Tissue specific differences in the assembly of mitochondrial complex I is revealed by a novel ENU mutation in ECSIT

Aims: Mitochondrial complex I assembly is a multi-step process which necessitates the involvement of a variety of assembly factors and chaperones to ensure the final active enzyme is correctly assembled. The role of the assembly factor ECSIT was studied across various murine tissues to determine its role in this process and how this varied between tissues of varying energetic demands. We hypothesised that many of the known functions of ECSIT were unhindered by the introduction of an ENU induced mutation, whilst it's role in complex I assembly was affected on a tissue specific basis. Methods and results: Here we describe a mutation in the mitochondrial complex I assembly factor ECSIT which reveals tissue specific requirements for ECSIT in complex I assembly. Mitochondrial complex I assembly is a multi-step process dependent on assembly factors that organise and arrange the individual subunits, allowing for their incorporation into the complete enzyme complex. We have identified an ENU induced mutation in ECSIT (N209I) that exhibits a profound effect on complex I component expression and assembly in heart tissue, resulting in hypertrophic cardiomyopathy in the absence of other phenotypes. The dysfunction of complex I appears to be cardiac specific, leading to a loss of mitochondrial output as measured by Seahorse extracellular flux and various biochemical assays in heart tissue, whilst mitochondria from other tissues were unaffected. Conclusions: These data suggest that the mechanisms underlying complex I assembly and activity may have tissue specific elements tailored to the specific demands of cells and tissues. Our data suggest that tissues with high energy demands, such as the heart, may utilise assembly factors in different ways to low energy tissues in order to improve mitochondrial output. This data have implications for the diagnosis and treatment of various disorders of mitochondrial function as well as cardiac hypertrophy with no identifiable underlying genetic cause. Translational perspective: Mitochondrial diseases often present as multi system disorders with far reaching implications to the health and well being of patients. Diagnoses are often undertaken by characterisation of mitochondrial function from skin or muscle biopsy, with the expectation that any affect on mitochondrial function will be recognisable in all cell types. However, this study demonstrates that mitochondrial function may differ between cell types with the involvement of tissue specific proteins or isoforms, as such, current diagnostic techniques may miss diagnoses of a more specific mitochondrial dysfunction.