Time-resolved mitochondrial-focused screening identifies regulatory components of oxidative metabolism

Defects in mitochondrial oxidative metabolism contribute to various genetic inherited disorders, termed mitochondrial diseases, with limited treatment options. Given the lack of functional annotation for numerous mitochondrial proteins, there is a necessity for an extensive gene inventory related to mitochondrial function, with special interest in Oxidative Phosphorylation (OXPHOS). To address this gap, we developed a CRISPR/Cas9 loss-of-function library targeting nuclear-encoded mitochondrial genes and conducted galactose-based screenings at various time points to uncover novel regulators of mitochondrial function. Our study resulted in a gene catalog essential for mitochondrial oxidative metabolism, and constructed a dynamic timeline mapping a broad network of mitochondrial pathways, with a particular focus on the OXPHOS complexes. Computational analysis pinpointed RTN4IP1 and ECHS1 as key genes strongly associated with OXPHOS and whose mutations are associated with mitochondrial diseases in humans. RTN4IP1 was found to be crucial for mitochondrial respiration, with complexome profiling revealing its role as an assembly factor required for the complete assembly of complex I. Furthermore, we discovered that ECHS1 controls oxidative metabolism independently of its canonical function in fatty acid oxidation. Deletion of ECHS1 leads to reduced catabolism of branched-chain amino acids (BCAAs), which impairs the activity of lipoic acid-dependent enzymes such as pyruvate dehydrogenase (PDH). This deleterious phenotype can be rescued by restricting valine intake or catabolism in ECHS1-deficient cells.