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Ying Hsu, Janelle Garrison, Poppy Datta, Gunhee Kim, Darryl Nishimura, Charles C Searby, Seongjin Seo, Val Sheffield; Photoreceptors show age-dependent ability to regain function after restoration of target gene expression in mouse models of Bardet Biedl Syndrome. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4542.
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Treatment effectiveness at different stages of retinal degeneration is a key consideration for gene therapy. In Bardet Biedl Syndrome (BBS), photoreceptor degeneration leads to blindness. We will address whether photoreceptors, particularly those found at later stages of retinal degeneration, can regain function and regenerate or remodel their defective outer segments (OSs) upon restoration of inactivated genes. The effect of timing on rescue efficiency is investigated in two BBS mouse models.
We used Bbs8 and Lztfl1 (Bbs17) mutant mice, in which the gene trap cassette that blocks the target gene expression can be eliminated by tamoxifen inducible FLP recombinase. Tamoxifen was injected to induce restoration of target gene expression at 5 different ages: a) postnatal days 9-15 (P9-15) when the OS develops, b) P21-P30 with 0-10%, c) at 6 weeks of age with 15-25%, d) at 3 months of age with ~50%, and e) at 6 months of age with 80-90% photoreceptor cell loss. We performed immunohistochemistry, electroretinography (ERG), and transmission electron microscopy (TEM) to measure rescue effectiveness.
Restoration of gene expression prior to retinal degeneration in Groups a) and b) completely restored retinal functions. Restoration of gene expression in Groups c) and d) significantly improved ERG amplitudes and arrested degeneration. Restoration of gene expression at a later stage of degeneration in Group e) showed no significant improvement in retinal functions. In Group c), preliminary data suggests that re-localization of proteins mislocalized to the OSs of BBS mutant retinas including STX3 and STXBP1 is minimal despite the restoration of BBS protein levels, and that surprisingly mislocalized STX3 and STXBP1 remain in the OS for longer than the expected duration considering the rate of normal OS renewal in mice.
Restoration of gene function in photoreceptor cells found at a later stages of retinal degeneration cannot restore normal OS morphology. Furthermore, BBS mutant mice may have slower OS renewal compared to control mice. Understanding the ability of photoreceptor cells for functonal recovery and OS remodeling or regeneration as disease progresses will contribute to the optimal design of gene therapies and aid in critical decision making in clinical studies.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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