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Results for “Genomic instability”

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PubMed2001

Replication status as a possible marker for genomic instability in cells originating from genotypes with balanced rearrangements.

Most allelic pairs of DNA replicate synchronously during the S phase of the cell cycle. However, some genes frequently replicate asynchronously, i.e. genes on the X chromosome and imprinted genes. Earlier studies demonstrated an asynchronous pattern of replication in some precancerous and invasive squamous carcinoma of the cervix as well as in multiple myeloma. A high rate of asynchronous pattern was found in: (1) lymphocytes of individuals with solid tumors as well as in other malignancies; (2) amniocytes of genotypes with an extra chromosome 13, 18 and 21; (3) lymphocytes of young mothers of a Down syndrome pregnancy. The asynchronic pattern was not locus specific and was found in all loci analyzed. These findings suggested that the mechanism controlling the temporal order of replication could be altered in cells with a genetic predisposition to cancer or aneuploidy. In this study, we found a higher rate of asynchronous pattern in genotypes carrying inversions 2 and 9 and in balanced heritable translocations (p < 0.01) and an even higher rate in cases with a de-novo balanced translocation. The process of tumorigenesis may begin with a change in cell cycle regulation which includes the duplication, replication and segregation of genetic information. However, it remains unknown whether individuals with balanced chromosome rearrangements are at increased risk of developing cancer later in life.

Alleles↗
PubMed2001

The 3'-->5' exonuclease of DNA polymerase delta can substitute for the 5' flap endonuclease Rad27/Fen1 in processing Okazaki fragments and preventing genome instability.

Many DNA polymerases (Pol) have an intrinsic 3'-->5' exonuclease (Exo) activity which corrects polymerase errors and prevents mutations. We describe a role of the 3'-->5' Exo of Pol delta as a supplement or backup for the Rad27/Fen1 5' flap endonuclease. A yeast rad27 null allele was lethal in combination with Pol delta mutations in Exo I, Exo II, and Exo III motifs that inactivate its exonuclease, but it was viable with mutations in other parts of Pol delta. The rad27-p allele, which has little phenotypic effect by itself, was also lethal in combination with mutations in the Pol delta Exo I and Exo II motifs. However, rad27-p Pol delta Exo III double mutants were viable. They exhibited strong synergistic increases in CAN1 duplication mutations, intrachromosomal and interchromosomal recombination, and required the wild-type double-strand break repair genes RAD50, RAD51, and RAD52 for viability. Observed effects were similar to those of the rad27-null mutant deficient in the removal of 5' flaps in the lagging strand. These results suggest that the 3'-->5' Exo activity of Pol delta is redundant with Rad27/Fen1 for creating ligatable nicks between adjacent Okazaki fragments, possibly by reducing the amount of strand-displacement in the lagging strand.

Alleles↗
PubMed2003

Induction of genome instability by DNA damage in Saccharomyces cerevisiae.

The accumulation of gross chromosomal rearrangements (GCRs) is a characteristic of many types of cancer cells, although it is unclear what defects cause these rearrangements and how the different types of GCRs observed are formed. In the present study, we have used a Saccharomyces cerevisiae system for measuring GCRs to analyze the ability of a variety of DNA damaging agents to induce GCRs. The two most potent inducers of GCRs observed were methyl methane sulfonate (MMS) and HO-endonuclease-induced double strand breaks (DSBs). Bleomycin, camptothecan and gamma-irradiation induced intermediate levels of GCRs and cisplatin induced very low levels of GCRs whereas N-methyl-NPRIME;-nitro-N-nitrosoguanidine (MNNG) and ethyl methane sulfonate (EMS) primarily induced base substitution mutations. MMS treatment primarily induced rearrangements in which the end of a chromosome was deleted and a new telomere was added (telomere additions) and also induced translocations. Consistent with this GCR spectrum, the formation of MMS-induced GCRs was primarily dependent on telomere maintenance functions and were completely eliminated in mutants that were defective for both telomere maintenance functions and non-homologous end joining (NHEJ). In contrast, HO-endonuclease DSBs induced mostly translocations and interstitial deletions whereas few telomere additions were observed. Genetic analysis indicated that HO DSB-induced GCRs were suppressed by a number of pathways including the DNA damage checkpoints, DSB repair pathways and NHEJ.

