Coenzyme Q (CoQ) is a conserved polyprenylated lipid composed of a redox-active benzoquinone ring and a long polyisoprenyl tail that serves as a membrane anchor. CoQ biosynthesis involves multiple steps, including multiple modifications of the precursor ring 4-hydroxybenzoic acid. Mutations in the enzymes involved in CoQ biosynthesis pathway result in primary coenzyme Q deficiencies, mitochondrial disorders whose clinical heterogenicity reflects the multiple biological function of CoQ. Patients with these disorders do not always respond to CoQ supplementation, and CoQ analogs have not been successful as alternative approaches. Progress made in understanding the CoQ biosynthesis pathway and studies of supplementation with 4-hydroxybenzoic acid ring analogs have opened a new area in the field of primary CoQ deficiencies treatment. Here, we will review these studies, focusing on efficacy of the different 4-hydroxybenzoic acid ring analogs, models in which they have been tested, and their mechanisms of action. Understanding how these compounds ameliorate biochemical, molecular, and/or clinical phenotypes of CoQ deficiencies is important to develop the most rational treatment for CoQ deficient patients, depending on their molecular defects.

Objective: Mitochondrial DNA (mtDNA) depletion/deletions syndrome (MDDS) comprises a group of diseases caused by primary autosomal defects of mtDNA maintenance. Our objective was to study the etiology of MDDS in 4 patients who lack pathogenic variants in known genetic causes.Methods: Whole exome sequencing of the probands was performed to identify pathogenic variants. We validated the mitochondrial defect by analyzing mtDNA, mitochondrial dNTP pools, respiratory chain activities, and GUK1 activity. To confirm pathogenicity of GUK1 deficiency, we expressed 2 GUK1 isoforms in patient cells.Results: We identified biallelic GUK1 pathogenic variants in all 4 probands who presented with ptosis, ophthalmoparesis, and myopathic proximal limb weakness, as well as variable hepatopathy and altered T‐lymphocyte profiles. Muscle biopsies from all probands showed mtDNA depletion, deletions, or both, as well as reduced activities of mitochondrial respiratory chain enzymes. GUK1 encodes guanylate kinase, originally identified as a cytosolic enzyme. Long and short isoforms of GUK1 exist. We observed that the long isoform is intramitochondrial and the short is cytosolic. In probands’ fibroblasts, we noted decreased GUK1 activity causing unbalanced mitochondrial dNTP pools and mtDNA depletion in both replicating and quiescent fibroblasts indicating that GUK1 deficiency impairs de novo and salvage nucleotide pathways. Proband fibroblasts treated with deoxyguanosine and/or forodesine, a purine phosphatase inhibitor, ameliorated mtDNA depletion, indicating potential pharmacological therapies.Interpretation: Primary GUK1 deficiency is a new and potentially treatable cause of MDDS. The cytosolic isoform of GUK1 may contribute to the T‐lymphocyte abnormality, which has not been observed in other MDDS disorders. ANN NEUROL 2024;96:1209–1224

Background and Objectives Coenzyme Q₁₀ (CoQ₁₀)–deficient cerebellar ataxia can be due to pathogenic variants in genes encoding for CoQ₁₀ biosynthetic proteins or associated with defects in protein unrelated to its biosynthesis. Diagnosis is crucial because patients may respond favorably to CoQ₁₀ supplementation. The aim of this study was to identify through whole-exome sequencing (WES) the pathogenic variants, and assess CoQ₁₀ levels, in fibroblasts from patients with undiagnosed cerebellar ataxia referred to investigate CoQ₁₀ deficiency. Methods WES was performed on genomic DNA extracted from 16 patients. Sequencing data were filtered using a virtual panel of genes associated with CoQ₁₀ deficiency and/or cerebellar ataxia. CoQ₁₀ levels were measured by high-performance liquid chromatography in 14 patient-derived fibroblasts. Results A definite genetic etiology was identified in 8 samples of 16 (diagnostic yield = 50%). The identified genetic causes were pathogenic variants of the genes COQ8A (ADCK3) (n = 3 samples), ATP1A3 (n = 2), PLA2G6 (n = 1), SPG7 (n = 1), and MFSD8 (n = 1). Five novel mutations were found (COQ8A n = 3, PLA2G6 n = 1, and MFSD8 n = 1). CoQ₁₀ levels were significantly decreased in 3/14 fibroblast samples (21.4%), 1 carrying compound heterozygous COQ8A pathogenic variants, 1 harboring a homozygous pathogenic SPG7 variant, and 1 with an unknown molecular defect. Discussion This work confirms the importance of COQ8A gene mutations as a frequent genetic cause of cerebellar ataxia and CoQ₁₀ deficiency and suggests SPG7 mutations as a novel cause of secondary CoQ₁₀ deficiency.

