Hypomethylation of ras oncogenes in primary human cancers

The role of methionine in carcinogenesis in vivo

The effects of methionine on carcinogenesis and tumor development have been studied intermittently for over 35 years. These studies have generally shown that methionine confers some degree of protection against the development of liver tumors by hepatocarcinogens. Such protective effects by supplemental dietary methionine are more pronounced in animals fed methionine- and choline-deficient diets rather than methionine- and choline-adequate diets. To date few if any protective effects of methionine have been observed against tumor formation in extrahepatic tissues. The effects of methionine on hepatocarcinogenesis appear to correlate well with its effects on the liver content of S-adenosylmethionine, the chief physiologic methyl donor. Perturbation of the methyl pool is known to alter the extent of methylation of membrane phospholipids, RNA, and DNA. Thus several plausible mechanisms by which methionine may modify the carcinogenic process center upon the aberrant methylation of macromolecules.

Expression-linked demethylation of 5-methylcytosines in the chicken vitellogenin gene region

We have studied the methylation status of the estradiol-controlled chicken vitellogenin (Vtg) gene, which is expressed in the liver. A 30-kb region was investigated, containing 17 HpaII and 18 HhaI sites, of which 21 are in the 22-kb gene. Of these 21 sites, 9 were found to be demethylated in laying-hen liver relative to immature chicken liver. Outside the transcribed region, only one site was found to be relatively undermethylated in laying-hen liver. This site, at 0.6 kb in front of the gene, is, as shown earlier, also demethylated in rooster or immature chicken liver upon primary hormone stimulation, as well as in the non-expressing estradiol target organ oviduct. In this respect, this site sharply contrasts with those in the transcribed region, which appear to become demethylated only upon prolonged transcription of the gene.

DNA methyltransferases in normal and avian sarcoma virus-transformed rat cells. Quantitation of 5-methyldeoxycytidine in DNA and enzyme kinetics study

In rat kidney cells transformed by avian sarcoma virus (B77 strain) DNA is hypomethylated (2.61 +/- 0.07%) when compared to DNA extracted from normal cells (3.33 +/- 0.11%) as revealed by high-performance liquid chromatography analysis. Kinetics studies showed that no significant differences could be detected between DNA methyltransferase activities from normal and transformed cells with regard to apparent Vmax, apparent Km for S-adenosylmethionine (2.32 X 10(-6) M and 6.64 X 10(-6) M respectively) and apparent Ki for S-adenosylhomocysteine (9.2 X 10(-7) M and 7.8 X 10(-7) M respectively), when unmethylated duplex DNA was used as second substrate. Equivalent ratios of S-adenosylmethionine over S-adenosylhomocysteine were measured in each cell type and DNA methyltransferase activities from both sources were found to be strictly additive. These results show that the hypomethylation of DNA detected in transformed cells is related neither to alterations of enzymatic activities extracted from nuclei nor to unbalanced S-adenosylmethionine/S-adenosylhomocysteine ratios.

The accessibility of 5-methylcytosine to specific antibodies in double-stranded DNA of Xanthomonas phage XP12

Antibodies specifically directed to 5-methylcytidine were raised in rabbits and purified by affinity chromatography. The accessibility of 5-methyldeoxycytidine (m5dCyd) to such antibodies was studied with DNAs from various origins. The reaction was followed by measuring the retention of radiolabelled DNA by antibodies on nitrocellulose filters, by immunoprecipitation, by gel filtration and was visualized with the electron microscope. Antibodies did not bind to Escherichia coli B DNA, which is deficient in m5dCyd. Denatured and native DNA from calf thymus, which contains m5dCyd as a minor nucleoside, was weakly retained on the filters whereas DNA extracted from Xanthomonas oryzae XP12 bacteriophage, which is rich in m5dCyd, was well recognized even in the native form.

Purification of DNA from formaldehyde fixed and paraffin embedded human tissue

The ability to isolate DNA from preserved human tissues would provide numerous experimental opportunities. In this report it is shown that DNA can be extracted from tissues prepared for routine histopathological examination (i.e., fixed with formaldehyde and embedded in paraffin). Although the extracted DNA is not intact, it is double stranded, cleavable with restriction endonucleases, and suitable for a variety of standard techniques used in molecular biology.

Hypomethylation of DNA from benign and malignant human colon neoplasms

The methylation state of DNA from human colon tissue displaying neoplastic growth was determined by means of restriction endonuclease analysis. When compared to DNA from adjacent normal tissue, DNA from both benign colon polyps and malignant carcinomas was substantially hypomethylated. With the use of probes for growth hormone, gamma-globin, alpha-chorionic gonadotropin, and gamma-crystallin, methylation changes were detected in all 23 neoplastic growths examined. Benign polyps were hypomethylated to a degree similar to that in malignant tissue. These results indicate that hypomethylation is a consistent biochemical characteristic of human colonic tumors and is an alteration in the DNA that precedes malignancy.