Chromosome Aberrations↗
PubMed1999

Absence of Brca2 causes genome instability by chromosome breakage and loss associated with centrosome amplification.

Women heterozygous for mutations in the breast-cancer susceptibility genes BRCA1 and BRCA2 have a highly elevated risk of developing breast cancer [1]. BRCA1 and BRCA2 encode large proteins with no sequence similarity to one another. Although involvement in DNA repair and transcription has been suggested, it is still not understood how loss of function of these genes leads to breast cancer [2]. Embryonic fibroblasts (MEFs) derived from mice homozygous for a hypomorphic mutation (Brca2(Tr2014)) within the 3' region of exon 11 in Brca2 [3], or a similar mutation (Brca2(Tr)) [4], proliferate poorly in culture and overexpress the tumour suppressor p53 and the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). These MEFs have intact p53-dependent DNA damage G(1)-S [3] [4] and G(2)-M checkpoints [4], but are impaired in DNA double-strand break repair [3] and develop chromosome aberrations [4]. Here, we report that Brca2(Tr2014/Tr2014) MEFs frequently develop micronuclei. These abnormal DNA-containing bodies were formed through both loss of acentric chromosome fragments and by chromosome missegregation, which resulted in aneuploidy. Absence of Brca2 also led to centrosome amplification, which we found associated with the formation of micronuclei. These data suggest a potential mechanism whereby loss of BRCA2 may, within subclones, drive the loss of cell-cycle regulation genes, enabling proliferation and tumourigenesis.

Aneuploidy↗
PubMed2004

Genomic instability in bone marrow failure syndromes.

Aneuploidy is frequently seen in leukemia and myelodysplasia (MDS) but was thought to be uncommon in aplastic anemia (AA). We examined marrow cells from 96 unselected patients with bone marrow failure syndromes to assess the frequency of undetected aneuploidy for chromosomes 7 and 8 by fluorescence in situ hybridization (FISH) as compared to routine cytogenetic analysis. Twenty-eight percent (27/96) of patients had an abnormal karyotype. FISH identified an additional 27 patients with undetected monosomy 7 or trisomy 8. Those patients with undetected monosomy 7 generally had a poor clinical outcome, suffering from lack of response to medical therapy or early death. In one AA/MDS patient with normal cytogenetics, FISH identified a large population of monosomy 7 cells, which clearly heralded a clinical relapse. In another patient, FISH studies were used to delineate instability of chromosome 8, with apparent disease progression from AA to MDS. We conclude that undetected aneuploidy exists in marrow cells of a significant percentage of patients with bone marrow failure syndromes.

Adolescent↗
PubMed2002

Molecular cytogenetic analysis of genomic instability at the 1q12-22 chromosomal site in B-cell non-Hodgkin lymphoma.

Abnormalities of chromosome arm 1q have frequently been reported in B-cell non-Hodgkin lymphoma (NHL), and correlated with poor outcome. Five genes mapped to this region (BCL9, MUC1, FCGR2B, IRTA1, and RTA2) have been shown to be deregulated by juxtaposition with the IG genes. However, abnormalities of the 1q21-22 region that are not involved in translocations with the IG genes have not been addressed. We performed a molecular cytogenetic analysis of 1q12-22 abnormalities in 24 B-cell NHL cases. The cases analyzed were in two groups: one, composed of 18 cases with the single break in the 1q12-22 region, and another, composed of six cases with multiple breaks in the 1q12-22 region. The involvement of heterochromatin and its vicinity was observed most frequently in the single-break cases (13 of 18 cases). In this group, the recurring partner region was 1q32, which resulted in dup(1)(q12-21q32) or trp(1)(q12q32) in 5 cases. The 6 cases with multiple breaks showed an unexpected level of instability along with complex combinations of abnormalities, especially sequential duplication and inversion, in the 1q12-22 region. The BCL9 locus was deleted by complex aberration in 2 of 6 cases. High-level amplification of the WI-16757 locus was found in 2 cases. Our studies demonstrate a high level of instability of the 1q12-22 region, possibly stemming from its chromatin organization. Chromosome arm 1q is gene-rich, and characterization of aberrations described in this study can be expected to lead to the discovery of additional functionally significant genetic changes.

Chromosome Aberrations↗
PubMed2002

Impact of genomic instability in risk assessment and chemoprevention of oral premalignancies.

