The mechanisms by which embryonic stem (ES) cells self-renew while maintaining the ability to differentiate into virtually all adult cell types are not well understood. Polycomb group (PcG) proteins are transcriptional repressors that help to maintain cellular identity during metazoan development by epigenetic modification of chromatin structure. PcG proteins have essential roles in early embryonic development and have been implicated in ES cell pluripotency, but few of their target genes are known in mammals. Here we show that PcG proteins directly repress a large cohort of developmental regulators in murine ES cells, the expression of which would otherwise promote differentiation. Using genome-wide location analysis in murine ES cells, we found that the Polycomb repressive complexes PRC1 and PRC2 co-occupied 512 genes, many of which encode transcription factors with important roles in development. All of the co-occupied genes contained modified nucleosomes (trimethylated Lys 27 on histone H3). Consistent with a causal role in gene silencing in ES cells, PcG target genes were de-repressed in cells deficient for the PRC2 component Eed, and were preferentially activated on induction of differentiation. Our results indicate that dynamic repression of developmental pathways by Polycomb complexes may be required for maintaining ES cell pluripotency and plasticity during embryonic development.

NEWS こうした細胞を単離するには、幹細胞に 特徴的なタンパク質が発現された場合に だけ活性化される抗生物質耐性遺伝子を 組み込む。これらの細胞に抗生物質を投 与すれば、幹細胞になりそこねた細胞を 死滅させ取り除くことができる。 山中が昨年、幹細胞のマーカーとして 用いたタンパク質は、再プログラミング された細胞を見つけだすのに最適なもの ではなかった。今回、3 つの研究グルー プはいずれも、別の 2 つのタンパク質マー カー （NanogとOct4） を用いて成功した。 3 つの研究グループとも、この方法で単 離した iPS 細胞を用いてキメラマウスを 作り出すことができ、このマウスから子 孫へとiPS のDNA が受け継がれた。 Jaenisch はさらに、特殊な胚を使っ て、完全にiPS細胞に由来する細胞をもっ た胎児を生み出した。 「これは最高の胚 性幹細胞にしかできないことだ」と彼は いう。5 月 31日にババリアで開催された 会議でJaenischの発表する成果を聞いた Schöler は、 「信じがたい、まさに驚きだ」 という。 「私にいわせれば、これはドリー （クローン動物の第一号） みたいなものだ。 それくらい、たいへんな功績なのだ。 」 この手法は魅力的である。ヒトのクロー ン作製は、入手できる卵細胞の数や、習 得に約 6 か月もかかる技術のむずかしさ のために制限されていたが、中山の手法 だと、最もありふれた細胞を使うことが でき、しかも簡単な実験技術で行える。 しかし、ヒト細胞へのこの手法の適用は まだ成功していない。 「我々は必死で研究 している。それこそ昼夜を問わず」と山中 はいう。おそらく、さらに他の転写因子が 必要なのだろう、と彼は付け加えた。 もしヒト細胞でうまくいけば、パーキ ンソン病や糖尿病といった疾患の患者か ら iPS 細胞を作り出すことができるかも しれず、これらの細胞が発生するときに 細胞内でどのような分子レベルの変化が 起こるかを観察できるかもしれない。こ の「シャーレ内で再現された疾患」は、 さまざまな環境要因が病態にどうかかわ るかを調べたり、薬剤が疾患進行をどれ くらい抑制できるかを調べたりする糸口 になるだろう。 しかし、このiPS細胞でも完全無欠とは このキメラマウスが誕生したということは、使われた細胞が胚性幹細胞のようにふるまったということだ。 S. OGDEN www.nature.com/naturedigest

SUMMARY MicroRNAs (miRNAs) are crucial for normal embryonic stem (ES) cell self-renewal and cellular differentiation, but how miRNA gene expression is controlled by the key transcriptional regulators of ES cells has not been established. We describe here a new map of the transcriptional regulatory circuitry of ES cells that incorporates both protein-coding and miRNA genes, and which is based on high-resolution ChIP-seq data, systematic identification of miRNA promoters, and quantitative sequencing of short transcripts in multiple cell types. We find that the key ES cell transcription factors are associated with promoters for most miRNAs that are preferentially expressed in ES cells and with promoters for a set of silent miRNA genes. This silent set of miRNA genes is co-occupied by Polycomb Group proteins in ES cells and expressed in a tissue-specific fashion in differentiated cells. These data reveal how key ES cell transcription factors promote the miRNA expression program that contributes to normal self-renewal and cellular differentiation, and integrate miRNAs and their targets into an expanded model of the regulatory circuitry controlling ES cell identity.

It has recently been demonstrated that mouse and human fibroblasts can be reprogrammed into an embryonic stem cell-like state by introducing combinations of four transcription factors. However, the therapeutic potential of such induced pluripotent stem (iPS) cells remained undefined. By using a humanized sickle cell anemia mouse model, we show that mice can be rescued after transplantation with hematopoietic progenitors obtained in vitro from autologous iPS cells. This was achieved after correction of the human sickle hemoglobin allele by gene-specific targeting. Our results provide proof of principle for using transcription factor-induced reprogramming combined with gene and cell therapy for disease treatment in mice. The problems associated with using retroviruses and oncogenes for reprogramming need to be resolved before iPS cells can be considered for human therapy.

Pluripotency can be induced in differentiated murine and human cells by retroviral transduction of Oct4, Sox2, Klf4, and c-Myc. We have devised a reprogramming strategy in which these four transcription factors are expressed from doxycycline (dox)-inducible lentiviral vectors. Using these inducible constructs, we derived induced pluripotent stem (iPS) cells from mouse embryonic fibroblasts (MEFs) and found that transgene silencing is a prerequisite for normal cell differentiation. We have analyzed the timing of known pluripotency marker activation during mouse iPS cell derivation and observed that alkaline phosphatase (AP) was activated first, followed by stage-specific embryonic antigen 1 (SSEA1). Expression of Nanog and the endogenous Oct4 gene, marking fully reprogrammed cells, was only observed late in the process. Importantly, the virally transduced cDNAs needed to be expressed for at least 12 days in order to generate iPS cells. Our results are a step toward understanding some of the molecular events governing epigenetic reprogramming.

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Bovine in vitro-produced (IVP) and nuclear transfer (NT)-derived embryos differ from their in vivo-developed counterparts in a number of characteristics. A preeminent observation is the occurrence of the large offspring syndrome, which is correlated with considerable embryonic fetal and postnatal losses. We summarize here results from our studies in which we compared gene expression patterns from IVP and NT-derived embryos with those from their IVP counterparts. Numerous aberrations were found in IVP and NT-derived embryos, including a complete lack of expression, an induced expression, or a significant up- or downregulation of a specific gene. These alterations may affect a number of physiological functions and are considered as a kind of stress response of the embryos to deficient environmental conditions. We hypothesize that the alterations are caused by epigenetic modifications, primarily by changes in the methylation patterns. Unravelling these epigenetic modifications is promising to reveal the underlying mechanisms of the large offspring syndrome.

Due to a production error, the first sentence of the fifth paragraph of the main text reads, "Second, both models of the associated cancers involve hypomethylation rather than hypermethylation." It should have read "Second, both models or the associated cancers involve hypomethylation rather than hypermethylation.