Genomic instability is one of the hallmarks of cancer cells. We are studying telomere function to both[unreadable] understand how chromosomes are maintained and to explore their role in the initiation and growth of[unreadable] cancer cells. Telomeres distinguish natural chromosome ends from double stranded DNA breaks. They[unreadable] maintain chromosome stability by protecting chromosome ends from processes that normally occur at[unreadable] breaks such as fusion, translocation and DNA degradation. Telomeres are maintained by telomerase,[unreadable] which adds the simple sequence repeat TTAGGG onto all ends. In the next five years we will take[unreadable] advantage of the telomerase null mouse, mTR-/-, to dissect the role of telomeres in tumor initiation and[unreadable] growth. Using this mouse we previously established that under some conditions short telomeres induce[unreadable] apoptosis and thus decrease tumor growth. In other genetic settings, however, short telomeres initiate[unreadable] chromosomal rearrangements and increase tumor formation. We will examine the proteins that influence[unreadable] which pathway will predominate in cells that have short, dysfunctional telomeres. We will use shRNAs[unreadable] to knock down the level of specific proteins that signal DNA damage, to determine their role in signaling[unreadable] the loss of telomere function. We will examine the role of these genes in both cultured cells in vitro and[unreadable] in tumor formation. To identify new proteins that may recognize dysfunctional telomeres we will screen[unreadable] for shRNAs that interfere with the response to short telomeres. Although telomerase is required for the[unreadable] growth of many tumor cells, some cells have mechanisms that maintain telomeres (alternative telomere[unreadable] lengthening , ALT) and allow growth in the absence of telomerase. We will examine which recombination[unreadable] pathway(s) play a role in the growth of ALT cells and whether interfering with these pathways[unreadable] inhibits the growth of mTR-/- tumors. Understanding the role of these ALT pathways in tumors is[unreadable] essential for potential cancer therapeutics since these alternative mechanisms will be selected for in[unreadable] human tumors that are treated with telomerase inhibitors. Finally, we will examine the role of telomerase[unreadable] activity in stem cells and how that may potentiate stem cell derived tumors. The hedgehog signaling[unreadable] pathway, which is active in many stem cells, is required for the growth of a variety of common human[unreadable] tumors. We will examine whether telomerase is a target of the hedgehog signaling pathway and what role[unreadable] telomerase activity plays in these stem cell derived tumors.