This proposal aims to clarify mitochondrial contribution to obesity and thinness, using carefully characterized mitochondrial disease and obese patient materials, and genetically modified disease models. Manifestations of mitochondrial respiratory chain (RC) defects range from infantile multisystem disorders to adult-onset myopathies or neurodegeneration, and even aging-related wasting. Why defects in oxidative ATP production can lead to such variety of manifestations and tissue specificity is unknown. We have previously identified a number of gene defects that lead to RC disorders. In addition to neurological symptoms, these patients often show various metabolic manifestations: specific gene defects associate with short stature and thinness, whereas others with metabolic syndrome or obesity. This implies that specific mitochondrial defects can have opposing effects for fat storage or utilization. The involved pathways may contribute to mitochondrial disease progression, but are unknown.We propose to a) undertake a major clinical study on genetically defined, obese or thin, mitochondrial patients, and examine their metabolic phenotype in finest detail. These data will be compared to those from normal obesity, to search for common mechanisms between mitochondrial and general obesity. b) generate a set of disease models for mitochondrial disorders associated with obesity, and knock-out models for specific signallers for crossing with the disease models. c) identify in detail the involved regulatory pathways, and utilize these for searching chemical compounds that could modulate the response, and have therapeutic potential. The project has potential for major breakthroughs in the fields of mitochondrial disease pathogenesis and treatment, neurodegeneration and obesity.