Polycomb group (PcG) proteins are involved in epigenetic silencing where they function as major determinants of cell identity, stem cell pluripotency and the epigenetic gene silencing involved in cancer development. Recently numerous PcG proteins, including CBX4, have been shown to accumulate at sites of DNA damage. However, it remains unclear whether or not CBX4 or its E3 sumo ligase activity is directly involved in the DNA damage response (DDR). Here we define a novel role for CBX4 as an early DDR protein that mediates SUMO conjugation at sites of DNA lesions. DNA damage stimulates sumoylation of BMI1 by CBX4 at lysine 88, which is required for the accumulation of BMI1 at DNA damage sites. Moreover, we establish that CBX4 recruitment to the sites of laser micro-irradiation-induced DNA damage requires PARP activity but does not require H2AX, RNF8, BMI1 nor PI-3-related kinases. The importance of CBX4 in the DDR was confirmed by the depletion of CBX4, which resulted in decreased cellular resistance to ionizing radiation. Our results reveal a direct role for CBX4 in the DDR pathway.

Phosphorylation of histone H2AX on serine 139 (gamma-H2AX, γH2AX) occurs at sites flanking DNA double-strand breaks (DSBs) and can provide a measure of the number of DSBs within a cell. Here we describe a rapid and simple flow-cytometry-based method, optimized to measure gamma-H2AX in non-fixed peripheral blood cells. No DSB induced signal was observed in H2AX−/− cells indicating that our FACS method specifically recognized gamma-H2AX accumulation. The gamma-H2AX assay was capable of detecting DNA damage at levels 100-fold below the detection limit of the alkaline comet assay. The gamma-H2AX signal was quantitative with a linear increase of the gamma-H2AX signal over two orders of magnitude. We found that all nucleated blood cell types examined, including the short-lived neutrophils induce gamma-H2AX in response to DSBs. Interindividual difference in the gamma-H2AX signal in response to ionizing radiation and the DSB-inducing drug calicheamicin was almost 2-fold in blood cells from patients, indicating that the amount of gamma-H2AX produced in response to a given dose of radiation varies significantly in the human population. This simple method could be used to monitor response to radiation or DNA-damaging drugs.

Direct modulation of gene expression by targeting oncogenic transcription factors is a new area of research for cancer treatment. ERG, an ETS-family transcription factor, is commonly over-expressed or translocated in leukaemia and prostate carcinoma. In this work, we selected the di-(thiophene-phenyl-amidine) compound DB1255 as an ERG/DNA binding inhibitor using a screening test of synthetic inhibitors of the ERG/DNA interaction followed by electrophoretic mobility shift assays (EMSA) validation. Spectrometry, footprint and biosensor-surface plasmon resonance analyses of the DB1255/DNA interaction evidenced sequence selectivity and groove binding as dimer. Additional EMSA evidenced the precise DNA-binding sequence required for optimal DB1255/DNA binding and thus for an efficient ERG/DNA complex inhibition. We further highlighted the structure activity relationships from comparison with derivatives. In cellulo luciferase assay confirmed this modulation both with the constructed optimal sequences and the Osteopontin promoter known to be regulated by ERG and which ERG-binding site was protected from DNaseI digestion on binding of DB1255. These data showed for the first time the ERG/DNA complex modulation, both in vitro and in cells, by a heterocyclic diamidine that specifically targets a portion of the ERG DNA recognition site.

Human polynucleotide kinase/phosphatase (PNKP) is a dual specificity 5′-DNA kinase/3′-DNA phosphatase, with roles in base excision repair, DNA single-strand break repair and non-homologous end joining (NHEJ); yet precisely how PNKP functions in the repair of DNA double strand breaks (DSBs) remains unclear. We demonstrate that PNKP is phosphorylated by the DNA-dependent protein kinase (DNA-PK) and ataxia-telangiectasia mutated (ATM) in vitro. The major phosphorylation site for both kinases was serine 114, with serine 126 being a minor site. Ionizing radiation (IR)-induced phosphorylation of cellular PNKP on S114 was ATM dependent, whereas phosphorylation of PNKP on S126 required both ATM and DNA-PK. Inactivation of DNA-PK and/or ATM led to reduced PNKP at DNA damage sites in vivo. Cells expressing PNKP with alanine or aspartic acid at serines 114 and 126 were modestly radiosensitive and IR enhanced the association of PNKP with XRCC4 and DNA ligase IV; however, this interaction was not affected by mutation of PNKP phosphorylation sites. Purified PNKP protein with mutation of serines 114 and 126 had decreased DNA kinase and DNA phosphatase activities and reduced affinity for DNA in vitro. Together, our results reveal that IR-induced phosphorylation of PNKP by ATM and DNA-PK regulates PNKP function at DSBs.

Heterocyclic diamidines are compounds with antiparasitic properties that target the minor groove of kinetoplast DNA. The mechanism of action of these compounds is unknown, but topological changes to DNA structures are likely to be involved. In this study, we have developed a polyacrylamide gel electrophoresis-based screening method to determine topological effects of heterocyclic diamidines on four minor groove target sequences: AAAAA, TTTAA, AAATT and ATATA. The AAAAA and AAATT sequences have the largest intrinsic bend, whereas the TTTAA and ATATA sequences are relatively straight. The changes caused by binding of the compounds are sequence dependent, but generally the topological effects on AAAAA and AAATT are similar as are the effects on TTTAA and ATATA. A total of 13 compounds with a variety of structural differences were evaluated for topological changes to DNA. All compounds decrease the mobility of the ATATA sequence that is consistent with decreased minor groove width and bending of the relatively straight DNA into the minor groove. Similar, but generally smaller, effects are seen with TTTAA. The intrinsically bent AAAAA and AAATT sequences, which have more narrow minor grooves, have smaller mobility changes on binding that are consistent with increased or decreased bending depending on compound structure.

Plants produce small cysteine-rich antimicrobial peptides as an innate defense against pathogens. Based on amino acid sequence homology, these peptides were classified mostly as α-defensins, thionins, lipid transfer proteins, cyclotides, snakins and hevein-like. Although many antimicrobial plant peptides are now well characterized, much information is still missing or is unavailable to potential users. The compilation of such information in one centralized resource, such as a database would therefore facilitate the study of the potential these peptide structures represent, for example, as alternatives in response to increasing antibiotic resistance or for increasing plant resistance to pathogens by genetic engineering. To achieve this goal, we developed a new database, PhytAMP, which contains valuable information on antimicrobial plant peptides, including taxonomic, microbiological and physicochemical data. Information is very easy to extract from this database and allows rapid prediction of structure/function relationships and target organisms and hence better exploitation of plant peptide biological activities in both the pharmaceutical and agricultural sectors. PhytAMP may be accessed free of charge at http://phytamp.pfba-lab.org.