Activated oncogenes in human tumors

The search for the genetic targets responsible for tumorigenesis has led to the identification of a number of cancer genes or cellular oncogenes (c-oncogenes). The oncogenes are activated forms of the proto-oncogenes, which are normal cellular genes and are scattered throughout the cellular genome. The number of known cellular proto-oncogenes and associated oncogenes now exceeds 30. There are different proto-oncogene families and their products have different functions and cellular localisation. They may function in normal cells in the process of proliferation, regulation of cellular metabolism through signal transfer, or cell differentiation. Activation of proto-oncogenes in man is now assumed to be due to: 1) point mutation; 2) overexpression or 3) gene rearrangement. The observation that in some tumors multiple oncogenes are altered could be interpreted in terms of a multigene hypothesis. However, in some cases, a single properly-activated oncogene may be able to trigger the whole process of malignant conversion. It is difficult to correlate, without ambiguity, tumor induction to specific types of DNA lesions in human tumors. Xeroderma pigmentosum (XP), a rare recessive autosomal skin disorder characterized biochemically as a DNA repair-deficient disease, is the first example in which unrepaired UV-induced DNA lesions are directly responsible for tumorigenesis. In two independent XP skin tumors, isolated from the same patient, we have detected several (N-ras, c-myc, Ha-ras) altered oncogenes in the same tumor. We postulate that the modifications we have found in these tumors are primarily due to the presence of unrepaired UV-adducts. Long term treatment of human tumoral cell lines bearing an activated ras oncogene, with Interferon-alpha (IFN), showed that IFN can affect the phenotype of the tumor cells without altering the expression of the activated ras gene. IFN may have the capacity to affect diverse cellular pathways. Consequently, the nature of the biological response of a given type of tumor cell to IFN may depend on its inherent properties.     

Activated oncogenes in human tumors

ras oncogenes are cellular genes altered by point mutation in 10 to 30% of human tumors. Under this mutated form they play a role in the malignant process, probably in association with other oncogenes. The different ras genes identified in human cancers, the point mutations that activate the ras genes and the properties of the ras proteins are described.     

Activated N-ras oncogenes in human neuroblastoma

Fifteen primary neuroblastomas and four bone marrow samples from neuroblastoma patients, representing clinical Stages I to IV, have been screened for mutations in codons 12, 13, and 61 of N-ras. Neuroblastoma DNAs were subjected to the polymerase chain reaction to amplify the relevant sequences and were then hybridized with specific oligonucleotides to locate and identify point mutations. The results show that one Stage I tumor contained an N-ras gene that was activated by a GC-CG transversion of the first base of codon 61, while in one Stage II tumor, activation of N-ras involved a GC-CG transversion of the first position of codon 13. N-ras activations were not detected in the six Stage III tumors and eight Stage IV tumors that were examined.     

Activated ras oncogenes in human thyroid cancers

Human thyroid epithelial (follicular) cells give rise to two malignant tumors--"follicular" carcinomas, which metastasize almost exclusively via the bloodstream, and "papillary" carcinomas, which metastasize predominantly via lymphatics (Williams, E. D. In: W. Duncan (ed.), Recent Results in Cancer Research: Thyroid Cancer, pp. 47-55. Berlin: Springer-Verlag, 1980). We have investigated whether this contrast in biological behavior might be associated with different patterns of oncogene activation. DNA transfection analysis of five follicular and ten papillary cancers indeed showed a statistically significant difference in the pattern of genes responsible, activated ras oncogenes being found in 80% of follicular tumors but only 20% of papillary tumors. In addition, in follicular cancers we have found activation of all three ras oncogenes (H-ras, K-ras, and N-ras), the first time that this has been demonstrated in a primary human tumor type (as opposed to cell lines). We suggest therefore that ras activation may be an important determinant of metastatic capability in these epithelial cancers.     

