Causes and Consequences of Chromosomal Instability

The DNA of every organism is split into a number of distinct units, called chromosomes. During cell division, each of the daughter cells needs to obtain the correct and equal complement of the chromosomes. Nevertheless, daughter cells sometimes end up with the wrong number of chromosomes, a state called aneuploidy. Generally, aneuploidy is damaging, and often fatal to the cell. In human development, it can result in miscarriages or genetic disorders like Down syndrome. Mysteriously, this does not seem to apply to cancer cells, which survive and even thrive with aneuploidy. The first question we wish to address is: How do healthy cells prevent aneuploidy? Normally, there are mechanisms to distinguish between chromosomes that are properly aligned and those that are misaligned and therefore need correction. These mechanisms, however, do not always work. When chromosomes are not aligned, there is an increased likelihood that they are improperly distributed to daughter cells, with one cell receiving two copies and the other receiving none. Such mistakes are called chromosome missegregation events. One of the mechanisms that prevents chromosome misalignment involves the protein Aurora B. Aurora B is a member of the chromosomal passenger complex (CPC). We have been studying the CPC in the budding yeast Saccharomyces cerevisiae to determine how it identifies chromosomes that are misaligned. Budding yeast are an extremely powerful tool for performing genetic studies, which we will combine with advanced microscopy and biochemistry to gain a comprehensive view of CPC function. We believe the results will provide significant insight into the mechanisms of the CPC, which should then lead to a better understanding of how cells avoid chromosome missegregation. The second question that we will address is: How do cells evolve to survive with the missegregation of chromosomes? Missegregation can lead to a cell without any copies of a chromosome, which nearly always kills the cell. Additionally, if a cell gains a third copy of a chromosome, all of that chromosomes gene products are produced at about 50 percent higher levels. There are thousands of genes on each chromosome, each one making a little contribution in disrupting how the cell functions. We aim to identify mutations in budding yeast that allow cells to overcome the defects associated with chromosome missegregation. We will start with cells that are extremely sick due to frequent errors in chromosome alignment, and select for cells with increased health over many hundreds of generations. The short life cycle of yeast allows us to conduct such evolutionary experiments in weeks, instead of years. Identification of mutations that allow cells to cope with chromosome missegregation will enable a greater understanding of chromosome missegregation in general, and potentially reveal insights into how cancer cells mutate to survive under similar conditions.

The DNA of every organism is divided distinct units, called chromosomes. During cell division, each of the daughter cells needs to obtain the correct and equal number of chromosomes. However, daughter cells sometimes end up with the wrong number of chromosomes, a state called aneuploidy. Generally, aneuploidy is damaging, and often fatal to the cell. In human development, it can result in miscarriages or genetic disorders like Down syndrome. Paradoxically, most cancer cells have aneuploidy, and somehow survive and even thrive with it. Cancer cells contain the wrong number of chromosomes because they frequently make errors equally distributing the chromosomes to the two daughter cells. Such errors are termed chromosome missegregation events. Chromosome missegregation, and the resulting aneuploidy, would typically be very bad for cells. We therefore wanted to determine how cells could evolve to survive with the missegregation of chromosomes. We first set up a system to study aneuploid cells using a simple cell type - yeast. Some species of yeast have a similar number of chromosomes to humans, and are therefore useful for studying aneuploidy. We started with cells that were extremely sick due to frequent chromosome missegregation. We then grew them in the lab for hundreds of generations to see how they would change over time. The fast growth of yeast allowed us to conduct such evolutionary experiments in only a couple of weeks. We then sequenced the entire genomes of the yeast to determine what changes had occurred to their DNA and chromosomes. We found that there were specific chromosomes that were very frequently present with extra copies. Aneuploidy of these chromosomes decreased the amount of chromosome missegregation, indicating that aneuploidy itself contributes to the adaptation process. We next wanted to determine if similar things happen in human cells. Using the knowledge we had obtained from yeast, we set up adaptation experiments in human cells to determine if they also gain or lose specific chromosomes over time after increasing the rate of chromosome missegregation. Indeed, certain chromosomes were gained or lost in nearly every adapted cell population. Amazingly, the most frequently gained chromosomes were also the most frequently aneuploid chromosomes in cancer! We were therefore able to recapitulate aneuploidy patterns previously identified in cancer cells in human cells for the first time. Furthermore, for two of the frequently aneuploid chromosomes, we were able to identify individual genes on those chromosomes that contributed to their selection. In the future, we believe that these types of experiments will continue to provide insights into the impact of aneuploidy on cells, and we hope that such insights will provide the foundation for future cancer treatments based on the specific aneuploid chromosomes present in individual tumors.