Molekulare Mechanismen der Spindel-Kontrollpunkt-Inhibierung und der chromosomalen Instabilität

During mitosis, sister chromatids are equally segregated onto two daughter cells, thus ensuring euploidy. Chromosomal instability, the perpetual missegregation of whole chromosomes, is closely associated with aneuploidy and a major hallmark of human cancer. Therefore, it is most important to understand the molecular mechanisms contributing to chromosomal instability and to identify those genes that contribute to an aberrant progression of mitosis in cancer cells. We identified CHK2 as a tumorsuppressor gene that is essential for the maintenance of chromosomal stability and my work characterizes a novel mitotic function of Chk2. The Chk2 kinase, which has been implicated in the DNA-damage signaling pathway before, is activated not only in response to DNA damage but also during mitosis. Moreover, Chk2 is essential for the normal and timely progression of mitosis and for the accurate assembly of mitotic spindles. Depletion of CHK2 or abrogation of its kinase activity causes abnormal mitotic spindle assembly, which is associated with a transient delay in prometaphase. The transient spindle defects upon loss of CHK2 promote subsequently the generation of isolated, so called „lagging“ chromosomes during anaphase that are not correctly segregated onto daughter cells. Significantly, both, lagging chromosomes and chromosomal instability could be suppressed by ectopic expression of MCAK, which acts as a microtubule depolymerase involved in correcting faulty attachments of chromosomes to the mitotic spindle. Remarkably, chromosomes that were not correctly attached to the mitotic spindle due to the loss of CHK2 can evade detection by a mitotic control mechanism, known as the spindle assembly checkpoint. Thus, loss of CHK2 can induce CIN in the presence of a functional spindle checkpoint. My results show that Chk2 plays unexpectedly a key role in mitotic spindle assembly, which is a requirement for accurate chromosome attachment and for maintenance of chromosomal stability. Therefore, this new mitotic function of Chk2 might contribute to its tumorsuppressor function, which is so far little understood. In addition to its fundamental role for the maintenance of chromosomal stability, mitosis is an important target for anti-cancer therapy. Anti-mitotic drugs including various Taxanes or Vinca alkaloids, which are used in the clinic for many years, bind to microtubules and interfere with the spindle dynamics. This leads to impaired chromosome alignment and to the activation of the spindle checkpoint resulting in a mitotic cell cycle arrest. Previously, our lab could demonstrate that the spindle checkpoint is not only important for the mitotic arrest in response to spindle damaging drugs but also for the chemotherapy-induced apoptosis. Hence, malfunctions of the spindle checkpoint as observed in human cancer not only lead to chromosome missegregation but also to chemotherapy resistance. To target such therapy resistant tumor cells, the spindle checkpoint itself could be exploited as a new therapeutic target, because it is essential for cell viability. In fact, our lab has identified the indolocarbazol compound Gö6976 as an inhibitor for the spindle checkpoint that induces apoptosis in various tumor cells. In my work, I have characterized this pharmacological inhibitor of the spindle checkpoint and I could show that Gö6976 inhibits the mitotic kinases Aurora A and Aurora B in vitro and in vivo. Inhibition of Aurora A by Gö6976 causes defects in spindle assembly and chromosome alignment and the concomitant inhibition of Aurora B causes an override of the spindle checkpoint, which leads to an exit from an aberrant mitosis. This concomitant inhibition of two mitotic kinases, which might both be involved in spindle checkpoint function, finally leads to an efficient, tumor-selective and spindle checkpoint independent induction of apoptosis. Thus, the mitotic spindle checkpoint and its associated mitotic kinases are indeed attractive new chemotherapeutic targets that avoid mechanisms of resistance due to malfunction of the spindle checkpoint.