Evidence that the familial adenomatous polyposis gene is involved in a subset of colon cancers with a complementable defect in c-myc regulation

Genetic Events Responsible for Colorectal Tumorigenesis: Achievements and Challenges

Colorectal carcinogenesis is a multistep process that is accompanied by accumulation of changes in proto-oncogenes and tumor-suppressor genes. APC/MCC, RAS, DCC, p53 mutations and/or allelic losses, hyperexpression of c-MYC and RB genes, as well as other genomic alterations appear at characteristic stages of tumor development and are observed in most neoplasms. However, consideration of each of these abnormalities leaves many unanswered questions. The striking data on recurrent amplification of the RB tumor-suppressor gene as well as suppressive activities of protein kinase C and activated RAS genes, at least in some colon carcinoma cell lines, suggest the unusual effects of some signalling pathways in colonic epithelial cells. The results obtained to date indicate that distinct sets of genetic changes may underlie the development of colorectal tumors.

Upstream stimulating factor-1 (USF1) and USF2 bind to and activate the promoter of the adenomatous polyposis coli (APC) tumor suppressor gene

The adenomatous polyposis coli (APC) gene product is involved in cell cycle arrest and apoptosis, and loss of function is associated with the development of colorectal carcinogenesis. Although it has been demonstrated that the APC gene is inducible, its transcriptional regulation has not been elucidated. Therefore, we characterized the promoter region of the APC gene and transcription factors required for basal expression. The APC gene has a TATA-less promoter and contains consensus binding sites for Octamer, AP2, Sp1, a CAAT-box, and three nucleotide sequences for E-box A, B, and M. The E-boxes are functional in several cancer cell lines and upstream stimulating factor-1 (USF1) and USF2 interact with these sites, with a preferred sequence-specificity for the B site. Analysis of activation of the cloned APC promoter by USF1 and USF2 in transient transfection assays in HCT-116 cells demonstrated that mutation of the E-box B site completely abolished the basal promoter activity. Further, the ectopic USF1 and USF2 expression in HCT-116 cells with deletion mutations of E-box A, B, and M sites showed that these E-boxes contribute to USF1- and USF2-mediated transcriptional activation of the APC promoter, with maximum promoter activity being associated with the E-box B site. Thus, USF1 and USF2 transcription factors are critical for APC gene expression.

APC and intestinal carcinogenesis. Insights from animal models

The APC protein is a crucial regulator of intestinal cell growth, and mutations in the APC gene are a common initial event in the process of human colorectal carcinogenesis. Animals bearing germline mutations in Apc are therefore important models for human colorectal cancer. These animals have been used both to understand the biology of human colorectal cancer and to screen for agents able to prevent malignant transformation of susceptible intestinal cells.

Function of the c-Myc oncogenic transcription factor

The c-myc gene and the expression of the c-Myc protein are frequently altered in human cancers. The c-myc gene encodes the transcription factor c-Myc, which heterodimerizes with a partner protein, termed Max, to regulate gene expression. Max also heterodimerizes with the Mad family of proteins to repress transcription, antagonize c-Myc, and promote cellular differentiation. The constitutive activation of c-myc expression is key to the genesis of many cancers, and hence the understanding of c-Myc function depends on our understanding of its target genes. In this review, we attempt to place the putative target genes of c-Myc in the context of c-Myc-mediated phenotypes. From this perspective, c-Myc emerges as an oncogenic transcription factor that integrates the cell cycle machinery with cell adhesion, cellular metabolism, and the apoptotic pathways.

MYC oncogenes and human neoplastic disease

c-myc, N-myc and L-myc are the three members of the myc oncoprotein family whose role in the pathogenesis of many human neoplastic diseases has received wide empirical support. In this review, we first summarize data, derived mainly from non-clinical studies, indicating that these oncoproteins actually serve quite different roles in vivo. This concept necessarily lies at the heart of the basis for the observation that the deregulated expression of each MYC gene is reproducibly associated with only certain naturally occurring malignancies in humans and that these genes are not interchangeable with respect to their aberrant functional consequences. We also review evidence implicating each of the above MYC genes in specific neoplastic diseases and have attempted to identify unresolved questions which deserve further basic or clinical investigation. We have made every attempt to review those diseases for which significant and confirmatory evidence, based on studies with primary tumor material, exists to implicate MYC members in their causation and/or progression.

