Inherited retinal dystrophies (IRDs) are monogenic forms of blindness that affect 1 in 4000 individuals and are caused by mutations in over 200 genes. This genetic heterogeneity has seriously hampered the development of a treatment for these diseases. IRDs constitute an unmet medical need.

The main aim of this project is to test whether the group of retinal dystrophies caused by an abnormal elevation of cGMP levels in photoreceptor cells (referred to as "cGMP disorders") can be treated by inhibiting the enzyme inosine monophosphate dehydrogenase 1 (IMPDH1), required for the synthesis of GTP. "cGMP disorders" include forms of hereditary blindness caused by mutations in the PDE6A, PDE6B, PDE6G, AIPL1, and IMPDH1 genes, which represent a significant fraction of the affected families in Spain. This project addresses three specific aims. Specific aim 1 involves developing a medicament, referred to as IMPDH1-shRNA-SLP, that efficiently silences IMPDH1 activity in cell lines. This reagent will be based on an shRNA for IMPDH1 encapsulated in solid lipid nanoparticles. A collection of shRNAs will be synthesized and tested in 661W and ARPE19 cell lines to select the most effective shRNA against IMPDH1 retinal splicing isoforms. The selected shRNA will be cloned into an expression vector and encapsulated in solid lipid nanoparticles for further in vivo testing via intravitreal administration.

This strategy will result in a drug formulation that could be easily transferred to patients. Specific aim 2 consists on evaluating the potential of the developed medicament to attain the phenotypic rescue of a preclinical mouse model of type 10 retinitis pigmentosa that expresses the D226N mutation in the IMPDH1 gene. This preclinical model constitutes an "avatar" for the majority of the RP10 affected individuals in Spain and worldwide, and was established in our laboratory. Specific aim 3 consists on assessing the capacity of IMPDH1-shRNA-SLP to attain the phenotypic rescue of rd10 mice (an established model of autosomal recessive retinitis pigmentosa by mutations in PDE6B), to obtain the proof of concept that IMPDH1-shRNA-SLP could constitute a generic treatment for "cGMP disorders" regardless of the causal gene. Two major advantages of using lipid nanoparticles over viral vectors are: i) their ability to target gene expression to the photoreceptor cell layer of the retina through intravitreal injection; and ii) a more favorable safety profile compared to viral vectors, regarding eliciting an immune response.