RPE65 surface epitopes, protein interactions, and expression in rod- and cone-dominant species

RPE65 is an abundant protein expressed in the RPE where it functions in the visual cycle necessary for the synthesis of 11-cis retinal, the chromophore of the visual pigments [1,2]. Developmentally, RPE65 is an important marker for the differentiated phenotype of the RPE [3]. Mutations in the gene encoding RPE65 are responsible for childhood-onset forms of autosomal recessive severe retinal dystrophy, including Leber congenital amaurosis, in an estimated 11% of cases [4]. A number of research groups are currently involved in efforts to develop therapeutic methods specific for RPE65 loss-offunction [5-7], with clinical trials of gene replacement therapy planned for the near future. Recent studies show that RPE65 functions as the isomerohydrolase in the RPE that converts all-trans retinyl esters to 11-cis retinol [8-10] by coupling the free energy of ester hydrolysis to the trans to cis isomerization reaction [11]. This role is compatible with the findings of earlier studies showing that RPE65 is a retinoid-binding protein [12,13] that acts a molecular switch, binding all-trans retinyl esters when palmitoylated, and all-trans retinol when depalmitoylated by transfer to lecithin retinol acyl transferase (LRAT) [14]. Pathogenesis associated with RPE65 loss-of-function is proposed to result, in part, from constitutive opsin activity due to loss of chromophore [15], although other interpretations exist [16]. More than half of all known RPE65 mutations are missense substitutions affecting over 30 different amino acid residues [4]. The relatively high incidence of RPE65 mutations in patients with early-onset disease, as well as its central role in current therapeutic efforts, creates a strong incentive to achieve a mechanistic understanding of RPE65 participation in the visual cycle. Among the many critical issues to resolve include elucidating the mechanism(s) by which missense mutations ©2005 Molecular Vision