Coenzyme Q10 (CoQ10) is a small lipophilic molecule composed of a benzoquinone ring and a hydrophobic isoprenoid tail which is present in virtually all cell membranes. In the mitochondrial respiratory chain, CoQ10 is vital for the transport of electrons from complex I and complex II to complex III. It is also an antioxidant, membrane stabilizer, and modulator of apoptosis. Human CoQ10-deficiency has been associated with four clinical phenotypes. Patients with all forms of CoQ10-deficiency have improved with oral supplementation, therefore recognition of this treatable genetic condition is important. In the last decade, the candidate and her mentors have collected biological samples from 84 patients (71 families) with documented CoQ10 deficiency in muscle and/or fibroblasts, or suspected CoQ10 deficiency based on the clinical manifestations as well as the response to CoQ10 supplementation. A total of 54 patients (48 families) have documented CoQ10 deficiency in muscle, fibroblasts, or both. In 2006, the investigative team reported the first mutations in CoQ10 biosynthetic genes, COQ2, which encodes 4-para-hydroxybenzoate: polyprenyl transferase; and PDSS2, which encodes subunit 2 of decaprenyl diphosphate synthase. In addition, in a family with four individuals with cerebellar ataxia and CoQ10 deficiency, they identified a pathogenic mutation in the APTX gene, which encodes a protein involved in single-strand break repair. Thus, these studies have revealed that CoQ10 deficiency can be primary or secondary. Not surprisingly, CoQ10 deficiency causes defects of respiratory chain activities (reduced activities of complexes I+III and II+III). The relative importance of respiratory chain defects, ROS production, and apoptosis in the pathogenesis of CoQ10-deficiency is unknown. The investigative team studied the consequences of severe CoQ10 deficiency on bioenergetics, oxidative stress, and antioxidant defenses in cultured skin fibroblasts harboring COQ2 and PDSS2 mutations. Defects in the first two committed steps of the CoQ10 biosynthetic pathway produce different biochemical alterations. PDSS2 mutant fibroblasts have 12% CoQ10 relative to control cells and markedly reduced ATP synthesis, but do not show increased reactive oxygen species (ROS) production, signs of oxidative stress, or increased antioxidant defense markers. In contrast, COQ2 mutant fibroblasts have 30% CoQ10 with partial defect in ATP synthesis, and significantly increased ROS production and oxidation of lipids and proteins. To better understand the pathogenesis of CoQ10 deficiency, the investigative team has characterized the effects of varying severity of CoQ10 deficiency on ROS production and mitochondrial bioenergetics in cells harboring different genetic defects of CoQ10 biosynthesis. They confirmed their previous findings and further observed that the correlation between level of CoQ10 and ROS production follows a parabolic curve; 10-15% residual CoQ10 and 60-70% are not associated with significant ROS production, whereas 30-50% residual CoQ10 is associated with the maximum increases in ROS production. Moreover, increase in reactive oxygen species appears to be associated with initial hyperpolarization followed by depolarization and cell death. These data are corroborated by preliminary results of treatment with CoQ10 and other antioxidants in fibroblasts from the CoQ10 deficient patients. To better understand the pathogenesis of human CoQ10 deficiency the candidate proposes the following three specific aims: Aim 1: To identify novel genetic causes of CoQ10 deficiency. Aim 2: To understand the mitochondrial bioenergetics and oxidative stress consequences of different degrees of CoQ10 deficiency in the same genetic background, she will modulate COQ2 and PDSS2 expression using RNA interference (RNAi). Aim 3: To test ROS scavenging as a potential therapeutic strategy, she will overexpress the enzyme superoxide manganese dismutase (MnSOD) in COQ2 mutant fibroblasts and will assess level of ROS, oxidative stress, and apoptosis. NARRATIVE: Defects of mitochondria cause diverse human diseases. A subtype of mitochondrial disease is caused by deficiency of coenzyme Q10 (CoQ10), an essential component of the mitochondria involved in energy production. Patients with CoQ10 deficiency often improve dramatically with CoQ10 supplementation. The candidate will study patients with this disease and she will attempt to understand why the mutations cause CoQ10 deficiency. Knowing the cause of CoQ10 deficiency will likely enhance our scientific knowledge of CoQ10 biosynthesis, and will provide molecular tests for accurate genetic counseling, prenatal diagnosis, and more rapid initiation of the therapy.