Molecular Mechanisms that Govern Stem Cell Differentiation and their Implications in Cancer
Abstract
Mammalian development is orchestrated by global transcriptional changes, which drive cellular differentiation, giving rise to diverse cell types. The mechanisms that mediate the temporal control of early differentiation can be studied using embryonic stem cell (ESCs) and embryonal carcinoma cells (ECCs) as model systems. In these stem cells, differentiation signals induce transcriptional repression of genes that maintain pluripotency (PpG) and activation of genes required for lineage specification. Expression of PpGs is controlled by these genes’ proximal and distal regulatory elements, promoters and enhancers, respectively. Previously published work from our laboratory showed that during differentiation of ESCs, the repression of PpGs is accompanied by enhancer silencing mediated by the Lsd1/Mi2-NuRD-Dnmt3a complex. The enzymes in this complex catalyze histone H3K27Ac deacetylation and H3K4me1/2 demethylation followed by a gain of DNA methylation mediated by the DNA methyltransferase, Dnmt3a. The absence of these chromatin changes at PpG enhancers during ESC differentiation leads to their incomplete repression. In cancer, abnormal expression of PpG is commonly observed. Our studies show that in differentiating F9 embryonal carcinoma cells (F9 ECCs), PpG maintain substantial expression concomitant with an absence of Lsd1-mediated H3K4me1 demethylation at their respective enhancers. The continued presence of H3K4me1 blocks the downstream activity of Dnmt3a, leading to the absence of DNA methylation at these sites. The absence of Lsd1 activity at PpG enhancers establishes a “primed” chromatin state distinguished by the absence of DNA methylation and the presence of H3K4me1. We further established that the activity of Lsd1 in these cells was inhibited by Oct3/4, which was partially repressed post-differentiation. Our data reveal that sustained expression of the pioneer pluripotency factor Oct3/4 disrupts the enhancer silencing mechanism. This generates an aberrant “primed” enhancer state, which is susceptible to activation and supports tumorigenicity.
Degree
Ph.D.
Advisors
Gowher, Purdue University.
Subject Area
Physiology|Biochemistry|Bioengineering|Bioinformatics|Cellular biology|Developmental biology|Genetics
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