Transient loss of Polycomb components induces an epigenetic cancer fate.

Although cancer initiation and progression are generally associated with the accumulation of somatic mutations1,2, substantial epigenomic alterations underlie many aspects of tumorigenesis and cancer susceptibility3-6, suggesting that genetic mechanisms might not be the only drivers of malignant transformation7. However, whether purely non-genetic mechanisms are sufficient to initiate tumorigenesis irrespective of mutations has been unknown. Here, we show that a transient perturbation of transcriptional silencing mediated by Polycomb group proteins is sufficient to induce an irreversible switch to a cancer cell fate in Drosophila. This is linked to the irreversible derepression of genes that can drive tumorigenesis, including members of the JAK-STAT signalling pathway and zfh1, the fly homologue of the ZEB1 oncogene, whose aberrant activation is required for Polycomb perturbation-induced tumorigenesis. These data show that a reversible depletion of Polycomb proteins can induce cancer in the absence of driver mutations, suggesting that tumours can emerge through epigenetic dysregulation leading to inheritance of altered cell fates.

Spatiotemporal characterization of ionizing radiation induced DNA damage foci and their relation to chromatin organization.

DNA damage sensing proteins have been shown to localize to the sites of DNA double strand breaks (DSB) within seconds to minutes following ionizing radiation (IR) exposure, resulting in the formation of microscopically visible nuclear domains referred to as radiation-induced foci (RIF). This review characterizes the spatiotemporal properties of RIF at physiological doses, minutes to hours following exposure to ionizing radiation, and it proposes a model describing RIF formation and resolution as a function of radiation quality and chromatin territories. Discussion is limited to RIF formed by three interrelated proteins ATM (Ataxia telangiectasia mutated), 53BP1 (p53 binding protein 1) and gammaH2AX (phosphorylated variant histone H2AX), with an emphasis on the later. This review discusses the importance of not equating RIF with DSB in all situations and shows how dose and time dependence of RIF frequency is inconsistent with a one to one equivalence. Instead, we propose that RIF mark regions of the chromatin that would serve as scaffolds rigid enough to keep broken DNA from diffusing away, but open enough to allow the repair machinery to access the damage site. We review data indicating clear kinetic and physical differences between RIF emerging from dense and uncondensed regions of the nucleus. We suggest that persistent RIF observed days following exposure to ionizing radiation are nuclear marks of permanent rearrangement of the chromatin architecture. Such chromatin alterations may not always lead to growth arrest as cells have been shown to replicate these in progeny. Thus, heritable persistent RIF spanning over tens of Mbp may reflect persistent changes in the transcriptome of a large progeny of cells. Such model opens the door to a "non-DNA-centric view" of radiation-induced phenotypes.

Role of homologous recombination in trabectedin-induced DNA damage.

Trabectedin (ET-743, Yondelis) is a natural marine compound with antitumour activity currently undergoing phase II/III clinical trials. The mechanism of the drug's action is still to be defined, even though it has been clearly demonstrated the key role of Nucleotide Excision Repair (NER). To get further insights into the drug's mode of action, we studied the involvement of the DNA-double strand break repair (DNA-DSB) pathways: homologous and non-homologous recombination, both in budding yeasts and in mammalian cells and the possible cross-talk between NER and these repair pathways. Budding yeasts and mammalian cells deficient in the non-homologous end-joining pathway were moderately sensitive to trabectedin, while systems deficient in the homologous recombination pathway were extremely sensitive to the drug, with a 100-fold decrease in the IC50, suggesting that trabectedin-induced lesions are repaired by this pathway. The induction of Rad51 foci and the appearance of gamma-H2AX were chosen as putative markers for DNA-DSBs and were studied at different time points after trabectedin treatment in NER proficient and deficient systems. Both were clearly detected only in the presence of an active NER, suggesting that the DSBs are not directly caused by the drug, but are formed during the processing/repair of the drug- induced lesions.

Srs2 DNA helicase is involved in checkpoint response and its regulation requires a functional Mec1-dependent pathway and Cdk1 activity.

In Saccharomyces cerevisiae the rate of DNA replication is slowed down in response to DNA damage as a result of checkpoint activation, which is mediated by the Mec1 and Rad53 protein kinases. We found that the Srs2 DNA helicase, which is involved in DNA repair and recombination, is phosphorylated in response to intra-S DNA damage in a checkpoint-dependent manner. DNA damage-induced Srs2 phosphorylation also requires the activity of the cyclin-dependent kinase Cdk1, suggesting that the checkpoint pathway might modulate Cdk1 activity in response to DNA damage. Moreover, srs2 mutants fail to activate Rad53 properly and to slow down DNA replication in response to intra-S DNA damage. The residual Rad53 activity observed in srs2 cells depends upon the checkpoint proteins Rad17 and Rad24. Moreover, DNA damage-induced lethality in rad17 mutants depends partially upon Srs2, suggesting that a functional Srs2 helicase causes accumulation of lethal events in a checkpoint-defective context. Altogether, our data implicate Srs2 in the Mec1 and Rad53 pathway and connect the checkpoint response to DNA repair and recombination.