Molecular mechanisms for the final step of dissolving sister chromatid cohesion at anaphase onset

To make a human body from a single cell (a fertilized egg), an enormous number of cell divisions must take place. Each cell division produces two daughter cells, which equally inherit genetic information from their mother cell. Human cells store their genetic information within 46 structures called chromosomes. To maintain this genetic information, all chromosomes must be precisely copied and each daughter cell must receive one copy of each chromosome when cell division takes place. Failure in this process may cause cancers and genetic abnormalities. The aim of our research is to clarify the fundamental mechanisms by which all chromosomes are properly inherited by daughter cells.

In this research project, we focus on the special glue between copied chromosomes. As mentioned above, all chromosomes must be copied and each copy must be given to newly born daughter cells. It is crucial that copied chromosomes are held together with the special glue. However, at right timing, the glue must be quickly removed so that each chromosome copy is given to each daughter cell. If the glue is lost too early, chromosomes are mixed up and not correctly given to daughter cells. On the other hand, if removal of the glue is delayed, chromosome copies do not separately at right time, and both copies could be given to one daughter cell (rather than each copy to each daughter). We will study how cells regulate removal of the glue between copied chromosomes and what mechanisms ensure efficient removal of the glue. We will use both yeast and human cells in our study since they have many similarities in regulation of the chromosome glue. Use of yeast often allows us to reach conclusion more quickly. We will then address if the glue is regulated in the same way in human cells.

Sister chromatid cohesion is established during DNA replication and is removed when cells enter anaphase. Cohesion relies on the cohesin ring complex, composed of Scc1 (also called Mcd1 or Rad21), Scc3, Smc1, Smc3 and Pds5. At anaphase onset, the cysteine protease separase is activated and cleaves cohesin Scc1 in half, causing opening of the cohesin ring complex and removal of sister chromatid cohesion, thus allowing chromosome segregation to opposite spindle poles.

It has been thought that Scc1 cleavage by separase is the final regulatory step for dissolution of sister chromatid cohesion at the anaphase onset. However, our recent study has revealed that there is an additional regulatory step for this process; i.e. deacetylation of Smc3 by Hos1 facilitates timely removal of cohesins from chromosomes in budding yeast (Li et al 2017). However, it is still unknown how Smc3 deacetylation promotes removal of cohesins at anaphase onset. In this project, we will address this question. In particular, we will address whether Smc3 deacetylation prompts de-repression of the ATPase activity and disengagement of Smc1-3 heads at anaphase onset. We will also test an alternative possibility that Smc1-3 heads are inter-bridged by other proteins (in addition to Scc1 that is cleaved at anaphase onset), such as Scc3 and Pds5, which may be subsequently destabilized by Smc3 deacetylation at anaphase onset.

Meanwhile, HDAC8 has been identified as the Smc3 deacetylase in human cells. Mutations in HDAC8 are associated with Cornelia de Lange syndrome, a dominantly inherited congenital disorder. We will address whether Smc3 deacetyaltion is important for efficient dissolution of sister chromatid cohesion at anaphase onset in human cells. If so, we will investigate how Smc3 deacetylation facilitates cohesin removal from chromosome and if the defects in this process are involved in development of Cornelia de Lange syndrome.

If funded, the proposed research will bring forth the following impacts:

Academic impact:

1) In the proposed project, we will investigate and reveal final regulatory steps of dissolving sister chromatid cohesion at anaphase onset. Our research will discover novel mechanisms of dissolving cohesion, which will benefit research in chromosome biology. Our research should open up new aspects of chromosome regulation, which will benefit not only research in chromosome biology but also wider research in cell biology and life sciences.

2) Our research will produce useful reagents for research community. Such reagents include DNA constructs, yeast strains and human cell lines. For example, N. castellii yeast strains with tagged cohesins will be useful and popular tools for calibration of ChIP-seq in S. cerevisiae, which allows quantitative comparison of chromosome-associated cohesins between different samples. We will swiftly share our reagents with research community following publication.

3) We will investigate fundamental mechanisms ensuring proper chromosome segregation. High-fidelity chromosome segregation is a key to genetic integrity of cells. So, our project should give important information to university students and early-career researchers, who are interested with genetics and cell biology. We hope our research will give them inspiration and encouragement to become scientists in life sciences.


Economic and societal impact:

1) We will investigate whether (if so, how) Smc3 deacetylation by HDAC8 deacetylase facilitates dissolution of sister chromatid cohesion at anaphase onset in human cells. HDAC8 is one of the genes, whose mutations cause Cornelia de Lange Syndrome (CdLS). We will investigate how HDAC8 mutations, associated with CdLS, change efficiency in dissolving cohesion. Our research could shed new lights on the mechanisms of CdLS and could provide clues to its diagnosis and treatment.

2) We will investigate mechanisms of dissolving sister chromatid cohesion at anaphase onset. The defects in this process cause chromosome missegregation, which are associated with a variety of human diseases characterized by chromosome instability, such as cancers and congenital disorders. Our research could find clues to mechanisms of these diseases. In the long term, our research could bring forth the clinical impact on their diagnosis and treatment.

3) Our research will provide excellent materials for public engagement regarding cell biology and life sciences. We will study the process of sister chromatid separation and chromosome segregation; these processes are arguably most dynamic and eye-catching cellular events we observe under the microscopes. Moreover failure in these processes is associated with various human diseases. So, these biological events should attract interest of public. We will organize inspiring public engagement events using materials from our research.