produce healthy red blood cells. “You can think of the fetal globin genes as training wheels on a bicycle,” said Professor Crossley. “We believe we can get them working again in people who need new wheels.”
However, the study also faces challenges. Although the demethylation effect is durable in HUDEP2 cells, the level of HBG activation is relatively low (approximately 35%) in erythroid cells derived from primary CD34⁺ cells, and it remains uncertain whether the demethylated state can be maintained long-term in vivo. Hematopoietic stem cells may retain de novo methylation capacity, necessitating further research to determine whether periodic treatment is required. Furthermore, the changes in expression of some off-target genes observed after TETv4 treatment (although they did not affect the overall transcriptome) also require further evaluation of their in vivo impact.
Professor Kate Quinlan, a co-author of the study, stated, “We are excited about the future of epigenetic editing. Our research demonstrates that it enables us to enhance gene expression without modifying the DNA sequence. Therapies based on this technology are expected to carry a lower risk of unintended adverse effects compared to first- or second-generation CRISPR.” In addition to beta-hemoglobinopathies, many genetic disorders involve the abnormal activation or silencing of genes; regulating methylation status may offer non-DNA-disrupting therapeutic options for these conditions. “Perhaps most importantly, molecules can now be targeted to individual genes,” remarked Professor Crossley. “Here, we have removed or added methyl groups, but this is just the beginning. Other modifications can further enhance our ability to regulate gene output for therapeutic and agricultural purposes. This marks the dawn of a new era.”
From the initial observation by scientists in 1979 of the association between DNA methylation and globin gene silencing, to the precise validation of this causal relationship using epigenetic editing in 2025, nearly half a century of exploration has unveiled the intricate logic underlying life regulation. This study not only confirms the core proposition that “methylation is the anchor of gene silencing,” but also opens a new pathway for treating genetic disorders without cutting DNA. With the continuous optimization and safety validation of epigenetic editing technologies, this “gentle” approach to gene regulation holds promise for providing millions of β-hemoglobinopathy patients worldwide with therapeutic options that offer fewer side effects and greater long-term safety.