Effects of OPA1 mutations mitochondrial morphology and apoptosis:: Relevance to ADOA pathogenesis

To characterize the molecular links between type-I autosomal dominant optic atrophy (ADOA) and OPAI dysfunctions, the effects of pathogenic alleles of this dynamin on mitochondrial morphology and apoptosis were analyzed, either in fibroblasts from affected individuals, or in HeLa cells transfected with similar mutants. The alleles were missense substitutions in the GTPase domain (OPAI(G300E) and OPAI(R290Q)) or deletion of the GTPase effector domain (OPAI(Delta 58)). Fragmentation of mitochondria and apoptosis increased in OPAI(R290Q) fibroblasts and in OPAI(G300E) transfected HeLa cells. OPAI(Delta 58) did not influence mitochondrial morphology, but increased the sensitivity to staurosporine of fibroblasts. In these cells, the amount of OPAI protein was half of that in control fibroblasts. We conclude that GTPase mutants exert a dominant negative effect by competing with wild-type alleles to integrate into fusion-competent complexes, whereas C-terminal truncated alleles act by haplo-insufficiency. We present a model where antagonistic fusion and fission forces maintain the mitochondrial network, within morphological limits that are compatible with cellular functions. In the retinal ganglion cells (RGCs) of patients suffering from type-I ADOA, OPAI-driven fusion cannot adequately oppose fission, thereby rendering them more sensitive to apoptotic stimuli and eventually leading to optic nerve degeneration.