Showing papers on "Carcinogenesis published in 1994"

Mutations in the p53 Tumor Suppressor Gene: Clues to Cancer Etiology and Molecular Pathogenesis

Abstract: The p53 tumor suppressor gene has come to the forefront of cancer research because it is commonly mutated in human cancer and the spectrum of p53 mutations in these cancers is providing clues to the etiology and molecular pathogenesis of neoplasia (1—3). Detection of p53 abnormalities may have diagnostic, prognostic, and therapeutic implications (4). The 15-year history of p53 investigations is a paradigm in cancer research, illustrating the convergence of previously parallel lines of basic, clinical, and epidemiological investigation and the rapid trans fer of research findings from the laboratory to the clinic. p53 is clearly a component in biochemical pathways central to human carcinogen esis; p53 protein alterations due to missense mutations and loss of p53 protein by nonsense or frameshift mutations provide a selective ad vantage for clonal expansion of preneoplastic and neoplastic cells (5). The potential for a missense mutation to cause loss of tumor suppres sor function and gain of oncogenic activity, i.e., to transform cells by two mechanisms, is one explanation for the commonality of p53 mutations in human cancer. Recent studies investigating the mecha nisms underlying the biological activity of p53 indicate that the protein is involved in gene transcription, DNA synthesis and repair, genomic plasticity, and programmed cell death (1—6).These complex biochemical processes are performed by multicomponent protein ma chines; therefore, it is not surprising that the p53 protein forms complexes with other cellular proteins (Fig. 1) and that some viral oncoproteins alter the functions of these machines by binding to p53 and perturbing its interaction with other cellular protein components. In this Perspective, we will focus on the origin of p.53 mutations, the mutational spectrum of p.53 in human cancers, and the hypotheses generated by the analysis of p53 mutations in premalignant and malignant cells. The interpretation ofp53 mutations in human cancers is based on observations of the patterns of DNA damage induced by chemical and physical mutagens in model systems. In this Introduc tion, we will review these data, which provide the background for many of the inferences drawn from p53 mutational analysis.

A cell cycle regulator potentially involved in genesis of many tumor types

Abstract: A putative tumor suppressor locus on the short arm of human chromosome 9 has been localized to a region of less than 40 kilobases by means of homozygous deletions in melanoma cell lines. This region contained a gene, Multiple Tumor Suppressor 1 (MTS1), that encodes a previously identified inhibitor (p16) of cyclin-dependent kinase 4. MTS1 was homozygously deleted at high frequency in cell lines derived from tumors of lung, breast, brain, bone, skin, bladder, kidney, ovary, and lymphocyte. Melanoma cell lines that carried at least one copy of MTS1 frequently carried nonsense, missense, or frameshift mutations in the gene. These findings suggest that MTS1 mutations are involved in tumor formation in a wide range of tissues.

Cell cycle control and cancer

Abstract: Multiple genetic changes occur during the evolution of normal cells into cancer cells. This evolution is facilitated in cancer cells by loss of fidelity in the processes that replicate, repair, and segregate the genome. Recent advances in our understanding of the cell cycle reveal how fidelity is normally achieved by the coordinated activity of cyclin-dependent kinases, checkpoint controls, and repair pathways and how this fidelity can be abrogated by specific genetic changes. These insights suggest molecular mechanisms for cellular transformation and may help to identify potential targets for improved cancer therapies.

Chronic infections and inflammatory processes as cancer risk factors : possible role of nitric oxide in carcinogenesis

Abstract: Infection by bacteria, parasites or viruses and tissue inflammation such as gastritis, hepatitis and colitis are recognized risk factors for human cancers at various sites. Nitric oxide (NO) and other oxygen radicals produced in infected and inflamed tissues could contribute to the process of carcinogenesis by different mechanisms, which are discussed on the basis of authors' studies on liver fluke infection and cholangiocarcinoma development. A similar mechanism could apply to other suspected and known cancer-causing agents including Helicobacter pylori infection (stomach cancer) or asbestos exposure (lung mesothelioma). Studies on the type of tissue and DNA damage produced by NO and by other reactive oxygen species are shedding new light on the molecular mechanisms by which chronic inflammatory processes may initiate or enhance carcinogenesis in humans.

Abnormal kidney development and hematological disorders in PDGF beta-receptor mutant mice.

