Mitochondrial disease: mechanisms, signalling, and therapeutic opportunities.

Mitochondria are central hubs of cellular metabolism and signalling, and their dysfunction underlies a broad spectrum of human diseases, including rare mitochondrial disorders as well as common neurodegenerative and metabolic conditions. Mitochondrial diseases are genetically heterogeneous disorders caused by mutations in nuclear or mitochondrial DNA that impair oxidative phosphorylation (OXPHOS), resulting in reduced ATP production and cellular energy failure. Despite a shared bioenergetic defect, these diseases display marked clinical variability, and the mechanisms underlying this heterogeneity remain poorly understood. At present, no curative therapies are available, although several metabolic and experimental approaches have shown promise in preclinical models. Mitochondrial dysfunction is commonly associated with altered redox homeostasis and increased production of reactive oxygen species (ROS), which can damage mitochondrial components, including mitochondrial DNA, and further impair respiratory chain function. At the same time, ROS also act as context-dependent signalling molecules, with effects that vary according to concentration, localization, and cell type complicating their interpretation in disease mechanisms and therapy development. In this review, we summarize current concepts in mitochondrial disease pathophysiology focusing on unresolved questions that limit mechanistic understanding and clinical translation. We critically evaluate the role of ROS in disease progression and signalling, discuss how the alternative oxidase (AOX) has emerged as a valuable experimental tool to dissect ROS-related mechanisms and reveal unexpected aspects of mitochondrial dysfunction and disease variability.