Primary coenzyme Q10 (CoQ10) deficiency is a rare mitochondrial disorder caused by mutations in genes involved in CoQ biosynthesis (e.g., COQ4) that result in impaired mitochondrial respiration, oxidative stress, and dysfunction across multiple organ systems because of decreased mitochondrial levels of CoQ10. Although oral CoQ10 supplementation has been examined for standard of care, poor absorption and inadequate tissue and intracellular distribution have resulted in a lack of clinically significant efficacy. BPM31510 is a lipid nanoparticle containing oxidized CoQ10 designed to improve bioavailability and targeted uptake into the mitochondria. In the current study, we assessed the efficacy of BPM31510 to increase CoQ levels in Coq4^F147C mice, a novel genetic knock-in model of primary CoQ deficiency. Coenzyme Q9, the main form of CoQ in mice, and CoQ10 were significantly decreased in the brain, kidney, heart, and muscle of Coq4^F147C mice compared with Coq4^+/+ mice. BPM31510 treatment significantly increased oxidized CoQ10 levels across all tissues, mediated by the nanoliposome biodistribution of oxidized CoQ10 in BPM31510. MALDI-MS imaging demonstrated regional and spatial restoration of CoQ10 within the brain, including the cerebellum, myocardium, and renal cortex of Coq4^F147C mice. These results demonstrate that BPM31510 successfully concentrates pharmacologically active CoQ10 in target tissues that are not reachable with oral therapy in a genetic model of primary CoQ deficiency. We enabled the visualization of suborgan CoQ10 localization to specifically demonstrate CoQ10 restoration. This study establishes proof of concept for spatial quinomics, a new methodology that combines spatial metabolomics with quinomics to evaluate next-generation CoQ10-based therapeutics for mitochondrial disorders.

According to the developmental origins of health and disease hypothesis, perinatal exposure to an environmental toxicant during the development of the nervous system could cause a permanent cellular modification that may promote the appearance of neurodegenerative diseases at an older age. Tributyltin chloride is an environmental pollutant that, among other effects, provokes a dysfunction of the oxidative phosphorylation system and has adverse effects on the nervous system. We studied neuronal differentiation of human neuroblastoma cells and neural stem cells in the presence of tributyltin chloride concentrations found in human blood (≤ 100 nM), and brain development in two‐year‐old mice after perinatal exposure to tributyltin chloride (≤ 1000 nM). Pregnant mice (8–9 weeks old) were exposed to TBTC (0, 100, 500, or 1000 nM) via drinking water throughout gestation and lactation and ended upon weaning of the pups. Genetic‐molecular, biochemical and cellular studies were performed on human SH‐SY5Y neuroblastoma cells and on neural stem cells differentiating into neurons. In addition to these studies, histological studies of the brain and functional tests were performed in two‐year‐old mice. A decrease in the oxidative phosphorylation activity, essential for the proper function of the nervous system, affected neuronal differentiation of human neural stem cells and neuroblastoma cells in vitro. Exposure to this compound during pregnancy and lactation resulted in a modification of global deoxyribonucleic acid methylation levels in 2‐year‐old mice. Additionally, various histological changes were detected in the brains of these mice. Therefore, the alteration of brain development with long‐term consequences may be one of the manifestations of early exposure to tributyltin.