Retinoschisin Gene Therapy and Natural History in theRs1h-KO Mouse: Long-term Rescue from Retinal Degeneration

Abstract

The authors characterized the natural history of a retinoschisin gene knockout (Rs1h-KO) mouse model and evaluated the long-term effects of retinal rescue after AAV(2/2)-CMV-Rs1h gene delivery. Full-field scotopic electroretinograms (ERGs) were recorded from 44 male hemizygous Rs1h-KO and 44 male wild-type (WT) C57BL/6J mice at six ages between 1 and 16 months. Retinal morphometry included outer segment layer (OSL) width, photoreceptor cell count, and grading of schisis cavity severity. One eye each of seven Rs1h-KO mice at age 14 days was injected with AAV(2/2)-CMV-Rs1h, and retinal histology and ERG findings at 14 months were analyzed. The outer nuclear layer (ONL) of 1-month-old Rs1h-KO mice was disorganized but had nearly normal cell counts. The OSL was thinned, rod outer segments were misaligned, and abundant schisis cavities spanned the inner nuclear and outer plexiform layers in all retinas. ERG a- and b-wave amplitudes at this age were reduced by 33% and 50%, respectively. ERG and ONL cell numbers decreased further between 1 and 16 months, with unequal changes in the a- and b-waves with age. The a-wave reduction correlated well with the steady decline in ONL cell number, whereas a rapid decline in the b-wave and a (b/a-wave) ratio less than in WT were associated with increasing severity of schisis cavities at young ages. At 4 months, the cavities were maximal, but they coalesced and disappeared at older ages. The (b/a-wave) ratio was inversely correlated with cavity severity across all ages (r = -0.74; P < 0.0001; n = 22). Considerable heterogeneity was observed at each age in the ERG amplitudes and retinal morphology. Mice injected with AAV-Rs1h at 14 days showed considerable structural and functional rescue at age 14 months, including improved rod outer and inner segment integrity, less photoreceptor cell loss, and larger ERG amplitudes compared with untreated fellow eyes. The ERG of the Rs1h-KO mouse at early ages reflects disruption of photoreceptor and second-order neuron function. In mid to late ages, the ERG decline reflects primarily photoreceptor degeneration. The Rs1h-KO mouse is consistent with human clinical X-linked juvenile retinoschisis (XLRS) in showing schisis cavities, which affect primarily the b-wave, the regression of schisis cavities at older ages, and a considerable range in phenotypic severity across individuals. This mouse model also indicates the critical roll of RS-protein in photoreceptor survival consistent with decreased a-waves in some patients with XLRS. Long-term rescue of retinal morphology and function by AAV-Rs1h gene transfer may provide a basis for considering intervention in the homologous human XLRS condition.