Population cytogenetics of aphidicolin-induced fragile sites

Cytogenetic characterization of HB2 epithelial cells from the human breast

HB2 is a cell line originated by subcloning of MTSV1-7 mammary luminal epithelial cells isolated from human milk and immortalization via introduction of the gene encoding simian virus 40 (SV40) large T antigen. Despite its wide utilization as non-neoplastic counterpart in assays aimed to elucidating various biochemical and genetical aspects of normal and tumoral breast cells, to our knowledge no literature data have so far appeared concerning the chromosomal characterization of the HB2 cells. Here, we report the cytogenetic characterization of the karyotype of HB2 cells, which puts in evidence the occurrence of changes in chromosomal number and structure and the presence of unidentified chromosomal markers in variable amount. Our results do not detract from the utility of HB2 cells in illustrating fundamental aspects of breast cell biology, but rather interject a note of caution into generalizing results obtained with this cell line to other non-immortalized epithelial cell populations from the human breast. Therefore, this work represents a useful resource for all who want to perform appropriate and focused future studies on this cell line and proposes precise indications for a knowledgeable use of HB2 cells.

Common fragile sites in colon cancer cell lines: role of mismatch repair, RAD51 and poly(ADP-ribose) polymerase-1

Common fragile sites (CFS) are specific chromosomal areas prone to form gaps and breaks when cells are exposed to stresses that affect DNA synthesis, such as exposure to aphidicolin (APC), an inhibitor of DNA polymerases. The APC-induced DNA damage is repaired primarily by homologous recombination (HR), and RAD51, one of the key players in HR, participates to CFS stability. Since another DNA repair pathway, the mismatch repair (MMR), is known to control HR, we examined the influence of both the MMR and HR DNA repair pathways on the extent of chromosomal damage and distribution of CFS provoked by APC and/or by RAD51 silencing in MMR-deficient and -proficient colon cancer cell lines (i.e., HCT-15 and HCT-15 transfected with hMSH6, or HCT-116 and HCT-116/3+6, in which a part of a chromosome 3 containing the wild-type hMLH1 allele was inserted). Here, we show that MMR-deficient cells are more sensitive to APC-induced chromosomal damage particularly at the CFS as compared to MMR-proficient cells, indicating an involvement of MMR in the control of CFS stability. The most expressed CFS is FRA16D in 16q23, an area containing the tumour suppressor gene WWOX often mutated in colon cancer. We also show that silencing of RAD51 provokes a higher number of breaks in MMR-proficient cells with respect to their MMR-deficient counterparts, likely as a consequence of the combined inhibitory effects of RAD51 silencing on HR and MMR-mediated suppression of HR. The RAD51 silencing causes a broader distribution of breaks at CFS than that observed with APC. Treatment with APC of RAD51-silenced cells further increases DNA breaks in MMR-proficient cells. The RNAi-mediated silencing of PARP-1 does not cause chromosomal breaks or affect the expression/distribution of CFS induced by APC. Our results indicate that MMR modulates colon cancer sensitivity to chromosomal breaks and CFS induced by APC and RAD51 silencing.

Cloning of genetically tagged chromosome break sequences reveals new fragile sites at 6p21 and 13q22

Fragile sites are specific genomic loci that are especially prone to chromosome breakage. For the human genome there are 31 rare fragile sites and 88 common fragile sites listed in the National Center for Biotechnology Information database; however, the exact number remains unknown. In this study, unstable DNA sequences, which have been previously tagged with a marker gene, were cloned and provided starting points for the characterization of two aphidicolin inducible common fragile sites. Mapping of these unstable regions with six-color fluorescence in situ hybridization revealed two new fragile sites at 6p21 and 13q22, which encompass genomic regions of 9.3 and 3.1 Mb, respectively. According to the fragile site nomenclature they were consequently entitled as FRA6H and FRA13E. Both identified regions are known to be associated with recurrent aberrations in malignant and nonmalignant disorders. It is conceivable that these fragile sites result in genetic damage that might contribute to cancer phenotypes such as osteosarcoma, breast and prostate cancer.

