Expression of Wild-Type Rp1 Protein in <em>Rp1</em> Knock-in Mice Rescues the Retinal Degeneration Phenotype

Abstract

<div><p>Mutations in the retinitis pigmentosa 1 (<em>RP1</em>) gene are a common cause of autosomal dominant retinitis pigmentosa (adRP), and have also been found to cause autosomal recessive RP (arRP) in a few families. The 33 dominant mutations and 6 recessive <em>RP1</em> mutations identified to date are all nonsense or frameshift mutations, and almost exclusively (38 out of 39) are located in the 4^th and final exon of <em>RP1</em>. To better understand the underlying disease mechanisms of and help develop therapeutic strategies for RP1 disease, we performed a series of human genetic and animal studies using gene targeted and transgenic mice. Here we report that a frameshift mutation in the 3^rd exon of <em>RP1</em> (c.686delC; p.P229QfsX35) found in a patient with recessive <em>RP1</em> disease causes RP in the homozygous state, whereas the heterozygous carriers are unaffected, confirming that haploinsufficiency is not the causative mechanism for <em>RP1</em> disease. We then generated <em>Rp1</em> knock-in mice with a nonsense Q662X mutation in exon 4, as well as <em>Rp1</em> transgenic mice carrying a wild-type BAC <em>Rp1</em> transgene. The <em>Rp1</em>-Q662X allele produces a truncated Rp1 protein, and homozygous <em>Rp1</em>-Q662X mice experience a progressive photoreceptor degeneration characterized disorganization of photoreceptor outer segments. This phenotype could be prevented by expression of a normal amount of Rp1 protein from the BAC transgene without removal of the mutant Rp1-Q662X protein. Over-expression of Rp1 protein in additional BAC <em>Rp1</em> transgenic lines resulted in retinal degeneration. These findings suggest that the truncated Rp1-Q662X protein does not exert a toxic gain-of-function effect. These results also imply that in principle gene augmentation therapy could be beneficial for both recessive and dominant <em>RP1</em> patients, but the levels of RP1 protein delivered for therapy will have to be carefully controlled.</p> </div