Increased replication initiation and conflicts with transcription underlie Cyclin E-induced replication stress.

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.

MYBL2 haploinsufficiency increases susceptibility to age-related haematopoietic neoplasia.

The haematopoietic system is prone to age-related disorders ranging from deficits in functional blood cells to the development of neoplastic states. Such neoplasms often involve recurrent cytogenetic abnormalities, among which a deletion in the long arm of chromosome 20 (del20q) is common in myeloid malignancies. The del20q minimum deleted region contains nine genes, including MYBL2, which encodes a key protein involved in the maintenance of genome integrity. Here, we show that mice expressing half the normal levels of Mybl2 (Mybl2(+/Δ)) develop a variety of myeloid disorders upon ageing. These include myeloproliferative neoplasms, myelodysplasia (MDS) and myeloid leukaemia, mirroring the human conditions associated with del20q. Moreover, analysis of gene expression profiles from patients with MDS demonstrated reduced levels of MYBL2, regardless of del20q status and demonstrated a strong correlation between low levels of MYBL2 RNA and reduced expression of a subset of genes related to DNA replication and checkpoint control pathways. Paralleling the human data, we found that these pathways are also disturbed in our Mybl2(+/Δ) mice. This novel mouse model, therefore, represents a valuable tool for studying the initiation and progression of haematological malignancies during ageing, and may provide a platform for preclinical testing of therapeutic approaches.