title: Stoichiometry of base excision repair proteins correlates with increased somatic CAG instability in striatum over cerebellum in Huntington's disease transgenic mice.
abstract: Huntington's disease (HD) is a progressive neurodegenerative disorder caused by expansion of an unstable CAG repeat in the coding sequence of the Huntingtin (HTT) gene. Instability affects both germline and somatic cells. Somatic instability increases with age and is tissue-specific. In particular, the CAG repeat sequence in the striatum, the brain region that preferentially degenerates in HD, is highly unstable, whereas it is rather stable in the disease-spared cerebellum. The mechanisms underlying the age-dependence and tissue-specificity of somatic CAG instability remain obscure. Recent studies have suggested that DNA oxidation and OGG1, a glycosylase involved in the repair of 8-oxoguanine lesions, contribute to this process. We show that in HD mice oxidative DNA damage abnormally accumulates at CAG repeats in a length-dependent, but age- and tissue-independent manner, indicating that oxidative DNA damage alone is not sufficient to trigger somatic instability. Protein levels and activities of major base excision repair (BER) enzymes were compared between striatum and cerebellum of HD mice. Strikingly, 5'-flap endonuclease activity was much lower in the striatum than in the cerebellum of HD mice. Accordingly, Flap Endonuclease-1 (FEN1), the main enzyme responsible for 5'-flap endonuclease activity, and the BER cofactor HMGB1, both of which participate in long-patch BER (LP-BER), were also significantly lower in the striatum compared to the cerebellum. Finally, chromatin immunoprecipitation experiments revealed that POLbeta was specifically enriched at CAG expansions in the striatum, but not in the cerebellum of HD mice. These in vivo data fit a model in which POLbeta strand displacement activity during LP-BER promotes the formation of stable 5'-flap structures at CAG repeats representing pre-expanded intermediate structures, which are not efficiently removed when FEN1 activity is constitutively low. We propose that the stoichiometry of BER enzymes is one critical factor underlying the tissue selectivity of somatic CAG expansion.
authors.affiliation: Department of Neurobiology and Genetics, Institute of Genetics and Molecular and Cellular Biology, UMR 7104-CNRS/INSERM/UdS, Illkirch, France.
authors.forename: Agathi-Vassiliki
authors.initials: AV
authors.lastname: Goula
authors.forename: Brian R
authors.initials: BR
authors.lastname: Berquist
authors.forename: David M
authors.initials: DM
authors.lastname: Wilson
authors.forename: Vanessa C
authors.initials: VC
authors.lastname: Wheeler
authors.forename: Yvon
authors.initials: Y
authors.lastname: Trottier
authors.forename: Karine
authors.initials: K
authors.lastname: Merienne
mesh_terms: D000375:Aging / Q000378:metabolism; D000818:Animals; D001483:Base Sequence; D002531:Cerebellum / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D004247:DNA / Q000737:chemistry; D004249:DNA Damage; D045647:DNA Glycosylases / Q000378:metabolism; D019951:DNA Polymerase beta / Q000378:metabolism; D004260:DNA Repair / Q000235:genetics; D045643:DNA Repair Enzymes / Q000235:genetics / Q000378:metabolism*; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase / Q000378:metabolism; D045585:Flap Endonucleases / Q000378:metabolism; D042822:Genomic Instability / Q000235:genetics*; D006816:Huntington Disease / Q000235:genetics*; D051379:Mice; D008822:Mice, Transgenic; D008957:Models, Genetic; D008969:Molecular Sequence Data; D017072:Neostriatum / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D009690:Nucleic Acid Conformation; D009928:Organ Specificity / Q000235:genetics; D013379:Substrate Specificity; D019680:Trinucleotide Repeat Expansion / Q000235:genetics*
chemical_list: D004247:DNA; D019951:DNA Polymerase beta; D045585:Flap Endonucleases; D045647:DNA Glycosylases; C485990:Ogg1 protein, mouse; C486216:Apex1 protein, mouse; D043603:DNA-(Apurinic or Apyrimidinic Site) Lyase; D045643:DNA Repair Enzymes