Enhanced flexibility and aphidicolin-induced DNA breaks near mammalian replication origins: implications for replicon mapping and chromosome fragility

Genomic hotspots of chromosome rearrangements explain conserved synteny despite high rates of chromosome evolution in a holocentric lineage

Holocentric organisms, unlike typical monocentric organisms, have kinetochore activity distributed along almost the whole length of the chromosome. Because of this, chromosome rearrangements through fission and fusion are more likely to become fixed in holocentric species, which may account for the extraordinary rates of chromosome evolution that many holocentric lineages exhibit. Long blocks of genome synteny have been reported in animals with holocentric chromosomes despite high rates of chromosome rearrangements. Nothing is known from plants, however, despite the fact that holocentricity appears to have played a key role in the diversification of one of the largest angiosperm genera, Carex (Cyperaceae). In the current study, we compared genomes of Carex species and a distantly related Cyperaceae species to characterize conserved and rearranged genome regions. Our analyses span divergence times ranging between 2 and 50 million years. We also compared a C. scoparia chromosome-level genome assembly with a linkage map of the same species to study rearrangements at a population level and suppression of recombination patterns. We found longer genome synteny blocks than expected under a null model of random rearrangement breakpoints, even between very distantly related species. We also found repetitive DNA to be non-randomly associated with holocentromeres and rearranged regions of the genome. The evidence of conserved synteny in sedges despite high rates of chromosome fission and fusion suggests that conserved genomic hotspots of chromosome evolution related to repetitive DNA shape the evolution of recombination, gene order and crossability in sedges. This finding may help explain why sedges are able to maintain species cohesion even in the face of high interspecific chromosome rearrangements.

High-resolution mapping of mitotic DNA synthesis regions and common fragile sites in the human genome through direct sequencing

DNA replication stress, a feature of human cancers, often leads to instability at specific genomic loci, such as the common fragile sites (CFSs). Cells experiencing DNA replication stress may also exhibit mitotic DNA synthesis (MiDAS). To understand the physiological function of MiDAS and its relationship to CFSs, we mapped, at high resolution, the genomic sites of MiDAS in cells treated with the DNA polymerase inhibitor aphidicolin. Sites of MiDAS were evident as well-defined peaks that were largely conserved between cell lines and encompassed all known CFSs. The MiDAS peaks mapped within large, transcribed, origin-poor genomic regions. In cells that had been treated with aphidicolin, these regions remained unreplicated even in late S phase; MiDAS then served to complete their replication after the cells entered mitosis. Interestingly, leading and lagging strand synthesis were uncoupled in MiDAS, consistent with MiDAS being a form of break-induced replication, a repair mechanism for collapsed DNA replication forks. Our results provide a better understanding of the mechanisms leading to genomic instability at CFSs and in cancer cells.

The role of fork stalling and DNA structures in causing chromosome fragility

Alternative non-B form DNA structures, also called secondary structures, can form in certain DNA sequences under conditions that produce single-stranded DNA, such as during replication, transcription, and repair. Direct links between secondary structure formation, replication fork stalling, and genomic instability have been found for many repeated DNA sequences that cause disease when they expand. Common fragile sites (CFSs) are known to be AT-rich and break under replication stress, yet the molecular basis for their fragility is still being investigated. Over the past several years, new evidence has linked both the formation of secondary structures and transcription to fork stalling and fragility of CFSs. How these two events may synergize to cause fragility and the role of nuclease cleavage at secondary structures in rare and CFSs are discussed here. We also highlight evidence for a new hypothesis that secondary structures at CFSs not only initiate fragility but also inhibit healing, resulting in their characteristic appearance.

Non Random Distribution of DMD Deletion Breakpoints and Implication of Double Strand Breaks Repair and Replication Error Repair Mechanisms

BACKGROUND

Dystrophinopathies are mostly caused by copy number variations, especially deletions, in the dystrophin gene (DMD). Despite the large size of the gene, deletions do not occur randomly but mainly in two hot spots, the main one involving exons 45 to 55. The underlying mechanisms are complex and implicate two main mechanisms: Non-homologous end joining (NHEJ) and micro-homology mediated replication-dependent recombination (MMRDR).

OBJECTIVE

Our goals were to assess the distribution of intronic breakpoints (BPs) in the genomic sequence of the main hot spot of deletions within DMD gene and to search for specific sequences at or near to BPs that might promote BP occurrence or be associated with DNA break repair.

