Molecular analysis of cellular loci disrupted by papillomavirus 16 integration in cervical cancer: frequent viral integration in topologically destabilized and transcriptionally active chromosomal regions

Impact of Replication Stress in Human Papillomavirus Pathogenesis

The inactivation of critical cell cycle checkpoints by the human papillomavirus (HPV) oncoprotein E7 results in replication stress (RS) that leads to genomic instability in premalignant lesions. Intriguingly, RS tolerance is achieved through several mechanisms, enabling HPV to exploit the cellular RS response for viral replication and to facilitate viral persistence in the presence of DNA damage. As such, inhibitors of the RS response pathway may provide a novel approach to target HPV-associated lesions and cancers.

Mechanisms by which HPV Induces a Replication Competent Environment in Differentiating Keratinocytes

Human papillomaviruses (HPV) are the causative agents of cervical cancer and are also associated with other genital malignancies, as well as an increasing number of head and neck cancers. HPVs have evolved their life cycle to contend with the different cell states found in the stratified epithelium. Initial infection and viral genome maintenance occurs in the proliferating basal cells of the stratified epithelium, where cellular replication machinery is abundant. However, the productive phase of the viral life cycle, including productive replication, late gene expression and virion production, occurs upon epithelial differentiation, in cells that normally exit the cell cycle. This review outlines how HPV interfaces with specific cellular signaling pathways and factors to provide a replication-competent environment in differentiating cells.

Playing with fire: consequences of human papillomavirus DNA replication adjacent to genetically unstable regions of host chromatin

Papillomaviruses are small DNA viruses that replicate persistently in the stratified epithelial surfaces of the host. They have minimal coding capacity and must hijack many cellular processes to complete their life cycle. For example, viral genomes are tethered to host chromatin to ensure that they are effectively partitioned in dividing cells, and the host DNA damage and repair pathways are usurped to replicate viral DNA in differentiated cells. These processes result in the close juxtaposition of viral DNA with host DNA that is undergoing replication stress. This could explain the propensity of oncogenic human papillomaviruses (HPVs) to accidently integrate into common fragile sites in host DNA.

Detection of the human papillomavirus 58 physical state using the amplification of papillomavirus oncogene transcripts assay

HPV 58 is detected commonly in cervical cancer in East Asian countries. To evaluate the HPV 58 physical state, the amplification of papillomavirus oncogene transcripts (APOT) and hybridisation assays were established. Episome- and integrate-derived transcripts were confirmed by direct sequencing. Twenty-nine HPV 58 positive samples from various cervical lesions were used. The results showed that the episome-derived transcripts were recognised as two major specific amplified products (1040 and 714 bp). Two splice donor sites were mapped to the 5' splice site of the E1 gene on SD898 and SD899 and spliced to the 3' acceptor site of the E4 gene on SA3353, SA3356 and SA3365. The episome-derived transcripts were found 100% in normal cervical epithelia and low-grade lesions (9/9 cases) while the integrate-derived transcripts were detected in 13.3% of high-grade lesions (2/15 cases) and in 20% of carcinomas (1/5 cases). HPV 58 integration sites were found on chromosomes 4q21, 12q24 and 18q12. Using the established APOT assay, the results revealed not only novel information on the HPV 58 transcription patterns of episomal transcripts, but also integration site. The APOT assay is a reliable and useful tool for the detection of the HPV 58 physical state and its oncogene expression.

Systems responses of rats to aflatoxin B1 exposure revealed with metabonomic changes in multiple biological matrices

Exposure to aflatoxins causes liver fibrosis and hepatocellular carcinoma posing a significant health risk for human populations and livestock. To understand the mammalian systems responses to aflatoxin-B1 (AFB1) exposure, we analyzed the AFB1-induced metabonomic changes in multiple biological matrices (plasma, urine, and liver) of rats using (1)H NMR spectroscopy together with clinical biochemistry and histopathologic assessments. We found that AFB1 exposure caused significant elevation of glucose, amino acids, and choline metabolites (choline, phosphocholine, and glycerophosphocholine) in plasma but reduction of plasma lipids. AFB1 also induced elevation of liver lipids, amino acids (tyrosine, histidine, phenylalanine, leucine, isoleucine, and valine), choline, and nucleic acid metabolites (inosine, adenosine, and uridine) together with reduction of hepatic glycogen and glucose. AFB1 further caused decreases in urinary TCA cycle intermediates (2-oxoglutarate and citrate) and elevation of gut microbiota cometabolites (phenylacetylglycine and hippurate). These indicated that AFB1 exposure caused hepatic steatosis accompanied with widespread metabolic changes including lipid and cell membrane metabolisms, protein biosynthesis, glycolysis, TCA cycle, and gut microbiota functions. This implied that AFB1 exposure probably caused oxidative-stress-mediated impairments of mitochondria functions. These findings provide an overview of biochemical consequences of AFB1 exposure and comprehensive insights into the metabolic aspects of AFB1-induced hepatotoxicity in rats.

