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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2006.10.045 Byline: Henrietta Szutorisz (1), Andrew Georgiou (1), Laszlo Tora (2), Niall Dillon (1) Abstract: The ability of stem cells to activate different gene expression programs requires the choreographed assembly of trans-acting factors at enhancers and promoters during cell differentiation. In this study, we show that the proteasome acts on specific regulatory regions in embryonic stem (ES) cells to prevent incorrect transcriptional initiation. Chemical or siRNA-mediated inhibition of proteasome activity results in increased transcription factor and RNA polymerase II binding and leads to activation of cryptic promoters. Analysis of the binding profiles of different proteasome subunits in normal ES cells and following RNAi knockdown of individual subunits provides evidence for a targeted assembly of the 26S proteasome at specific regulatory elements. Our results suggest that the proteasome promotes a dynamic turnover of transcription factor and Pol II binding at tissue-specific gene domains in ES cells, thereby restricting permissive transcriptional activity and keeping the genes in a potentiated state, ready for activation at later stages. Author Affiliation: (1) Gene Regulation and Chromatin Group, MRC Clinical Sciences Centre, Faculty of Medicine, Imperial College, Hammersmith Campus, Du Cane Road, London W12 ONN, UK (2) Institut de Genetique et de Biologie Moleculaire et Cellulaire (IGBMC), CNRS UMR 7104, INSERM, ULP, 1 rue Laurent Fries, BP 10142, 67404 Illkirch Cedex, France Article History: Received 13 April 2006; Revised 28 August 2006; Accepted 13 October 2006 Article Note: (miscellaneous) Published: December 28, 2006

The different types of model that can explain pluripotency at the level of gene are examined. A novel epigenetic model is proposed to explain how genes are primed in embryonic stem cells to express at later stages of development.

Activation of eukaryotic genes requires a tight temporal control of trans-acting-factor binding to different types of sequence elements. General transcription factors (GTFs) have a central role in the regulation of RNA polymerase II (Pol II) function because they are involved in the initiation of transcription at all class II promoters. Recent studies have shown that GTFs and Pol II are recruited to enhancer elements and that this binding is an early event in gene activation. We propose that an important role of some enhancers is to function as nucleation centres for the assembly of the pre-initiation complex to regulate the timing of gene activation during development, differentiation and the cell cycle.

Many developmental disorders are thought to arise from an interaction between genetic and environmental risk factors. The Hedgehog (HH) signaling pathway regulates myriad developmental processes, and pathway inhibition is associated with birth defects, including holoprosencephaly (HPE). Cannabinoids are HH pathway inhibitors, but little is known of their effects on HH-dependent processes in mammalian embryos, and their mechanism of action is unclear. We report that the psychoactive cannabinoid Δ9- tetrahydrocannabinol (THC) induces two hallmark HH loss-offunction phenotypes (HPE and ventral neural tube patterning defects) in Cdon mutant mice, which have a subthreshold deficit in HH signaling. THC therefore acts as a 'conditional teratogen', dependent on a complementary but insufficient genetic insult. In vitro findings indicate that THC is a direct inhibitor of the essential HH signal transducer smoothened. The canonical THC receptor, cannabinoid receptor-type 1, is not required for THC to inhibit HH signaling. Cannabis consumption during pregnancy may contribute to a combination of risk factors underlying specific developmental disorders. These findings therefore have significant public health relevance. [ABSTRACT FROM AUTHOR]

Background: Cannabis remains one of the most widely abused drugs during pregnancy. In utero exposure to its principal psychoactive component, Δ 9 -tetrahydrocannabinol (THC), can result in long-term neuropsychiatric risk for the progeny. This study investigated epigenetic signatures underlying these enduring consequences., Methods: Rat dams were exposed daily to THC (0.15 mg/kg) during pregnancy, and adult male offspring were examined for reward and depressive-like behavioral endophenotypes. Using unbiased sequencing approaches, we explored transcriptional and epigenetic profiles in the nucleus accumbens (NAc), a brain area central to reward and emotional processing. An in vitro CRISPR (clustered regularly interspaced short palindromic repeats) activation model coupled with RNA sequencing was also applied to study specific consequences of epigenetic dysregulation, and altered molecular signatures were compared with human major depressive disorder transcriptome datasets., Results: Prenatal THC exposure induced increased motivation for food, heightened learned helplessness and anhedonia, and altered stress sensitivity. We identified a robust increase specific to males in the expression of Kmt2a (histone-lysine N-methyltransferase 2A) that targets H3K4 (lysine 4 on histone H3) in cellular chromatin. Normalizing Kmt2a in the NAc rescued the motivational phenotype of prenatally THC-exposed animals. Comparison of RNA- and H3K4me3-sequencing datasets from the NAc of rat offspring with the in vitro model of Kmt2a upregulation revealed overlapping, significant disturbances in pathways that mediate synaptic plasticity. Similar transcriptional alterations were detected in human major depressive disorder., Conclusions: These studies provide direct evidence for the persistent effects of prenatal cannabis exposure on transcriptional and epigenetic deviations in the NAc via Kmt2a dysregulation and associated psychiatric vulnerability., (Copyright © 2021 Society of Biological Psychiatry. Published by Elsevier Inc. All rights reserved.)

Telomerase is crucial for human carcinogenesis. The limiting component of telomerase activity is telomerase reverse transcriptase (hTERT), undetectable in differentiated somatic cells but present in most tumor cells. There is evidence that hTERT transcription is shut down by a repressor in normal cells, but the mechanisms that turn on or maintain expression in tumor cells are not understood. To identify cis-acting regulatory elements, we scanned the hTERT gene for nuclease sensitive sites. In tumor cells and in in vitro transformed fibroblasts that contain hTERT mRNA, we detected a pattern of nuclease-sensitive sites in the second intron different from that in normal fibroblasts. To test whether the chromatin state characterized by the increased nuclease sensitivity plays a role in tumor-specific hTERT expression, we used a telomerase-positive breast carcinoma line, 21NT. Introduction of a normal chromosome 3 into these cells is known to down-regulate hTERT expression, probably through transcriptional silencing. 21NT cells displayed a similar pattern of micrococcal nuclease (MNase) sensitivity to other telomerase-positive lines, whereas the hTERT chromatin of the chromosome 3-hybrids resembled that of normal fibroblasts. In segregants that had lost the normal chromosome 3, the MNase sensitivity pattern characteristic of telomerase-positive cells was restored, and some (but not all) re-expressed the hTERT gene. The simplest model compatible with these results, and with data on the mapping of an hTERT repressor on chromosome 3, is that hTERT expression in tumor cells depends on an open state of intron 2-chromatin. We propose that, during the development of the breast carcinoma from which the 21NT cell line was derived, loss of function of this repressor led to chromatin remodeling necessary (but probably not sufficient) for expression of the hTERT gene. An improved understanding of the precise mechanism of hTERT dysregulation in human cancer may well find applications in the development of antitelomerase cancer therapy.