Physiological/molecular causes of age-related egg aneuploidy

The maternal age effect is the phenomenon that pregnancies of trisomic fetuses like Down syndrome increase dramatically with maternal age. Trisomic pregnancies occur at a frequency of around 3% for women in their 20s, the frequency rises steeply for women in their 30s and exceeds 30% for women in their 40s. Trisomies are the leading cause of mental retardation and miscarriages. A trisomy, or three copies of a chromosome, is most often caused by cell division errors in egg precursor cells called oocytes, leading to the inheritance of two rather than one copy of a maternal chromosome in the embryo. The third copy is provided by sperm upon fertilization. Although it has been known for decades that chromosome segregation errors in oocytes lead to trisomic pregnancies, what has remained a mystery is why these errors increase with maternal age. It is likely that part of the answer can be found in the special life span of oocytes. Unlike other cells, oocytes are generated during fetal development and arrest for months in the mouse and decades in the human until ovulation triggers the division of the adult oocyte. Therefore, the oocytes of a 30-year-old women are potentially 30-years-old. This poses a particular challenge for a protein complex called cohesin, which embraces replicated chromosomes during oogenesis in the fetus and holds these chromosomes together until the oocyte divides. Unknown mechanisms cause an age-related loss of this protein complex and this is thought to eventually lead to chromosome segregation errors and aneuploid eggs, with too many or too few chromosomes. The age-related protein loss occurs over different time scales in mouse and presumably human, suggesting that physiological rather than chronological ageing is important. We propose a new encompassing hypothesis to explain the molecular and physiological causes of the maternal age effect. Since the wound-generating process of ovulation releases reactive oxygen species (ROS) that can attack and damage proteins, we hypothesize that ovulations cause cumulative damage to cohesin, leading to its loss, and thereby ovulation frequency contributes to maternal age-related egg aneuploidy and infertility. We will test this hypothesis using several approaches and study the effects on chromosome segregation by time-lapse microscopy of live mouse oocytes. We will also use genetically modified mouse strains to test the contribution of ROS to division errors in oocytes. Lastly, we will use our state-of-the-art genomics method to study how genome organization changes in truly aged cells, namely >30-year-old human oocytes. Altogether, these studies will provide new insights into the molecular and physiological causes of the maternal age effect. A better understanding of this phenomenon has important implications for womens reproductive health in a society with an increasing trend towards delaying childbirth to maternal age of 30 or older.

The "maternal age effect" is the phenomenon that describes the dramatically increased frequency of conceived embryos, which carry an abnormal number of chromosomes, with maternal age. The causes of this have remained enigmatic. The cells that can be used for fertilization are generated during embryonic life and the available pool, located in the ovary, is defined soon after birth. These cells age together with the entire body during its lifetime and cannot be replenished. Once puberty begins, several of those cells are recruited, leave the ovary with ovulation, and undergo a cell division process necessary for the generation of the egg that can be fertilized. When eggs are fertilized after such errors, they can generate embryos that are incompatible with life with few exceptions such as trisomy 21 (Down's syndrome). We considered that the ovary, where the eggs reside, can be thought to be undergoing accelerated aging since ovulations involve the rupture of the ovarian tissue, for the eggs to exit, and requires subsequent repair of the ruptured tissue. For ovulation to take place an equilibrium of oxidants and antioxidants is required. Oxidants can damage important molecules within the cells, some permanently, while antioxidants help protect against oxidant damage. It is conceivable that with age there is accumulation of damage to the ovarian tissue which hosts the finite number of eggs to be released for fertilization at a later stage of reproductive life. Could the ovulation process or a reduction of antioxidant protection contribute to the observed errors during the production of eggs for fertilization? To answer this, we used five different experimental conditions. In three conditions we limited the number of ovulations. In the remaining two, we perturbed the availability of protective antioxidants. All three methods used to suppress the ovulatory process led to a reduction in the frequency of errors during egg production, suggesting that the ovulation process contributes to the "maternal age effect". Conversely, the conditions that reduced protective antioxidant molecules led to an increase in errors within the ovulated eggs. The conclusions of our work are that ovulations contribute to the "maternal age effect" and egg ageing can be delayed in mice. An implication of this work is that long-term ovulation-suppressing conditions can potentially reduce the risk of errors occurring during the preparation of eggs for fertilization, at advanced maternal age.