Abstract: Platelet-derived growth factor, a major mitogen and chemoattractant for a number of cell types, is implicated in the processes of wound healing, tumorigenesis, and differentiation and is recognized by two receptors, alpha and beta. To begin understanding the role of these receptors in development, beta-receptor-deficient mice were generated by gene targeting in ES cells. Mutant mice are hemorrhagic, thrombocytopenic, and severely anemic, exhibit a defect in kidney glomeruli because of a lack of mesangial cells, and die at or shortly before birth. However, many cell types and tissues that express the receptor, including major blood vessels and the heart, appear normal in the absence of the receptor. These results indicate that whereas the beta receptor is essential in certain cell types during embryonic development, its broader role may be masked because of compensation by the alpha-subunit.

Cytidine methylation of regulatory sequences near the pi-class glutathione S-transferase gene accompanies human prostatic carcinogenesis

Abstract: Hypermethylation of regulatory sequences at the locus of the pi-class glutathione S-transferase gene GSTP1 was detected in 20 of 20 human prostatic carcinoma tissue specimens studied but not in normal tissues or prostatic tissues exhibiting benign hyperplasia. In addition, a striking decrease in GSTP1 expression was found to accompany human prostatic carcinogenesis. Immunohistochemical staining with anti-GSTP1 antibodies failed to detect the enzyme in 88 of 91 prostatic carcinomas analyzed. In vitro, GSTP1 expression was limited to human prostatic cancer cell lines containing GSTP1 alleles with hypomethylated promoter sequences; a human prostatic cancer cell line containing only hypermethylated GSTP1 promoter sequences did not express GSTP1 mRNA or polypeptides. Methylation of cytidine nucleotides in GSTP1 regulatory sequences constitutes the most common genomic alteration yet described for human prostate cancer.

Anticarcinogenic activities of sulforaphane and structurally related synthetic norbornyl isothiocyanates.

Abstract: Sulforaphane [1-isothiocyanato-4-(methyl-sulfinyl)butane] was recently isolated from one variety of broccoli as the major and very potent inducer of phase 2 detoxication enzymes in murine hepatoma cells in culture Since phase 2 enzyme induction is often associated with reduced susceptibility of animals and their cells to the toxic and neoplastic effects of carcinogens and other electrophiles, it was important to establish whether sulforaphane could block chemical carcinogenesis In this paper we report that sulforaphane and three synthetic analogues, designed as potent phase 2 enzyme inducers, block the formation of mammary tumors in Sprague-Dawley rats treated with single doses of 9,10-dimethyl-1,2-benzanthracene The analogues are exo-2-acetyl-exo-6-isothiocyanatonorbornane, endo-2-acetyl-exo-6-isothiocyanatonorbornane, and exo-2-acetyl-exo-5-isothiocyanatonorbornane When sulforaphane and exo-2-acetyl-exo-6-isothiocyanatonorbornane were administered by gavage (75 or 150 mumol per day for 5 days) around the time of exposure to the carcinogen, the incidence, multiplicity, and weight of mammary tumors were significantly reduced, and their development was delayed The analogues endo-2-acetyl-exo-6-isothiocyanatonorbornane and exo-2-acetyl-exo-5-isothiocyanatonorbornane were less potent protectors Thus, a class of functionalized isothiocyanates with anticarcinogenic properties has been identified These results validate the thesis that inducers of phase 2 enzymes in cultured cells are likely to protect against carcinogenesis

Two molecular pathways to transitional cell carcinoma of the bladder.

Abstract: Noninvasive transitional cell carcinomas of the bladder can have two distinct morphologies suggesting they contain different genetic alterations. Papillary transitional cell carcinomas (T(a) tumors) are often multifocal and only occasionally progress, whereas flat tumors (carcinomas in situ, CIS), frequently progress to invasive disease. We examined 216 bladder tumors of various stages and histopathologies for two genetic alterations previously described to be of importance in bladder tumorigenesis. Loss of heterozygosity of chromosome 9 was observed in 24 of 70 (34%) T(a) tumors but was present in only 3 of 24 (12%) CIS and dysplasia lesions (P = 0.04). In contrast, only 1 of 36 (3%) T(a) tumors contained a p53 gene mutation compared to 15 of 23 (65%) CIS and dysplasias (P < 0.001), a frequency comparable to that observed in muscle invasive tumors (25 of 49; 51%). The presence of p53 mutations in CIS and dysplasia could explain their propensities to progress since these mutations are known to destabilize the genome. Analysis of several tumor pairs involving a CIS and an invasive cancer provided evidence that the chromosome 9 alteration may in some cases be involved in the progression of CIS to more invasive tumors, in addition to its role in the initiation of T(a) tumors. However, the CIS and secondary tumor were found to contain different genetic alterations in some patients suggesting divergent progression pathways. Bladder carcinogenesis may therefore proceed through two distinct genetic alteration pathways responsible for generating superficial tumors with differing morphologies and pathologies.

