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PubMed1996

Curable and non-curable malignancies: lessons from paediatric cancer.

The tremendous progress achieved in understanding the molecular basis of cancer, was unfortunately not followed by a mutual improvement in the morbidity and mortality of adult cancer. In contrast, the success rate achieved in paediatric oncology has increased significantly during the past 30 years, and more than two-thirds of the children with cancer can now be cured. p53 has been shown to have a central role on apoptosis in various cells. As apoptosis is a final common pathway for much of our anti cancer therapy, resistance to apoptosis due to a normal activity of p53 is an important mechanism of tumor resistance and treatment failure. Contrary to the findings in most adult tumors, where about 50% of the tumors lack p53 activity, the rate of p53 mutations in childhood cancer is surprisingly low. This may be the key to the much better prognosis of children with cancer. In most adult tumors, multiple genetic events, between five and seven, are usually involved. The oncogenes involved in such tumors usually represent those located upstream of the nuclear transcription factors. In most paediatric tumors, in contrast, the initiating event is the activation of nuclear transcription factors secondary to chromosomal translocations. It can be speculated that multiple events activating various components of the signal transduction machinery are needed for tumorigenesis, and hence the evolution and progression of such tumors is slow. Moreover, if the malignant cell has to accumulate multiple mutations, the chances of crippling the apoptotic mechanism are higher. Genomic instability evidenced by microsatellite variation has been found in colon, pancreas, breast, liver and ovarian adult tumors, and not in paediatric tumors. As multiple somatic mutations are needed for the initiation and progression of the common adult malignancies, inherent genomic instability can dispose to accumulation of multiple mutations. All these molecular interactions are discussed with relevance to the difference between non-curable, mostly adult tumors, and curable, mostly paediatric tumors.

Antineoplastic Agents↗

PubMed1990

A comparison of levels of intrinsic single strand breaks/alkali labile sites associated with human melanoma cell invasion.

Intrinsic levels of protein-free single strand breaks/alkali-labile sites in human melanoma cell populations of varying in vitro invasive capacity have been assayed with DNA filter elution methodology. DNA from two human melanoma cell lines, A375P and C8161, and from a subpopulation selected from A375P, A375P-5, were assayed to test the hypothesis that increased levels of DNA damage may be associated with the phenotype of increased invasive and metastatic capacities. The elution profiles obtained reveal statistically significant increases in the level of single strand breaks and/or alkali-labile sites (SSB/ALS) which correlate with increasing invasive and metastatic capacities. The increased levels of SSB/ALS in A375P-5 observed in freshly selected cells decline as these cells are maintained in culture. The stability of this A375P-5 phenotype correlates with previously reported levels of double minute chromosomes, an indicator of genomic instability. Alterations in average intrinsic levels of cellular lesions are therefore an additional factor to be considered in the phenotypic characterization of invasive and metastatic tumor cells and may reflect or contribute to the genomic instability characteristic of tumor cell populations.

Alkalies↗

PubMed2000

Ageing and the mismatch repair system.

Age-related accumulation of mutations has been extensively documented, and it has been proposed as one of the prominent causes of malignancies in old age. The present review is focused on the particular case of DNA mismatch repair system (MMR), that has drawn increased attention for its possible relevance to malignancy. We also report on our own observations on an age-associated genomic instability that develops with age in the MMR system. Our study was performed on DNA samples that were prepared from peripheral blood cells, obtained at a 10-year interval from young and old human subjects. The two DNA samples from each individual were examined comparatively. The older individuals showed a significantly higher rate of microsatellite instability (MSI) in several loci examined, whereas no difference was found between the paired samples of any of the young subjects. We suggest that this increase in MSI with age may indicate an overall genomic instability in the elderly.

Aging↗

PubMed2003

Shaping of tumor and drug-resistant genomes by instability and selection.

Tumors with defects in mismatch repair (MMR) show fewer chromosomal changes by cytogenetic analyses than most solid tumors, suggesting that a greater proportion of the alterations required for malignancy occur in genes with nucleotide sequences susceptible to errors normally corrected by MMR. Here, we used genome-wide microarray comparative genomic hybridization to carry out a higher resolution evaluation of the effect of MMR competence on genomic alterations occurring in 20 cell lines and to determine if characteristic aberrations arise in MMR-proficient and -deficient HCT116 cells undergoing selection for methotrexate resistance. We observed different spectra of aberrations in MMR-proficient compared to -deficient cell lines, as well as among cell lines with different types of MMR-deficiency. We also observed different genetic routes to drug resistance. Resistant MMR-deficient cells most frequently displayed no copy number alterations (16/29 cell pools), whereas all MMR-proficient cells had unique abnormalities involving chromosome 5, including amplicons centered on the target gene, DHFR and/or a neighboring novel locus (7/13 pools). These observations support the concept that tumor genomes are shaped by selection for alterations that promote survival and growth advantage, as well as by the particular dysfunctions in genes responsible for maintenance of genetic integrity.

