Genome-wide demethylation promotes triplet repeat instability independently of homologous recombination

Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA.
DNA Repair (Impact Factor: 3.11). 03/2008; 7(2):313-20. DOI: 10.1016/j.dnarep.2007.11.002
Source: PubMed


Trinucleotide repeat instability is intrinsic to a family of human neurodegenerative diseases. The mechanism leading to repeat length variation is unclear. We previously showed that treatment with the demethylating agent 5-aza-2'-deoxycytidine (5-aza-CdR) dramatically increases triplet repeat instability in mammalian cells. Based on previous reports that demethylation increases homologous recombination (HR), and our own observations that HR destabilizes triplet repeats, we hypothesized that demethylation alters repeat stability by stimulating HR. Here, we test that hypothesis at the adenosine phosphoribosyl transferase (Aprt) locus in CHO cells, where CpG demethylation and HR have both been shown to increase CAG repeat instability. We find that the rate of HR at the Aprt locus is not altered by demethylation. The spectrum of recombinants, however, was shifted from the usual 6:1 ratio of conversions to crossovers to more equal proportions in 5-aza-CdR-treated cells. The subtle influences of demethylation on HR at the Aprt locus are not sufficient to account for its dramatic effects on repeat instability. We conclude that 5-aza-CdR promotes triplet repeat instability independently of HR.

Download full-text


Available from: Vincent Dion
  • [Show abstract] [Hide abstract]
    ABSTRACT: A coal-based activated carbon was used to adsorb nitric oxide (NO) and SO2 at temperatures between 20-140°C. At a relative NO/SO2 ratio of 40/1, the rate of NO uptake and the conversion of NO-to-NO2 was decreased by approximately 15% as compared to the case when no SO2 was present. Adsorbed NO2 consisted of two species, the more strongly bound of which could be removed by desorption at temperatures between 100-150°C. Adsorbed SO2 also consisted of two species, the more strongly bound of which required a temperature of 300°C to desorb. The concentration of the more weakly held SO2 decreased in the presence of adsorbed NO2 while the overall concentration of adsorbed SO2 increased in the presence of NO2 as compared to without NO2. The data suggest that the sites which catalyze NO -> NO2 are either poisoned or eliminated by adsorbed SO2.
    No preview · Article · Dec 1995
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Expanded CAG repeat tracts are the cause of at least a dozen neurodegenerative disorders. In humans, long CAG repeats tend to expand during transmissions from parent to offspring, leading to an earlier age of disease onset and more severe symptoms in subsequent generations. Here, we show that the maintenance DNA methyltransferase Dnmt1, which preserves the patterns of CpG methylation, plays a key role in CAG repeat instability in human cells and in the male and female mouse germlines. SiRNA knockdown of Dnmt1 in human cells destabilized CAG triplet repeats, and Dnmt1 deficiency in mice promoted intergenerational expansion of CAG repeats at the murine spinocerebellar ataxia type 1 (Sca1) locus. Importantly, Dnmt1(+/-) SCA1 mice, unlike their Dnmt1(+/+) SCA1 counterparts, closely reproduced the intergenerational instability patterns observed in human SCA1 patients. In addition, we found aberrant DNA and histone methylation at sites within the CpG island that abuts the expanded repeat tract in Dnmt1-deficient mice. These studies suggest that local chromatin structure may play a role in triplet repeat instability. These results are consistent with normal epigenetic changes during germline development contributing to intergenerational instability of CAG repeats in mice and in humans.
    Full-text · Article · Jun 2008 · Human Molecular Genetics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Trinucleotide repeat expansion underlies at least 17 neurological diseases. In affected individuals, the expanded locus is characterized by dramatic changes in chromatin structure and in repeat tract length. Interestingly, recent studies show that several chromatin modifiers, including a histone acetyltransferase, a DNA methyltransferase and the chromatin insulator CTCF can modulate repeat instability. Here, we propose that the unusual chromatin structure of expanded repeats directly impacts their instability. We discuss several potential models for how this might occur, including a role for DNA repair-dependent epigenetic reprogramming in increasing repeat instability, and the capacity of epigenetic marks to alter sense and antisense transcription, thereby affecting repeat instability.
    Full-text · Article · Jul 2009 · Trends in Genetics
Show more