Understanding the mechanisms of aneuploid cell elimination during early mammalian development

During early development errors in cell division causes the daughter cells to have either fewer or extra numbers of chromosomes. This condition is called aneuploidy and is highly detrimental to embryogenesis. During development, aneuploid cells are normally removed from the embryo by the quality control mechanisms that ensure tissue fitness. When this is not possible aneuploidy causes miscarriage. However, in spite of aneuploidy being the leading cause of spontaneous pregnancy loss in humans, we still know surprisingly little about the consequences of chromosomal abnormalities during embryonic development.

In this proposal we aim to identify what happens to aneuploid cells during early mammalian development. For this we will do three things:
First, we will analyse which are the most important properties of aneuploid cells during early embryo development. To achieve this, we will use the mouse embryo as our model system as its early development recapitulates many features of human development. Using this system, we will artificially induce aneuploidy in cells and embryos and analyse what types of stress pathways are activated in these cells. Once the specific stress pathways that are induced by aneuploidy are identified, these will manipulated to test if activation reproduces the adverse effects of aneuploidy. We will also test if inhibiting these stresses prevents aneuploid cell elimination in the embryo.

The second thing that we will do to study what happens to aneuploid cells during development is to analyse what signals are activated by aneuploidy induced stress and cause the death of aneuploid cells and embryos. Our previous work has identified the mTOR pathway, that is a key regulator of cell growth, as important for the elimination of abnormal cells during embryonic development. Here we will test the importance of the mTOR pathway for the elimination of aneuploid cells by asking how its repression prevents aneuploid cell propagation.

The third thing that we will do to study aneuploidy during embryogenesis is to analyse exactly when aneuploid cells are eliminated during embryo development. For this we will study the levels of aneuploidy at different stages of mouse embryogenesis.
Together we anticipate that our studies will provide a comprehensive overview of what the fate of aneuploidy is in the embryo and what are the adverse effects of aneuploidy.

Aneuploidy is a type of chromosomal aberration in which the chromosome number is abnormal. During early human development aneuploidy is remarkably frequent, with about fifty to eighty percent of pre-implantation embryos having some level of aneuploidy. In contrast to this aneuploid cells are very infrequent in embryos at later development, indicating that either cells with aneuploidy are removed from the embryo, or if they persist, they lead to embryo loss. In spite of the significant knowledge that we have gained over the last few years on how aneuploidy arises during meiosis and the early mitotic divisions, we still know very little about the type of stresses that aneuploid cells exhibit and what pathways lead to the elimination of aneuploid cells.

In this proposal we will use a mouse model of induced aneuploidy and combine experiments in embryonic stem cells with studies in the early mouse embryo to study the mechanism of aneuploid cell elimination in the embryo. For this we will aim to identify when aneuploid cells are lost during embryogenesis, as well as analyse the most prominent cell stress pathways that become activated in aneuploid cells prior to their elimination. We will also study what pathways act downstream of these cellular stresses to induce the elimination of cells with aneuploidy. Our preliminary work has found that the mTOR pathway, a major regulator of cell growth, is important for the elimination of aberrant cells during early mouse development. For this reason, here we will first study the importance of mTOR for the elimination of aneuploid cells. Together, our work will help uncover the mechanisms underlying an important cause of pregnancy cause in humans.

Our studies will help uncover the mechanisms that ensure the elimination of aneuploid cells during embryonic development. In humans 50 to 80% of embryos show mosaic aneuploidy during pre-implantation stages of development. These aneuploid cells can have two fates: they can either be eliminated from the embryo before having overt adverse effects on its development or if this is not possible cause pregnancy loss. This proposal aims to establish what determines each one of these outcomes. We will do this by shedding light into the cellular consequences of aneuploidy during early mouse embryonic development as well as into the mechanisms by which aneuploid cells are eliminated from the embryo.

The beneficiaries of this work can be classed into the following groups of people:

1) In the short term the primary beneficiaries are those with the following research interests:
- Researchers interested in understanding chromosome segregation and embryonic development.
- Clinicians interested in understanding the causes of pregnancy loss.
- As aneuploid cells frequently arise during human pluripotent stem cell (PSC) culture, our work on how aneuploid cells evade elimination during normal embryogenesis will also be relevant to those interested in the use PSCs for regenerative medicine.

2) The commercial sector and clinicians will benefit from insight into how to develop tools that are more sensitive, more reliable and less invasive than those already available to detect aneuploidy during pre-implantation development.

3) This project will train early-career researchers in emerging methodologies in embryology and stem cell biology, contributing to their career development, as well as producing individuals capable of carrying out future research in the biomedical sciences. At a time when industry is moving in the direction of interdisciplinary research, such individuals will be highly sought-after not only in academia but also in the commercial sector

4) Teachers and students interested in reproductive biology will benefit from knowledge about the fate of aneuploid cells during development and how they affect normal embryogenesis.

5) A final beneficiary of our work will be the lay public. By highlighting the medical relevance of understanding developmental processes and their potential impact on pregnancy outcome, our work will provide factual input to and therefore benefit the public discussion about the importance of basic research for human health.

The results of this research will be conveyed to other researchers through the publication of findings in peer-reviewed journals, by reporting unpublished work at conferences and through personal communication with other scientists. Though the results will primarily be disseminated through scientific journals, we will liaise with dedicated Media Teams at Imperial College to issue a press release when appropriate. We take seriously the responsibility of scientists to engage with the lay public, to raise awareness among them of the results of publicly funded research, to openly debate ethical issues relating to our research so that public opinion may be formed in an informed manner and to take the excitement of our research to the children of today, who will be the scientists of tomorrow. For these reasons we engage through the University but also as individuals in activities aimed at the public dissemination of science.