Investigating the Mechanism of Optic Nerve disorders associated with Down Syndrome

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PROJECT ABSTRACT The prevalence of optic nerve hypoplasia (ONH) among individuals with Down syndrome (DS) is more than ~100 folds higher than in the general population. ONH is characterized by a thin, underdeveloped optic nerve that often results from secondary loss of retinal ganglion cells (RGCs) and is the leading cause of childhood legal blindness in developed nations. We have developed a murine model of ONH by manipulating the CASK (calcium/calmodulin dependent serine protein kinase) gene. The ONH pathology of CASK mutant mice recapitulates many aspects of human ONH, including the timing of the onset of pathology (after RGC development, i.e., secondary loss) and the non-progressive nature of the pathology. In addition to ONH, DS and pathology associated with CASK mutation share other similarities such as microcephaly. Biochemical experiments suggest that similar to DS, CASK mutation alters mitochondrial function, resulting in aberrant fatty acid metabolism. We have uncovered a deficiency of the ω-6 polyunsaturated fatty acid arachidonic acid (ARA) in the central nervous system (CNS) of CASK mutant mice. A specific reduction of ARA-containing lipids has been also reported in postmortem human DS brains. In both CASK mutant mice and the DS murine model visual contrast sensitivity is reduced. Based on the literature and our observations, we hypothesize that ARA deficiency in the optic nerve (ON) leads to ONH in DS. If this hypothesis is correct, dietary ARA supplementation could ameliorate ONH in DS. In this proposal, we plan to use two independent murine DS models (Ts65Dn and DP16 mice) to investigate retinal circuit and retinogeniculate connectivity using appropriate markers and tracers along with imaging techniques. Three-dimensional ultrastructural morphometry of the ON in DS murine models will be performed using serial block-face scanning electron microscopy (SBFSEM). Behavioral experiments will document visual defects. Visual circuit function will be evaluated with a novel in vivo electrophysiological technique we have named Network Response to Visual Excitation (NeRVE), a method that will be applicable beyond this proposal to better understand rodent vision processing. Mitochondrial metabolism, reactive oxygen species generation, oxidative damage, and fatty acid metabolic defects in the retina, optic nerve and brain of two independent DS mouse models will be measured. We will also quantify the levels of two ω- polyunsaturated fatty acids (docosahexaenoic acid and ARA), as well as phospholipids, in the ON of both types of DS mice. Finally, we will test if ARA supplementation ameliorates known defects in visual acuity and contrast sensitivity in these two DS mouse models. Our study is likely to provide data on DS pathobiology and provide a clear mechanism for the co-occurrence of ONH in DS. Furthermore, promising results from our ARA supplementation study would be immediately translatable into clinical practice for the amelioration of ONH in individuals with DS.