Human papillomavirus type 16 (HPV-16) genomes integrated in head and neck cancers and in HPV-16-immortalized human keratinocyte clones express chimeric virus-cell mRNAs similar to those found in cervical cancers

Many human papillomavirus (HPV)-positive high-grade lesions and cancers of the uterine cervix harbor integrated HPV genomes expressing the E6 and E7 oncogenes from chimeric virus-cell mRNAs, but less is known about HPV integration in head and neck cancer (HNC). Here we compared viral DNA status and E6-E7 mRNA sequences in HPV-16-positive HNC tumors to those in independent human keratinocyte cell clones derived from primary tonsillar or foreskin epithelia immortalized with HPV-16 genomes. Three of nine HNC tumors and epithelial clones containing unintegrated HPV-16 genomes expressed mRNAs spliced from HPV-16 SD880 to SA3358 and terminating at the viral early gene p(A) signal. In contrast, most integrated HPV genomes in six HNCs and a set of 31 keratinocyte clones expressed HPV-16 major early promoter (MEP)-initiated mRNAs spliced from viral SD880 directly to diverse cellular sequences, with a minority spliced to SA3358 followed by a cellular DNA junction. Sequence analysis of chimeric virus-cell mRNAs from HNC tumors and keratinocyte clones identified viral integration sites in a variety of chromosomes, with some located in or near growth control genes, including the c-myc protooncogene and the gene encoding FAP-1 phosphatase. Taken together, these findings support the hypothesis that HPV integration in cancers is a stochastic process resulting in clonal selection of aggressively expanding cells with altered gene expression of integrated HPV genomes and potential perturbations of cellular genes at or near viral integration sites. Furthermore, our results demonstrate that this selection also takes place and can be studied in primary human keratinocytes in culture.

Genomic Organization of AmplifiedMYCGenes Suggests Distinct Mechanisms of Amplification in Tumorigenesis

Integration of the human papillomavirus (HPV) genome into the host genome is associated with the disruption of the HPV E2 gene and with amplification and rearrangement of the viral and flanking cellular sequences. Molecular characterization of the genomic structures of coamplified HPV sequences and oncogenes provides essential information concerning the mechanisms of amplification and their roles in carcinogenesis. Using fluorescent hybridization on stretched DNA molecules in two cervical cancer-derived cell lines, we have elucidated the genomic structures of amplified regions containing HPV/myc genes over several hundreds of kilobases. Direct visualization of hybridization signals on individual DNA molecules suggests that overreplication and breakage-fusion-bridge-type mechanisms are involved in the genomic instability associated with HPV cervical cancers. Further analysis from two other genital cancer-derived cell lines reveals a recurrent motif of amplification, probably generated by a common mechanism involving overreplication upon viral integration. Interestingly, different amplification patterns seem to be correlated with the disease outcome, thus providing new insights into HPV-related cancer development and tumor progression.

Mapping and analysis of HPV16 integration sites in a head and neck cancer cell line

