Abstract
Purpose :
The efficacy of retinal gene therapy has been demonstrated in many preclinical studies. However, its long-term efficacy, particularly when treatments are given at advanced disease stages, is debated. Inconsistency of subretinal injection and partial treatment may contribute to the variable outcomes. The current study is to test whether gene therapy has lasting effects regardless of disease stages, using a genetic rescue approach to “optimally treat” retinal degeneration.
Methods :
We used a Bardet-Biedl syndrome type 17 (BBS17) mouse model, in which expression of Bbs17 (also known as Lztfl1) was blocked by an insertion of a gene-trap cassette. This gene-trap was removed at various disease stages by using tamoxifen-inducible FLP recombinase, restoring Lztfl1 expression with its own promoter. Restoration of Lztfl1 expression was validated by laser-capture microdissection, PCR, and immunoblotting. Immunohistochemistry, electron microscopy, and electroretinography (ERG) were used to assess rescue of molecular and cellular phenotypes and visual functions. Rescued animals were followed for 6 months after treatment, and disease phenotypes were compared with the natural history of retinal degeneration in unrescued animals.
Results :
Tamoxifen injection efficiently restored Lztfl1 expression at all ages tested. Treatments at early stages (post-natal (P) day P5 and P21; less than 10% cell loss at the time of treatment) resulted in long-term preservation of both rods and cones and markedly improved ERG. However, protein mislocalization phenotype was not completely rescued until 6-7 months of age, by which all unrescued photoreceptors had died, and retinal degeneration continued slowly. When Lztfl1 expression was restored at a mid-disease stage (P45; 20-30% cell loss), there was a transient delay in retinal degeneration and moderate improvement in ERG, but retinal degeneration continued. Rescue at P90 (40-60% cell loss) had no beneficial effect on vision despite successful restoration of Lztfl1 expression and male fertility, a BBS phenotype unrelated to vision.
Conclusions :
Our study shows that therapeutic effects of gene therapy diminish rapidly in our BBS17 mouse model as treatments are given at later stages. Our study also suggests that at least for certain retinal degenerations there is a limited time window for successful gene therapy. Therefore, early treatment should be encouraged.
This is a 2020 ARVO Annual Meeting abstract.