Mammalian nuclear function depends on the complex interaction of genetic and epi-genetic elements coordinated in space and time. Structure and function overlap to such a degree that they are usually considered as being inextricably linked. In this work I combine an experimental approach with a computational one in order to answer two main questions in the field of mammalian chromosome organization. In the first section of this thesis, I attempted to answer the question, to what extent does chromatin from different chromosome territories share the same space inside the nucleus? This is a relatively open question in the field of chromosome territories. It is well-known and accepted that interphase chromosomes are spatially constrained inside the nucleus and that they occupy their own territory, however, the degree of spatial interaction between neighbouring chromosomes is still under debate. Using labelling methods that directly incorporate halogenated DNA precursors into newly replicated DNA without the need for immuno-detection or in situ hybridization, we show that neighbouring chromosome territories colocalise at very low levels. We also found that the native structure of DNA foci is partially responsible for constraining the interaction of chromosome territories as disruption of the innate architecture of DNA foci by treatment with TSA resulted in increased colocalisation signal between adjacent chromosomes territories. The second major question I attempted to answer concerned the correlation between nuclear function and the banding pattern observed in human mitotic chromosomes. Human mitotic chromosomes display characteristic patterns of light and dark bands when visualized under the light microscope using specific chemical dyes such as Giemsa. Despite the long standing use of the Giemsa banding pattern in human genetics for identifying chromosome abnormalities and mapping genes, little is known about the molecular mechanisms that generate the Giemsa banding pattern or its biological relevance. The recent availability of many genetic and epigenetic features mapped to the human genome permit a high-resolution investigation of the molecular correlates of Giemsa banding. Here I investigate the relationship of more than 50 genomic and epigenomic features with light (R) and dark (G) bands. My results confirm many classical results, such as the low gene density of the most darkly staining G bands and their late replication time, using genome-wide data. Surprisingly, I found that for virtually all features investigated, R bands show intermediate properties between the lightest and darkest G bands, suggesting that many R bands contain G-like sequences within them. To identify R bands that show properties of G bands, I employed an unsupervised learning approach to classify R bands on their genomic and epigenomic properties and show that the smallest R bands show a tendency to have characteristics typical of G bands. I revisit the evidence supporting the boundaries of G and R bands in the current cytogenomic map and conclude that inaccurate placement of weakly supported band boundaries can explain the intermediate pattern of R bands. Finally, I propose an approach based on aggregating data from multiple genomic and epigenomic features to improve the positioning of band boundaries in the human cytogenomic map. My results suggest that contiguous domains showing a high degree of uniformity in the ratio of heterochromatin and euchromatin sub-domains define the Giemsa banding pattern in human chromosomes.

When a cell divides into two, it is imperative that each resultant daughter receives a full complement of chromosomes; DNA is ultimately responsible for all cellular processes. Cohesion between sister chromatids from the moment of their generation in S phase is central to ensuring the fidelity of chromosome segregation. Smc1 and Smc3 proteins interact with each other via their hinges and with a bridging kleisin subunit via their heads to form the cohesin ring. It is cohesin, through entrapment of sister chromatid within its ring, that confers sister chromatid cohesion. The process of cohesin’s loading onto DNA is poorly understood. While it is thought to depend on ATP hydrolysis, opening of the ring at one of its three interfaces, and the as yet undefined action of the kollerin complex, comprising Scc2 and Scc4 proteins, the sequence of events as they occur are yet to be defined. A recent screen for suppressors of a thermosensitive scc4 allele in budding yeast revealed a mutation within Smc1’s hinge that could bypass the kollerin subunit. Here, the Smc1 suppressor mutation is investigated. Through targeted mutagenesis, the Smc1D588Y mutant identified in the screen and two additional point mutants, Smc1D588F and Smc1D588W, are herein proven able to bypass Scc4 function completely. Thus we provide the strongest evidence to date to suggest that cohesin’s hinge is a critical factor in its loading. Biochemical evidence shows that isolated Smc1 hinge mutants are defective in their binding to Smc3 hinges. This, together with the genetic link made between the hinge and loading complex, suggests that hinge opening might be a requisite for loading. Through mutagenesis of Scc2 and Scc4 we show that the N-terminus of each protein is responsible for their dimerisation. Furthermore, the N- terminus of Scc2 confers no function other than in its binding to Scc4. Finally, we show that Scc4 is required for the enrichment of both Scc2 and cohesin at centromeres, but not at arm loci. Our results are therefore indicative of there being two different pathways of cohesin loading.

