Hepatic gene replacement restores energy metabolism and doubles the survival in mouse model of GRACILE syndrome, a neonatal mitochondrial disease

Preclinical gene therapy studies of mitochondrial diseases remain limited due to their multisystemic manifestations and the scarcity of physiologically relevant animal models. Mutations in BCS1L , a nuclear gene encoding an assembly factor for mitochondrial complex III (CIII), are the most common cause of pediatric CIII deficiency. The most severe phenotype, GRACILE syndrome, is caused by a homozygous Finnish founder mutation ( c.A232G , p.S78G ). The corresponding Bcs1l ^p.S78G knock-in mouse model effectively recapitulates the human disease, with juvenile-onset hepatopathy, tubulopathy, growth restriction, segmental progeria, and short survival. Here, we performed recombinant adeno-associated virus (rAAV)-mediated gene replacement in this model, which features a postnatal presymptomatic window until weaning. A single intraperitoneal injection of rAAV encoding wild-type Bcs1l restored CIII assembly and activity in the liver, preventing hepatopathy. Hepatocyte-specific correction was sufficient to ameliorate hypoglycemia, improve growth, normalize systemic metabolism, and extend survival by nearly two-fold, despite persistent CIII deficiency in other tissues. Remarkably, liver-directed rescue also prevented skeletal muscle transcriptomic changes, particularly those linked to altered energy substrate utilization. These results underscore the central role of the liver in systemic energy homeostasis and growth regulation in multiorgan mitochondrial diseases and demonstrate the therapeutic potential of hepatocyte-directed gene replacement in phenotypes with prominent hepatopathy.