Activation of H-ras oncogenes in preneoplastic mouse mammary tissues

The mammary hyperplastic outgrowth (HOG) line C4, resulted from serial transplantation of a hyperplastic alveolar nodule which arose in a dimethylbenz(a)anthracene (DMBA) treated mouse. The immortalized C4 outgrowth line, on transplantation into syngeneic mice, develops as preneoplastic, hyperplastic outgrowths and subsequently into malignant carcinomas after a long latent period (greater than 6 months). Treatment of mice carrying C4 HOG transplants with DMBA resulted in a reduced latent period for tumor development (less than 3 months) and an increased tumor incidence. DNA's from C4 HOGs and mammary carcinomas of untreated as well as DMBA-treated mice were analyzed for the presence of oncogenes by the NIH3T3 focus forming assay. Transforming H-ras genes were detected in two of 6 preneoplastic HOGs and 10 of 12 carcinomas from DMBA-treated mice. DNAs from neither the HOGs nor the tumors from untreated mice were positive in this assay. The H-ras locus was then directly examined in the 61st codon by in vitro amplification of each of the tissue DNAs using PCR. The location of the activating mutation was determined by hybridization of amplified DNA to mixed sequence oligonucleotide probes. The specific nature of the mutation was defined by RFLPs using XbaI, TaqI and Sau96I restriction enzymes. Six of the H-ras oncogenes in DMBA-promoted tumors were activated by commonly observed A to T transversions at the 61st codon, while five (including an additional tumor with H-ras oncogene revealed by PCR analysis) contained novel A to G transitions. The H-ras oncogene in one DMBA-treated HOG sample was activated by A to T while the second contained an A to G mutations, representative of both modes of mutational activation involved in this model of mammary tumorigenesis. In summary, DMBA-induced point mutated H-ras oncogenes appear to potentiate the progression of hyperplastic outgrowths (HOG) to mammary carcinomas.     

Activation of H-ras oncogenes in male B6C3F1 mouse liver tumors induced by vinthionine or 2-chloroethyl methyl sulfide

Vinthionine (S-vinyl-DL-homocysteine) is hepatocarcino-genicin rats and mice. [Vinyl-¹⁴C]vinthionine binds cova-ently torat liver DNA, RNA and protein in vivo, but not in vitro. Thisamino acid is directly mutagenic in Salmonella typhimurium TA100and TA1535; the mechanism of its metabolic activation in vivoin bacteria and liver is under study. In the present study livertumors were induced in 12-day-old male B6C3F1 mice by singlei.p. injections of vinthionine or the alkylating agent 2-chloroethylmethylsulfide (CEMS).At 10 months the gross tumors were examinedfor the presence of activated H-ras oncogenes. DNA was isolatedfrom single tumors per mouse from 37mice treated with vinthionineand from 31 mice treated with CEMS. These DNAs were screenedfor codon 61 mutations by restriction fragment length polymorphismof PCR-amplified H-ras gene fragments. Thirty seven of 37 vinthionine-inducedhepatomas had H-ras mutations in this codon, which consistedof seven C     

Overexpression of human H-ras transgene is responsible for tumors induced by chemical carcinogens in mice

Level of human prototype H-ras transgene expression in tumors induced by chemical carcinogens (N-ethyl-N-nitrosourea and N-methyl-N-nitrosourea) was analyzed in human H-ras transgenic mice (CB6F1-TgrasH2 Jic mice). All forestomach tumors examined revealed about 2-fold overexpression of the human H-ras transgene with or without point mutation at codon 12 or codon 61. However, endogenous mouse H- and K-ras genes exhibited neither point mutation nor overexpression. These results suggested that increased levels of ras gene products in the cell played an important role in facilitating chemical carcinogenesis in transgenic mice.     

Ras基因激活与乳腺癌发生及临床预后的关系

目的　探讨ras基因激活与乳腺癌发生及临床预后的关系。方法　采用LSAB免疫组化法对 8例乳腺纤维腺瘤、76例乳腺癌、3 4例淋巴结转移灶进行P2 1蛋白检测 ;用PCR -RFLP法对 6例乳腺纤维腺瘤、2 2例乳腺癌 ( 6例新鲜标本 )组织进行H ras12位点点突变检测。结果　 76例乳腺癌P2 1蛋白表达率为 76.3 % ,伴淋巴结转移组阳性率明显高于无转移组 (P <0 .0 5 ) ;与组织学分级比较 ,Ⅰ级P2 1蛋白阳性率明显低于Ⅱ、Ⅲ级 (P <0 .0 5 ) ;随着临床分期的升高P2 1蛋白表达率有增高的趋势。H ras12位点点突变率在乳腺纤维腺瘤为 16.7%、乳腺癌为 2 7.3 % ,其中伴淋巴结转移组 3 7.5 %、无转移组 2 8.6%。结论　ras基因激活与乳腺癌的发生及转移有关。H ras12位点点突变率明显低于P2 1蛋白表达率 ,说明该位点突变并非是乳腺癌的主要激活方式。     

Activated c-K-ras and c-N-ras oncogenes in 3-methylcholanthrene-induced BALB/c fibrosarcomas

DNAs from fourteen fibrosarcomas induced by 3-methyl-cholanthrene(MCA) in BALB/c mice were analyzed for the presence of transformingoncogenes following transfection on NIH3T3 cells. Six transfection-positiveDNAs contained an activated ras gene: four c-K-ras and two c-N-ras.These results demonstrate that c-K-ras is not the only oncogeneof the ras family activated in MCA-induced murine fibrosarcomasas was previously indicated.     

