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Telomere dysfunction-induced senescence as a barrier for the development of breast carcinomas
Tumor Cell Biology V
Human breast cancer arises through the acquisition of genetic changes that endow precursor cancer cells with a critical threshold of cancer-relevant genetic lesions. These complex genomic alterations confer upon precursor cancer cells the ability to grow indefinitely and to metastasize to distant sites. One important mechanism underlying a cell?s tumorigenic potential is the status of its telomere. Telomeres are G-rich simple repeat sequences that serve to prevent chromosomal ends from being recognized as DNA double-strand breaks (DSBs). Dysfunctional telomeres resemble DSBs, leading to the formation of dicentric chromosomes that fuel high degrees of genomic instability. In the setting of an intact p53- dependent DNA damage response (DDR) pathway, this instability promotes cellular senescence, a potent tumor suppressor mechanism. However, rare cells that stochastically lose p53 function emerge from this sea of genomic instability and progress towards cancer. In human breast carcinomas, the observation that telomere dysfunction is associated with the transition from benign ductal hyperplasia to malignant DCIS strongly supports the notion that dysfunctional telomere-driven genomic instability initiates the development of breast cancer. In addition, telomerase reactivation is required for progression to full malignancy and metastasis. Our study design aims to generate mouse models that faithfully recapitulate the initiation and progression of human breast cancer in vivo. The telomere binding protein POT1 (protection of telomeres 1) is a single-stranded telomere binding protein that is essential for chromosomal end protection. We generated a Pot1 conditional knockout mouse, and show that deletion of Pot1 induces a potent DNA damage response at telomeres that triggers a senescence phenotype in the absence of p53. Deletion of Pot1 also results in extensive chromosomal fusions and progression to cancer in the setting of p53 deficiency. In this proposal we will address the following specific aims using Pot1 and p53 conditional knockout mice: (1) To develop mouse models to examine the role of telomere dysfunction in the pathogenesis of mammary carcinoma in the settings of p53 competence or deficiency (2) To investigate the mechanisms by which Pot1 loss induces a DNA damage response to initiate replicative senescence in mammary epithelial cells and (3) To characterize chromosomal aberrations in mouse breast tumor samples, and to investigate the function of Pot1 loss in human breast cancers. Our proposal should provide mechanistic insights into how the DDR pathway senses dysfunctional telomeres to promote senescence in vivo. The finding that senescence inhibits breast cancer formation could have therapeutic implications. It is likely that upregulation of p53 function by compounds currently undergoing clinical trials would favor the activation of the cellular senescence program, resulting in suppression of breast cancer.
The growth potential of all eukaryotic cells is critically dependent upon the maintenance of functional telomeres-protein-DNA complexes that cap the ends of chromosomes. Telomeres serve to protect eukaryotic chromosomal ends from being recognized as damaged DNA, and growing evidence suggests that critically shortened (dysfunctional) telomeres may help trigger the onset of cancer. Chromosomes with dysfunctional telomeres can fuse with one another, generating cancer causing chromosomal translocation common in human breast carcinomas. A set of DNA damage response (DDR) proteins has evolved to eliminate cells possessing damaged DNA by stopping cell proliferation (senescence) and promoting cell death (apoptosis). We have previously shown that dysfunctional telomeres potently engage the DDR pathway, leading to the onset of senescence when p53 is functional. p53, the "guardian of the genome," provides protection to the organism by inducing genes that allow time to repair damaged DNA or eliminating cells that are too damaged and harmful to the organism. However, in the absence of p53, dysfunctional telomeres can initiate cancer by promoting genomic instability. In this proposal, we will use mouse models to explore in detail the interplay between telomere dysfunction and the development of breast carcinoma in the setting of an intact or mutated p53-dependent DDR pathway. We hypothesize that dysfunctional telomeres will trigger senescence when p53 is functional, thereby protecting breast tissues from cancer. However, when p53 is mutated, dysfunctional telomeres will promote cancer formation by increasing genomic instability. We recently generated a novel mouse model in which the critical telomere binding protein POT1 (protection of telomeres 1) can be conditionally deleted in breast tissues, to allow us to study the effect of telomere dysfunction in the presence or absence of p53 function. For breast carcinomas that escape senescence, we will use molecular cytogenetic and genomic techniques to identify oncogenes and tumor suppressors that promote tumorigenesis and determine whether they are disrupted in human breast cancers as well. Our proposal should provide mechanistic insights into how the DDR pathway senses and processes dysfunctional telomeres, and whether senescence can indeed suppress breast cancer development in vivo. The finding that induction of a senescence program can inhibit breast cancer initiation may have therapeutic implications. It is likely that upregulation of p53 function by compounds currently undergoing clinical trials would favor the activation of the cellular senescence program, resulting in suppression of breast tumorigenesis.