A tissue-specific fragile site associated with the sex reversed (Sxr) mutation in the mouse

Direct cloning and analysis of DNA sequences from a region of the Chinese hamster genome associated with aphidicolin-sensitive fragility

Fragile sites are reproducibly expressed and chemically induced decondensations on mitotic chromosomes observed under cytological conditions. They are classified both on the basis of the frequency with which they occur (rare and common) and in terms of the chemical agent used to induce expression in tissue culture cells. Aphidicolin-sensitive common fragile sites appear to be ubiquitous in humans and other mammals and have been considered as candidates of pathological importance. Recently DNA from FRA3B, the most highly expressed constitutive fragile site in the human genome, has been cloned although as yet the cause of the underlying fragility has not been identified. In this study we describe the isolation, using a direct cloning approach, of DNA from a region of the Chinese hamster genome associated with aphidicolin-inducible fragility. Cells of a human-hamster somatic cell hybrid were transfected with a pSV2HPRT vector while exposed to aphidicolin, an inhibitor of DNA polymerases alpha, delta and epsilon. FISH analysis of stable transfectant clones revealed that the ingoing plasmid DNA had preferentially integrated into fragile site-containing chromosomal bands. Plasmid rescue was used to recover DNA sequences flanking one such integration site in the hamster genome. We demonstrate by FISH analysis of metaphase cells induced with aphidicolin that the rescued DNA is from a region of fragility on Chinese hamster chromosome 2, distal to the DHFR locus. Analysis of the DNA sequences flanking the integration site revealed the overall A+T content of the 3,725 bp region sequenced to be 63.3%, with a highly [A].[T]-rich 156 bp region (86.5%) almost adjacent to the integration site. Computational analyses have identified strong homologies to Saccharomyces cerevisiae autonomous replicating sequences (ARS), polypyrimidine tracts, scaffold attachment site consensus sequences and a 24 bp consensus sequence highly conserved in eukaryotic replication origins, all of which appear to cluster around the [A].[T]-rich sequences. This domain also possesses structural characteristics which are common to both prokaryotic and eukaryotic origins of replications, in particular an unusually straight conformation of low thermal stability flanked either side by highly bent DNA segments. Further isolation and characterisation of DNA sequences from common fragile sites will facilitate studies into the underlying nature of these enigmatic regions of the mammalian genome, leading to a greater understanding of chromatin structure.

Folate-sensitive and aphidicolin-inducible fragile sites are expressed in the genome of the domestic cat

Peripheral blood lymphocytes from three clinically normal domestic cats were cultured for folate-sensitive and aphidicolin-inducible fragile site expression. Induction of folate-sensitive fragile sites was accomplished by culturing cells with trimethoprim plus caffeine. Chromosomes from all cats expressed both folate-sensitive and aphidicolin-inducible breaks and gaps. There were no significant differences between the two methods of fragile site induction in the percentage of cells expressing chromosome breaks and gaps or the mean number of breaks and gaps per cell. All three cats expressed specific chromosome breaks resembling fragile sites at A1q21-22, A1p22, and B1q32. All three sites were induced by aphidicolin. The sites at A1q21-22 and B1q32 were also induced in folate-deficient medium. This is the first report of the induction of chromosomal fragile sites in a feline species.

Chromosomal fragile site expression in dogs: I. Breed specific differences

Peripheral blood lymphocytes from clinically normal Doberman pinscher and boxer dogs were cultured for folate-sensitive and, in preliminary studies, aphidicolin-inducible fragile site expression. Both autosomal and X chromosomal fragile sites were observed in canine cells cultured under folate/thymidine depletion and in cells cultured in medium containing aphidicolin. Results from the three dogs evaluated for both folate-sensitive and aphidicolin-inducible fragile site expression showed that the frequency of fragile site expression was significantly (P less than 0.05) greater in cells cultured in medium containing aphidicolin than in cells cultured in folate/thymidine-depleted medium. Cells from the boxer dog expressed a high percentage (66.67%) of aphidicolin-inducible fragile sites in contrast to the Doberman pinscher dog in which only 21.10% of the lymphocytes expressed aphidicolin-inducible fragile sites. The frequencies of spontaneous and folate-sensitive fragile site expression did not vary significantly by breed of dog. Age of dog was significantly and positively correlated with frequency of folate-sensitive fragile site expression in dogs of the boxer breed, but not in dogs of the Doberman pinscher breed. The dog X chromosome expressed three folate-sensitive and aphidicolin-inducible fragile sites. The G-band location of these three fragile sites showed homology with three recognized constitutive common fragile sites on the human X chromosome: Xp22, Xq21, and Xq27.2. Two specific autosomal fragile sites were identified, one on the distal end of the long arm of chromosome 1 and one on the distal end of the long arm of chromosome 8. Other autosomal fragile sites were also apparent but could not be assigned reliably to specific chromosomes.

Rodent common fragile sites: are they conserved? Evidence from mouse and rat

Nineteen fragile sites induced by aphidicolin in lymphocyte cultures from the laboratory mouse are documented. These sites are compared with previously described fragile sites induced in mouse fibroblast systems, and then with those reported on chromosomes which have been evolutionarily conserved between the mouse and the laboratory rat. Of a total of 38 fragile sites thus far identified in mouse fibroblasts and lymphocytes, only 4 sites are common to the two cell types; 34 sites show no correspondence of loci. The reason for this discrepancy is unclear, but it is possible that these data may indicate some degree of tissue specificity of fragile site expression in the mouse. Eight autosomes in the mouse and rat retain straightforward and nearly complete banding homology. To test the hypothesis that fragile sites are conserved between the two species, we compared these eight autosomes with regard to number and distribution of fragile site loci. A total of 30 fragile sites is distributed over the conserved chromosomes. Only 4 (possibly 5) are common to both species; 18 are found in the rat but not the mouse, and 4 are found in the mouse but not the rat. Of the 4 shared sites, notable differences in frequency of expression exist. Our comparisons show that: (1) a small number of fragile sites is conserved; (2) a large number of fragile sites is not conserved, and (3) some sites which are conserved are quite different in the frequency at which they are expressed in the two species, indicating that the sites themselves may have undergone evolutionary change.(ABSTRACT TRUNCATED AT 250 WORDS)