Whether genotoxic stress exposures increase or decrease such heterogeneity in gene expression among

distinct hESCs is still unknown. However, the stochasticity of intranuclear molecular reactions and biochemical processes may control the ultimate decision of cell fate associated with DNA damage[40]. CHANGES IN MICRORNA GENE EXPRESSION IN HESCS EXPOSED Valproic acid TO RADIATION Gene expression alterations might be heavily influenced by epigenetic changes, such as DNA methylations, histone modifications, and perturbations in miRNA gene expression[31]. It was found that dozens of miRNA genes were overexpressed after UV-exposures in hESCs, including genes belonging to miR-302 and miR-371-372 clusters thought to be human pluripotent stem cell-specific[41]. Importantly, miR-302a, miR-302b, miR-302c, miR-302d, and mir-372 genes were implicated in regulating the expression of p21 in hESCs, governing

crucial self-renewal and cell cycle processes[41,42]. The comprehensive data on epigenetic alterations in stressed hESCs are lacking; however, our recent study addressed hESC responses to IR exposures at a level of global microRNAome changes[43]. By employing DNA microarray approach, we showed for the first time, that the microRNAome undergoes global alterations in hESCs after IR. We profiled expression of 1090 miRNA species in irradiated H1 and H9 lines of hESCs, and our analysis revealed statistically significant changes in expression of 54 genes following 1Gy of IR exposures[43]. Noteworthy, global microRNAome alterations in hESCs were both time-dependent and cell-line-dependent. “Late” transcriptional response at 16h post-IR exposures of hESCs was shown to be quite robust at a level of global microRNAome. Just a few miRNA genes, such as miR-15b, mir-1973, etc., were IR-responsive at 2h post IR in both hESC lines we examined. The level of miRNA gene expression alterations at this “early”

timepoint was modest at best (usually less than 2-fold)[43]. Our global analysis of microRNAome changes reinforced the idea that miRNA gene expression after genotoxic stress exposures maintains the pluripotent state Brefeldin_A of surviving hESCs; and, for the most part, implicates the cell cycle-, and alternative splicing-related biological processes. Importantly, the identification of novel molecular targets of genotoxic stress exposure in hESCs will aid in understanding the underlying mechanisms governing the fundamental principles of human pluripotent stem cell behavior and plasticity for application in health science and as a remedy to cure diseases. CHANGES IN GENE EXPRESSION IN HESCS EXPOSED TO GENOTOXIC DRUGS In general, data on sensitivity and gene expression changes in human pluripotent stem cells in response to different genotoxic agents/drugs are still very limited.