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Causes and Consequences of Aneuploidy
Summary: Angelika Amon is interested in characterizing the regulatory networks that control chromosome segregation during mitosis and meiosis and examining the consequences of errors in these processes on the cellular and organismal physiology.
We study the mechanisms that prevent chromosome missegregation and the consequences of aneuploidy on cell growth and proliferation. We focus on how the anaphase-G1 transition, also known as exit from mitosis, is regulated and integrated with other cell cycle events, and on how a specialized cell division, the meiotic cell division, is established. Furthermore, we study the consequences of aneuploidy on cell growth and proliferation in yeast and mammals.
Regulation of Exit from Mitosis
Exit from mitosis is triggered by the inactivation of mitotic cyclin-dependent kinases (CDKs). We showed that this inactivation of CDKs is brought about by the conserved protein phosphatase Cdc14. Our studies furthermore revealed an intricate regulation of this phosphatase. Cdc14 is regulated by the inhibitor Cfi1 (also known as Net1), which binds to and sequesters Cdc14 in the nucleolus during G1, S phase, G2, and metaphase. During anaphase, Cdc14 is released from its inhibitor and spreads throughout the nucleus and cytoplasm, where it dephosphorylates its targets. Two pathways control the association between Cdc14 and its inhibitor. The Cdc14 early-anaphase release network (FEAR network) promotes Cdc14 release from the nucleolus during early anaphase. The mitotic exit network (MEN) maintains Cdc14 in its released state during late stages of anaphase. The MEN resembles a Ras-like signaling cascade, and its activity is controlled by nuclear position. Currently, we are investigating the mechanisms whereby nuclear position and other cellular events control the MEN and determine how this signal is transmitted through the pathway.
Regulation of the Meiotic Cell Cycle
Meiosis leads to the formation of gametes. During the meiotic cell cycle, a single S phase is followed by two consecutive nuclear divisions. During meiosis I, separation of homologous chromosomes occurs; segregation of sister chromatids takes place during meiosis II. For the meiotic chromosome segregation program to succeed, protein complexes known as cohesin complexes, which hold sister chromatids together, must be lost from chromosomes in a stepwise manner: from chromosome arms during meiosis I and from centromeric regions during meiosis II. Furthermore, kinetochore orientation must change during meiosis. Sister kinetochores have to attach to microtubules so that they face the same spindle pole (coorientation) during meiosis I. During meiosis II, sister kinetochores attach to microtubules emanating from opposite poles (biorientation). Our study of the regulation of the meiotic stepwise loss of cohesion and meiosis I sister kinetochore coorientation is focused on the regulation of sister-chromatid cohesion factors and coorientation factors by phosphorylation. These studies show that the Polo kinase Cdc5 and CDKs are key regulators of both events. Furthermore, we are defining conditions sufficient to induce the stepwise loss of cohesion and sister kinetochore coorientation during mitosis.
We are also interested in the relationship between organismal age and meiosis. In humans, the fidelity of gamete formation decreases dramatically with age. We recently found that age also affects the meiotic program in budding yeast. Our studies show that aged mother cells exhibit a decreased ability to initiate the meiotic program and fail to express the meiotic inducer IME1. The few aged mother cells that do enter meiosis complete this developmental program but exhibit defects in meiotic chromosome segregation and spore formation. Furthermore, we find that mutations that extend the life span of yeast cells also extend the sexual reproductive life span. We are investigating how cellular age and developmental signals control entry into the meiotic program.
Effects of Aneuploidy on Normal Physiology and Tumorigenesis
Aneuploidy, a change in chromosome number that is not a multiple of the haploid complement, is frequently associated with disease in humans. The leading cause of miscarriages and mental retardation, aneuploidy is also associated with cancer. More than 90 percent of all solid human tumors are aneuploid. We study the effects of aneuploidy on cellular and organismal physiology in budding yeast and in the mouse, with a particular focus on the relationship between aneuploidy and disease, especially cancer.
Effects of aneuploidy on cellular physiology in yeast. We have created a collection of haploid yeast strains that each bear an extra copy of one or more of almost all of the yeast chromosomes. Characterization of these strains revealed that the additional chromosomes are active and elicit a number of phenotypes that are independent of the identity of the additional chromosome. These shared phenotypes include defects in cell cycle progression, increased demand for energy, and sensitivity to conditions interfering with protein synthesis, turnover, and folding. These phenotypes are observed only in strains carrying additional yeast chromosomes, indicating that they reflect the consequences of imbalances in cellular protein composition. Our studies suggest that aneuploidy puts significant stress on the cell, which responds to this condition. We continue to characterize the effects of aneuploidy in cells, with special emphasis on the protein degradation and folding machinery. We are also characterizing mutations that allow yeast cells to tolerate aneuploidy or that specifically kill aneuploid cells. This characterization may not only provide insights into the effects of aneuploidy on yeast cell proliferation but also reveal mechanisms whereby tumor cells, which are frequently aneuploid, escape the adverse effects this condition brings with it.
Trisomy as a model for the effects of aneuploidy on mammalian cell physiology and tumorigenesis. We also examined the consequences of aneuploidy on cellular proliferation and physiology in the mouse. We generated mouse embryos trisomic for chromosome 1, 13, 16, or 19. Analysis of the cell lines established from these aneuploid embryos revealed that cell proliferation was hampered compared to euploid controls. Furthermore, the characterization of the trisomic mouse embryonic fibroblasts (MEFs) revealed a number of shared traits that include increased cell size and metabolic alterations. These changes resemble the phenotypes observed in yeast strains carrying one or several additional chromosomes, suggesting that in the mouse too, aneuploidy results in a stress response. We are using chimeric approaches to examine the consequences of trisomy in vivo. Furthermore, we are identifying genes that, when inactivated, selectively hamper the proliferation of aneuploid cells. The characterization of these genes may provide new avenues for the treatment of cancer.
This work is supported in part by a grant from the National Institutes of Health.
Last updated June 23, 2009
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