Correction of Beta-Thalassemia IVS-II-654 Mutation in a Mouse Model Using Prime Editing

Gene editing of the endogenous cryptic 3' splice site corrects the RNA splicing defect in the β654-thalassemia mouse model

β654-thalassemia is caused by a point mutation in the second intron (IVS-II) of the β-globin gene that activates a cryptic 3' splice site, leading to incorrect RNA splicing. Our previous study demonstrated that when direct deletion of the β654 mutation sequence or the cryptic 3' splice site in the IVS-II occurs, correct splicing of β-globin mRNA can be restored. Herein, we conducted an in-depth analysis to explore a more precise gene-editing method for treating β654-thalassemia. A single base substitution of the cryptic 3' acceptor splice site was introduced in the genome of a β654-thalassemia mouse model using CRISPR-Cas9-mediated homology-directed repair (HDR). All of the HDR-edited mice allow the detection of correctly spliced β-globin mRNA. Pathological changes were improved compared with the non-edited β654 mice. This resulted in a more than two-fold increase in the survival rate beyond the weaning age of the mice carrying the β654 allele. The therapeutic effects of this gene-editing strategy showed that the typical β-thalassemia phenotype can be improved in a dose-dependent manner when the frequency of HDR is over 20%. Our research provides a unique and effective method for correcting the splicing defect by gene-editing the reactive splicing acceptor site in a β654 mouse model.

β-Thalassemia gene editing therapy: Advancements and difficulties

β-Thalassemia is the world's number 1 single-gene genetic disorder and is characterized by suppressed or impaired production of β-pearl protein chains. This results in intramedullary destruction and premature lysis of red blood cells in peripheral blood. Among them, patients with transfusion-dependent β-thalassemia face the problem of long-term transfusion and iron chelation therapy, which leads to clinical complications and great economic stress. As gene editing technology improves, we are seeing the dawn of a cure for the disease, with its reduction of ineffective erythropoiesis and effective prolongation of survival in critically ill patients. Here, we provide an overview of β-thalassemia distribution and pathophysiology. In addition, we focus on gene therapy and gene editing advances. Nucleic acid endonuclease tools currently available for gene editing fall into 3 categories: zinc finger nucleases, transcription activator-like effector nucleases, and regularly interspaced short palindromic repeats (CRISPR-Cas9) nucleases. This paper reviews the exploratory applications and exploration of emerging therapeutic tools based on 3 classes of nucleic acid endonucleases in the treatment of β-thalassemia diseases.

Recent Therapeutic Gene Editing Applications to Genetic Disorders

Recent years have witnessed unprecedented progress in therapeutic gene editing, revolutionizing the approach to treating genetic disorders. In this comprehensive review, we discuss the progression of milestones leading to the emergence of the clustered regularly interspaced short palindromic repeats (CRISPR)-based technology as a powerful tool for precise and targeted modifications of the human genome. CRISPR-Cas9 nuclease, base editing, and prime editing have taken center stage, demonstrating remarkable precision and efficacy in targeted ex vivo and in vivo genomic modifications. Enhanced delivery systems, including viral vectors and nanoparticles, have further improved the efficiency and safety of therapeutic gene editing, advancing their clinical translatability. The exploration of CRISPR-Cas systems beyond the commonly used Cas9, such as the development of Cas12 and Cas13 variants, has expanded the repertoire of gene editing tools, enabling more intricate modifications and therapeutic interventions. Outstandingly, prime editing represents a significant leap forward, given its unparalleled versatility and minimization of off-target effects. These innovations have paved the way for therapeutic gene editing in a multitude of previously incurable genetic disorders, ranging from monogenic diseases to complex polygenic conditions. This review highlights the latest innovative studies in the field, emphasizing breakthrough technologies in preclinical and clinical trials, and their applications in the realm of precision medicine. However, challenges such as off-target effects and ethical considerations remain, necessitating continued research to refine safety profiles and ethical frameworks.

Successful Correction by Prime Editing of a Mutation in the RYR1 Gene Responsible for a Myopathy

We report the first correction from prime editing a mutation in the RYR1 gene, paving the way to gene therapies for RYR1-related myopathies. The RYR1 gene codes for a calcium channel named Ryanodine receptor 1, which is expressed in skeletal muscle fibers. The failure of this channel causes muscle weakness in patients, which leads to motor disabilities. Currently, there are no effective treatments for these diseases, which are mainly caused by point mutations. Prime editing allows for the modification of precise nucleotides in the DNA. Our results showed a 59% correction rate of the T4709M mutation in the RYR1 gene in human myoblasts by RNA delivery of the prime editing components. It is to be noted that T4709M is recessive and, thus, persons having a heterozygous mutation are healthy. These results are the first demonstration that correcting mutations in the RYR1 gene is possible.

