How do tumors make ends meet?
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Chromosome Aberrations↗
Biomedical subjects
C Lengauer
Publications and source records attributed to C Lengauer.
A phosphatase associated with metastasis of colorectal cancer.
To gain insights into the molecular basis for metastasis, we compared the global gene expression profile of metastatic colorectal cancer with that of primary cancers, benign colorectal tumors, and normal colorectal epithelium. Among the genes identified, the PRL-3 protein tyrosine phosphatase gene was of particular interest. It was expressed at high levels in each of 18 cancer metastases studied but at lower levels in nonmetastatic tumors and normal colorectal epithelium. In 3 of 12 metastases examined, multiple copies of the PRL-3 gene were found within a small amplicon located at chromosome 8q24.3. These data suggest that the PRL-3 gene is important for colorectal cancer metastasis and provide a new therapeutic target for these intractable lesions.
Adenoma↗
Securin is required for chromosomal stability in human cells.
Abnormalities of chromosome number are the most common genetic aberrations in cancer. The mechanisms regulating the fidelity of mitotic chromosome transmission in mammalian cells are therefore of great interest. Here we show that human cells without an hSecurin gene lose chromosomes at a high frequency. This loss was linked to abnormal anaphases during which cells underwent repetitive unsuccessful attempts to segregate their chromosomes. The abnormal mitoses were associated with biochemical defects in the activation of separin, the sister-separating protease, rendering it unable to cleave the cohesin subunit Scc1 efficiently. These results illuminate the function of mammalian securin and show that it is essential for the maintenance of euploidy.
Amino Acid Sequence↗
Carcinogen-specific induction of genetic instability.
It has been proposed recently that the type of genetic instability in cancer cells reflects the selection pressures exerted by specific carcinogens. We have tested this hypothesis by treating immortal, genetically stable human cells with representative carcinogens. We found that cells resistant to the bulky-adduct-forming agent 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) exhibited a chromosomal instability (CIN), whereas cells resistant to the methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) exhibited a microsatellite instability (MIN) associated with mismatch repair defects. Conversely, we found that cells purposely made into CIN cells are resistant to PhIP, whereas MIN cells are resistant to MNNG. These data demonstrate that exposure to specific carcinogens can indeed select for tumor cells with distinct forms of genetic instability and vice versa.
Blotting, Western↗
Mechanisms underlying losses of heterozygosity in human colorectal cancers.
Losses of heterozygosity are the most common molecular genetic alteration observed in human cancers. However, there have been few systematic studies to understand the mechanism(s) responsible for losses of heterozygosity in such tumors. Here we report a detailed investigation of the five chromosomes lost most frequently in human colorectal cancers. A total of 10,216 determinations were made with 88 microsatellite markers, revealing 245 chromosomal loss events. The mechanisms of loss were remarkably chromosome-specific. Some chromosomes displayed complete loss such as that predicted to result from mitotic nondisjunction. However, more than half of the losses were associated with losses of only part of a chromosome rather than a whole chromosome. Surprisingly, these losses were due largely to structural alterations rather than to mitotic recombination, break-induced replication, or gene conversion, suggesting novel mechanisms for the generation of much of the aneuploidy in this common tumor type.
Alleles↗
Evidence that genetic instability occurs at an early stage of colorectal tumorigenesis.
Chromosomal instability is believed to be a common feature of most human tumors, but the stage at which such instability originates has not been defined. At the molecular level, chromosomal instability is characterized by allelic imbalance (AI), representing losses or gains of defined chromosomal regions. We have assessed AI in early colorectal tumors using newly developed methods for assessing AI in complex cell populations. A total of 32 adenomas of average size (2 mm; range, 1-3 mm) were studied. AI of chromosome 5q markers occurred in 55% of tumors analyzed, consistent with a gatekeeping role of the adenomatous polyposis coli tumor suppressor gene located at chromosomal position 5q21. AI was also detected in each of the other four chromosomes tested. The fractions of adenomas with AI of chromosomes 1p, 8p, 15q, and 18q were 10,19, 28, and 28%, respectively. Over 90% of the tumors exhibited AI of at least one chromosome, and 67% had allelic imbalance of a chromosome other than 5q. These findings demonstrate that AI is a common event, even in very small tumors, and suggest that chromosomal instability occurs very early during colorectal neoplasia.
