Project Summary Coenzyme Q10 (CoQ10) is a lipophillic molecule composed of a benzoquinone ring and a hydrophobic decaprenyl tail. In the mitochondrial respiratory chain, CoQ10 transports electrons from complexes I and II to complex III. It is also an antioxidant, co-factor for pyrimidine biosynthesis, and modulator of apoptosis. Human CoQ10 deficiency is a genetically and clinically heterogeneous disease. Response to therapy is variable. Our studies of cultured fibroblasts from patients carrying mutations in PDSS2, COQ2, ADCK3, and COQ9, have revealed correlations between level of CoQ10 and oxidative stress; <15% and >60% residual CoQ10 are not associated with oxidative stress, whereas 30-50% residual CoQ10 is associated with oxidative stress, mitochondrial hyperpolarization followed by depolarization and cell death. In cell lines with primary CoQ10 deficiency, incubation of CoQ10 deficient fibroblasts for 1 week with 5mM CoQ10 (but not short-tail ubiquinone analogs) improved bioenergetics indicating pharmacokinetic constraints and dose of CoQ10 may limit efficacy in CoQ10 deficient patients. Based on our studies, we hypothesize that: 1) genotypic heterogenity contributes to the phenotypic variability of CoQ10-deficiency; 2) levels of oxidative stress, respiratory chain deficiency, and apoptosis differ depending on the severity CoQ10 deficiency, and 3) CoQ10 and its analogs have variable efficacy in CoQ10 deficiencies. To test these hypotheses, we propose the following two specific aims: Aim 1) To test genetic and pharmacologic therapies for CoQ10 deficiency in vitro by assessing ADCK3 overexpression and analogs of 4- hydroxybenzoic acid, as approaches to rescue endogenous CoQ10 biosynthesis defects; Aim 2) To test therapies in two mouse model of CoQ10 deficiency by comparing efficacies of oral and intratechal administration of CoQ10 and idebenone in preventing or delaying the onset of the disease in the Pdss2 kd/kd and Coq9X/X mutant mice.