Kops Group

Research

The main aim of our research is to understand how the cell division process gives rise to two genetically identical daughter cells. We are particularly interested in the processes that ensure correct chromosome segregation during mitosis. This is not only fascinating from a molecular cell biological perspective (how does a cell do that?) but also has implications for health and disease: errors in chromosome segregation is a major cause for birth defects and embryonic lethality in humans, and the most common genetic alteration in human tumors is aberrant chromosome numbers, aka aneuploidy. Aneuploidy furthermore shows strong correlations with tumor aggressiveness, therapy resistance, and tumor recurrence, and as such with poor overall prognosis for the patient.

Figure: Chromosomes from a single healthy cell (left) or cancer cell (right). Note the extensive alterations to both chromosome numbers and chromosome integrity in the cancer cell. Images from Janssen et al., Science 333, 1895-8 (2011)

Some Mitosis Essentials

During the mitotic phase of the cell cycle, the duplicated genome in the form of condensed chromosomes is partitioned to the daughter cells. From S-phase onward, the two copies of each chromosome (referred to as sister chromatids, or sister) are connected by molecular rings and each copy attempts to establish attachments to spindle microtubules that emanate from two spindle poles. If one sister has attached to one pole, the other needs to attach to the other pole. Since the two poles are on opposite sides of the cell, such a ‘bipolar’ attachment allows the cell to drag each sister to opposite sides as soon as the linkages are removed. The cell only initiates this phase of sister separation when each and every chromosome is attached in a bipolar fashion. After chromosome segregation, the cell creates a cleavage site in between the two segregating genomes, resulting in the generation of two distinct but genetically identical daughter cells.

The process to generate a bipolar attachment is known as biorientation. Biorientation is a highly dynamic and relatively fast process that is an interplay between the microtubule-based pulling apparatus (aka the mitotic spindle), the spindle attachment site on each sister (aka the kinetochore), and proteins that regulate the interaction between kinetochore and spindle. To prevent genetic imbalances after cell division, the cell has to make sure that all 46 duplicated chromosomes (in case of a diploid human cell) are bioriented and ready to be pulled in opposite directions. For this, it has evolved a surveillance mechanism (aka a checkpoint) that monitors the attachments status of each individual kinetochore to spindle microtubules. The cell is not allowed to proceed with chromosome segregation if even a single kinetochore is not properly attached to the spindle (and thus the chromosome pair not properly bioriented). Our investigations focus on the proteins and signaling networks that regulate chromosome biorientation and the mitotic checkpoint.

Figure: Overall theme of fundamental research question in the lab.

Projects

Molecular Principles of the Mitotic Checkpoint and Chromosomes Biorientation Molecular Principles of the Mitotic Checkpoint and Biorientation The unattached kinetochore generates a signal that prevents mitotic progression. In reality, the signal is a protein complex that inhibits an enzyme that initiates the chromosome segregation process. This enzyme is known as the Anaphase Promoting Complex/Cyclosome (APC/C), a degradation machine (E3 ubiquitin ligase) that destroys key proteins that keep a cell in mitosis and the sisters linked. We wish to know how the checkpoint transmits the attachment status of kinetochores to inhibition of the APC/C. We furthermore wish to understand how correct, end-on attachments are generated and how the machineries that promote attachment and that correct attachment errors interact with the mitotic checkpoint. For recent examples of our work, see Suijkerbuijk et al (in press), Suijkerbuijk et al and Saurin et al.

Mitotic Checkpoint Silencing As soon as all chromosomes have bioriented, generation of the inhibitory complex from the kinetochores needs to be shut down and the inhibitory complex itself disassembled. Various mechanisms contribute to checkpoint silencing, including but not limited to transport-mediated depletion of proteins from kinetochores, and inhibitory ‘capping’ of an essential checkpoint protein by another protein called p31comet. [Our recent data] has shown that Mps1 can counter one or more of these activities and that removal of Mps1 from kinetochores is a prerequisite for checkpoint silencing. We wish to understand the regulation of checkpoint silencing in a broad sense: what activities are turned on or off when the cell is ready to undergo chromosome segregation, and what do these activities accomplish on a molecular level. For recent examples of our work, see Kops et al and Jelluma et al.

Mitotic Checkpoint Evolution Mitotic checkpoint evolution In collaboration with the group of Berend Snel at the department of theoretical biology at Utrecht University, we are tracing origins and evolution of the major checkpoint and kinetochore protein modules, with the goal of defining the ancient machineries and pinpoint conserved and acquired functional domains and motifs. For recent examples of our work, see Suijkerbuijk et al and Vleugel et al.

figure: Identification of mitotic checkpoint protein homologs in various species throughout the six supergroups of eukaryotes. Image from Vleugel et al. Dev Cell 23, 239-50 (2012).

APC/C Biology The APC/C is the most complex E3 ubiquitin ligase, consisting of at least 14 subunits. Of these, 12 are conserved in eukaryotes, while 2 subunits appear to be specific to certain lineages. We recently discovered an APC/C subunit, APC16, that is present in animals and plants but not fungi. APC16 is essential for APC/C function in mitosis, as APC16 RNAi caused mitotic arrest due to inability to degrade the mitotic regulators. We are interested in examining APC/C biology with particular focus on how it is inhibited by the checkpoint and how it promotes checkpoint silencing. 

Aneuploidy and Cancer Aneuploidy (an abnormal amount of chromosomes) is the most common genetic alteration in human tumors and a major cause for birth defects. Aneuploidy is a manifestation of the Chromosomal Instability (CIN) phenotype: the tendency to make frequent errors in chromosome segregation. CIN impacts on tumor formation or tumor development by promoting loss of tumor suppressor function.


Causes of aneuploidy  
Mutations in BUBR1 cause the hereditary cancer predisposition syndrome MVA (OMIM 257300). We wish to understand how these mutations disable BUBR1 function. Furthermore, deregulation of the mitotic kinase MPS1 is strongly correlated to poor clinical outcome, and we wish to understand how MPS1 is deregulated in cancer cells and what the consequences of this are for cell division. For recent examples of our work, see Suijkerbuijk et al and Jelluma et al.

Consequences of aneuploidy  Besides causing gains and losses of whole chromosomes, we have recently shown that CIN also causes secondary genomic alterations including DNA double strand breaks that lead to chromosomal translocations (see Janssen et al). This shows that a missegregation event can have extensive consequences for genome integrity.

Design of novel cancer therapy  Importantly, the extensive genomic damage described above can be the Achilles heel of the cancer cell. One the one hand, it has used it to drive tumor cell evolution and still needs it for adaptation purposes. On the other hand, aggravating the damage will cause cell death. Enhancing chromosome segregation errors indeed causes cell death, and does so more efficiently in cancer cells than in tumor cells. This strategy is therefore a potential novel cancer therapy. Tumor mouse models are being examined for tumor cell death after conditional and tissue-specific reduction in Mps1 activity and treatment with therapeutically relevant spindle poisons. For recent examples of our work, see Janssen et al, Jelluma et al and Sliedrecht et al.