Wild type N-ras as a tumor suppressor in human sporadic medullary thyroid cardnoma

视网膜母细胞瘤（Rb）通路的畸变与人类癌变密切相关。然而，Rb失活的遗传证据在出乎意料的有限种类的恶性肿瘤中被发现，这意味着Rb缺陷细胞需要额外的改变来实现强大的克隆扩增。Rb-杂合小鼠在甲状腺产生c细胞腺瘤是Rb双等位基因缺失的结果。我们之前报道了rb杂合小鼠中N-ras的额外缺失导致c细胞腺瘤的恶性转化。为了阐明野生型N-ras肿瘤抑制功能的机制，我们对来自转基因小鼠和人类甲状腺髓样癌患者的原发肿瘤进行了深入的表征。作为我们的研究结果，我们在这里提出了一种机制，通过N-ras位点的存在，至少可以保护小鼠c细胞和成纤维细胞免受rb丢失诱导的癌变。在缺乏Rb的情况下，N-Ras活性以e2f依赖的方式增加，诱导DNA损伤反应和细胞衰老，但其方式与其组成激活形式明显不同。这些发现表明野生型N-ras在控制pRb失活后肿瘤的恶性转化中起着关键作用。在人类散发性甲状腺髓样癌中，ret癌基因突变的频率低于家族病例，我们之前通过使用每个基因两侧的微卫星标记，在18例患者中检测到10例Rb-N-ras双LOH。在这项研究中，我们从东京女子医学院获得了更多的样本（18个腺癌和2个腺瘤），目的是进行更详细的免疫组织化学和遗传分析。这一系列的研究解决了N-ras在人类癌症中是否作为肿瘤抑制因子。目前，我们正在从新获得的临床样本中迅速积累大量信息。

Aberration of the retinoblastoma (Rb) pathway critically pertains to human carcinogenesis. However, the genetic evidence of Rb inactivation is found in an unexpectedly limited variety of malignant tumors, implicating a requirement of additional changes for robust clonal expansion by Rb-deficient cells. Rb-heterozygous mice produce C-cell adenoma in thyroid as a consequence of biallelic loss of Rb. We previously reported that additional deletion of N-ras in Rb-heterozygous mice resulted in malignant conversion in C-cell adenoma. To clarify the mechanism underlying the tumor suppressor function of wild type N-ras, we intensively characterized primary tumors from our genetically modified mice and also from human medullary thyroid carcinoma patients. As the results of our study, we propose here the mechanism by which at least murine C-cells and fibroblasts are protected from Rb-loss-induced carcinogenesis by the presence of N-ras loci. In the absence of Rb, N-Ras activity is increased in E2F-dependent manner, inducing DNA damage response and cellular senescence yet in a manner clearly distinct from its constitutively activated form. These findings assign a critical role for wild type N-ras in controlling malignant conversion of tumors upon pRb inactivation. In human sporadic medullary thyroid carcinomas in which ret oncogene mutation is less frequent than in familial cases, we previously detected 10 cases with Rb-N-ras double LOH out of 18 cases by using micro satellite markers flanking each gene. In this study, we obtained more number of samples (18 adenocarcinomas and 2 adenomas) from Tokyo Women's Medical College in an aim to perform more detailed immunohistochemical and genetic analyses. This line of investigation addresses whether N-ras acts as tumor suppressor in human cancers. Currently, we are acutely accumulating considerable amount of information from newly obtained clinical samples.