The Role of Histone Modifications in Chromosome Breakage and Translocation.
Abstract: Chromosome translocations are genetic hallmarks of most cancer cells. Translocations require the formation of DNA double-strand breaks (DSBs) at two or more genomic loci, followed by the illegitimate joining of broken chromosomal ends through DNA repair. There is increasing evidence that translocations occur at non-random sites in the genome, suggesting that certain regions of the genome... read moreare more susceptible to DNA breakage than others. We hypothesize that altered chromatin properties predispose genomic sites to DNA breakage and translocations. Using large-scale computational analysis, we identified altered levels of specific histone modifications compared to baseline levels at common leukemia and lymphoma breakpoints in hematopoietic stem cells. To probe the physiological relevance of these modifications, we mapped histone modifications and chromatin structure at translocation-prone regions in anaplastic large cell lymphoma (ALCL) precursor cells. We find enrichment of histone marks associated with open chromatin and a decrease in marks associated with closed chromatin near frequent translocation breakpoints. In order to directly test the role of chromatin features in DNA breakage susceptibility, we developed a protein-DNA tethering system that allows us to create local chromatin domains at pre-defined sites in the genome containing inducible DSB sites in vivo. By measuring the amount of DSBs using ligation-mediated PCR, we find that histone modifying enzymes that create active chromatin marks generally increase breakage susceptibility. Finally, we developed a high-throughput break-apart FISH (hiBA-FISH) assay to detect low frequency chromosome breakage and translocation events in lymphocytic cells expressing chromatin modifying enzymes. Experimental elevation of H3K4 methylation promotes chromosome breaks and specific translocations in response to genotoxic stress. Taken together, these experiments provide first insights into the role of histone modifications in the formation of nonrandom chromosomal breaks and the mechanisms that lead to translocations. Clarifying the role of epigenetic changes in translocations has significant clinical implications, particularly with regards to understanding cancer predisposition and susceptibility, and because reversal of aberrant epigenetic changes has emerged as a promising strategy for the treatment of cancer.
Thesis (Ph.D.)--Tufts University, 2016.
Submitted to the Dept. of Cell, Molecular & Developmental Biology.
Advisors: Tom Misteli, and Grace Gill.
Committee: Gavin Schnitzler, James Schwob, and Charlotte Kuperwasser.
Keyword: Cellular biology.read less