Year 2

Public Progress Report
A major risk in the regenerative medicine field is the potential of tumorigenesis in transplanted tissues. To minimize this risk, it is essential that we understand the mechanisms that normally suppress tumorigenesis in adult tissues. When transplanted, human embryonic stems invariably form tumors showing the natural tendency of these cells for uncontrolled cell growth. However, the natural counterparts of these cells that occur during human embryonic development rarely ever form tumors. This suggests that there are powerful embryonic programs that suppress unwanted cell growth as these cells differentiate into adult tissues. We have established a likely role for small molecules called microRNAs in this process. In the absence of microRNAs, the embryonic stem cells fail to differentiate into adult tissues and hence proliferate indefinitely. The goals of our CIRM grant are to uncover the specific microRNAs that are responsible for differentiation and the suppression of cell growth. In the past year, we have made significant progress toward this goal. First, we have completed studies from the previous year and have now published one and have another press, both in high impact journals. One study was published in Nature Biotechnology where we show that a family of microRNAs can promote the de-differentiation of specialized cells back to unspecialized embryonic cells that than can be differentiated to all adult cell types. The other study is in press in Nature and shows how another family promotes differentiation and suppresses proliferation of embryonic stem cells. Furthermore, it shows that the two families: one promoting de-differentiaton and the other promoting differentiation antagonize one another in the decision of whether to continue to remain undifferentiated and proliferate. Second, we have performed large screens to identify many other miRNAs that promote differentiation both in mouse and human ES cells. We are now following the promising miRNAs and expect to uncover various different means of promoting ES cell differentiation and suppressing their tumor forming potential. Our findings provide insights into how one might use small RNAs to clinically manipulate cells in order to expand stem cells or promote their differentiation as needed to produce stable replacement tissues with diminished risk for tumor growth.