Cancer genetics: tumor suppressor meets oncogene

The adenomatous polyposis coli (APC) tumor suppressor protein is inactivated by mutations in the majority of colorectal cancers. A recent study has revealed that alterations in the APC signaling pathway can result in the transcriptional activation of the c-MYC gene.

Tumores colorretais hereditários

Cerca de 4% a 15% dos tumores colorretais são hereditários e divididos em dois grupos: polipose adenomatosa familiar (FAP) e câncer colorretal hereditário sem polipose (HNPCC). Ambas são doenças autossômicas dominantes, com transmissão vertical, geração após geração, sem preferência por sexo. A FAP tem penetrância praticamente completa, caracterizada por mais de cem pólipos adenomatosos no intestino grosso, que aparecem em geral após a puberdade e se transformam em câncer em todos os casos não tratados, levando o paciente ao óbito em tomo dos 45 anos de idade. Manifestações extracolônicas são comuns, tais como: pólipos em estômago e duodeno, sarcomas abdominais, pigmentação de retina, osteomas, entre outras. A FAP é causada por mutação no gene APC, que está localizado no cromossomo 5q. Seu tratamento é basicamente cirúrgico, com retirada do intestino grosso, podendo-se preservar o reto, se este não apresentar muitos pólipos. O HNPCC tem penetrância em torno de 80% e não apresenta os pólipos benignos como na FAP, que permitem identificar pacientes com o fenótipo da doença. Geralmente, o diagnóstico da lesão colônica é realizado já na fase maligna, em torno dos 45 anos de idade, com preferência para o lado direito do cólon. Pode haver associação com tumores de endométrio na mulher, estômago, pâncreas, entre outros. É causada por mutação em genes de reparo do DNA (hMSH2, hMLH1, hPMS1, hPMS2, hMSH6/GTBP). A colectomia total deve ser realizada em pacientes com câncer de cólon e HNPCC. Se o tumor estiver localizado no reto, a proctocolectomia total pode ser uma opção. Em indivíduos portadores do defeito genético predisponente ao HNPCC, porém, assintomáticos, a indicação de cirurgias profiláticas é controversa. Atualmente, podem-se identificar indivíduos portadores de defeito genético herdado tanto na FAP como no HNPCC. Esses testes baseiam-se no estudo direto dos genes responsáveis pela respectiva doença ou pela proteína produto dos mesmos. É de suma importância uma abordagem multidisciplinar de pacientes portadores de FAP ou HNPCC, pois existe uma preocupação ética muito grande na realização dos testes genéticos de predisposição, considerando suas conseqüências psicológicas e sociais.

Evidence for a mutual regulation of p53 and c-myc expression in human colorectal cancer metastases

BACKGROUND

Alterations of the c-myc and the p53 genes occur in a majority of human colorectal cancers, and functional interaction between these two genes has recently been suggested.

PATIENTS AND METHODS

We analyzed p53 sequence and c-myc and p53 mRNA expression in 26 metastases and 4 advanced primaries of human colorectal cancer.

RESULTS

Twenty-one of 30 tumors (=70%) carried mutations of the p53 gene. In these samples, c-myc and p53 were overexpressed in 70% (15/21) and 71% (14/20) of evaluable cases, respectively, while in tumors carrying only wild-type p53, overexpression of c-myc and p53 was observed in only 33% (3/9; p < 0.05) and 22% (2/9; p < 0.01), respectively. Expression of p53 and c-myc were positively correlated (p = 0.014; r = 0.563) in tumors carrying a p53 mutation, but not in those with only wild-type p53.

CONCLUSION

We conclude that c-myc might induce p53 expression in human colorectal cancer and that wild-type but not mutant p53 might be involved in a negative feedback regulation of c-myc expression. The abrogation of this normal control mechanism seems to be an essential step during colorectal tumorigenesis and metastatic progression.

