Are centrosomes or aneuploidy the key to cancer?

Karyotype Aberrations in Action: The Evolution of Cancer Genomes and the Tumor Microenvironment

Cancer is a disease of cellular evolution. For this cellular evolution to take place, a population of cells must contain functional heterogeneity and an assessment of this heterogeneity in the form of natural selection. Cancer cells from advanced malignancies are genomically and functionally very different compared to the healthy cells from which they evolved. Genomic alterations include aneuploidy (numerical and structural changes in chromosome content) and polyploidy (e.g., whole genome doubling), which can have considerable effects on cell physiology and phenotype. Likewise, conditions in the tumor microenvironment are spatially heterogeneous and vastly different than in healthy tissues, resulting in a number of environmental niches that play important roles in driving the evolution of tumor cells. While a number of studies have documented abnormal conditions of the tumor microenvironment and the cellular consequences of aneuploidy and polyploidy, a thorough overview of the interplay between karyotypically abnormal cells and the tissue and tumor microenvironments is not available. Here, we examine the evidence for how this interaction may unfold during tumor evolution. We describe a bidirectional interplay in which aneuploid and polyploid cells alter and shape the microenvironment in which they and their progeny reside; in turn, this microenvironment modulates the rate of genesis for new karyotype aberrations and selects for cells that are most fit under a given condition. We conclude by discussing the importance of this interaction for tumor evolution and the possibility of leveraging our understanding of this interplay for cancer therapy.

Stage-specific frequency and prognostic significance of aneuploidy in patients with sporadic colorectal cancer--a meta-analysis and current overview

PURPOSE

Aneuploidy has long been suggested as an independent prognostic marker for colorectal cancer (CRC) patients and could thus aid for individualized medicine. However, due to a large spectrum of deviating studies, expert panels do not recommend ploidy assessment. In order to clarify a potential bias of stage-specific frequency of aneuploidy, we now conducted a meta-analysis combined with a systematic review regarding aneuploidy and prognosis.

METHODS

A systematic, web-based search process retrieved 1935 studies published in English between 1990 and 2011. The defined endpoint for the meta-analysis was an increase in aneuploidy frequency between early- (Dukes A, B and UICC I, II; n = 3632 samples) and late-stage (Dukes C, D and UICC III, IV; n = 3440 samples) colorectal carcinomas.

RESULTS

Of 1935 studies initially identified, 17 image (2130 patients) and 20 (7023 patients) flow cytometric studies were analyzed in detail. The meta-analysis (7072 patients) revealed late-stage CRC to be more frequently aneuploid than early-stage CRC (odds ratio 1.51, 95 % CI 1.37-1.67; p = 0.0007). Independent of tumor stage, the overall range of aneuploidy was 39 to 81 % (median 58 %), and altogether, 21 (54.1 %) studies described a significant prognostic impact of aneuploidy for overall, disease-specific, and recurrence-free survival, respectively.

CONCLUSIONS

A substantial number of studies showed a prognostic importance of aneuploidy in CRC. Furthermore, the higher frequency of aneuploidy in late-stage CRC implies an increase in genomic instability with CRC progression, indicating aneuploidy to be also a stage-specific prognostic marker.

Cytomixis--a unique phenomenon in animal and plant

Cytomixis is reported to be a uniform phenomenon in the context of fertilization during spermatogenesis of animals and in some lower groups of plants where oogamous reproduction prevails. However, the phenomenon is versatile in flowering taxa as it lacks uniformity in occurrences, causes, formation of intercellular bridges, involvement of number of cells in a cluster, evolutionary significance among others. A review on cytomixis is conducted with an objective that it may offer a scope to unravel some of the ambiguities associated with it and provide further information on cell, reproductive, structural and evolutionary biology.