Molecular determinants of dysplasia in colorectal lesions.

Abstract: One hallmark of malignant potential is dysplasia, the disruption of normal morphology. While it is generally recognized that cancer is the result of a series of genetic changes, the relationship of these alterations and their timing to the advent of dysplasia remains obscure. To address this issue, 54 small benign colorectal lesions of various malignant potential were analyzed for APC and K-RAS mutations, two alterations which have been implicated in the early stages of colorectal tumorigenesis. APC mutations were closely associated with dysplasia. In contrast, K-RAS mutations were found to be remarkably common in small nondysplastic lesions which apparently have a limited potential to progress to larger tumors. These results provide evidence that the nature and order of genetic changes can have a specific impact on both tumor morphology (e.g., dysplasia) and the likelihood of tumor progression.

Genomic instability in repeated sequences is an early somatic event in colorectal tumorigenesis that persists after transformation.

Abstract: Genomic instability at simple repeated sequences (SRS) is a landmark for some sporadic and hereditary cancers of the colon. We have identified several human tumour cell lines with up to 1,000–fold increases in mutation rates for endogenous microsatellite sequences, relative to normal cells or tumour cells without the mutator phenotype and show that they are very early events in tumorigenesis. Our in vivo and in vitro results show that the genomic instability persists after transformation and that microsatellite mutations accumulate as consecutive somatic slippage events of a single or a few repeated units. This mechanism may account for the repeat expansions in triplet hereditary diseases and the same defect in replication fidelity in non–polyposis colon cancer could also contribute to the non–mendelian anticipation in these diseases.

Reactive Oxygen Species in Tumorigenesis

Abstract: In this review we will summarize recent data on reactive oxygen species-induced mutagenesis and consider its relationship to tumorigenesis in humans. With the use of a single-stranded DNA template it has been possible to correlate oxygen radical-induced chemical alterations at specific nucleotides with the types of mutations that occur when these altered bases are copied by DNA polymerases. This has allowed us to identify the types of mutations that occur secondary to a variety of oxidative stresses and study several of the mechanisms by which they arise. The most frequent mutations that result from reactive oxygen species-induced damage to DNA in bacteria are C to T transitions. These mutations, however, are not pathoneumonic for mutagenesis by oxygen-free radicals since they result from DNA damage caused by other genotoxic agents as well as by DNA polymerase errors. One type of mutation, a tandem CC to TT double substitution, has been shown to be induced by reactive oxygen species generated by a variety of systems and may be diagnostic for such damage. In studies with mammalian DNA polymerases, DNA damaged by reactive oxygen species yields mutations different from those observed in Escherichia coli . This diversity of mutagenic changes in these in vitro studies highlights the role of DNA replicating enzymes in specifying the types of mutations produced by reactive oxygen species. In conclusion, we will consider the role of reactive oxygen species in the pathogenesis of three common tumors, carcinoma of the liver, lung, and prostate with consideration on the possible use of antioxidant preventive therapy to slow tumorigenesis sufficiently to prevent clinical presentation of these cancers during the life span of a patient.

Deletion of p16 and p15 Genes in Brain Tumors

Abstract: We have used molecular genetic methods to examine the status of cell cycle-inhibitory genes in human brain tumors. We found that p16 and a neighboring gene, p15, were often homozygously deleted in glioblastoma multiformes but not in medulloblastomas or ependymomas. The deletions occurred in both primary tumors and their derived xenografts, but no intragenic mutations in either of the two genes were found. The p15 gene was expressed in a more widespread pattern in normal tissues than p16, but the products of both genes had similar capacities to bind to cyclin D-dependent kinases 4 and 6. These data suggest that the target of deletion in glioblastoma multiforme includes both p15 and p16 genes. The reason that homozygous deletions, rather than intragenic mutations, are so common in these tumors may be that deletion is a more efficient mechanism for simultaneous inactivation of both genes.

