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. ' 2007 Wiley-Liss, Inc.Key words: fragile site; FRA6H; FRA13E; genomic instability; chromosome rearrangements Fragile sites are specific genomic loci that are especially prone to express genomic instability. They can be visualized as gaps and breaks on metaphase chromosomes after culturing cells under conditions of replication stress. Based on their incidence in the human population, they are divided into rare fragile sites, occurring in less than 5% of all individuals, and common fragile sites being a constitutive feature of the genome of probably all individuals. 1According to the National Center for Biotechnology Information (NCBI), there are 31 rare fragile sites and 88 common fragile sites in the human genome. However, the exact number of fragile sites remains unclear, since there are no stringent criteria for inclusion and there is no regular update of the database. 2The molecular basis for the expression of rare fragile sites is the dynamic mutation of expanding CGG trinucleotide or AT-rich minisatellite sequences that after reaching a certain threshold expansion account for the observed instability. [3][4][5][6] In contrast, the molecular basis for breakage of common fragile sites is still unclear. 12 However, analysis of the identified sequences did not reveal any particular sequence structure; an ATrichness and an enrichment of DNA flexibility structures seem to be the only shared features.13,14 It has been hypothesized that the AT-richness leads to an accumulation of DNA secondary structures, which might cause a delayed replication at fragile sites. [15][16][17][18] This perturbed replication at fragile sites was shown to activate cell cycle checkpoints in an ATR-dependent manner. 19 In line with this, several targets and modifiers of the ATR-pathway, such as BRCA1, SMC1, CHK1 and the Fanconi anemia pathway proteins, were reported to be involved in maintenance of fragile sites stability. [20][21][22][23] The replication perturbation may result in doubl...

Fragile sites are specific genomic loci that are particularly prone to chromosomal breakage. Based on their incidence in the human population, they are divided into rare fragile sites occurring in less than 5% of all individuals and common fragile sites being a constitutional feature of the genome of probably all individuals. In this study, cloning of unstable DNA sequences, which have been previously genetically tagged with a marker gene, was the basis for defining the genomic localization of the common fragile site FRA11G at 11q23.3. Mapping of the fragile site with six-color fluorescence in situ hybridization (FISH) resulted in the precise genomic localization of FRA11G to a 4.5 Mb region. The chromosomal subband 11q23.3 harbors both the common fragile site FRA11G and the rare fragile site FRA11B. Here, we show that FRA11G maps 0.8 Mb proximal to the genomic region previously defined to be affected by expression of FRA11B; thus, the common and the rare fragile sites at 11q23.3 encompass distinct genomic regions. The region of FRA11G is known to be involved in somatic and germline recurrent aberrations, and it is conceivable that genetic damage resulting from this fragile site might contribute to clinical phenotypes.

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