Recovery of ERG Components in the Cngb1-/- Model of RP Following Gene Therapy

Abstract

Purpose: : The lack of the CNG channel subunit CNGB1 leads to a loss of light-mediated electrical activity of rod photoreceptors and eventually to retinitis pigmentosa (RP). In the respective Cngb1^-/- mouse model, visual function and retinal morphology could be restored using rAAV-mediated gene replacement therapy. Here we assess contributions of photoreceptor populations with different functional status (i.e. from treated and untreated regions within the same eye) on the sum responses of the Ganzfeld electroretinogram (ERG).

Methods: : Cngb1^-/- knock-out mice were treated via subretinal injection of rAAV vector expressing murine CNGB1 under control of a rhodopsin promoter. Animals were injected at two weeks postnatally (PW2), and examined functionally and morphologically at PW10. ERGs were recorded under both scotopic and photopic conditions using a Jaeger/Toennies Multiliner Vision and a Roland Consult Ganzfeld system. In vivo retinal morphology was examined with optical coherence tomography (OCT, HE Spectralis™), and animals subsequently underwent histological analysis.

Results: : Cngb1^-/- knock-out mice feature a small rod response even in the absence of the CNGB1 subunit (Huettl et al., J Neurosci 2005). The respective ERG component present in both treated and untreated eyes is in agreement with the hypothesis that homomeric CNG channels, composed of the CNGA1 subunit only, reach the outer segment membrane in a very small number. In treated eyes, we found an additional signal component from a proportion of rods responding with practically regular timing and sensitivity, which we attribute to the rescue area. Since the treated region in this study amounts to approximately 1/3-1/4 of the retina, signals were much smaller than normal rod ERGs. This difference was also reflected in the mixed rod and cone ERG responses.Although the rod ERG was strongly reduced, the morphological appearance of the retina was not much altered. The major difference in the OCT was a preservation of outer segment markers (OLM and IS/OS border) in treated eyes of Cngb1^-/- mice.

Conclusions: : Here we show that gene replacement therapy in Cngb1^-/- mice results in photoreceptor populations with different functional status. The respective ERG components will be of high diagnostic importance and may serve as endpoints in treatment trials.

Keywords: electroretinography: non-clinical • gene transfer/gene therapy • imaging/image analysis: non-clinical