Rebuilding the road to cancer

Bronchogenic and alveologenic tumors in mice induced by N-nitrosopiperidine

The aim of this study was to explore the histogenesis and carcinogenesis of pulmonary cancer induced by N-nitrosopiperidine (NPIP) in mice. NPIP is a form of N-nitrosamine found in tobacco smoke, which has been shown to be a genotoxic chemical as well as a mutagenic compound for inducing chromosome aberrations and severe clastogenicity. In this study, 80 BALB/C strain mice were injected with 0.2 mmol/kg NPIP intraperitoneally for 8 weeks, and experiments were conducted for a further 16 weeks. For the control group, 40 mice were injected with an equal volume of 0.9% NaCl. Pulmonary tissues and tumors in the NPIP-treated group were examined by light microscopy and transmission electron microscopy and compared with the control group at 4-week intervals. The mRNA levels of p53 (mutant), bcl-2, c-myc, ras, and subunits of telomerase - telomerase reverse transcriptase (TERT) and an RNA component, TR - were assayed by mPCR or RT-PCR. Twenty-two mice in the experimental group were found to develop pulmonary tumors, but none in the control group. All tumors found in the experimental group originated from alveolar type II epithelial cells. In addition, 6 of the 22 mice also developed tumors of bronchogenic origin. The expression of p53, bcl-2, c-myc, ras, and the subunits of telomerase were found to increase in all pulmonary tissues and tumors formed thereafter upon NPIP treatment. In summary, NPIP-induced mouse lung tumors exhibited morphological changes during carcinogenesis, which may be the consequence of overexpression of some genes associated with the development of carcinoma and changes in subunits of telomerase. This mouse model of lung tumor formation may be a useful tool to delineate the histogenesis and carcinogenesis of human pulmonary cancer.

Effect of ellagic acid on the expression of human telomerase reverse transcriptase (hTERT) alpha+beta+ transcript in estrogen receptor-positive MCF-7 breast cancer cells

OBJECTIVES

To evaluate the potential of ellagic acid to inhibit the expression of human telomerase reverse transcriptase (hTERT) alpha+beta+ splice variant in MCF-7 breast cancer cells.

DESIGN AND METHODS

MCF-7 cells were incubated with ellagic acid (10(-)(9) M-10(-5) M) in the absence and in the presence of 17beta-estradiol (10(-8) M), a known inducer of hTERT transcription, and hTERT alpha+beta+ mRNA expression was quantified by real-time RT-PCR. 17beta-estradiol and ICI182780, a known estrogen antagonist, served as positive and negative controls respectively.

RESULTS

Ellagic acid, when alone, increased hTERT alpha+beta+ mRNA while its coexistence with 17beta-estradiol reduced significantly the 17beta-estradiol-induced increase in hTERT alpha+beta+ mRNA, implicating thus both its estrogenic and anti-estrogenic effects in breast cancer cells.

CONCLUSIONS

The potential of ellagic acid to down-regulate the 17beta-estradiol-induced hTERT alpha+beta+ mRNA expression may be a mechanism via which ellagic acid exerts, at least in part, its chemopreventive effects in breast cancer.

Canis familiaris telomerase reverse transcriptase undergoes alternative splicing

The enzyme telomerase is essential for cell proliferation and tumorigenesis. Telomerase reverse transcriptase (TERT) represents the catalytic subunit of the enzyme. In humans, TERT expression is regulated by several different mechanisms, including alternative splicing. Canis familiaris TERT (dogTERT) has been shown to have a high level of sequence similarity with human TERT, indicating that the dog may represent a suitable animal model for telomerase studies. In the present report we sought to investigate whether dogTERT undergoes alternative splicing. During the analysis of canine mammary tissues (both tumor and paired adjacent to the tumor normal tissues) for dogTERT expression by RT-PCR, we identified eight samples-one tumor and seven adjacent normal-which gave PCR products of unexpected sizes. DNA sequencing revealed two insertions (175 and 28 bp long) and two deletions (17 and 32 bp long), which were encountered in different combinations and gave rise to five different transcripts. The generation of all variants could be explained by the employment of alternative splicing sites within dogTERT genomic sequences. The 175-bp and 28-bp insertions, identified between exons 7 and 8 and between 8 and 9, respectively, constituted unspliced sequences of introns 7 and 8, respectively. Both deletions originated from exon 8 sequence removals due to alternative splicing. All five variants encoded truncated proteins, which lacked essential motifs for reverse transcription and might have thus lost their ability to compose active telomerase enzymes. This is the first identification of alternative splicing events within dogTERT. The results presented here may provide the basis for more thorough studies on the regulation of telomerase activity in canine normal and cancer cells.

