The Effect of the Polymorphic (AT)n Dinucleotide Repeat Length on the Fragility of the Flex1 Region within the Human Common Fragile Site FRA16D
Lee, Soo Mi Alison.
- Common fragile sites are chromosomal regions that are susceptible to breakage. The second most highly expressed common fragile site, FRA16D, is located within the WWOX tumor suppressor gene and has been associated with chromosomal instability in various cancers. Flex1 is a ~500 bp AT-rich region located within FRA16D and contains a polymorphic (AT)n dinucleotide repeat. The Flex1 region has been s... read morehown to increase chromosome fragility in a yeast model system. We used a direct repeat recombination assay in S. cerevisiae to examine the effect of the length of the perfect AT repeat embedded within the Flex1 region on chromosome breakage. We observed an increase in the rate of chromosome breakage that is dependent on the length of the perfect AT repeat. However, for the Flex1 sequences that lack 102 bp of the 3'-end sequence that is predicted to easily extrude to form a 17 bp hairpin, the rate of breakage was significantly lower. These results indicate that the 17 bp hairpin-prone sequence strongly inhibits recovery of broken chromosomes in the assay. Though the exact mechanism of inhibition is unknown, a potential mechanism is that the 17 bp hairpin interferes with resection at a double stranded break needed to expose complementary single stranded DNA for homologous recombination. The main finding of this study is that there is a correlation between the rate of chromosome breakage and the secondary structure-forming propensities among the perfect AT repeats within the AT-rich Flex1 sequence. Therefore, we propose that a hairpin (from a single-stranded DNA) and/or cruciform (from dsDNA) structure formed by the AT repeat in the Flex1 region contributes to chromosome breakage.read less