Chapter 31: Gene Therapy

As a key component of translational medicine, the goal is to move from identifying a disease-causing gene to implementing an effective treatment. Successful trials require the disease gene to be identified and capable of being cloned. Genetic conditions ranging from single-gene disorders like hemophilia, to blood disorders (e.g., leukemia), hereditary blindness (e.g., Leber congenital amaurosis or LCA), neurodegenerative diseases, and cancers (which currently make up the majority of clinical trials) are primary candidates for treatment. Therapeutic genes are delivered via two primary strategies: ex vivo gene therapy, where cells (such as hematopoietic stem cells, HSCs) are removed from the patient, genetically modified in a laboratory, and transplanted back without immune rejection; and in vivo gene therapy, where therapeutic DNA is introduced directly into the affected cells within the body. Viral vectors, including genetically modified retroviruses (like lentivirus) and adeno-associated viruses (AAV), are the main delivery tools because they efficiently transfer therapeutic DNA into cells. Retroviruses offer long-term gene expression by integrating the therapeutic gene into the host genome, but this integration is random and risks causing insertional mutations or activating oncogenes, a flaw demonstrated in the X-SCID trials that led to leukemia. Conversely, AAV vectors are generally nonpathogenic and popular because they typically do not integrate, residing instead as circular DNA episomes, which lowers the risk of insertional mutation but necessitates repeat treatments for dividing cells. Gene therapy faced significant setbacks, notably the death of Jesse Gelsinger (1999) due to a massive immune reaction to an adenovirus vector, leading to stricter regulations. Despite early struggles, successes have emerged, including the initial treatment of ADA-SCID using T cells (Ashanti DeSilva) and later using HSCs; the successful in vivo delivery of the Factor IX gene for Hemophilia B using AAV8; and the use of modified lentiviruses with HSCs to halt the progression of neurodegenerative disorders like MLD. The field has been revolutionized by gene editing techniques, which focus on targeted removal, correction, or replacement of defective genes, moving beyond traditional gene addition. Earlier editing tools, such as Zinc-Finger Nucleases (ZFNs) and TALENs, use engineered nucleases to create site-specific double-stranded breaks, showing promise in creating HIV resistance by disrupting the CCR5 gene. However, the CRISPR-Cas system has generated unparalleled optimism due to its ease, accuracy, and efficiency in genome editing using a guide RNA (sgRNA) and Cas9 nuclease to precisely target and cleave DNA. CRISPR-Cas is now being utilized in promising applications like cancer immunotherapy (CAR-T cell editing to disrupt the PD-1 gene) and preclinical studies for conditions such as Duchenne muscular dystrophy (DMD) and sickle-cell disease. Finally, RNA-based therapeutics are gaining traction as gene-silencing approaches, using antisense RNAs or RNA interference (RNAi) to block translation or degrade target mRNA; a major success being the approval of Spinraza for spinal muscular atrophy (SMA), which alters SMN2 splicing to produce functional protein. Ethical considerations remain paramount, drawing a strict line between approved somatic gene therapy and unapproved, highly controversial approaches like germ-line therapy (editing sperm/egg cells or embryos) and enhancement gene therapy (e.g., gene doping for athletic performance).