AAVs usher in the new era of Gene Therapy
Adeno-associated virus (AAV), initially discovered as a contaminant of adenovirus preparations (1), is the choice vehicle for gene therapy due to its ability to transduce proliferating and non-dividing cells with no known pathogenicity to humans. As well as this, recombinant AAVs (rAAVs) are incapable of integrating into the host genome while still producing long term, stable expression. The existence of multiple rAAV serotypes (2) allows hybrid vectors to be designed to target specific cell types while many researchers and companies are harnessing the power of synthetic biology to engineer new and improved AAV variants (3). With the explosion in new genome editing techniques such as CRISPR/Cas9, we are entering a new era of gene therapy.
Duchenne muscular dystrophy (DMD) is a fatal disease that affects boys leading to progressive loss of muscle strength and function. DMD is caused by mutations in the gene encoding the protein dystrophin, often leading to frameshift mutations resulting in loss of expression. Researchers have focused on restoring expression by “exon skipping” therapies but with little success to date (4). A recent paper by Amoasii et al (5), encouraged by previous results (6) utilized the power of AAV9 to deliver Cas9 to corrupt exon 51’s splice acceptor site in a canine model (deltaE50-MD) of DMD. Remarkably, injection of AAV9-sgRNA-51 leading to splicing of exon 49 to 52, restored dystrophin expression by as much as 90%.
While this study is preliminary and used non-primates, a recent press release detailed early findings from the first ever “In-body” gene editing clinical trial (7). Sangamo Therapeutics used AAV6 carrying a zinc finger nuclease (ZFN; SB-913) to deliver the gene IDS (iduronate-2-sulfatase) to the liver of patients with Hunter syndrome. Loss of IDS results in an inability to break down complex sugars such as glycosaminoglycans (GAGs) leading to accumulation of GAGs and subsequent organ failure. While it is not yet possible to conclude whether the trial is successful, it has revealed no serious side effects from administration of the AAV6 virus.
This study follows hot on the heels of several clinical trials in 2017 that utilized viruses to correct faulty genes. This includes treatment of sickle-cell anemia (lentivirus) (8), AAV2-mediated administration of LUXTURNA to restore eye sight by correcting mutations in the gene RPE65 (9) and AAV5 carrying factor VIII (AAV5-hFVIII-SQ) to treat hemophilia A (10). Lastly, the excitement around CAR-T therapies where patient’s own immune cells are transduced with AAVs to express engineered T cell receptors to target cancers has been confounded by a deal between the NHS in the UK and Novartis to provide CAR-T therapy to children with leukaemia (11).
Further advances in gene editing technologies with new generation AAVs will no doubt bring us closer to single dose gene therapies for a number of human diseases.
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1. Rose JA, et al. Nucleic acid from an adeno-associated virus: chemical and physical studies. Proc Natl Acad Sci USA. 1966;56(1):86–92.
2. Balakrishnan B., Jayandharan G.R. Basic biology of adeno-associated virus (AAV) vectors used in gene therapy. Curr. Gene Ther. 2014;14:86–100
3. Chan KY, et al. Engineered AAVs for efficient noninvasive gene delivery to the central and peripheral nervous systems. Nat Neurosci. 2017; 20(8): 1172–1179.
4. Aartsma-Rus A, et al. Theoretic applicability of antisense-mediated exon skipping for Duchenne muscular dystrophy mutations. Hum. Mutat. 2009; 30, 293–299.
5. Amoasii L, et al. Gene editing restores dystrophin expression in a canine model of Duchenne muscular dystrophy. Science. 2018; 10.1126/aau1549 .
6. Long C, et al. Postnatal genome editing partially restores dystrophin expression in a mouse model of muscular dystrophy. Science. 2016 Jan 22;351(6271):400-3
8. Ribeil JA, et al. Gene Therapy in a Patient with Sickle Cell Disease. N Engl J Med. 2017 Mar 2;376(9):848-85.
10. Rangarajan S, et al. AAV5-Factor VIII Gene Transfer in Severe Hemophilia A. N Engl J Med. 2017 Dec 28;377(26):2519-2530.