Influence of protein tyrosine phosphorylation on the expression of the c-myc oncogene in cancer of the large bowel

We tested the potential impact of tyrosine phosphorylation on the expression of the c-myc gene in two colon cancer cell lines, HCT8 and SW837. We found that the protein tyrosine kinase inhibitor genistein causes a decrease in the abundance of c-myc RNA and an inhibition of proliferation with a similar dose response. Geldanamycin, a mechanistically different tyrosine kinase inhibitor, also causes a decrease in both the expression of c-myc RNA and proliferation. Genistein has also been found to inhibit topoisomerase II, but the topoisomerase II inhibitor novobiocin did not lower the expression of c-myc. The most likely interpretation is that inhibition of protein tyrosine kinase activity caused a decrease in c-myc expression in these cells. The impact of tyrosine phosphorylation on the expression of the c-myc gene is further supported by the finding that inhibition of phosphotyrosine phosphatase using orthovanadate causes an increase in the level of c-myc RNA. The effect of genistein on HCT8 cells is not dependent on the synthesis of new protein and does not involve an alteration in the stability of the message. Analysis of transcription in the c-myc gene reveals a more complicated picture with a decrease in initiation and an increase in elongation but no net change in transcription. We speculate that the genistein induced reduction in myc expression is the result of a posttranscriptional intranuclear event(s).

Familial adenomatous polyposis

Familial adenomatous polyposis (FAP) is an autosomal dominant condition resulting in the development of more than 100 adenomatous polyps in the large bowel. In addition, a number of extracolonic manifestations of the condition may occur. Recently, increasing knowledge relating to the extracolonic abnormalities, and localization and sequencing of the gene for FAP, have had important implications for screening and long-term follow-up of those affected. In this review the natural history of the disease and the extracolonic manifestations associated with it are considered. Surgical management and advances in understanding at a molecular level are discussed, as well as the problems relating to screening for FAP and the implications of the new knowledge.

DNA lesions, inducible DNA repair, and cell division: three key factors in mutagenesis and carcinogenesis

DNA lesions that escape repair have a certain probability of giving rise to mutations when the cell divides. Endogenous DNA damage is high: 10(6) oxidative lesions are present per rat cell. An exogenous mutagen produces an increment in lesions over the background rate of endogenous lesions. The effectiveness of a particular lesion depends on whether it is excised by a DNA repair system and the probability that it gives rise to a mutation when the cell divides. When the cell divides, an unrepaired DNA lesion has a certain probability of giving rise to a mutation. Thus, an important factor in the mutagenic effect of an exogenous agent whether it is genotoxic or non-genotoxic, is the increment it causes over the background cell division rate (mitogenesis) in cells that appear to matter most in cancer, the stem cells, which are not on their way to being discarded. Increasing their cell division rate increases mutation and therefore cancer. There is little cancer from nondividing cells. Endogenous cell division rates can be influenced by hormone levels, decreased by calorie restriction, or increased by high doses of chemicals. If both the rate of DNA lesions and cell division are increased, then there will be a multiplicative effect on mutagenesis (and carcinogenesis), for example, by high doses of a mutagen that also increases mitogenesis through cell killing. The defense system against reactive electrophilic mutagens, such as the glutathione transferases, are also almost all inducible and buffer cells against increments in active forms of chemicals that can cause DNA lesions. A variety of DNA repair defense systems, almost all inducible, buffer the cell against any increment in DNA lesions. Therefore, the effect of a particular chemical insult depends on the level of each defense, which in turn depends on the past history of exposure. Exogenous agents can influence the induction and effectiveness of these defenses. Defenses can be partially disabled by lack of particular micronutrients in the diet (e.g., antioxidants).

Over-expression of the c-myc proto-oncogene in colorectal carcinoma

Alterations in the c-myc proto-oncogene in colorectal cancer were studied at the level of RNA expression, gene amplification and rearrangements. One hundred cases of colorectal cancer, stratified by Dukes' stage were examined. The level of messenger RNA expression was measured in tumours and matched normal mucosa from the same patient. Between 5 and 400 fold over-expression was found in 66% of tumours. Neither the presence nor the level of over-expression correlated with tumour staging. A significant correlation (P < 0.01) was found between over-expression of c-myc in tumours and the presence of synchronous adenomas elsewhere in the colon. In contrast to other tumours, no rearrangements of the gene were found on Southern analysis of colorectal cancers. Similarly, amplification of the gene was not found in the cancers examined.