Toxicogenomic evaluation of chemically induced chromosomal imbalance using DNA image analysis

The study of carcinogenic potential of a variety of chemical agents such as food additives and drugs of abuse via the application of various in vitro methodologies constitutes the first step for the evaluation of their toxicogenomic profile. Considering the chromosomal theories of carcinogenesis, where it is stated that aneuploidy and chromosomal imbalance (instability) are among the main causes of carcinogenesis, chemicals capable to induce such changes in the cells could be considered as potential carcinogens. Chromosomal imbalance and aneuploidy directly affect the overall DNA content of the exposed cell as well as other cellular morpho- and densitometric features. These features can be measured by means of computerized DNA image analysis technologies and include DNA content (DNA Index), Proliferation Index, Ploidy Balance, Degree of Aneuploidy, Skewness and Kurtosis. Considering the enormous number of untested chemicals and drugs of abuse that follow non-genotoxic mechanisms of carcinogenesis, the establishment of a reliable technology for the estimation of chemically induced chromosomal imbalance is of particular importance in toxicogenomic studies. In the present article and based on our previously published work, we highlight the advantages of the applications of DNA image analysis technology in an easy-to-use experimental model for the evaluation of the potential risk of various chemicals. The use of this technology for the detection of chemically induced chromosomal instability will contribute to the development of safer regulatory directives concerning the use of chemicals in food and pharmaceutical industry, as well as in the clarification of mechanisms of action of drugs of abuse.

Alzheimer Abeta peptide induces chromosome mis-segregation and aneuploidy, including trisomy 21: requirement for tau and APP

Chromosome aneuploidy, especially trisomy 21, arises in both familial and sporadic Alzheimer's disease. Expression of FAD genes or exposure to Aβ peptide induces aneuploidy in tg-mice and cultured cells. The requirement for GSK-3β, calpain, and Tau in Aβ-induced chromosome mis-segregation points to MT dysfunction as contributing to AD pathogenesis.

Both sporadic and familial Alzheimer's disease (AD) patients exhibit increased chromosome aneuploidy, particularly trisomy 21, in neurons and other cells. Significantly, trisomy 21/Down syndrome patients develop early onset AD pathology. We investigated the mechanism underlying mosaic chromosome aneuploidy in AD and report that FAD mutations in the Alzheimer Amyloid Precursor Protein gene, APP, induce chromosome mis-segregation and aneuploidy in transgenic mice and in transfected cells. Furthermore, adding synthetic Aβ peptide, the pathogenic product of APP, to cultured cells causes rapid and robust chromosome mis-segregation leading to aneuploid, including trisomy 21, daughters, which is prevented by LiCl addition or Ca²⁺ chelation and is replicated in tau KO cells, implicating GSK-3β, calpain, and Tau-dependent microtubule transport in the aneugenic activity of Aβ. Furthermore, APP KO cells are resistant to the aneugenic activity of Aβ, as they have been shown previously to be resistant to Aβ-induced tau phosphorylation and cell toxicity. These results indicate that Aβ-induced microtubule dysfunction leads to aneuploid neurons and may thereby contribute to the pathogenesis of AD.

Overexpression and mislocalization of the chromosomal segregation protein separase in multiple human cancers

PURPOSE

Separase, an endopeptidase, plays a pivotal role in chromosomal segregation by separating sister chromatids during the metaphase to anaphase transition. Using a mouse mammary tumor model we have recently shown that overexpression of Separase induces aneuploidy and tumorigenesis (Zhang et al., Proc Natl Acad Sci 2008;105:13033). In the present study, we have investigated the expression level of Separase across a wide range of human tumors.

EXPERIMENTAL DESIGN

To examine the expression levels and localization of Separase in human tumors, we have performed immunofluorescence microscopy using human Separase antibody and tumor tissue arrays from osteosarcoma, colorectal, breast, and prostate cancers with appropriate normal controls.

RESULTS

We show that Separase is significantly overexpressed in osteosarcoma, breast, and prostate tumor specimens. There is a strong correlation of tumor status with the localization of Separase into the nucleus throughout all stages of the cell cycle. Unlike the normal control tissues, where Separase localization is exclusively cytoplasmic in nondividing cells, human tumor samples show significantly higher number of resting cells with a strong nuclear Separase staining. Additionally, overexpression of Separase transcript strongly correlates with high incidence of relapse, metastasis, and lower 5-year overall survival rate in breast and prostate cancer patients.

CONCLUSION

These results further strengthen our hypothesis that Separase might be an oncogene, whose overexpression induces tumorigenesis, and indicates that Separase overexpression and aberrant nuclear localization are common in many tumor types and may predict outcome in some human cancers.