Adenoma↗
Loss of chromosome arms 3p and 9p and inactivation of P16 (INK4a) in normal epithelium of patients with primary lung cancer.
The accumulation of genetic alterations in the respiratory epithelium may give rise to cancer and often is accompanied by a series of histologic alterations over a period of several years. Recent studies have identified some molecular alterations in histologically normal-appearing epithelium among patients with lung cancer. To extend these observations, we investigated clonal genetic alterations by using fluorescence in situ hybridization (FISH) analysis and immunohistochemistry in 69 biopsy samples of histologically normal-appearing bronchial epithelium from 22 patients with or without lung cancer. Thirty-seven biopsy specimens from 13 patients were examined for loss of 3p14, and 48 biopsy specimens from 18 patients were examined for loss at 9p21 by FISH. P16(INK4a) expression was analyzed in 54 biopsy samples from 19 patients. In at least one biopsy specimen from five of the 13 patients with primary lung cancer, FISH or immunohistochemistry detected loss of the 3p14 or 9p21 region. In contrast, no alterations were detected for the same regions in the nine patients without primary lung cancer. Our results support the concept that the normal epithelial surface of large bronchi of patients with lung cancer has molecular changes suggestive of the outgrowth of numerous clonal foci.
Bronchi↗
Chromosome segregation and cancer: cutting through the mystery.
Mitosis is the most dramatic--and potentially dangerous--event in the cell cycle, as sister chromatids are irreversibly segregated to daughter cells. Defects in the checkpoints that normally maintain the fidelity of this process can lead to chromosomal instability (CIN) and cancer. However, CIN--a driving force of tumorigenesis--could be the cancer cell's ultimate vulnerability. An important goal is to identify novel anticancer compounds that directly target the mitotic errors at the heart of CIN.
Aneuploidy↗
Facilitating haplotype analysis by fully automated analysis of all chromosomes in human-mouse hybrid cell lines.
Recent evidence suggests that haplotype analysis is essential in recognizing genetic factors involved in the tendency toward a particular disease or pharmacogenetic phenotype, as well as to identify genes involved in multigenic disorders. Because of the increasing need for efficient haplotype tests, a new hybrid system, called conversion technology, was developed. Conversion technology aims at converting the diploid chromosome content into a haploid state so that hybrids contain a single copy of any desired chromosome. A number of mutations can now be identified easily, as they are no longer obscured by the normal sequence present on the other copy of the chromosome. However, the efficient use of this hybrid system depends on a complete analysis of both human and mouse chromosome complements in order to assess the stability of the hybrid cells and to accurately determine their human chromosome content. We describe a new multicolor FISH-based method capable of analyzing both genomes simultaneously in a single hybridization. This new technique should become an instrumental part of inexpensive, reliable haplotype tests.
Animals↗
Genes expressed in human tumor endothelium.
To gain a molecular understanding of tumor angiogenesis, we compared gene expression patterns of endothelial cells derived from blood vessels of normal and malignant colorectal tissues. Of over 170 transcripts predominantly expressed in the endothelium, 79 were differentially expressed, including 46 that were specifically elevated in tumor-associated endothelium. Several of these genes encode extracellular matrix proteins, but most are of unknown function. Most of these tumor endothelial markers were expressed in a wide range of tumor types, as well as in normal vessels associated with wound healing and corpus luteum formation. These studies demonstrate that tumor and normal endothelium are distinct at the molecular level, a finding that may have significant implications for the development of anti-angiogenic therapies.
Biomarkers, Tumor↗
Cloning and chromosomal localization of the human BARX2 homeobox protein gene.
The human BARX2 gene encodes a homeodomain-containing protein of 254 amino acids, which binds optimally to the DNA consensus sequence YYTAATGRTTTTY. BARX2 is highly expressed in adult salivary gland and is expressed at lower levels in other tissues, including mammary gland, kidney, and placenta. The BARX2 gene consists of four exons, and is located on human chromosome 11q25. This chromosomal location is within the minimal deletion region for Jacobsen syndrome, a syndrome including craniosynostosis and other developmental abnormalities. This chromosomal location, along with the reported expression of murine barx2 in craniofacial development, suggests that BARX2 may be causally involved in the craniofacial abnormalities in Jacobsen syndrome.
Amino Acid Sequence↗
CpG methylation is maintained in human cancer cells lacking DNMT1.
