8-hydroxyguanine and Huntington-Disease

The age at onset of Huntington disease (HD) shows a strong, negative correlation with the number of CAG repeats within the huntingtin (HTT) gene. However, this does not account for all the inter-individual variability seen among patients. In order to assess whether single-nucleotide polymorphisms (SNPs) in the OGG1 and XPC genes, both implicated in responses to oxidative stress, are associated with the age of onset of HD, 9 SNPs have been genotyped in 299 individuals with HD and 582 controls. After correction for multiple testing, two OGG1/XPC haplotypes were found to be associated with younger age at onset independently of the number of CAG repeats within the HTT gene. Both haplotypes contain XPC coding variants that would be expected to impact on protein function and/or variants in the 3'UTR that could result in altered protein levels via allele-specific mIR binding. One haplotype also contains the OGG1-326Cys (rs1052133) allele that has been associated with a lower 8-oxoG repair activity and is particularly sensitive to the cellular redox status. These results highlight the potential role of oxidative stress in determining the age at onset of HD.

Topics: 3' Untranslated Regions; Adult; Age of Onset; DNA Damage; DNA Glycosylases; DNA Repair; DNA-Binding Proteins; Female; Genotype; Guanine; Haplotypes; Humans; Huntingtin Protein; Huntington Disease; Male; MicroRNAs; Middle Aged; Nerve Tissue Proteins; Oxidative Stress; Polymorphism, Single Nucleotide; RNA, Messenger; Trinucleotide Repeats

In the phenomenon of trinucleotide repeat (TNR) expansion, an important interplay exists between DNA damage repair of 8-oxo-7,8-dihydroguanine (8-oxoG) and noncanonical structure formation. We show that TNR DNA adapts its structure to accommodate 8-oxoG. Using chemical probe analysis, we find that CAG repeats composing the stem-loop arm of a three-way junction alter the population of structures in response to 8-oxoG by positioning the lesion at or near the loop. Furthermore, we find that oligonucleotides composed of odd-numbered repeat sequences, which form populations of two structures, will also alter their structure to place 8-oxoG in the loop. However, sequences with an even number of repeats do not display this behavior. Analysis by differential scanning calorimetry indicates that when the lesion is located within the loop, there are no significant changes to the thermodynamic parameters as compared to the DNA lacking 8-oxoG. This contrasts with the enthalpic destabilization observed when 8-oxoG is base-paired to C and indicates that positioning 8-oxoG in the loop avoids the thermodynamic penalty associated with 8-oxoG base-pairing. Since formation of stem-loop hairpins is proposed to facilitate TNR expansion, these results highlight the importance of defining the structural consequences of DNA damage.

Topics: Base Sequence; Calorimetry, Differential Scanning; DNA Damage; DNA Repair; DNA, Single-Stranded; Guanine; Humans; Huntington Disease; Molecular Sequence Data; Nucleic Acid Conformation; Reactive Oxygen Species; Thermodynamics; Trinucleotide Repeat Expansion

Several human neurodegenerative disorders are characterized by the accumulation of 8-oxo-7,8-dihydroguanine (8-oxodG) in the DNA of affected neurons. This can occur either through direct oxidation of DNA guanine or via incorporation of the oxidized nucleotide during replication. Hydrolases that degrade oxidized purine nucleoside triphosphates normally minimize this incorporation. hMTH1 is the major human hydrolase. It degrades both 8-oxodGTP and 8-oxoGTP to the corresponding monophosphates. To investigate whether the incorporation of oxidized nucleic acid precursors contributes to neurodegeneration, we constructed a transgenic mouse in which the human hMTH1 8-oxodGTPase is expressed. hMTH1 expression protected embryonic fibroblasts and mouse tissues against the effects of oxidants. Wild-type mice exposed to 3-nitropropionic acid develop neuropathological and behavioural symptoms that resemble those of Huntington's disease. hMTH1 transgene expression conferred a dramatic protection against these Huntington's disease-like symptoms, including weight loss, dystonia and gait abnormalities, striatal degeneration, and death. In a complementary approach, an in vitro genetic model for Huntington's disease was also used. hMTH1 expression protected progenitor striatal cells containing an expanded CAG repeat of the huntingtin gene from toxicity associated with expression of the mutant huntingtin. The findings implicate oxidized nucleic acid precursors in the neuropathological features of Huntington's disease and identify the utilization of oxidized nucleoside triphosphates by striatal cells as a significant contributor to the pathogenesis of this disorder.

Topics: 8-Hydroxy-2'-Deoxyguanosine; Animals; Corpus Striatum; Deoxyguanosine; DNA Damage; DNA Repair Enzymes; DNA, Complementary; Embryo, Mammalian; Fibroblasts; Guanine; Humans; Huntingtin Protein; Huntington Disease; Mice; Mice, Transgenic; Nerve Tissue Proteins; Neurodegenerative Diseases; Nitro Compounds; Nuclear Proteins; Oxidation-Reduction; Oxidative Stress; Phosphoric Monoester Hydrolases; Propionates; Stem Cells