METHODS

Using comparative genomic hybridization microarray, 57 deletions within the intron 44 to 55 region were mapped. Moreover, 21 junction fragments were sequenced to search for specific sequences.

RESULTS

Non-randomly distributed BPs were found in introns 44, 47, 48, 49 and 53 and 50% of BPs clustered within genomic regions of less than 700bp. Repeated elements (REs), known to promote gene rearrangement via several mechanisms, were present in the vicinity of 90% of clustered BPs and less frequently (72%) close to scattered BPs, illustrating the important role of such elements in the occurrence of DMD deletions. Palindromic and TTTAAA sequences, which also promote DNA instability, were identified at fragment junctions in 20% and 5% of cases, respectively. Micro-homologies (76%) and insertions or deletions of small sequences were frequently found at BP junctions.

CONCLUSIONS

Our results illustrate, in a large series of patients, the important role of RE and other genomic features in DNA breaks, and the involvement of different mechanisms in DMD gene deletions: Mainly replication error repair mechanisms, but also NHEJ and potentially aberrant firing of replication origins. A combination of these mechanisms may also be possible.

Decondensation of chromosomal 45S rDNA sites in Lolium and Festuca genotypes does not result in karyotype instability

Fragile sites (FSs) in plants have been described for species like Lolium and other grasses. Whereas in humans FSs were shown to be involved in genome instabilities; the consequences of FSs expression in plants are not known yet. To evaluate whether FSs cause karyotype instabilities, we assessed the frequency of micronuclei and lagging chromosomes in meristematic cells, the stability of the DNA content, and the occurrence of neocentromeres in the presumed chromosomal fragments of Lolium perenne, Lolium multiflorum, Festuca arrundinacea, and two Festulolium hybrids. The cell cycle analysis along with flow cytometric genome size measurements showed high stability in all genomes evaluated. Neocentromeric activity was neither observed in the presumed fragments nor in any other chromosomal region, then this is not the mechanism responsible by the stability. However, Fluorescence in situ hybridization (FISH) with a 45S ribosomal DNA (rDNA) probe in combination with YOYO staining of metaphasic chromosomes showed that many extended nucleolus organizing region (NOR) form very thin YOYO-positive chromatin fibers connecting the acentric 'fragment' with the centromere-containing chromosome region. The obtained data indicate that the expression of FSs does not result in genome instabilities or neocentromere formation. The FS-containing 45S rDNA carrying chromatin fibers undergo a cell cycle and gene activity-dependent dynamic decondensation process.

Karyotype structure and chromosome fragility in the grass Phleum echinatum Host

Phleum echinatum Host (2n = 2x = 10) is an annual Mediterranean species which differs from other representatives of the genus Phleum by reduced chromosome number, asymmetric karyotype and unusually high amount of DNA in the genome. Chromosomes of this plant were studied using conventional acetic-orcein staining and fluorescence in situ hybridization (FISH). FISH showed the major 35S ribosomal DNA (rDNA) site at the secondary constriction of satellite chromosome (3) and the minor 35S rDNA site near 5S rDNA cluster in the monobrachial chromosome 5. Telomeric repeats were detected at all chromosome ends within secondary constriction in satellited chromosome 3 and at the centromeric regions of chromosomes 1 and 2. Intrachromosomally located telomeric repeats are probably traces of chromosomal rearrangements that have shaped P.echinatum genome; they were prone to breakage which was manifested in chromosome fragmentation. The most distinct telomeric signals, suggesting massive amplification of interstitial telomeric sequences (ITRs), were observed at the nucleolar organizer region (NOR) of the third chromosome pair. Double FISH confirmed co-localization of telomeric and 35S rDNA repeats in this locus characterized by the biggest fragility in the karyotype. Fragile sites of P.echinatum, composed of amplified telomeric repeats, may bear a resemblance to metazoan rare fragile sites enriched in microsatellite repeats.

