title: Functional defects due to spacer-region mutations of human mitochondrial DNA polymerase in a family with an ataxia-myopathy syndrome.
abstract: Defects of mitochondrial polymerase gamma (POLG) underlie neurological diseases ranging from myopathies to parkinsonism and infantile Alpers syndrome. The most severe manifestations have been associated with mutations of the 'spacer' region of POLG, the function of which has remained unstudied in humans. We identified a family, segregating three POLG amino acid variants, A467T, R627Q and Q1236H. The first two affect the spacer region and the third is a polymorphism, allelic with R627Q. Three grades of disease severity appeared to correlate with the genotypes. The patient with the most severe outcome, cerebellar ataxia syndrome, had all three variants, those with R627Q and Q1236H had juvenile-onset ptosis and gait disturbance and those with a single A467T allele had late-onset ptosis. To evaluate the molecular pathogenesis of these spacer defects, we expressed and purified the mutant proteins and studied their catalytic properties in vitro. The A467T substitution resulted in clearly decreased activity, DNA binding and processivity of the polymerase. Our biochemical data, the dominant manifestation of A467T and its previously reported high frequency in the Belgian population (0.6%), emphasize the role of this mutation as a common cause of neurological disease. Further, biochemical evidence that a polymorphic variant may modify the function of a mutant POLG, if occurring in the same polypeptide, is shown here. Finally, and surprisingly, other pathogenic spacer mutants showed DNA-binding affinities and processivities similar to or higher than the controls, suggesting that the disease-causing mechanisms of spacer mutations extend beyond the basic catalytic functions of POLG.
authors.affiliation: Programme of Neurosciences, Biomedicum-Helsinki, Helsinki University, Finland.
authors.forename: Petri T
authors.initials: PT
authors.lastname: Luoma
authors.forename: Ningguang
authors.initials: N
authors.lastname: Luo
authors.forename: Wolfgang N
authors.initials: WN
authors.lastname: Löscher
authors.forename: Carol L
authors.initials: CL
authors.lastname: Farr
authors.forename: Rita
authors.initials: R
authors.lastname: Horvath
authors.forename: Julia
authors.initials: J
authors.lastname: Wanschitz
authors.forename: Stefan
authors.initials: S
authors.lastname: Kiechl
authors.forename: Laurie S
authors.initials: LS
authors.lastname: Kaguni
authors.forename: Anu
authors.initials: A
authors.lastname: Suomalainen
mesh_terms: D000328:Adult; D000368:Aged; D000595:Amino Acid Sequence; D000818:Animals; D001259:Ataxia / Q000235:genetics*; D001483:Base Sequence; D017931:DNA Primers; D004259:DNA-Directed DNA Polymerase / Q000235:genetics*; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D006801:Humans; D008297:Male; D008854:Microscopy, Electron; D008875:Middle Aged; D008928:Mitochondria / Q000201:enzymology* / Q000648:ultrastructure; D008969:Molecular Sequence Data; D009135:Muscular Diseases / Q000235:genetics*; D009154:Mutation; D010375:Pedigree; D017386:Sequence Homology, Amino Acid; D013577:Syndrome
chemical_list: D017931:DNA Primers; D004259:DNA-Directed DNA Polymerase