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

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.

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.