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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 cell's 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 cycle, the meiotic cell cycle, 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). In 1998, we showed that this inactivation of CDKs is brought about by the conserved protein phosphatase Cdc14. We also found that 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.
Subsequently, we identified two pathways that 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. We are investigating the mechanisms whereby nuclear position regulates the MEN and determining how this signal is transmitted through the pathway. (This work is also supported by a grant from the National Institutes of Health.)
Regulation of the Meiotic Cell Cycle
Meiosis leads to the formation of gametes. Defects in meiotic chromosome segregation are the leading cause of miscarriages and one of the leading causes of birth defects in humans. 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 that 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 changes during meiosis. Sister kinetochores 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). We study how the stepwise loss of cohesins and kinetochore orientation are regulated during meiosis.
Stepwise loss of cohesins during meiosis. The stepwise loss of cohesins is essential for meiotic chromosome segregation. A screen aimed at discovering genes required for this loss of cohesins identified IML3, CHL4, and SGO1. All three proteins localize to centromeric regions, suggesting that they are intimately involved in maintaining cohesins around centromeres during meiosis I. We furthermore showed that phosphorylation of the cohesin subunit Rec8, the cohesin protector Sgo1, and meiotic recombination function together to bring about the stepwise loss of cohesins from chromosomes. We are now determining how Sgo1 singles out cohesins around kinetochores to prevent their removal during meiosis I, and how the protein is itself regulated.
Sister kinetochore orientation during meiosis. Kinetochores of sister chromatids attach to microtubules emanating from the same pole (coorientation) during meiosis I and to microtubules emanating from opposite poles (biorientation) during meiosis II. We recently found that the Aurora B kinase Ipl1 regulates kinetochore-microtubule attachment during both meiotic divisions and that a complex known as the monopolin complex ensures that the protein kinase coorients sister chromatids during meiosis I. Furthermore, the defining of conditions sufficient to induce sister kinetochore coorientation during mitosis provided insight into the function of the monopolin complex, which joins sister kinetochores independently of cohesins. We propose that this function helps Aurora B to coorient sister chromatids during meiosis I.
By generating meiosis-specific loss-of-function alleles we were also able to characterize the role of the polo kinase Cdc5 in sister kinetochore coorientation. In the absence of CDC5, sister kinetochores attach to microtubules emanating from opposite poles rather than the same pole. In addition, proteins required for proper kinetochore orientation, such as Mam1, are mislocalized in Cdc5-depleted cells. We are addressing the molecular mechanisms whereby the monopolin complex is targeted to kinetochores and how it fuses sister kinetochores. (This work is supported by a grant from the National Institutes of Health.)
The Effects of Aneuploidy on Cell Proliferation
The importance of understanding the effects of aneuploidy on cell growth and proliferation is highlighted by the observation that aneuploidy is frequently found in tumor cells. Recently, we have begun to investigate what happens to yeast cells that acquire extra chromosomes and hence are aneuploid. 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 they share a number of phenotypes, including defects in cell cycle progression, increased glucose uptake, and sensitivity to conditions interfering with protein synthesis and folding. These phenotypes are observed only in strains carrying additional yeast genes, indicating that they reflect the consequences of additional transcription and translation, as well as the resulting imbalances in cellular protein composition.
We conclude that aneuploidy causes not only a proliferative disadvantage but also a set of phenotypes that is independent of the identity of the individual extra chromosomes. We are now in the process of identifying the mechanisms that cause these phenotypes and isolating mutants that allow yeast cells to tolerate aneuploidy or that specifically kill aneuploid cells. As with our work on chromosome segregation, we will also test whether the mechanisms that select against aneuploidy in yeast operate in mammalian cells. These studies may shed light on how aneuploidy is tolerated in tumor cells.
Last updated: June 18, 2007
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