The m.11778 A > G variant associated with the coexistence of Leber's hereditary optic neuropathy and multiple sclerosis-like illness dysregulates the metabolic interplay between mitochondrial oxidative phosphorylation and glycolysis.

Little is known about the molecular mechanism of the rare coexistence of Leber's Hereditary Optic Neuropathy (LHON) and multiple sclerosis (MS), also known as the Harding's syndrome. In this study, we provide novel evidence that the m.11778A > G variant causes a defective metabolic interplay between mitochondrial oxidative phosphorylation and glycolysis. We used dermal fibroblasts derived from a female proband exhibiting clinical symptoms compatible with LHON-MS due to the presence of the pathogenic m.11778A > G variant at near homoplasmic levels. Our mitochondrial morphometric analysis reveals abnormal cristae architecture. Live-cell respiratory studies show stunted metabolic potential and spare respiratory capacity, vital for cell survival upon a sudden energy demand. The m.11778 A > G variant also alters glycolytic activities with a diminished compensatory glycolysis, thereby preventing an efficient metabolic reprogramming during a mitochondrial ATP crisis. Our collective results provide evidence of limited bioenergetic flexibility in the presence of the m.11778 A > G variant. Our study sheds light on the potential pathophysiologic mechanism of the m.11778 A > G variant leading to energy crisis in this patient with the LHON-MS disease.