title: Overexpression of human mitochondrial valyl tRNA synthetase can partially restore levels of cognate mt-tRNAVal carrying the pathogenic C25U mutation.
abstract: Phenotypic diversity associated with pathogenic mutations of the human mitochondrial genome (mtDNA) has often been explained by unequal segregation of the mutated and wild-type genomes (heteroplasmy). However, this simple hypothesis cannot explain the tissue specificity of disorders caused by homoplasmic mtDNA mutations. We have previously associated a homoplasmic point mutation (1624C>T) in MTTV with a profound metabolic disorder that resulted in the neonatal deaths of numerous siblings. Affected tissues harboured a marked biochemical defect in components of the mitochondrial respiratory chain, presumably due to the extremely low (<1%) steady-state levels of mt-tRNA(Val). In primary myoblasts and transmitochondrial cybrids established from the proband (index case) and offspring, the marked respiratory deficiency was lost and steady-state levels of the mutated mt-tRNA(Val) were greater than in the biopsy material, but were still an order of magnitude lower than in control myoblasts. We present evidence that the generalized decrease in steady-state mt-tRNA(Val) observed in the homoplasmic 1624C>T-cell lines is caused by a rapid degradation of the deacylated form of the abnormal mt-tRNA(Val). By both establishing the identity of the human mitochondrial valyl-tRNA synthetase then inducing its overexpression in transmitochondrial cell lines, we have been able to partially restore steady-state levels of the mutated mt-tRNA(Val), consistent with an increased stability of the charged mt-tRNA. These data indicate that variations in the levels of VARS2L between tissue types and patients could underlie the difference in clinical presentation between individuals homoplasmic for the 1624C>T mutation.
authors.affiliation: Mitochondrial Research Group, Institute of Neuroscience, Medical School, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne, NE2 4HH, UK.
authors.forename: Joanna
authors.initials: J
authors.lastname: Rorbach
authors.forename: Abdul Aziz
authors.initials: AA
authors.lastname: Yusoff
authors.forename: Helen
authors.initials: H
authors.lastname: Tuppen
authors.forename: Dyg P
authors.initials: DP
authors.lastname: Abg-Kamaludin
authors.forename: Zofia M A
authors.initials: ZM
authors.lastname: Chrzanowska-Lightowlers
authors.forename: Robert W
authors.initials: RW
authors.lastname: Taylor
authors.forename: Douglass M
authors.initials: DM
authors.lastname: Turnbull
authors.forename: Robert
authors.initials: R
authors.lastname: McFarland
authors.forename: Robert N
authors.initials: RN
authors.lastname: Lightowlers
mesh_terms: D001483:Base Sequence; D002460:Cell Line; D002478:Cells, Cultured; D006680:HLA Antigens / Q000378:metabolism*; D006801:Humans; D008928:Mitochondria / Q000201:enzymology; D017240:Mitochondrial Myopathies / Q000235:genetics*; D024101:Mitochondrial Proteins / Q000378:metabolism*; D008969:Molecular Sequence Data; D017354:Point Mutation; D012313:RNA / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D020871:RNA Stability; D000077278:RNA, Mitochondrial; D012366:RNA, Transfer, Val / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D014637:Valine-tRNA Ligase / Q000378:metabolism*
chemical_list: D006680:HLA Antigens; D024101:Mitochondrial Proteins; D000077278:RNA, Mitochondrial; D012366:RNA, Transfer, Val; C070977:VARS2 protein, human; D012313:RNA; D014637:Valine-tRNA Ligase