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CGH detection of chromosomal rearrangements in diploid yeast cells exposed to ionizing radiation

ABSTRACT: 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

ORGANISM(S): Saccharomyces cerevisiae

PROVIDER: GSE6984 | GEO | 2007/02/08

SECONDARY ACCESSION(S): PRJNA99281

REPOSITORIES: GEO

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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.

Project description:DNA double strand breaks (DSBs) in repetitive sequences are a potent source of genomic instability, due to the possibility of non-allelic homologous recombination (NAHR). Repetitive sequences are especially at risk during meiosis, when numerous programmed DSBs are introduced into the genome to initiate meiotic recombination 1. Within the budding yeast repetitive ribosomal (r)DNA array, meiotic DSB formation is prevented in part through Sir2-dependent heterochromatin 2,3. Here, we demonstrate that the edges of the rDNA array are exceptionally susceptible to meiotic DSBs, revealing an inherent heterogeneity within the rDNA array. We find that this localised DSB susceptibility necessitates a border-specific protection system consisting of the meiotic ATPase Pch2 and the origin recognition complex subunit Orc1. Upon disruption of these factors, DSB formation and recombination specifically increased in the outermost rDNA repeats, leading to NAHR and rDNA instability. Strikingly, the Sir2-dependent heterochromatin of the rDNA itself was responsible for the induction of DSBs at the rDNA borders in pch2? cells. Thus, while Sir2 activity globally prevents meiotic DSBs within the rDNA, it creates a highly permissive environment for DSB formation at the heterochromatin/euchromatin junctions. Heterochromatinised repetitive DNA arrays are abundantly present in most eukaryotic genomes. Our data define the borders of such chromatin domains as distinct high-risk regions for meiotic NAHR, whose protection may be a universal requirement to prevent meiotic genome rearrangements associated with genomic diseases and birth defects. This SuperSeries is composed of the following subset Series: GSE30071: ssDNA mapping in dmc1 strains GSE30072: ChIP-chip of DSB factors in wild type and pch2 strains Two types of study were undertaken to understand the regulation of meiotic DSB formation close to repetitive DNA elements in yeast. First, DSBs were mapped using ssDNA enrichment in strains isogenic for a dmc1 mutation, and also including pch2 delete, orc1-161, rdna delete and a reciprocal translocation between chromosomes 2 and 12 (trans2to12). Second, the association of the DSB factors Hop1, Rec114, Mer2, and Mre1, as well as total histone H3 and H3K4-trimethylation were measured by ChIP-chip analysis in wild-type and pch2 delete strains.

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CGH detection of chromosomal rearrangements in diploid yeast cells exposed to ionizing radiation

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