title: Impact of the mitochondrial genetic background in complex III deficiency.
abstract: BACKGROUND
  In recent years clinical evidence has emphasized the importance of the mtDNA genetic background that hosts a primary pathogenic mutation in the clinical expression of mitochondrial disorders, but little experimental confirmation has been provided. We have analyzed the pathogenic role of a novel homoplasmic mutation (m.15533 A>G) in the cytochrome b (MT-CYB) gene in a patient presenting with lactic acidosis, seizures, mild mental delay, and behaviour abnormalities.
  
  
  METHODS
  Spectrophotometric analyses of the respiratory chain enzyme activities were performed in different tissues, the whole muscle mitochondrial DNA of the patient was sequenced, and the novel mutation was confirmed by PCR-RFLP. Transmitochondrial cybrids were constructed to confirm the pathogenicity of the mutation, and assembly/stability studies were carried out in fibroblasts and cybrids by means of mitochondrial translation inhibition in combination with blue native gel electrophoresis.
  
  
  RESULTS
  Biochemical analyses revealed a decrease in respiratory chain complex III activity in patient's skeletal muscle, and a combined enzyme defect of complexes III and IV in fibroblasts. Mutant transmitochondrial cybrids restored normal enzyme activities and steady-state protein levels, the mutation was mildly conserved along evolution, and the proband's mother and maternal aunt, both clinically unaffected, also harboured the homoplasmic mutation. These data suggested a nuclear genetic origin of the disease. However, by forcing the de novo functioning of the OXPHOS system, a severe delay in the biogenesis of the respiratory chain complexes was observed in the mutants, which demonstrated a direct functional effect of the mitochondrial genetic background.
  
  
  CONCLUSIONS
  Our results point to possible pitfalls in the detection of pathogenic mitochondrial mutations, and highlight the role of the genetic mtDNA background in the development of mitochondrial disorders.
authors.affiliation: Centro de Investigación, Hospital Universitario 12 de Octubre, Madrid, Spain.
authors.forename: Mari Carmen
authors.initials: MC
authors.lastname: Gil Borlado
authors.forename: David
authors.initials: D
authors.lastname: Moreno Lastres
authors.forename: Maritza
authors.initials: M
authors.lastname: Gonzalez Hoyuela
authors.forename: Maria
authors.initials: M
authors.lastname: Moran
authors.forename: Alberto
authors.initials: A
authors.lastname: Blazquez
authors.forename: Rosa
authors.initials: R
authors.lastname: Pello
authors.forename: Lorena
authors.initials: L
authors.lastname: Marin Buera
authors.forename: Toni
authors.initials: T
authors.lastname: Gabaldon
authors.forename: Juan Jose
authors.initials: JJ
authors.lastname: Garcia Peñas
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Martín
authors.forename: Joaquin
authors.initials: J
authors.lastname: Arenas
authors.forename: Cristina
authors.initials: C
authors.lastname: Ugalde
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D045303:Cytochromes b / Q000235:genetics / Q000378:metabolism; D014450:Electron Transport Complex III / Q000172:deficiency* / Q000235:genetics; D003576:Electron Transport Complex IV / Q000235:genetics / Q000378:metabolism; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D008928:Mitochondria / Q000235:genetics* / Q000378:metabolism; D028361:Mitochondrial Diseases / Q000235:genetics* / Q000378:metabolism; D008969:Molecular Sequence Data; D017354:Point Mutation
chemical_list: D045303:Cytochromes b; D003576:Electron Transport Complex IV; D014450:Electron Transport Complex III