Gene augmentation therapy delays NMNAT1-associated retinal degeneration in a mouse model

Abstract

Purpose : No treatment is available for NMNAT1-associated retinal degeneration, an early-onset recessive disease that causes severe vision loss. Here, we tested the hypothesis that supplementing retinal cells with a functional copy of NMNAT1 via viral-mediated gene augmentation therapy is protective in mice that are homozygous for the Nmnat1-p.V9M mutation.

Methods : A codon-optimized human NMNAT1 cDNA, driven by the ubiquitously-expressing CAG promoter, was packaged into AAV2/Anc80 (AAV2/Anc80.CAG.NMNAT1) and injected unilaterally into the sub-retinal space (0.75µL containing 7.5x10⁸ genomic copies) of two-week-old Nmnat1^V9M/V9M mice and littermate controls. At 1.5, 3, and 4 months of age, retinal architecture was assessed histologically. Outer nuclear layer (ONL) thickness was measured and transgene expression was localized using a custom human-specific α-NMNAT1 antibody that does not cross-react with the endogenous mouse Nmnat1.

Results : At 4 weeks post-injection (age 1.5 months), the ONL of the treated region of Nmnat1^V9M/V9M retinas are 4x (p=0.001) thicker than non-treated retina of the same animals (corresponding to ~8 versus 2 rows of nuclei, respectively), and immunolabeling shows transgene expression primarily in photoreceptors. This structural rescue has not been observed at the later time points. In age-matched control mice, ONL thickness of the treated and non-treated retina within the same animals are equivalent (p=0.52).

Conclusions : Gene augmentation therapy delays retinal degeneration in the Nmnat1-p.V9M mouse. The eventual loss of efficacy is not caused by toxicity of AAV2/Anc80.CAG.NMNAT1, but instead may be due to an inadequate distribution of transgene expression across retinal cells types. The experiments indicate that providing gene augmentation mainly to photoreceptors, the first cells to degenerate in Nmnat1-associated disease, is necessary but not sufficient for a long-term intervention. In this case, multiple cell types are independently vulnerable to mutant Nmnat1, any of which would serve as a disease locus if left untreated. Identifying these cell types should be possible using additional AAV serotypes and cell-specific promoters separately and alongside AAV2/Anc80.CAG. A dose response-validated selection of these vectors could then be used in combination to create a gene augmentation therapy with a cell targeting profile tailored for sustained efficacy.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.