The Tumorigenicity of Human Embryonic Stem Cells
Introduction
Human embryonic stem cells (HESCs) are pluripotent cells derived from the inner cell mass (ICM) of a human blastocyst stage embryo (Thomson et al., 1998). They are characterized by their ability to self‐renew by cellular divisions and their ability to differentiate to all somatic tissue of the embryo (pluripotency). HESCs grow in tightly packed colonies and, if supplied with specific culture requirements such as a supportive feeder layer (usually mitotically arrested mouse embryonic fibroblasts), can remain undifferentiated indefinitely. HESCs are also defined by the expression of a battery of typical genes, the most renown among them are Oct4, Nanog, Sox2, high telomerase activity, and typical cell surface markers such as SSEA3, SSEA4, TRA‐1–60, TRA‐1–81, and tissue‐specific alkaline phosphatase (Adewumi et al., 2007).
HESCs currently open some of the most promising avenues in the field of regenerative medicine, and efforts are being made to establish HESC‐based therapy for various diseases such as Parkinson's disease, heart failures, and diabetes (Blum and Benvenisty, 2005). Accordingly, reports on the successful differentiation of HESCs to CNS neurons, cardiomyocytes, insulin‐secreting cells, and many other cell types are rapidly accumulating (Blum and Benvenisty, 2005). HESCs can spontaneously differentiate in vitro in the form of embryoid bodies (EBs) (Itskovitz‐Eldor et al., 2000) (Fig. 1). However, HESCs ability to spontaneously differentiate is best manifested when these cells are transplanted in vivo into immunosuppressed mice, where they form typical gross looking tumors termed teratomas, in which the cells differentiate disorderedly to various tissue types of the embryo (Przyborski, 2005) (Fig. 1). This tumorigenic nature of HESCs is considered a major hurdle for their clinical utilization, but it can be valuable for other purposes, such as studying early human development. This tumorigenic nature of HESCs is also important for the assessment of the differentiation potential of newly derived pluripotent cells, since blastocyst injection of HESCs is obviously impractical (Lensch et al., 2007).
Almost two decades before HESCs were first successfully derived from human embryos (Thomson et al., 1998), the first mouse embryonic stem (ES) cells were derived (Evans 1981, Martin 1981). These successful establishments of blastocyst‐derived ES cells are based on work previously performed on pluripotent cells that were isolated from teratocarcinomas (Andrews 2002, Damjanov 2005, Solter 2006). These tumor cells are very close counterparts of HESCs because of their ability to self‐renew and to differentiate in culture. HESCs are unique in that they are tumorigenic yet perfectly normal in every other aspect. They thus can make an excellent tool for the understanding of tumorigenicity. Accordingly, HESC and many tumor cells hold some similarities, such as the aforementioned self‐renewal and undifferentiated phenotype, along with the expression of telomerase, the ability for in vivo angiogenesis, shortened cell cycle and, of course, the ability to generate tumors upon transplantation in vivo (Dreesen and Brivanlou, 2007).
In this review we will discuss the relations between HESCs and their tumor counterparts, outlining the differences and similarities between them. We will also discuss cellular and molecular aspects of HESC tumorigenicity, the use of HESC‐induced tumors for studying human embryonic development, and conclude the discussion by reviewing some of the approaches for evading the appearance of these tumors in clinical utilization.
Section snippets
Spontaneous Teratomas and Teratocarcinomas
Spontaneously occurring teratomas and teratocarcinomas represent a unique set of tumors. They are categorized among the group of germ cell tumors (GCTs), and are characterized by the presence of haphazardly arranged differentiated tissues representing the three embryonic germ layers. This points to their origin from a pluripotent precursor (Ulbright, 2005). GCTs appear both in gonadal and extragonadal sites along the body midline, and are classified into five pathological groups (Looijenga 2007
In Vivo Differentiation of Embryonic Carcinoma Cells
As is known from every knockout mouse thus far made, normal euploid mouse ES cells lose their tumorigenicity by incorporation into age‐complemented embryonic environment. This is conclusively manifested by their incorporation into the blastocyst to form completely normal, germ line transmitting chimera (Bradley et al., 1984). Aneuploid mouse teratocarcinoma‐derived EC cells, however, could not be completely reversed by injection to the blastocyst. It appeared that some of the chimera progeny
HESC‐Induced Teratomas as a Model for Early Human Development
In line with the notion that normal HESCs can differentiate to form teratoma‐like tumors without being transformed, HESC‐derived tumors are seen by some not as a tumor at all, but rather as failed progress of normal embryonic development, due to the incorrect localization of the developing cells (Lensch 2007, Lensch 2007). In that context, this tumorigenic activity of HESCs can function as a very promising tool by which one can study the very early stages of human embryonic development. These
HESC‐Induced Teratomas as a Clinical Hurdle
There are still several obstacles on the way to the implementation of HESCs in cellular therapy, the most prominent being immune rejection and the tumorigenicity of HESCs‐based tissues (Parson 2006, Vogel 2005). Recently, the issue of immune rejection of HESC‐based grafts has made enormous progress towards resolution with the report of two different approaches to generating patient‐specific pluripotent stem cells. First, Byrne et al. (2007) have succeeded in the cloning of a nonhuman primate, a
Concluding Remarks
Blastocyst‐derived ES cells are the in vitro counterparts of the malignant EC cells found in spontaneous teratocarcinomas. The study of HESCs has emerged from pivotal experiments and observations on teratocarcinoma and teratoma GCTs, and is now coming of age.
Being the in vitro successors of the pluripotent ICM cells, it is hypothesized that the differentiation of nonadapted HESCs in vivo resembles normal embryonic processes, albeit in a disorganized manner. It is thus attractive to use
acnowledgments
We thank Yoav Mayshar and Rina Klinov for critically reading the manuscript and Tamar Golan‐Lev for assistance with preparation of the figures. This work was partially supported by funds from Bereshit Consortium, the Israeli Ministry of Trade and Industry (Grant number 37675), and by the European Community (ESTOOLS, Grant number 018739).
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