Genome-wide erasure of DNA cytosine-5 methylation has been reported to occur along the paternal pronucleus in fertilized oocytes in an apparently replication-independent manner, but the mechanism of this reprogramming process has remained enigmatic. Recently, considerable amounts of 5-hydroxymethylcytosine (5hmC), most likely derived from enzymatic oxidation of 5-methylcytosine (5mC) by TET proteins, have been detected in certain mammalian tissues. 5hmC has been proposed as a potential intermediate in active DNA demethylation. Here, we show that in advanced pronuclear-stage zygotes the paternal pronucleus contains substantial amounts of 5hmC but lacks 5mC. The converse is true for the maternal pronucleus, which retains 5mC but shows little or no 5hmC signal. Importantly, 5hmC persists into mitotic one-cell, two-cell, and later cleavage-stage embryos, suggesting that 5mC oxidation is not followed immediately by genome-wide removal of 5hmC through excision repair pathways or other mechanisms. This conclusion is supported by bisulfite sequencing data, which shows only limited conversion of modified cytosines to cytosines at several gene loci. It is likely that 5mC oxidation is carried out by the Tet3 oxidase. Tet3 , but not Tet1 or Tet2 , was expressed at high levels in oocytes and zygotes, with rapidly declining levels at the two-cell stage. Our results show that 5mC oxidation is part of the early life cycle of mammals.

In this Opinion article, we summarize how changes in DNA methylation occur during aging in mammals and discuss examples of how such events may contribute to the aging process. We explore mechanisms that could facilitate DNA methylation changes in a site-specific manner and highlight a model in which region-specific DNA hypermethylation during aging is facilitated in a competitive manner by destabilization of the Polycomb repressive complex.

DNA methylation in mammals is highly dynamic during germ cell and preimplantation development but is relatively static during the development of somatic tissues. 5-hydroxymethylcytosine (5hmC), created by oxidation of 5-methylcytosine (5mC) by Tet proteins and most abundant in the brain, is thought to be an intermediary toward 5mC demethylation. We investigated patterns of 5mC and 5hmC during neurogenesis in the embryonic mouse brain. 5hmC levels increase during neuronal differentiation. In neuronal cells, 5hmC is not enriched at enhancers but associates preferentially with gene bodies of activated neuronal function-related genes. Within these genes, gain of 5hmC is often accompanied by loss of H3K27me3. Enrichment of 5hmC is not associated with substantial DNA demethylation, suggesting that 5hmC is a stable epigenetic mark. Functional perturbation of the H3K27 methyltransferase Ezh2 or of Tet2 and Tet3 leads to defects in neuronal differentiation, suggesting that formation of 5hmC and loss of H3K27me3 cooperate to promote brain development.

Ultraviolet (UV) irradiation from the sun has been epidemiologically and mechanistically linked to skin cancer, a spectrum of diseases of rising incidence in many human populations. Both non-melanoma and melanoma skin cancers are associated with sunlight exposure. In this review, we discuss the UV wavelength-dependent formation of the major UV-induced DNA damage products, their repair and mutagenicity and their potential involvement in sunlight-associated skin cancers. We emphasize the major role played by the cyclobutane pyrimidine dimers (CPDs) in skin cancer mutations relative to that of (6–4) photoproducts and oxidative DNA damage. Collectively, the data implicate the CPD as the DNA lesion most strongly involved in human cancers induced by sunlight.