Molecular Determinants of Chromosome Transmission and Cell Cycle Regulation

Division Of Basic Sciences - Nci

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Abstract

We use multi-organismal (yeast, mouse and human cells) and multi-disciplinary (genetic, cell biology, biochemical and genome-wide) approaches to study faithful chromosome segregation, a fundamental process of every living cell. Genetic screens served as a starting point and in-depth mechanistic studies have provided evidence for new roles for kinetochore genes and the identification of new kinetochore genes. We also identified and defined roles for post-translational modifications (acetylation, methylation, phosphorylation, sumoylation and ubiquitination) of Cse4 in chromosome segregation. Our current research is aimed at understanding the role of Cse4-associated proteins in chromosome segregation and defining pathways that prevent mislocalization of Cse4 to non-centromeric regions. In the first project we have defined roles for Scm3, Pat1, Cdc5 and Sgo1 for the assembly of centromeric chromatin and characterized role of post-translational modifications of centromeric histones in faithful chromosome segregation. Our results show that imbalanced stoichiometry of a Cse4 chaperone, Scm3 (HJURP in humans) leads to chromosome mis-segregation in both human and yeast cells thereby providing a link between HJURP overexpression and mitotic defects in cancers (Mishra et al., 2011). Scm3 interacts with Pat1 (Protein associated with topoisomerase II) and we have shown that Pat1 regulates the topology of centromeric chromatin (Mishra et al., 2013). We used a pat1 deletion strain to define the number of Cse4 molecules at the yeast kinetochore (Hasse, Mishra 2013, Mishra et al., 2015) and provided evidence for a structural role for Pat1 in the structural integrity of centromeric chromatin and localization of Cse4 for faithful chromosome segregation. In addition to kinetochore proteins, association of cohesins with centromeres and along the length of the chromosomes ensures faithful segregation of sister chromatids during mitosis. We have shown that evolutionarily conserved polo kinase, Cdc5 associates with centromeric chromatin to facilitate the removal of centromeric cohesins (Mishra et al., 2016) and Cdc5-mediated phosphorylation of Cse4 regulates faithful chromosome segregation (Mishra et al., 2019). Furthermore, we have determined that evolutionarily conserved Sgo1 which protects centromeric cohesion interacts with Cse4 and this is required for faithful chromosome segregation. Biochemical approaches have allowed us to provide a comprehensive analysis of Post-translational modifications (PTMs) of Cse4. Conserved sites for acetylation, methylation, and phosphorylation were identified in Cse4 (Boeckmann et al., 2013). We generated a phospho-specific antibody and showed the association of phosphorylated Cse4 with centromeres and determined that evolutionarily conserved Aurora B/Ipl1 kinase phosphorylates Cse4 in vivo and in vitro for faithful chromosome segregation. Using budding yeast with a single nucleosome we provided the first evidence that yeast centromeres contain hypoacetylated histone H4 and that increased acetylation of histone H4 on lysine 16 (H4K16) leads to chromosome mis-segregation (Choy et al., 2011). Even though HDAC inhibitors (HDACi) are used in clinical trials we do not fully understand their mode of action. Hence, we performed a genome-wide screen with an HDACi to identify pathways that are vulnerable to altered histone acetylation. Our results showed that chromosome segregation mutants are more sensitive to HDACi (Choy et al., 2015). Future studies will allow us to understand the molecular role of PTMs of Cse4 in chromosome segregation and determine if these PTMs are conserved in human CENP-A. In the second project we have focused on the identification of pathways that regulate cellular levels of Cse4 thereby preventing its mislocalization and CIN. We showed previously that S. cerevisiae spt4 mutants show mislocalization of Cse4 and chromosome segregation defects that are complemented by human SPT4 (Basrai etal, 1996 and Crotti and Basrai 2004). We established the cause and effect of Cse4 mislocalization by showing that altered histone dosage and mislocalization of Cse4 to non-centromeric chromatin correlate with chromosome loss (Au et al., 2008). One mechanism that prevents mislocalization of Cse4 is ubiquitin-mediated proteolysis of Cse4 by E3 ligase Psh1. We identified a novel role for the N terminus of Cse4 in ubiquitin (Ub)-mediated proteolysis for faithful chromosome segregation (Au et al., 2013). We recently reported that Cse4 is sumoylated and ubiquitination of sumoylated Cse4 by Slx5 regulates its proteolysis to prevent mislocalization to euchromatin (Ohkuni et al., 2016, 2018). We have undertaken genome-wide approaches to identify regulators that prevent mislocalization of Cse4 to euchromatin. Our studies have revealed a role for histone chaperones (Ciftci-Yilmaz et al., 2018) and essential E3 Ub ligases in Cse4 proteolysis. Our ongoing studies are aimed at in-depth analysis of the yeast genes identified in the screen to understand the molecular mechanisms that prevent mislocalization of Cse4 for chromosomal stability. Mislocalization of CENP-A contributes to CIN in human cells. Given the clinical significance of high CENP-A expression and its correlation with cancer, it is critical to understand how CENP-A overexpression contributes to tumorigenesis and whether CENP-A expression can be exploited for prognosis, diagnosis and targeted treatment of CENP-A overexpressing cancers. We established cell lines and optimized cell biology-based assays to address a long-standing question of whether mislocalization of overexpressed CENP-A contributes to CIN. We determined that constitutive or inducible expression of CENP-A in HeLa and stable diploid RPE1 cells results in mislocalization of CENP-A to non-centromeric regions. Comprehensive analysis for mitotic effects showed a dose-dependent effect of CENP-A overexpression on chromosome segregation defects and higher incidence of micronuclei. Altered localization of kinetochore proteins contributes to a weakening of the native kinetochore in CENP-A overexpressing cells. Depletion of the histone chaperone DAXX prevents CENP-A mislocalization and rescues the CIN phenotype in CENP-A overexpressing cells. These results show that mislocalization of CENP-A is one of the major contributors for CIN in CENP-A overexpressing cells. Our studies provide the first evidence for how mislocalization of CENP-A to non-centromeric chromatin contributes to CIN in human cells and provide mechanistic insights into how CENP-A overexpression may contribute to aneuploidy in CENP-A overexpressing cancers (Shrestha et al., 2017). We are pursuing studies with human homologs of the yeast genes identified in genome wide screens and using other approaches to identify and characterize pathways that prevent mislocalization of CENP-A for genome stability. In summary, our studies using multi-organismal and multi-disciplinary approaches will provide mechanistic insights for how defects in kinetochore function contribute to aneuploidy in human cancers. We are optimistic that our in vivo studies with the mouse model will help translate basic science research to the clinic and aid in the diagnosis, prognosis and treatment of cancers that show overexpression of CENP-A.