A new breakthrough in genome editing: the story of Casgevy

Twenty years after the first clinical trial demonstrating the efficacy of retroviral vectors in correcting adenosine deaminase-deficient severe combined immunodeficiency [1Blaese RM Culver KW Miller AD Carter CS Fleisher T Clerici M et al.T Lymphocyte-Directed Gene Therapy for ADA− SCID: Initial Trial Results After 4 Years.Science (80-). 1995; 270: 475-480https://doi.org/10.1126/SCIENCE.270.5235.475Crossref PubMed Google Scholar], Strimvelis was approved by an amazing effort led by Alessandro Aiuti (Fondazione Telethon (Telethon) Ospedale San Raffaele (OSR)) in collaboration with GlaxoSmithKline (GSK). This, along with other studies aimed at correcting different blood disorders, based first on gamma-retroviral vectors and later on lentiviral vectors [2Tucci F, Galimberti S, Naldini L, Grazia Valsecchi M, Aiuti A. A systematic review and meta-analysis of gene therapy with hematopoietic stem and progenitor cells for monogenic disorders n.d. https://doi.org/10.1038/s41467-022-28762-2.Google Scholar], not only facilitated the successful treatment of hundreds of patients but also paved the way for the more rapid application of novel technologies. This is the case with gene editing approaches, where it took only 10 years from the description of this novel technology to the approval of the first gene editing-based treatment, Casgevy. The story of Casgevy (exagamglogene autotemcel) is one of so many examples highlighting the relevance of basic research in the development of novel therapeutic approaches for patients: Francis Mojica described the CRISPR locus, coined the term CRISPR and hypothesized its role as a bacterial immune system [3Mojica FJM Díez-Villaseñor C Soria E Juez G. Biological significance of a family of regularly spaced repeats in the genomes of Archaea, Bacteria and mitochondria.Mol Microbiol. 2000; 36: 244-246https://doi.org/10.1046/J.1365-2958.2000.01838.XCrossref PubMed Scopus (0) Google Scholar,4Mojica FJM Díez-Villaseñor C García-Martínez J Soria E. Intervening sequences of regularly spaced prokaryotic repeats derive from foreign genetic elements.J Mol Evol. 2005; 60: 174-182https://doi.org/10.1007/S00239-004-0046-3Crossref PubMed Scopus (0) Google Scholar]. Virginijus Siksnys [5McAllister KA Bennett LM Houle CD Ward T Malphurs J Collins NK et al.Cancer susceptibility of mice with a homozygous deletion in the COOH-terminal domain of the Brca2 gene.Cancer Res. 2002; 62: 990-994PubMed Google Scholar], Emmanuele Charpentier and Jennifer Doudna [6Jinek M Chylinski K Fonfara I Hauer M Doudna JA Charpentier E. A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity.Science. 2012; 337: 816-821https://doi.org/10.1126/SCIENCE.1225829Crossref PubMed Scopus (0) Google Scholar] elucidated the mechanism of action of Cas9 and finally, Feng Zhang [7Cong L Ran FA Cox D Lin S Barretto R Hsu PD Habib N. Zhang F. et al.Multiplex Genome Engineering Using CRISPR/Cas Systems.Science. 2013; 2013Google Scholar] and George Church [8Mali P Yang L Esvelt KM Aach J Guell M DiCarlo JE et al.RNA-guided human genome engineering via Cas9.Science. 2013; 339 (80-): 823-826https://doi.org/10.1126/science.1232033Crossref PubMed Scopus (7189) Google Scholar] reported in parallel the possibility of targeting eukaryotic cells using CRISPR/Cas9 system. These complex studies led to the development of a versatile and user-friendly CRISPR/Cas9 system enabling to target almost any region of the genome. This system consists of a RNA-guided endonuclease (Cas9 nuclease) and a guide RNA (gRNA) that binds to the complementary region in the genome, adjacent to a protospacer adjacent motif sequence. The Cas9 cleaves the targeted DNA, creating a double-strand break that can be repaired by the endogenous cell machinery, resulting in gene disruption in most cases or gene correction when a donor template is used. The simplicity of the system has broadened the application of this genome editing tool in both research and clinical translation for the treatment of inherited disorders. In this context, many studies focused on reversing the most common monogenic diseases worldwide: β-thalassemia and sickle cell disease (SCD) caused by mutations in the β-globin gene (HBB). Hemoglobin production is tightly regulated during development, with ɣ-globin expression during fetal stages decreasing after birth as β-globin and adult hemoglobin levels increase. Mutations in BCL11A, a transcription factor repressor of the ɣ-globin gene and fetal hemoglobin, are associated with a significant reduction in the severity of both diseases [9Bauer DE Kamran SC Lessard S Xu J Fujiwara Y Lin C et al.An erythroid enhancer of BCL11A subject to genetic variation determines fetal hemoglobin level.Science. 