Submitted by: sdemir   Date: 2009-09-16 10:42
DNA Damage Responses: Mechanisms and Roles in Human Disease
2007 G.H.A. Clowes Memorial Award Lecture
Michael B. Kastan

Significant progress has been made in recent years in elucidating the molecular controls of cellular responses to DNA damage in mammalian cells. Much of our understanding of the mechanisms involved in cellular DNA damage response pathways has come from studies of human cancer susceptibility syndromes that are altered in DNA damage responses. Ataxia-telangiectasia mutated (ATM), the gene mutated in the disorder ataxia-telangiectasia, codes for a protein kinase that is a central mediator of responses to DNA double-strand breaks (DSB) in cells. Once activated, ATM phosphorylates numerous substrates in the cell that modulate the response of the cell to the DNA damage. We recently developed a novel system to create DNA DSBs at defined endogenous sites in the human genome and used this system to detect protein recruitment and loss at and around these breaks by chromatin immunoprecipitation. Results from this system showed the functional importance of ATM kinase activity and phosphorylation in the response to DSBs and supported a model in which ordered chromatin structure changes that occur after DNA breakage and that depend on functional NBS1 and ATM facilitate DNA DSB repair. Insights about these pathways provide us with opportunities to develop new approaches to benefit patients. Examples and opportunities for developing inhibitors that act as sensitizers to chemotherapy or radiation therapy or activators that could improve responses to cellular stresses, such as oxidative damage, are discussed. Relevant to the latter, we have shown benefits of an ATM activator in disease settings ranging from metabolic syndrome to cancer prevention.

Schematic representation of the signal transduction pathways initiated by ATM following ionizing irradiation (IR) and the functional roles of the ATM targets. Following ionizing radiation, the ATM kinase is activated via intermolecular autophosphorylation of Ser1981 and subsequent dissociation of the ATM homodimer. The ATM kinase is then free to circulate in the cell and subsequently phosphorylate chk2, p53, and mdm2 to initiate the G1 arrest; NBS1, FANCD2, BRCA1, and SMC1 to initiate the S-phase arrest; and BRCA1 and hRad17 to cause a G2 arrest. SMC1 is the only target of ATM where mutation of the ATM phosphorylation sites affects radiosensitivity. BRCA1 and NBS1 seem to be required for SMC1 phosphorylation; thus, absence of either of these proteins results in radiosensitivity.

Proposed model for an ionizing radiation–induced signaling pathway. Chromatin or nuclear structure changes caused by DNA breakage or other mechanisms lead to intermolecular autophosphorylation of ATM dimers, resulting in release of phosphorylated and active ATM monomers. If DNA strand breaks are present, several proteins, including NBS1 and BRCA1, are recruited to the sites of the breaks independent of the ATM activation process. After activation, monomeric ATM can phosphorylate nucleoplasmic substrates, such as p53, and if NBS1 and BRCA1 have localized to DNA breaks, activated ATM is recruited to the break. At the DNA break, activated ATM can phosphorylate substrates, including SMC1, NBS1, and BRCA1. The phosphorylation of SMC1 reduces chromosomal breakage and enhances cell survival.
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