Sequence conservation at human and mouse orthologous common fragile regions, FRA3B/FHIT and Fra14A2/Fhit

It has been suggested that delayed DNA replication underlies fragility at common human fragile sites, but specific sequences responsible for expression of these inducible fragile sites have not been identified. One approach to identify such cis-acting sequences within the large nonexonic regions of fragile sites would be to identify conserved functional elements within orthologous fragile sites by interspecies sequence comparison. This study describes a comparison of orthologous fragile regions, the human FRA3B/FHIT and the murine Fra14A2/Fhit locus. We sequenced over 600 kbp of the mouse Fra14A2, covering the region orthologous to the fragile epicenter of FRA3B, and determined the Fhit deletion break points in a mouse kidney cancer cell line (RENCA). The murine Fra14A2 locus, like the human FRA3B, was characterized by a high AT content. Alignment of the two sequences showed that this fragile region was stable in evolution despite its susceptibility to mitotic recombination on inhibition of DNA replication. There were also several unusual highly conserved regions (HCRs). The positions of predicted matrix attachment regions (MARs), possibly related to replication origins, were not conserved. Of known fragile region landmarks, five cancer cell break points, one viral integration site, and one aphidicolin break cluster were located within or near HCRs. Thus, comparison of orthologous fragile regions has identified highly conserved sequences with possible functional roles in maintenance of fragility.