Mechanical evolution of DNA double-strand breaks in the nucleosome

Summary Double strand breaks in the DNA backbone are the most lethal type of defect that can be induced in the cell nucleus by chemical and radiation treatments of cancer. However, little is known about the potentially large differences in the outcomes of damage between free and nucleosomal DNA, leading to corresponding differences in damage repair capability. We performed microsecond-length molecular dynamics computer simulations of nucleosomes including double-strand breaks (DSB) at various sites, to characterize the early stages of the evolution of this important DNA lesion right after its formation. We find that all DSB configurations tend to remain compact, with only the terminal bases interacting with histone proteins; the interacting molecular structures are studied by looking at the essential dynamics of the relevant DNA and histone fragments, and compared to the intact nucleosome, thus exposing key features of the interactions. Moreover, we show that the broken DNA ends at the DSB must overcome a free-energy barrier to detach from the nucleosome core, as measured by means of umbrella sampling of the potential of mean force. Finally, by using state-of-the-art calculation of the covariant mechanical stress at the molecular scale, we demonstrate that, depending on the DNA-core separation distance, the coupled bending and torsional stress stored in the detached DNA can force the free end to either stick back to the nucleosome core surface, or to open up straight, thus making it accessible to damage signalization proteins.