Chapter 065. Gene Therapy in Clinical Medicine (Part 3)

Long-Term Expression in Genetic Disease: In Vivo Gene Transfer with Recombinant Adeno-Associated Viral (AAV) VectorsRecombinant AAV vectors have emerged as attractive gene delivery vehicles for genetic disease. Engineered from a small replication-defective DNA virus, they are devoid of viral coding sequences and trigger very little immune response in experimental animals. They are capable of transducing nondividing target cells, and the donated DNA is stabilized primarily in an episomal form, thus minimizing risks associated with insertional mutagenesis. Because the vector has a tropism for certain long-lived cell types, such as skeletal muscle, the central nervous system (CNS), and hepatocytes, long-term...
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Chapter 065. Gene Therapy in Clinical Medicine (Part 3) Chapter 065. Gene Therapy in Clinical Medicine (Part 3) Long-Term Expression in Genetic Disease: In Vivo Gene Transfer withRecombinant Adeno-Associated Viral (AAV) Vectors Recombinant AAV vectors have emerged as attractive gene deliveryvehicles for genetic disease. Engineered from a small replication-defective DNAvirus, they are devoid of viral coding sequences and trigger very little immuneresponse in experimental animals. They are capable of transducing nondividingtarget cells, and the donated DNA is stabilized primarily in an episomal form, thusminimizing risks associated with insertional mutagenesis. Because the vector has atropism for certain long-lived cell types, such as skeletal muscle, the centralnervous system (CNS), and hepatocytes, long-term expression can be achievedeven in the absence of integration. Clinical trials using recombinant AAV vectors are now ongoing formuscular dystrophies, α1-antitrypsin deficiency, lipoprotein lipase deficiency,hemophilia B, and a form of congenital blindness called Lebers congenitalamaurosis. Hemophilia is often considered a promising disease model for genetransfer, as the gene product does not require precise regulation of expression andbiologically active clotting factors can be synthesized in a variety of tissue types,permitting latitude in choice of target tissue. Moreover, raising circulating factorlevels from 5 years) expression of factor VIII or factor IX in the hemophilic dogmodel. Administration to skeletal muscle of an AAV vector expressing factor IXin patients with hemophilia was safe and resulted in long-term expression asmeasured by muscle biopsy, but circulating levels never rose >1% for sustainedperiods, and a large number of IM injections (>80–100) was required to access alarge muscle mass. Intravascular vector delivery has been employed to accesslarge areas of skeletal muscle in animal models of hemophilia and will likely betested in upcoming trials. Administration of an AAV vector expressing factor IXto the liver in humans with hemophilia resulted in therapeutic circulating levels atthe highest dose tested, but expression at these levels (>5%) lasted for only 6–10weeks before declining to baseline (hosts for the virus), may be a contributing factor in the loss of expression.Fortunately, triggering of the memory T cell response appears tissue-specific, andit is possible that introduction of the vector into immunoprivileged sites, such asthe CNS (e.g., for Parkinsons disease) or the retina, will avoid this complication. Lebers congenital amaurosis (LCA) is a form of retinal degenerativedisease, characterized by severe early-onset blindness. This disease, not currentlytreatable, is caused by mutations in several different genes; ~15% of cases of LCAare due to a mutation in a gene, RPE65, encoding a retinal pigment epithelialprotein. In dogs with a null mutation in RPE65, sight has been restored aftersubretinal injection of an AAV vector expressing RPE65. Transgene expressionappears to be stable, with the first animals treated >5 years ago continuing tomanifest electrophysiologic and behavioral evidence of visual function. As is thecase for X-linked SCID, gene transfer must occur relatively early in life to achievecorrection of the genetic disease, although the exact limitations imposed by ageawait clinical studies. AAV-RPE65 trials have now been approved in both theUnited States and Great Britain. Other inherited retinal degenerative disorders mayalso be amenable to correction by gene transfer, as are certain complex acquireddisorders such as age-related macular degeneration, which affects several millionpeople worldwide. The neovascularization that occurs in age-related maculardegeneration can be inhibited by expression of vascular endothelial growth factor(VEGF) inhibitors such as angiostatin, or through the use of RNAi-mediatedknockdown of VEGF. Early-phase trials of siRNAs that target VEGF RNA areunderway, but these require repeated intravitreal injection of the siRNAs; an AAVvector–mediated approach might allow long-term knockdown of VEGF. Gene Therapy for Cancer The majority of clinical gene transfer experience has been in subjects withcancer (Fig. 65-1). As a general rule, a feature that distinguishes gene therapiesfrom conventional cancer therapeutics is that the former are less toxic, in somecases because they are delivered locally (e.g., intratumoral injections), and in othercases because they are targeted specifically to elements of the tumor(immunotherapies, anti-angiogenic approaches). Cancer gene therapies can be divided into local and systemic approaches(Table 65-2). Some of the earlie ...