Do secondary mitochondrial DNA defects cause retinal ganglion cell death in dominant optic atrophy?

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

Abstract

Dominant optic atrophy (DOA) is one of the commonest form of inherited blindness and it results in significant visual disability. DOA is caused by irreversible damage to the optic nerve, which connects the eye to the vision centres within the brain. It is a specialised cable made up of about 2 million, highly specialised cells known as retinal ganglion cells (RGCs). In 60-70% of cases, DOA is due to a mutation in the OPA1 gene but we still do not know how this genetic defect leads to disease and why only RGCs are affected. Our preliminary findings in one large family with DOA indicate that the OPA1 mutation compromises the normal function of mitochondria. Mitochondria are essential components of all human cells and they are responsible for energy production. If insufficient energy is produced, cells cannot function properly and die. To test my hypothesis further, I will analyse blood and muscle biopsies from additional families with DOA for evidence of mitochondrial dysfunction. I will also analyse RGCs from a mouse model of DOA and determine if they contain high levels of mitochondrial DNA abnormalities. Finally, I will try to identify the gene(s) responsible for DOA in those families who not carry an OPA1 mutation. RGCs are affected in other eye conditions like glaucoma and a better understanding of their selective vulnerability will help us develop more effective treatment strategies for these blinding diseases.

Technical Summary

Dominant optic atrophy (DOA) is the most common inherited optic neuropathy and it is characterised by the progressive, focal neurodegeneration of the retinal ganglion cell (RGC) layer. DOA causes significant visual handicap, has no cure, and ~60% of cases are due to mutations in the OPA1 gene (3q28-q29). However, it is not yet established how the mutant Opa1 protein leads to disease and why RGCs are selectively targeted.
Mitochondria contain multiple copies of their own DNA (mtDNA) and both mutations (deletions and point mutations) and a reduction in the number of mtDNA molecules (depletion) can trigger a bioenergetic defect. I have recently characterised a large OPA1 family with histochemical evidence of mitochondrial dysfunction and cytochrome c oxidase negative (COX-negative) fibres in limb muscle which contained high-levels of pathogenic mtDNA deletions. Five other families with DOA have since been found to harbour multiple mtDNA deletions, only three of which had OPA1 mutations.
Our preliminary data therefore suggest that (i) the Opa1 protein is involved in mtDNA maintenance (ii) the accumulation of secondary (2 ) mtDNA defects is central to the pathophysiology of DOA and (iii) some families with DOA are likely to harbour as yet unidentified nuclear genes involved in mtDNA maintenance.
(1) Do secondary mtDNA defects lead to RGC death in DOA?
I will sequence the OPA1 gene in ten additional families with a clinical diagnosis of DOA and look for COX deficiency in limb muscle biopsies. Using laser microdissection, I will then capture single COX-negative muscle fibres and assess whether detrimental levels of 2 mtDNA defects are present (point mutations, deletions and depletion). I will also compare the range of mtDNA defects observed in these families with those seen in other disorders of mtDNA maintenance and attempt genotype-phenotype correlations. Finally, using a mouse model of DOA, I will determine whether the level of apoptosis and proportion of COX-negative cells is higher in RGCs compared to other tissues.
(2) Is DOA caused by other nuclear genes involved in mtDNA maintenance?
Through an existing collaborative network, I have access to a group of families with DOA but no OPA1 mutations. I will perform linkage analysis on these families to search for novel DOA loci and using bioinformatic techniques, I will identify and sequence candidate genes implicated in mtDNA maintenance within these regions.
Other optic neuropathies, including glaucoma, preferentially affect RGCs and understanding why these cells are selectively vulnerable will have broad implications.

Publications

10 25 50
 
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 What disease mechanisms contribute to multisystem tissue involvement in dominant optic atrophy due to OPA1 mutations?
Amount £1,126,871 (GBP)
Funding ID G1002570 
Organisation Medical Research Council (MRC) 
Sector Academic/University
Country United Kingdom
Start 03/2012 
End 08/2016
 
Title Inherited Eye Diseases Database 
Description An extensive up-to-date database of patients with inherited eye diseases, with a primary focus on mitochondrial genetic disorders. 
Type Of Material Database/Collection of Data/Biological Samples 
Provided To Others? No  
Impact DOA and LHON are the two most commmon inherited optic nerve disorders in the general population. This database has allowed us to define the epidemiology of autosomal dominant optic atrophy (DOA) in the North of England and has contributed to the successful completion of a Phase II randomised controlled trial for Leber Hereditary Optic Neuropathy (LHON). 
 
Title OPA1 Tissue Biobank 
Description Setting up of fibroblasts and myoblast cell lines carrying pathogenic OPA1 mutations. 
Type Of Material Cell line 
Provided To Others? No  
Impact This valuable tissue biobank will be used for future projects looking at OPA1 disease mechanisms. 
 
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 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 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 ARVO - Florida 2008 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Oral presentation at ARVO, which is the largest annual scientific meeting devoted to basic research into eye disorders.

New collaborations
Year(s) Of Engagement Activity 2008
 
Description EUROMIT 7 - Stockholm 2008 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Posters presented at EUROMIT 7 which is the largest meeting devoted to research into mitochondrial disorders.

New collaborations
Year(s) Of Engagement Activity 2008
 
Description Fighting Blindness - Dublin 2009 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact Meeting on translational eye research. The participants came from various backgrounds: clinicians, scientists, and patient groups - both local and international.

New collaborations - especially with international patient associations involved in the fight against blindness.
Year(s) Of Engagement Activity 2009