What OPA1 models teach us about potential therapy

Abstract

Autosomal dominant optic atrophy (DOA) is the most common inherited optic neuropathy, affecting approximately 1 in 25 000 people. DOA has an insidious onset in early childhood. It presents typically with bilateral central painless vision loss, dyschromatopsia, and optic disc pallor due to gradual retinal ganglion cell (RGC) loss and optic nerve degeneration. 60%–70% of genetically confirmed DOA cases are associated with variants in OPA1. OPA1 is an ubiquitously expressed GTPase that regulates mitochondrial homeostasis through coordination of inner membrane fusion, maintenance of cristae structure, and maintenance of bioenergetic output. Over 250 known pathogenic variants in OPA1 have been reported, with missense variants within the GTPase domain associated with an increased susceptibility to developing DOA plus (DOA+), a multisystemic syndromic form of the disease with extraocular features, including sensorineural hearing loss and ataxia. Current therapeutic options to treat DOA are limited. Idebenone has been shown to protect against visual loss in Leber's hereditary optic neuropathy (LHON). In DOA off ‐label retrospective series show some promise in some patients.Animal models for OPA1 DOA, have been reported, including Drosophila, C elegans, Danio rerio and mouse models. They have cast light on some potential therapeutic modalities. Despite limitations, murine models of DOA are currently indispensable for understanding pathophysiology of disease, drug testing and development. They replicate many symptoms and signs of human disease. However, there are caveats. Chemically induced Complex I deficits are very toxic. Genetic models of mitochondrial mutation affect protein integrity and function and can cause severe pathologies. This is made more problematic if there is a need for aging the mouse to bring out the full phenotype. We shall review the mouse models and describe some features that aid or our understanding of pathophysiological mechanisms. Our research has characterized the earliest morphological changes as a progressive loss of synapses, dendrite retraction and RGC remodelling and loss of connectivity. In the DOA mouse Opa1+/− mouse (B6;C3‐Opa1Q285STOP), heterozygous Opa1+/− mice develop visual defects, on psychophysics (optokinetic response) and electrophysiology (VEP, PhNR), consistent with RGC dysfunction. There is depressed mitochondrial Complex I and IV activity. Visible structural changes and disrupted axon morphology in the optic nerve are seen. We have carried out a randomized, placebo‐controlled trial of the benzoquinone idebenone in our ADOA mouse, which revealed a limited therapeutic effect on RGC dendropathy and visual function.It is apparent that there are large differences between the rodent and primate visual system. The fovea of the retina, a critical component for central vision is one of the only primate‐specific components of the mammalian nervous system and is thus, not found in rodent. For these reasons, there is a strong case to use human cells. We shall review data on induced human pluripotent stem cell (hiPSC) with OPA1 mutation, with or without differentiation to RGCs, as models, and on CRISPRCas9 correction of patient‐derived OPA1 iPSC lines.