Sterol homeostasis in mammalian cells and tissues involves balancing three fundamental processes: de novo sterol biosynthesis; sterol import (e.g., from blood-borne lipoproteins); and sterol export. In complex tissues, composed of multiple different cell types (such as the retina), import and export also may involve intra-tissue, inter-cellular sterol exchange. Disruption of any of these processes can result in pathologies that impact the normal structure and function of the retina. Here, we provide a brief overview of what is known currently about sterol homeostasis in the vertebrate retina and offer a proposed path for future experimental work to further our understanding of these processes, with relevance to the development of novel therapeutic interventions for human diseases involving defective sterol homeostasis.

Retinitis pigmentosa-59 (RP59) is a rare, recessive form of RP, caused by mutations in the gene encoding DHDDS (dehydrodolichyl diphosphate synthase). DHDDS forms a heterotetrameric complex with Nogo-B Receptor (NgBR; gene NUS1) to form a cis-prenyltransferase (CPT) enzyme complex, which is required for synthesis of dolichol, which in turn is required for protein N-glycosylation as well as other glycosylation reactions in eukaryotic cells. Herein, we review the published phenotypic characteristics of RP59 models extant, with an emphasis on their ocular phenotypes, based primarily upon knock-in of known RP59-associated DHDDS mutations as well as cell type- and tissue-specific knockout of DHDDS alleles in mice. We also briefly review findings in RP59 patients with retinal disease and other patients with DHDDS mutations causing epilepsy and other neurologic disease. We discuss these findings in the context of addressing “knowledge gaps” in our current understanding of the underlying pathobiology mechanism of RP59, as well as their potential utility for developing therapeutic interventions to block the onset, or to dampen the severity or progression, of RP59.

Congenital Disorders of Glycosylation (CDG) are a heterogenous group of primarily autosomal recessive mendelian diseases caused by disruptions in the synthesis of lipid linked oligosaccha-rides and their transfer to proteins. CDGs affect multiple organ systems and vary in presentation, even within families. Here we describe a chemically induced mouse mutant, tvrm76, with early onset photoreceptor degeneration. The recessive mutation was mapped to Chromosome 9 and as-sociated with a missense mutation in the Dpagt1 gene encoding UDP-N-acetyl-D-glucosamine:dolichyl-phosphate N-acetyl-D-glucosaminephosphotransferase (EC 2.7.8.15). The mutation is predicted to cause a substitution of aspartic acid with glycine at residue 166 of DPAGT1. Increased expression of Ddit3, and elevated levels of HSPA5 (BiP) sug-gest the presence of early-onset endoplasmic reticulum (ER) stress. These changes were associated with induction of photoreceptor apoptosis in tvrm76 retinas. Mutations in human DPAGT1 cause Myasthenic Syndrome 13 and severe forms of Congenital Disorder of Glycosylation Type Ij. In contrast, Dpagt1tvrm76 homozygous mice present with congenital photoreceptor degeneration without overt muscle or muscular junction involvement. Our results suggest the possibility of DPAGT1 mutations in human patients that present primarily with retinitis pigmentosa with little or no muscle disease. Variants in DPAGT1 should be considered when evaluating cases of non-syndromic retinal degeneration.