Russell S, Bennett J, Wellman JA, et al. Efficacy and safety of voretigene neparvovec (AAV2-hRPE65v2) in patients with RPE65-mediated inherited retinal dystrophy: a randomised, controlled, open-label, phase 3 trial. Lancet. 2017;390(10097):849-860. doi:10.1016/S0140-6736(17)31868-8

This trial is the culmination of a long process of bringing a drug to market. It provides a report of a study of patients where 21 were assigned to the therapy and 10 to the control group. Endpoints investigated in this study are intended to support the approval of a gene therapy for a type of genetic blindness called Leber congenital amaurosis type 2 (RPE65-/-). Treatment involved the surgical injection of a recombinant adeno-associated virus (AAV) into the retina between the layer of sensory cells and the layer of pigment cells. Statistical measurements were calibrated to detect improvements in a functional test for vision which amounted to a type of modular obstacle course which could be shuffled and repeated in different light levels. More conventional test for vision were also employed. Safety assessments included general physical and eye exams, lab tests, and self reporting of adverse events. After 1-year of study participants demonstrated improved vision and no serious adverse events.

The gene RPE65 encodes for an enzyme that participates in the visual cycle. The cells in the retina that sense light are called rods and cones. They are made of stacks of membrane proteins which are sensitive to light. Specifically these proteins bind a pigment molecule called retinal. When light of a correct wavelength excites the pigment it isomerizes and induces a conformational change in the protein. This results in a signal which is processed in the brain into sight. The retinal isomer is then removed from the protein and regenerated into a form which can be reloaded onto the protein. A key enzyme in this regeneration is the product of RPE65. Without this enzyme an individual will eventually go blind.

Monogenic diseases are good candidates for gene therapy as there is a single genetic factor responsible for the disease phenotype. In order to be successful a gene therapy needs to be delivered into the appropriate cell where the disease causing lesion needs to be repaired. There are many different potential ways to accomplish this delivery. The most commonly used is a recombinant virus; however, recombinant bacteria, electricity, physical force, and chemical transfection products could be used.

Adeno-associated virus cannot replicate by itself. It requires confection with a helper virus (which contains the necessary machinery to replicate itself) to be able to replicate. This virus injects it's DNA into a host cell and this usually maintained as an episomal concatamer. This means the injected DNA remains as a “loose” piece if DNA outside of the chromosomes.

A good overview of the way in which synthetic DNA is delivered into a human by adeno-associated virus is the reference below:

Schultz BR, Chamberlain JS. Recombinant adeno-associated virus transduction and integration. Mol Ther. 2008;16(7):1189-1199. doi:10.1038/mt.2008.103

— Daniel Chan 2020/01/13 19:24