**1. Introduction**

Retinitis pigmentosa (RP) represents a group of hereditary retinal degenerative disorders of diverse genetic origins that have as their common trait the progressive, irreversible dysfunction, degeneration, and demise of retinal photoreceptor cells, with rods initially undergoing these pathological changes followed eventually by cones [1,2]. Relatively recently, a K42E point mutation in the dehydrodolichyl diphosphate synthase (DHDDS) gene was shown to cause a rare, recessive form of RP (RP59; OMIM #613861) [3–5]. DHDDS catalyzes *cis*-prenyl chain elongation in the synthesis of dolichyl diphosphate (Dol-PP), which is required for protein *<sup>N</sup>*-glycosylation [6,7]. DHDDS catalyzes the condensation of multiple units of isopentenyl pyrophosphate (IPP, also called isopentenyl diphosphate) to farnesyl pyrophosphate (FPP, also called farnesyl diphosphate) to produce Dol-PP [8,9]. This is used

as the "lipid carrier" onto which oligosaccharide chains are built that are ultimately transferred to specific asparagine (*N*) residues on nascent polypeptide chains in the lumen of the endoplasmic reticulum (ER) to form *N*-linked glycoproteins [10]. The monophosphate (Dol-P) is used as a sugar carrier, transferring sugars from their corresponding sugar-nucleotide adducts (e.g., UDP-glucose, GDP-mannose, etc.) to the growing Dol-PP-linked oligosaccharide chains in the ER. Mutations in rhodopsin that block its glycosylation have been shown to cause retinal degeneration in vertebrate animals [11,12]. In addition, pharmacological inhibition of protein *<sup>N</sup>*-glycosylation with tunicamycin has been shown to disrupt retinal photoreceptor outer segmen<sup>t</sup> (OS) disc membrane morphogenesis in vitro [13], as well as to cause retinal degeneration with progressive shortening and loss of photoreceptor OSs in vivo [14].

In the present study, we created a DHDDS K42E homozygous knock-in mouse model (hereafter called *Dhdds*K42E/K42E) of RP59—since K42E is the most prevalent point mutation in the RP59 patient population [3–5]—to study its underlying pathological mechanism, with the working hypothesis that defective protein *<sup>N</sup>*-glycosylation underlies the retinal dysfunction and degeneration observed in human RP59. Herein, we present a description of the generation and initial characterization of the phenotypic features of the *Dhdds*K42E/K42E mouse model. Surprisingly, although we expected to observe an early onset, progressive, and potentially severe retinal degeneration, this was not the case. The retina appeared histologically intact and normal according to spectral domain optical coherence tomography (SD-OCT) analysis for up to at least one year of age. However, there was evidence of gliotic reactivity (glial fibrillary acidic protein (GFAP) immunostaining), despite the lack of obvious neuronal degeneration or cell death/loss. Also, despite the homozygous mutation in *Dhdds*, we found no evidence of compromised protein *<sup>N</sup>*-glycosylation in mutant mouse retinas.
