**1. Introduction**

Retinitis pigmentosa (RP) and related disorders are characterized by degeneration and loss of photoreceptors, attenuation of retinal blood vessels, pigment deposits, and a waxy pallor of the optic disc that result in impaired night vision and peripheral and central vision loss [1]. Defective protein glycosylation in the retinal pigment epithelium (RPE) has been associated with retinal degeneration elicited by photoreceptor abnormalities and impaired phagocytosis of aging photoreceptor outer segmen<sup>t</sup> (OS) membranes [2–5]. One such example, dehydrodolichyl diphosphate synthase (DHDDS; OMIM #608172), is an essential enzyme in the mevalonate pathway where it functions ubiquitously in

isoprenoid chain elongation to form dolichols that comprise 17–20 isoprene units. The phosphorylated form of dolichol, dolichol pyrophosphate is necessary for N-glycosylation at specific residues in many membrane proteins [6]. There is no known unique function for DHDDS in the retina, or in any other ocular tissue. There are numerous N-glycosylated proteins in the retina and photoreceptors, such as rhodopsin [7] and the cGMP-gated cation channel in rods. Additionally, in the RPE, there are several glycosylated structural proteins, ion channels and transport proteins that contribute to the transepithelial potential of the polarized RPE monolayer [8–10].

Mutations in the gene encoding DHDDS lead to a recessive form of RP called RP59 (OMIM #613861, DHDDS, K42E), which was first identified in several families of Ashkenazi Jewish origin [11–13]. RP59 is considered to belong to the family of human genetic diseases known as "congenital disorders of glycosylation" (CDGs) [14]. Two other mutations in the DHDDS gene (T206A and R98W), which occur heterozygously with the K42E mutation, were also reported, but have not been studied in detail [15,16]. Patients with the K42E mutation in DHDDS exhibit all the cardinal features of RP as well as macular changes [12], suggesting possible RPE involvement. While the K42E, T206A, and R98W mutations are only associated with known RP symptoms, another mutation in the DHDDS gene has led to infant morbidity at seven months of age [17].

As a first step to assess the role of DHDDS in specific retinal cell types and to understand the molecular mechanism of RP59, we created a Cre-lox dependent line of mice that allows targeted, cell type-specific deletion of *Dhdds* in cells of interest. Using a conditional knockout was necessary to circumvent embryonic lethality associated with global knockout of *Dhdds* [18,19]. Here, we describe the generation and characterization of a *Dhddsflx*/*flx* CreRPE mouse line (i.e., RPE-specific *Dhdds* knockout) and its validation as a model of RPE atrophy and retinal degeneration. We show that RPE-specific deletion of *Dhdds* induces structural and functional deficits in the RPE and the photoreceptors, which suggests that RPE pathology may be a significant contributor to the retinal degeneration observed in patients with RP59 mutations.
