Most cancers arise by an evolutionary process as mutations accumulate in chromosomes of somatic cells allowing them to escape the proliferative restrains that control cell growth. To counteract uncontrolled cellular proliferation, metazoans developed a number of innate tumor suppressing mechanisms, one of them being a terminal growth arrest called cellular senescence. In recent years, as we and others identified novel senescence markers, the biological role and importance of this stress response in arresting cells in pre- malignant lesions has been demonstrated. Why cells undergo senescence in vivo and thereby prevent tumor progression in humans however, remains poorly understood. Studies in human cell cultures revealed that cellular senescence is triggered by a number of stresses including dysfunction of telomeres, the physical ends of linear chromosomes. Our studies revealed that when telomeres become critically short, due to continuous cell proliferation and other stresses, they become recognized as double strand DNA breaks and initiate a signaling cascade that results in telomere dysfunction induced cellular senescence (TDIS). In this proposal we demonstrate that the majority of cells in three human cancer precursor lesions, but not in their malignant cancer counterparts, display dysfunctional telomeres and other markers of cellular senescence. Our data therefore indicate that TDIS is a critical and universal tumor suppressing mechanism that limits the growth of pre-malignant human neoplasias. In addition, we discovered that oncogenic signals, often associated with initiation of cancer growth, dramatically accelerate telomere erosion and dysfunction in pre-malignant human cells. It is therefore possible that telomeres act as sentinels of hyperproliferative stresses that rapidly induce cellular senescence once cellular growth control mechanisms become compromised. To test these predictions we will 1) analyze a number of common cancer precursor lesions as well as their malignant counterparts for markers of TDIS by fluorescence microscopy and 2) generate a mouse tumor model system that can analyze whether TDIS suppresses malignant progression of transformed human cells in tumor cell xenografts. In addition we will 3) determine the causes for telomere dysfunction under conditions that cause aberrant cell proliferation. We will use assays to measure telomere-shortening, -dysfunction and - structure, employ forward- and reverse- genetic interventions to manipulate telomere dysfunction, and identify telomere maintenance factors critical for triggering TDIS. The proposed experiments will reveal the impact of TDIS in preventing human tumor progression, identify novel diagnostic markers for tumor stage, and provide detailed insights into the causes of telomere dysfunction in pre-malignant human cells. This knowledge is critical for developing novel therapies that prevent the malignant progression of human cancer.