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Gene therapy can provide transformative disease-modifying effects, with potentially lifelong clinical benefits after a single therapeutic administration. The most advanced retinal gene therapy program in the United States is in phase III study.
Take-home message: Gene therapy can provide transformative disease-modifying effects, with potentially lifelong clinical benefits after a single therapeutic administration. The most advanced retinal gene therapy program in the United States is in phase III study.
By Jeffrey D. Chulay, MD, Special to Ophthalmology Times
Alachua, FL-Genes encode proteins that perform an array of functions. Many diseases have a genetic aspect whereby a mutated gene is passed down from generation to generation. Mutated genes may encode abnormal proteins or disable the production of a protein completely, either of which can cause disease.
Gene therapy introduces a functional copy of the gene into a patient’s own cells. By correcting the underlying genetic defect that causes disease, gene therapy can potentially provide lifelong clinical benefits after a single administration.
Gene therapy uses viral vectors to deliver genes into cells affected by disease. Viral vectors have been optimized for this purpose by removing pathogenic elements and severely impairinghttp://www.modernmedicine.com/tag/retinitis-pigmentosa the viruses’ ability to replicate.
Adeno-associated virus (AAV) vectors are well suited for treating retinal diseases. AAV is a small, simple, non-enveloped virus with only two native genes, making the virus straightforward to work with from a vector-engineering standpoint. AAV vectors can carry gene sequences up to 4,000 base pairs in length.1
More than 90% of human genes have coding sequences less than 3,000 base pairs in length.
AAV vectors have been used in more than 100 human clinical trials with no serious adverse events traced to the use of AAV as the gene delivery vector and are considered to be the safest viral vector for use in human gene therapy. AAV has never been linked to human disease and elicits only a mild immune response. AAV vectors have no viral genes remaining, virtually eliminating the possibility that any viral genes will cause an adverse event.1
AAV vectors can now be produced at a commercial scale in compliance with current Good Manufacturing Practice for use in clinical trials and future marketed products. As many as 2.4 x 1014 vector copies per liter have been produced, with batches up to 100 liters, using the latest advances in large-scale viral vector production technology.2
Studies have confirmed that the purification process eliminates all but a trace amount of the raw materials used during manufacture, in many cases below assay detection levels.3
Leber congenital amaurosis (LCA) is an inherited, early-onset retinal degenerative disease caused by mutations in any one of 16 genes. One form is caused by mutations in the RPE65 gene. The safety of AAV vector expressing RPE65 is supported by multiple phase I clinical trials.4-6
The safety and efficacy of RPE65 gene therapy for LCA continues to be evaluated in ongoing clinical trials including a phase III study.
Wet age-related macular degeneration (AMD) is a major cause of visual impairment in older adults that is associated with neovascularization and macular edema. Current standard of care is intravitreal injection of vascular endothelial growth factor (VEGF) inhibitors that must be administered monthly or bimonthly. An emerging wet AMD gene therapy approach uses an AAV vector to deliver the sFLT01 gene into retinal cells. The sFLT01 gene is an engineered version of the VEGF receptor that binds and inhibits VEGF ligand.7
The safety and tolerability of sFLT01 gene therapy for wet AMD is under investigation in an active phase I trial. Other AMD gene therapy programs include an active phase I/II trial evaluating the safety of an AAV vector expressing the naturally occurring VEGF decoy receptor sFlt-1, and an active phase I trial evaluating the safety of a lentiviral vector expressing the anti-angiogenic proteins endostatin and angiostatin.
Choroideremia is an inherited X-linked degenerative disease of the retina and choroid caused by mutations in the CHM gene. An ongoing phase I/II clinical trial reported promising 6-month findings with an AAV vector expressing the CHM gene.8Recruitment for a second phase I/II trial has recently commenced.
X-linked retinoschisis (XLRS) is an inherited retinal disease associated with splitting of the retinal layers and caused by mutations in the RS1 gene, which encodes the retinoschisin protein (Figure 1).
Two phase I/II trials are planned to evaluate safety and tolerability of AAV vectors expressing RS1 (Figure 2).
One trial is currently recruiting and the other is scheduled to start recruitment in the first half of 2015.
