New Publication: Failure of Origin Activation in Response to Fork Stalling Leads to Chromosomal Instability at Fragile Sites

Molecular Cell, Volume 43, Issue 1, 122-131, 8 July 2011

Efrat Ozeri-Galai¹, Ronald Lebofsky^{2, 3}, Ayelet Rahat^{1, 4}, Assaf C. Bester¹, Aaron Bensimon^{2, 5} and Batsheva Kerem^{1, , 1} Department of Genetics, The Life Sciences Institute, Edmond J. Safra Campus, The Hebrew University, Jerusalem 91904,    Israel

Highlights

► Forks stall at AT-rich sequences in FRA16C ► Origin density at FRA16C is higher than the entire genome ► Under stress FRA16C fails to activate additional origins for replication completion ► Decrease in potential origins by MCM inhibition increases fragile site expression

Graphical Abstract
Summary

Perturbed DNA replication in early stages of cancer development induces chromosomal instability preferentially at fragile sites. However, the molecular basis for this instability is unknown. Here, we show that even under normal growth conditions, replication fork progression along the fragile site, FRA16C, is slow and forks frequently stall at AT-rich sequences, leading to activation of additional origins to enable replication completion. Under mild replication stress, the frequency of stalling at AT-rich sequences is further increased. Strikingly, unlike in the entire genome, in the FRA16C region additional origins are not activated, suggesting that all potential origins are already activated under normal conditions. Thus, the basis for FRA16C fragility is replication fork stalling at AT-rich sequences and inability to activate additional origins under replication stress. Our results provide a mechanism explaining the replication stress sensitivity of fragile sites and thus, the basis for genomic instability during early stages of cancer development.