Oxidation products of 5-methyl cytosine are decreased in senescent cells and tissues of progeroid mice

5-Hydroxymethylcytosine and 5-formylcytosine are stable DNA base modifications generated from 5-methylcytosine by the ten-eleven translocation protein family that function as epigenetic markers. 5-Hydroxymethyluracil may also be generated from thymine by ten-eleven translocation enzymes. Here, we asked if these epigenetic changes accumulate in senescent cells, since they are thought to be inversely correlated with proliferation. Testing this in ERCC1-XPF-deficient cells and mice also enabled discovery if these DNA base changes are repaired by nucleotide excision repair. Epigenetic marks were measured in proliferating, quiescent and senescent wild-type (WT) and Ercc1−/− primary mouse embryonic fibroblasts. The pattern of epigenetic marks depended more on the proliferation status of the cells than their DNA repair capacity. The cytosine modifications were all decreased in senescent cells compared to quiescent or proliferating cells, whereas 5-(hydroxymethyl)-2′-deoxyuridine was increased. In vivo, both 5-(hydroxymethyl)-2′-deoxyuridine and 5-(hydroxymethyl)-2′-deoxycytidine were significantly increased in liver tissues of aged WT mice compared to young adult WT mice. Livers of Ercc1-deficient mice with premature senescence and aging had reduced level of 5-(hydroxymethyl)- 2′-deoxycytidine and 5-formyl-2′-deoxycytidine compared to aged-matched WT controls. Taken together, we demonstrate for the first time, that 5-(hydroxymethyl)-2′-deoxycytidine is significantly reduced in senescent cells and tissue, potentially yielding a novel marker of senescence

Sublethal RNA Oxidation as a Mechanism for Neurodegenerative Disease

Although cellular RNA is subjected to the same oxidative insults as DNA and other cellular macromolecules, oxidative damage to RNA has not been a major focus in investigations of the biological consequences of free radical damage. In fact, because it is largely single-stranded and its bases lack the protection of hydrogen bonding and binding by specific proteins, RNA may be more susceptible to oxidative insults than is DNA. Oxidative damage to protein-coding RNA or non-coding RNA will, in turn, potentially cause errors in proteins and/or dysregulation of gene expression. While less lethal than mutations in the genome, such sublethal insults to cells might be associated with underlying mechanisms of several chronic diseases, including neurodegenerative disease. Recently, oxidative RNA damage has been described in several neurodegenerative diseases including Alzheimer disease, Parkinson disease, dementia with Lewy bodies, and prion diseases. Of particular interest, oxidative RNA damage can be demonstrated in vulnerable neurons early in disease, suggesting that RNA oxidation may actively contribute to the onset of the disease. An increasing body of evidence suggests that, mechanistically speaking, the detrimental effects of oxidative RNA damage to protein synthesis are attenuated, at least in part, by the existence of protective mechanisms that prevent the incorporation of the damaged ribonucleotides into the translational machinery. Further investigations aimed at understanding the processing mechanisms related to oxidative RNA damage and its consequences may provide significant insights into the pathogenesis of neurodegenerative and other degenerative diseases and lead to better therapeutic strategies

Cellular level of 8-oxo-2'-deoxyguanosine in DNA does not correlate with urinary excretion of the modified base/nucleoside.

We assessed a relationship between the level of 8-oxodG in leukocyte DNA measured with the high performance liquid chromatography with electrochemical detection (HPLC/EC) technique and urinary excretion of the modified nucleoside/base analysed with a recently developed methodology involving HPLC prepurification followed by gas chromatography with isotope dilution mass spectrometric detection. No correlation was found between these markers of oxidative DNA damage commonly used in epidemiological studies. Several possible explanations of this finding are discussed

Genomic Uracil and Aberrant Profile of Demethylation Intermediates in Epigenetics and Hematologic Malignancies

DNA of all living cells undergoes continuous structural and chemical alterations resulting from fundamental cellular metabolic processes and reactivity of normal cellular metabolites and constituents. Examples include enzymatically oxidized bases, aberrantly methylated bases, and deaminated bases, the latter largely uracil from deaminated cytosine. In addition, the non-canonical DNA base uracil may result from misincorporated dUMP. Furthermore, uracil generated by deamination of cytosine in DNA is not always damage as it is also an intermediate in normal somatic hypermutation (SHM) and class shift recombination (CSR) at the Ig locus of B-cells in adaptive immunity. Many of the modifications alter base-pairing properties and may thus cause replicative and transcriptional mutagenesis. The best known and most studied epigenetic mark in DNA is 5-methylcytosine (5mC), generated by a methyltransferase that uses SAM as methyl donor, usually in CpG contexts. Oxidation products of 5mC are now thought to be intermediates in active demethylation as well as epigenetic marks in their own rights. The aim of this review is to describe the endogenous processes that surround the generation and removal of the most common types of DNA nucleobase modifications, namely, uracil and certain epigenetic modifications, together with their role in the development of hematological malignances. We also discuss what dictates whether the presence of an altered nucleobase is defined as damage or a natural modification