Ribodysgenesis: sudden genome instability in the yeast Saccharomyces cerevisiae arising from RNase H2 cleavage at genomic-embedded ribonucleotides
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ABSTRACT: Yeast Saccharomyces cerevisiae has been widely used as a model system for studying genome instability. Here, heterozygous S. cerevisiae zygotes were generated to determine the genomic alterations induced by sudden introduction of active RNase H2. In combination of a custom SNP microarray, the patterns of chromosomal instability could be explored at a whole genome level. Ribonucleotides can be incorporated into DNA during replication by the replicative DNA polymerases. These aberrant DNA subunits are efficiently recognized and removed by Ribonucleotide Excision Repair, which is initiated by the heterotrimeric enzyme RNase H2. While RNase H2 is essential in higher eukaryotes, the yeast Saccharomyces cerevisiae can survive without RNase H2 enzyme, although the genome undergoes mutation, recombination and other genome instability events at an increased rate. Although RNase H2 can be considered as a protector of the genome from the deleterious events that can ensue from recognition and removal of embedded ribonucleotides, under conditions of high ribonucleotide incorporation and retention in the genome in a RNase H2-negative strain, sudden introduction of active RNase H2 causes massive DNA breaks and genome instability in a condition which we term “ribodysgenesis”. The DNA breaks and genome instability arise solely from RNase H2 cleavage directed to the ribonucleotide-containing genome. Survivors of ribodysgenesis have massive loss of heterozygosity events stemming from recombinogenic lesions on the ribonucleotide-containing DNA, with increases of over 1000X from wild-type. DNA breaks are produced over one to two divisions and subsequently cells adapt to RNase H2 and ribonucleotides in the genome and grow with normal levels of genome instability.
Project description:We report here the consequences of loss of Saccharomyces cerevisiae RNase H2 by comparing mRNA expression profiles in wild type yeast versus a strain lacking the RNH201 gene, encoding the catalytic subunit. Deleting RHN201 alters mRNA expression for hundreds of genes. Expression changes were based on seven biological replicates (3 from one batch and 4 from another batch) and are seen for many genes involved in stress responses and genome maintenance, consistent with a role of RNase H2 in removing ribonucleotides incorporated into DNA during replication. Differentially expressed genes include, those involved in ribosomal RNA processing and biogenesis and in tRNA modification, supports a role for RNase H2 in resolving R-loops formed during transcription of rRNA and tRNA. Other differentially expresses genes include putative or known helicases and nucleases maintenance of dsRNA viruses, which could be relevant to how mammalian RNase H2 dysfunction elicits an innate immune response leading to neurodegeneration. We examine the consequences of loss of Saccharomyces cerevisiae RNase H2 by comparing mRNA expression profiles in wild type yeast versus a strain lacking the RNH201 gene encoding the catalytic subunit. Expression changes were based on seven biological replicates (3 from one batch and 4 from another batch). For strain reference please see S.A. Nick McElhinny, et al, Genome instability due to ribonucleotide incorporation into DNA, Nat Chem Bio 6, 774-781 (2010).
Project description:We report the application of high through-put tag sequencing to measure the location and strand of DNA embedded ribonucleotides in the yeast genome. Mutations in the catalytic subunits of the polymerases (pol1-L868M, pol2-M644G and pol3-L612M) lead to the increased incorporation of ribonucleotides during DNA replication, providing an in vivo label with which to track the contribution of each polymerase to the fully replicated genome. Yeast strains used in this study are deleted for rnh201, encoding the catalytic subunit of the RNase H2 gene so that embedded ribonucleotides are not rapidly removed by ribonucleotide excision repair following DNA replication. Analysis of this data demonstrates that polymerase alpha contributes to the fully replicated genome. Sequencing of DNA embedded ribonucleotides in S. cerevisiae strains to map the contribution of replicative polymerases to the fully replicated genome.
Project description:Two types of RNA:DNA associations can lead to genome instability: the formation of R-loops during transcription and the incorporation of ribonucleotide monophosphates (rNMPs) into DNA during replication. Both ribonuclease (RNase) H1 and RNase H2 degrade the RNA component of R-loops, whereas only RNase H2 can remove one or a few rNMPs from DNA. We performed high-resolution mapping of mitotic recombination events throughout the yeast genome in diploid strains of Saccharomyces cerevisiae lacking RNase H1 (rnh1Δ), RNase H2 (rnh201Δ), or both RNase H1 and RNase H2 (rnh1Δ rnh201Δ). We found little effect on recombination in the rnh1Δ strain, but elevated recombination in both the rnh201Δ and the double-mutant strains; levels of recombination in the double mutant were about 50% higher than in the rnh201 single-mutant strain. An rnh201Δ mutant that additionally contained a mutation that reduces rNMP incorporation by DNA polymerase ε (pol2-M644L) had a level of instability similar to that observed in the presence of wild-type Polε. This result suggests that the elevated recombination observed in the absence of only RNase H2 is primarily a consequence of R loops rather than misincorporated rNMPs.
Project description:We report the application of high through-put tag sequencing to measure the location and strand of DNA embedded ribonucleotides in the yeast genome. Mutations in the catalytic subunits of the polymerases (pol1-L868M, pol2-M644G and pol3-L612M) lead to the increased incorporation of ribonucleotides during DNA replication, providing an in vivo label with which to track the contribution of each polymerase to the fully replicated genome. Yeast strains used in this study are deleted for rnh201, encoding the catalytic subunit of the RNase H2 gene so that embedded ribonucleotides are not rapidly removed by ribonucleotide excision repair following DNA replication. Analysis of this data demonstrates that polymerase alpha contributes to the fully replicated genome.
