Restoration of mitochondrial complex III function in hepatocytes highlights the liver as a key thermogenic organ independent of brown adipocyte activation

The liver is the key hub of systemic energy metabolism and growth, yet it is surprisingly rarely a major affected organ in mitochondrial diseases that compromise oxidative phosphorylation. Bcs1l ^p.S78G knock-in mice, carrying a respiratory complex III (CIII)-deficiency patient mutation, present juvenile-onset liver and kidney disease, growth restriction, lipodystrophy and premature death. To explore the as yet poorly understood systemic metabolic roles of the liver in mitochondrial diseases, we utilized this model and performed recombinant adeno-associated viruses (rAAV)-based gene therapy using a hepatocyte-specific promoter to drive wild-type BCS1L expression. A single rAAV -Bcs1l intraperitoneal injection into presymptomatic 3-week-old mice transduced the liver, restored hepatocyte CIII assembly and activity, prevented liver disease and was sufficient to improve growth, prevent lethal hypoglycemia, and extend survival by 100%. Unexpectedly, the hepatocyte-specific gene replacement also prevented severe hypothermia. Mechanistically, the mutant mice lacked signs of thermogenic BAT activation and thermoneutral housing did not correct the hypothermia. The mice exhibited impaired thermosensation, implying failure to respond to hypothermic cues. Immunostaining revealed grossly abnormal foot pad innervation, suggesting sensory neuropathy and impaired thermosensing. The rAAV- Bcs1l -treated mice maintained near-normal body temperature without induction of BAT thermogenesis, demonstrating that mitochondrial respiration in hepatocytes is both essential and sufficient for euthermia in juvenile mice. These findings emphasize the crucial role of the liver in thermoregulation, suggest sensory neuropathy as an unexpected cause of hypothermia, and demonstrate the therapeutic potential of tissue-specific gene therapy in a preclinical model of a multiorgan mitochondrial disease.