An important advantage of gene therapy for Usher Syndrome is that the retina can be reached with one syringe and the AVV vector can be applied very locally and specifically. Besides, only a small quantity is needed. Another advantage is that the immune system in the eye is less responsive. This reduces the chance of an immune response and infection that would block the effect of the treatment.
Research has shown that after gene therapy, the retina (part of the eye) can despite the presence of dead cells make normal connections with other cells, which leads to a normal response to light.
Also take a look at the fact sheet about gene therapy of the Life Sciences and Society Foundation (BWM = Biowetenschappen en Maatschappij). (Unfortunately only in Dutch)