Novel aphidicolin-inducible common fragile siteFRA9G maps to 9p22.2, within theC9orf39 gene

Common fragile sites represent a component of normal chromosome structure that form gaps and breaks on metaphase chromosomes after partial inhibition of DNA synthesis. In humans, cytogenetic locations of 89 common fragile sites are listed in the Genome Database; however, the exact number of fragile sites remains unknown. The application of high resolution mapping approaches continues to reveal new common fragile sites in the human genome. Here, we identified a novel aphidicolin-inducible common fragile site FRA9G, which maps to chromosomal band 9p22.2. We have characterized the structure of the fragile DNA sequence that extends over a genomic region of approximately 300 kb within the C9orf39 (chromosome 9 open reading frame 39) gene. Analysis of incidence in healthy individuals showed that FRA9G is commonly expressed in the population. Heterozygous BRCA2 mutation carriers exhibit an almost sevenfold increase of FRA9G expression compared to an unrelated control population group. Identification of a novel aphidicolin-inducible common fragile site at 9p22 may have implications for understanding the mechanism of genetic instability in tumorigenesis and other genetic disorders.

Low-frequency common fragile sites: Link to neuropsychiatric disorders?

Common fragile sites are unstable chromosomal regions that predispose chromosomes to breakage and rearrangements. Recombinogenic DNA sequences encompassing these sites may contribute to both germinal and somatic genomic mutations, and the genomic instability at these regions might cause severe inherited disorders or predispose to cancer. In this review, we discuss the characterization of common fragile site FRA13A within the neurobeachin gene, which is involved in development and function of the central nervous system. We raise the possibility of an implication of common fragile sites in neuropsychiatric disorders and overview previous and recent reports concerning individual variability of expression of common fragile sites in human populations.

Loss of heterozygosity at 3p in benign lesions preceding invasive breast cancer

BACKGROUND

Loss of heterozygosity (LOH) at chromosome 3p is one of the most common genetic abnormalities identified in human cancers and has occasionally been noted in benign proliferative lesions predisposing to breast cancer. If the frequency of LOH at 3p in benign proliferative lesions correlates with the subsequent development of breast cancer, it may be possible to develop powerful tools for molecular risk assessment based on this technology.

MATERIALS AND METHODS

Archival paraffin-embedded tissues from benign breast biopsies in five women who have developed breast cancer and three women who have not developed breast cancer were microdissected and allelotyped at 3p using six microsatellite markers.

RESULTS

No LOH was detected in the biopsies from women who have not developed breast cancer. For women developing breast cancer, the proportion of informative loci showing LOH in the benign proliferative lesions was 0.47 as compared to 0.57 for the associated breast cancers. There was no LOH detected in epithelial DNA from a fibroadenoma. Of 15 informative loci, 4 (27%) showed LOH in both the benign proliferative lesion and the associated cancer; however, the actual parental allele lost was different in three of these four cases.

CONCLUSIONS

These results suggest that there are specific patterns of genetic instability common to preneoplastic lesions and the breast cancers that subsequently develop even when the paired lesions are not clonally related. LOH analysis of benign breast epithelium may provide a tool for molecular risk assessment and a surrogate endpoint for breast cancer chemoprevention trials.

Instability at chromosomal fragile sites

Chromosomal fragile sites are loci that are especially prone to forming gaps or breaks on metaphase chromosomes when cells are cultured under conditions that inhibit DNA replication or repair. The relationship of "rare" folate sensitive fragile sites with (CCG)n expansion and, in some cases, genetic disease is well established. Although they comprise the vast majority of fragile sites, much less is known at the molecular level about the "common" fragile sites. These fragile sites may be seen on all chromosomes as a constant feature. In addition to forming fragile sites on metaphase chromosomes, they have been shown to display a number of characteristics of unstable and highly recombinogenic DNA in vitro, including chromosome rearrangements, sister chromatid exchanges and, more recently, intrachromosomal gene amplification. Only one such fragile site, FRA3B at 3p14.2, has been extensively investigated at the molecular level. It extends over a broad region of possibly 500 kb, and no trinucleotide or other simple repeat motifs have been identified in the region. FRA3B has recently been shown to lie within the FHIT gene locus. This region and the FHIT gene are unstable in a number of tumors and tumor cell lines. It thus appears that common fragile sites are also associated with unstable regions of DNA in vivo, at least in some tumor cells, and may cause this instability. Current challenges include determining the mechanism of fragile site expression and instability, and both the environmental and genetic factors that influence this process. Candidate factors include those genes involved in DNA repair and cell cycle and common carcinogens such as those in cigarette smoke.

Bleomycin-induced chromosome aberrations in lymphocytes derived from patients with lamellar ichthyosis

Patients affected by some genetic skin defects, for example, dyskeratosis congenita or scleroderma, may present spontaneous or induced chromosomal fragility. Hence we performed a cytogenetic analysis in families of patients affected by lamellar ichthyosis, an autosomal recessive disease not yet fully characterized at the cellular and molecular levels. Chromosomal fragility was assayed in untreated lymphocyte cultures and in those supplemented with aphidicolin or bleomycin. Cells from some affected patients and some of their parents showed hypersensitivity to the radiomimetic agent bleomycin.