Hypermethylation is associated with the silencing of tumour susceptibility genes in several forms of cancer; however, the mechanisms responsible for this aberrant methylation are poorly understood. The prototypic DNA methyltransferase, DNMT1, has been widely assumed to be responsible for most of the methylation of the human genome, including the abnormal methylation found in cancers. To test this hypothesis, we disrupted the DNMT1 gene through homologous recombination in human colorectal carcinoma cells. Here we show that cells lacking DNMT1 exhibited markedly decreased cellular DNA methyltransferase activity, but there was only a 20% decrease in overall genomic methylation. Although juxtacentromeric satellites became significantly demethylated, most of the loci that we analysed, including the tumour suppressor gene p16INK4a, remained fully methylated and silenced. These results indicate that DNMT1 has an unsuspected degree of regional specificity in human cells and that methylating activities other than DNMT1 can maintain the methylation of most of the genome.
5-Methylcytosine↗
Conversion of diploidy to haploidy.
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Adaptor Proteins, Signal Transducing↗
14-3-3Sigma is required to prevent mitotic catastrophe after DNA damage.
14-3-3Sigma is a member of a family of proteins that regulate cellular activity by binding and sequestering phosphorylated proteins. It has been suggested that 14-3-3sigma promotes pre-mitotic cell-cycle arrest following DNA damage, and that its expression can be controlled by the p53 tumour suppressor gene. Here we describe an improved approach to the generation of human somatic-cell knockouts, which we have used to generate human colorectal cancer cells in which both 14-3-3sigma alleles are inactivated. After DNA damage, these cells initially arrested in the G2 phase of the cell cycle, but, unlike cells containing 14-3-3sigma, the 14-3-3sigma-/- cells were unable to maintain cell-cycle arrest. The 14-3-3sigma-/- cells died ('mitotic catastrophe') as they entered mitosis. This process was associated with a failure of the 14-3-3sigma-deficient cells to sequester the proteins (cyclin B1 and cdc2) that initiate mitosis and prevent them from entering the nucleus. These results may indicate a mechanism for maintaining the G2 checkpoint and preventing mitotic death.
14-3-3 Proteins↗
Characterization of MAD2B and other mitotic spindle checkpoint genes.
Aneuploidy is a characteristic of the majority of human cancers, and recent work has suggested that mitotic checkpoint defects play a role in its development. To further explore this issue, we isolated a novel human gene, MAD2B (MAD2L2), which is homologous to the spindle checkpoint gene MAD2 (MAD2L1). We determined the chromosomal localization of it and other spindle checkpoint genes, including MAD1L1, MAD2, BUB3, TTK (MPS1L1), and CDC20. In addition, we resolved the genomic intron-exon structure of the human BUB1 gene. We then searched for mutations in these genes in a panel of 19 aneuploid colorectal tumors. No new mutations were identified, suggesting that genes yet to be discovered are responsible for most of the checkpoint defects observed in aneuploid cancers.
Amino Acid Sequence↗
Disruption of p53 in human cancer cells alters the responses to therapeutic agents.
We have examined the effects of commonly used chemotherapeutic agents on human colon cancer cell lines in which the p53 pathway has been specifically disrupted by targeted homologous recombination. We found that p53 had profound effects on drug responses, and these effects varied dramatically depending on the drug. The p53-deficient cells were sensitized to the effects of DNA-damaging agents as a result of the failure to induce expression of the cyclin-dependent kinase inhibitor p21. In contrast, p53 disruption rendered cells strikingly resistant to the effects of the antimetabolite 5-fluorouracil (5-FU), the mainstay of adjuvant therapy for colorectal cancer. The effects on 5-FU sensitivity were observed both in vitro and in vivo, were independent of p21, and appeared to be the result of perturbations in RNA, rather than DNA, metabolism. These results have significant implications for future efforts to maximize therapeutic efficacy in patients with defined genetic alterations.
Animals↗
Genetic instability and darwinian selection in tumours.
Genetic instability has long been hypothesized to be a cardinal feature of cancer. Recent work has strengthened the proposal that mutational alterations conferring instability occur early during tumour formation. The ensuing genetic instability drives tumour progression by generating mutations in oncogenes and tumour-suppressor genes. These mutant genes provide cancer cells with a selective growth advantage, thereby leading to the clonal outgrowth of a tumour. Here, we discuss the role of genetic instability in tumour formation and outline future work necessary to substantiate the genetic instability hypothesis.
Animals↗