Telomerase inhibition combined with other chemotherapeutic reagents to enhance anti-cancer effect

Genetic experiments using a dominant-negative form of human telomerase (DN-hTERT) demonstrated that telomerase inhibition can result in telomeric shortening followed by proliferation arrest and cell death by apoptosis. Neoplastic cells from telomerase RNA null (mTERC-/-) mice showed enhanced chemosensitivity to doxorubicin or related double-strand DNA break (DSB)-inducing agents. Telomerase dysfunction, rather than telomerase inhibition, is proposed to be the principal determinant governing chemosensitivity specifically to DSB-inducing agents. We observed that imatinib and vincristine (VCR), in addition to DSB-inducing agents, also enhanced chemosensitivity in telomestatin-treated K562 cells. This observation suggests that combined use of telomerase inhibitors and imatinib or other chemotherapeutic agents may be a very useful approach to treatment of BCR-ABL-positive leukemia.

Are oncoantigens suitable targets for anti-tumour therapy?

When a vaccine-elicited immune response is directed against oncoantigens--proteins required for the neoplastic process--the chance that the tumour will evade the vaccine should be reduced. But how can these causal oncoantigens be identified? One approach is to find tumour-associated and microenvironment-associated oncoantigens required for progression from one tumour stage to the next by comparing gene signatures isolated from the different stages of tumour progression in cancer-prone transgenic mice. Mouse oncoantigens subsequently shown to be involved in human cancer can then be validated in mouse vaccination experiments. This provides the groundwork for the rational design of cancer vaccines for clinical trials.

Real-Time RT-PCR Quantification of Human Telomerase Reverse Transcriptase Splice Variants in Tumor Cell Lines and Non–Small Cell Lung Cancer

Background: We developed and validated a real-time reverse transcription (RT)–PCR for the quantification of 4 individual human telomerase reverse transcriptase (TERT) splice variants (α+β+, α−β+, α+β−, α−β−) in tumor cell lines and non–small cell lung cancer (NSCLC).

Methods: We used in silico designed primers and a common TaqMan probe for highly specific amplification of each TERT splice variant, PCR transcript–specific DNA external standards as calibrators, and the MCF-7 cell line for the development and validation of the method. We then quantified TERT splice variants in 6 tumor cell lines and telomerase activity and TERT splice variant expression in cancerous and paired noncancerous tissue samples from 28 NSCLC patients.

Results: In most tumor cell lines, we observed little variation in the proportion of TERT splice variants. The α+β− splice variant showed the highest expression and α−β+ and α−β− the lowest. Quantification of the 4 TERT splice variants in NSCLC and surrounding nonneoplastic tissues showed the highest expression percentage for the α+β− variant in both NSCLC and adjacent nonneoplastic tissue samples, followed by α+β+, with the α−β+ and α−β− splice variants having the lowest expression. In the NSCLC tumors, the α+β+ variant had higher expression than other splice variants, and its expression correlated with telomerase activity, overall survival, and disease-free survival.

Conclusions: Real-time RT-PCR quantification is a specific, sensitive, and rapid method that can elucidate the biological role of TERT splice variants in tumor development and progression. Our results suggest that the expression of the TERT α+β+ splice variant may be an independent negative prognostic factor for NSCLC patients.