Spontaneous breakage and fragile site expression in chronic lymphocytic leukemia

It has been suggested that genetic predisposition to cancer might be related to spontaneous chromosome instability or to fragile site expression. Therefore, spontaneous breakage and fragile sites were analyzed in nine untreated chronic lymphocytic leukemia (CLL) patients to determine their relation to cancer rearrangements. Five cases presented spontaneous gaps and breaks with a random distribution of breakpoints. In cultures treated with fluorodeoxyuridine or aphidicolin, 29 specific bands could be defined as fragile sites. A significant clustering of these sites was found with known common fragile sites (c-fra) and cancer breakpoints described in the literature. Most of these cancer breakpoints were involved in structural abnormalities associated with CLL (p < 0.00001). These data suggest that the expression of specific fragile sites might be related to structural chromosomal aberrations in CLL.

Chromosome imbalance at the 3p14 region in human breast tumours: high frequency in patients with inherited predisposition due to BRCA2

Our previous studies have indicated that genetic aberrations in the 3p14 region are more frequent in malignant tumours from hereditary breast cancer patients than sporadic breast cancers. The main purpose of this study was to test if BRCA2 susceptibility alleles contribute to imbalance in the 3p14 region. We mapped allelic imbalance at 3p14 in tumours from Icelandic sisters affected with breast cancer using a set of 10 microsatellite markers (tel-D3S1295-D3S1234-D3S1300-D3S1600-D3S1233+ ++-D3S1217-D3S1261-D3S1296-D3S1210- D3S1284-cen). The patients were of known carrier status with respect to the 999del5 mutation in BRCA2 which is the most common cause of hereditary breast cancer in Iceland. Of 103 patients, 32 in the group were mutation carriers. A high degree of imbalance was observed in tumours from BRCA2 mutation carriers, ranging from 44 to 88% for individual markers. This was significantly higher than the percentage of imbalance in tumours from non-carriers, where the frequency ranged from 25 to 43%. In both groups, we noted elevated 3p14 imbalance in patients with bilateral disease. Allelic imbalance was most commonly observed near the marker D3S1210 (3p14.1-p12) and the FHIT gene (3p21.1-p14.2) for both groups. We conclude that genomic aberrations in 3p14 are especially frequent in tumours with BRCA2 gene defects, and suggest that this is caused by regional loss of chromosome stability rather than selection.

An IVF fallacy: multiple pregnancy risk is lower for older women

INTRODUCTION

Multiple pregnancy is one of the most important and preventable complications of in vitro fertilization (IVF) and embryo transfer. The general clinical practice in many IVF clinics is to transfer four or five embryos to older women if available, since pregnancy rates are lower in women older than 35 years of age. However, it is not clear whether the risk for multiple pregnancy is also lower.

OBJECTIVE

Our objective was to investigate whether transferring a higher number of embryos actually improves pregnancy outcome in older women, without increasing the risk for multiple pregnancy and to investigate other factors that may affect the occurrence of multiple pregnancy.

SETTING

The setting was university-based IVF program at The Toronto Hospital.

DESIGN

The design was a retrospective case series.

PATIENTS AND METHODS

The outcome of 1116 IVF cycles between January 1992 and December 1993 was investigated according to different age groups.

MAIN OUTCOME MEASURE

The main outcome measure was multiple pregnancies.

RESULTS

Seventy multiple pregnancies resulted from a total of 242 pregnancies. Overall pregnancy and multiple pregnancy rates were inversely correlated with age. However, when the data were adjusted for the number of embryos transferred, this trend disappeared. The result of multiple regression analysis showed that the multiple pregnancy rate was higher without improving the pregnancy rate when the number of embryos transferred exceeded three, regardless of the age of the patients, especially when more embryos were available than the number of transferred ones.

CONCLUSIONS

The number of embryos transferred should be limited to a maximum of three regardless of the age of patients, to reduce the high frequency of multiple gestations in an IVF program.

The enigma of common fragile sites

One hundred and fifty breakpoint sites were recorded during an analysis of aphidicolin-ethanol inducible fragile sites (FS) in 56 blood samples and 13 amniocyte cultures and were classified according to the criteria formulated by the Chromosome Coordinating Meeting. The finding of previously unlisted FS in this sample, the altered expression of FS in conditions not usually associated with chromosomal abnormalities and the apparent lack of tissue specificity indicate the importance of one or more fundamental mechanisms operating to produce the diverse associated clinical phenotypes, with the chromosomal fragility representing an intermediate phenotype. Several lines of evidence converge towards the conclusion that FS are a manifestation of an altered state of genetic activity at areas associated with transcriptional regulation, because of their concordance with CpG islands, nuclease sensitive sites, replication origins, zinc finger protein domains and viral integration sites. An investigation is required whether this phenomenon could contribute both to evolutionary diversity through increased recombination, the formation of unstable repeat sequences and variable methylation, and to the expression of multigene disease processes resulting in the production of variable and complex phenotypes, even within families.