Base Pair Mismatch↗

PubMed1998

Radiation-induced transmissable chromosomal instability in haemopoietic stem cells.

Heritable radiation-induced genetic alterations have long been assumed to be "fixed" within the first cell division. However, there is a growing body of evidence that a considerable fraction of cells surviving radiation exposure appear normal, but a variety of mutational changes arise in their progeny due to a transmissible genomic instability. In our investigations of G-banded metaphases, non-clonal cytogenetic aberrations, predominantly chromatid-type aberrations, have been observed in the clonal descendants of murine and human haemopoietic stem cells surviving low doses (approximately l track per cell) of alpha-particle irradiations. The data are consistent with a transmissible genetic instability induced in a stem cell resulting in a diversity of chromosomal aberrations in its clonal progeny many cell divisions later. Recent studies have demonstrated that the instability phenotype persists in vivo and that the expression of chromosomal instability has a strong dependence on the genetic characteristics of the irradiated cell. At the time when cytogenetic aberrations are detected, an increased incidence of hprt mutations and apoptotic cells have been observed in the clonal descendants of (alpha-irradiated murine haemopoietic stem cells. Thus, delayed chromosomal abnormalities, delayed cell death by apoptosis and late-arising specific gene mutations may reflect diverse consequences of radiation-induced genomic instability. The relationship, if any, between these effects is not established. Current studies suggest that expression of these delayed heritable effects is determined by the type of radiation exposure, type of cell and a variety of genetic factors.

Alpha Particles↗

PubMed2000

DNA rearrangements at the extremities of the Streptomyces ambofaciens linear chromosome: evidence for developmental control.

In Streptomyces, a genomic instability results from frequent recombination events which occur at the ends of the linear chromosomal DNA. These events are believed to be responsible for the variability observed in these regions among Streptomyces species and strains. In order to identify functions able to control this type of genome plasticity, mutators as well as mutants produced at different stages of development have been characterized in S. ambofaciens. Their characterization suggests the existence of a relationship between genomic instability and colony development.

Chromosomes, Bacterial↗

PubMed2007

Molecular pathogenesis of hepatitis C virus-associated hepatocellular carcinoma.

Chronic infection with hepatitis C virus (HCV) is causally associated with the development of hepatocellular carcinoma (HCC). HCV is not cytolytic and replicates entirely in the cytoplasm. Viral interaction with the host leads to subversion of immune response and other defense mechanisms. The recent development of robust cell culture systems for HCV infection provides new opportunities for the study of virus-cell interaction and viral pathogenesis. HCV infection causes active inflammation and fibrosis, which ultimately progresses to cirrhosis. The onset of cirrhosis usually precedes the multistage process of tumor development, in which common themes of viral carcinogenesis can be identified. While chronic inflammation and cirrhosis are thought to play an important role in tumor initiation, the underlying mechanisms are incompletely understood. Recent studies have revealed that infection with HCV induces genome instability, leading to further genetic and epigenetic alterations which contribute to the full development of HCC tumor. The expression of viral oncoproteins such as C and NS5A is critically involved both in the induction of genome instability and in dysregulating cellular control of growth and signal transduction. A better understanding of the molecular pathogenesis of HCV will reveal novel strategies for the prevention and treatment of related diseases including HCC.

Carcinoma, Hepatocellular↗

PubMedDate not supplied

DNA repair and tumorigenesis: lessons from hereditary cancer syndromes.

The discovery that alterations of the DNA mismatch repair system (MMR) were linked to the common human cancer susceptibility syndrome hereditary nonpolyposis colon cancer (HNPCC) resulted in the declaration of a third class of genes involved in tumor development. In addition to oncogenes and tumor suppressors, alterations of DNA repair genes involved in maintaining genomic stability were found to be a clear cause of tum the level of the single nucleotides or chromosomes. This observation suggested that the establishment of genomic instability, termed the Mutator Phenotype, was an important aspect of tumor development.(1,2) Since the initial identification of the human MutS homolog hMSH2 nearly a decade ago,(3,4) more links have been described between human cancers and genes involved in maintaining genomic stability. Work in recent years has revealed that DNA repair proteins may also function in signaling pathways that provoke cell cycle arrest and apoptosis. This review will focus on the genetic and biochemical functions of DNA repair genes linked to hereditary cancer predisposition characterized by genomic instability (Table 1). Interestingly, the protein products of these genes have been directly or indirectly linked to the DNA damage-induce cell cycle arrest and apoptosis. We conclude that a robust connection between DNA repair proteins and damage-induced apoptosis may be as important for tumorigenesis as their role in maintaining genome stability.