K-ras and p53 Gene Mutations in Pancreatic Cancer: Ductal and Nonductal Tumors Progress through Different Genetic Lesions

Abstract: We studied K-ras and p53 gene mutations in a panel of 57 primary pancreatic cancers including ductal and nonductal tumors. DNAs were obtained from formalin-fixed, paraffin-embedded material. Target sequences were amplified by polymerase chain reaction and analyzed by denaturing gradient gel electrophoresis and sequencing. Both K-ras and p53 genes were frequently mutated in ductal cancers (25 of 35, 71.4%; 18 of 35, 51.4%, respectively). K-ras mutations were confined to the second position of codon 12 where base transitions and transversions were equally observed. p53 changes were mainly missense mutations. Transitions and transversions were found equally with a prevalence of G:C-->A:T changes among transitions. No gene alterations were present in the 6 exocrine nonductal tumors and (with one exception) in the 12 endocrine tumors analyzed. Our results indicate that mutated K-ras and p53 genes can cooperate in the establishment of ductal pancreatic cancers, whereas other genetic events have to be present in nonductal tumors. Moreover, K-ras alterations may represent an early event in ductal tumorigenesis, as suggested both by the high gene mutation frequency and by the presence of mutations in low-grade tumors. On the contrary, p53 gene changes seem to represent an event required for the malignancy progression of ductal tumors from lower to higher grades.

The 1993 Walter Hubert Lecture: the role of the p53 tumour-suppressor gene in tumorigenesis

Abstract: The p53 tumour-suppressor gene is mutated in 60% of human tumours, and the product of the gene acts as a suppressor of cell division. It is thought that the growth-suppressive effects of p53 are mediated through the transcriptional transactivation activity of the protein. Overexpression of the p53 protein results either in arrest in the G1 phase of the cell cycle or in the induction of apoptosis. Both the level of the protein and its transcriptional transactivation activity increase following treatment of cells with agents that damage DNA, and it is thought that p53 acts to protect cells against the accumulation of mutations and subsequent conversion to a cancerous state. The induction of p53 levels in cells exposed to gamma-irradiation results in cell cycle arrest in some cells (fibroblasts) and apoptosis in others (thymocytes). Cells lacking p53 have lost this cell cycle control and presumably accumulate damage-induced mutations that result in tumorigenesis. Thus, the role of p53 in suppressing tumorigenesis may be to rescue the cell or organism from the mutagenic effects of DNA damage. Loss of p53 function accelerates the process of tumorigenesis and alters the response of cells to agents that damage DNA, indicating that successful strategies for radiation therapy may well need to take into account the tissue of origin and the status of p53 in the tumour.

Bcl-2 blocks p53-dependent apoptosis.

Abstract: Adenovirus E1A expression recruits primary rodent cells into proliferation but fails to transform them because of the induction of programmed cell death (apoptosis). The adenovirus E1B 19,000-molecular-weight protein (19K protein), the E1B 55K protein, and the human Bcl-2 protein each cause high-frequency transformation when coexpressed with E1A by inhibiting apoptosis. Thus, transformation of primary rodent cells by E1A requires deregulation of cell growth to be coupled to suppression of apoptosis. The product of the p53 tumor suppressor gene induces apoptosis in transformed cells and is required for induction of apoptosis by E1A. The ability of Bcl-2 to suppress apoptosis induced by E1A suggested that Bcl-2 may function by inhibition of p53. Rodent cells transformed with E1A plus the p53(Val-135) temperature-sensitive mutant are transformed at the restrictive temperature and undergo rapid and complete apoptosis at the permissive temperature when p53 adopts the wild-type conformation. Human Bcl-2 expression completely prevented p53-mediated apoptosis at the permissive temperature and caused cells to remain in a predominantly growth-arrested state. Growth arrest was leaky, occurred at multiple points in the cell cycle, and was reversible. Bcl-2 did not affect the ability of p53 to localize to the nucleus, nor were the levels of the p53 protein altered. Thus, Bcl-2 diverts the activity of p53 from induction of apoptosis to induction of growth arrest, and it is thereby identified as a modifier of p53 function. The ability of Bcl-2 to bypass induction of apoptosis by p53 may contribute to its oncogenic and antiapoptotic activity.

