title: Abnormal DNA methylation in oocytes could be associated with a decrease in reproductive potential in old mice.
abstract: OBJECTIVE
  This study was designed to evaluate DNA methylation and the expression of DNA methyltransferases (Dnmt1, Dnmt3a, Dnmt3b and Dnmt3L) in metaphaseII (MII) oocytes and the DNA methylation of pre-implantation embryos during mouse aging to address whether such aging-related changes are associated with decreased reproductive potential in aged mice.
  
  
  METHODS
  Oocytes (MII) from 6 to 8 weeks old female mice are referred to as the 'young group'; oocytes from the same group that were maintained until 35-40 weeks old are referred to as the 'old group.' The oocytes were fertilized both in vitro and in vivo to obtain embryos. The DNA methylation levels in the oocytes (MII) and pre-implantation embryos were assessed using fluorescence staining. The expression levels of the Dnmt genes in the oocytes (MII) were assessed using Western blotting.
  
  
  RESULTS
  The DNA methylation levels in the oocytes and pre-implantation embryos (in vivo and in vitro) decreased significantly during the aging of the mice. The expression levels of all of the examined Dnmt proteins in the old group were lower than young group. Both the cleavage and blastocyst rate were significantly lower in the oocytes of the older mice (69.9 % vs. 80.9 %, P < 0.05; 33.9 % vs. 56.4 %, P < 0.05). The pregnancy rate of the old mice was lower than that of the young mice (46.7 % vs. 100 %, P < 0.05). The stillbirth and fetal malformation rate was significantly higher in the old group than in the young group (17.2 % vs. 2.9 %, P < 0.05).
  
  
  CONCLUSIONS
  The decreased expression of Dnmt1, Dnmt3a, Dnmt3b and Dnmt3L in oocytes (MII) and the change of genome-wide DNA methylation in oocytes and pre-implantation embryos due to aging may be related to lower reproductive potential in old female mice.
authors.affiliation: Key Laboratory of Animal Genetics, Breeding and Reproduction, Ministry of Agriculture and National Engineering Laboratory for Animal Breeding, College of Animal Science and Technology, China Agricultural University, Beijing, People's Republic of China.
authors.forename: Ming-xing
authors.initials: MX
authors.lastname: Yue
authors.forename: Xiang-wei
authors.initials: XW
authors.lastname: Fu
authors.forename: Guang-bin
authors.initials: GB
authors.lastname: Zhou
authors.forename: Yun-peng
authors.initials: YP
authors.lastname: Hou
authors.forename: Ming
authors.initials: M
authors.lastname: DU
authors.forename: Liang
authors.initials: L
authors.lastname: Wang
authors.forename: Shi-en
authors.initials: SE
authors.lastname: Zhu
mesh_terms: D000367:Age Factors; D000375:Aging / Q000235:genetics*; D000818:Animals; D001755:Blastocyst / Q000166:cytology / Q000502:physiology*; D019175:DNA Methylation; D015254:DNA Modification Methylases / Q000096:biosynthesis / Q000235:genetics; D047108:Embryonic Development; D005260:Female; D051379:Mice; D009865:Oocytes / Q000166:cytology* / Q000502:physiology*; D011247:Pregnancy
chemical_list: D015254:DNA Modification Methylases