The Stanley Scott Cancer Center, Louisiana State University Health Sciences Center, New Orleans, Louisiana, United States of America.
PLoS One. 2013;8(2):e57871. doi: 10.1371/journal.pone.0057871. Epub 2013 Feb 22.
We previously showed that DNA fragmentation factor, which comprises a caspase-3-activated DNase (CAD) and its inhibitor (ICAD), may influence the rate of cell death by generating PARP-1-activating DNA breaks. Here we tested the hypothesis that ICAD-deficient colon epithelial cells exhibiting resistance to death stimuli may accumulate additional genetic modifications, leading to a tumorigenic phenotype. We show that ICAD deficiency may be associated with colon malignancy in humans. Indeed, an examination of ICAD expression using immunohistochemistry in an array of both colon cancer and normal tissues revealed that ICAD expression levels were severely compromised in the cancerous tissues. Upon DNA damage caused by a low dose of irradiation, ICAD cells acquire a tumorigenic phenotype. Colon epithelial cells derived from ICAD mice showed a significant resistance to death induced by the colon carcinogen dimethylhydrazine in vitro and in mice. Such resistance was associated with a decrease in PARP-1 activation. In an animal model of dimethylhydrazine-induced colon tumorigenesis, ICAD(-/-) mice developed significantly higher numbers of tumors with markedly larger sizes than the wild-type counterparts. Interestingly, the phenotype of the ICAD(-/-) mice was not associated with a significant increase in the precancerous aberrant crypt foci suggesting a potential link to tumor progression rather than initiation. More importantly, ICAD deficiency was associated with severe genomic instability as assessed by array comparative genomic hybridization. Such genomic instability consisted most prominently of amplifications but with sizable deletions as compared to the wild-type counterparts affecting several cancer-related genes including RAF-1, GSN, LMO3, and Fzd6 independently of p53. Altogether, our results present a viable case for the involvement of ICAD deficiency in colon carcinogenesis and show that apoptosis and genomic instability may comprise the means by which such deficiency may contribute to the process of increasing susceptibility to carcinogen-induced tumorigenesis.
我们之前曾表明，包含半胱天冬酶-3 激活的 DNA 酶（CAD）及其抑制剂（ICAD）的 DNA 片段化因子可能会通过产生激活 PARP-1 的 DNA 断裂来影响细胞死亡的速度。在这里，我们检验了这样一个假设，即对死亡刺激表现出抗性的 ICAD 缺陷结肠上皮细胞可能会积累额外的遗传修饰，从而导致肿瘤发生表型。我们表明，ICAD 缺陷可能与人类结肠癌有关。事实上，通过对结肠癌和正常组织的阵列进行免疫组织化学检查，发现 CAD 的表达水平在癌变组织中严重受损。在低剂量辐射引起的 DNA 损伤后，ICAD 细胞获得了致瘤表型。从 ICAD 小鼠衍生的结肠上皮细胞在体外和小鼠中对结肠致癌物二甲基肼诱导的死亡表现出明显的抗性。这种抗性与 PARP-1 激活的减少有关。在二甲基肼诱导的结肠癌动物模型中，ICAD(-/-) 小鼠比野生型小鼠形成的肿瘤数量明显更多，且肿瘤体积明显更大。有趣的是，ICAD(-/-) 小鼠的表型与癌前异常隐窝焦点的显著增加无关，这表明与肿瘤进展而不是肿瘤起始有关。更重要的是，如通过比较基因组杂交的阵列评估的那样，ICAD 缺陷与严重的基因组不稳定性有关。与野生型相比，这种基因组不稳定性主要由扩增组成，但也有大量缺失，影响了几个与癌症相关的基因，包括 RAF-1、GSN、LMO3 和 Fzd6，而与 p53 无关。总之，我们的结果提出了一个可行的案例，即 ICAD 缺陷参与结肠癌的发生，并表明凋亡和基因组不稳定性可能构成了这种缺陷促进致癌物诱导的肿瘤发生易感性增加的机制。