Cyclo[n]pyrroles: size and site-specific binding to G-quadruplexes

Inhibiting the enzyme telomerase by stabilizing the G-quadruplex has potential in anticancer drug design. Diprotonated cyclo[n]pyrroles represent a set of expanded porphyrin analogues with structures similar to that of telomestatin, a natural product known to bind to and stabilize G-quadruplexes. As a first step toward testing whether cyclo[n]pyrroles display a similar function, a series of diprotonated cyclo[n]pyrroles (where n = 6, 7, and 8) was each added to the human telomere repeat sequence d(T(2)AG(3))(4) and examined with mass spectrometry, ion mobility, and molecular dynamics calculations. Nano-ESI-MS indicated that the smaller the cyclo[n]pyrrole, the more strongly it binds to the telomeric sequence. It was also found that cyclo[6]pyrrole bound to d(T(2)AG(3))(4) better than octaethylporphyrin, a finding rationalized by cyclo[6]pyrrole having a 2+ charge, while octaethylporphyrin bears no charge. Ion mobility measurements were used to measure the collision cross section of each d(T(2)AG(3))(4)/cyclo[n]pyrrole complex. Only one peak was observed in the arrival time distributions for all complexes, and the experimental cross sections indicated that only structures with d(T(2)AG(3))(4) in an antiparallel G-quadruplex arrangement and each cyclo[n]pyrrole externally stacked below the G-quartets occur under these experimental conditions. When the cyclo[n]pyrroles were intercalated or nonspecifically bound to the quadruplex, or if conformations different than antiparallel were considered for d(T(2)AG(3))(4), the theoretical cross sections did not match experiment. On this basis, it is inferred that (1) external stacking represents the dominant binding mode for the interaction of cyclo[n]pyrroles with d(T(2)AG(3))(4) and (2) the overall size and charge of the cyclo[n]pyrroles play important roles in defining the binding strength.

Routine expression profiling of microarray gene signatures in acute leukaemia by real-time PCR of human bone marrow

Cancer subtype diagnosis using microarray signatures has the potential to transform pathological diagnosis but the routine measurement of genes signatures remains difficult. Reverse transcription polymerase chain reaction (RT-PCR) measurement of Indicator genes for acute myeloid leukaemia (AML) and acute lymphoblastic leukaemia (ALL) was used to determine gene signatures. Bone marrow (BM) mononuclear cells were sorted into total, CD34(+) and CD34(-) fractions, and mRNAs globally amplified from each fraction using polyA PCR. The expression profile of the 17 top-ranked genes distinguishing AML and ALL were measured by RT-PCR in five ALL, 26 AML, 12 AML remission, four chronic myeloid leukaemia (CML) and nine morphologically normal BM samples. All but two of the genes measured showed similar expression in AML and ALL to that reported previously. Specifically, c-MYB (P </= 0.04) was significantly increased in ALL in the total fraction, whilst HOXA9 (P </= 0.19) and cystatin c (P </= 0.01) were increased in AML in the CD34(+) and CD34(-) fractions, respectively. c-MYB, hSNF2, RBAP48, HKRT-1, LYN, CD33, Adipsin and HOXA9 were increased in AML compared with remission AML, indicating an ability to determine disease activity. The method used is simple, sensitive and robust, enabling routine clinical use, and it can also be extended to other tumours types with gene signatures.

Use of human tissue to assess the oncogenic activity of melanoma-associated mutations

Multiple genetic alterations occur in melanoma, a lethal skin malignancy of increasing incidence. These include mutations that activate Ras and two of its effector cascades, Raf and phosphoinositide 3-kinase (PI3K). Induction of Ras and Raf can be caused by active N-Ras and B-Raf mutants as well as by gene amplification. Activation of PI3K pathway components occurs by PTEN loss and by AKT3 amplification. Melanomas also commonly show impairment of the p16(INK4A)-CDK4-Rb and ARF-HDM2-p53 tumor suppressor pathways. CDKN2A mutations can produce p16(INK4A) and ARF protein loss. Rb bypass can also occur through activating CDK4 mutations as well as by CDK4 amplification. In addition to ARF deletion, p53 pathway disruption can result from dominant negative TP53 mutations. TERT amplification also occurs in melanoma. The extent to which these mutations can induce human melanocytic neoplasia is unknown. Here we characterize pathways sufficient to generate human melanocytic neoplasia and show that genetically altered human tissue facilitates functional analysis of mutations observed in human tumors.