Animals↗

PubMed2005

Two functional coding single nucleotide polymorphisms in STK15 (Aurora-A) coordinately increase esophageal cancer risk.

STK15/Aurora-A is a serine/threonine kinase essential for chromosome segregation and cytokinesis, and is considered to be a cancer susceptibility gene in mice and humans. Two coding single nucleotide polymorphisms in Aurora-A, 91T>A [phenylalanine/isoleucine (F/I)] and 169G>A [valine/isoleucine (V/I)], create four haplotypes, 91T-169G, 91A-169G, 91T-169A, and 91A-169A. We evaluated the association between these coding single nucleotide polymorphisms and esophageal cancer risk by genotyping 197 esophageal cancer cases and 146 controls. Haplotype 91A-169A (I31/I57) was observed to be statistically more frequent in cancer cases (odds ratio, 3.1452; 95% confidence interval, 1.0258-9.6435). Functional differences among the four isoforms were then analyzed to reveal the source of the cancer risk. Kinase activity levels of I31/I57 and F31/I57 were reduced to 15% and 40% compared with I31/V57 in vivo and in vitro. We considered the differences between the kinase activities and divided individuals into four categories of Aurora-A haplotype combination. Category I had 57.5% or less kinase activity compared with the most common category, category III, and had a significantly higher estimated cancer risk (odds ratio, 5.5328; 95% confidence interval, 1.8149-16.8671). Abnormal nuclear morphology, a characteristic of genomic instability, was observed to be 30 to 40 times more frequent in human immortalized fibroblast cells overexpressing I31/I57 or F31/I57 compared with the others. Furthermore, significantly higher levels of chromosomal instability were observed in cancers in category I (homozygote 91T-169A) than those in category III (homozygous 91A-169G). These results indicate that the less kinase active Aurora-A haplotype combinations might induce genomic instability and increase esophageal cancer risk either in a recessive or a dominant manner.

Aurora Kinase A↗

PubMed2003

Werner syndrome protein limits MYC-induced cellular senescence.

The MYC oncoprotein is a transcription factor that coordinates cell growth and division. MYC overexpression exacerbates genomic instability and sensitizes cells to apoptotic stimuli. Here we demonstrate that MYC directly stimulates transcription of the human Werner syndrome gene, WRN, which encodes a conserved RecQ helicase. Loss-of-function mutations in WRN lead to genomic instability, an elevated cancer risk, and premature cellular senescence. The overexpression of MYC in WRN syndrome fibroblasts or after WRN depletion from control fibroblasts led to rapid cellular senescence that could not be suppressed by hTERT expression. We propose that WRN up-regulation by MYC may promote MYC-driven tumorigenesis by preventing cellular senescence.

Cell Division↗

PubMed2001

Telomeric associations and chromosome instability in ataxia telangiectasia T cells characterized by TCL1 expression.

T-cell tumors in ataxia telangiectasia (AT), such as T-PLL/T-CLL, are first preceded by the development of a large clone of T-lymphocytes, characterized by chromosomal rearrangements, which usually involve specific regions such as the 14q11 region. Malignancy develops years later, after additional chromosomal changes resulting from the genomic instability consequent to ATM disruption and to the activation of the TCL1 oncogene. Here we report the results of a cytogenetic follow-up of an AT patient (AT94-1), still without signs of hematological abnormalities, bearing a T-lymphocyte clone characterized by the t(14;14)(q11;q32) rearrangement and having TCL1 expression. We demonstrated that in clonal cells TCL1 expression correlates with increasing genomic instability and in time this mainly induces chromosomal rearrangements and telomeric associations (tas). Chromosome 21 is not randomly involved; in particular, an i(21q) indicates that it is a subclone prone to additional genetic changes and could represent an early chromosomal rearrangement involved in tumorigenesis. With regard to the increase in tas, we observed that: (i) it is inversely correlated with the proliferative ability of AT94-1 lymphocytes in PHA-stimulated short-term cultures (cell aging in vitro); (ii) this increase is not due to changes either in cell radiosensitivity (measured as bleomycin (BML)-sensitivity) or due to an illegitimate recombination (measured as adriamycin-sensitivity), which may not be sufficient for tumor development.