Plant 45S rDNA clusters are fragile sites and their instability is associated with epigenetic alterations

Our previous study demonstrated that 45S ribosomal DNA (45S rDNA) clusters were chromosome fragile sites expressed spontaneously in Lolium. In this study, fragile phenotypes of 45S rDNA were observed under aphidicolin (APH) incubation in several plant species. Further actinomycin D (ActD) treatment showed that transcriptional stress might interfere with chromatin packaging, resulting in 45S rDNA fragile expression. These data identified 45S rDNA sites as replication-dependent as well as transcription-dependent fragile sites in plants. In the presence of ActD, a dramatic switch to an open chromatin conformation and accumulated incomplete 5′ end of the external transcribed spacer (5′ETS) transcripts were observed, accompanied by decreased DNA methylation, decreased levels of histone H3, and increased histone acetylation and levels of H3K4me2, suggesting that these epigenetic alterations are associated with failure of 45S rDNA condensation. Furthermore, the finding that γ-H2AX was accumulated at 45S rDNA sites following ActD treatment suggested that the DNA damage signaling pathway was associated with the appearance of 45S rDNA fragile phenotypes. Our data provide a link between 45S rDNA transcription and chromatin-packaging defects and open the door for further identifying the molecular mechanism involved.

Impaired replication dynamics at the FRA3B common fragile site

Chromosomal common fragile sites (CFSs) are genetically unstable regions of the genome that are induced by conditions that impair DNA replication. In this report, we show that treatment with the DNA polymerase inhibitor, aphidicolin (APH), slows the replication rate throughout S phase. To investigate the unusual sensitivity of CFSs to APH-induced replication stress, we examined replication dynamics within a 50 kb region of the most frequently expressed CFS, FRA3B. We mapped four origins of replication, ori 1-4, using two independent methods. In untreated cells, we detected significantly less newly replicated DNA at FRA3B ori 1-3, as compared with three control origins located within non-fragile regions (NCFSs). In APH-treated cells, all FRA3B and control origins tested were active; however, there was a significant increase of nascent strand DNA at the control origins and, to a lesser extent, at the FRA3B ori 1-3. On the basis of these observations and the theoretical modeling of the nascent strand abundance assay developed in this study, we hypothesize that CFS origins may be less efficient, and that APH treatment slows replication fork movement near these origins to a greater extent, resulting in impaired DNA replication and, ultimately, leading to the genetic instability characteristic of CFSs.

Common fragile sites are characterized by histone hypoacetylation

Common fragile sites (CFSs) represent large, highly unstable regions of the human genome. CFS sequences are sensitive to perturbation of replication; however, the molecular basis for the instability at CFSs is poorly understood. We hypothesized that a unique epigenetic pattern may underlie the unusual sensitivity of CFSs to replication interference. To examine this hypothesis, we analyzed chromatin modification patterns within the six human CFSs with the highest levels of breakage, and their surrounding non-fragile regions (NCFSs). Chromatin at most of the CFSs analyzed has significantly less histone acetylation than that of their surrounding NCFSs. Trichostatin A and/or 5-azadeoxycytidine treatment reduced chromosome breakage at CFSs. Furthermore, chromatin at the most commonly expressed CFS, the FRA3B, is more resistant to micrococcal nuclease than that of the flanking non-fragile sequences. These results demonstrate that histone hypoacetylation is a characteristic epigenetic pattern of CFSs, and chromatin within CFSs might be relatively more compact than that of the NCFSs, indicating a role for chromatin conformation in genomic instability at CFSs. Moreover, lack of histone acetylation at CFSs may contribute to the defective response to replication stress characteristic of CFSs, leading to the genetic instability characteristic of this regions.

Oncogene-induced replication stress preferentially targets common fragile sites in preneoplastic lesions. A genome-wide study

Common fragile sites (CFSs) are regions of the genome prone to breakage by replication inhibitors (extrinsic replication stress). Recently, we and others observed that oncogene-induced replication stress (RS) induces DNA damage from the earliest stages of cancer. Our aim was to perform a genome-wide analysis in precancerous and cancerous experimental models to examine whether allelic imbalance occurs within CFSs. Subsequently, CFSs sequence characteristics were assessed. We used a growth-factor-induced human skin hyperplasia and a H-ras-induced mouse hyperplastic urothelium as preneoplastic models, along with an inducible U2OS-CDT1(Tet-ON) cancer cell line model, all bearing established oncogene-induced RS stimuli. Human DNA was analysed with Affymetrix SNP microarrays, while mouse DNA was analysed with Nimblegen array CGH. We studied 56 aphidicolin-type CFSs and 1914 regions of control, nonfragile DNA. Our theoretical in silico analysis spanned 2.16 billion nonoverlapping bases on human chromosomes 1-22. Our results provide direct experimental evidence indicating that genomic alterations were more common within CFSs in epidermal and urothelial preneoplastic lesions as well as in cancer. CFSs were on average less flexible than nonfragile regions, contained more guanine-cytosine (GC) and Alu sequences. Importantly, regions with loss-of-heterozygosity were also less flexible and had a higher Alu percentage.

