The genotype/phenotype conundrum of inherited mitochondrial disorders: Insights from a survey of mtDNA mutations associated with Leigh syndrome in complex I

Mitochondrial disorders encompass a broad spectrum of genetic disorders impairing mitochondrial function. Considerable advancements have been made in the diagnosis and clinical management of these primary mitochondrial diseases. However, diagnosis and treatment have remained largely empirical, because the pathogenic mechanisms are still poorly understood by which any of the numerous known mutations lead to a specific phenotype in patients. To make inroads into this central challenge of mitochondrial medicine, we performed a focused survey of a cohort of published cases of Leigh syndrome caused by point mutations in subunits of respiratory chain complex I encoded by the mitochondrial genome. Leigh syndrome is one of the most severe mitochondrial disorders and is characterized by clinical and genetic manifestations predominantly affecting the central nervous system and the brain. We found that even basic correlations between a specific molecular defect and disease severity and progression are often obscured by the heterogeneity of the available diagnostic data. Still, our analysis showed that in order to understand the specific pathogenic impact it entails, for each mutation one has to carefully differentiate which functional domain of complex I is actually affected. It seems evident that much more comprehensive and differentiated studies of representative mutations as well as far more complete and standardized diagnostic data from patients should be obtained. This will be prerequisite for understanding and discriminating pathogenic mechanisms as a way to develop effective rational therapies for Leigh syndrome and other mitochondrial disorders.