Human papillomavirus (HPV) is a circular double-stranded DNA virus implicated in at least 90% of cervical and anogenital cancers and has been observed in approximately 20% of squamous cell carcinomas of the head and neck (SCCHN). Transcription of the viral oncogenes E6 and E7 is regulated by expression of the E2 protein. Disruption of the E2 gene sequence due to viral integration results in upregulation of E6 and E7, which promote tumorigenesis by abrogating p53 and pRb functions. HPV integration sites in cervical and anogenital cancers have been mapped primarily to chromosomal fragile sites and in some cases have been shown to integrate within tumor suppressor genes or other cancer-related genes. To study viral integration sites in SCCHN, we examined an HPV16-infected SCCHN cell line cultured from a tongue-base tumor. HPV fluorescence in situ hybridization (FISH) revealed multiple integrated viral DNA copies in blocks throughout the genome. Sequential FISH and spectral karyotyping identified integration sites on chromosomes 3, 6, 9q, 13q and t(1;8)(q;?). Restriction site-polymerase chain reaction (RS-PCR) was performed to identify the viral-cellular junctions. Sequence analyses confirmed integration sites at 9q31.1 and 6p21 and revealed a novel junction at 16p12.3. Subsequent chromosome breakage studies suggested that the observed viral-cellular integration sites may have occurred within common fragile sites. Additional studies using RT-PCR for E6--E7 viral transcripts showed oncoprotein expression from episomal and integrated viral sequences. Our results suggest that viral integration of HPV in SCCHN appears to occur nonrandomly through targeting specific chromosomal sequences prone to breakage.

Systematic review of genomic integration sites of human papillomavirus genomes in epithelial dysplasia and invasive cancer of the female lower genital tract

Cancers of the anogenital tract as well as some head and neck cancers are caused by persistent infections with high-risk type human papillomaviruses (HPVs). Two viral oncogenes, E6 and E7, induce severe chromosomal instability associated with centrosome aberrations, anaphase bridges, chromosome lagging, and breaking. This occurs early in preneoplastic lesions, when the viral genome still persists in an episomal state. In most invasive cancers and also in a few high-grade dysplastic lesions, however, integration of high-risk HPV genomes into the host genome is observed. Integration seems to be a direct consequence of chromosomal instability and an important molecular event in the progression of preneoplastic lesions. Disruption or deregulation of defined critical cellular gene functions by insertional mutagenesis by integrated HPV genome fragments has been hypothesized as one major promoting factor in the pathogenesis of HPV-associated cancers. This hypothesis was based on the detection of HPV integration events in the area of tumor-relevant genes in few cases. Here, we reviewed >190 reported integration loci with respect to changes in the viral structure and the targeted genomic locus. This analysis confirms that HPV integration sites are randomly distributed over the whole genome with a clear predilection for genomic fragile sites. No evidence for targeted disruption or functional alteration of critical cellular genes by the integrated viral sequences could be found.

Study of viral integration of HPV-16 in young patients with LSIL

AIMS

To investigate the physical status of human papillomavirus 16 (HPV-16) in low grade squamous intraepithelial lesions (LSILs) as a means of determining the percentage of viral integration.

METHODS

Ninety two LSIL/HPV positive Thin Prep(TM) samples were initially tested for the E6 gene by the polymerase chain reaction (PCR) to identify the HPV-16 virus. To avoid false positive results, the specificity of the bands obtained from PCR was confirmed by Southern blot hybridisation with internal oligonucleotide probes. Next, a PCR screen for the E2 gene was performed to identify those samples in which the virus was integrated. Viral integration was detected in just over half of them.

RESULTS

Twenty of the 92 samples were HPV-16 positive, as shown by PCR for the E6 gene. Southern blot analysis confirmed that 13 of these samples were positive for the viral E6 gene. Thus, viral integration was detected in just over a half of the samples positive for HPV-16.

CONCLUSIONS

These data show that HPV-16 integration occurs in a subset of LSILs. The measurement of HPV-16 integration would be a helpful complementary tool for cytological evaluation in primary cervical screening to identify those patients at risk of developing high grade squamous intraepithelial lesions and cervical cancer.

A comprehensive analysis of HPV integration loci in anogenital lesions combining transcript and genome-based amplification techniques

Persistent infections with high-risk human papillomaviruses (HPVs) induce dysplastic lesions of the lower genital tract. Some of these lesions eventually progress to invasive cancers, particularly of the uterine cervix. In many advanced preneoplastic cervical lesions and most derived carcinomas, HPV genomes are found to be integrated into the host cell chromosomes. Although HPV integration seems to play an important role in the progression of cervical dysplasia, the underlying mechanisms are still unclear. To investigate the pathogenic role of genomic integration of HPV genomes in greater detail, we analysed integration sites of HPV16 and 18 genomes in 21 anogenital precancerous and cancerous lesions using a ligation-mediated chain reaction (DIPS) and the recently described amplification of papilloma virus oncogene transcripts (APOT) assay. On the genomic level, only singular integration events were observed in individual neoplastic cell clones. At many integration sites, a short overlap between HPV and genomic sequences was observed, suggesting that the integration of HPV genomes is mediated by nonhomologous sequence-specific recombination. APOT analysis revealed that the majority of integrated HPV genomes was actively transcribed. These data suggest that in the progression of cervical preneoplasia to invasive carcinomas, integration of viral genomes occurs only at single or few chromosomal loci in a given cell clone. Disruption of cellular genes might support malignant transformation in rare cases; however, it is not a pathogenic prerequisite. The main function of HPV integration seems to be the stabilization of oncogene transcription.

