Unknown,Transcriptomics,Genomics,Proteomics

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Chromosome fragility at the expanded GAA/TTC tracks depends on repeat orientation and requires mismatch repair system.

ABSTRACT: Expansion of triplex-forming GAA/TTC repeats in the first intron of FRDA gene is known to cause Friedreich’s ataxia. Besides FRDA, there are a number of other highly polymorphic GAA/TTC loci in the human genome where the size variations so far were considered to be a neutral event. Using yeast as a model system, we demonstrate that expanded GAA/TTC repeats represent a threat to eukaryotic genome integrity by triggering double-strand breaks and gross chromosomal rearrangements. The fragility potential strongly depends on the length of the track and orientation of the repeats relative to the replication origin which correlates with their propensity to adopt secondary structure and to block replication progression. We show that fragility is mediated by mismatch repair machinery and requires the MutS(beta) and endonuclease activity of MutL(alpha). We suggest that the mechanism of GAA/TTC-induced chromosome aberrations defined in yeast can also operate in human carriers with expanded tracks. Keywords: CGH-array Genomic DNA from each of 15 strains was competitively hybridized to DNA from the parent diploid strain (Cy3/green). Gains of genomic segments in the survivors were detected as continuous regions of positive Log2 Red:Green ratios, while losses were detected as negative Log2 Red:Green ratios.

ORGANISM(S): Saccharomyces cerevisiae

SUBMITTER: Piotr Mieczkowski

PROVIDER: E-GEOD-11425 | biostudies-arrayexpress |

REPOSITORIES: biostudies-arrayexpress

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Project description:At the organismal level, genome rearrangements are usually deleterious and are often associated with disease. Yet, on an evolutionary scale, they can be beneficial as they provide for rapid genetic diversification. DNA lesions, particularly double-strand breaks (DSBs), are sources of genome instability that can be rectified by various repair processes. Homologous recombination (HR) is highly effective at protecting the genome from DSBs and provides for accurate repair between sister chromatids and homologous chromosomes. Here we show that although random DSBs induced by ionizing radiation in yeast chromosomes are repaired efficiently by HR in G-2 diploid cells, rearrangements are frequent. The chromosome aberrations (ABs) primarily resulted from recombination between Ty retrotransposable elements, the most abundant class of dispersed repetitive DNAs in the genome, while some rearrangements involved other classes of repetitive DNA. Few, if any, of the ABs could be attributed to nonhomologous end-joining (NHEJ). We conclude that only those few DSBs that fall at or near the 3-5% of the genome composed of repetitive DNA elements are effective at generating rearrangements, while other lesions that appear in unique (single copy) sequences are correctly repaired. Thus, by successfully competing with repair that normally occurs between large homologous chromosomal DNAs, the combination of repetitive elements and DSBs provides genome plasticity and a rich source of evolutionary opportunities. Keywords: CGH-array Diploid G-2 yeast cells were exposed to 80 krad of ionizing radiation and plated on rich media to obtain survivor colonies. Genomic DNA from each of 37 survivors (Cy5/red; JW1 to JW13, and A1 to A24) was competitively hybridized to DNA from the parent diploid strain (Cy3/green). Gains of genomic segments in the survivors were detected as continuous regions of positive Log2 Red:Green ratios, while losses were detected as negative Log2 Red:Green ratios.

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Chromosome fragility at the expanded GAA/TTC tracks depends on repeat orientation and requires mismatch repair system.

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