OXPHOS bioenergetic compensation does not explain disease penetrance in Leber hereditary optic neuropathy

Leber hereditary optic neuropathy (LHON) is one of the most common primary mitochondrial diseases. It is caused by point mutations in mitochondrial DNA (mtDNA) genes and in some cases, it can result in irreversible vision loss, primarily in young men. It is currently unknown why LHON mutations affect only some carriers and whether bioenergetic compensation enables asymptomatic carriers to overcome mitochondrial impairment and preserve cellular function. Here, we conducted bioenergetic metabolic assays and RNA sequencing to address this question using male-only, age-matched, m.11778G>A lymphoblasts and primary fibroblasts from both carriers and affected individuals. Our work indicates that OXPHOS bioenergetic compensation in LHON peripheral cells does not explain disease phenotype. We show that complex I impairment is similar in carrier and affected cells, despite a transcriptional downregulation of metabolic pathways including glycolysis in affected cells relative to carriers detected by RNA sequencing. Although we did not detect OXPHOS bioenergetic compensation in carrier cells, our results indicate that affected cells suffer a growth impairment under metabolic challenge compared to carrier cells, which were unaffected by metabolic challenge. If recapitulated in retinal ganglion cells, decreased susceptibility to metabolic challenge in carriers may help preserve metabolic homeostasis in the face of the mitochondrial complex I bioenergetic defect. The m.11778G>A mitochondrial polymorphism contributes to the development of LHON, but the majority of individuals with the mutation do not develop LHON. We obtained fibroblasts from individuals with LHON (n=3 affected), individuals with the polymorphism but no signs of LHON (n=3 carriers), and individuals without the polymorphism. Fibroblasts underwent RNA-seq.