Common fragile sites are preferential targets for HPV16 integrations in cervical tumors

The development of cervical cancer is highly associated with human papillomavirus (HPV) infection. HPV integration into the genome of infected cervical cells is temporally associated with the acquisition of the malignant phenotype. A relationship between the sites of HPV integration in cervical cancer and the position of the common fragile sites (CFSs) has been observed at both the cytogenetic and molecular levels. To further explore this relationship at the molecular level, we used RS-PCR to rapidly isolate cellular sequences flanking the sites of HPV16 integration in 26 primary cervical tumors. Human bacterial artificial chromosome clones were isolated based on these flanking sequences and used as probes for fluorescence in situ hybridization on aphidicolin-stimulated metaphases. Our data demonstrate that 11/23 HPV16 integrations in cervical tumors occurred within CFSs (P&<0.001). In addition, we show that deletions and complex rearrangements frequently occur in the cellular sequences targeted by the integrations and that integrations cluster in FRA13C (13q22), FRA3B (3p14.2), and FRA17B (17q23). Finally, our data suggest that cellular genes, such as Notch 1, are disrupted by the HPV16 integrations, which may contribute to the malignant phenotype.

Single copy heterozygote integration of HPV 33 in chromosomal band 5p14 is found in an epithelial cell clone with selective growth advantage

Infection with human papillomavirus (HPV) of specific high-risk type triggers a series of events in target cells, which will eventually lead to development of genital neoplasia. The integration of high-risk HPV DNA into the cell genome has been regarded as a crucial event in tumor progression. With respect to different HPV types, the knowledge of HPV integrated loci is still limited. We have now determined the genomic variation and chromosomal location of HPV 33 DNA in the cell line UT-DEC-1, established from a vaginal mild dysplasia lesion. The viral sequence of the cell line was determined, and a variant of the prototype HPV 33 strain was identified, showing nucleotide substitutions resulting in amino acid changes in the E2, L2 and E4 open reading frames. In late passage UT-DEC-1 cells, a deletion of more than half of the 3' part of E1 and major parts of the E2 and E4 genes provided evidence for integration. The flanking sequences of the integration site were completely homologous to published sequences from chromosomal band 5p14, and remained unchanged in all subclones established from late passage cells. There were no chromosomal deletions or gross rearrangements at the integration site, and only a single heterozygotic copy of HPV 33 was detected. The karyotype of late passage cells showed only minor changes compared with early passage cells. During passaging of the cell line, there were progressive changes towards a malignant phenotype, and in parallel to this, the cells carrying episomal HPV 33 of the early passages was completely superseded by cells containing the integrated virus. Thus, our results show that this single copy heterozygote integration of HPV 33 into chromosome band 5p14 appears to be associated with emergence of cells escaping senescence, and with growth advantage compared with cells carrying episomal virus.

Molecular evolution of ribosomal intergenic spacers in Odontophrynus americanus 2n and 4n (Amphibia: Anura)

Ribosomal intergenic spacers (IGSs) of Odontophrynus americanus 2n and 4n were cloned, restriction mapped, and partially sequenced. Three distinct regions, namely alpha, beta, and delta, were identified in the IGSs. The alpha and beta regions flanked the 28S and 18S rRNA genes, respectively, conserving an identical restriction pattern at each ploidy level. The delta region, located between alpha and beta, was highly variable in size and restriction pattern, enclosing different BamHI subrepeats (B-SR), 87- to 530-bp-long. Sequence analysis showed that B-SRs were composed mainly of different arrangements of similar blocks of sequences. Another family of repetitive sequences was found in the delta region, clustered inside large BamHI fragments. These subrepeats are 189-bp-long and, although very similar in diploid and tetraploid IGSs, show a pattern of concerted evolution. A hypothetical functional role for the 189-bp repeats is discussed in view of their predicted secondary structure and presence of potential E2 binding sites inside diploid subrepeats. Although the same structural elements were present both in diploid and tetraploid IGSs, the higher level of repeatability of tetraploid IGSs suggests that common ancestor sequences have undergone several rounds of amplification after O. americanus polyploidy.