CRISPR/Cas-based gene editing in therapeutic strategies for beta-thalassemia

Beta-thalassemia (β-thalassemia) is an autosomal recessive disorder caused by point mutations, insertions, and deletions in the HBB gene cluster, resulting in the underproduction of β-globin chains. The most severe type may demonstrate complications including massive hepatosplenomegaly, bone deformities, and severe growth retardation in children. Treatments for β-thalassemia include blood transfusion, splenectomy, and allogeneic hematopoietic stem cell transplantation (HSCT). However, long-term blood transfusions require regular iron removal therapy. For allogeneic HSCT, human lymphocyte antigen (HLA)-matched donors are rarely available, and acute graft-versus-host disease (GVHD) may occur after the transplantation. Thus, these conventional treatments are facing significant challenges. In recent years, with the advent and advancement of CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 (CRISPR-associated protein 9) gene editing technology, precise genome editing has achieved encouraging successes in basic and clinical studies for treating various genetic disorders, including β-thalassemia. Target gene-edited autogeneic HSCT helps patients avoid graft rejection and GVHD, making it a promising curative therapy for transfusion-dependent β-thalassemia (TDT). In this review, we introduce the development and mechanisms of CRISPR/Cas9. Recent advances on feasible strategies of CRISPR/Cas9 targeting three globin genes (HBB, HBG, and HBA) and targeting cell selections for β-thalassemia therapy are highlighted. Current CRISPR-based clinical trials in the treatment of β-thalassemia are summarized, which are focused on γ-globin reactivation and fetal hemoglobin reproduction in hematopoietic stem cells. Lastly, the applications of other promising CRISPR-based technologies, such as base editing and prime editing, in treating β-thalassemia and the limitations of the CRISPR/Cas system in therapeutic applications are discussed.

Current trends of clinical trials involving CRISPR/Cas systems

The CRISPR/Cas9 system is a powerful genome editing tool that has made enormous impacts on next-generation molecular diagnostics and therapeutics, especially for genetic disorders that traditional therapies cannot cure. Currently, CRISPR-based gene editing is widely applied in basic, preclinical, and clinical studies. In this review, we attempt to identify trends in clinical studies involving CRISPR techniques to gain insights into the improvement and contribution of CRISPR/Cas technologies compared to traditional modified modalities. The review of clinical trials is focused on the applications of the CRISPR/Cas systems in the treatment of cancer, hematological, endocrine, and immune system diseases, as well as in diagnostics. The scientific basis underlined is analyzed. In addition, the challenges of CRISPR application in disease therapies and recent advances that expand and improve CRISPR applications in precision medicine are discussed.

Combined approaches for increasing fetal hemoglobin (HbF) and de novo production of adult hemoglobin (HbA) in erythroid cells from β-thalassemia patients: treatment with HbF inducers and CRISPR-Cas9 based genome editing

Genome editing (GE) is one of the most efficient and useful molecular approaches to correct the effects of gene mutations in hereditary monogenetic diseases, including β-thalassemia. CRISPR-Cas9 gene editing has been proposed for effective correction of the β-thalassemia mutation, obtaining high-level "de novo" production of adult hemoglobin (HbA). In addition to the correction of the primary gene mutations causing β-thalassemia, several reports demonstrate that gene editing can be employed to increase fetal hemoglobin (HbF), obtaining important clinical benefits in treated β-thalassemia patients. This important objective can be achieved through CRISPR-Cas9 disruption of genes encoding transcriptional repressors of γ-globin gene expression (such as BCL11A, SOX6, KLF-1) or their binding sites in the HBG promoter, mimicking non-deletional and deletional HPFH mutations. These two approaches (β-globin gene correction and genome editing of the genes encoding repressors of γ-globin gene transcription) can be, at least in theory, combined. However, since multiplex CRISPR-Cas9 gene editing is associated with documented evidence concerning possible genotoxicity, this review is focused on the possibility to combine pharmacologically-mediated HbF induction protocols with the "de novo" production of HbA using CRISPR-Cas9 gene editing.

