Given the proven ability of

Given the proven ability of transcription factors to artificially induce pluripotency in somatic order Sunitinib Malate (Takahashi and Yamanaka, 2006), the high expression of these factors in the germline (Kurimoto et al., 2008) raises the question of how PGCs are constrained from becoming pluripotent and thereby tumorigenic in vivo. Our findings point to the primacy of LIF/STAT3 signaling in driving fate conversion. We propose that activation of the STAT3 pathway in PGCs can result in reacquisition of pluripotency in two contexts—in vitro enabling the derivation of EG cells and in vivo allowing the formation of pluripotent GCTs (Figure 4C). The observation that STAT3 targets are underrepresented in PGCs suggests that the pathway is normally either silent or is antagonized. Indeed the LIF receptor gp130 is not required during PGC development (Molyneaux et al., 2003). This may be an important safeguard against acquisition of ectopic pluripotency. STAT3 targets are upregulated in those GCTs that have a pluripotent compartment, or that have transited through a pluripotent state, suggesting that this pathway may play a previously unappreciated role in teratocarcinogenesis. This may merit further investigation, notably because inhibitors of the Jak/Stat pathway are being developed as chemotherapeutic agents against hematological and solid tumors (Liu et al., 2012; O’Shea et al., 2013; Quintás-Cardama et al., 2011). Although, GCTs are generally responsive to cisplatin therapy, resistance does occur particularly in teratomas (Oosterhuis and Looijenga, 2005) and conceivably might be reduced by targeting the STAT3 pathway in combination therapy. Finally, re-evaluation of PGCs as a robust source of pluripotent stem cells and the pivotal role played by LIF in the conversion process raises the possibility that the early human germline might be a promising source of LIF-responsive pluripotent stem cells. We speculate that rebuilding pluripotency directly from in vivo or in vitro derived human PGCs may allow capture of the hypothetical human naive state (De Los Angeles et al., 2012), which has so far proved elusive starting from preimplantation embryos.
Experimental Procedures
Acknowledgments
Introduction Parental imprinting is a form of epigenetic regulation by which genes are expressed from only one of the two parental alleles. In humans, loss of imprinting is associated with several diseases (e.g., Prader-Willi/Angelman syndromes) and malignancies (e.g., Wilm’s tumor) (Yamazawa et al., 2010). The generation of mouse embryos containing only maternal (parthenogenetic) or paternal (androgenetic) alleles (McGrath and Solter, 1984; Surani and Barton, 1983; Surani et al., 1984) demonstrated the importance of imprinting for restricting asexual form of reproduction in placental mammals. Parthenogenesis may occur naturally in humans resulting in parthenogenetic ovarian teratomas. We have recently generated human-parthenogenetic-induced pluripotent stem cells (PgHiPSCs) by reprogramming of parthenogenetic ovarian teratomas (Stelzer et al., 2011). Studying the gene expression of PgHiPSCs enabled us to identify novel paternally expressed genes (PEGs), and to study the developmental potential of these cells (Stelzer et al., 2011). Differential marking of DNA methylation in the gametes is considered the hallmark mechanism controlling parental imprinting as it establishes germline DMRs (gDMRs), which are then maintained throughout the life of the embryo (Proudhon et al., 2012; Reik et al., 2001; Smith et al., 2012). In the past few years, global surveys of imprinted DMRs in the mouse were reported (Hiura et al., 2010; Kelsey et al., 1999; Proudhon et al., 2012; Singh et al., 2011), and recently DNA methylation analysis at single-base resolution, performed on reciprocal crosses of inbred-mice, identified dozens of novel DMRs (Xie et al., 2012). In humans, however, due to ethical and technical limitations, only few low-resolution surveys were achieved thus far (Choufani et al., 2011). Moreover, the vast majority of DMRs in humans were identified by association with certain diseases or by sharing synteny with mouse DMRs. In this study, we aimed to perform a comprehensive analysis of imprinted DMRs in humans. We thus analyzed global DNA methylation of our PgHiPSCs and their parental fibroblasts by reduced representation bisulfite sequencing (RRBS) (Gu et al., 2011; Meissner et al., 2008) and compared the methylation signature to that of a large panel of human embryonic stem cells (HESCs) and induced pluripotent stem cells (HiPSCs) (Bock et al., 2011).