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      Oocyte ageing and epigenetics

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          Abstract

          It has become a current social trend for women to delay childbearing. However, the quality of oocytes from older females is compromised and the pregnancy rate of older women is lower. With the increased rate of delayed childbearing, it is becoming more and more crucial to understand the mechanisms underlying the compromised quality of oocytes from older women, including mitochondrial dysfunctions, aneuploidy and epigenetic changes. Establishing proper epigenetic modifications during oogenesis and early embryo development is an important aspect in reproduction. The reprogramming process may be influenced by external and internal factors that result in improper epigenetic changes in germ cells. Furthermore, germ cell epigenetic changes might be inherited by the next generations. In this review, we briefly summarise the effects of ageing on oocyte quality. We focus on discussing the relationship between ageing and epigenetic modifications, highlighting the epigenetic changes in oocytes from advanced-age females and in post-ovulatory aged oocytes as well as the possible underlying mechanisms.

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          Most cited references 168

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          Conversion of 5-methylcytosine to 5-hydroxymethylcytosine in mammalian DNA by MLL partner TET1.

          DNA cytosine methylation is crucial for retrotransposon silencing and mammalian development. In a computational search for enzymes that could modify 5-methylcytosine (5mC), we identified TET proteins as mammalian homologs of the trypanosome proteins JBP1 and JBP2, which have been proposed to oxidize the 5-methyl group of thymine. We show here that TET1, a fusion partner of the MLL gene in acute myeloid leukemia, is a 2-oxoglutarate (2OG)- and Fe(II)-dependent enzyme that catalyzes conversion of 5mC to 5-hydroxymethylcytosine (hmC) in cultured cells and in vitro. hmC is present in the genome of mouse embryonic stem cells, and hmC levels decrease upon RNA interference-mediated depletion of TET1. Thus, TET proteins have potential roles in epigenetic regulation through modification of 5mC to hmC.
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            Epigenetic differences arise during the lifetime of monozygotic twins.

            Monozygous twins share a common genotype. However, most monozygotic twin pairs are not identical; several types of phenotypic discordance may be observed, such as differences in susceptibilities to disease and a wide range of anthropomorphic features. There are several possible explanations for these observations, but one is the existence of epigenetic differences. To address this issue, we examined the global and locus-specific differences in DNA methylation and histone acetylation of a large cohort of monozygotic twins. We found that, although twins are epigenetically indistinguishable during the early years of life, older monozygous twins exhibited remarkable differences in their overall content and genomic distribution of 5-methylcytosine DNA and histone acetylation, affecting their gene-expression portrait. These findings indicate how an appreciation of epigenetics is missing from our understanding of how different phenotypes can be originated from the same genotype.
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              Perceptions of epigenetics.

              Geneticists study the gene; however, for epigeneticists, there is no obvious 'epigene'. Nevertheless, during the past year, more than 2,500 articles, numerous scientific meetings and a new journal were devoted to the subject of epigenetics. It encompasses some of the most exciting contemporary biology and is portrayed by the popular press as a revolutionary new science--an antidote to the idea that we are hard-wired by our genes. So what is epigenetics?
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                Author and article information

                Affiliations
                Reproductive Medicine Center, Henan Provincial People's Hospital , #7 Weiwu Road, Jinshui District, Zhengzhou, Henan Province, 450003, People's Republic of China
                State Key Laboratory of Reproductive Biology , Institute of Zoology, Chinese Academy of Sciences , #1 Beichen West Road, Chaoyang District, Beijing, 100101, People's Republic of China
                Reproductive Medicine Center, People's Hospital of Zhengzhou University , Zhengzhou, Henan Province, 450003, People's Republic of China
                Department of Veterinary Pathobiology , University of Missouri , Columbia, Missouri, 65211, USA
                Author notes
                Correspondence should be addressed to C-L Zhang; zcl6086@ 123456gmail.com or to Q-Y Sun; Email: sunqy@ 123456ioz.ac.cn
                Journal
                Reproduction
                Reproduction
                REPRO
                Reproduction (Cambridge, England)
                Bioscientifica Ltd (Bristol )
                1470-1626
                1741-7899
                March 2015
                15 July 2014
                : 149
                : 3
                : R103-R114
                REP140242
                10.1530/REP-14-0242
                4397590
                25391845
                © 2015 The authors

                This work is licensed under a Creative Commons Attribution 3.0 Unported License

                Categories
                Review

                Obstetrics & Gynecology

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