Alzheimer's presenilin 1 causes chromosome missegregation and aneuploidy

Mutations in the presenilin 1 gene cause most early onset familial Alzheimer's disease (FAD). Here, we report that a defect in the cell cycle - improper chromosome segregation - can be caused by abnormal presenilin function and therefore may contribute to AD pathogenesis. Specifically we find that either over-expression or FAD mutation in presenilin 1 (M146L and M146V) leads to chromosome missegregation and aneuploidy in vivo and in vitro: (1) Up to 20% of lymphocytes and neurons of FAD-PS-1 transgenic and knocking mice are aneuploid by metaphase chromosome analysis and in situ hybridization. (2) Transiently transfected human cells over-expressing normal or mutant PS-1 develop similar aneuploidy within 48 h, including trisomy 21. (3) Mitotic spindles in the PS-1 transfected cells contain abnormal microtubule arrays and lagging chromosomes. Several mechanisms by which chromosome missegregation induced by presenilin may contribute to Alzheimer's disease are discussed.

Dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA), two naturally occurring benzoxazinones contained in sprouts of Gramineae are potent aneugens in human-derived liver cells (HepG2)

Benzoxazinoids (BAs) are toxic constituents of sprouts of Gramineae such as wheat, maize and rye and are part of the plant defence system against pests. In the last years, sprouts have been increasingly consumed as health foods and are also used for the production of dietary supplements. In the present study we investigated the mutagenic activities of the two most abundant BAs, namely 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) in the Salmonella/microsome assay and additionally, in micronucleus (MN) assay and single cell gel electrophoresis (SCGE) assay in a human-derived liver cell line (HepG2). DIBOA caused significant induction of his(+) revertants in all three strains in the range between 0.02 and 0.50 mg/plate; the highest activity was observed in TA100 (fivefold increase over the background at the highest dose level). The effect in YG1024 (a derivative of TA98 with increased acetyltransferase activity) was only slightly higher than the effect in the parental strain indicating that acetylation plays no crucial role in the activation of this BA. DIMBOA was in general less active and a positive result was only seen in the base substitution strain (TA100). Addition of rat liver homogenate (S9-mix) led to a significant (ca. twofold) increase of the mutagenic activities of both BAs. In SCGE assays with HepG2 cells consistently negative results were obtained with both compounds whereas in MN assays significant dose dependent effects were observed under similar experimental conditions. DIMBOA caused significant effects already at concentrations > or =1 microM; at the highest dose (20 microM) the MN frequency was sevenfold higher than the background level. DIBOA caused weaker effects and was positive at doses > or =2.5 microM, the maximal induction (twofold over background) was observed with 20 microM. Overall, DIMBOA was ca. 30-fold more active as DIBOA. Subsequent experiments with pancentromeric probes showed that >80% of the MN induced at the highest doses gave a centromere positive signal indicating that both BAs are aneugenic. This is an interesting observation as it is assumed that aneuploidy is a key event in cancer induction and at present no other aneugenic plant-derived substances of dietary relevance are known.

Aneuploidy, stem cells and cancer

Telomeres which protect the individual chromosomes from disintegration, end-to-end fusion and maintain the genomic integrity during the somatic cell divisions play an important role in cellular aging. Aging and cancer development are linked with each other because cancer is considered a group of complex genetic diseases that develop in old cells and, in both, telomere attrition is involved. Numeric chromosome imbalance also known as aneuploidy is the hallmark of most solid tumors, whether spontaneous or induced by carcinogens. We provide evidence in support of the hypothesis that telomere attrition is the earliest genetic alteration responsible for the induction of aneuploidy. Dysfunctional telomeres are highly recombinogenic leading to the formation of dicentric chromosomes. During cell divisions, such complex chromosome alterations undergo breakage fusion bridge cycles and may lead to loss of heterozygosity (LOH) and gene amplification. Furthermore, we have provided evidence in support of the hypothesis that all types of cancer originate in the organ- or tissue-specific stem cells present in a particular organ. Cancer cells and stem cells share many characteristics, such as, self-renewal, migration, and differentiation. Metaphases with abnormal genetic constitution present in the lymphocytes of cancer patients and in some of their asymptomatic family members may have been derived from the organ-specific stem cells. In addition, evidence and discussion has been presented for the existence of cancer-specific stem cells. Successful treatment of cancer, therefore, should be directed towards these cancer stem cells.

