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

视网膜母细胞瘤(RB)通路的异常与人类的癌变密切相关。然而，Rb失活的遗传证据在出人意料的有限的多种恶性肿瘤中被发现，这意味着需要额外的改变来使Rb缺乏的细胞进行强劲的克隆性扩张。由于Rb双等位基因丢失，Rb杂合子小鼠在甲状腺中产生C细胞腺瘤。我们之前曾报道，在RB杂合子小鼠中，N-ras基因的额外缺失导致C细胞腺瘤的恶性转化。为了阐明野生型N-ras抑制肿瘤功能的机制，我们集中研究了我们的转基因小鼠和人类甲状腺髓样癌患者的原发肿瘤。作为我们研究的结果，我们在这里提出了至少通过N-ras基因座的存在来保护小鼠C细胞和成纤维细胞免受Rb缺失诱导的癌变的机制。在没有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，但方式明显不同于其结构性激活形式。这些发现指定了野生型N-ras在控制肿瘤在pRb失活后的恶性转化中的关键作用。在Rit癌基因突变频率低于家族性的散发性甲状腺髓样癌中，我们先前利用每个基因两侧的微卫星标记检测了18例中的10例Rb-N-ras双重杂合性缺失。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例腺癌和2例腺瘤)，目的是进行更详细的免疫组织化学和遗传学分析。这一系列研究解决了N-ras在人类癌症中是否起到肿瘤抑制作用。目前，我们正在从新获得的临床样本中敏锐地积累大量信息。

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.