Transformation of human bronchial epithelial cells transfected by Harvey ras oncogene

Transfection of normal human bronchial epithelial (NHBE) cells with a plasmid carrying the ras oncogene of Harvey murine sarcoma virus (v-Ha ras) changed the growth requirements, terminal differentiation, and tumorigenicity of the recipient cells. One of the cell lines isolated after transfection (TBE-1) was studied extensively and shown to contain v-Ha ras DNA. Total cellular RNA from TBE-1 cells hybridized to v-Ha ras structural gene fragment probes five to eight times more than RNA from parental NHBE cells. The TBE-1 cells expressed phosphorylated v-Ha ras polypeptide p21, showed a reduced requirement for growth-factor supplements, and became aneuploid as an early cellular response to v-Ha ras expression. As the transfectants acquire an indefinite life-span and anchorage independence they became transplantable tumor cells and showed many phenotypic changes suggesting a pleiotropic mechanism for the role of Ha ras in human carcinogenesis.

Expression of p21ras in fresh primary and metastatic human colorectal tumors

Activation of the cellular oncogene ras has been implicated in many types of human malignancies. In this study, the relative levels of p21 protein product of ras (p21ras) in primary and metastatic colon tumors were compared to those in adjacent normal tissues. Nine of the 17 primary tumors had substantially elevated levels of p21ras with respect to adjacent normal tissues. Eight of these tumors were from Dukes' B and C stages. Four of the five tumors classified as "D" stage (in which distant metastases are present) did not show elevated levels of p21ras. In metastases from primary colon tumors, nine of nine were considerably reduced in p21ras expression regardless of the site of metastasis. These data suggest that elevation of p21ras may be a common event in early stages of colon tumors, and tumor progression may lead to a more autonomous population of cells in which other growth factors supplant the role of this protein.

Therapy of gastrointestinal cancer

Cancer of the gastrointestinal tract represents a major international health problem. At the present time surgical resection for limited stages of disease represents the only treatment which can consistently provide long-term disease-free survival. Unfortunately, the majority of patients present with either microscopic metastatic disease in distant sites or advanced tumour growth which exceeds the limits of surgical resection. Relatively little progress has been made in the development of effective forms of non-surgical therapy. Gastric cancer, however, has been demonstrated to have greater sensitivity to forms of chemotherapy and radiation therapy than was previously appreciated. During the past decade, more effective forms of palliative therapy have been developed for patients with advanced disease, and approximately 15% of the cases with locally unresectable gastric cancer can now achieve long-term disease-free survival with combined forms of treatment. Unfortunately, similar progress has not been made in the management of pancreatic cancer or advanced colon cancer. The recent experience of the Gastrointestinal Tumor Study Group with the use of combined radiotherapy and chemotherapy for rectal cancer has demonstrated that improved disease-free survival can be achieved for patients with Dukes B and C disease. Overall, the current limited efficacy and considerable toxicity of conventional therapies strongly support the development of new approaches to the management of gastrointestinal cancer; this includes the exploitation of the recent progress that has been made in our understanding of cell proliferation and cell cycle control, and the importance of oncogenes and growth factors for regulation of these processes. Ultimately, our understanding of the molecular genetics of gastrointestinal cancer might allow for development of more effective means for both prevention and treatment at the molecular level.

Purification of human DNA (cytosine-5-)-methyltransferase

We have developed a facile procedure for the purification of DNA methyltransferase activity from human placenta. The procedure avoids the isolation of nuclei and the dialysis and chromatography of large volumes. A purification of 38,000-fold from the whole cell extract has been achieved. The procedure employs ion exchange, affinity, and hydrophobic interaction chromatography coupled with preparative glycerol gradient centrifugation. A protein of 126,000 daltons was found to copurify with the activity and was the major band seen in the most highly purified material after SDS gel electrophoresis. This observation, coupled with an observed sedimentation coefficient of 6.3S, suggests that the enzyme is composed of a single polypeptide chain of this molecular weight. Hemimethylated DNA was found to be the preferred substrate for the enzyme at each stage in the purification. The ratio of the activity of the purified product on hemimethylated to that on unmethylated M13 duplex DNA was about 12 to 1. Thus, the purified activity has the properties postulated for a maintenance methyltransferase. The availability of highly purified human DNA methyltransferase should facilitate many studies on the structure, function, and expression of these activities in both normal and transformed cells.

In vitro enzymatic methylation of DNA substituted by N-2-aminofluorene

Both the initial velocity and the overall methylation of DNA substituted by aminofluorene, by a rat liver DNA(cytosine-5-)-methyltransferase, are increased as compared to native DNA. The Km and Vmax of the modified DNA for the enzyme increase as a function of the extent of modification. The carcinogen may induce a secondary structure favouring the 'walking' of the enzyme along the DNA. The hypermethylation caused by this carcinogen could have a significance in gene activity, cellular differentiation and cancer induction.