Adult↗

PubMed1998

Genetic instability and chromosomal aberrations in colorectal cancer: a review of the current models.

Our understanding of the pathogenesis of cancer has undergone a revolution over the past decade. Tumors develop by the accumulation of damage to genes that regulate cell growth. Many of the genes responsible for disregulation of cell growth have been identified, as have the processes that lead to the genetic damage. One of the most important concepts that has facilitated our understanding of carcinogenesis is that of genetic or "genomic" instability, which is required to permit a sufficient amount of genetic damage to accumulate to permit the neoplastic phenotype to emerge and evolve. Two mechanisms that lead to genomic instability--one of which involves the loss of chromosomal fragments from the nucleus, and a second which is characterized by microsatellite instability--are discussed.

Chromosome Aberrations↗

PubMed1999

Beta-catenin mutations are specific for colorectal carcinomas with microsatellite instability but occur in endometrial carcinomas irrespective of mutator pathway.

Some colorectal tumors with wild-type adenomatous polyposis coli gene have activating mutations in beta-catenin (encoded by CTNNB1) that result in decreased phosphorylation by GSK-3beta and increased signaling through the Tcf/Lef transcription factors. To investigate the relationship between CTNNB1 mutations and underlying pathways of genomic instability, we examined 80 colorectal cancers stratified by the presence or absence of microsatellite instability (MSI). CTNNB1 mutations were identified in 13 (25%) of 53 cancers with high frequency MSI (MSI-H), including 12 point mutations at exon 3 phosphorylation sites (codons 41 and 45) and one deletion of the entire exon 3 degradation box. No CTNNB1 mutations were identified in 27 microsatellite stable or low frequency MSI (MSI-L) colorectal cancers (P < 0.01). In contrast, CTNNB1 mutations were identified in 3 of 9 (33%) MSI-H and 10 of 20 (50%) MSS/MSI-L endometrial carcinomas, suggesting a more generalized involvement in these tumors. Only six (46%) of the endometrial carcinoma CTNNB1 mutations occurred at residues directly phosphorylated by GSK-3beta, and only one of these was at either codon 41 or 45. All point mutations in MSI-H cancers were transitions, whereas 64% of those in MSS/MSI-L cancers were transversions (P < 0.01). The differences in the mutation profiles suggest that there may be molecular fingerprints of CTNNB1 mutations, determined by biological factors related to both tumor type and underlying pathways of genomic instability.

Adenocarcinoma↗

PubMed2000

Unwinding of a DNA triple helix by the Werner and Bloom syndrome helicases.

Bloom syndrome and Werner syndrome are genome instability disorders, which result from mutations in two different genes encoding helicases. Both enzymes are members of the RecQ family of helicases, have a 3' --> 5' polarity, and require a 3' single strand tail. In addition to their activity in unwinding duplex substrates, recent studies show that the two enzymes are able to unwind G2 and G4 tetraplexes, prompting speculation that failure to resolve these structures in Bloom syndrome and Werner syndrome cells may contribute to genome instability. The triple helix is another alternate DNA structure that can be formed by sequences that are widely distributed throughout the human genome. Here we show that purified Bloom and Werner helicases can unwind a DNA triple helix. The reactions are dependent on nucleoside triphosphate hydrolysis and require a free 3' tail attached to the third strand. The two enzymes unwound triplexes without requirement for a duplex extension that would form a fork at the junction of the tail and the triplex. In contrast, a duplex formed by the third strand and a complement to the triplex region was a poor substrate for both enzymes. However, the same duplex was readily unwound when a noncomplementary 5' tail was added to form a forked structure. It seems likely that structural features of the triplex mimic those of a fork and thus support efficient unwinding by the two helicases.

Binding Sites↗

PubMed2000

Telomere dynamics and genetic instability in disease progression of chronic myeloid leukemia.

Chronic myeloid leukemia (CML) is characterized by a Philadelphia (Ph) translocation creating a novel BCR-ABL oncoprotein, and CML patients have a chronic phase for several years followed by an intractable blast cell proliferation, called blast transformation. In the blast phase, more than 60% of patients show additional cytogenetic changes, e. g., double Ph, +8, i(17q). In this review, we would like to address genetic changes, including genome instability, cytogenetic changes, and telomere dynamics that relate to karyotypic instability. In the chronic phase, approximately 60% of CML patients show reduced telomere length without highly elevated telomerase activity or microsatellite alterations, indicating that telomere reduction may be linked to cell replication. Therefore, the Ph translocation might be a first event to immortalize cell proliferation. In the blast phase, 50% of CML patients have high levels of elevated telomerase activity and the same number of patients had microsatellite changes. Of note is that most patients with telomerase up-regulation in the blast phase had additional cytogenetic changes and >60% of them showed microsatellite changes at least at one locus. In contrast, most patients without telomerase activity did not show microsatellite changes. These findings may indicate that telomerase up-regulation in the blast phase of CML patients is closely associated with microsatellite changes (representative of genome instability), while blast cells in the remaining patients (30%) maintain their proliferative capability without microsatellite changes and telomerase up-regulation. This further suggests that there is also an unknown mechanism for genome stability without the process of telomerase up-regulation in some patients with CML in blast crisis.