Actin-binding proteins required for reliable chromosome segregation in mitosis

While studying mitosis in Dictyostelium mutants with deficiencies in actin-binding proteins, we found that two of these proteins, cortexillin and Aip1, are required for the precise segregation of chromosomes. Atypical spindles and nuclei with varying DNA content indicate that mutants lacking cortexillin or Aip1 are genetically unstable. These aberrations are caused by the detachment and irregular reattachment of centrosomes to the nuclear surface. Live imaging showed how coalescing mitotic complexes give rise to a multipolar spindle, and how excess centrosomes can be eliminated by mitotic cleavage between anucleate and nucleated portions of a cell. We hypothesize that mutations in regulatory proteins of the actin network might be one cause of genetic instability of malignant tumor cells.

Absence of p53 in Clara cells favours multinucleation and loss of cell cycle arrest

BACKGROUND

The p53 oncosuppressor protein is a critical mediator of the response to injury in mammalian cells and is mutationally inactivated in the majority of lung malignancies. In this analysis, the effects of p53-deficiency were investigated in short-term primary cultures of murine bronchiolar Clara cells. Clara cells, isolated from gene-targeted p53-deficient mice, were compared to cells derived from wild type littermates.

RESULTS

p53 null cultures displayed abnormal morphology; specifically, a high incidence of multinucleation, which increased with time in culture. Multinucleated cells were proficient in S phase DNA synthesis, as determined by BrdU incorporation. However, multinucleation did not reflect altered rates of S phase synthesis, which were similar between wild type and p53-/- cultures. Nucleation defects in p53-/- Clara cells associated with increased centrosome number, as determined by confocal microscopy of pericentrin-stained cultures, and may highlight a novel role of p53 in preserving genomic integrity in lung epithelial cells. Effects of p53-deficiency were also studied following exposure to DNA damage. A p53-dependent reduction in the BrdU index was observed in Clara cells following ionizing radiation. The reduction in BrdU index in wild type cells displayed serum-dependency, and occurred only in the absence of serum. Taken together, these findings demonstrate that in murine primary Clara cell culture, cell cycle arrest is a p53-mediated response to DNA damage, and that extracellular factors, such as serum, influence this response.

CONCLUSION

These findings highlight functions of wild type p53 protein in bipolar spindle formation, centrosome regulation, and growth control in bronchiolar Clara cells.

Myc-Is this the oncogene from Hell?

A new paper implicates the Myc oncoprotein in the direct induction of DNA damage and consequent genome instability in cultured cells. However, it is less clear whether Myc induces the same genetic pandemonium in vivo.

Loss of heterozygosity as a predictor to map tumor suppressor genes in cancer: molecular basis of its occurrence

High frequency of chromosomal deletions elicited as losses of heterozygosity is a hallmark of genomic instability in cancer. Functional losses of tumor suppressor genes caused by loss of heterozygosity at defined regions during clonal selection for growth advantage define the minimally lost regions as their likely locations on chromosomes. Loss of heterozygosity is elicited at the molecular or cytogenetic level as a deletion, a gene conversion, single or double homologous and nonhomologous mitotic recombinations, a translocation, chromosome breakage and loss, chromosomal fusion or telomeric end-to-end fusions, or whole chromosome loss with or without accompanying duplication of the retained chromosome. Because of the high level of specificity, loss of heterozygosity has recently become invaluable as a marker for diagnosis and prognosis of cancer. The molecular defects for the occurrence of loss of heterozygosity are derived from disabled caretaker genes, which protect the integrity of DNA, or chromosome segregator genes, which mediate faithful chromosome disjunction.

Cell cycle regulation of c-Jun N-terminal kinase activity at the centrosomes

The c-Jun N-terminal kinase (JNK), a subgroup of the mitogen-activated protein kinase (MAPK) family of serine/threonine kinases, has established functions in cell growth and apoptosis. While the mechanisms are unclear, JNK has also been also implicated in signaling pathways that initiate cell cycle checkpoints and cell cycle progression. By following the localization of active and inactive JNK during the cell cycle, we have found that the majority of cellular JNK is soluble and present in the cytoplasm and the nucleus. Interestingly, insoluble fractions of JNK are also localized in nuclear and cytoplasmic speckles, and to the centrosomes. While JNK is associated with the centrosome throughout the cell cycle, it is only active at the centrosome from S phase through anaphase. This novel localization of centrosomal JNK is a possible link between JNK-activating stimuli and centrosome or cell cycle events.

