Protein Inhibitor of Retinal Membrane Guanylyl Cyclase Rescues Mouse Rod Photoreceptors from GUCY2D Retinal Dystrophy

Mutations in GUCY2D gene coding for retinal membrane guanylyl cyclase 1 (RetGC1, or GC-E) can deregulate cGMP synthesis in photoreceptors by guanylyl cyclase activating proteins (GCAPs) via negative Ca 2+ feedback and elevate cGMP production in the dark, thus causing autosomal-dominant cone-rod dystrophy ( GUCY2D adCORD). We expressed an engineered p rotein i nhibitor of retinal g uanylyl cy clase (PIGCY) in transgenic mice of either sex harboring the GUCY2D adCORD RetGC1 mutant Arg838Ser ( R838S T g ) in order to suppress aberrant production of cGMP in their rods and prevent their fast degeneration. PIGCY did not return the abnormal Ca 2+ sensitivity of cGMP synthesis in PIGCY Tg R838S Tg retinas to the normal physiological range, but it reduced the overall rate of cGMP production. Decelerated cGMP production strongly suppressed degeneration of functional rods. By the age of 6 months, 70% of photoreceptor nuclei remained in PIGCY Tg R838S Tg versus 20% in R838S Tg . Retinal response to light detected by electroretinography was preserved in PIGCY Tg R838S Tg mice significantly better than in R838S Tg . The remaining altered calcium feedback affected the shape of PIGCY Tg R838S Tg rod photoresponses, but overall light sensitivity was not drastically different from normal. Single-photon response amplitude and time-to-peak in PIGCY Tg R838S Tg rods were increased and more variable, but were similar to wild type in fractional light sensitivity and half-saturating light intensity. Consistent with the still abnormal calcium feedback, desensitization under background light in PIGCY Tg R838S Tg rods was increased and their noise was elevated under moderate background illumination as compared to wild type. In summary, PIGCY effectively rescued degenerating GUCY2D adCORD rods while maintaining their functional light sensitivity. Significance statement Elevated cGMP synthesis in photoreceptors caused by mutations deregulating the calcium-sensitive activity of human membrane guanylyl cyclase 1 (RetGC1, GUCY2D ) in the dark causes progressing congenital blindness, autosomal dominant cone-rod dystrophy (adCORD). This work is the first to demonstrate that degeneration of transgenic mouse rod photoreceptors harboring a human GUCY2D adCORD RetGC1 mutant can be effectively suppressed in vivo by an engineered protein inhibitor. Reduction of cGMP synthesis rescuing adCORD rods from degeneration moderately affects the shape of light responses but does not disable rod light sensitivity. The findings open the possibility that such an approach could be utilized to treat presently incurable GUCY2D adCORD.