Chromosomal variations within aneuploid cancer lines

Aneuploid cancers exhibit a wide spectrum of clinical aggressiveness, possibly because of varying chromosome compositions. To test this, karyotypes from the diploid CCD-34Lu fibroblast and the aneuploid A549 and SUIT-2 cancer lines underwent fluorescence in situ hybridization (FISH) and DAPI counterstaining. The number of DAPI-stained and FISH-identified chromosomes, 1-22, X,Y, as well as structural abnormalities, were counted and compared using the chi(2), Mann-Whitney rank sum test and the Levene's equality of variance. Virtually all of the evaluable diploid CCD-34Lu karyotypes had 46 chromosomes with two normal-appearing homologues. The aneuploid chromosome numbers per karyotype were highly variable, averaging 62 and 72 for the A549 and SUIT-2 lines, respectively. However, the A549 chromosome numbers were more narrowly distributed than the SUIT-2 karyotype chromosome numbers. Furthermore, 25% of the A549 chromosomes had structural abnormalities compared to only 7% of the SUIT-2 chromosomes. The chromosomal compositions of the aneuploid A549 and SUIT-2 cancer lines are widely divergent, suggesting that diverse genetic alterations, rather than chance, may govern the chromosome makeups of aneuploid cancers.

A comparative study on the interaction of acridine and synthetic bis-acridine with G-quadruplex structure

DNA from the telomeres contains a stretch of simple tandemly repeated sequences in which clusters of G residues alternate with clusters of T/A sequences along one DNA strand. Model telomeric G-clusters form four-stranded structures in presence of Na(I), K(I) and NH(4)(I) ions. Electrophoretic and spectroscopic studies were made with the telomeric related sequences d(T6G16) or d(G4T2G4T2G4T2G4). It was noticed earlier that G-quadruplex may either be inter-molecular, or intra-molecular, or a mixture of both. CD spectral characteristics of various G-quadruplex DNA suggests that the CD maximum at 293 nm corresponds to that of an intra-molecular G-quadruplex structure or hairpin dimers. Fluorescence titration studies also show that acridine and the bis-acridine are interacting with G-quadruplex DNA and destabilize the K(I)-quadruplex structure more efficiently than the quadruplex formed by NH(4)(I) ion. Among the two drugs studied, acridine is more capable of breaking the G-quadruplex structure than bis-acridine. This result is further confirmed by the CD experiments.

Telomerase inhibition enhances apoptosis in human acute leukemia cells: possibility of antitelomerase therapy

Telomerase is a ribonucleoprotein enzyme that maintains protective structures at the ends of eukaryotic chromosomes. We examined the impact of telomerase inhibition by the dominant-negative human catalytic subunit of telomerase (DN-hTERT) on the biological features of acute leukemia. We introduced vectors encoding dominant- negative (DN)-hTERT, wild-type (WT)-hTERT, or a control vector expressing only a drug-resistant marker into a telomerase-positive human acute lymphoblastic leukemia cell line, HAL-01. Expression of DN-hTERT dramatically inhibited telomerase activity, leading to apoptotic cell death. Mutant telomerase expression also enhanced daunorubicin-induced apoptosis. Nude mice (n=5 per group) received subcutanous implants of HAL-01 cells expressing the control vector or DN-hTERT or WT-hTERT. Implantation of HAL-01 cells expressing control vector (n=5) rapidly produced tumors, whereas implantation of those expressing DN-hTERT (n=5) did not. Thus, telomerase inhibition both growth of HAL-01 cells in vitro and tumorigenic capacity in vivo. Furthermore, the G-quadruplex-interactive telomerase-specific inhibitor, telomestatin, shortened the telomere length and induced apoptosis in freshly isolated primary acute leukemia cells. These results suggest that antitelomerase therapy may be useful in some acute leukemias in combination with antileukemic agents such as daunorubicin.

Progressive transformation of immortalized esophageal epithelial cells

AIM: To investigate the progressive transformation of immortal cells of human fetal esophageal epithelium induced by human papillomavirus, and to examine biological criteria of sequential passage of cells, including cellular phenotype, proliferative rate, telomerase, chromosome and tumorigenicity.