Altered growth of human colon cancer cell lines disrupted at activated Ki-ras

Point mutations that activate the Ki-ras proto-oncogene are presented in about 50 percent of human colorectal tumors. To study the functional significance of these mutations, the activated Ki-ras genes in two human colon carcinoma cell lines, DLD-1 and HCT 116, were disrupted by homologous recombination. Compared with parental cells, cells disrupted at the activated Ki-ras gene were morphologically altered, lost the capacity for anchorage-independent growth, grew more slowly both in vitro and in nude mice, and showed reduced expression of c-myc. Thus, the activated Ki-ras gene plays a key role in colorectal tumorigenesis through altered cell differentiation and cell growth.

Characterization of the DiFi rectal carcinoma cell line derived from a familial adenomatous polyposis patient

The DiFi human colorectal cancer cell line was recently established from a familial adenomatous polyposis patient with extracolonic features characteristic of the Gardner syndrome. These cells have now been propagated for 150 passages in standard culture media and vessels without feeder layers or collagen coatings. They retain features of colonic epithelial cells such as surface microvilli, secretory vesicles, and desmosomes. Cytosol of DiFi cells contains a high level (502 U/mg protein) of the mucin CA 19-9. In addition, DiFi cells produce carcinoembryonic antigen, and induce tumors in athymic mice. Cytoskeleton analysis of DiFi cells by fluorescence microscopy showed a pronounced disorganization of actin cable structure. The isozyme genetic signature of DiFi cells is unique (0.01 probability of finding the same genetic signature in a different cell line), differs from that of HeLa cells, and has expressional features seen in other colorectal cell lines. The DiFi cell karyotype is tetraploid, contains many marker chromosomes, and shows numerous episomal particles. Two copies of chromosome 18 were absent, and only a single normal chromosome 17 was found. This parallels detection of allelic losses from DiFi cell DNA at loci on chromosomes 17p and 18 using molecular (cDNA) probes. DiFi cells clearly express transcripts for the c-myc proto-oncogene, the c-myb proto-oncogene, and the p53 tumor suppressor gene. Transforming growth factor beta inhibits DiFi cell growth in soft agar and suppresses c-myc expression in these cells. The value of this cell line in the study of genetic alterations in colorectal cancer is discussed.

Cellular localisation of c-myc product in human colorectal epithelial neoplasia

Aberrant expression of c-myc has been implicated in the development of colorectal carcinomas. We have used monoclonal antibodies 6E10 and 9E10, raised against mid-sequence and C-terminal peptides of the c-myc protein, to study the distribution of myc protein in normal and diseased bowel at the light microscope and ultrastructural levels. Normal mucosa showed staining only of some nuclei in the proliferative zones of crypts. In adenomas, staining varied from predominantly nuclear to pancellular to focal or pancytoplasmic. Moderately well differentiated areas of carcinomas gave strong focal cytoplasmic staining, while in poorly differentiated tumours staining was pancytoplasmic. Electron microscopy with these antibodies detected myc protein associated with dense chromatin and, where cytoplasmic staining occurred, with polyribosomes. Tumours showed a reduced staining of nuclear pores compared with normal tissue. Comparison of staining patterns with 6E10 and 9E10 in normal tissue, adenomas, and tumours suggests that tumour progression is associated with an accumulation of cytoplasmic c-myc protein, perhaps resulting from alterations to the C-terminus which reduce the efficiency of nuclear targeting of the protein and thus disrupt the regulation of the cell cycle.

