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Epstein RJ, Lin FPY, Brink RA, Blackburn J. Synonymous alterations of cancer-associated Trp53 CpG mutational hotspots cause fatal developmental jaw malocclusions but no tumors in knock-in mice. PLoS One 2023; 18:e0284327. [PMID: 37053216 PMCID: PMC10101519 DOI: 10.1371/journal.pone.0284327] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2022] [Accepted: 03/28/2023] [Indexed: 04/14/2023] Open
Abstract
Intragenic CpG dinucleotides are tightly conserved in evolution yet are also vulnerable to methylation-dependent mutation, raising the question as to why these functionally critical sites have not been deselected by more stable coding sequences. We previously showed in cell lines that altered exonic CpG methylation can modify promoter start sites, and hence protein isoform expression, for the human TP53 tumor suppressor gene. Here we extend this work to the in vivo setting by testing whether synonymous germline modifications of exonic CpG sites affect murine development, fertility, longevity, or cancer incidence. We substituted the DNA-binding exons 5-8 of Trp53, the mouse ortholog of human TP53, with variant-CpG (either CpG-depleted or -enriched) sequences predicted to encode the normal p53 amino acid sequence; a control construct was also created in which all non-CpG sites were synonymously substituted. Homozygous Trp53-null mice were the only genotype to develop tumors. Mice with variant-CpG Trp53 sequences remained tumor-free, but were uniquely prone to dental anomalies causing jaw malocclusion (p < .0001). Since the latter phenotype also characterises murine Rett syndrome due to dysfunction of the trans-repressive MeCP2 methyl-CpG-binding protein, we hypothesise that CpG sites may exert non-coding phenotypic effects via pre-translational cis-interactions of 5-methylcytosine with methyl-binding proteins which regulate mRNA transcript initiation, expression or splicing, although direct effects on mRNA structure or translation are also possible.
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Affiliation(s)
- Richard J Epstein
- University of New South Wales, St Vincent's Hospital Campus, Sydney, Australia
- Garvan Institute of Medical Research, Sydney, Australia
| | - Frank P Y Lin
- University of New South Wales, St Vincent's Hospital Campus, Sydney, Australia
- Centre for Clinical Genomics, The Kinghorn Cancer Centre, Sydney, Australia
| | - Robert A Brink
- University of New South Wales, St Vincent's Hospital Campus, Sydney, Australia
- Garvan Institute of Medical Research, Sydney, Australia
| | - James Blackburn
- University of New South Wales, St Vincent's Hospital Campus, Sydney, Australia
- Garvan Institute of Medical Research, Sydney, Australia
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Zhang M, Yang D, Gold B. Origins of nonsense mutations in human tumor suppressor genes. Mutat Res 2021; 823:111761. [PMID: 34461460 DOI: 10.1016/j.mrfmmm.2021.111761] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2019] [Revised: 07/27/2020] [Accepted: 08/12/2021] [Indexed: 11/26/2022]
Abstract
Understanding the origins of mutations in tumor suppressor genes and oncogenes associated with cancers in different tissues is critical to the development of potential prevention strategies. Analysis of >10,000 nonsense mutations in 63 tumor suppressor genes based on the ratio of the number of nonsense mutations per codon type is reported for each gene. The ratio for C•G→T•A nonsense mutations at Arg CGA codons to the number of CGA codons in all cancers is 23 (3088 total nonsense mutations for 134 CGA codons in the 63 suppressor genes). The ratio for this codon, which is attributed to hydrolytic deamination of 5-methylcytosine at CpG sites based on the sequence context, is 6-fold higher than the next highest ratio that involves a C•G→T•A transition at Trp TGG codons. C•G→A•T transversions at Glu, Ser, Tyr, Gly and Cys codons account for 25 % of the total nonsense mutations but the mutation per codon ratio for these codons is 1.0. Analysis of the bases 5' of the mutated CGA codons in the 63 tumor suppressor genes in all cancers shows a preference of 5'-G > C ∼ T ∼ A, which is not indicative of a role for enzymatic deamination by deaminases. Overall C•G→T•A mutations account for 61 % of all of the nonsense mutations in the collection of tumor suppressor genes. It is demonstrated that the ratio of C•G→T•A deamination-associated nonsense mutations at CGA codons (hydrolytic deamination) to the number of frame shift insertion/deletion mutations (i.e., replication based) for 5 major tumor suppressors genes are very similar in 3 different tissues that undergo a wide range of stem cell divisions. Therefore, the frequency of deamination mutations parallels the number of stem cell replications. This may reflect the generation of more solvent accessible single-stranded DNA regions during polymerization that are kinetically more prone to deamination.
