Identification of the mechanisms controlling aneuploidy

Chromosome Capture

We have developed a method to study chromosome capture in fission yeast. After cold shock, spindles disassemble and chromosomes are often lost during this process. Using this technique, we showed by live cell analysis that lost kinetochores interact laterally with intranuclear microtubules (INMs) and that both microtubule depolymerization (end-on pulling) and minus-end-directed movement (microtubule sliding) contribute to chromosome retrieval to the spindle pole body (SPB). We find that the minus-end-directed motor Klp2 colocalizes with the kinetochore during its transport to the SPB and contributes to the effectiveness of retrieval by affecting both end-on pulling and lateral sliding. Furthermore, we provide in vivo evidence that Dam1, a component of the DASH complex, also co-localizes with the kinetochore during its transport and is essential for its retrieval by either of these mechanisms.

Merotelic attachment, seeing is believing.

Faithful segregation of sister chromatids requires attachment of each kinetochore to microtubules that extend from opposite spindle poles. Merotelic kinetochore orientation is a kinetochore-microtubule mis-attachment in which a single kinetochore binds microtubules to both spindle poles rather than just one. We showed that merotelic kinetochore attachment can be genetically induced in fission yeast. Merotelic attachment during anaphase leads to intra-kinetochore stretching followed by either correction or kinetochore disruption. Laser ablation of spindle microtubules revealed that intra-kinetochore stretching is imposed by microtubule-dependent forces. Interestingly, the presence of multiple merotelic chromosomes linearly antagonizes spindle elongation rate and this phenomenon can be numerically solved using a simple force balance model. Based on the predictions of our mechanical model, we provided in vivo evidence that correction of merotelic attachment in anaphase is dependent on Ase1/Prc1/Map65 to  prevent spindle collapse, asymmetric division or cut phenotype.

Chromosome segregation and telomere separation

The segregation of centromeres and telomeres at mitosis are coordinated at multiple levels to prevent the formation of aneuploid cells, a phenotype frequently observed in cancer. One potential source of mitotic instability arises from chromosome segregation defects, giving rise to chromatin bridges at anaphase and chromosome loss. Most of these defects are corrected before anaphase onset by a mechanism involving Aurora B kinase, a key regulator of mitosis in a wide range of organisms, including fission yeast. Here, we describe a new role for Aurora B in telomere dispersion and disjunction during mitosis. Telomere dispersion is initiated before anaphase onset while disjunction takes place later in mitosis. Dispersion is promoted by the dissociation of Swi6/HP1 and cohesin Rad21 from telomeres while disjunction occurs at anaphase following the phosphorylation of condensin subunit Cnd2.

Strikingly, we demonstrate that deletion of Ccq1, a telomeric shelterin component, rescues cell death after Aurora inhibition revealing an essential role for telomeres in chromosome arms segregation. Thus, our findings demonstrate that Aurora B targets distinct heterochromatin domains, centromeres and telomeres to control chromosome segregation.