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A Neurodegenerative Perspective on Mitochondrial Optic Neuropathies

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A Neurodegenerative Perspective on Mitochondrial Optic Neuropathies Acta Neuropathol DOI 10.1007/s00401-016-1625-2 REVIEW A neurodegenerative perspective on mitochondrial optic neuropathies Patrick Yu-Wai-Man1,2,3 · Marcela Votruba4,5 · Florence Burté1 · Chiara La Morgia6,7 · Piero Barboni8,9 · Valerio Carelli6,7 Received: 21 May 2016 / Revised: 24 September 2016 / Accepted: 25 September 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Mitochondrial optic neuropathies constitute an mutations within the nuclear gene OPA1 that encodes for important cause of chronic visual morbidity and registrable a mitochondrial inner membrane protein. The defining blindness in both the paediatric and adult population. It is neuropathological feature is the preferential loss of retinal a genetically heterogeneous group of disorders caused by ganglion cells (RGCs) within the inner retina but, rather both mitochondrial DNA (mtDNA) mutations and a grow- strikingly, the smaller calibre RGCs that constitute the pap- ing list of nuclear genetic defects that invariably affect a illomacular bundle are particularly vulnerable, whereas mel- critical component of the mitochondrial machinery. The two anopsin-containing RGCs are relatively spared. Although classical paradigms are Leber hereditary optic neuropathy the majority of patients with LHON and DOA will present (LHON), which is a primary mtDNA disorder, and autoso- with isolated optic nerve involvement, some individuals will mal dominant optic atrophy (DOA) secondary to pathogenic also develop additional neurological complications pointing towards a greater vulnerability of the central nervous sys- tem (CNS) in susceptible mutation carriers. These so-called Electronic supplementary material The online version of this article (doi:10.1007/s00401-016-1625-2) contains supplementary “plus” phenotypes are mechanistically important as they put material, which is available to authorized users. the loss of RGCs within the broader perspective of neuronal loss and mitochondrial dysfunction, highlighting common * Patrick Yu‑Wai‑Man pathways that could be modulated to halt progressive neu- Patrick.Yu‑Wai‑[email protected] rodegeneration in other related CNS disorders. The manage- 1 Wellcome Trust Centre for Mitochondrial Research, Institute ment of patients with mitochondrial optic neuropathies still of Genetic Medicine, Newcastle University, Newcastle upon remains largely supportive, but the development of effec- Tyne, NE1 3BZ, UK tive disease-modifying treatments is now within tantalis- 2 Newcastle Eye Centre, Royal Victoria Infirmary, Newcastle ing reach helped by major advances in drug discovery and upon Tyne, NE1 4LP, UK delivery, and targeted genetic manipulation. 3 NIHR Biomedical Research Centre at Moorfields Eye Hospital and UCL Institute of Ophthalmology, Keywords Dominant optic atrophy · Leber London, EC1V 2PD, UK hereditary optic neuropathy · Mitochondrial diseases · 4 School of Optometry and Vision Sciences, Cardiff University, Neurodegenerative diseases · OPA1 · Retinal ganglion cell Cardiff, UK 5 Cardiff Eye Unit, University Hospital of Wales, Cardiff, UK 6 IRCCS Institute of Neurological Sciences of Bologna, Introduction Bellaria Hospital, Bologna, Italy 7 Unit of Neurology, Department of Biomedical Mitochondrial diseases affect at least 1 in 4300 of the and Neuromotor Sciences (DIBINEM), University population and patients can present with either isolated or of Bologna, Bologna, Italy multisystemic organ involvement [58]. Despite these wide 8 Studio Oculistico d’Azeglio, Bologna, Italy phenotypic manifestations, the eye is particularly vulner- 9 San Raffaele Scientific Institute, Milan, Italy able and over half of all patients will develop significant 1 3 Acta Neuropathol ocular complications, in particular involvement of the and despite three decades of intensive research, there are optic nerve [35, 143]. Unsurprisingly, the risk of blindness still several unsolved mysteries, in particular the marked ranks high on patients’ fears about the prognosis associ- incomplete penetrance and the sex bias associated with ated with their specific mitochondrial genetic defect [12, this mitochondrial disorder [134]. Penetrance can vary 74]. The optic nerve consists of the projecting axons from widely between families or even within different branches about 1.2 million retinal ganglion cells (RGCs) and the of the same family, but as a rule of thumb, the penetrance pathophysiological pathways that drive the selective loss for a male LHON carrier is ~50 % compared with ~10 % of this highly specialised neuronal population are provid- for a female LHON carrier [35, 143]. All the evidence so ing important new insights into the broader link between