title: Hypoxia induces mitochondrial mutagenesis and dysfunction in inflammatory arthritis.
abstract: OBJECTIVE
  To assess the levels and spectrum of mitochondrial DNA (mtDNA) point mutations in synovial tissue from patients with inflammatory arthritis in relation to in vivo hypoxia and oxidative stress levels.
  
  
  METHODS
  Random Mutation Capture assay was used to quantitatively evaluate alterations of the synovial mitochondrial genome. In vivo tissue oxygen levels (tPO(2)) were measured at arthroscopy using a Licox probe. Synovial expression of lipid peroxidation (4-hydroxynonenal [4-HNE]) and mitochondrial cytochrome c oxidase subunit II (CytcO II) deficiency were assessed by immunohistochemistry. In vitro levels of mtDNA point mutations, reactive oxygen species (ROS), mitochondrial membrane potential, and markers of oxidative DNA damage (8-oxo-7,8-dihydro-2'-deoxyguanine [8-oxodG]) and lipid peroxidation (4-HNE) were determined in human synoviocytes under normoxia and hypoxia (1%) in the presence or absence of superoxide dismutase (SOD) or N-acetylcysteine (NAC) or a hydroxylase inhibitor (dimethyloxalylglycine [DMOG]). Patients were categorized according to their in vivo tPO(2) level (<20 mm Hg or >20 mm Hg), and mtDNA point mutations, immunochemistry features, and stress markers were compared between groups.
  
  
  RESULTS
  The median tPO(2) level in synovial tissue indicated significant hypoxia (25.47 mm Hg). Higher frequency of mtDNA mutations was associated with reduced in vivo oxygen tension (P = 0.05) and with higher synovial 4-HNE cytoplasmic expression (P = 0.04). Synovial expression of CytcO II correlated with in vivo tPO(2) levels (P = 0.03), and levels were lower in patients with tPO(2) <20 mm Hg (P < 0.05). In vitro levels of mtDNA mutations, ROS, mitochondrial membrane potential, 8-oxo-dG, and 4-HNE were higher in synoviocytes exposed to 1% hypoxia (P < 0.05); all of these increased levels were rescued by SOD and DMOG and, with the exception of ROS, by NAC.
  
  
  CONCLUSIONS
  These findings demonstrate that hypoxia-induced mitochondrial dysfunction drives mitochondrial genome mutagenesis, and antioxidants significantly rescue these events.
authors.affiliation: Dublin Academic Medical Centre, St. Vincent's University Hospital, and The Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Dublin, Ireland.
authors.forename: Monika
authors.initials: M
authors.lastname: Biniecka
authors.forename: Edward
authors.initials: E
authors.lastname: Fox
authors.forename: Wei
authors.initials: W
authors.lastname: Gao
authors.forename: Chin Teck
authors.initials: CT
authors.lastname: Ng
authors.forename: Douglas J
authors.initials: DJ
authors.lastname: Veale
authors.forename: Ursula
authors.initials: U
authors.lastname: Fearon
authors.forename: Jacintha
authors.initials: J
authors.lastname: O'Sullivan
mesh_terms: D015535:Arthritis, Psoriatic / Q000235:genetics / Q000378:metabolism*; D001172:Arthritis, Rheumatoid / Q000235:genetics / Q000378:metabolism*; D002478:Cells, Cultured; D006801:Humans; D000860:Hypoxia / Q000235:genetics* / Q000378:metabolism; D007249:Inflammation / Q000235:genetics / Q000378:metabolism; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism*; D016296:Mutagenesis; D018384:Oxidative Stress / Q000235:genetics; D017382:Reactive Oxygen Species / Q000378:metabolism; D013582:Synovial Fluid / Q000378:metabolism; D013583:Synovial Membrane / Q000378:metabolism
chemical_list: D017382:Reactive Oxygen Species