Mechanism of rat liver DNA methyltransferase interaction with anti-benzo[a]pyrenediol epoxide modified DNA templates

We investigated the methylation reaction catalyzed by 1500-fold purified rat liver DNA methyltransferase (DMase) on native Micrococcal luteus DNA (ML-DNA) and poly(dC-dG) templates containing covalently bound (+)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE), the strongly carcinogenic, principal metabolite of benzo[a]pyrene. Since eukaryotic DNA methyltransferases recognize the dinucleotide 5'd[CG] in DNA as a substrate for methylation, the model polynucleotide poly(dC-dG) was used to study in more detail the mode of interaction and effect on incorporation. With either of these BPDE-modified templates, a progressive inhibition of methylation was correlated with increasing amount of BPDE substitution. The effect of BPDE-dG adducts did not alter the apparent km with respect to the concentration of d[CG] in either unmodified or BPDE-modified poly(dC-dG) (km = 10 microM) but lowered the relative apparent Vmax. In assays in which perturbation by salt of preformed enzyme-DNA complex is measured, no change in the relative stability to either unsubstituted or the carcinogen-modified template was noted, thus, excluding any change in the ionic component of this interaction. However, in competition-type experiments, BPDE-DNA is an inhibitor of the methylation reaction on native DNA. When BPDE-DNA is allowed to interact with the enzyme before the addition of native competitor DNA, the methylation rate is not stimulated, suggesting very tight hydrophobic binding of the enzyme to BPDE-DNA and an inhibition in the dissociation of DMase from the template following a methylation event.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypomethylation of DNA in meiotic and postmeiotic rooster testis cells

To study whether changes in methylation of DNA are related to the structural and functional changes that chromatin undergoes throughout rooster spermatogenenis, we analyzed, by high-performance liquid chromatography, the 5-methylcytosine content of DNA purified from rooster testis cell nuclei at successive stages of the cell differentiation process. The DNA of meiotic and postmeiotic cells appears partially under-methylated, containing approximately 30% less methylcytosines than the DNA obtained from premeiotic and somatic cells.

Expression of cellular oncogenes in human malignancies

Cellular oncogenes have been implicated in the induction of malignant transformation in some model systems in vitro and may be related to malignancies in vivo in some vertebrate species. This article describes a study of the expression of 15 cellular oncogenes in fresh human tumors from 54 patients, representing 20 different tumor types. More than one cellular oncogene was transcriptionally active in all of the tumors examined. In 14 patients it was possible to study normal and malignant tissue from the same organ. In many of these patients, the transcriptional activity of certain oncogenes was greater in the malignant than the normal tissue. The cellular fes (feline sarcoma) oncogene, not previously known to be transcribed in mammalian tissue, was found to be active in lung and hematopoietic malignancies.

Inhibition of malignant transformation in vitro by inhibitors of poly(ADP-ribose) synthesis

Malignant transformation in vitro of hamster embryo cells and mouse C3H 10T 1/2 cells by x-rays, ultraviolet light, and chemical carcinogens was inhibited by benzamide and by 3-aminobenzamide at concentrations that are specific for inhibition of poly(ADP-ribose) formation. These compounds slow the ligation stage of repair of x-ray and alkylation damage but not of ultraviolet light damage. At high concentrations they also inhibited de novo synthesis of DNA purines and DNA methylation by S-adenosylmethionine. The suppression of transformation by the benzamides is in striking contrast to their reported effectiveness in enhancing sister chromatid exchange, mutagenesis, and killing in cells exposed to alkylating agents. Our results suggest that mechanisms regulating malignant transformation are different from those regulating DNA repair, sister chromatid exchange, and mutagenesis and may be associated with changes in gene regulation and expression caused by alterations in poly(ADP-ribosyl)ation.

The 5-methylcytosine content of DNA from human tumors

The over-all 5-methylcytosine (m5C) content of DNA from normal tissues varies considerably in a tissue-specific manner. By high-performance liquid chromatography, we have examined the m5C contents of enzymatic digests of DNA from 103 human tumors including benign, primary malignant and secondary malignant neoplasms. The diversity and large number of these tumor samples allowed us to compare the range of DNA methylation levels from neoplastic tissues to that of normal tissues from humans. Most of the metastatic neoplasms had significantly lower genomic m5C contents than did most of the benign neoplasms or normal tissues. The percentage of primary malignancies with hypomethylated DNA was intermediate between those of metastases and benign neoplasms. These findings might reflect an involvement of extensive demethylation of DNA in tumor progression. Such demethylation could be a source of the continually generated cellular diversity associated with cancer.

Mutation affecting the 12th amino acid of the c-Ha-ras oncogene product occurs infrequently in human cancer

A point mutation alters the 12th amino acid of the c-Ha-ras oncogene product p21 in a human bladder cancer cell line. This is, at present, the only mutation known to result in a human transforming gene. This mutation may therefore represent a possible target for mutagenesis leading to carcinogenesis in humans. By means of restriction enzyme analysis, 29 human cancers, including 20 primary tumor tissues, derived from organs commonly exposed to environmental carcinogens, were tested for the presence of this mutation. None of ten primary bladder carcinomas exhibited the mutation; nor did nine colon carcinomas or ten carcinomas of the lung. Thus the point mutation affecting the 12th amino acid of the c-Ha-ras gene product, while a valuable model for carcinogenesis, does not appear to play a role in the development of most human epithelial cancers of the bladder, colon, or lung.