High-resolution mapping identifies a commonly amplified 11q13.3 region containing multiple genes flanked by segmental duplications

DNA amplification of the 11q13 region is observed frequently in many carcinomas. Within the amplified region several candidate oncogenes have been mapped, including cyclin D1, TAOS1 and cortactin. Yet, it is unknown which gene(s) is/are responsible for the selective pressure enabling amplicon formation. This is probably due to the use of low-resolution detection methods. Furthermore, the size and structure of the amplified 11q13 region is complex and consists of multiple amplicon cores that differ between different tumor types. We set out to test whether the borders of the 11q13 amplicon are restricted to regions that enable DNA breakage and subsequent amplification. A high-resolution array of the 11q13 region was generated to study the structure of the 11q13 amplicon and analyzed 29 laryngeal and pharyngeal carcinomas and nine cell lines with 11q13 amplification. We found that boundaries of the commonly amplified region were restricted to four segments. Three boundaries coincided with a syntenic breakpoint. Such regions have been suggested to be putatively fragile. Sequence comparisons revealed that the amplicon was flanked by two large low copy repeats known as segmental duplications. These segmental duplications might be responsible for the typical structure and size of the 11q13 amplicon. We hypothesize that the selection for genes through amplification of the 11q13.3 region is determined by the ability to form DNA breaks within specific regions and, consequently, results in large amplicons containing multiple genes.

Copy number variation in the genome; the human DMD gene as an example

Recent developments have yielded new technologies that have greatly simplified the detection of deletions and duplications, i.e., copy number variants (CNVs). These technologies can be used to screen for CNVs in and around specific genomic regions, as well as genome-wide. Several genome-wide studies have demonstrated that CNV in the human genome is widespread and may include millions of nucleotides. One of the questions that emerge is which sequences, structures and/or processes are involved in their generation. Using as an example the human DMD gene, mutations in which cause Duchenne and Becker muscular dystrophy, we review the current data, determine the deletion and duplication profile across the gene and summarize the information that has been collected regarding their origin. In addition we discuss the methods most frequently used for their detection, in particular MAPH and MLPA.

Chromosomal fragility, structural rearrangements and mobile element activity may reflect dynamic epigenetic mechanisms of importance in neurobehavioural genetics

Advances in human genome analyses have not yet allowed identification of specific genetic mechanisms underlying the expression of human neurobehavioural disorders. There is an increasing awareness that several genes may contribute to behavioural phenotypes and these genes appear to interact in as yet undetermined ways. It has been suggested that the problem needs elucidation from an epigenetic, gene expression perspective. Cytogenetic instability manifesting as chromosomal fragile sites, translocations, duplications, deletions and inversions, when co-occurring with neurobehavioural disorders, may offer a doorway to the investigation of such chromatin level, regulatory region, epigenetic processes. Due to earlier indications of non-specificity of chromosomal aberrations, poor phenotype:genotype correlations and a shift to analysing candidate coding regions on high resolution map level, the only utility of chromosomal breakpoints came to be seen as harbouring possible candidate genes of interest when segregating together with particular neurobehavioural disorders. More recent findings of the expression of highly specific subsets of fragile sites in association with Tourette and Rett syndromes need to be extended to other neurobehavioural disorders to ascertain whether observed patterns can be considered representative of 'chromatin endophenotypes' correlating with discrete sets of neurobehavioural symptoms. Environmental/epigenetic factors could affect the chromatin characteristics of the genome arising through DNA strand breakage, mobile element activity and retroinsertion, establishing new architectural features of regulatory control networks very rapidly in comparison to coding region evolution rates. Microarray-based techniques for the genome-wide mapping of in vivo protein-DNA interactions offer increasingly comprehensive views of genetic and epigenetic regulatory networks. It may be informative to include functionally significant chromatin structural variation analyses when considering candidate genes for neurobehavioural disorders.

An inactive X specific replication origin associated with a matrix attachment region in the human X linked HPRT gene

Early in female mammalian embryogenesis, one of the two X chromosomes is inactivated to compensate the gene dosage between males and females. One of the features of X chromosome inactivation (XCI) is the late replication of the inactivated X chromosome. This study reports the identification, by competitive PCR of nascent DNA, of a replication origin in intron 2 of the human X-linked HPRT gene, that is functional only on the inactive X. Features frequently associated with replication origins, including a peak of enhanced DNA flexibility, a perfect match to the yeast ACS sequence, a 14/15 match to the Drosophila topoisomerase II consensus, and a 20/21 match to an initiation region consensus sequence, were identified close to the replication origin. The origin is located approximately 2 kb upstream of a matrix attachment region (MAR) and also contains two A:T-rich elements, thought to facilitate DNA unwinding.