Frequent somatic mutation of the MTS1/CDK4I (multiple tumor suppressor/cyclin-dependent kinase 4 inhibitor) gene in esophageal squamous cell carcinoma.

Abstract: We previously reported frequent loss of heterozygosity on chromosome 9p in esophageal carcinomas and suggested that a tumor suppressor gene located on this chromosomal arm might be involved in development of these cancers. Since recently published studies have shown that a gene mapped on chromosome 9p21, MTS1/CDK4I (multiple tumor suppressor 1/cyclin-dependent kinase 4 inhibitor), is frequently mutated in various types of tumors, we chose to examine esophageal squamous cell carcinomas for mutations in this candidate gene. DNA sequence analyses revealed somatic mutations of MTS1/CDK4I in 14 of 27 tumors examined; 8 were frame-shift mutations and 6 were missense mutations. These results suggested that the MTS1/CDK4I gene is a tumor suppressor the inactivation of which plays an important role during carcinogenesis of the squamous cell type of esophageal carcinoma.

Somatic mutations of the von Hippel — Lindau disease tumour suppressor gene in non-familial clear cell renal carcinoma

Abstract: Loss of heterozygosity (LOH) studies have suggested that somatic mutations of a tumour suppressor gene or genes on chromosome 3p are a critical event in the pathogenesis of non-familial renal cell carcinoma (RCC). Germline mutations of the von Hippel-Lindau (VHL) disease gene predispose to early onset and multifocal clear cell renal cell carcinoma, and the mechanism of tumorigenesis in VHL disease is consistent with a one-hit mutation model. To investigate the role of somatic VHL gene mutations in non-familial RCC, we analysed 99 primary RCC for VHL gene mutations by SSCP and heteroduplex analysis. Somatic VHL gene mutations were identified in 30 of 65 (46%) sporadic RCC with chromosome 3p allele loss and one of 34 (3%) tumours with no LOH for chromosome 3p. The VHL gene mutations were heterogeneous (17 frameshift deletions, eight missense mutations, four frameshift insertions, one nonsense and one splice site mutation), but no mutations were detected in the first 120 codons of cloned coding sequence. Most RCCs with somatic VHL mutations (23 of 27 (85%) informative cases) had chromosome 3p25 allele loss in the region of the VHL gene so that both alleles of the VHL gene had been inactivated as expected from a two-hit model of tumorigenesis. Detailed histopathology was available for 59 of the tumours investigated: 18 of 43 (42%) RCC with a clear cell appearance had a somatic VHL gene mutation but none of 16 non-clear cell RCC (eight chromophilic, three chromophobe and five oncocytoma) (chi2 = 7.77, P < 0.025).(ABSTRACT TRUNCATED AT 250 WORDS)

UV and skin cancer: specific p53 gene mutation in normal skin as a biologically relevant exposure measurement.

Abstract: Many human skin tumors contain mutated p53 genes that probably result from UV exposure. To investigate the link between UV exposure and p53 gene mutation, we developed two methods to detect presumptive UV-specific p53 gene mutations in UV-exposed normal skin. The methods are based on mutant allele-specific PCRs and ligase chain reactions and designed to detect CC to TT mutations at codons 245 and 247/248, using 10 micrograms of DNA samples. These specific mutations in the p53 gene have been reported in skin tumors. CC to TT mutations in the p53 gene were detected in cultured human skin cells only after UV irradiation, and the mutation frequency increased with increasing UV dose. Seventeen of 23 samples of normal skin from sun-exposed sites (74%) on Australian skin cancer patients contained CC to TT mutations in one or both of codons 245 and 247/248 of the p53 gene, and only 1 of 20 samples from non-sun-exposed sites (5%) harbored the mutation. None of 15 biopsies of normal skin from non-sun-exposed or intermittently exposed sites on volunteers living in France carried such mutations. Our results suggest that specific p53 gene mutations associated with human skin cancer are induced in normal skin by solar UV radiation. Measurement of these mutations may be useful as a biologically relevant measure of UV exposure in humans and as a possible predictor of risk for skin cancer.

p53-dependent G1 arrest involves pRB-related proteins and is disrupted by the human papillomavirus 16 E7 oncoprotein.