2013; 342: 253-257https://doi.org/10.1126/SCIENCE.1242088Crossref PubMed Scopus (0) Google Scholar]. Targeting the erythroid-specific-enhancer region of BCL11A using the CRISPR/Cas9 system led to a very similar phenotype in preclinical studies and paved the way for the development of the first clinical application of the CRISPR/Cas9 system as a therapeutic strategy in β-thalassemia and SCD by VERTEX and CRISPR therapeutics [10Frangoul H Altshuler D Cappellini MD Chen Y-S Domm J Eustace BK et al.CRISPR-Cas9 Gene Editing for Sickle Cell Disease and β-Thalassemia.N Engl J Med. 2020; https://doi.org/10.1056/nejmoa2031054Crossref Google Scholar]. The protocol designed was based on harvesting mobilized peripheral blood HSCs from the patient, modifying them ex vivo using the CRIPSR/Cas9 system, freezing the cells to perform quality control tests. Viability, purity, content, potency and sterility studies were performed to confirm that the product met the release criteria. Importantly, safety studies to discard potential off-target activity of the CRISPR/Cas9 system were performed by GUIDE-seq and computational prediction to identify regions with high sequence similarity to the sgRNA sequence. Finally, manufactured product was infused into the patients after myeloablative conditioning. Impressive results were achieved in both diseases [11Frangoul H Locatelli F Sharma A Bhatia M Mapara M Molinari L et al.Exagamglogene Autotemcel for Severe Sickle Cell Disease.N Engl J Med. 2024; 390: 1649-1662https://doi.org/10.1056/NEJMOA2309676/SUPPL_FILE/NEJMOA2309676_DATA-SHARING.PDFCrossref Google Scholar,12Locatelli F Lang P Wall D Meisel R Corbacioglu S Li AM et al.Exagamglogene Autotemcel for Transfusion-Dependent β-Thalassemia.N Engl J Med. 2024; 390: 1663-1676https://doi.org/10.1056/NEJMOA2309673Crossref Google Scholar]. All 44 SCD patients who had experienced at least two severe vaso-occlusive episodes in the previous two years engrafted. Twenty-nine out of the 30 patients (97%) with sufficient follow-up achieved the primary efficacy endpoint of freedom from severe vaso-occlusive episodes for at least 12 consecutive months. In the trial involving 52 transfusion-dependent β-thalassemia patients, 32 out of the 35 patients (91%) with sufficient follow-up became transfusion independent for at least 12 months. One of the three patients who did not achieve transfusion independence had a significant reduction in red cell transfusions, and the other two patients stop receiving red cell transfusions at 14.5 months and 12.2 months after the exa-cel infusion, respectively. These results led to the first regulatory approval of a CRISPR/Cas9-based therapeutic system by the Medicines and Healthcare Products Regulatory Agency (MHRA) in the UK on November 16th, 2023 for Casgevy as a new treatment for SCD and transfusion-dependent β-thalassemia. This was followed by an FDA approval for SCD in patients aged 12 years and older on December 8th, 2023, and an EMA recommendation for a conditional marketing authorization for the treatment of SCD and transfusion-dependent β-thalassemia in patients aged 12 years and older, for whom hematopoietic stem cell transplantation is appropriate and a suitable donor is not available. The opinion will now be forwarded to the European Commission for a decision on an EU-wide marketing authorization. Finally, on January 16th, 2024, the FDA approved a Biologics License Application (BLA) for transfusion-dependent β-thalassemia.Long-term follow-up of these patients will be critical to confirm the efficacy and safety, mainly related to the potential off-target activity of the CRISPR/Cas9 system, of this gene editing strategy to correct SCD and β-thalassemia. Once safety and efficacy are demonstrated in this first-in-human CRISPR-based therapy, it will pave the way for the application of this technology to treat other hematologic disorders. The main concern now is how to make this novel therapy accessible to patients worldwide, given its high cost. This problem hinders the application of advanced therapies to patients. Several initiatives are currently underway to reach agreements with pharmaceutical companies, researchers, patients, advocacy groups and regulatory agencies with the ultimate goal of extending the application of these innovative therapies to patients worldwide [13Fox T Bueren J Candotti F Fischer A Aiuti A Lankester A et al.Access to gene therapy for rare diseases when commercialization is not fit for purpose.Nat Med. 2023; 29https://doi.org/10.1038/S41591-023-02208-8Crossref Google Scholar]. After the preparation of this work the authors used DeepL in order to correct the text to native English. After using this tool, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication. BE declares no conflict of interest PR has licensed medicinal products and receives funding and equity from Rocket Pharmaceuticals, Inc., Patents & Royalties, Research & consulting Funding.