Spindle checkpoint function is required for mitotic catastrophe induced by DNA-damaging agents

Mitotic catastrophe is an important mechanism for the induction of cell death in cancer cells by antineoplastic agents that damage DNA. This process is facilitated by defects in the G1 and G2 checkpoints of the cell cycle that are apparent in most cancer cells and which allow the cells to enter mitosis with DNA damage. We have now characterized the dynamics of mitotic catastrophe induced by DNA-damaging agents in p53-deficient cancer cells. Cells that entered mitosis with DNA damage transiently arrested at metaphase for more than 10 h without segregation of chromosomes and subsequently died directly from metaphase. In those metaphase arrested precatastrophic cells, anaphase-promoting complex appeared to be inactivated and BubR1 was persistently localized at kinetochores, suggesting that spindle checkpoint is activated after the DNA damage. Furthermore, suppression of spindle checkpoint function by BubR1 or Mad2 RNA interference in the DNA damaged cells led to escape from catastrophic death and to subsequent abnormal mitosis. Dysfunction of the spindle checkpoint in p53-deficient cancer cells is thus likely a critical factor in resistance to DNA-damaging therapeutic agents.

Comparative analysis of the PDCD2–TBP–PSMB1 region in vertebrates

Three orthologous genes encoding programmed cell death 2 (PDCD2), TATA-binding protein (TBP), and proteasomal subunit C5 (PSMB1) proteins have been shown previously to be nonrandomly distributed in both mammalian and invertebrate genomes. Here we analyze a conserved synteny of the PDCD2, TBP, and PSMB1 orthologs in four nonmammalian vertebrates. Homologous genes of the chicken, zebrafish, fugu, and Tetraodon nigroviridis were identified. A chicken cosmid harboring the orthologs of these three genes was completely sequenced. The fish genes were analyzed in silico. In all seven vertebrates thus far investigated, the PDCD2 and TBP genes are located tail-to-tail. In six tested species but the zebrafish, the PSMB1 gene mapped head-to-head or in the close vicinity to the TBP, but even in the zebrafish, all three genes were syntenic. In contrast, a three times reused synteny breakpoint in the 5'-region from PDCD2 was detected. A comparative analysis revealed the distribution of putative matrix-attached regions (MARs), which may affect the synteny conservation.

Enhanced expression of MYCN leads to centrosome hyperamplification after DNA damage in neuroblastoma cells

Centrosomes play important roles in cell polarity, regulation of cell cycle and chromosomal stability. Centrosome abnormality is frequently found in many cancers and contributes to chromosomal instability (including aneuploidy, tetraploidy, and/or micronuclei) in daughter cells through the assembly of multipolar or monopolar spindles during mitosis. It has recently been reported that loss of tumor suppressor genes or overexpression of oncogenes causes centrosome hyperamplification. Amplification and overexpression of the MYCN oncogene is found in a subgroup of neuroblastomas. In this study, we examined whether overexpression of MYCN causes centrosome hyperamplification in neuroblastoma cells. We show that ectopic expression of MYCN alone in a neuroblastoma cell line did not cause centrosome hyperamplification. However, centrosome hyperamplification and micronuclei formation were seen in these cells after DNA damage. These findings suggest that overexpression of MYCN abrogates the regulation of the centrosome cycle after DNA damage.

Matrix attachment region (MAR) properties and abnormal expansion of AT island minisatellites in FRA16B fragile sites in leukemic CEM cells

