Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ChromoCellDev (Chromosome Architecture and the Fidelity of Mitosis during Development)

Teaser

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...

Summary

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. The present proposal tests the “active chromosome” hypothesis by investigating how chromosome morphology influences the fidelity of mitosis. are using 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 investigate the role of mitotic chromosomes in the fidelity of mitosis at three different levels. The first one uses novel approaches to uncover the process of mitotic chromosome assembly, which is still largely unknown. The second explores how mitotic chromosomes take an active part in mitosis by examining how chromosome condensation and cohesion influence chromosome movement and the signalling of the surveillance mechanisms that control nuclear division. Lastly we evaluate how mitotic errors arising from abnormal chromosome structure impact on development. We aim to evaluate, at the cellular and organism level, how the cell perceives such errors and how (indeed if) they tolerate mitotic abnormalities. By conceptually challenging the passive chromosome view this project has the potential to redefine the role of chromatin during mitosis and further understand how chromosomal abnormalities may underlay several human pathologies, such as cancer, infertility and developmental disorders.

Work performed

During the first reporting period (18 months) most efforts were concentrated on the development of critical experimental systems/tools to dissect the various biological questions. Major achievements include the development of TEV protease-mediated inactivation systems for condensin I and II in the living fly and the dissection of condensin I role in the maintenance of chromosome organization (Piskadlo et al eLife 2017). The work has also focused on the further development of these tools to study the role of cohesin and condensin at a highly quantitative level, as well as in the context of the developing organism.

Final results

Mitotic chromosome assembly remains a big mystery in biology. Condensin complexes are pivotal for chromosome architecture yet how they shape mitotic chromatin remains unknown. We uncovered that chromosome structure is linked to the state of sister chromatid resolution. We found that the enzyme responsible for resolving DNA entanglements, Topoisomerase II, is capable of re-intertwining previously separated DNA molecules. Correct separation of DNA molecules relies on condensin I activity, continuously required to counteract this erroneous activity. These findings challenge current views on chromosome resolution maintenance and highlight that this is a highly dynamic bidirectional process. Understanding how cells avoid DNA damage during division clarifies why errors in this process cause diseases. For example, changes to condensin I are common in certain cancers and can also lead to disrupted brain development (e.g. microcephaly).

Website & more info

More info: http://www.chromosomedynamicslab.pt.