Two Novel Mouse Models to Study the Potential Cause of Nystagmus

Abstract

Infantile Nystagmus Syndrome (INS) is a gaze holding disorder characterized by uncontrollable oscillations of the eyes. INS has a prevalence of 14 out of 10,000 humans, with 1.9 of those considered idiopathic. INS is associated with albinism, sensory afferent defects, a mutation in the FRMD7 gene, and a number of syndromic mutations such as PAX6. Nystagmus cannot be characterized by just one gene or group of genes as there are many different comorbidities associated with nystagmus. Because there are many molecular causes that are associated with the development of nystagmus, effective curative treatments are lacking. Models to study nystagmus vary from albino mice to Frmd7 knockout mice. Additional models to study nystagmus in mice are needed to characterize other potential causes that underlie development of this disorder. I hypothesized that knocking out the skeletal muscle regulatory factor MyoD would affect the function of the extraocular muscles and induce nystagmus. I also hypothesized that looking into a simplified albino strain compared to a pigmented mouse would produce a transcriptome analysis useful in identifying genes associated with nystagmus.I tested the eye movements of the mice with MyoD knocked out (MyoD-/- mice) using optokinetic nystagmus (OKN) and determined that the MyoD knockout mice had uncontrolled oscillatory movements during stimuli and no stimuli. Their wildtype siblings used as controls had normal eye movements during stimuli, and no movements during the absence of stimuli. To study the extraocular function of these mice I used fiber number and cross-sectional area counting, which both were significantly smaller in the MyoD knockouts, I counted the number of satellite cells in each genotype and found that there was a greater number of PAX7 cells in aged MyoD knockout mice and an increase of PITX2 positive cells outside the dystrophin layer compared to inside in MyoD knockout mice. I hypothesize that this is due to their inability to differentiate in the absence of being able to turn on MyoD. I tested the eye movements of B6(CG)-Tyr(c-2J)/J mice that are genetically identical to a control pigmented mouse, the C57BL/6 mouse, but are phenotypically albino due to a mutation in the tyrosinase gene (B6 Albino) using OKN. The eye movements of the C57BL/6 mice were normal during presentation of stimuli, and absent during no stimuli. The eye movements of the B6 Albino mice are uncontrolled and oscillatory during both stimuli and no stimuli, which is a characteristic of nystagmus. To determine the transcriptome of the extraocular muscles, the cranial oculomotor neurons, and the abducens motor neurons I performed an RNA-Seq experiment and found that there were differentially expressed genes in all the tissues, with two genes overlapping in each tissue type, wdfy1 and nnt. WDFY1 contains a FYVE zinc finger binding domain and multiple WD-40 repeat domains, and it localizes to early endosomes mediated by the FYVE domain after forming a complex that recognizes damaged lysosomes (Teranishi et al. 2022, Ridley et al. 2001). NNT, or nicotinamide nucleotide transhydrogenase, is an integral part of the inner mitochondrial membrane for proton translocation across the inner mitochondrial membrane (Hoek et al., 1988). Further work is needed to determine if expression of these molecules is causative or a consequence of the nystagmus. I determined that the MyoD knockout mouse and the B6 Albino mouse are both excellent models for the study of nystagmus. These two novel ways to look at nystagmus through a model organism will provide future research to determine if the genes identified, when specifically knocked out, result in nystagmus. They might also be found in some human patients with an as yet unidentified mutation that is causative for the development or maintenance of nystagmus.