AAV-Delivered Tulp1 Supplementation Therapy Targeting Photoreceptors Provides Minimal Benefit in Tulp1-/- Retinas

With marketing approval of the first ocular gene therapy, and other gene therapies in clinical trial, treatments for inherited retinal degenerations (IRDs) have become a reality. Biallelic mutations in the tubby like protein 1 gene (TULP1) are causative of IRDs in humans; a mouse knock-out model (Tulp1-/-) is characterized by a similar disease phenotype. We developed aTulp1supplementation therapy forTulp1-/-mice. Utilizing subretinal AAV2/5 delivery at postnatal day (p)2-3 and rhodopsin-kinase promoter (GRK1P) we targetedTulp1to photoreceptor cells exploring three doses, 2.2E9, 3.7E8, and 1.2E8 vgs.Tulp1mRNA and TULP1 protein were assessed by RT-qPCR, western blot and immunocytochemistry, and visual function by electroretinography. Our results indicate that TULP1 was expressed in photoreceptors; achieved levels ofTulp1mRNA and protein were similar to wild type levels at p20. However, the thickness of the outer nuclear layer (ONL) did not improve in treatedTulp1-/-mice. There was a small and transient electroretinography benefit in the treated retinas at 4 weeks of age (not observed by 6 weeks) when using 3.7E8 vg dose. Dark-adapted mixed rod and cone a- and b-wave amplitudes were 24.3 +/- 13.5 mu V and 52.2 +/- 31.7 mu V in treatedTulp1-/-mice, which were significantly different (p< 0.001,t-test), from those detected in untreated eyes (7.1 +/- 7.0 mu V and 9.4 +/- 15.1 mu V, respectively). Our results indicate thatTulp1supplementation in photoreceptors may not be sufficient to provide robust benefit inTulp1-/-mice. As such, further studies are required to fine tune theTulp1supplementation therapy, which, in principle, should rescue theTulp1-/-phenotype.