The farnesyltransferase inhibitor L744,832 reduces hypoxia in tumors expressing activated H-ras

Many tumors contain extensive regions of hypoxia. Because hypoxic cells are markedly more resistant to killing by radiation, repeated attempts have been made to improve the oxygenation of tumors to enhance radiotherapy. We have studied the oxygenation of tumor xenografts in nude mice after treatment with the farnesyltransferase inhibitor L744,832. Hypoxia was assessed by measuring the binding of the hypoxic cell marker pentafluorinated 2-nitroimidazole. We show that xenografts from two tumor cell lines with mutations in H-ras had markedly improved oxygenation after farnesyltransferase treatment. In contrast, xenografts from two tumors without ras mutations had equivalent hypoxia regardless of treatment. The effect on tumor oxygenation could be detected at 3 days and remained after 7 days of treatment. These results indicate that treatment with farnesyltransferase inhibitors can alter the oxygenation of certain tumors and suggest that such treatment might be useful in the radiosensitization of these tumors.     

Rapid detection of ras oncogenes in human tumors: applications to colon, esophageal, and gastric cancer

We have developed a rapid, nonradioactive large scale method for the detection of ras oncogenes in human tumors. DNA is amplified by the polymerase chain reaction (PCR), and then digested with specific restriction enzymes to detect either endogenous or primer-mediated Restriction Fragment Length Polymorphisms (RFLPs). We report here that three of 15 colon tumors tested contain K-ras codon 12 aspartic acid mutations and one, along with the HCT 116 colon carcinoma cell line, contains a K-ras codon 13 aspartic acid mutation. On the other hand, we did not detect H- or K-ras codon 12 mutations or the K-ras codon 13 aspartic acid mutation in 25 esophageal and 27 gastric cardia tumors isolated from patients in Lin-xing County, China. By incorporating nucleotide substitutions in PCR primers, this method can be applied towards the rapid, non-radioactive screening of virtually any genetic disease caused by known point mutations.     

Rare activation of ras oncogenes in radiation induced rat skin tumors

We examined a large panel of radiation induced rat skin tumors for activation of the H- or K-ras oncogenes. Using oligonucleotide hybridization analysis of tumor DNA we found that only 1 out of 96 tumors tested had an activated K-ras gene, and none of the 78 tumors examined for H-ras mutations were positive. Tumors were induced by high and low LET, and included lesions of various sizes and histologic type. DNA sequencing of tumors and NIH3T3 transfectants from a previous study gave results consistent with a rare occurence of ras activation in this system.     

Expression of oncogenes in human breast cancer specimens

More than 60 breast cancer specimens were screened for their expression status of 25 different proto-oncogenes. The screening method is based on in vitro synthesis of a radioactive cDNA copied from the total cellular RNA of tumor tissue. This cDNA is hybridized to cloned oncogene probes which are immobilized to a GeneScreen membrane. Frequently multiple oncogenes were found expressed although expression levels were rather moderate. 25-30% of the analyzed tumors showed significant expression of either erbB, src, raf1, lck or H-ras. Although neu expression--an oncogene believed to be particular relevant as prognostic parameter for mamma carcinoma--was screened for most of the tumors with a heterologous gene probe, expression signals could be detected in about 20% cases. The only notable correlation with classical clinical parameters such as tumor size and proliferation stage, hormone receptor status and different DNA indices was the observation that tumors lacking the progesterone receptor frequently express multiple oncogenes. Advantages and limitations of the cDNA/dot-blot screening for oncogene expression are discussed.     

ras oncogenes in human cancer: a review

Mutations in codon 12, 13, or 61 of one of the three ras genes, H-ras, K-ras, and N-ras, convert these genes into active oncogenes. Rapid assays for the detection of these point mutations have been developed recently and used to investigate the role mutated ras genes play in the pathogenesis of human tumors. It appeared that ras gene mutations can be found in a variety of tumor types, although the incidence varies greatly. The highest incidences are found in adenocarcinomas of the pancreas (90%), the colon (50%), and the lung (30%); in thyroid tumors (50%); and in myeloid leukemia (30%). For some tumor types a relationship may exist between the presence of a ras mutation and clinical or histopathological features of the tumor. There is some evidence that environmental agents may be involved in the induction of the mutations.