Dysregulated mitophagy and mitochondrial organization in optic atrophy due to OPA1 mutations

Objective To investigate mitophagy in five patients with severe dominantly inherited optic atrophy (DOA), caused by depletion of OPA1 (a protein that is essential for mitochondrial fusion), compared with healthy controls. Methods Patients with severe DOA (DOA plus) had peripheral neuropathy, cognitive regression and epilepsy in addition to loss of vision. We quantified mitophagy in dermal fibroblasts, using two high throughput imaging systems, by visualising co-localisation of mitochondrial fragments with engulfing autophagosomes. Results Fibroblasts from three bi-allelic OPA1(-/-) patients with severe DOA had increased mitochondrial fragmentation and mitochondrial DNA (mtDNA) depleted cells due to decreased levels of OPA1 protein. Similarly, in siRNA-treated control fibroblasts, profound OPA1 knock-down caused mitochondrial fragmentation, loss of mtDNA, impaired mitochondrial function and mitochondrial mis-localisation. Compared to controls, basal mitophagy (abundance of autophagosomes co-localising with mitochondria) was increased in (i) bi-allelic patients, (ii) mono-allelic patients with DOA plus (iii) OPA1 siRNA treated control cultures Mitophagic flux was also increased. Genetic knock-down of the mitophagy protein ATG7 confirmed this by eliminating differences between patient and control fibroblasts. Conclusions We demonstrated increased mitophagy and excessive mitochondrial fragmentation in primary human cultures associated with DOA plus due to bi-allelic OPA1 mutations. We previously found that increased mitophagy (mitochondrial recycling) was associated with visual loss in another mitochondrial optic neuropathy, Leber’s Hereditary Optic Neuropathy (LHON). Combined with our LHON findings, this implicates excessive mitochondrial fragmentation, dysregulated mitophagy and impaired response to energetic stress in the Liao 8 pathogenesis of mitochondrial optic neuropathies, potentially linked with mitochondrial mislocalisation and mtDNA depletion.