Characterization of human embryonic stem cells in vivo in the SCID-hu mouse model.
Human stem cells hold great promise to treat a variety of devastating diseases, but there are many barriers to successful clinical trials. One major challenge in embryonic stem (ES) cell research is to maintain cultures of human ES cells. There are variations in culture conditions that affect the quality of human ES cells, as well as cellular differentiation and genomic alterations. To advance ES cell research, it is essential to establish optimal culture conditions for ES cells. A second major challenge is to direct ES cells to differentiate into specific lineages. Since the development of mouse ES cells in 1980, there has not been a single reported case in which differentiation of an ES cell was guided to a specific cell type. A better understand is needed of the intrinsic and extrinsic factors that governing ES cell differentiation.
Over the past two decades, several laboratories including ours have shown that a mouse model implanted with intact human tissues provides the best physiologically-relevant model to study human stem cell biology. Based on our experience, we hypothesized that the engrafted human tissues in SCID mice could provide a physiologically relevant microenvironment to direct human ES cells to differentiate into mature cells of corresponding tissues. To test this, we used two NIH-approved ES cell lines (H1 and HSF-6) and three types of human fetal tissues (thymus, lung and pancreas). Human ES cells were injected directly into each tissue type engrafted in SCID mice. In most engrafted human tissues, we observed an aggressively growing tumor, which displayed characteristics of primitive, undifferentiated tumors rather than teratomas.
A better understanding the basis for this interesting phenomenon observed in our experimental system could have significant impacts in two important areas related to human ES cell research: tumorigenicity and expansion of human ES cells. In Aim 1, we will characterize these tumor-like cells as being ES cell-like or EC cell-like. If the cells resemble ES cells, it would suggest that we have identified a physiologically-relevant model system in which human ES cells could be expanded while maintaining their ES cell activity. In Aim 2, we will identify human tissue-derived factors responsible for ES cell expansion, which could mean the feasibility to develop a MEF-free system for robust human ES cell expansion. In addition, the identification of human tissue-derived factors could also provide an important tool to investigate intrinsic and extrinsic factors that govern self-renewal of human ES cells. However, if these tumor-like cells turn out to resemble EC cells, then it would suggest that EC cells are possible outcomes and could raise serious safety concerns for clinical use of ES cells. The identification of human tissue-derived factors (Aim 2), may lead to investigations of intrinsic and extrinsic factors important in EC development and of ways to prevent ES to EC transitions.
Over the past two decades, several laboratories, including ours, have shown that the SCID-hu mouse, an animal model that has intact human tissues implanted, provides the best physiologically-relevant model to study human stem cell biology. Based on our extensive experience with SCID-hu mice, we hypothesized that this model might also provide a physiologically-relevant microenvironment to direct human ES cells to differentiate into mature cells of corresponding tissues. Using two NIH-certified cell lines and three different human tissues in SCID mice, we observed aggressive tumor growth in most engrafted human tissues. These tumors displayed histological characteristics of primitive, undifferentiated tumors rather than teratomas, and molecular studies suggest that these tumor-like cells may be ES cells. A better understanding of the basis for this interesting phenomenon could have significant impacts in two important areas related to human ES cell research: tumorigenicity and expansion of human ES cells.
Benefit to California: Currently, culturing and expanding human ES cells in vitro is a major challenge. In order to advance human ES cell research, it is essential to establish optimal human ES cell culture conditions. One of the possible outcome of our proposal is the identification of human ES cell proliferation factor(s). By working with the Geron Corporation (Menlo Park, CA), we may be able to develop an optimal ex vivo culture system to expand high quality human ES cells for potential clinical application and to distribute to researchers in California who are interested in doing human ES cell research. Another possible outcome of our proposal is to alleviate the risks and concerns of using human ES cell-derived products in clinical application. Again, we will work with the Geron Corporation and medical centers in California, including the City of Hope, to investigate this issue. The outcome of this proposal could help to establish California as a premier state in human ES cell research.