Suppression of deregulated c-MYC expression in human colon carcinoma cells by chromosome 5 transfer

Two-thirds of sporadic colon carcinomas express elevated levels of the c-MYC protooncogene. In addition, most colon carcinoma cell lines show constitutive elevated expression (10- to 40-fold over normal) of MYC RNA and protein that is not modulated in response to a mitogenic stimulus. Indirect immunofluorescence has been used to detect c-MYC protein in such cell lines, in hybrid cells resulting from fusions of such lines with cells that regulate MYC normally, and in carcinoma cells to which a normal copy of chromosome 5 has been transferred by microcell fusion. The deregulated expression of c-MYC is suppressed by fusion with a cell that regulates MYC normally. In addition, transfer of chromosome 5 by microcell fusion results in suppression of deregulated expression. Suppressed cells are no longer tumorigenic in nude mice. Loss of the transferred chromosome results in reexpression of the tumorigenic phenotype and in constitutive elevated expression of MYC. These data indicate that function of a tumor-suppressor gene on chromosome 5 is necessary for the regulated expression of MYC in at least some colon cells. Loss of this suppressor results in deregulated MYC expression and is a necessary, but most likely not sufficient, event for the expression of the tumorigenic phenotype in a subset of colon carcinomas.

Posttranscriptional regulation of c-myc proto-oncogene expression and growth inhibition by recombinant human interferon-beta ser17 in a human colon carcinoma cell line

Recombinant human interferon-beta ser17 (IFN-beta ser17), a cytokine that exhibits both antiviral and antiproliferative activity against a wide variety of cell types, causes a time- and dose-dependent inhibition of monolayer growth and of the expression of the c-myc proto-oncogene in DLD-1 Clone A human colon-carcinoma cells. The suppression of c-myc expression mediated by IFN-beta ser17 is due to a posttranscriptional destabilization of c-myc mRNA rather than to an inhibition of c-myc mRNA transcription. There is evidence suggesting that the selective reduction in the half-life of c-myc mRNA in IFN-beta ser17-treated cells occurs through an increase in the activity of the 2',5'-oligoadenylate synthetase/RNase L [2',5'-oligo (A) synthetase] pathway in DLD-1 Clone A cells. Cotreatment of these cells with IFN-beta ser17 and the anticancer agent N-methylformamide leads to the partial abrogation of 2',5'-oligo (A) synthetase activity and the stabilization of c-myc mRNA. These findings suggest that there is a correlation between the IFN-beta ser17-mediated suppression of c-myc expression and the induction of 2',5'-oligo (A) synthetase activity in DLD-1 clone A cells.

c-myc protein product is a marker of DNA synthesis but not of malignancy in human gastrointestinal tissues and tumours

c-myc is a conserved cellular gene. The gene product is a nuclear-bound 62,000 molecular weight phosphoprotein (p62c-myc). Although p62c-myc levels have been measured in colorectal cancers, little is known about the expression of the protein in upper gastrointestinal tumours and tissues. Studies were performed on tumour and mucosal specimens from 87 patients with colorectal cancer, from two with polyposis coli, from six with squamous oesophageal carcinomas and from 18 with gastric carcinomas. The mean p62c-myc content was measured in units of fluorescence in the G1 diploid and G2 diploid peaks of the cell cycle by multiparameter flow cytometry using the 6E10 antibody. The nuclear p62c-myc content increased with DNA synthesis in tumours and mucosa. G2 levels of p62c-myc were higher in glandular mucosa than in adenocarcinomas. No differences in peak nuclear c-myc expression were found in relation to histological grade or to anatomical site of colorectal tumours. There was a broadly inverse relationship between G2 p62c-myc levels in tumours and mucosa and their in vivo 5-bromo-2'-deoxyuridine labelling indices. Nuclear p62c-myc levels are cell cycle related but the protein has not been shown to be a marker of increased tissue proliferation or of gastrointestinal malignancy. The reduction of the nuclear p62c-myc content of many adenocarcinoma cells compared with glandular mucosa cells suggests that reduced synthesis or nuclear retention of the normal protein may be a factor in the development of gastrointestinal adenocarcinomas, although the mechanism by which this may occur is not clear.