Vertebrate genomes are punctuated by CpG islands regions, which have an elevated frequency of CpG dinucleotides. CpG islands are associated with over 70% of mammalian promoters suggesting they may contribute to the regulation of transcription. However, despite being discovered over 30 years ago, the function of CpG islands is still not understood. Unlike the majority of the genome, CpG islands are resistant to DNA methylation. This provides a binding site for CFP1 which binds specifically to non-methylated DNA via its zinc-finger CXXC (zf-CXXC) domain. CFP1 is a subunit of the SET1 methyltransferase complex, and is thought to direct the activating histone modification H3K4me3 to CpG islands. Interestingly, CFP1 also contains a PHD domain which is proposed to bind the H3K4me3 mark, potentially producing a feedback loop between H3K4me3 and the SET1 complex. Although the structural basis for discrimination of non-methylated CpGs is known, it is not clear how zf-CXXC proteins distinguish CpG islands amongst the irregular nucleosomal landscape which exists within the nucleus. This thesis is focused on the role of CFP1 in the relationship between CpG islands, SET1 and H3K4me3. To address these questions, it was important to mechanistically dissect the contribution of the PHD and zf-CXXC domains. The proposal that the PHD domain of CFP1 binds selectively to H3K4me3 was confirmed by in vitro experiments, however this study demonstrates that the PHD domain is insufficient for stable interactions with chromatin. Using complementary genome-wide and live cell imaging approaches, the zf-CXXC domain shown to be required for PHD-dependent interactions. Genome-wide snapshots of binding interactions, together with spatial and temporal details, expose a surprising contribution of the SET1 complex to the nuclear mobility of CFP1, providing a new perspective on the role of CFP1 in H3K4 methylation.

The three-dimensional organization of the human genome is non-random in interphase cells. Heterochromatin is highly clustered at the nuclear periphery, adjacent to nucleoli, and near centromeres. These localizations are reshuffled during mitosis when the chromosomes are condensed, nucleoli disassembled, and the nuclear envelope broken down. After cytokinesis, heterochromatin is re-localized to the domains described above. However, the mechanisms by which this localization is coordinated are not well understood. This dissertation will present evidence showing that both CAF-1 p150 and Ki-67 regulate nuclear structure throughout the human cell cycle. Chromatin Assembly Factor 1 (CAF-1) is a highly conserved three-subunit protein complex which deposits histones (H3/H4)2 heterotetramers onto replicating DNA during S-phase of the cell cycle. The N-terminal domain of the largest subunit of CAF-1 (p150N) is dispensable for histone deposition, and instead regulates the localization of specific loci (Nucleolar-Associated Domains, or “NADs”) and several proteins to the nucleolus during interphase. One of the proteins regulated by p150N is Ki-67, a protein widely used as a clinical marker of cellular proliferation. Depletion of Ki-67 decreases the association of NADs to the nucleolus in a manner similar to that of p150. Ki-67 is also a fundamental component of the perichromosomal layer (PCL), a sheath of proteins that surrounds all condensed chromosomes during mitosis. A subset of p150 localizes to the PCL during mitosis, and depletion of p150 disrupts Ki-67 localization to the PCL. This activity was mapped to the Sumoylation Interacting Motif (SIM) within p150N, which is also required for the localization of NADs and Ki-67 to the nucleolus during interphase. Together, these studies indicate that p150N coordinates the three-dimensional arrangement of both interphase and mitotic chromosomes via Ki-67.

Topoisomerase 2α (Topo2α) is an essential protein with DNA decatenating enzymatic properties, indispensable for chromosome decatenation and segregation. It is a target for a plethora of antitumour drugs and Topo2α protein levels have been associated with the success of treatment, but also drug resistance and secondary malignancies. Although unique in its ability to resolve catenated chromosomes, the role of Topo2α in other steps of DNA metabolism, such as DNA replication elongation and termination have been elusive. A thorough understanding of the role of Topo2α in the cell will not only allow for increased insight into the mechanisms it is involved in, but it will also shed light on proteins and pathways that can act as back-up in its absence, and therefore hopefully expand the basis on which to improve treatment options. Through a synthetic lethal interaction (SLI) screen with an siRNA library targeting 200 DNA repair and signalling genes, Topo2α emerged as being synthetic lethal to Werner protein (WRN), a RecQ helicase involved in maintaining genome integrity mainly in S phase, and the loss of which leads to Werner Syndrome (WS), a segmental progeroid syndrome. The screen was performed in WRN deficient cells, with the initial aim to find proteins that act to buffer against loss of viability, which is the central idea in the concept of synthetic lethality in the absence of WRN. The screen revealed an SLI between WRN and Topo2α and although we were unable to fully validate this, it spurred the question of Topo2α’s role in DNA replication. The findings in this thesis suggest that Topo2α is not required for DNA elongation and timely completion of S phase, and that simultaneous loss of the closely related isoform Topo2β does not affect replication, suggesting that these proteins do not act in parallel back-up pathways during replication. Interestingly, cells accumulate in the polyploid fraction after both depletion and inhibition of Topo2α, albeit with different kinetics. The mechanistic basis of this phenotype remains to be understood through further research, but it is highly interesting as aneuplidity and polyploidy are implicated in the initial stages of tumour development.