METHODS: The SHEE cell series consisted of immortalized embryonic esophageal epithelium which was in malignant transformation when cultivated over sixty passages without co-carcinogens. Cells of the 10th, 31st, 60th and 85th passages were present in progressive development after being transfected with HPV. Cells were cultivated in a culture flask and 24-hole cultural plates. Progressive changes of morphology, cell growth, contact-inhibition, and anchorage-dependent growth characteristics were examined by phase contrast microscopy. The cell proliferation rate was assayed by flow cytometry. The modal number of chromosomes was analyzed. HPV18E₆E₇ was detected by Western blot methods and activities of telomerase were analyzed by TRAP. Tumorigenicity of cells was detected with soft agar plates cultivated and with tumor formation in SCID mice.

RESULTS: In morphological examination the 10th passage cells were in good differentiation, the 60th and 85th passages cells were in relatively poor differentiation, and the 31st passage cells had two distinct differentiations. The characteristics of the 85th and 60th passage cells were weakened at contact-inhibition and anchorage-dependent growth. Karyotypes of four stages of cells belonged to hyperdiploid or hypotriploid, and bimodal distribution of chromosomes appeared in the 31st and 60th passage cells. All of these characteristics combined with a increasing trend. The activities of telomerase were expressed in the latter three passages. Four fourths of SCID mice in the 85th passage cells and one fourth of SCID mice in the 60th passage cells developed tumors, but the cells in the 10th and 31st passage displayed no tumor formation.

CONCLUSION: In continual cultivation of fetal esophageal epithelial cells with transduction of HPV18E₆E₇, cells from the 10th to the 85th passage were changed gradually from preimmortal, immortal, precancerous to malignantly transformed stages. All of these changes were in a dynamic progressive process. The establishment of a continuous line of esophageal epithelium may provide a in vitro model of carcinogenesis induced by HPV.

Clusterin/apolipoprotein J in human aging and cancer

Clusterin/Apolipoprotein J (ApoJ) is a heterodimeric highly conserved secreted glycoprotein being expressed in a wide variety of tissues and found in all human fluids. Despite being cloned since 1989, no genuine function has been attributed to ApoJ so far. The protein has been reportedly implicated in several diverse physiological processes such as sperm maturation, lipid transportation, complement inhibition, tissue remodeling, membrane recycling, cell-cell and cell-substratum interactions, stabilization of stressed proteins in a folding-competent state and promotion or inhibition of apoptosis. ApoJ gene is differentially regulated by cytokines, growth factors and stress-inducing agents, while another defining prominent and intriguing ApoJ feature is its upregulation in many severe physiological disturbances states and in several neurodegenerative conditions mostly related to advanced aging. Moreover, ApoJ accumulates during the viable growth arrested cellular state of senescence, that is thought to contribute to aging and to tumorigenesis suppression; paradoxically ApoJ is also upregulated in several cases of in vivo cancer progression and tumor formation. This review focuses on the reported data related to ApoJ cell-type and signal specific regulation, function and site of action in normal and cancer cells. We discuss the role of ApoJ during cellular senescence and tumorigenesis, especially under the light of the recently demonstrated various ApoJ intracellular protein forms and their interaction with molecules involved in signal transduction and DNA repair, raising the possibility that its overexpression during cellular senescence might cause a predisposition to cancer.

Frequent deletions of tumor suppressor genes in pure pancreatic juice from patients with tumoral or nontumoral pancreatic diseases

BACKGROUND/AIMS

K-ras codon 12 mutation is the most frequent genetic alteration in pancreatic cancer. Sensitivity and specificity of K-ras are not high enough to detect all pancreatic cancers, especially at early stage. This study investigated whether detection of p16 and/or DPC4 deletions along with K-ras mutation in DNA samples could improve the definition of patients at risk of pancreatic cancer.

METHODS

K-ras mutations were investigated by sequencing. p16 and DPC4 homozygous deletions were studied using comparative multiplex polymerase chain reaction of DNA in pancreatic juice sampled during endoscopic retrograde pancreatography in 57 patients with either pancreatic cancer (group I, 18 patients), chronic pancreatitis (group II, 20 patients), or nontumoral pancreatobiliary disease (group III, 19 patients).

RESULTS

The frequencies of Ki-ras mutations were 61% in group I, 10% in group II, and 10.5% in group III. The frequencies of p16 exon 2 and DPC4 deletions were, respectively, 28 and 36% in group I, 50 and 58% in group II, and 24 and 36% in group III.