Small tumor virus genomes are integrated near nuclear matrix attachment regions in transformed cells

More than 15% of human cancers have a viral etiology. In benign lesions induced by the small DNA tumor viruses, viral genomes are typically maintained extrachromosomally. Malignant progression is often associated with viral integration into host cell chromatin. To study the role of viral integration in tumorigenesis, we analyzed the positions of integrated viral genomes in tumors and tumor cell lines induced by the small oncogenic viruses, including the high-risk human papillomaviruses, hepatitis B virus, simian virus 40, and human T-cell leukemia virus type 1. We show that viral integrations in tumor cells lie near cellular sequences identified as nuclear matrix attachment regions (MARs), while integrations in nonneoplastic cells show no significant correlation with these regions. In mammalian cells, the nuclear matrix functions in gene expression and DNA replication. MARs play varied but poorly understood roles in eukaryotic gene expression. Our results suggest that integrated tumor virus genomes are subject to MAR-mediated transcriptional regulation, providing insight into mechanisms of viral carcinogenesis. Furthermore, the viral oncoproteins serve as invaluable tools for the study of mechanisms controlling cellular growth. Similarly, our demonstration that integrated viral genomes may be subject to MAR-mediated transcriptional effects should facilitate elucidation of fundamental mechanisms regulating eukaryotic gene expression.

Physical state of HPV16 and chromosomal mapping of the integrated form in cervical carcinomas

Using a procedure based on restriction enzyme cleavage, self-ligation, and inverse polymerase chain reaction (rliPCR), the authors investigated 18 cervical intraepithelial neoplasia III (CIN III) cases and 37 invasive squamous carcinomas for integration of human papillomavirus type 16 (HPV16). All eighteen CIN III cases (severe dysplasia or high-grade squamous intraepithelial lesion) were found to harbor episomal HPV, but one of the samples contained mixed episomal and integrated forms. Seventeen of 37 invasive cervical carcinoma samples were identified previously as containing the completely integrated HPV16 genome by using PCR covering the entire E1/E2 gene, and this was confirmed by rliPCR in 16 cases. One case, however, showed a low level of episomal deoxyribonucleic acid in addition to the predominant integrated form. Of the remaining 20 carcinoma samples showing episomal forms in the previous analysis, 14 were found to contain integrated forms using rliPCR, and four contained multimeric episomal forms. Thus, in total, 31 of 37 of the carcinomas (84%) showed the integrated HPV16 genome. The rliPCR product from five carcinoma cases was cloned into a plasmid vector and used as a template for "primer walking" deoxyribonucleic acid sequencing to deduce human sequences flanking the integrated HPV genome. Based on this information, bacterial artificial chromosome (BAC) and P1-derived artificial chromosome (PAC) clones were obtained and used as probes in fluorescent in situ hybridization experiments on human metaphase chromosomes. The results of the fluorescent in situ hybridization experiments showed evidence for HPV16 integration in chromosome regions 1q25, 3q28, 6p25, 11p13, and 18q22. Sixteen carcinoma samples, containing episomal HPV16, were sequenced in the long control region. Evidence for changes in E2 binding or silencer YY1 sequences was found in only two samples.

The integration of HPV-18 DNA in cervical carcinoma

AIMS

Little information is available on the patterns of integration into the host chromosomal DNA of cervical carcinomas of human papillomavirus type 18 (HPV-18) DNA, which is associated with up to 20% of these carcinomas. Because integration of the viral genome may be extremely important in the pathogenesis of cervical carcinoma, the aim of this study was to investigate which regions of HPV-18 DNA are integrated into the cellular DNA of cervical carcinomas.

METHODS

Southern analysis using four subgenomic probes covering the entire HPV-18 genome was used to map viral DNA integrated within cellular DNA. The polymerase chain reaction (PCR) was used to confirm the presence of specific regions of the viral genome.