Abstract: The cell cycle regulatory tumor suppressor proteins p53 and pRB are targeted for inactivation by several tumor viruses, including the high-risk types of human papillomaviruses (HPVs) via interactions of the HPV E6 and E7 oncoproteins with p53 and pRB, respectively. p53 plays a central role in a signal transduction pathway that mediates G1 arrest after DNA damage, though the mechanism by which G1 arrest occurs has not been elucidated. The cyclin-associated protein p21waf1/cip1 has recently been shown to be induced by p53 and to inhibit cyclin complex-mediated phosphorylation of pRB in vitro. Thus, we investigated a possible role for pRB in the p53-mediated DNA damage response. After gamma-irradiation, cells expressing wild-type p53 arrested in G1, contained increased levels of WAF1/CIP1 mRNA, and demonstrated accumulation of hypophosphorylated pRB. In contrast, cell lines with abnormal p53 genes or with p53 functionally inactivated by the E6 oncoprotein of HPV16 (a high-risk HPV) failed to arrest in G1, did not elevate WAF1/CIP1 mRNA, and did not accumulate hypophosphorylated pRB. Despite apparently normal elevation of p53 protein and WAF1/CIP1 mRNA after irradiation, cells expressing HPV16 E7 also failed to arrest in G1 and did not accumulate hypophosphorylated pRB. Disruption of RB genes alone did not totally abrogate this G1 arrest. Our results suggest that p53 indirectly regulates phosphorylation of pRB and that pRB and/or other pRB-like molecules that bind to HPV16 E7 participate in the DNA damage-mediated G1 arrest signal. In the process of HPV infection, the HPV E6 and E7 oncoproteins may undermine this cell cycle checkpoint, contributing to the accumulation of genetic alterations during tumorigenesis.

Dysregulation of Epidermal Growth Factor Receptor Expression in Premalignant Lesions during Head and Neck Tumorigenesis

Abstract: The development of head and neck cancer, believed to result from field cancerization and a multistep process of tumorigenesis, is often associated with an accumulation of genotypic and phenotypic alterations. The phenotypic changes could be the result of dysregulation of growth control genes such as epidermal growth factor receptor (EGFR). With the goal of identifying a potential biomarker of the multistep process of tumorigensis, we studied specimens of 36 head and neck squamous cell carcinomas from 5 different sites that contained normal epithelia and/or premalignant lesions adjacent to the tumors. Almost all of the individuals from whom these specimens were obtained had been exposed to first-hand smoking and/or alcohol consumption. Using a monoclonal anti-EGFR antibody for immunohistochemical analysis on paraffin-embedded sections with attached 886 cells for internal control, the levels of EGFR expression were assessed by image analysis. The relative staining intensity of EGFR in normal epithelia adjacent to tumors was 2-fold higher than that in normal control epithelium (P = 0.021), suggesting that, even in histologically normal epithelium, EGFR was already up-regulated in tissues surrounding tumors. These findings supported the theory of field cancerization in head and neck tumorigenesis. As tissue progressed from normal tissue adjacent to tumor to hyperplasia and to dysplasia, EGFR expression remained elevated. However, in the step from dysplasia to squamous cell carcinoma, EGFR expression was further and dramatically up-regulated (P = 0.01). Therefore, these results indicate that EGFR dysregulation happens in two steps, the moderate up-regulation of EGFR expression in normal epithelium adjacent to tumor and the further up-regulation of EGFR expression in the change from dysplasia to squamous cell carcinoma. In summary, the studies presented here indicate that EGFR dysregulation might be a useful marker for identifying individuals at risk of tumor development and an intermediate end point in chemoprevention trials.

p16 alterations and deletion mapping of 9p21-p22 in malignant mesothelioma.

Abstract: To determine whether p16 is altered in human malignant mesothelioma (MM), molecular analysis of multiple 9p loci was performed on 40 cell lines and 23 primary tumors from 42 MM patients. We identified homozygous deletions of p16 in 34 (85%) cell lines and a point mutation in 1 line. Down-regulation of p16 was observed in 4 of the remaining cell lines, 1 of which displayed a DNA rearrangement of p16. Homozygous deletions of p16 were identified in 5 of 23 (22%) primary tumors; no mutations or rearrangements were found in these specimens. Four cell lines displayed a single homozygous deletion proximal to or distal to p16; 4 others had 2 nonoverlapping deletions, one involving p16 and the other involving a region proximal to this locus. These data indicate that alterations of p16 are a common occurrence in MM cell lines and, to a lesser extent, in primary tumors. Furthermore, deletions of 9p21-p22 outside of the p16 locus may reflect the involvement of other putative tumor suppressor genes that could also contribute to the pathogenesis of some MMs.

