To analyze relationships between replication and homologous recombination in mammalian cells, we used replication inhibitors to treat mouse and hamster cell lines containing tandem repeat recombination substrates. In the first step, few double-strand breaks (DSBs) are produced, recombination is slightly increased, but cell lines defective in non-homologous end-joining (NHEJ) affected in ku86 (xrs6) or xrcc4 (XR-1) genes show enhanced sensitivity to replication inhibitors. In the second step, replication inhibition leads to coordinated kinetics of DSB accumulation, Rad51 foci formation and RAD51-dependent gene conversion stimulation. In xrs6 as well as XR-1 cell lines, Rad51 foci accumulate more rapidly compared with their respective controls. We propose that replication inhibition produces DSBs, which are first processed by the NHEJ; then, following DSB accumulation, RAD51 recombination can act.

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Non-homologous end joining (NHEJ) and homologous recombination (HR) are two alternative/competitor pathways for the repair of DNA double-strand breaks (DSBs). To gain further insights into the regulation of DSB repair, we detail here the different HR pathways affected by (i) the inactivation of DNA-PK activity, by treatment with Wortmannin, and (ii) a mutation in the xrcc4 gene, involved in a late NHEJ step, using the XR-1 cell line. Here we have analyzed not only the impact of NHEJ inactivation on recombination induced by a single DSB targeted to the recombination substrate (using I-SceI endonuclease) but also on gamma-ray- and UV-C-induced and spontaneous recombination and finally on Rad51 foci formation, i.e. on the assembly of the homologous recombination complex, at the molecular level. The results presented here show that in contrast to embryonic stem cells, the xrcc4 mutation strongly stimulates I-SceI-induced HR in adult hamster cells. More precisely, we show here that both single strand annealing and gene conversion are stimulated. In contrast, Wortmannin does not affect I-SceI-induced HR. In addition, gamma-ray-induced recombination is stimulated by both xrcc4 mutation and Wortmannin treatment in an epistatic-like manner. In contrast, neither spontaneous nor UV-C-induced recombination was affected by xrcc4 mutation, showing that the channeling from NHEJ to HR is specific to DSBs. Finally, we show here that xrcc4 mutation or Wortmannin treatment results in a stimulation of Rad51 foci assembly, thus that a late NHEJ step is able to affect Rad51 recombination complex assembly. The present data suggest a model according to which NHEJ and HR do not simply compete for DSB repair but can act sequentially: a defect in a late NHEJ step is not a dead end and can make DSB available for subsequent Rad51 recombination complex assembly.

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TRAIL induces selective tumor cell death through TRAIL-R1 and TRAIL-R2. Despite the fact that these receptors share high structural homologies, induction of apoptosis upon ER stress, cell autonomous motility and invasion have solely been described to occur through TRAIL-R2. Using the TALEN gene-editing approach, we show that TRAIL-R1 can also induce apoptosis during unresolved unfolded protein response (UPR). Likewise, TRAIL-R1 was found to co-immunoprecipitate with FADD and caspase-8 during ER stress. Its deficiency conferred resistance to apoptosis induced by thaspigargin, tunicamycin or brefeldin A. Our data also demonstrate that tumor cell motility and invasion-induced by TRAIL-R2 is not cell autonomous but induced in a TRAIL-dependant manner. TRAIL-R1, on the other hand, is unable to trigger cell migration owing to its inability to induce an increase in calcium flux. Importantly, all the isogenic cell lines generated in this study revealed that apoptosis induced TRAIL is preferentially induced by TRAIL-R1. Taken together, our results provide novel insights into the physiological functions of TRAIL-R1 and TRAIL-R2 and suggest that targeting TRAIL-R1 for anticancer therapy is likely to be more appropriate owing to its lack of pro-motile signaling capability.

We have previously developed an assay to measure DNA homologous pairing activities in crude extracts: The POM blot. In mammalian nuclear extracts, we detected two major DNA homologous pairing activities: POMp100 and POMp75. Here, we present the puri®ca-tion and identi®cation of POMp75 as the pro-oncoprotein TLS/FUS. Because of the pro-oncogene status of TLS/FUS, we studied in addition, the relationships between cell proliferation and POM activities. We show that transformation of human ®broblasts by SV40 large T antigen results in a strong increase of both POMp100 and TLS/POMp75 activities. Although detectable levels of both POMp100 and TLS/POMp75 are observed in non-immortalized ®broblasts or lymphocytes, ®broblasts at mid con¯uence or lymphocytes stimulated by phytohaemaglutinin, show higher levels of POM activities. Moreover, induction of dierentiation of mouse F9 line by retinoic acid leads to the inhibition of both POMp100 and TLS/POMp75 activities. Comparison of POM activity of TLS/FUS with the amount of TLS protein detected by Western blot, suggests that the POM activity could be regulated by post-translation modi®ca-tion. Taken together, these results indicate that POMp100 and TLS/POMp75 activitites are present in normal cells but are connected to cell proliferation. Possible relationship between cell proliferation, response to DNA damage and DNA homologous pairing activity of the pro-oncoprotein TLS/FUS are discussed.

The ability to specifically engineer the genome of living cells at precise locations using rare-cutting designer endonucleases has broad implications for biotechnology and medicine, particularly for functional genomics, transgenics and gene therapy. However, the potential impact of chromosomal context and epigenetics on designer endonuclease-mediated genome editing is poorly understood. To address this question, we conducted a comprehensive analysis on the efficacy of 37 endonucleases derived from the quintessential I-CreI meganuclease that were specifically designed to cleave 39 different genomic targets. The analysis revealed that the efficiency of targeted mutagenesis at a given chromosomal locus is predictive of that of homologous gene targeting. Consequently, a strong genome-wide correlation was apparent between the efficiency of targeted mutagenesis (≤0.1% to ∼6%) with that of homologous gene targeting (≤0.1% to ∼15%). In contrast, the efficiency of targeted mutagenesis or homologous gene targeting at a given chromosomal locus does not correlate with the activity of individual endonucleases on transiently transfected substrates. Finally, we demonstrate that chromatin accessibility modulates the efficacy of rare-cutting endonucleases, accounting for strong position effects. Thus, chromosomal context and epigenetic mechanisms may play a major role in the efficiency rare-cutting endonuclease-induced genome engineering.

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