Replication delay along FRA7H, a common fragile site on human chromosome 7, leads to chromosomal instability

Common fragile sites are specific chromosomal loci that show gaps, breaks, or rearrangements in metaphase chromosomes under conditions that interfere with DNA replication. The mechanism underlying the chromosomal instability at fragile sites was hypothesized to associate with late replication time. Here, we aimed to investigate the replication pattern of the common fragile site FRA7H, encompassing 160 kb on the long arm of human chromosome 7. Usi ng in situ hybridization on interphase nuclei, we revealed that the replica tion of this region is initiated relatively early, before 30% of S phase is completed. However, a high fraction (similar to 35%) of S-phase nuclei sho wed allelic asynchrony, indicating that the replication of FRA7H is accompl ished at different times in S phase. This allelic asynchrony is not the res ult of a specific replication time of each FRA7H allele. Analysis of the re plication pattern of adjacent clones along FRA7H by using cell population a nd two color fluorescent in situ hybridization analyses showed significant differences in the replication of adjacent clones, under normal growth cond ition and upon aphidicolin treatment. This pattern significantly differed f rom that of two nonfragile regions which showed a coordinated replication u nder both conditions. These results indicate that aphidicolin is enhancing an already existing difference in the replication time along the FRA7H regi on. Based on our replication analysis of FRA7H and on previous analysis of the common fragile site FRA3B, we suggest that delayed replication is under lying the fragility at aphidicolin-induced common fragile sites.