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Affiliation(s)
- Min Zhang
- Department of Pharmaceutical Sciences, University of Pittsburgh, Salk Hall, Pittsburgh, PA, 15261, United States
| | - Da Yang
- Department of Pharmaceutical Sciences, University of Pittsburgh, Salk Hall, Pittsburgh, PA, 15261, United States
| | - Barry Gold
- Department of Pharmaceutical Sciences, University of Pittsburgh, Salk Hall, Pittsburgh, PA, 15261, United States.
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Blackburn J, Roden DL, Ng R, Wu J, Bosman A, Epstein RJ. Damage-inducible intragenic demethylation of the human TP53 tumor suppressor gene is associated with transcription from an alternative intronic promoter. Mol Carcinog 2015; 55:1940-1951. [PMID: 26676339 PMCID: PMC5111752 DOI: 10.1002/mc.22441] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2015] [Revised: 10/29/2015] [Accepted: 11/17/2015] [Indexed: 12/23/2022]
Abstract
Wild-type TP53 exons 5-8 contain CpG dinucleotides that are prone to methylation-dependent mutation during carcinogenesis, but the regulatory effects of methylation affecting these CpG sites are unclear. To clarify this, we first assessed site-specific TP53 CpG methylation in normal and transformed cells. Both DNA damage and cell ageing were associated with site-specific CpG demethylation in exon 5 accompanied by induction of a truncated TP53 isoform regulated by an adjacent intronic promoter (P2). We then synthesized novel synonymous TP53 alleles with divergent CpG content but stable encodement of the wild-type polypeptide. Expression of CpG-enriched TP53 constructs selectively reduced production of the full-length transcript (P1), consistent with a causal relationship between intragenic demethylation and transcription. 450K methylation comparison of normal (TP53-wildtype) and cancerous (TP53-mutant) human cells and tissues revealed focal cancer-associated declines in CpG methylation near the P1 transcription start site, accompanied by rises near the alternate exon 5 start site. These data confirm that site-specific changes of intragenic TP53 CpG methylation are extrinsically inducible, and suggest that human cancer progression is mediated in part by dysregulation of damage-inducible intragenic CpG demethylation that alters TP53 P1/P2 isoform expression. © 2015 The Authors. Molecular Carcinogenesis Published by Wiley Periodicals, Inc.
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Affiliation(s)
- James Blackburn
- Laboratory of Genome Evolution, The Kinghorn Cancer Centre, Garvan Institute for Medical Research, Sydney, Australia.,UNSW Medicine, St. Vincent's Clinical School, Darlinghurst, Sydney, Australia
| | - Daniel L Roden
- Laboratory of Cancer Biology, The Kinghorn Cancer Centre, Garvan Institute for Medical Research, Sydney, Australia
| | - Robert Ng
- Laboratory of Genome Evolution, The Kinghorn Cancer Centre, Garvan Institute for Medical Research, Sydney, Australia.,UNSW Medicine, St. Vincent's Clinical School, Darlinghurst, Sydney, Australia
| | - Jianmin Wu
- Laboratory of Cancer Bioinformatics, The Kinghorn Cancer Centre, Garvan Institute for Medical Research, Sydney, Australia
| | - Alexis Bosman
- Laboratory of Developmental and Stem Cell Biology, Victor Chang Cardiac Research Institute, Sydney, Australia
| | - Richard J Epstein
- Laboratory of Genome Evolution, The Kinghorn Cancer Centre, Garvan Institute for Medical Research, Sydney, Australia.,UNSW Medicine, St. Vincent's Clinical School, Darlinghurst, Sydney, Australia.,Clinical Informatics & Research Centre, Department of Oncology, St. Vincent's Hospital, Sydney, Australia
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Kucab JE, van Steeg H, Luijten M, Schmeiser HH, White PA, Phillips DH, Arlt VM. TP53 mutations induced by BPDE in Xpa-WT and Xpa-Null human TP53 knock-in (Hupki) mouse embryo fibroblasts. Mutat Res 2015; 773:48-62. [PMID: 25847421 PMCID: PMC4547099 DOI: 10.1016/j.mrfmmm.2015.01.013] [Citation(s) in RCA: 34] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/08/2014] [Revised: 12/18/2014] [Accepted: 01/18/2015] [Indexed: 01/13/2023]
Abstract