Prime editing: A potential treatment option for β-thalassemia

The potential to therapeutically alter the genome is one of the remarkable scientific developments in recent years. Genome editing technologies have provided an opportunity to precisely alter genomic sequence(s) in eukaryotic cells as a treatment option for various genetic disorders. These technologies allow the correction of harmful mutations in patients by precise nucleotide editing. Genome editing technologies such as CRISPR (clustered regularly interspaced short palindromic repeat) and base editors have greatly contributed to the practical applications of gene editing. However, these technologies have certain limitations, including imperfect editing, undesirable mutations, off-target effects, and lack of potential to simultaneously edit multiple loci. Recently, prime editing (PE) has emerged as a new gene editing technology with the potential to overcome the above-mentioned limitations. Interestingly, PE not only has higher specificity but also does not require double-strand breaks. In addition, a minimum possibility of potential off-target mutant sites makes PE a preferred choice for therapeutic gene editing. Furthermore, PE has the potential to introduce insertion and deletions of all 12 single-base mutations at target sequences. Considering its potential, PE has been applied as a treatment option for genetic diseases including hemoglobinopathies. β-Thalassemia, for example, one of the most significant blood disorders characterized by reduced levels of functional hemoglobin, could potentially be treated using PE. Therapeutic reactivation of the γ-globin gene in adult β-thalassemia patients through PE technology is considered a promising therapeutic strategy. The current review aims to briefly discuss the genome editing strategies and potential applications of PE for the treatment of β-thalassemia. In addition, the review will also focus on challenges associated with the use of PE.

Molecular Basis and Genetic Modifiers of Thalassemia

Thalassemia syndromes are common monogenic disorders and represent a significant health issue worldwide. In this review, the authors elaborate on fundamental genetic knowledge about thalassemias, including the structure and location of globin genes, the production of hemoglobin during development, the molecular lesions causing α-, β-, and other thalassemia syndromes, the genotype-phenotype correlation, and the genetic modifiers of these conditions. In addition, they briefly discuss the molecular techniques applied for diagnosis and innovative cell and gene therapy strategies to cure these conditions.

Prime Editing for Human Gene Therapy: Where Are We Now?

Gene therapy holds tremendous potential in the treatment of inherited diseases. Unlike traditional medicines, which only treat the symptoms, gene therapy has the potential to cure the disease by addressing the root of the problem: genetic mutations. The discovery of CRISPR/Cas9 in 2012 paved the way for the development of those therapies. Improvement of this system led to the recent development of an outstanding technology called prime editing. This system can introduce targeted insertions, deletions, and all 12 possible base-to-base conversions in the human genome. Since the first publication on prime editing in 2019, groups all around the world have worked on this promising technology to develop a treatment for genetic diseases. To date, prime editing has been attempted in preclinical studies for liver, eye, skin, muscular, and neurodegenerative hereditary diseases, in addition to cystic fibrosis, beta-thalassemia, X-linked severe combined immunodeficiency, and cancer. In this review, we portrayed where we are now on prime editing for human gene therapy and outlined the best strategies for correcting pathogenic mutations by prime editing.

Hematopoietic stem and progenitors cells gene editing: Beyond blood disorders

Lessons learned from decades-long practice in the transplantation of hematopoietic stem and progenitor cells (HSPCs) to treat severe inherited disorders or cancer, have set the stage for the current ex vivo gene therapies using autologous gene-modified hematopoietic stem and progenitor cells that have treated so far, hundreds of patients with monogenic disorders. With increased knowledge of hematopoietic stem and progenitor cell biology, improved modalities for patient conditioning and with the emergence of new gene editing technologies, a new era of hematopoietic stem and progenitor cell-based gene therapies is poised to emerge. Gene editing has the potential to restore physiological expression of a mutated gene, or to insert a functional gene in a precise locus with reduced off-target activity and toxicity. Advances in patient conditioning has reduced treatment toxicities and may improve the engraftment of gene-modified cells and specific progeny. Thanks to these improvements, new potential treatments of various blood- or immune disorders as well as other inherited diseases will continue to emerge. In the present review, the most recent advances in hematopoietic stem and progenitor cell gene editing will be reported, with a focus on how this approach could be a promising solution to treat non-blood-related inherited disorders and the mechanisms behind the therapeutic actions discussed.