In the last few years, several studies have shown that gene therapy, especially if applied early, can stop or reverse some forms of congenital blindness in model organisms. A recent study indicates it may be effective even if applied in mid-course stages of blindness. A team of Chinese and American researchers led by Dr. Ji-jing Pang of the Eye Hospital, School of Ophthalmology and Optometry, Wenzhou Medical College, Wenzhou, PR China, and including Jackson Laboratory scientist Bo Chang, M.D., used gene therapy to restore cone function in the retinas of the B6(A)-Rpe65rd12/J (005379) mouse, a model of Leber congenital amaurosis – even if the disease has progressed for three months (Li et al. 2011; Pang et al. 2005).
Leber congenital amaurosis
Leber congenital amaurosis (LCA) is a group of early onset and hereditary retinal diseases that makes children partially or totally blind by 6 to 12 months of age. Mutations in at least 14 genes have been linked to LCA. One of the genes is RPE65, which, when mutated, causes Leber congenital amaurosis type 2 (LCA2). Expressed in the retinal pigment epithelium, RPE65 plays a critical role in regenerating "11-cis retinal," the color-giving component (chromophore) in rods and cones, after it is bleached during light absorption. Its loss destroys all photoreceptor function.
To correct RPE65 mutation damage, scientists are evaluating drugs, cell transplants and gene delivery in Rpe65 knockout, transgenic and spontaneous mutant mice, and spontaneous Rpe65-mutant dogs. Initially, because cones were thought to be completely degenerated early in Rpe65-deficient mice, scientists focused on restoring rod function. However, RPE65 gene therapy was found to restore cone function in dogs, and Pang and his colleagues demonstrated that it can restore cone function in Rpe65-deficient mice if administered early —14 days after birth (Pang et al. 2006; Pang et al. 2010).
A better vector
Recently, Pang and his team found that though cone degeneration in Rpe65-deficient mice begins by 14 days after birth, a few cones in the peripheral dorsal and temporal quadrants of the retina still function at five weeks and perhaps later, when virtually all other cones are degenerated. Yet, their efforts to rescue cone function by gene therapy in mid-course LCA stages failed: when applied to 35-day old Rpe65-deficient mice, gene therapy restores rod function only (Pang et al. 2010).
Pang and his team hypothesized that the RPE65 gene vector they were using did not transfect the few cones that survive into later LCA stages. To more efficiently target those cones, they used a more powerful vector – a self-complementary RPE65-expressing adeno-associated virus (AAV5) vector – and injected it sub-retinally. They injected both 14- and 90-day old Rpe65-deficient mice and, two months later, evaluated only mice in which at least 95% of the retina had been transfected. They found that the cone function and morphology in mice that had been injected at 14 days were nearly normal, and that the function and morphology of the surviving cones in the mice that had been injected at 90 days had improved – and the number of functioning cones had increased.
The findings by Dr. Pang and his colleagues indicated that gene therapy might be able to stop cone degeneration during early Leber congenital amaurosis and restore the function and morphology of the cones still surviving in later stages of human LCA. These results have important implications for the ongoing LCA2 clinical trials, which are focusing on restoring rod function in children and young adults.
Li X, Li W, Dai X, Kong F, Zheng Q, Zhou X, Lu F, Chang B, Rohrer B, Hauswirth WW, Qu J, Pang JJ. 2011. Gene therapy rescues cone structure and function in the 3-month-old rd12 mouse: a model for midcourse RPE65 leber congenital amaurosis. Invest Ophthalmol Vis Sci 52:7-15.
Pang J, Chang B, Hawes N, Hurd RE, Davisson MT, Li J, Noorwez SM, Malhotra R, McDowell JH, Kaushal S, Hauswirth WW, Nusinowitz S, Thompson DA, Heckenlively JR. 2005. Retinal degeneration 12 (rd12): a new, spontaneously arising mouse model for human Leber congenital amaurosis (LCA). Mol Vis 11:152-62.
Pang J, Chang B, Kumar A, Nusinowitz S, Noorwez SM, Li J, Rani A, Foster TC, Chiodo VA, Doyle T, Malhotra R, McDowell H, Li SHMQ, Kaushal S, Hauswirth WW. 2006. Gene therapy restores vision-dependent behavior as well as retinal structure and function in a mouse model of RPE65 Leber congenital amaurosis. Mol Ther 13:565-72.
Pang J, Boye SE, Lei B, Boye SL, Everhart D, Umino Y, Rohrer B, Alexander J, Li J, Dai X, Li Q, Chang B, Barlow R, Hauswirth WW. 2010. Early gene therapy confers structural and functional rescue to cones in two models of RPE65 deficiency: rd12 and Rpe65-/-Rho-/- mice. Gene Ther 17815-26.