The p53 gene and protein in human brain tumors.

Abstract: Because p53 gene alterations are commonplace in human tumors and because p53 protein is involved in a number of important cellular pathways, p53 has become a topic of intensive investigation, both by basic scientists and clinicians. p53 was initially identified by two independent laboratories in 1979 as a 53 kilodalton (kD) protein that complexes with the large T antigen of SV40 virus. Shortly thereafter, it was shown that the E1B oncoprotein of adenovirus also binds p53. The binding of two different oncogenic viral tumor proteins to the same cellular protein suggested that p53 might be integral to tumorigenesis. The human p53 cDNA and gene were subsequently cloned in the mid-1980s, and analysis of p53 gene alterations in human tumors followed a few year later. During these 10 years, researchers grappling with the vagaries of p53 first characterized the gene as an oncogene, then as a tumor suppressor gene, and most recently as both a tumor suppressor gene and a so-called [open quotes]dominant negative[close quotes] oncogene. The last few years have seen an explosion in work on this single gene and its protein product. A review of a computerized medical database revealed approximately 650 articles on p53 in 1992 alone. p53more » has assumed importance in neuro-oncology because p53 mutations and protein alterations are frequent in the common diffuse, fibrillary astrocytic tumors of adults. p53 mutations in astrocytomas were first described in 1989 and were followed by more extensive analyses of gene mutations and protein alterations in adult astrocytomas. The gene has also been studied in less common brain tumors. Elucidating the role of p53 in brain tumorigenesis will not only enhance understanding of brain tumor biology but may also contribute to improved diagnosis and therapy. This discussion reviews key aspects of the p53 gene and protein, and describe their emerging roles in central nervous system neoplasia. 102 refs., 6 figs., 1 tab.« less

Somatic APC and K-ras codon 12 mutations in aberrant crypt foci from human colons.

Abstract: Aberrant crypt foci (ACF) are microscopic lesions which have been postulated to precede the development of adenomatous polyps, the precursors to colorectal cancer. APC and ras gene mutations have been shown to be important early molecular events in the development of colorectal neoplasms. The objective of this study was to establish the nature and frequency of these two genetic alterations in ACF harvested from human colorectal resection specimens. One hundred and fifty-four ACF comprised of between 1 and 56 crypts were harvested from the grossly normal mucosa of colorectal resection specimens of 28 patients with varying pathological diagnoses. One hundred and twenty-five ACF from 20 colons were screened for the presence of K- ras codon 12 mutations with a polymerase chain reaction/restriction enzyme-based method. The APC gene mutation cluster region was screened in 65 ACF from 20 colons using a polymerase chain reaction/single strand conformation polymorphism technique. Putative mutations were confirmed by direct sequencing. K- ras codon 12 mutations were identified in 13% (16 of 125) of ACF. We also identified APC mutations in 4.6% (3 of 65) of ACF. The results of this study demonstrate that both APC and K- ras mutations occur in ACF. These observations support the role of the ACF as a colorectal cancer precursor and provide further insight into the early genetic changes which occur during colorectal tumorigenesis.

Epigenetic lesions at the H19 locus in Wilms' tumour patients.

Abstract: To test the potential role of H19 as a tumour suppressor gene we have examined its expression and DNA methylation in Wilms' tumours (WTs). In most WTs (18/25), H19 RNA was reduced at least 20–fold from fetal kidney levels. Of the expression–negative tumours ten retained 11p15.5 heterozygosity: in nine of these, H19 DNA was biallelically hypermethylated and in two cases hypermethylation locally restricted to H19 sequences was also present in the non–neoplastic kidney parenchyma. IGF2 mRNA was expressed in most but not all WTs and expression patterns were consistent with IGF2/H19 enhancer competition without obligate inverse coupling. These observations implicate genetic and epigenetic inactivation of H19 in Wilms' tumorigenesis.