Project description:Ribodysgenesis: sudden genome instability in the yeast Saccharomyces cerevisiae arising from RNase H2 cleavage at genomic-embedded ribonucleotides
Project description:Insufficient repair of DNA lesions results in the acquisition of somatic mutations and displays the driving force in cancerogenesis. Ribonucleotide incorporation by eukaryotic DNA polymerases occurs during every round of genome duplication and represents by far the most frequent type of naturally occurring DNA lesions. RNAse H2 removes misincorporated ribonucleotides from genomic DNA in a process termed ribonucleotide excision repair (RER). Whether intestinal epithelial proliferation requires RER and whether abrogation of RER is involved in the etiology of cancerogenesis at all is unknown. Mice with an epithelial specific deletion of RNase H2 subunit b (H2bΔIEC) and co-deletion of the tumor suppressor p53 (H2b/p53ΔIEC) were generated and phenotyped at young and old age. RNA sequencing was performed in isolated epithelial cells and intestinal organoids. Mutational signature of spontaneous tumors from H2b/p53ΔIEC mice were characterized using exome sequencing. Association of tumor RNase H2 expression and patient survival was assessed in transcriptome data from 467 CRC patients. H2bΔIEC mice display chronic epithelial DNA damage and develop a p53-dependent proliferative exhaustion of the intestinal stem cell compartment. H2b/p53ΔIEC mice have restored epithelial proliferation and spontaneously develop small intestinal carcinomas. Resulting tumors display a distinct mutational signature characterized by T>G base substitutions at GpTpG trinucleotides. Transcriptome data from human colorectal cancer patients indicate that reduced RNase H2 expression is associated with poor survival in CRC. Conclusion: We propose a hitherto unappreciated role for RNase H2 as a tumor suppressor gene in CRC. Our mouse model provides a novel tool to study the impact of abrogated RER on intestinal carcinogenesis.
Project description:Large numbers of ribonucleotides are incorporated into the eukaryotic nuclear genome during S-phase due to imperfect discrimination against ribonucleoside triphosphates by the replicative DNA polymerases. Ribonucleotides, by far the most common DNA lesion in replicating cells, destabilize the DNA, and an evolutionarily conserved DNA repair machinery, ribonucleotide excision repair (RER), ensures ribonucleotide removal. Complete lack of RER is embryonically lethal. Partial loss-of-function mutations in the genes encoding subunits of RNase H2, the enzyme essential for initiation of RER, cause the SLE-related type I interferonopathy Aicardi-Goutières syndrome. Here we establish that selective inactivation of RER in mouse epidermis results in spontaneous DNA damage, epidermal hyperproliferation associated with loss of hair follicle stem cells and hair follicle function. The animals develop keratinocyte intraepithelial neoplasia and invasive squamous cell carcinoma with complete penetrance, despite potent type I interferon production and skin inflammation. Compromised RER-mediated genome maintenance might represent an important tumor-promoting principle in human cancer.
Project description:Large numbers of ribonucleotides are incorporated into the eukaryotic nuclear genome during S-phase due to imperfect discrimination against ribonucleoside triphosphates by the replicative DNA polymerases. Ribonucleotides, by far the most common DNA lesion in replicating cells, destabilize the DNA, and an evolutionarily conserved DNA repair machinery, ribonucleotide excision repair (RER), ensures ribonucleotide removal. Complete lack of RER is embryonically lethal. Partial loss-of-function mutations in the genes encoding subunits of RNase H2, the enzyme essential for initiation of RER, cause the SLE-related type I interferonopathy Aicardi-Goutières syndrome. Here we establish that selective inactivation of RER in mouse epidermis results in spontaneous DNA damage, epidermal hyperproliferation associated with loss of hair follicle stem cells and hair follicle function. The animals develop keratinocyte intraepithelial neoplasia and invasive squamous cell carcinoma with complete penetrance, despite potent type I interferon production and skin inflammation. Compromised RER-mediated genome maintenance might represent an important tumor-promoting principle in human cancer.
Project description:Misincorporation of ribonucleotides into DNA during genome replication has recently become recognized as a significant source of genomic instability. The frequency of ribonucleotides in the genome is determined by dNTP/rNTP ratios, the ability of the DNA polymerases to discriminate against ribonucleotides, and by the capacity of repair mechanisms to remove misincorporated ribonucleotides. To simultaneously compare how the nuclear and mitochondrial genomes incorporate and remove ribonucleotides, we challenged these processes by imbalancing cellular dNTP pools. Using a collection of yeast strains with altered dNTP pools, we discovered an inverse relationship between the concentration of individual dNTPs and the amount of the corresponding ribonucleotides incorporated in mitochondrial DNA, while in nuclear DNA the ribonucleotide pattern was only altered in the absence of ribonucleotide excision repair. Our analysis uncovers major differences in ribonucleotide repair between the two genomes and provides concrete evidence that yeast mitochondria lack mechanisms for repair of misincorporated ribonucleotides.
Project description:We report here the consequences of loss of Saccharomyces cerevisiae RNase H2 by comparing mRNA expression profiles in wild type yeast versus a strain lacking the RNH201 gene, encoding the catalytic subunit. Deleting RHN201 alters mRNA expression for hundreds of genes. Expression changes were based on seven biological replicates (3 from one batch and 4 from another batch) and are seen for many genes involved in stress responses and genome maintenance, consistent with a role of RNase H2 in removing ribonucleotides incorporated into DNA during replication. Differentially expressed genes include, those involved in ribosomal RNA processing and biogenesis and in tRNA modification, supports a role for RNase H2 in resolving R-loops formed during transcription of rRNA and tRNA. Other differentially expresses genes include putative or known helicases and nucleases maintenance of dsRNA viruses, which could be relevant to how mammalian RNase H2 dysfunction elicits an innate immune response leading to neurodegeneration.