Researchers at the Berkeley Stem Cell Centre at the University of California Berkeley, led by David Schaffer, have developed a virus that restores sight by inserting genes into hard-to-reach cells in the retina. The retina is the tissue inside the back of the eye that consists of photosensitive cells that can identify light and colours.
The researchers believe that this type of gene therapy could be used to restore vision to a variety of diseases affecting the retina, including hereditary defects and illnesses brought on by old age, such as macular degeneration. In either case, the virus would deliver a correct copy of the gene to the retinal cells and replace the defective genes, whether they were defective at birth or became defective through excessive cellular reproduction with age.
It’s not an entirely new development – in 2007, researchers successfully restored some sight to over a dozen people with Leber’s congenintal amaurosis, a hereditary disease causing major loss of vision in early adulthood, by attaching the gene into benign respiratory adeno-associated viruses and injecting them directly into the retina. However, the virus could not reach all the cells via retinal injection. Furthermore, the needle can cause retinal detachment, which incidentally cannot be cured with gene therapy and must be resolved physically through surgery.
It should be noted that gene therapy also will not restore sight to eyes that are completely blind and have dead retinal cells since those dead cells cannot be revived to produce the proper proteins after fixing the genes.
The new technique requires injection of the virus into the liquid vitreous humour of the eye in a fifteen minute procedure. So far, it has been shown to be much more effective than injecting the virus into the retina of rodents and monkeys. Although the virus penetrates spottily across the retina, penetration in the fine-vision area called the fovea has been far more successful than current viral injection techniques so far, which do not significantly penetrate the foveal cells at all. The researchers are currently collaborating with physicians to determine which patients would best benefit from the gene delivery technique before identifying candidates for clinical trials.
This is the culmination of over 14 years of work, starting from the refining of the adeno-associated viruses to penetrate tissues in a specific way and modified so that they would not attack tissues or reproduce. Over 100 million variants of viruses were developed, each with slightly different proteins on its surface, some more suitable for penetrating livers and others for retinas. The researchers then selected the viruses best suited for delivery to the retinal cells.
There remains a risk that patients may be allergic to the adeno-associated virus but the new viral delivery method makes gene therapy more viable as a future treatment for blindness with genetic causes.
Leave a Reply