A high-throughput small molecule screen identifies synergism between DNA methylation and Aurora kinase pathways for X reactivation.

X-chromosome inactivation is a mechanism of dosage compensation in which one of the two X chromosomes in female mammals is transcriptionally silenced. Once established, silencing of the inactive X (Xi) is robust and difficult to reverse pharmacologically. However, the Xi is a reservoir of >1,000 functional genes that could be potentially tapped to treat X-linked disease. To identify compounds that could reactivate the Xi, here we screened ∼367,000 small molecules in an automated high-content screen using an Xi-linked GFP reporter in mouse fibroblasts. Given the robust nature of silencing, we sensitized the screen by "priming" cells with the DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (5azadC). Compounds that elicited GFP activity include VX680, MLN8237, and 5azadC, which are known to target the Aurora kinase and DNA methylation pathways. We demonstrate that the combinations of VX680 and 5azadC, as well as MLN8237 and 5azadC, synergistically up-regulate genes on the Xi. Thus, our work identifies a synergism between the DNA methylation and Aurora kinase pathways as being one of interest for possible pharmacological reactivation of the Xi.

Xist repression shows time-dependent effects on the reprogramming of female somatic cells to induced pluripotent stem cells.

Although the reactivation of silenced X chromosomes has been observed as part of the process of reprogramming female somatic cells into induced pluripotent stem cells (iPSCs), it remains unknown whether repression of the X-inactive specific transcript (Xist) can greatly enhance female iPSC induction similar to that observed in somatic cell nuclear transfer studies. In this study, we discovered that the repression of Xist plays opposite roles in the early and late phases of female iPSCs induction. Our results demonstrate that the downregulation of Xist by an isopropyl ß-d-1-thiogalactopyranoside (IPTG)-inducible short hairpin RNA (shRNA) system can greatly impair the mesenchymal-to-epithelial transition (MET) in the early phase of iPSC induction but can significantly promote the transition of pre-iPSCs to iPSCs in the late phase. Furthermore, we demonstrate that although the knockdown of Xist did not affect the H3K27me3 modification on the X chromosome, macroH2A was released from the inactivated X chromosome (Xi). This enables the X chromosome silencing to be a reversible event. Moreover, we demonstrate that the supplementation of vitamin C (Vc) can augment and stabilize the reversible X chromosome by preventing the relocalization of macroH2A to the Xi. Therefore, our study reveals an opposite role of Xist repression in the early and late stages of reprogramming female somatic cells to pluripotency and demonstrates that the release of macroH2A by Xist repression enables the transition from pre-iPSCs to iPSCs.

Valproic acid confers functional pluripotency to human amniotic fluid stem cells in a transgene-free approach.

Induced pluripotent stem cells (iPSCs) with potential for therapeutic applications can be derived from somatic cells via ectopic expression of a set of limited and defined transcription factors. However, due to risks of random integration of the reprogramming transgenes into the host genome, the low efficiency of the process, and the potential risk of virally induced tumorigenicity, alternative methods have been developed to generate pluripotent cells using nonintegrating systems, albeit with limited success. Here, we show that c-KIT+ human first-trimester amniotic fluid stem cells (AFSCs) can be fully reprogrammed to pluripotency without ectopic factors, by culture on Matrigel in human embryonic stem cell (hESC) medium supplemented with the histone deacetylase inhibitor (HDACi) valproic acid (VPA). The cells share 82% transcriptome identity with hESCs and are capable of forming embryoid bodies (EBs) in vitro and teratomas in vivo. After long-term expansion, they maintain genetic stability, protein level expression of key pluripotency factors, high cell-division kinetics, telomerase activity, repression of X-inactivation, and capacity to differentiate into lineages of the three germ layers, such as definitive endoderm, hepatocytes, bone, fat, cartilage, neurons, and oligodendrocytes. We conclude that AFSC can be utilized for cell banking of patient-specific pluripotent cells for potential applications in allogeneic cellular replacement therapies, pharmaceutical screening, and disease modeling.