far points towards LHON being a complex disease that is mitochondrial dysfunction and neurodegeneration within determined by additional mitochondrial and nuclear genetic the central nervous system (CNS) [71]. The two classical risk factors as well as environmental triggers such as smok- mitochondrial optic neuropathies are Leber hereditary optic ing, which is associated with significantly increased risk of neuropathy (LHON) and autosomal dominant optic atrophy visual loss [66, 75, 88]. Oestrogens could also have a neu- (DOA), which share overlapping clinical and pathologi- roprotective effect on RGCs and this feature could account, cal features, despite being caused by mitochondrial DNA at least partly, for the relative protection of female LHON (mtDNA) point mutations and a growing list of nuclear carriers against visual loss [55]. genetic defects, respectively. The defining neuropathologi- Patients with LHON typically present with bilateral sub- cal feature of LHON and DOA is the early loss of RGCs acute loss of central vision and the time course is defined within the papillomacular bundle resulting in an expand- by an expanding dense scotoma and a period of worsening ing field defect, known as a scotoma, within the patient’s visual acuities until a nadir is reached. In unilateral cases, central vision [52]. Interestingly, a subgroup of patients can the fellow eye usually converts within 3-6 months after first develop a syndromic form of LHON and DOA that is char- disease onset (Fig. 1). Spontaneous visual recovery can acterised by prominent neurological deficits in addition to occur, usually in the first year, but the prognosis remains optic atrophy. This review will focus on the disease mecha- guarded with most patients remaining within the legal cri- nisms that underpin the selective vulnerability of RGCs teria for blind registration [97]. Limited post-mortem stud- in mitochondrial optic neuropathies, but also how these ies of optic nerves from patients with LHON confirm that could relate to the more extensive CNS neurodegenera- the neuropathology is limited to the RGC layer [82, 116]. tion observed in patients with the more severe “plus” phe- The rapid loss of RGCs in the acute phase of the disease is notypes. Crucially for patients and their families, there are thought to be mediated via apoptotic pathways, but further currently limited treatment options and a number of strate- work is needed to determine whether the primary site of the gies are being considered to prevent or halt neuronal loss in pathology lies within the cell body or the proximal unmy- this group of disorders, all of which will need to be rigor- elinated axonal segments. The acute phase of LHON is ously assessed for safety and efficacy. characterised by the early loss of RGCs within the papillo- macular bundle and this interesting pathological feature has been ascribed to their smaller than average axonal calibre Leber hereditary optic neuropathy [118]. The basis for this anatomical vulnerability has been modelled using a mathematical equation that integrates the LHON is a primary mitochondrial genetic disorder that surface area (energy demand) and volume (energy supply) affects approximately 1 in 30,000 people in the population ratio of the RGC axons, in relation to the amount of ATP and three point mutations within the mitochondrial genome needed to maintain axonal conduction [101]. It must be account for over 90 % of cases, namely m.3460G>A emphasised that there is no absolute experimental proof, (MTND1), m.11778G>A (MTND4) and m.14484T>C but the greater surface area/volume ratio in smaller fibres (MTND6) [87]. These three primary mtDNA mutations all is thought to result in a more limited mitochondrial bioen- affect complex I subunits and the disturbed flux of elec- ergetic reserve capacity and proportionally more ROS pro- trons along the mitochondrial respiratory chain results duction compared with larger fibres [83]. in impaired oxidative phosphorylation (OXPHOS) and increased levels of reactive oxygen species (ROS) [32]. LHON is the classical paradigm of a mitochondrial optic LHON plus phenotypes neuropathy and the preferential loss of retinal ganglion cells (RGCs) raises intriguing questions relating to tissue Although the neuropathological hallmark of LHON is specificity and the secondary modulatory factors that dic- dominated by the loss of RGCs within the inner retina, a tate disease expression. The m.11778G>A LHON mutation subgroup of patients will develop extraocular features as was the first mtDNA variant linked with human disease part of a more severe LHON “plus” syndromic phenotype 1 3 Acta Neuropathol Fig. 1 Colour fundus and OCT findings in a patient with acute LHON protocol, Cirrus HD-OCT, Carl Zeiss Meditec). The left optic disc compared with a healthy subject. a An 18-year-old man harbouring
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