Mutational and Functional Analysis of the p53 Tumor Suppressor Gene

我们正在研究P53介导的细胞凋亡的分子机制。我们以前的研究发现了一种新的细胞凋亡途径，涉及P53与DNA解旋酶、XPB和XPD的功能和物理相互作用。我们已经将这些研究扩展到RecQ解旋酶家族的其他成员BLM和WRN，它们分别与癌症易感综合征Bloom和Werner有关。WRN的胚系突变在早衰和癌症易感性综合征(Werner综合征(WS))的患者中被发现。在体内和体外，P53通过其羧基末端与WRN蛋白结合。WS成纤维细胞具有减弱的P53介导的凋亡反应，这一缺陷可以通过表达野生型WRN来挽救。这些数据支持这样的假设，即P53可以通过调节特定的含DExH的DNA解旋酶来诱导细胞凋亡，并可能与WS患者观察到的癌症易感性有关。P53与BLM或WRN结合并抑制其DNA解旋酶活性。野生型p53比突变体(248W或273H)更大的p53阻止了BLM或WRN对Holliday Junctions(HJ)的解离。通过磷酸化或Pab421抗体修饰P53的羧基末端，可减少P53与HJ的结合，以及P53介导的WRN或BLM对HJ解离的抑制。这些结果与以下假设相一致，即P53有助于感知可供选择的DNA结构，例如HJ，修复小体的组装，以及DNA重组修复和凋亡的调控。Bloom综合征(BS)是一种常染色体隐性遗传性基因组不稳定综合征，以生长迟缓、免疫缺陷和癌症易感性为特征。与XPB、XPD或WS患者的细胞一样，BS成纤维细胞中P53介导的细胞凋亡也是缺陷的。野生型(Wt)BLM基因的表达可以从功能上挽救这一凋亡途径。来自BS供者的淋巴母细胞系(LCLS)对伽玛射线或阿霉素诱导的细胞杀伤具有抵抗力，也可以被wt BLM拯救。相比之下，BS细胞有正常的Fas介导的凋亡，以及正常的DNA损伤诱导的P53积聚和G1-S和G2-M细胞周期检查点。BLM定位于PML核小体(NBS)，该结构还含有早幼粒细胞白血病蛋白(PML)、Rb、SUMO-1等，并可能参与细胞凋亡。携带p53胚系突变的Li-Fraumeni综合征(LFS)患者的细胞BLM灶数量减少。P53的诱导增加了BLM病灶的数量，但不改变BLM水平和NBS的数量。这些结果表明了P53的一种新的功能，并与P53介导的BLM向NBS的核转运有助于其凋亡活性的假设相一致。

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.