Origin of multidrug resistance in cells with and without multidrug resistance genes: chromosome reassortments catalyzed by aneuploidy

Cancer cells and aneuploid cell lines can acquire resistance against multiple unrelated chemotherapeutic drugs that are over 3,000-fold those of normal levels and display spontaneous resistances up to 20-fold of normal levels. Two different mechanisms were proposed for this phenotype: (i) classical mutation of drug metabolizing genes or (ii) chromosome reassortments, catalyzed by cancer- and cell line-specific aneuploidy, which generate, via new gene dosage combinations, a plethora of cancer phenotypes, including drug resistance. To distinguish between these mechanisms, we have asked whether three mouse cell lines can become drug resistant, from which two or three genes have been deleted, and on which multidrug resistance is thought to depend: Mdr1a, Mdr1b, and Mrp1. Because all three lines could acquire multidrug resistance and were aneuploid, whereas diploid mouse cells could not, we conclude that aneuploid cells become drug resistant via specific chromosome assortments, independent of putative resistance genes. We have asked further whether aneuploid drug-resistant Chinese hamster cells revert spontaneously to drug sensitivity in the absence of cytotoxic drugs at the high rates that are typical of chromosome reassortments catalyzed by aneuploidy or at the very low or zero rates (i.e., deletion) of gene mutation. We found that four drug-resistant hamster cell lines reverted to drug sensitivity at rates of about 2-3% per generation, whereas two closely related lines remained resistant under our conditions. Thus, the karyotypic instability generated by aneuploidy emerges as the common source of the various levels of drug resistance of cancer cells: minor spontaneous resistances reflect accidental chromosome assortments, the high selected resistances reflect complex specific assortments, and multidrug resistance reflects new combinations of unselected genes located on the same chromosomes as selected genes.

Mitotic checkpoint protein hsMAD2 as a marker predicting liver metastasis of human gastric cancers

hsMAD2, the human homologue of mitotic arrest deficient 2 (MAD2), is a key component of the mitotic checkpoint system. Recently, mutations and decreased expression of mitotic checkpoint genes including hsMAD2 have been reported in cancer cell lines with defective mitotic checkpoint. However, the genetic alterations in the genomic hsMAD2 gene have not been determined in gastric cancers. Moreover, the biological implications of the overexpressed hsMAD2 in primary cancers are unknown. In this study, we analyzed 32 primary gastric cancers with polymerase chain reaction (PCR) amplification of all exons, including flanking intronic sequences, of the genomic hsMAD2 gene followed by direct DNA sequencing. We also measured the hsMAD2 protein levels in cancer and normal tissues by semi-quantitative immunoblotting. No mutations were found in the coding sequences, although three single nucleotide polymorphisms (SNPs) were identified in the noncoding sequences in 13 of 32 patients. These SNPs were not associated with either hsMAD2 expression or disease progression. The semi-quantitative western blot analysis showed hsMAD2 was significantly overexpressed in gastric cancer tissues compared with corresponding normal tissues (P < 0.001). The calculated ratio of the hsMAD2 protein in cancer tissue (C) to that in corresponding normal tissue (N) (C / N ratio) was significantly higher in patients with well differentiated adenocarcinoma (P = 0.0274) or with synchronous liver metastasis (P = 0.0025). A C / N ratio greater than 3 was observed more frequently in patients with synchronous liver metastasis. Therefore, C / N ratio > 3 may be clinically important as a predictive indicator for metachronous liver metastasis of gastric cancers.

Induction of aneuploidy by nickel sulfate in V79 Chinese hamster cells

The ability of nickel sulfate (NiSO(4)) to induce chromosome aneuploidy was investigated in vitro using the V79 Chinese hamster cell line. V79 cells were treated with 100-400 microM NiSO(4) for 24h, and monitored up to 72 h following treatment with a chromosome aberration assay, a micronuclei assay using antikinetochore antibodies (CREST assay) and an anaphase/telophase assay. Aneuploid cells were induced in a significant fraction of the cell population 24-48 h following treatment with nickel sulfate. The majority of these cells were hyperdiploid. In addition, nickel sulfate caused increased frequency of cells with kinetochore-positive micronuclei as well as kinetochore-negative micronuclei. Abnormal chromosome segregation such as lagging chromosomes, chromosome bridges and asymmetric segregation were also observed in more than 50% of anaphase or telophase cells following treatment with NiSO(4). The incidences of these abnormalities were dose-dependent in general, although the effects were prominent in a sublethal dose. These results indicate that NiSO(4) has the ability to induce aneuploidy in V79 cells. In addition, the results in anaphase/telophase assay suggest that the compound may have an effect on spindle apparatus, which could result in aneuploidy following abnormal chromosome segregation.

