Spatial genome organization: Interrelationships of chromosome territories and genes to nuclear architecture
Marella, Narasimharao V.
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The mammalian genome is housed in a membrane bound organelle referred to as the nucleus. The three dimensional structural organization of the nucleus has been implicated to affect various genomic functions. Each chromosome in the interphase cell nuclei occupies a distinct region called the chromosome territory. Advances in cytogenetic techniques including fluorescence insitu hybridization and development of chromosome specific probes have allowed visualization of these individual territories within the interphase nuclei. The organization of the chromosome territories within the nuclear environment is highly debatable as it seems to be influenced by chromosome size or by gene density. Changes in the spatial organization of the chromosomes during differentiation and conservation of territorial associations within various tissue and cell types are also less understood aspects of genomic organization. It is known that aberrations in the spatial and temporal organization of the genome leads to expression of disease phenotypes like cancer. However this phenomenon has been exemplified in only a few studies. In order to provide a deeper understanding of the above mentioned aspects of spatial genomic organization and its influence on gene regulation we have performed chromosome territory labeling experiments on a subset of six human chromosomes by adopting a RE-FISH (repeated fluorescence insitu hybridization) in a normal diploid human fibroblast (WI38) and a normal breast epithelial (MCF10A) cell line. We identified a tissue specific organization for these chromosomes within each of these cell lines by employing a novel computer graphing algorithm referred to as the generalized median graph (GMG). The radial positioning of the chromosomes showed a linear correlation with the chromosome size in both cell lines. We were also able to measure the chromosome-chromosome associations for our subset of chromosomes using in house developed algorithms (Chapter 2). Our study on chromosome 18 and 19 organization during keratinocyte differentiation suggests significant stage specific shifts in chromosome territory spatial positions during differentiation (Chapter 3). We further extended our investigations on genome organization from chromosome territories to individual genes. FISH experiments were performed with individual cosmid probes as well as BAC probes to elucidate the organization of the human type I interferon gene cluster on metaphase chromosomes of the human osteosarcoma cell line (MG63) and normal diploid fibroblasts (Chapter 4). Both the cosmid and BAC probes consistently showed a six fold ladder-like genomic amplification of the interferon gene cluster on one chromosome in the MG63 cell line termed the 'interferon chromosome'. This amplification was absent on WI38 metaphase chromosomes. Comparative genomic hybridization (CGH) analysis also confirmed this gene amplification. We also found that centromere and whole chromosome regions of chromosomes 4 and 9 were interspersed with the amplified gene cluster on the interferon chromosome. Based on the results of our study, we propose a model involving the breakage- fusion -bridge theory for the generation of the interferon chromosome in the MG63 cell line (Chapter 4). Finally in this thesis, we investigate the relationship of alterations in spatial organization and genomic amplification to aberrant changes in gene expression in cancer. The MCF10A series of breast epithelial cell lines consisting of a normal MCF10A, premalignant MCF10At1 and malignant MCF10CA1a were utilized in these studies. Spectral Karyotyping (SKY) and CGH analyses were performed on all three cell lines. Two color gene expression analyses were carried out on mRNA isolated from normal MCF10A and malignant MCF10CA1a cell lines. A total of 8000 genes were identified that showed at least two fold changes- either up or down regulated. Structural changes observed by CGH and SKY were correlated with the gene expression changes. Our results showed that a direct correlation between modifications in genomic structure and changes in gene expression does not exist in a majority of the observed genes (Chapter 5). Overall, the experiments done in this thesis highlight and explore the relationships between the spatial and temporal organization in the nucleus and its influence on genomic function. The thesis is divided into the following six chapters: Chapter1: Introduction. Chapter 2: Tissue specific chromosome organization in normal and cancer cell nuclei. Chapter 3: Distinct changes in chromosome arrangements during human epidermal keratinocyte differentiation. Chapter 4: Ladder-like amplification of the type I interferon gene cluster in the human osteosarcoma cell line MG63. Chapter 5: Cytogenetic and functional analysis of breast cancer progression: Integration of spectral karyotyping, comparative genomic hybridization and cDNA microarray approaches. Chapter 6: Future Aims