Head-and-neck cancer is a disfiguring disease with increasing incidence rates even in young people, whose exposure to known risk factors is limited. This emphasizes the importance of early identification, on an individual basis, of precursor lesions that will develop into carcinomas. The clinical value of identifying individuals at high risk of oral cancer is emphasized by the fact that these patients are likely to benefit from available chemopreventive measures, largely without adverse effects.

Antineoplastic Agents↗
PubMed1997

Lymphocyte-specific genomic instability and risk of lymphoid malignancy.

Genetic instability is a force that links evolution, development and cancer. We have developed a quantifiable assay for a particular kind of lymphocyte-specific genetic instability (antigen receptor trans-rearrangements) mediated by the V(D)J recombinase complex. We find that the level of this type of instability correlates (at the population level) with risk for lymphoid malignancy. We have developed a murine model for this measure of instability to allow a more refined analysis of the genetic and environmental factors that sum to define population cancer risk.

Animals↗
PubMed1995

Mitotic and post mitotic consequences of genomic instability induced by oncogenic Ha-ras.

Induced expression of a mutant human Ha-ras oncogene in NIH3T3 cells leads to the rapid production of multicentric chromosomes, acentric chromosome fragments, double minute chromosomes, increased heteroploidy, and increased capacity to undergo gene amplification. In this study we have used fluorescent-in-situ hybridization (FISH) to demonstrate that induction of the Ha-ras oncogene also leads to disruption of the mitotic machinery, resulting in aberrant mitoses and abnormal daughter cells. Cells induced to express an oncogenic Ha-ras transgene accumulate chromosomes that lag outside of the rest of the chromosomal architecture, chromosomes that form bridges between daughter nuclei at anaphase, and that form micronuclei. Many of these mitotic aberrations contain structurally abnormal chromosomes. These ras-induced changes were suppressed by the introduction of a gene encoding the dominant negative effector of ras, raf 301. Expression of raf301 in cells induced to express Ha-ras reduced the level of growth in soft agar, chromosome aberrations, mitotic aberrations, and frequency of gene amplification. These data provide evidence for an association between Ha-ras induced transformation, chromosome aberrations and gene amplification. Furthermore they offer insight into how the cell responds to the formation of aberrant chromosomes, and how disrupting chromosomal architecture could lead to further imbalances in the distribution of genetic material between daughter cells.

3T3 Cells↗
PubMed2000

Non-homologous recombination between Alu and LINE-1 repeats caused a 430-kb deletion in the dystrophin gene: a novel source of genomic instability.

Although large deletions in the dystrophin gene have been identified in more than two-thirds of Duchenne and Becker muscular dystrophy patients, the molecular mechanisms that lead to the generation of these deletions are largely unknown. Here, Alu and LINE-1 (L1) repetitive elements were shown to be present at one or other of the two ends, respectively, of a 430-kb deletion in the dystrophin gene. The breakpoint of the deletion, which stretches from exons 2 to 7, was defined more precisely by polymerase chain reaction (PCR) walking on introns 1 and 7. Finally, the region containing the breakpoint was amplified as a fragment of more than 10kb. Sequencing of the deletion endpoint revealed the presence of an Alu sequence in intron 1, 25 kb downstream from the 3' end of exon 1 that was joined directly to an L1 sequence in intron 7, 4.5kb downstream from the 3' end of exon 7. The deletion was calculated to be 430kb. To our knowledge, this is a novel recombination event joining non-homologous Alu and L1 repeats, and is the largest known intrachromosomal deletion that is thought to involve repetitive genetic elements. Sequence characteristics around the breakpoint are discussed.

Alu Elements↗
PubMed2001

The medaka as a model for studying germ-cell mutagenesis and genomic instability.

To study germ-cell mutagenesis from the viewpoint of biodiversity, we developed a nonmammalian specific-locus test system using the Japanese medaka, Oryzias latipes. The genetic end points available are dominant lethal mutations, total specific-locus mutations, and viable specific-locus mutations. We examined 1,091,824 F(1) embryos that correspond to 3,135,999 genetic loci using gamma rays and ethylnitrosourea as mutagens. The results indicated the feasibility of using the medaka test system to detect genotoxic substances in the aquatic environment. The data also indicated the presence of in vivo safeguards against germ-cell mutagenesis. We present a brief summary of our medaka specific-locus test system raising perspectives on its value.

Journal Article↗