The contribution of epigenetic changes to abnormal centrosomes and genomic instability in breast cancer

The centrosome is the major microtubule organizing center of the cell and as such it plays an important role in cytoskeletal organization and in the formation of the bipolar mitotic spindle. Centrosome defects, characterized by abnormal size, number, and microtubule nucleation capacity, are distinguishing features of most high grade breast tumors and have been implicated as a possible cause for the loss of tissue architecture and the origin of mitotic abnormalities seen in solid tumors in general. Centrosome defects arise through uncoupling of centriole duplication and the cell cycle as a result of either genetic alterations or through physical or chemical perturbation of centrosome function. Centrosomes manifest unique epigenetic properties whereby positional or structural information can be propagated through somatic cell lineages by way of nongenetic pathways. Because aberrant centrosome function can result in chromosomal instability, these properties may have important implications for the origin of malignant breast tumors.

Centrosome maturation

In the past, centrosome maturation has been described as the change in microtubule nucleation potential that occurs as cells pass through specific phases of the cell cycle. It is suggested that the idea of centrosome maturation be expanded to include gain of functions that are not necessarily related to microtubule nucleation. Some of these functions could be transient and dependent on the temporary association of molecules with the centrosome as cells progress through the cell cycle. Thus, the centrosome may best be viewed as a site for mediating macromolecular interactions, perhaps as a central processing station within the cell. The centromatrix, a relatively stable lattice of polymers within the centrosome's PCM, could serve as a scaffold for the transient binding of mediator molecules, as well as allow the dynamic exchange of centrosome constituents with a soluble cytoplasmic pool. New evidence adds support to the idea that centrioles are crucial for the maintenance of PCM structure. However, significant evidence indicates that aspects of centrosome structure and function can be maintained in the absence of centrioles. In the case of paternal centrosome maturation, sperm centrioles may not contain an associated centromatrix. It is proposed that regulation of paternal centrioles or centriole associated proteins could mediate centriole-dependent centromatrix assembly following fertilization. Thus, regulation of centromatrix-centriole interactions could be involved in maintaining the integrity of the centrosome's PCM and play an important role in centrosome disassembly during cell differentiation and morphogenesis.

Explaining the high mutation rates of cancer cells to drug and multidrug resistance by chromosome reassortments that are catalyzed by aneuploidy

The mutation rates of cancer cells to drug and multidrug resistance are paradoxically high, i.e., 10(-3) to 10(-6), compared with those altering phenotypes of recessive genes in normal diploid cells of about 10(-12). Here the hypothesis was investigated that these mutations are due to chromosome reassortments that are catalyzed by aneuploidy. Aneuploidy, an abnormal number of chromosomes, is the most common genetic abnormality of cancer cells and is known to change phenotypes (e.g., Down's syndrome). Moreover, we have shown recently that aneuploidy autocatalyzes reassortments of up to 2% per chromosome per mitosis because it unbalances spindle proteins, even centrosome numbers, via gene dosage. The hypothesis predicts that a selected phenotype is associated with multiple unselected ones, because chromosome reassortments unbalance simultaneously thousands of regulatory and structural genes. It also predicts variants of a selected phenotype based on variant reassortments. To test our hypothesis we have investigated in parallel the mutation rates of highly aneuploid and of normal diploid Chinese hamster cells to resistance against puromycin, cytosine arabinoside, colcemid, and methotrexate. The mutation rates of aneuploid cells ranged from 10(-4) to 10(-6), but no drug-resistant mutants were obtained from diploid cells in our conditions. Further selection increased drug resistance at similar mutation rates. Mutants selected from cloned cells for resistance against one drug displayed different unselected phenotypes, e.g., polygonal or fusiform cellular morphology, flat or three-dimensional colonies, and resistances against other unrelated drugs. Thus our hypothesis offers a unifying explanation for the high mutation rates of aneuploid cancer cells and for the association of selected with unselected phenotypes, e.g., multidrug resistance. It also predicts drug-specific chromosome combinations that could become a basis for selecting alternative chemotherapy against drug-resistant cancer.

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.

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.