Stargardt disease is an inherited retinal dystrophy characterized by photoreceptor degeneration in the macula. Most cases are caused by mutations in the ABCA4 gene.9Patients are currently being enrolled in a phase I/II trial that will evaluate the safety and tolerability of a lentiviral vector expressing the ABCA4 gene.
Usher syndrome type 1 is an inherited disease associated with congenital deafness and early-onset photoreceptor loss. About half of Usher type 1 cases are caused by mutations in the MYO7A gene (type 1B). Patients are currently being enrolled in a phase I/II trial that will evaluate the safety and tolerability of a lentiviral vector expressing the MYO7A gene.
Leber hereditary optic neuropathy (LHON) is a maternally inherited disorder characterized by loss of retinal ganglion cells and optic nerve atrophy. Nearly all cases are caused by mutations in any one of three mitochondrial genes (ND1, ND4, and ND6). About half of LHON cases are caused by the same mutation in the ND4 gene.10,11
Recruitment is ongoing for two phase I trials that will evaluate the safety and tolerability of AAV vectors expressing the ND4 gene.
Achromatopsia is an inherited retinal disease characterized by lack of cone photoreceptor function. The most common causes are mutations in the CNGA3 and CNGB3 genes. A preclinical study in dog models of achromatopsia with CNGB3 mutations showed restoration of cone function and improved ability to navigate mazes under bright light conditions after treatment with AAV vector expressing CNGB3.12
Clinical gene therapy programs to develop treatments for CNGB3 and CNGA3 mutations are under way.
Retinitis pigmentosa is an inherited retinal dystrophy associated with progressive vision loss. In the United States about 10 to 15% of retinitis pigmentosa cases are X-linked (XLRP) and three-fourths of these cases are caused by mutations in the RPGR gene. Studies in dog models of XLRP with RPGR mutations showed delayed disease progression after treatment with AAV vector expressing RPGR.13Preclinical evaluation is ongoing.
Blue cone monochromacy (BCM) is an inherited color vision deficiency characterized by lack of functional long-wavelength (L) and medium-wavelength (M) cone photoreceptors. BCM is caused by mutations in an area of the X chromosome that regulates expression of the L and M opsin genes.14
Preclinical evaluations of gene therapy for color vision deficiencies, including BCM, are ongoing.
The once theoretical promise of gene therapy is evolving into clinical reality. Gene therapy can provide transformative disease-modifying effects by correcting the underlying defect that causes disease, potentially with lifelong clinical benefits after a single administration.
An additional advancement in the treatment of retinal degeneration is an approach that delivers a light-sensitive protein to neurons in the retina. One such light-sensitive protein is channelrhodopsin 2. When channelrhodopsin 2 is inserted into a neuron and the neuron is stimulated by light, the neuron is activated and can transmit a signal to the visual cortex.
This technique, called optogenetics, is being applied to the design of therapeutic genes that can be expressed by AAV vectors for the treatment of advanced retinal degeneration.15
The current status of preclinical and clinical evaluation supports continued investment in the development of AAV-based gene therapies for retinal diseases, with ongoing research bringing these treatments closer to the clinic.
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9. Testa F, Melillo P, Di Iorio V, et al. Macular function and morphologic features in juvenile stargardt disease: longitudinal study. Ophthalmology. 2014;121:2399-2405.
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12. Komaromy AM, Alexander JJ, Rowlan JS, et al. Gene therapy rescues cone function in congenital achromatopsia. Hum Mol Genet. 2010;19:2581-2593.
13. Beltran WA, Cideciyan AV, Lewin AS, et al. Gene therapy rescues photoreceptor blindness in dogs and paves the way for treating human X-linked retinitis pigmentosa. Proc Natl Acad Sci U S A. 2012;109:2132-2137.
14. Gardner JC, Michaelides M, Holder GE, et al. Blue cone monochromacy: causative mutations and associated phenotypes. Mol Vis. 2009;15:876-884.
15. Busskamp V, Picaud S, Sahel JA and Roska B. Optogenetic therapy for retinitis pigmentosa. Gene Ther. 2012;19:169-175.
Jeffrey D. Chulay, MD, is vice president and chief medical officer at Applied Genetic Technologies Corp. (AGTC).