[Show abstract][Hide abstract] ABSTRACT: Human embryonic stem cells (HESCs) and induced pluripotent stem cells (HiPSCs) offer an immense potential as a source of cells for regenerative medicine. However, the ability of undifferentiated HESCs and HiPSCs to produce tumors in vivo presents a major obstacle for the translation of this potential into clinical reality. Therefore, characterizing the nature of HESC- and HiPSC-derived tumors, especially their malignant potential, is extremely important in order to evaluate the risk involved in their clinical use. Here we review recent observations on the tumorigenicity of human pluripotent stem cells. We argue that diploid, early passage, HESCs produce benign teratomas without undergoing genetic modifications. Conversely, HESCs that acquired genetic or epigenetic changes upon adaptation to in vitro culture can produce malignant teratocarcinomas. We discuss the molecular mechanisms of HESC tumorigenicity and suggest approaches to prevent tumor formation from these cells. We also discuss the differences in the tumorigenicity between mouse embryonic stem cells (MESCs) and HESCs, and suggest methodologies that may help to identify cellular markers for culture adapted HESCs.
[Show abstract][Hide abstract] ABSTRACT: CpG island-like sequences are commonly thought to provide the sole signals for designating constitutively unmethylated regions in the genome, thus generating open chromatin domains within a sea of global repression. Using a new database obtained from comprehensive microarray analysis, we show that unmethylated regions (UMRs) seem to be formed during early embryogenesis, not as a result of CpG-ness, but rather through the recognition of specific sequence motifs closely associated with transcription start sites. This same system probably brings about the resetting of pluripotency genes during somatic cell reprogramming. The data also reveal a new class of nonpromoter UMRs that become de novo methylated in a tissue-specific manner during development, and this process may be involved in gene regulation. In short, we show that UMRs are an important aspect of genome structure that have a dynamic role in development.
[Show abstract][Hide abstract] ABSTRACT: Teratomas derived from human embryonic stem (hES) cells are unique among oncogenic phenomena as they are polyclonal and develop from apparently normal cells. A deeper understanding of this process should aid in the development of safer cell therapies and may help elucidate the basic principles of tumor initiation. We find that transplantation of diploid hES cells from four independent cell lines generates benign teratomas with no sign of malignancy or persisting embryonal carcinoma-like cells. In contrast, mouse embryonic stem (mES) cells from four cell lines consistently generate malignant teratocarcinomas. Global gene expression analysis shows that survivin (BIRC5), an anti-apoptotic oncofetal gene, is highly expressed in hES cells and teratomas but not in embryoid bodies. Genetic and pharmacological ablation of survivin induces apoptosis in hES cells and in teratomas both in vitro and in vivo. We suggest that continued expression of survivin upon differentiation in vivo may contribute to teratoma formation by hES cells.
[Show abstract][Hide abstract] ABSTRACT: Human embryonic stem cells (HESCs) are the in vitro descendants of the pluripotent inner cell mass (ICM) of human blastocyst stage embryos. HESCs can be kept undifferentiated in culture or be differentiated to tissues representing all three germ layers, both in vivo and in vitro. These properties make HESC-based therapy remarkably appealing for the treatment of various disorders. Upon transplantation in vivo, undifferentiated HESCs rapidly generate the formation of large tumors called teratomas. These are benign masses of haphazardly differentiated tissues. Teratomas also appear spontaneously in humans and in mice. When they also encompass a core of malignant undifferentiated cells, these tumors are defined as teratocarcinomas. These malignant undifferentiated cells are termed embryonic carcinoma (EC), and are the malignant counterparts of embryonic stem cells. Here we review the history of experimental teratomas and teratocarcinomas, from spontaneous teratocarcinomas in mice to induced teratomas by HESC transplantation. We then discuss cellular and molecular aspects of the tumorigenicity of HESCs. We also describe the utilization of HESC-induced teratomas for the modeling of early human embryogenesis and for modeling developmental diseases. The problem of HESC-induced teratomas may also impede or prevent future HESC-based therapies. We thus conclude with a survey of approaches to evade HESC-induced tumor formation.
Advances in Cancer Research 02/2008; 100:133-58. · 6.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Differentiation of human embryonic stem cells (HESCs) can be studied in vivo through the induction of teratomas in immune-deficient mice. Cells within the teratomas differentiate into all three embryonic germ layers. However, the exact nature of the proliferation and differentiation of HESCs within the teratoma is not fully characterized, and it is not clear whether the differentiation is cell autonomous or affected by neighboring cells. Here, we establish a genetic approach to study the clonality of differentiation in teratomas using a mixture of HESC lines. We first demonstrate, by means of 5-bromo-2'-deoxyuridine incorporation, that cell proliferation occurs throughout the teratoma, and that there are no clusters of undifferentiated-proliferating cells. Using a combination of laser capture microdissection and DNA fingerprinting analysis, we show that different cell lines contribute mutually to the same distinctive tissue structures. Further support for the nonclonal differentiation within the teratoma was achieved by fluorescence in situ hybridization analysis of sex chromosomes. We therefore suggest that in vivo differentiation of HESCs is polyclonal and, thus, may not be cell autonomous, stressing the need for a three-dimensional growth in order to achieve complex differentiation of HESCs. Disclosure of potential conflicts of interest is found at the end of this article.
[Show abstract][Hide abstract] ABSTRACT: The International Stem Cell Initiative characterized 59 human embryonic stem cell lines from 17 laboratories worldwide. Despite diverse genotypes and different techniques used for derivation and maintenance, all lines exhibited similar expression patterns for several markers of human embryonic stem cells. They expressed the glycolipid antigens SSEA3 and SSEA4, the keratan sulfate antigens TRA-1-60, TRA-1-81, GCTM2 and GCT343, and the protein antigens CD9, Thy1 (also known as CD90), tissue-nonspecific alkaline phosphatase and class 1 HLA, as well as the strongly developmentally regulated genes NANOG, POU5F1 (formerly known as OCT4), TDGF1, DNMT3B, GABRB3 and GDF3. Nevertheless, the lines were not identical: differences in expression of several lineage markers were evident, and several imprinted genes showed generally similar allele-specific expression patterns, but some gene-dependent variation was observed. Also, some female lines expressed readily detectable levels of XIST whereas others did not. No significant contamination of the lines with mycoplasma, bacteria or cytopathic viruses was detected.