Phosphorylation of the histone variant H2AX forms γ-H2AX that marks DNA double-strand break (DSB). Here, we generated the sequencing-based maps of H2AX and γ-H2AX positioning in resting and proliferating cells before and after ionizing irradiation. Genome-wide locations of possible endogenous and exogenous DSBs were identified based on γ-H2AX distribution in dividing cancer cells without irradiation and that in resting cells upon irradiation, respectively. γ-H2AX-enriched regions of endogenous origin in replicating cells included sub-telomeres and active transcription start sites, apparently reflecting replication- and transcription-mediated stress during rapid cell division. Surprisingly, H2AX itself, prior to phosphorylation, was specifically located at these endogenous hotspots. This phenomenon was only observed in dividing cancer cells but not in resting cells. Endogenous H2AX was concentrated on the transcription start site of actively transcribed genes but was irrelevant to pausing of RNA polymerase II (pol II), which precisely coincided with γ-H2AX of endogenous origin. γ-H2AX enrichment upon irradiation also coincided with actively transcribed regions, but unlike endogenous γ-H2AX, it extended into the gene body and was not specifically concentrated on the pausing site of pol II. Sub-telomeres were less responsive to external DNA damage than to endogenous stress. Our findings provide insight into DNA repair programs of cancer and may have implications for cancer therapy.

Block copolymers that combine a side-group liquid crystalline polymer (SGLCP) block and a pH-responsive hydrophilic block, poly(acrylic acid) (PAA), are shown to confer pH-dependent anchoring of the director orientation at the aqueous/LC interface. The SGLCP block, poly-(4-cyanobiphenyl-4-oxyundecylacrylate), was chosen based on its ability to influence the director field of the 5CB (4-cyano-4 0 -pentylbiphenyl). At low pH the PAA block collapses and the inherent, planar alignment tendency of 5CB at a water interface prevails. As pH increases, the polyelectrolyte block becomes increasingly charged and expands, producing a change to homeotropic anchoring. The change in anchoring occurs as quickly as the buffer can be changed (within $2 s) and is reversible, with a response that is repeatable over as many cycles as were tested (approximately 20 cycles). The polymer-mediated anchoring persists for 6 days, indicating that the SGLCP block secures the self-assembled layer on the 5CB, even under conditions that cause repulsive interactions among the PAA blocks. Thus, SGLCP blocks can translate conformational changes of a responsive hydrophilic block into rapid, reversible changes in the director field.