Somatic mutations in the tumour suppressor gene TP53 occur in more than 50% of human tumours; in some instances exposure to environmental carcinogens can be linked to characteristic mutational signatures. The Hupki (human TP53 knock-in) mouse embryo fibroblast (HUF) immortalization assay (HIMA) is a useful model for studying the impact of environmental carcinogens on TP53 mutagenesis. In an effort to increase the frequency of TP53-mutated clones achievable in the HIMA, we generated nucleotide excision repair (NER)-deficient HUFs by crossing the Hupki mouse with an Xpa-knockout (Xpa-Null) mouse. We hypothesized that carcinogen-induced DNA adducts would persist in the TP53 sequence of Xpa-Null HUFs leading to an increased propensity for mismatched base pairing and mutation during replication of adducted DNA. We found that Xpa-Null Hupki mice, and HUFs derived from them, were more sensitive to the environmental carcinogen benzo[a]pyrene (BaP) than their wild-type (Xpa-WT) counterparts. Following treatment with the reactive metabolite of BaP, benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), Xpa-WT and Xpa-Null HUF cultures were subjected to the HIMA. A significant increase in TP53 mutations on the transcribed strand was detected in Xpa-Null HUFs compared to Xpa-WT HUFs, but the TP53-mutant frequency overall was not significantly different between the two genotypes. BPDE induced mutations primarily at G:C base pairs, with approximately half occurring at CpG sites, and the predominant mutation type was G:C>T:A in both Xpa-WT and Xpa-Null cells. Further, several of the TP53 mutation hotspots identified in smokers' lung cancer were mutated by BPDE in HUFs (codons 157, 158, 245, 248, 249, 273). Therefore, the pattern and spectrum of BPDE-induced TP53 mutations in the HIMA are consistent with TP53 mutations detected in lung tumours of smokers. While Xpa-Null HUFs exhibited increased sensitivity to BPDE-induced damage on the transcribed strand, NER-deficiency did not enhance TP53 mutagenesis resulting from damage on the non-transcribed strand in this model.
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Affiliation(s)
- Jill E Kucab
- Analytical and Environmental Sciences Division, MRC-PHE Centre for Environment & Health, King's College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, United Kingdom.
| | - Harry van Steeg
- Center for Health Protection, National Institute for Public Health and the Environment (RIVM), Bilthoven 3721 MA, The Netherlands
| | - Mirjam Luijten
- Center for Health Protection, National Institute for Public Health and the Environment (RIVM), Bilthoven 3721 MA, The Netherlands
| | - Heinz H Schmeiser
- Division of Radiopharmaceutical Chemistry (E030), German Cancer Research Center (DKFZ), Im Neuenheimer Feld 280, 69120 Heidelberg, Germany
| | - Paul A White
- Environmental Health Science and Research Bureau, Health Canada, 50 Colombine Driveway, Ottawa, Ontario K1A 0K9, Canada
| | - David H Phillips
- Analytical and Environmental Sciences Division, MRC-PHE Centre for Environment & Health, King's College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, United Kingdom
| | - Volker M Arlt
- Analytical and Environmental Sciences Division, MRC-PHE Centre for Environment & Health, King's College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, United Kingdom
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Collings CK, Waddell PJ, Anderson JN. Effects of DNA methylation on nucleosome stability. Nucleic Acids Res 2013; 41:2918-31. [PMID: 23355616 PMCID: PMC3597673 DOI: 10.1093/nar/gks893] [Citation(s) in RCA: 70] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Methylation of DNA at CpG dinucleotides represents one of the most important epigenetic mechanisms involved in the control of gene expression in vertebrate cells. In this report, we conducted nucleosome reconstitution experiments in conjunction with high-throughput sequencing on 572 KB of human DNA and 668 KB of mouse DNA that was unmethylated or methylated in order to investigate the effects of this epigenetic modification on the positioning and stability of nucleosomes. The results demonstrated that a subset of nucleosomes positioned by nucleotide sequence was sensitive to methylation where the modification increased the affinity of these sequences for the histone octamer. The features that distinguished these nucleosomes from the bulk of the methylation-insensitive nucleosomes were an increase in the frequency of CpG dinucleotides and a unique rotational orientation of CpGs such that their minor grooves tended to face toward the histones in the nucleosome rather than away. These methylation-sensitive nucleosomes were preferentially associated with exons as compared to introns while unmethylated CpG islands near transcription start sites became enriched in nucleosomes upon methylation. The results of this study suggest that the effects of DNA methylation on nucleosome stability in vitro can recapitulate what has been observed in the cell and provide a direct link between DNA methylation and the structure and function of chromatin.
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Affiliation(s)
- Clayton K Collings
- Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA
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