DNA damage response in in vitro matured oocytes
Atamian, Elisa Karine
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The reproductive lifetime of a woman is limited primarily by her age. The state of an oocyte represents the central determinant of the fate of an ovarian follicle as well as embryo development throughout maturation. Oocyte reserve and oocyte quality are two major determinants of the likelihood of achieving pregnancy for a woman. Assisted Reproductive Technology (ART) has provided a valuable alternative for women attempting to conceive at an older age, however even with ART the likelihood of a live birth also decreases with increased age. Mammalian oocytes undergo meiotic maturation in preparation for ovulation and fertilization. Throughout most of its lifetime the oocyte remains arrested in the dictyate stage of prophase of meiosis I (MI), also called the germinal vesicle (GV) stage, until the follicle receives a hormonal signal to progress through meiosis. Only a small fraction of the follicles present in the ovaries receive this signal, while the rest remain unresponsive. The DNA damage response (DDR) is activated in the presence of double stranded breaks (DSBs) in DNA and can induce various cellular responses including senescence or cell cycle arrest, and/or apoptosis, also known as programmed cell death. Telomeres mediate senescence in most cells. Telomeres consist of tandem DNA repeats and associated proteins, which cap and protect chromosome ends. Telomeres and their associated proteins form a loop at the ends of chromosomes, which buries them. This telomere complex is called shelterin. Shelterin prevents the ends of chromosomes from triggering a DNA damage response. However, with each round of DNA replication chromosomes lose small segments of their telomeres. Telomere attrition also can arise in non-dividing cells via the action of oxygen radicals. We hypothesize that germinal vesicle arrest, which occurs in some oocytes retrieved for ART that fail to progress through meiosis, is associated with telomere attrition and the associated cellular senescence pathway induced by DNA damage. Previous studies have identified higher levels of DNA damage foci in isolated GV arrested oocytes compared to those that progress through the meiotic cell cycle. Our studies confirm the presence of the DNA damage response (DDR) regulator, ATM, at higher levels in GV stage oocytes versus those that have matured to later stages. Immunostaining shows a near 50% increase in presence of ATM in arrested oocytes. Confirming the role of the DDR in cell cycle arrest during oocyte maturation could highlight a new target for strategies to improve ART technology and increase the likelihood of achieving pregnancy later in life.