Mitochondrial complex I deficiency-associated diseases and models.

Mitochondrial complex I is the first and largest enzyme of the mitochondrial respiratory chain and thus plays a crucial role in cellular energy metabolism. Defects in the mitochondrial respiratory chain, and in particular CI deficiency, are the primary cause of human mitochondrial associated diseases, which most often presents as severe neurometabolic disorders with fatal outcome. Up to this date the diagnosis and treatment of CI deficiency-associated diseases is challenging, only limited symptomatic therapies exist and no cures are available. This review aims at summarizing current knowledge on the genetic basis of CI deficiency-associated diseases and available experimental disease models. Most common human disorders caused by CI deficiency range from Leigh syndrome to MELAS and LHON, all characterized by genetic and symptomatic heterogeneity. So far, in vivo studies on non-mammalian organisms and mouse models, as well as in vitro studies on patient derived fibroblasts, cybrids and human-induced pluripotent stem cells have mainly facilitated the research of CI deficiency. These model systems provide insights on molecular mechanisms in mitochondrial disease and approaches for potential therapeutic intervention strategies. However, current research is limited by translational relevance of existing disease models, varying degrees of heteroplasmy and tissue specific effects characteristic of mitochondrial diseases, so that basic disease mechanisms still remain poorly understood. To overcome these challenges there is an urgent need for in vivo and in vitro human relevant models to aid the development of effective therapeutic interventions and potential cures of CI deficiency-associated diseases.