Developing Models to Investigate Mechanisms of Genomic Imprinting

Genomic imprinting is a conserved, essential process in mammalian development that regulates the expression of a small number of genes in a monoallelic, parent-or-origin-specific manner. Misregulation of imprinted genes is associated with imprinting disorders including Beckwith-Wiedemann syndrome (BWS) and Silver-Russell syndrome (SRS) that exhibit abnormal growth phenotypes. These disorders are associated with aberrant regulation of the imprinted loci in human 11p15 including the H19/IGF2 locus. Finding various alterations of 11p15 associated with BWS and SRS inspires investigation of imprinting mechanisms in human, which could provide insights into therapeutics. Mouse models have been fundamental to the study of mechanisms of imprinting, serving as a proxy for the orthologous human locus. However, elements that regulate genomic imprinting, the imprinting control regions (ICRs), often diverge across species. Thus, it is essential to first understand whether the diverged ICR has a species-specific role in regulating imprinting. In Chapter 2, we generated a mouse in which the human ICR sequence replaces the orthologous mouse ICR at the H19/Igf2 locus. We show that the imprinting mechanism has partially diverged between mouse and human, depending on the parental origin of the human ICR sequence in mouse. Additionally, we find that this mouse model is optimal for studying specific alterations associated with BWS and SRS. The partially diverging imprinting mechanism between mouse and human suggests that entirely human models are compelling alternatives. In Chapter 3, we demonstrate the derivation of induced pluripotent stem cells (iPSCs) from BWS patient fibroblasts. We find that the iPSCs exhibit proper epigenetic and transcriptional signatures of BWS. Although we find that certain aspect of epigenetic perturbation is inevitable in our iPSCs, the consequence of this perturbation remains unknown. Therefore, we propose that the iPSCs can be differentiated into clinically-relevant cell types to elucidate molecular mechanisms leading to BWS. Overall, the work in this dissertation underscores the versatile and complementary use of different model systems in investigating imprinting mechanisms. In addition to serving as platforms to model imprinting disorders, these models provide insights into the evolutionary perspective of imprinting as well as the significance of various epigenetic mechanisms that regulate imprinting.