DNA damage response pathways in cancer causation and treatment

Many of the insights that we have gained into the mechanisms involved in cellular DNA damage response pathways have come from studies of human cancer susceptibility syndromes that are altered in DNA damage responses. One of these disorders, ataxia-telangiectasia (A-T), is characterized by multiple physiologic abnormalities, including neurodegeneration, immunologic abnormalities, cancer predisposition, sterility, and metabolic abnormalities. The gene mutated in this disorder, Atm, is a protein kinase that is activated by the introduction of DNA double-strand breaks in cells. Atm activity is required for cell cycle arrests induced by ionizing irradiation (IR) in G1, S, and G2 phases of the cell cycle. Several targets of the Atm kinase have been identified that participate in these IR-induced cell cycle arrests. For example, phosphorylation of p53, mdm2, and Chk2 participate in the G1 checkpoint; Nbs1, Brca1, FancD2, and Smc1 participate in the transient IR-induced S-phase arrest; and Brca1 and hRad17 have been implicated in the G2/M checkpoint. Although Atm is critical for cellular responses to IR, related kinases, such as Atr, appear to be important for responses to other cellular stresses [1]. Some substrates appear to be shared by the two kinases, with the major difference being which stimulus is present and which kinase is used to initiate the signaling pathway.Characterization of these Atm substrates permitted us to manipulate these proteins in cell lines and to selectively abrogate single or multiple checkpoints. Using this approach, we demonstrated that abrogation of checkpoints does not by itself result in radiosensitivity. Although this has been known for several years in regards to the S-phase checkpoint, it was a surprising finding that abrogation of the G2/M checkpoint did not cause radiosensitivity. This observation suggested that some other function of Atm, other than checkpoint control, was important for cellular survival following ionizing irradiation.