What disease mechanisms contribute to multisystem tissue involvement in dominant optic atrophy due to OPA1 mutations?

Lead Research Organisation: Newcastle University
Department Name: Institute of Neuroscience

Abstract

Autosomal dominant optic atrophy (DOA) affects the optic nerve, causing insidious visual loss from early childhood. It is the most common inherited optic nerve disorder and the majority of patients carry mutations in OPA1, a nuclear gene critical for normal mitochondrial function. Mitochondria are essential components of all human cells, containing multiple copies of their own DNA (MtDNA), and forming long branching networks which fuel the cell‘s energy requirements.

About 20% of patients with OPA1 mutations will develop a more severe form of the disease (DOA+), associated with a worse visual prognosis and additional neurological complications. The risk of developing DOA+ is three times higher with missense mutations located within the GTPase gene region. Using a range of human tissue samples, I will investigate whether this specific mutational subgroup results in greater fragmentation of the mitochondrial network and more marked mtDNA damage, thus contributing to accelerated cell death. I will also explore the possibility of rescuing the fragmentation defect by manipulating the level of expression of OPA1 and other mitochondrial proteins involved in network fusion and fission. Visual loss in DOA is progressive and the neurological features develop in later life, providing a clear window of opportunity for therapeutic intervention.

Technical Summary

Autosomal dominant optic atrophy (DOA) is the most common inherited optic nerve disorder and the majority of patients harbour pathogenic OPA1 mutations. Although DOA is characterised by the preferential loss of retinal ganglion cells (RGC), about 20% of OPA1 carriers will develop a more severe form of the disease (DOA+), associated with a worse visual prognosis and additional debilitating neurological complications. The risk of developing DOA+ is three times higher with missense OPA1 mutations affecting the catatytic GTPase domain. I will explore the pathogenenetic mechanisms underlying this key observation by comparing tissue samples; skeletal muscle biopsies, fibroblasts, and myoblasts, collected from DOA+ patients with missense GTPase mutations (n=10), and pure DOA patients carrying frameshift deletions (n=5) and splice site mutations (n=5).

(i) Do missense OPA1 GTPase mutations result in increased mitochondrial network fragmentation?
OPA1 is a mitochondrial inner membrane protein with important pro-fusional properties. In conjunction with the mitochondrial outer membrane proteins MFN1 and MFN2, OPA1 counterbalances the influence of hFIS1 and DRP1, two pro-fission proteins found in the mitochondrial outer membrane and cytosol, respectively. Our preliminary data suggest that missense GTPase mutations lead to greater mitochondrial fragmentation compared with other OPA1 mutational subgroups. I will confirm these initial findings, investigating at the same time the pathological consequences of the observed fragmentation defect on cellular function and viability.

(ii) Do missense OPA1 GTPase mutations lead to greater mitochondrial DNA (mtDNA) instability?
Our pilot data, based on next-generation ultra-deep-sequencing, suggest that OPA1 GTPase mutations result in greater mtDNA instability. I will therefore compare the pattern and mutational burden of somatic mtDNA abnormalities in a larger group of DOA+ and pure DOA patients, relating them to two classical disorders of mtDNA maintenance (POLG1 and PEO1), and to normal ageing. I will use the generated biological data to model possible OPA1 disease mechanisms leading to accelerated mtDNA mutagenesis and the development of DOA+ features.

(iii) Are these deleterious consequences secondary to a dominant-negative effect?
HEK293 cells will be transfected with mutant cDNA extracted from OPA1 fibroblasts harbouring missense GTPase mutations. If these mutations exert a dominant-negative effect, mitochondrial fragmentation will be observed despite normal levels of the endogenous, wild-type OPA1 protein. Finally, I will explore the possibility of rescuing the fragmentation phenotype by manipulating the expression of the major pro-fusion and pro-fission mitochondrial proteins. These experiments could pave the way for gene therapy aimed initially at halting RGC loss and progressive visual decline.

Publications

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Angebault C (2015) Recessive Mutations in RTN4IP1 Cause Isolated and Syndromic Optic Neuropathies. in American journal of human genetics

 
Description MRC Centenary Early Career Award
Amount £15,000 (GBP)
Organisation Medical Research Council (MRC) 
Sector Academic/University
Country United Kingdom
Start 10/2012 
End 09/2013
 
Description Moorfields Special Trustees Research Award
Amount £130,000 (GBP)
Organisation Moorfields Eye Charity 
Sector Charity/Non Profit
Country United Kingdom
Start 10/2016 
End 09/2019
 
Description NIHR Rare Diseases Translational Research Collaboration
Amount £310,000 (GBP)
Organisation National Institute for Health Research 
Department NIHR Biomedical Research Centre
Sector Academic/University
Country United Kingdom
Start 09/2016 
End 08/2018
 
Description NIHR Rare Diseases Translational Research Collaboration
Amount £250,000 (GBP)
Organisation National Institute for Health Research 
Sector Public
Country United Kingdom
Start 08/2014 
End 08/2016
 
Description Small Grants Award
Amount £15,000 (GBP)
Funding ID 24TP171 
Organisation Fight for Sight 
Sector Charity/Non Profit
Country United Kingdom
Start 01/2018 
End 12/2018
 
Description Angers OPA1 Collaboration 
Organisation Angers University Hospital (CHU Angers)
Department Department of Biochemistry and Genetics
Country France 
Sector Academic/University 
PI Contribution Exchange of patient data and tissue samples
Collaborator Contribution Exchange of patient data and tissue samples
Impact PMID:20974897 PMID:20157015
Start Year 2007
 