CONCLUSIONS

The combination of p16 and DPC4 deletions with K-ras mutation does not improve the diagnosis of pancreatic cancer based on K-ras mutation alone. These data suggest that tumor suppressor gene inactivation can occur with a high frequency during nonmalignant pancreatic diseases.

Adenovirus E1-transformed cells grow despite the continuous presence of transcriptionally active p53

The E1 region of adenovirus (Ad) type 5 is capable of transforming cells. According to current concepts, the Ad E1B 55 kDa (E1B 55K) protein enables transformed cells to grow by constantly binding and inactivating the p53 tumour suppressor protein. To test this model, the transcriptional activity of p53 was determined in Ad E1-transformed cells. Surprisingly, it was found that a p53-responsive promoter is highly active in Ad E1-transformed cells and further activated only 3- to 4-fold (compared to 200-fold in p53(-/-) cells) by exogenously expressed p53 or p53mt24-28, a p53 mutant that is transcriptionally active but unable to bind the E1B 55K. On the other hand, the transient overexpression of E1B 55K led to a strong downregulation of a p53-responsive promoter relative to its baseline activity in Ad E1-transformed cells but not in p53(-/-) cells. COS-7 cells, transformed by simian virus 40 (SV40), also showed constitutive p53 activity, whereas HeLa cells, transformed with oncogenic human papillomavirus, did not. Upon stable transfection, Ad E1-transformed cells but not p53(-/-) cells gave rise to colonies that expressed exogenous p53 or p53mt24-28 but, nonetheless, grew at near-wild-type rates. It is proposed that E1B 55K or the SV40 tumour antigen are saturated by the p53 protein, which accumulates in virus-transformed cells, leaving a proportion of active p53 molecules. The transformation of cells by the Ad E1 genes confers permissiveness for active p53, conceivably by inactivating the relevant products of p53 target genes that would otherwise prevent cell growth. Thus, Ad-transformed cells contain and tolerate active p53.

Induction of tubulogenesis in telomerase-immortalized human microvascular endothelial cells by glioblastoma cells

To facilitate the study of human endothelial cells we have used a replication defective retrovirus encoding the catalytic subunit of telomerase (hTERT) to derive populations of telomerase-immortalized human microvascular endothelial (TIME) cells. Whereas parental HMVECs became senescent on average within 35-45 population doublings (PDs), TIME cells have continued to proliferate for at least 200 PDs. TIME cells express readily detectable telomerase activity but display only a modest increase in telomere length. Karyotypic analysis reveals the cells to have a normal complement of human chromosomes with no evidence of gross genetic abnormalities. Furthermore, TIME cells retain many of the characteristics of the primary endothelial cells from which they were derived. For example, they express a panel of characteristic endothelial cell surface marker proteins such as CD31/PECAM-1 and alpha(v)beta3-integrin. In addition, TIME cells express receptors for low-density lipoprotein (LDL) receptor as they are competent for receptor-mediated endocytosis of fluorescent acetylated LDL. Importantly, when plated on matrigel, TIME cells undergo tubule formation. Moreover, when cocultured in the presence of human glioma cells, but not primary human astrocytes, TIME cells are induced to form stable tubules. Detachment of TIME cells from extracellular matrix leads to a form of programmed cell death known as anoikis. Conditional activation of the protein kinase Akt (Akt:ER*) significantly inhibited the onset of TIME cell anoikis under these conditions. We believe that the ability of hTERT to immortalize primary human endothelial cells, and the fact that such cells retain the endothelial characteristics of the cells from which they were derived, will greatly facilitate the analysis of human endothelial cell biology in vitro.

Increased epidermal tumors and increased skin wound healing in transgenic mice overexpressing the catalytic subunit of telomerase, mTERT, in basal keratinocytes

Telomerase transgenics are an important tool to assess the role of telomerase in cancer, as well as to evaluate the potential use of telomerase for gene therapy of age-associated diseases. Here, we have targeted the expression of the catalytic component of mouse telomerase, mTERT, to basal keratinocytes using the bovine keratin 5 promoter. These telomerase-transgenic mice are viable and show histologically normal stratified epithelia with high levels of telomerase activity and normal telomere length. Interestingly, the epidermis of these mice is highly responsive to the mitogenic effects of phorbol esters, and it is more susceptible than that of wild-type littermates to the development skin tumors upon chemical carcinogenesis. The epidermis of telomerase-transgenic mice also shows an increased wound-healing rate compared with wild-type littermates. These results suggest that, contrary to the general assumption, telomerase actively promotes proliferation in cells that have sufficiently long telomeres and unravel potential risks of gene therapy for age-associated diseases based on telomerase upregulation.