RESULTS

In all 11 carcinomas there was a single major HPV-18 DNA integrant, retaining approximately 4000 bp of HPV-18 DNA, indicating that approximately half of the virus genome had been lost upon integration. Southern analysis suggested strongly that the viral breakpoint was within the E1/E2 gene boundary, with concomitant loss of part or all of the E2 ORF (open reading frame), all of the E4, E5, and L2 ORFs and part of the L1 ORF. These data were supported by the PCR results, which confirmed that the region of integrated HPV-18 DNA from nucleotides 6558 to 162 was present in all the carcinoma samples studied. Assuming that no genomic rearrangements, deletions, or insertions had occurred, 4131 bp of integrated HPV-18 DNA could be accounted for in eight cervical carcinoma samples. The results of Southern analysis also suggested that integration of HPV-18 DNA may have occurred at a specific host chromosomal site.

CONCLUSIONS

Broadly, the viral sequences retained upon HPV-18 integration resemble those found when HPV-16 is integrated. However, it appears that the HPV-18 E2 region is more consistently deleted.

Human papilloma virus type 16 E6 gene expression in oral exophytic epithelial lesions as detected by in situ rtPCR

OBJECTIVE

We adopted an in situ reverse transcriptase polymerase chain reaction method of detecting and determining the frequency of early (E6) gene expression of human papilloma virus type 16 at the individual cell level in a sample of oral exophytic lesions with various degrees of epithelial hyperplasia and dysplasia in immunosuppressed and immunocompetent patients.

STUDY DESIGN

The significance of differences between the study groups was determined by Mantel-Haenszel chi-square analysis and calculation of odds ratios, accounting for immunosuppression and degree of dysplasia, respectively.

RESULTS

Grouped together, the lesions of dysplasia (mild to severe) and squamous cell carcinoma were found to be 16 times more likely to express human papilloma virus E6 mRNA than the benign lesions (P = .0013); in the lesions of immunosuppressed patients, human papilloma virus 16 E6 was roughly 10 times more likely to be expressed than in those of the immunocompetent patients (P = .0008, accounting for dysplasia).

CONCLUSIONS

Our data indicate that human papilloma virus 16 E6 gene expression, and perhaps integration of the virus in the host genome, might play a role in the development of oral neoplasia in association with immunosuppression.

Detection and identification of human papilloma viral DNA, types 16, 18, and 33, by a combination of polymerase chain reaction and a colorimetric solid phase capture hybridisation assay

A colorimetric microplate hybridization assay was developed previously to simplify detection procedures of DNA fragments resulting from polymerase chain reactions (PCR). This format has now been adapted for the simultaneous detection and identification of three human papillomavirus (HPV), types 16, 18 and 33, associated frequently with cervical cancer. This post-PCR detection system uses three type-specific capture oligonucleotides linked covalently to a single microplate well and three type-specific multibiotinylated oligonucleotidic probes for detection. It therefore offers a double specificity; the first is conferred by pairs of primers, specific of each type of virus tested, and the second, by the sets of capture and detection probes which are complementary to internal regions of the amplified DNA fragments. The detection format outperformed agarose gel electrophoresis of amplified DNA products in sensitivity and specificity. The rapidity and simplicity of this hybridisation system would justify its use in routine diagnostic examination of cervical specimens (smears and biopsies).

Interstitial deletions and intrachromosomal amplification initiated from a double-strand break targeted to a mammalian chromosome

Interstitial deletions of tumour suppressor genes and amplification of oncogenes are two major manifestations of chromosomal instability in tumour cells. The development of model systems allowing the study of the events triggering these processes is of major clinical importance. Using the properties of the I-SceI nuclease to introduce a localized double-strand break (DSB) in a mammalian chromosome carrying its target sequence, we demonstrate here that both types of mutations can be initiated by non-conservative DSB repair pathways. In our system, I-SceI activity dissociates a transfected gpt gene from its promoter, allowing the isolation of gpt- clones. Our results show that intrachromatid single-strand annealing events occur frequently, giving rise to interstitial deletions not accompanied by other chromosomal rearrangements. We also observed that, when present in the cells, extrachromosomal DNA molecules are integrated preferentially at the broken locus. Taking advantage of the insertion of the I-SceI recognition sequence telomeric to and close to the dihydrofolate reductase gene, we show that a less frequent outcome of I-SceI activity is the initiation of cycles of intrachromosomal amplification of this marker, from breaks at a site merging with the enzyme target.