AT-rich minisatellites (AT islands) are sites of genomic instability in cancer cells and targets for extremely lethal AT-specific drugs, such as bizelesin. Here we investigated the AT islands in the FRA16B fragile site region for their possible roles in the organization of DNA on the nuclear matrix. The FRA16B AT island nominally spans approximately 3 kb of mostly >90% A/T DNA. In silico analysis indicates that this domain exhibits characteristics of nuclear matrix attachment regions (MARs): an exceptionally intense computed 'MAR potential' and profound duplex destabilization and flexibility. FRA16B repeats specifically bind to isolated nuclear matrices, which indicates their in vitro MAR function. This binding is several-fold greater than that of a known MAR in the c-myc gene. AT islands in fragile sites FRA16B and FRA16D are significantly more abundant in CEM cells that are hypersensitive to bizelesin compared to normal WI-38 cells. FRA16B overabundance in CEM is due to an approximately 10-fold expansion of FRA16B repeats. The expanded FRA16B minisatellites in CEM cells preferentially localize to the nuclear matrix-associated DNA indicating their in vivo MAR function. The unexpanded repeats in WI-38 cells localize to the loop DNA. The c-myc MAR is also matrix-associated in CEM cells while localizing to loop DNA in WI-38 cells. These results are the first to demonstrate that AT islands in fragile sites can function as MARs both in vitro and in vivo. The ability of FRA16B-mediated MAR sites to rearrange depending on the repeat expansion status could be relevant to both genomic instability of cancer cells and their sensitivity to AT-island targeting drugs.

Characterization of the human common fragile site FRA2G

Common fragile sites are nonrandom loci that show gaps and breaks when cells are exposed to specific compounds. They are preferentially involved in recombination, chromosomal rearrangements, and foreign DNA integration. These sites have been suggested to play a role in chromosome instability observed in cancer. In this work we used a FISH-based approach to identify a BAC contig that spans the FRA2G fragile site located at the 2q31 region. Our observations indicate that a very fragile region spanning at least 450 kb is present within a large fragile region that extends over 1 Mb. At least seven genes are mapped in the fragile region. One of these seems to be a good candidate as a potential tumor suppressor gene impaired by the recurrent deletions observed at the 2q31 region in some neoplasms. In the fragile region, a considerable number of regions of high flexibility that may be related to the fragility are present.

Bending and flexibility of methylated and unmethylated EcoRI DNA

We used cyclization kinetics experiments and Monte Carlo simulations to determine a structural model for a DNA decamer containing the EcoRI restriction site. Our findings agree well with recent crystal and NMR structures of the EcoRI dodecamer, where an overall bend of seven degrees is distributed symmetrically over the molecule. Monte Carlo simulations indicate that the sequence has a higher flexibility, assumed to be isotropic, compared to that of a "generic" DNA sequence. This model was used as a starting point for the investigation of the effect of cytosine methylation on DNA bending and flexibility. While methylation did not affect bend magnitude or direction, it resulted in a reduction in bending flexibility and under-winding of the methylated nucleotides. We demonstrate that our approach can augment the understanding of DNA structure and dynamics by adding information about the global structure and flexibility of the sequence. We also show that cyclization kinetics can be used to study the properties of modified nucleotides.

Mammalian recombination hot spot in a DNA loop anchorage region: a model for the study of common fragile sites

We analyzed the replication pattern and the topological organization of a 200 kb long Chinese hamster polygenic locus, which spans the boundary of two isochores. One of them is G + C rich while the second one is highly A + T rich. Previous analysis of mutants amplified for this locus had identified, within the A + T rich isochore, a mitotic recombination hotspot and a replication origin separated by some 7 kb. The recombination hotspot exhibits structural features repeatedly observed at common fragile sites, including a typical enrichment in peaks of enhanced DNA helix flexibility. By studying the replication pattern of the same locus in the non-amplified CHO cells, we confirm here the localization of the replication origin and show that the mitotic recombination hotspot does not correspond to a replicon junction. This finding makes questionable current hypotheses correlating replication termination regions with recombination prone sequences. Using topoisomerase II-mediated DNA cleavage at matrix attachment sites, we identified a 40 kb-long DNA anchorage region extending all along a transcription unit nested within the A + T rich isochore. Both the recombination hotspot and the replication origin lie within this topoisomerase II sensitive region, which suggests that features essential for initiation of recombination and initiation of DNA replication cluster within DNA anchorage regions. Features common to this region and to common fragile sites are discussed. J. Cell. Biochem. Suppl. 36: 170-178, 2001.

Fragile and unstable chromosomes in cancer: causes and consequences

Cancer cells commonly exhibit various forms of genetic instability, such as changes in chromosome copy number, translocations and point mutations in particular genes. Although transmissible change seems to be an essential part of the neoplastic process, the extent to which DNA instability is a cause rather than a consequence of cancer is unclear. Chromosomal fragile sites have been proposed to be not only susceptible to DNA instability in cancer cells, but also associated with genes that contribute to the neoplastic process. Mutation at fragile site loci might therefore have a causative role in cancer. Recent studies on one class of human chromosomal fragile sites show that instability at fragile site loci can functionally contribute to tumor cell biology.