Aneuploidy precedes and segregates with chemical carcinogenesis

A century ago, Boveri proposed that cancer is caused by aneuploidy, an abnormal balance of chromosomes, because aneuploidy correlates with cancer and because experimental aneuploidy generates "pathological" phenotypes. Half a century later, when cancers were found to be nonclonal for aneuploidy, but clonal for somatic gene mutations, this hypothesis was abandoned. As a result, aneuploidy is now generally viewed as a consequence, and mutated genes as a cause of cancer. However, we have recently proposed a two-stage mechanism of carcinogenesis that resolves the discrepancy between clonal mutation and nonclonal karyotypes. The proposal is as follows: in stage 1, a carcinogen "initiates" carcinogenesis by generating a preneoplastic aneuploidy; in stage 2, aneuploidy causes asymmetric mitosis because it biases balance-sensitive spindle and chromosomal proteins and alters centrosomes both numerically and structurally (in proportion to the degree of aneuploidy). Therefore, the karyotype of an initiated cell evolves autocatalytically, generating ever-new chromosome combinations, including neoplastic ones. Accordingly, the heterogeneous karyotypes of "clonal" cancers are an inevitable consequence of the karyotypic instability of aneuploid cells. The notorious long latent periods, of months to decades, from carcinogen to carcinogenesis, would reflect the low probability of evolving by chance karyotypes that compete favorably with normal cells, in principle analagous to natural evolution. Here, we have confirmed experimentally five predictions of the aneuploidy hypothesis: (1) the carcinogens dimethylbenzanthracene and cytosine arabinoside induced aneuploidy in a fraction of treated Chinese hamster embryo cells; (2) aneuploidy preceded malignant transformation; (3) transformation of carcinogen-treated cells occurred only months after carcinogen treatment, i.e., autocatalytically; (4) preneoplastic aneuploidy segregated with malignant transformation in vitro and with 14 of 14 tumors in animals; and (5) karyotypes of tumors were heterogeneous. We conclude that, with the carcinogens studied, aneuploidy precedes cancer and is necessary for carcinogenesis.

K-Ras mutations and benign pancreatic disease

This review addresses the history of the ras oncogene, the techniques used to detect molecular alterations in the ras oncogene, and the application of polymerase chain reaction (PCR)-based methods to determine point mutations in clinical samples of patients with pancreatic diseases, namely pancreatic carcinoma and chronic pancreatitis. The frequency of ras mutations in pancreatic carcinoma is high, ranging from 70 to almost 100%. The frequence of ras mutations in chronic pancreatitis, either in pancreatic tissue or pancreatic secretions, vary between 0 and 100%. This wide range in part may be owing to differences in sampling, DNA extraction, or PCR method. The meaning of a k-ras mutation is under debate. Taking into account the positivity of ductal hyperplasias in normal pancreas and ras mutations in normal appearing duct cells, this molecular finding may not mean anything. In contrast, ras mutations are associated with smoking, one acknowledged risk factor for pancreatic carcinoma. The need for large prospective cohort studies is emphasized.

Aneuploidy vs. gene mutation hypothesis of cancer: recent study claims mutation but is found to support aneuploidy