Description Bologna OPA1 Collaboration 
Organisation University of Bologna
Department Department of Neurologial Sciences
Country Italy 
Sector Academic/University 
PI Contribution We are working closely with Dr Carelli's team in Bologa to understand the development of multisystem disease in patients harbouring OPA1 mutations.
Collaborator Contribution Dr Carelli's team have provided us with clinical data for a meta-analysis of the phenotypes seen in patients with OPA1 mutations.
Impact PMID: 20974897 PMID: 20157015
Start Year 2007
 
Description Cardiff OPA1 Mouse Collaboration 
Organisation Cardiff University
Department School of Optometry and Vision Sciences
Country United Kingdom 
Sector Academic/University 
PI Contribution I have analysed these OPA1 mouse tissues as part of my ongoing PhD research project.
Collaborator Contribution Our collaborators in Cardiff have provided us with tissues from an OPA1 mouse model of dominant optic atrophy.
Impact PMID: 19443720 PMID: 20157015
Start Year 2008
 
Description Jerusalem Hadassah Collaboration 
Organisation Hebrew University of Jerusalem
Department Hebrew University Hadassah Medical School
Country Israel 
Sector Academic/University 
PI Contribution Analysis of tissue samples from patients with inherited neurodegenerative diseases.
Collaborator Contribution Provision of tissue samples from patients with inherited neurodegenerative diseases.
Impact PMID: 26561570
Start Year 2015
 
Description Montpellier OPA1 Collaboration 
Organisation National Institute of Health and Medical Research (INSERM)
Department Institute of Neuroscience Montpellier
Country France 
Sector Academic/University 
PI Contribution We are working with Dr Guy Lenaers team in Montpellier to investigate the role of OPA1 in mitochondrial DNA replication.
Collaborator Contribution Dr Leaners' team have provided tissue samples and experimental data for our research in Newcastle.
Impact PMID: 20974897 PMID: 20157015
Start Year 2008
 
Description Moorfields BRC Collaboration 
Organisation Moorfields Eye Hospital NHS Foundation Trust
Department NIHR Moorfields Biomedical Research Centre
Country United Kingdom 
Sector Academic/University 
PI Contribution Patients with inherited optic neuropathies and a confirmed molecular diagnosis.
Collaborator Contribution Access to advanced psychophysical testing for the deep phenotyping of patients with inherited optic neuropathies.
Impact Joint application to the NIHR Rare Diseases Translational Research Collaboration.
Start Year 2013
 
Description Tuebingen OPA1 Mouse Collaboration 
Organisation Eberhard Karls University of Tubingen
Department Institute for Ophthalmic Research
Country Germany 
Sector Academic/University 
PI Contribution I have analysed these OPA1 mouse tissues as part of my ongoing PhD research project.
Collaborator Contribution Our collaborators in Tuebingen have provided us with tissues from a second OPA1 mouse model of dominant optic atrophy - different from the Cardiff one.
Impact PMID: 19815013 PMID: 19181907
Start Year 2008
 
Description ENMC workshop on neuromuscular disorders of mitochondrial fusion and fission 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Organisers:
Dr. Patrick Yu-Wai-Man (Newcastle, UK); Dr. Valerio Carelli (Bologna, Italia); Prof. Patrick F. Chinnery (Newcastle, UK)

Description:
The 197th ENMC workshop entitled "Neuromuscular disorders of mitochondrial fusion and fission - molecular mechanisms and therapeutic strategies" took place from the 26th to the 28thof April 2013 in Naarden, The Netherlands. A multidisciplinary group of 19 participants took part in this workshop, including 18 clinical and basic science researchers from 6 different countries (France, Germany, Italy, Spain, the UK, and the USA), and 1 patient representative from CMT UK.

The first half of this workshop reviewed the fundamental and interrelated roles mediated by the MFN2 and OPA1 proteins in normal cellular function. The pathogenetic mechanisms directly implicated in the development of CMT2A and DOA were discussed in the context of both in vitro and in vivo disease models. In the second half of this workshop, the participants collectively described the range of clinical features linked to MFN2 and OPA1 mutations, including novel disease manifestations and the natural history of this heterogeneous group of disorders. The final session focused on how to translate recent scientific advances for the benefit of patients and the best way to design future clinical trials in this challenging area of research.

The following key deliverables were achieved:

1.A comprehensive description of the expanding neuromuscular phenotypes associated with pathogenic MFN2 and OPA1 mutations.

2.Collaborative biobank access to patient tissue samples and animal models to further explore fundamental disease mechanisms in CMT-2A and DOA.

3.Pooled clinical registry of well-characterised patient cohorts for the purpose of future clinical studies, including treatment trials.
Year(s) Of Engagement Activity 2013
URL http://www.enmc.org/workshops/workshop-reports/neuromuscular-disorders-mitochondrial-fusion-and-fiss...
 
Description UK LHON Patient Network 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Participants in your research and patient groups
Results and Impact LHON (Leber hereditary optic neuropathy) is an important cause of irreversible blindness among young adults and treatment options are currently limited. Over the past two years, I have been working closely with families that have been affected by LHON in the UK through regular small group meetings and e-mail discussions.

The engagement activities described earlier have lead to the setting up a UK LHON charity to support patients and their families (http://www.lhonsociety.org/).
Year(s) Of Engagement Activity 2014
URL http://www.lhonsociety.org/