Increased expression of aphidicolin-induced common fragile sites in Tourette syndrome: the key to understand the genetics of comorbid phenotypes?

In a comparison of 80 common aphidicolin-induced fragile sites (FS) between 26 DSM-IV Tourette syndrome (TS) and 24 control individuals, the mean of the summed break frequencies following mild aphidicolin pretreatment was significantly higher in TS individuals than in controls (P < 0.001). Other breakpoints encountered during this study, i.e., random breaks, breaks corresponding to rare FS, and breakpoints recorded by others but not listed as common FS according to the Chromosome Coordinating Meeting [1992] were listed as category II breakpoints. By using the most significantly different mean FS breakage figures between TS and control individuals, further stepwise discriminant analysis allowed identification of TS individuals from only a few sites in both the common FS and category II breakpoint groups. Future research needs to focus on confirmation of altered common fragile site expression in association with behavioral variation, whether expression of certain discriminatory sites concurs with specific comorbid disorder expression; the nature of the molecular alterations at these FS and the implications of a genomic instability phenotype for the mapping of a primary TS gene or genes.

Characteristic chromosomal fragility of human embryonic cells exposed in vitro to aphidicolin

The frequency and distribution of aphidicolin (APC)-induced common fragile sites (cfs) were analyzed in human embryonic cells of different origins. Embryonic lung fibroblasts (MRC-5), amniocytes (AMINO) and embryonic retina cells (HERO790) are as sensitive to the APC-induced clastogenic effect as peripheral lymphocytes, whereas embryonic kidney cells (HEK) seem more resistant to the induction of chromosomal gaps and breaks by the drug. Analysis of the distribution of fragile sites confirmed that the expression of specific APC-induced cfs varies in different cells and that the embryonic cell strains show a greater similarity among themselves than to lymphocytes. In addition, HEK, MRC-5, HERO790 and AMINO cells show specific APC induction of the cfs at the 1p31.2 chromosomal band, which seems to be a distinctive feature of the embryonic stage of cells.

Expression of common fragile sites in untreated non-Hodgkin's lymphoma with aphidicolin and folate deficiency

The frequency and distribution of aphidicolin induced and folate sensitive common fragile sites on chromosomes of peripheral blood lymphocytes in untreated non-Hodgkin's lymphoma patients and healthy controls showed a considerable overlap in the expression of common fragile sites between the two groups. However, a significant increase in the expression of 16 aphidicolin induced common fragile sites was seen in untreated lymphoma patients. In the folate deficient cultures only, the common fragile sites 2q22, 8q24, 11q13, 12q21, 16q22, 17p12 and 20p12 were found in both the groups under study. The fragile sites at 8q22, 8q24, 11q13 and 18q21 in patients showed an increased expression over the control group. Interestingly these fragile sites were located in the same chromosomal bands as the oncogenes, MOS, MYC, BCL-1 and BCL-2 as well as cancer breakpoints specifically associated with non-Hodgkin's lymphoma, suggesting the possibility that fragile sites may play a critical role in the pathogenesis of non-Hodgkin's lymphoma.

Increased chromosome fragility in lymphocytes of short normal children treated with recombinant human growth hormone

A few years ago it was reported that some growth-hormone-deficient children had developed leukemia following therapy with human growth hormone. This raised concern that this therapy may stimulate tumor development. Since it is known that the tendency to develop cancer is closely related to chromosome breakage, we decided to investigate whether recombinant human growth hormone (rhGH) therapy can increase chromosome fragility. Ten short normal children were studied during their first year of treatment. Lymphocytes were collected at 0, 6 and 12 months of rhGH therapy, and we assessed the rate of spontaneous chromosome aberrations, the frequency of sister chromatid exchanges, the proliferative rate indices, the expression of common fragile sites induced by aphidicolin, and the sensitivity towards the radiomimetic action of bleomycin. At 6 months of therapy, there was a significant increase in bleomycin-induced chromosome aberrations, which remained unchanged after 1 year of treatment. An increase in spontaneous chromosome rearrangements at 6 and 12 months of therapy was also observed. These findings are further supported by data obtained from the analysis of 16 short normal children already on rhGH therapy.