For nearly a century, cancer has been blamed on somatic mutation. But it is still unclear whether this mutation is aneuploidy, an abnormal balance of chromosomes, or gene mutation. Despite enormous efforts, the currently popular gene mutation hypothesis has failed to identify cancer-specific mutations with transforming function and cannot explain why cancer occurs only many months to decades after mutation by carcinogens and why solid cancers are aneuploid, although conventional mutation does not depend on karyotype alteration. A recent high-profile publication now claims to have solved these discrepancies with a set of three synthetic mutant genes that "suffices to convert normal human cells into tumorigenic cells." However, we show here that even this study failed to explain why it took more than "60 population doublings" from the introduction of the first of these genes, a derivative of the tumor antigen of simian virus 40 tumor virus, to generate tumor cells, why the tumor cells were clonal although gene transfer was polyclonal, and above all, why the tumor cells were aneuploid. If aneuploidy is assumed to be the somatic mutation that causes cancer, all these results can be explained. The aneuploidy hypothesis predicts the long latent periods and the clonality on the basis of the following two-stage mechanism: stage one, a carcinogen (or mutant gene) generates aneuploidy; stage two, aneuploidy destabilizes the karyotype and thus initiates an autocatalytic karyotype evolution generating preneoplastic and eventually neoplastic karyotypes. Because the odds are very low that an abnormal karyotype will surpass the viability of a normal diploid cell, the evolution of a neoplastic cell species is slow and thus clonal, which is comparable to conventional evolution of new species.

Aneuploidy vs. gene mutation hypothesis of cancer: Recent study claims mutation but is found to support aneuploidy

For nearly a century, cancer has been blamed on somatic mutation. But it is still unclear whether this mutation is aneuploidy, an abnormal balance of chromosomes, or gene mutation. Despite enormous efforts, the currently popular gene mutation hypothesis has failed to identify cancer-specific mutations with transforming function and cannot explain why cancer occurs only many months to decades after mutation by carcinogens and why solid cancers are aneuploid, although conventional mutation does not depend on karyotype alteration. A recent high-profile publication now claims to have solved these discrepancies with a set of three synthetic mutant genes that "suffices to convert normal human cells into tumorigenic cells." However, we show here that even this study failed to explain why it took more than "60 population doublings" from the introduction of the first of these genes, a derivative of the tumor antigen of simian virus 40 tumor virus, to generate tumor cells, why the tumor cells were clonal although gene transfer was polyclonal, and above all, why the tumor cells were aneuploid. If aneuploidy is assumed to be the somatic mutation that causes cancer, all these results can be explained. The aneuploidy hypothesis predicts the long latent periods and the clonality on the basis of the following two-stage mechanism: stage one, a carcinogen (or mutant gene) generates aneuploidy; stage two, aneuploidy destabilizes the karyotype and thus initiates an autocatalytic karyotype evolution generating preneoplastic and eventually neoplastic karyotypes. Because the odds are very low that an abnormal karyotype will surpass the viability of a normal diploid cell, the evolution of a neoplastic cell species is slow and thus clonal, which is comparable to conventional evolution of new species.

Telomerase and mammalian ageing: a critical appraisal

The telomeres that occur at the end of chromosomes are maintained by the activity of telomerase and are thought to be important protective factors in maintaining the integrity of chromosomes. It now appears that in vitro replicative senescence, which has been observed in cultured somatic cells, is due to a loss of telomere length in those cells, caused by inactivity of telomerase. This has led to the proposition that telomerase activity is an important determinant in organismal ageing. However, many cells in the body do not proliferate regularly and therefore will not lose telomere length. Cells that do proliferate frequently have now been shown to have active telomerase. Other cells, such as fibroblasts, that do not have telomerase activity but proliferate only occasionally may not reach the Hayflick limit during the lifetime of an animal. There is also no correlation between telomere length and the maximal lifespan exhibited by different species. Studies of telomerase knock-out mice have reported some aspects of accelerated ageing after three generations, but the relevance of these observations to normal ageing remains unconvincing. The role of telomerase in producing immortal tumour cells and the possibility that activation of telomerase is an important event in malignant transformation is similarly controversial and open to alternative interpretations. The significance of these and other observations, and how they define the role of telomerase in ageing, is discussed.

Telomeres, telomerase, and cancer: life on the edge of genomic stability

The presence of telomerase activity in most human tumors, but not in many normal somatic tissues, has raised considerable interest in telomerase as a possible anticancer therapy. Recent advances in the cloning and characterization of mammalian telomerase components have paved the way for a more detailed understanding of the role of telomerase and telomere length maintenance in cell proliferation. Here, we summarize the most recent biochemical and genetic evidence suggesting that telomere length maintenance by telomerase is critical to the proliferative ability of some immortalized mammalian cells in culture and in vivo.