We are investigating the molecular mechanisms of p53-mediated apoptosis. Our previous studies have identified a novel pathway of apoptosis involving the functional and physical interaction of p53 with DNA helicases, XPB and XPD. We have extended these studies to other members of the RecQ helicase family, BLM and WRN, that are linked to cancer predisposition syndromes, Bloom and Werner, respectively. Germline mutations in WRN are found in patients with the premature aging and cancer susceptibility syndrome known as Werner syndrome (WS). p53 binds to the WRN protein in vivo and in vitro through its carboxyl terminus. WS fibroblasts have an attenuated p53-mediated apoptotic response, and this deficiency can be rescued by expression of wild-type WRN. These data support the hypothesis that p53 can induce apoptosis through the modulation of specific DExH-containing DNA helicases and may have implications for the cancer predisposition observed in WS patients. p53 binds to BLM or WRN and inhibits their DNA helicase activities. Wild-type p53 greater than p53 mutants (248W or 273H) prevents the unwinding of Holliday Junctions (HJ) by BLM or WRN. Modification of the carboxyl terminal of p53 by phosphorylation or the Pab421 antibody reduces p53 binding to HJ and p53-mediated inhibition of WRN or BLM unwinding of HJ. These results are consistent with the hypothesis that p53 contributes to the sensing of alternative DNA structures, e.g., HJ, assembly of a repairsome, and modulation of DNA recombination repair and apoptosis. Bloom syndrome (BS) is an autosomal recessive genomic instability syndrome characterized by growth retardation, immune deficiency and cancer predisposition. Similar to cells from XPB, XPD or WS individuals who have an attenuated p53-dependent apoptotic pathway, p53-mediated apoptosis also is defective in BS fibroblasts. This apoptotic pathway can be functionally rescued by the expression of the wild-type (wt) BLM gene. Lymphoblastoid cell lines (LCLs) derived from BS donors are resistant to either gamma-radiation or adriamycin-induced cell killing, and also can be rescued by the wt BLM. In contrast, BS cells have a normal Fas-mediated apoptosis, and a normal DNA damage-induced p53 accumulation and G1-S and G2-M cell cycle checkpoints. BLM localizes in nuclear foci identified as PML nuclear bodies (NBs), a structure that also contains the promyelocytic leukemia protein (PML), Rb, SUMO-1 and others, and may be involved in apoptosis. Cells from Li-Fraumeni syndrome (LFS) patients carrying p53 germline mutations have a decreased number of BLM foci. The induction of p53 increased the number of BLM foci, but did not alter either BLM levels or the number of NBs. These results indicate a novel function of p53 and are consistent with the hypothesis that, nuclear trafficking of BLM to NBs mediated by p53, contributes to its apoptotic activity.
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