The Cellular Response to DNA Double Strand Break Damage During Mitosis in Human Cells

DNA Double strand breaks (DSBs) represent a major threat to genomic integrity. Without timely, thorough resolution, DSBs can yield translocations, mutations, and ultimately tumorigenesis and cell death. Cells have developed elegant and elaborate means to address DSBs. Considerable effort has been expended towards elucidating the molecular nature and parameters of this response during interphase. Yet few studies address how cells cope with DSBs that arise during mitosis. Despite major advances in understanding the fate of DSBs in G1 and S/G2 phases, the cellular response to mitotic DSBs remains largely unknown.;Previously studies showed that laser induced DSB on mitotic chromosomes are conducive to aH2AX formation in animal cells. This phenomenon was repeatable and sufficient to activate the spindle checkpoint in a considerable portion of cells. Since gammaH2AX is typically associated with a robust DNA damage response in Interphase (2), it is logical that global activation of DSB damage recognition/repair occurs in response to mitotic DSBs.;Recent studies indicate extensive DSB recognition and checkpoint signaling during mitosis. We therefore hypothesized: (1) DSB response factors in addition to gammaH2AX can form foci on laser induced mitotic DSBs (2) DSB repair pathway activation can occur upon mitotic DSBs induction. (3) Cells with mitotic DSBs will exhibit DSB repair pathway selection preference as in Interphase. (4) The kinetics of the response to mitotic DSBs differs significantly from that of the interphase DSB response.;To this end we utilized nanosecond UVA optical scissors as a tool to induce DSBs on mitotic chromosomes. Immunostaining was then used to probe irradiated cells for DSB repair pathway factors to see if repair pathway activation occurs upon mitotic DSB induction. Our results indicate both Non Homologous End Joining (NHEJ) and Homologous Recombination (HR) pathways can be activated upon mitotic DSB induction. However a delay in the activation of the HR pathway occurs, possibly due to reduced end processing, preventing extensive activation prior to completion of mitosis. Together these results indicate a strong role for NHEJ and a limited role for HR in DSB resolution during mitosis.