Genome stability relies on accurate partition of the genome during nuclear division. Proper mitosis, in turn, depends on changes in chromosome organization, such as chromosome condensation and sister chromatid cohesion. Despite the importance of these structural changes, chromatin itself has been long assumed to play a rather passive role during mitosis and chromosomes are usually compared to a “corpse at a funeral: they provide the reason for the proceedings but do not take an active part in them.” (Mazia, 1961). Recent evidence, however, suggests that chromosomes play a more active role in the process of their own segregation. Our work tested the “active chromosome” hypothesis by investigating how chromosome morphology influences the fidelity of mitosis. We used innovative methods for acute protein inactivation, to evaluate the role of two key protein complexes involved in mitotic chromosome architecture - Condensins and Cohesins. Using a multidisciplinary approach, combining acute protein inactivation, 3D-live cell imaging and quantitative methods, we investigated the role of mitotic chromosomes in the fidelity of mitosis at three different levels. We established novel approaches that allowed us to uncover the maintenance of mitotic chromosome organization by inactivating Condensin in a fast and acute manner (TEV cleavage system). This work changed the view on chromosome architecture and emphasizes how controlled resolution of chromosome entanglements is required throughout mitosis (Piskadlo et al, 2017). It further allowed us to study the role of other players in mitotic chromosomes organization, including topoisomerase 2, histone tails and a poorly characterized helicase-like protein. At a cellular level, we also investigated how mitotic chromosomes take an active part in mitosis by examining how chromosome condensation and cohesion influence chromosome movement and the signaling of the surveillance mechanisms that control nuclear division. Our work elucidated how sister chromatid cohesion defects evade the checkpoint mechanisms that control mitosis (Mirkovic et al, 2015). Also, we discovered that mitotic defects associated with sister chromatid cohesion loss can be recovered by inactivation of the mitotic checkpoint (Silva et al, 2018). The execution of this project also allowed us to establish that cohesin loss induces previously unrecognized defects on mitotic fidelity (Carvalhal et al, 2018). Lastly, our work also evaluated the impact of abnormal chromosome structure and number in development and established the developing brain as the most sensitive tissue in response to aneuploidy (Mirkovic et al, 2019). By conceptually challenging the passive chromosome view this project redefined the role of chromatin during mitosis. It further proposed novel mechanisms for how defects in chromosome assembly may underlay several human pathologies, such as cancer, infertility and development disorders.