Genetics of colon cancer

Strikingly rapid advances in the identification of genetic events that are important in colonic carcinogenesis have been made in the past several years. Specific inherited (adenomatous polyposis coli gene) and acquired (ras gene point mutations; c-myc gene amplification; allelic deletion at specific sites on chromosomes 5, 17, and 18) genetic abnormalities appear to be capable of mediating steps in the progression from normal to malignant colonic mucosa. Understanding these genetic factors and how they influence cellular function will have a profound effect on medical practice. High-risk populations will be (and are being) identified by genetic markers, thus allowing prevention and screening to be more precisely targeted to the population at risk; intervention strategies will be designed on the basis of the known cellular defects of neoplastic colonic mucosa; and new molecular preventive and therapeutic approaches can be developed.

Karyotype peculiarities of human colorectal adenocarcinomas

The data of the chromosome abnormalities in 15 colorectal tumors are presented. Rearrangements of the short arm of chromosome 17, leading to deletions of this arm or its part were noted in 12 tumors; in 2 other cases, one of the homologs of pair 17 was lost. The losses of at least one homolog of other chromosomal pairs were also found: chromosome 18, in 12 out of 13 cases with fully identified numerical abnormalities; chromosome 5, in 6 tumors; chromosome 21, in 5 cases; chromosomes 4, 15, and 22, in 4 cases each. Additional homologs of pair 20 were observed in 6 tumors, extra 8q was found in 5 tumors, and extra 13q in 6 cases. Rearrangements of the short arm of chromosome 1 and the long arm of chromosome 11 characterized 6 tumors each. The data recorded in our series differ from the data of other authors in two respects: the high incidence of the loss of sex chromosomes and the rearrangements of the long arm of chromosome 9. X chromosomes were missing in 4 out of 7 tumors in females, and Y chromosomes were absent in 5 out of 8 tumors in males. The long arm of chromosome 9 was rearranged in 8 cases, in 5 of them the breakpoint being at 9q22. Cytological manifestations of gene amplification (double minutes or multiple microchromosomes) were noted in 6 tumors.

Correlation between chromosome 5q deletions and different mechanisms of c-myc overexpression in human colorectal cancer

Images

Chemical carcinogenesis: too many rodent carcinogens

The administration of chemicals at the maximum tolerated dose (MTD) in standard animal cancer tests is postulated to increase cell division (mitogenesis), which in turn increases rates of mutagenesis and thus carcinogenesis. The animal data are consistent with this mechanism, because a high proportion--about half--of all chemicals tested (whether natural or synthetic) are indeed rodent carcinogens. We conclude that at the low doses of most human exposures, where cell killing does not occur, the hazards to humans of rodent carcinogens may be much lower than is commonly assumed.

Dietary carcinogens, environmental pollution, and cancer: some misconceptions

Various misconceptions about dietary carcinogens, pesticide residues, and cancer causation are discussed. The pesticides in our diet are 99.99% natural, since plants make an enormous variety of toxins against fungi, insects, and animal predators. Although only 50 of these natural pesticides have been tested in animal cancer tests, about half of them are carcinogens. About half of all chemicals tested in animal cancer tests are positive. The proportion of natural pesticides positive in animal tests of clastogenicity is also the same as for synthetic chemicals. It is argued that testing chemicals in animals at the maximum tolerated dose primarily measures chronic cell proliferation, a threshold process. Cell proliferation is mutagenic in several ways, including inducing mitotic recombination, and therefore chronic induction of cell proliferation is a risk factor for cancer.

c-myc overexpression is a tumor-specific phenomenon in a subset of human colorectal carcinomas

The transcriptional activity of the c-myc proto-oncogene was examined in 25 primary human colorectal carcinomas and their corresponding normal mucosae. The purpose was to determine whether the elevated levels of c-myc expression, frequently detected in this type of tumor, might be the consequence of alterations in the cell growth rate or the effect of a real transcriptional deregulation of the gene. In about 44% of the tumors the elevated c-myc expression was consequent to the enhanced growth rate of the neoplastic tissue, as estimated by the expression of the S-phase-specific histone H3 gene. In the other 56%, c-myc overexpression did not entirely depend on the proliferative activity of the neoplastic population. In this latter group, c-myc deregulation did not reside in structural modifications of the putative regulatory regions of the gene. Therefore, c-myc overexpression, at least in a subset of colorectal cancer, seems to be consequent to alterations in transregulative phenomena exerted on the c-myc gene by other genetic loci.