Curing Sickle Cell Disease by Allogeneic Hematopoietic Stem Cell (HSC) Transplantation Toward In Vivo HSC Gene Therapy

Sickle cell disease comprises a group of prevalent inherited disorders defined by an underlying sickle cell allele that forms sickle hemoglobin. The incidence of this disease is rising, with more than 500,000 children born with it globally. The disease carries significant morbidity and mortality. Its only curative treatment was an allogeneic hematopoietic stem cell (HSC) transplant (HSCT) until late 2023, when two one-time gene therapies were approved for treating patients aged 12 years or older with severe sickle cell disease. This work aims to inform readers about these two gene therapies: one lentiviral-based and the other nonviral. The latter is based on the Nobel Prize-winning discovery of clustered, regularly interspaced, short, palindromic repeats (CRISPR)/CRISPR-associated (Cas)9 proteins and single-guide RNA (sgRNA)-based genome editing. Both approved gene therapies require an autologous HSCT with ex vivo genetically edited autologous hematopoietic stem and progenitor cells. Therefore, access to these gene therapies is limited to specialized centers with expertise in HSCTs. This review is meant for students, researchers, and clinical practitioners. It explains the basis for both approved gene therapies, their mechanisms of action, differences, risks, and other lentiviral-based and CRISPR-Cas9-based ex vivo gene therapies for sickle cell disease in clinical development. Additionally, it discusses the current state of preclinical studies for in vivo HSC gene therapy for sickle cell disease, which utilize advanced genome editing technologies developed after CRISPR-Cas9-sgRNA-based genome editing. In vivo HSC gene therapy, after it is clinically developed, would eliminate the need for an HSCT in receiving gene therapy and vastly increase access for numerous patients worldwide, even in low-income countries with the most significant disease burden.