Increased sensitivity to chromatid aberration induction by bleomycin and neocarzinostatin results from alterations in a DNA damage response pathway

DNA damage response pathways coordinate the cellular response to DNA damage . To investigate the roles of tumor suppressor genes in these pathways, hum an lymphoblastoid cells (wild-type, p53-/-, ATM-/-) were treated for 1 h wi th 0-3 mug/ml of the radiomimetic compound bleomycin (BLM), and cells treat ed in G(2) were analyzed for chromatid aberrations. BLM-induced aberration frequencies were significantly increased, to the greatest extent in the ATM -/- cells and, to a lesser extent, in the p53-/- cells compared to wild-typ e cells. These observations are consistent with p53 and ATM acting in a dam age response pathway activated by DNA strand breaks. The consequences of di srupting this pathway were further investigated by studies using wortmannin , a PI-3 kinase and DNA repair inhibitor. Wortmannin significantly increase d the BLM-induced aberration frequencies in all but the ATM-/- cells, eleva ting the sensitivity of p53-/- cells to ATM--/-levels and that of wild-type cells to intermediate levels. These differential sensitivities suggest tha t the ATM phenotype is the result of dual cellular defects, one involving p 53 and the other a wortmannin-sensitive component. Similar studies in Brca2 +/- and Brca2+/- human lymphoblasts showed no increased sensitization to BL M in the absence of inhibitor, and differential sensitization by wortmannin . To determine if there was any substrate specificity for p53- and ATM-medi ated DNA damage responses, chromatid aberrations were assessed in wild-type , p53-/-, and ATM-/- cells exposed to 0-0.4 mug/ml neocarzinostatin (NCS) f or 1 h. In contrast to results with BLM, the p53-/- cells exhibited a low s ensitivity to NCS-induced aberrations, similar to wild-type, while ATM-/- c ells remained highly sensitive. This suggests that the response to BLM-and NCS-induced lesions involves different mechanisms. Published by Elsevier Sc ience B.V.