New method reactivates fetal hemoglobin without gene editing
Study: It has potential to treat more SCD patients at a lower cost
Researchers have identified a new method to reactivate fetal hemoglobin, without the need for gene-editing therapy, that has the potential to treat more people with sickle cell disease (SCD) at a lower cost and with fewer risks, as reported in a new preclinical study.
While investigating the underlying mechanism of gene-editing therapies, the team discovered they could achieve the same outcome using antisense oligonucleotides (ASOs), a class of drugs that can influence protein production without altering DNA and that have shown promise in treating various genetic conditions.
“We think this could offer a more affordable, accessible, and scalable alternative to current gene therapies,” Jian Xu, PhD, the study’s senior author at St. Jude Children’s Research Hospital in Tennessee, said in a hospital news story.
The study, “Silencing of BCL11A by Disrupting Enhancer-Dependent Epigenetic Insulation,” was published in the journal Blood.
SCD-causing mutations disrupt adult form of hemoglobin
SCD-causing mutations disrupt the adult form of hemoglobin, the protein in red blood cells that carries oxygen, but do not affect fetal hemoglobin (HbF). This alternative form is naturally produced during fetal development.
Production of HbF is switched off soon after birth and replaced by production of adult hemoglobin. As such, therapies that boost HbF production to compensate for the mutated adult hemoglobin offer a promising strategy for SCD.
Casgevy (exagamglogene autotemcel), an approved SCD treatment, uses the CRISPR/Cas9 gene-editing machinery to disrupt the activity of the BCL11A gene, which encodes a protein of the same name that silences the genes that produce HbF.
By targeting a specific portion of BCL11A’s enhancer — a DNA region that, when bound to certain proteins, increases the gene’s activity — Casgevy increases HbF, thereby preventing the disease’s hallmark red blood cell sickling and easing disease symptoms.
Our motivation for this study was twofold. First, to find out how CRISPR genome editing effectively inactivates BCL11A for fetal hemoglobin reactivation. And second, to identify more cost-effective and accessible therapeutic strategies.
Still, the precise molecular mechanism behind the remarkable efficacy of CRISPR-based gene-editing therapies targeting the BCL11A enhancer remains unclear. Also, the high cost, limited availability, and potential risks associated with current gene-editing therapies make them inaccessible to many patients.
“Our motivation for this study was twofold,” Xu said. “First, to find out how CRISPR genome editing effectively inactivates BCL11A for fetal hemoglobin reactivation. And second, to identify more cost-effective and accessible therapeutic strategies.”
The team first discovered that the BCL11A enhancer folded into a three-dimensional structure called a chromatin rosette — a flower-like shape formed by strands of DNA wrapped around certain proteins.
This structure was essential for maintaining BCL11A production via epigenetic insulation. This process creates gene regulatory boundaries to protect genes from external signals that can either silence or enhance their activity.
“We found that this enhancer forms a chromatin ‘rosette’ structure, making multiple contacts with critical regulatory elements of the gene,” Xu explained. “This ensures high-level BCL11A [activity] and prevents its silencing in red blood cell precursors.”
ASO treatment reactivated production of fetal hemoglobin
Further analysis confirmed that the disruption of the BCL11A enhancer via a similar approach to Casgevy destabilized the rosette structure, affecting epigenetic insulation and leading to the silencing of the BCL11A gene and subsequent production of HbF.
When researchers examined how the rosette structure formed, they found that it required a special type of RNA molecule, called an enhancer RNA (eRNA). These RNA molecules are generated from a gene’s enhancer regions and play a role in facilitating interactions between the regulatory elements of the gene.
To evaluate the role of these eRNAs in BCL11A’s epigenetic insulation, the team used ASOs designed to target these eRNAs specifically. ASOs are short, lab-made molecules that bind to specific RNA molecules, including eRNAs, marking them for destruction.
ASO treatment degraded BCL11A’s eRNAs, thereby preventing epigenetic insulation, silencing BCL11A, and reactivating the production of HbF in red blood cell precursors from both healthy individuals and those with SCD.
“By delivering antisense oligonucleotides to both normal and sickle red blood cell precursors, we found we can selectively degrade the enhancer RNA, causing BCL11A silencing and fetal hemoglobin reactivation,” Xu said.
The researchers concluded: “Developing potent, specific inhibitors of BCL11A eRNAs, along with combinatorial approaches targeting multiple layers of BCL11A regulation and function, represents a promising path toward more accessible therapies for major hemoglobin disorders.”
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