Byun Do-Sun, Chae Kwon-Seok, Ryu Byung-Kyu, Lee Min-Goo, Chi Sung-Gil
Department of Pathology, College of Medicine, Kyung Hee University, Seoul, Korea.
Int J Cancer. 2002 Apr 10;98(5):718-23. doi: 10.1002/ijc.10253.
p53R2, a recently identified putative tumor suppressor located at 8q23.1, encodes a protein with striking similarity to a small subunit of ribonucleotide reductase. p53R2 is directly induced by wild-type p53 and involved in the p53 checkpoint for repair of damaged DNA, raising the possibility that mutational inactivation of p53R2 may contribute to the development and progression of human malignancies. To explore the p53R2's candidacy for a suppressor in gastric tumorigenesis, we examined the expression and mutation status of p53R2 in 166 gastric specimens including 90 primary adenocarcinomas and 15 cell lines. In response to genotoxic damages, p53R2 transcription was clearly activated in wild-type but not mutant-type p53-carrying cells while basal expression of p53R2 in undamaged cells showed no association with the mutational status of p53. Host cell reactivation assay revealed that p53R2 enhances DNA repair efficiency and plays a role in the p53-mediated repair of damaged DNA, whereas no significant effect of p53R2 on cell growth and apoptosis was detected in flow cytometry and [(3)H]thymidine incorporation assays. p53R2 transcript was expressed in all normal and tumor tissues and its expression levels were not significantly different between normal and malignant carcinoma tissues. p53R2 expression showed no correlation with stage, grade and histological types of tumors. Moreover, no tumor-specific reduction of p53R2 was detected in 30 matched sets. Mutational analysis of p53R2 in 105 carcinomas including 15 cell lines also failed to detect any evidences for genomic deletion or somatic mutations leading to amino acid substitutions or frameshift whereas 31% (28 of 90) of the same primary tumors showed p53 alterations. Whereas 82% (23 of 28) of the mutant p53-carrying primary tumors expressed abnormally low p21(Waf1), no association of p53R2 expression with the p53 status was recognized, suggesting that basal transcription of p53R2 is regulated through the p53-independent mechanism. Collectively, our study indicates that although p53R2 is induced in a p53-dependent manner and involved the p53-mediated DNA repair in gastric epithelial cells, it is not a critical target of genetic inactivation in gastric tumorigenesis.
p53R2是最近发现的一个假定的肿瘤抑制基因，位于8q23.1，编码一种与核糖核苷酸还原酶的一个小亚基具有显著相似性的蛋白质。p53R2由野生型p53直接诱导产生，并参与受损DNA修复的p53检查点，这增加了p53R2的突变失活可能有助于人类恶性肿瘤发生和发展的可能性。为了探究p53R2在胃癌发生中作为肿瘤抑制基因的可能性，我们检测了166份胃标本（包括90例原发性腺癌和15个细胞系）中p53R2的表达和突变状态。在对基因毒性损伤的反应中，p53R2转录在携带野生型而非突变型p53的细胞中被明显激活，而未受损细胞中p53R2的基础表达与p53的突变状态无关。宿主细胞再激活试验表明，p53R2提高了DNA修复效率，并在p53介导的受损DNA修复中发挥作用，而在流式细胞术和[³H]胸腺嘧啶核苷掺入试验中未检测到p53R2对细胞生长和凋亡有显著影响。p53R2转录本在所有正常和肿瘤组织中均有表达，其表达水平在正常组织和恶性癌组织之间无显著差异。p53R2的表达与肿瘤的分期、分级和组织学类型无关。此外，在30对匹配样本中未检测到p53R2的肿瘤特异性降低。对包括15个细胞系在内的105例癌组织中p53R2的突变分析也未发现任何基因组缺失或导致氨基酸替换或移码的体细胞突变的证据，而同一批原发性肿瘤中有31%（90例中的28例）显示p53改变。虽然82%（28例中的23例）携带突变型p53的原发性肿瘤异常低表达p21（Waf1），但未发现p53R2的表达与p53状态有关，这表明p53R2的基础转录是通过不依赖p53的机制调控的。总体而言，我们的研究表明，虽然p53R2在胃上皮细胞中以p53依赖的方式被诱导产生，并参与p53介导的DNA修复，但它不是胃癌发生过程中基因失活的关键靶点。
