Vorce R L, Goodman J I
Toxicol Appl Pharmacol. 1987 Aug;90(1):86-95. doi: 10.1016/0041-008x(87)90309-7.
Oncogenes have been implicated in the mechanism(s) involved in the different stages of carcinogenesis. Point mutations have been found to activate the ras family of oncogenes. However, these genes must also be expressed in order for phenotypic alterations to be seen. In addition, overproduction of the normal product of some oncogenes may be a factor in the development of cancer. This study was therefore designed to investigate the potential for activity that the Kirsten (Ki)-ras, Harvey (Ha)-ras, and myc oncogenes possess in normal rat liver; activation of these genes has previously been observed in hepatocellular carcinomas. The serum albumin gene was used as a positive control. Two possible control points of gene expression were examined: (1) gene methylation state, as there is an established positive correlation between hypomethylation and gene activity; and (2) DNase I sensitivity because transcriptionally active genes are preferentially sensitive to digestion by this enzyme. It was found that the serum albumin gene is hypomethylated and preferentially sensitive to DNase I, both states being consistent with gene activity. In contrast, Ki-ras is highly methylated and relatively resistant to DNase I digestion. The Ha-ras and myc oncogens exhibit a degree of methylation and DNase I sensitivity intermediate between the serum albumin gene and the Ki-ras oncogene. It thus appears that Ki-ras is not expressed in normal rat liver, whereas Ha-ras and myc may have the potential for activity. In addition, a correlation was found between the methylation state of a gene and its sensitivity to DNase I. These findings are compatible with the view that carcinogenesis is a multistage process. Our results indicate that possible control points for oncogene expression, as well as mutation, should be considered in investigations of the role these genes play in carcinogenesis.
癌基因与癌变不同阶段所涉及的机制有关。已发现点突变可激活癌基因的ras家族。然而，这些基因也必须表达才能观察到表型改变。此外，一些癌基因正常产物的过量产生可能是癌症发生发展的一个因素。因此，本研究旨在探讨 Kirsten（Ki）-ras、Harvey（Ha）-ras 和 myc 癌基因在正常大鼠肝脏中的潜在活性；先前已在肝细胞癌中观察到这些基因的激活。血清白蛋白基因用作阳性对照。研究了基因表达的两个可能控制点：（1）基因甲基化状态，因为低甲基化与基因活性之间已确立存在正相关；（2）DNase I 敏感性，因为转录活跃的基因优先对该酶的消化敏感。结果发现血清白蛋白基因是低甲基化的，并且优先对 DNase I 敏感，这两种状态都与基因活性一致。相比之下，Ki-ras 高度甲基化，对 DNase I 消化相对抗性。Ha-ras 和 myc 癌基因表现出介于血清白蛋白基因和 Ki-ras 癌基因之间的一定程度的甲基化和 DNase I 敏感性。因此，似乎 Ki-ras 在正常大鼠肝脏中不表达，而 Ha-ras 和 myc 可能具有活性潜力。此外，发现基因的甲基化状态与其对 DNase I 的敏感性之间存在相关性。这些发现与癌变是一个多阶段过程的观点一致。我们的结果表明，在研究这些基因在癌变中所起的作用时，应考虑癌基因表达以及突变的可能控制点。
