It has become increasingly clear that oncogenes not only provide aberrant growth signals to cells but also cause DNA damage at replication forks (replication stress), which activate the ataxia telangiectasia mutated (ATM)/p53-dependent tumor barrier. Here we studied underlying mechanisms of oncogene-induced replication stress in cells overexpressing the oncogene Cyclin E. Cyclin E overexpression is associated with increased firing of replication origins, impaired replication fork progression and DNA damage that activates RAD51-mediated recombination. By inhibiting replication initiation factors, we show that Cyclin E-induced replication slowing and DNA damage is a consequence of excessive origin firing. A significant amount of Cyclin E-induced replication slowing is due to interference between replication and transcription, which also underlies the activation of homologous recombination. Our data suggest that Cyclin E-induced replication stress is caused by deregulation of replication initiation and increased interference between replication and transcription, which results in impaired replication fork progression and DNA damage triggering the tumor barrier or cancer-promoting mutations.Oncogene ( Keywords: Cyclin E; replication fork; origin firing; DNA damage; homologous recombination INTRODUCTION Faithful replication of the genome is essential to maintain genomic stability and prevent cancer-promoting mutations. If the progression of replication forks is impaired, this can lead to replicationassociated DNA damage, also known as replication stress. It is now recognized that replication stress induced by oncogenes is an important factor that underlies the activation of the ataxia telangiectasia mutated (ATM)-and p53-mediated tumor barriers of senescence and apoptosis. [1][2][3][4][5][6][7] There is currently no coherent model to explain how oncogenes induce replication stress and DNA damage. Oncogenes of the growth factor signaling pathways generally activate cell proliferation by deregulating the transition from G1 to S phase of the cell cycle.8 Deregulated S-phase entry leading to aberrant DNA replication is therefore likely a principal mechanism of oncogene-induced replication stress. It has been reported that overexpression of the oncogenes HPV-16 E6/E7 and Cyclin E leads to S-phase entry in the presence of insufficient nucleotide pools, leading to impaired DNA replication fork progression and DNA damage.9 However, the underlying mechanism for the nucleotide depletion observed in the presence of oncogenes is not clear, and not all oncogenes that deregulate S-phase entry induce DNA damage. 10 The mechanisms of aberrant DNA replication and oncogene-induced replication stress therefore deserve further scrutiny.During a normal cell cycle, not only entry into S phase but also the timing of DNA replication initiation during S phase is carefully regulated by cell cycle and checkpoint signaling pathways.

A common feature of early embryo cells from the inner cell mass (ICM) and of ESCs is an absolute dependence on an atypical cell cycle in which the G1 phase is shortened to preserve their self-renewing and pluripotent nature. The transcription factor B-Myb has been attributed a role in proliferation, in particular during the G2/M phases of the cell cycle. Intriguingly, B-Myb levels in ICM/ESCs are greater than 100 times compared with those in normal proliferating cells, suggesting a particularly important function for this transcription factor in pluripotent stem cells. B-Myb is essential for embryo development beyond the preimplantation stage, but its role in ICM/ESCs remains unclear. Using a combination of mouse genetics, single DNA fiber analyses and high-resolution three-dimensional (3D) imaging, we demonstrate that B-Myb has no influence on the expression of pluripotency factors, but instead B-Myb ablation leads to stalling of replication forks and superactivation of replication factories that result in disorganization of the replication program and an increase in double-strand breaks. These effects are partly due to aberrant transcriptional regulation of cell cycle proliferation factors, namely c-Myc and FoxM1, which dictate normal S phase progression. We conclude that B-Myb acts crucially during the S phase in ESCs by facilitating proper progression of replication, thereby protecting the cells from genomic damage. Our findings have particular relevance in the light of the potential therapeutic application of ESCs and the need to maintain their genomic integrity. STEM CELLS

Argininosuccinate lyase (ASL) belongs to the hepatic urea cycle detoxifying ammonia, and the citrulline-nitric oxide (NO) cycle producing NO. ASL-deficient patients present argininosuccinic aciduria characterised by hyperammonaemia, multiorgan disease and neurocognitive impairment despite treatment aiming to normalise ammonaemia without considering NO imbalance. Here we show that cerebral disease in argininosuccinic aciduria involves neuronal oxidative/nitrosative stress independent of hyperammonaemia. Intravenous injection of AAV8 vector into adult or neonatal ASL-deficient mice demonstrates long-term correction of the hepatic urea cycle and the cerebral citrulline-NO cycle, respectively. Cerebral disease persists if ammonaemia only is normalised but is dramatically reduced after correction of both ammonaemia and neuronal ASL activity. This correlates with behavioural improvement and reduced cortical cell death. Thus, neuronal oxidative/nitrosative stress is a distinct pathophysiological mechanism from hyperammonaemia. Disease amelioration by simultaneous brain and liver gene transfer with one vector, to treat both metabolic pathways, provides new hope for hepatocerebral metabolic diseases.