title: A homoplasmic mtDNA variant can influence the phenotype of the pathogenic m.7472Cins MTTS1 mutation: are two mutations better than one?
abstract: Mutations in mitochondrial tRNA (mt-tRNA) genes are well recognized as a common cause of human disease, exhibiting a significant degree of clinical heterogeneity. While these differences are explicable, in part, by differences in the innate pathogenicity of the mutation, its distribution and abundance, other factors, including nuclear genetic background, mitochondrial DNA (mtDNA) haplotype and additional mtDNA mutations may influence the expression of mt-tRNA mutations. We describe the clinical, biochemical and molecular findings in a family with progressive myopathy, deafness and diabetes and striking respiratory chain abnormalities due to a well-characterized heteroplasmic mt-tRNA mutation in the mt-tRNA(Ser(UCN)) (MTTS1) gene. In addition to the m.7472Cins mutation, all individuals were homoplasmic for another variant, m.7472A > C, affecting the adjacent nucleotide in the mt-tRNA(Ser(UCN)) structure. In addition to available patient tissues, we have analysed transmitochondrial cybrid clones harbouring homoplasmic levels of m.7472A > C and varying levels of the m.7472Cins mutation in an attempt to clarify the precise role of the m.7472A > C transversion in the underlying respiratory chain abnormality. Evidence from both in vivo and in vitro studies demonstrate that the m.7472A > C is able to modify the expression of the m.7472Cins mutation and would suggest that it is not a neutral variant but appears to cause a biochemical defect by itself, confirming that homoplasmic mtDNA variants can modulate the phenotypic expression of pathogenic, heteroplasmic mtDNA mutations.
authors.affiliation: Mitochondrial Research Group, The Medical School, Newcastle University, Newcastle upon Tyne, UK.
authors.forename: Helen
authors.initials: H
authors.lastname: Swalwell
authors.forename: Emma L
authors.initials: EL
authors.lastname: Blakely
authors.forename: Ruth
authors.initials: R
authors.lastname: Sutton
authors.forename: Kasia
authors.initials: K
authors.lastname: Tonska
authors.forename: Matthias
authors.initials: M
authors.lastname: Elstner
authors.forename: Langping
authors.initials: L
authors.lastname: He
authors.forename: Tanja
authors.initials: T
authors.lastname: Taivassalo
authors.forename: Dennis K
authors.initials: DK
authors.lastname: Burns
authors.forename: Douglass M
authors.initials: DM
authors.lastname: Turnbull
authors.forename: Ronald G
authors.initials: RG
authors.lastname: Haller
authors.forename: Mercy M
authors.initials: MM
authors.lastname: Davidson
authors.forename: Robert W
authors.initials: RW
authors.lastname: Taylor
mesh_terms: D000368:Aged; D001483:Base Sequence; D001681:Biological Assay; D015152:Blotting, Northern; D002999:Clone Cells; D004252:DNA Mutational Analysis; D004272:DNA, Mitochondrial / Q000235:genetics*; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000378:metabolism; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D024101:Mitochondrial Proteins / Q000096:biosynthesis; D008969:Molecular Sequence Data; D018482:Muscle, Skeletal / Q000201:enzymology / Q000473:pathology; D009154:Mutation / Q000235:genetics*; D010375:Pedigree; D010641:Phenotype; D014176:Protein Biosynthesis; D012343:RNA, Transfer / Q000235:genetics*; D013385:Succinate Dehydrogenase / Q000378:metabolism
chemical_list: D004272:DNA, Mitochondrial; D024101:Mitochondrial Proteins; D012343:RNA, Transfer; D013385:Succinate Dehydrogenase; D003576:Electron Transport Complex IV