Cell Transformation, Neoplastic↗

PubMed2006

Ovarian cancer: homeobox genes, autocrine/paracrine growth, and kinase signaling.

Epithelial ovarian cancer, the fourth leading cause of cancer deaths in American women, is currently classified by surgical and histologic appearance. However, the predictive value of this classification is limited. The risk of epithelial ovarian cancer increases with the number of ovulatory events. It is now thought that different ovarian tumors are derived from a single ovarian surface epithelial precursor cell with the degree and pattern of differentiation determined by combinatorial expression of homeobox genes normally involved in differentiation of the female genital tract. This aberrant differentiation occurs in association with histology-specific genomic aberrations, genomic instability, and resultant chromosomal changes, and may be triggered by prolonged abnormal or excessive exposure of surface epithelial cells to autocrine/paracrine stimulation by sex steroids and other growth factors. As the disease progresses, activation of kinase pathways and continued abnormal autocrine/paracrine stimulation contribute to genomic instability but also identify potential targets for novel therapeutic intervention.

ErbB Receptors↗

PubMed2003

Regulated over-expression of DNA polymerase beta mediates early onset cataract in mice.

Base excision repair (BER) is a tightly coordinated mechanism for repair of DNA base damage (via alkylation and oxidation) and base loss. From E. coli to yeast to human cells, subtle alterations in expression of BER proteins lead to mutagenic or genome instability phenotypes. DNA polymerase beta (beta-pol), the major BER polymerase, has been found to be over-expressed in human tumor tissues and more recently it has been shown that over-expression of beta-pol results in a mutator and genome instability phenotype. These previous reports imply that beta-pol over-expression is deleterious and suggests that such an imbalance may cause an overall functional deficiency in the BER pathway. In the present study, we have developed a bicistronic tetracycline-responsive transgenic system to over-express beta-pol in mice. We find that over-expression of beta-pol in the lens epithelium results in the early onset of severe cortical cataract, with cataractogenesis beginning within 4 days after birth. In utero and post-natal suppression of transgenic Flag-beta-pol expression by doxycycline administration completely prevents cataract formation through adulthood, yet cataract is subsequently observed following removal of doxycycline and re-expression of the transgene. Cataract development accompanies increased expression of cyclooxygenase-2 in the lenticular fibers of the lens, implicating oxidative stress in the development of this cataractous phenotype. Although the mechanism for the transgene mediated cataractogenesis is not clear at this time, it is nevertheless intriguing that increased expression of beta-pol leads to such a phenotype. These results suggest that either a beta-pol expression imbalance negatively affects overall fidelity and/or BER capacity or that beta-pol has a role in lens epithelial cell differentiation.

Animals↗

PubMed2006

Analysis of genetic abnormalities provides insights into genetic evolution of hyperdiploid myeloma.

Aneuploidy is ubiquitous in human cancer and is seen as whole chromosome gains and losses, unbalanced translocations and inversions, duplications, deletions and loss of heterozygosity. Within this complexity, some subgroups of aneuploid tumors emerge as distinct biological and clinical entities. Hyperdiploid myeloma (H-MM), characterized by hyperdiploid chromosome numbers because of nonrandom trisomies, is one such example. We undertook a comprehensive survey of the karyotypes of a large number of H-MM (n = 469) to describe fully genomic instability in these tumors, to dissect pathways of genetic evolution, and identify distinct subgroups based on their genetic changes. While selective pressure apparently favors the emergence of clones with gains of chromosomes 3, 5, 7, 9, 11, 15, 19, and 21, a background of ongoing genomic instability results in gains of other chromosomes, albeit at a much lower prevalence. A deduced temporal analysis of these karyotypes indicates that selected gains are early events. Other events occurring later in the course of the disease include secondary chromosome translocations and monosomies. The development of these genetic aberrations is thus highly ordered and undoubtedly of biological relevance. Within this framework, we propose a model of genetic evolution in H-MM.

Chromosome Aberrations↗