title: Molecular and biochemical characterisation of a novel mutation in POLG associated with Alpers syndrome.
abstract: BACKGROUND
  DNA polymerase γ (POLG) is the only known mitochondrial DNA (mtDNA) polymerase. It mediates mtDNA replication and base excision repair. Mutations in the POLG gene lead to reduction of functional mtDNA (mtDNA depletion and/or deletions) and are therefore predicted to result in defective oxidative phosphorylation (OXPHOS). Many mutations map to the polymerase and exonuclease domains of the enzyme and produce a broad clinical spectrum. The most frequent mutation p.A467T is localised in the linker region between these domains. In compound heterozygote patients the p.A467T mutation has been described to be associated amongst others with fatal childhood encephalopathy. These patients have a poorer survival rate compared to homozygotes.
  
  
  METHODS
  mtDNA content in various tissues (fibroblasts, muscle and liver) was quantified using quantitative PCR (qPCR). OXPHOS activities in the same tissues were assessed using spectrophotometric methods and catalytic stain of BN-PAGE.
  
  
  RESULTS
  We characterise a novel splice site mutation in POLG found in trans with the p.A467T mutation in a 3.5 years old boy with valproic acid induced acute liver failure (Alpers-Huttenlocher syndrome). These mutations result in a tissue specific depletion of the mtDNA which correlates with the OXPHOS-activities.
  
  
  CONCLUSIONS
  mtDNA depletion can be expressed in a high tissue-specific manner and confirms the need to analyse primary tissue. Furthermore, POLG analysis optimises clinical management in the early stages of disease and reinforces the need for its evaluation before starting valproic acid treatment.
authors.affiliation: Division of Human Genetics, University Hospital Bern, Bern, Switzerland. andre.schaller@insel.ch
authors.forename: André
authors.initials: A
authors.lastname: Schaller
authors.forename: Dagmar
authors.initials: D
authors.lastname: Hahn
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Jackson
authors.forename: Ilse
authors.initials: I
authors.lastname: Kern
authors.forename: Christophe
authors.initials: C
authors.lastname: Chardot
authors.forename: Dominique C
authors.initials: DC
authors.lastname: Belli
authors.forename: Sabina
authors.initials: S
authors.lastname: Gallati
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Nuoffer
mesh_terms: D018929:Cell Culture Techniques; D002675:Child, Preschool; D000074002:DNA Polymerase gamma; D004272:DNA, Mitochondrial / Q000378:metabolism*; D004259:DNA-Directed DNA Polymerase / Q000235:genetics*; D002549:Diffuse Cerebral Sclerosis of Schilder / Q000235:genetics* / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D006801:Humans; D008099:Liver / Q000378:metabolism; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism; D009154:Mutation / Q000235:genetics*; D010085:Oxidative Phosphorylation; D017422:Sequence Analysis, DNA / Q000379:methods
chemical_list: D004272:DNA, Mitochondrial; D000074002:DNA Polymerase gamma; D004259:DNA-Directed DNA Polymerase