Characterization Of Epigenetic Plasticity And Chromatin Dynamics In Cancer Cell Models
Cancer progression is driven by cumulative changes that promote and maintain the malignant phenotype. Epigenetic alterations are central to malignant transformation and to the development of therapy resistance. Changes in DNA methylation, histone acetylation and methylation, noncoding RNA expression and higher-order chromatin structures are epigenetic features of cancer, which are independent of changes in the DNA sequence. Despite the knowledge that these epigenetic alterations disrupt essential pathways that protect cells from uncontrolled growth, how these modifications collectively coordinate cancer gene expression programs remains poorly understood. In this dissertation, I utilize molecular and informatic approaches to define and characterize the genome-wide epigenetic patterns of two important human cancer cell models. I further explore the dynamic alterations of chromatin structure and its interplay with gene regulation in response to therapeutic agents.
In the first part of this dissertation, pancreatic ductal adenocarcinoma (PDAC) cell models were used to characterize genome-wide patterns of chromatin structure. The effects of histone acetyltransferase (HAT) inhibitors on chromatin structure patterns were investigated to understand how these potential therapeutics influence the epigenome and gene regulation. Accordingly, HAT inhibitors globally target histone modifications and also impacted specific gene pathways and regulatory domains such as super-enhancers. Overall, the results from this study uncover potential roles for specific epigenomic domains in PDAC cells and demonstrate epigenomic plasticity to HAT inhibitors.
In the second part of this dissertation, I investigate the dynamic changes of chromatin structure in response to estrogen signaling over a time-course using Estrogen Receptor (ER) positive breast cancer cell models. Accordingly, I generated genome-wide chromatin contact maps, ER, CTCF and regulatory histone modification profiles and compared and integrated these profiles to determine the temporal patterns of regulatory chromatin compartments. The results reveal that the majority of alterations occur in regions that correspond to active chromatin states, and that dynamic chromatin is linked to genes associated with specific cancer growth and metabolic signaling pathways. To distinguish ER-regulated processes in tamoxifen-sensitive and in tamoxifen-resistant (TAMR) cell models, we determined the corresponding chromatin and gene expression profiles using ER-positive TAMR cancer cell derivatives. Comparison of the patterns revealed characteristic features of estrogen responsiveness and show a global reprogramming of chromatin structure in breast cancer cells with acquired tamoxifen resistance.
Taken together, this dissertation reveals novel insight into dynamic epigenomic alterations that occur with extrinsic stimuli and provides insight into mechanisms underlying the therapeutic responses in cancer cells.