The overarching goal of this study is to define novel roles for CREB/ATF transcription factors in genome protection. Members of the CREB (cAMP response element-binding protein) family, including CREB, CREM, and ATF1, mediate transcriptional response to cAMP, calcium, and growth factors. It has long been known that CREB plays an important role in the metabolic regulation-where glucagon stimulates CREB-dependent transcriptional programs that promote gluconeogenesis-and the brain, where CREB promotes synaptic long-term potentiation and memory formation. For the past several years, our group has been studying CREB in the noncanonical context of DNA damage. We discovered that CREB is regulated by the ATM (ataxia-telangiectasia-mutated) protein kinase, which is a tumor suppressor protein that functions as master regulator of the DNA damage response (DDR). ATM phosphorylates CREB on a highly conserved cluster of Ser/Thr residues, within the CREB transactivation domain termed the ATM/CK cluster. Phosphorylation of the ATM/CK cluster antagonizes CREB transcriptional functions in vitro; however, the physiologic ramifications of CREB phosphorylation in vivo have not been established. To ascertain the functional importance of ATM-CREB regulation during the DDR, we generated CREB gene targeted mice (CREBS111A/S111A) encoding a CREB protein that is resistant to phosphorylation by ATM. Tissues from CREBS111A/S111A mice show constitutively elevated CREB activity that is resistant to downregulation by DNA damage. We will now use the CREBS111A/S111A model to test our major hypothesis that ATM-mediated CREB phosphorylation is required for downregulation of CREB transcriptional programs and tumor suppression. A key aspect of these studies is to test whether the ATM-CREB pathway synergizes with a parallel ATM-p53 pathway to mediate tumor suppression in vivo. Finally, even though CREB is best known for regulating gene expression, we recently discovered that CREB is unexpectedly recruited to sites of DNA damage. One exciting idea to emerge from this finding is that CREB harbors a novel function in DNA double-strand break repair that might be regulated by ATM. Experiments outlined in this study will test this hypothesis, which has implications for genome protection and tumor suppression. In sum, our studies will define new functions for CREB family transcription factors in genome protection. The Specific Aims of the proposal are to: (i) Characterize gene expression mechanisms in CREBS111A/S111A mice; (ii) Elucidate mechanisms and functional consequences of CREB recruitment to DNA damage; and (iii) Test whether CREBS111A/S111A mice manifest genome instability.