DNA damage checkpoint genes, such as p53, are frequently mutated in human cancer, but the selective pressure for their inactivation remains elusive. We analysed a panel of human lung hyperplasias, all of which retained wild-type p53 genes and had no signs of gross chromosomal instability, and found signs of a DNA damage response, including histone H2AX and Chk2 phosphorylation, p53 accumulation, focal staining of p53 binding protein 1 (53BP1) and apoptosis. Progression to carcinoma was associated with p53 or 53BP1 inactivation and decreased apoptosis. A DNA damage response was also observed in dysplastic nevi and in human skin xenografts, in which hyperplasia was induced by overexpression of growth factors. Both lung and experimentally-induced skin hyperplasias showed allelic imbalance at loci that are prone to DNA double-strand break formation when DNA replication is compromised (common fragile sites). We propose that, from its earliest stages, cancer development is associated with DNA replication stress, which leads to DNA double-strand breaks, genomic instability and selective pressure for p53 mutations.

This report demonstrates that introduction of physiologically relevant miRNAs can enhance somatic cell reprogramming. The mouse embryonic stem (ES) cell specific microRNAs (miRNA) miR-291-3p, miR-294, and miR-295 enhanced the efficiency of Klf4, Oct4 and Sox2 induced pluripotency. These miRNAs did not further enhance reprogramming in the presence of cMyc. cMyc binds the promoter of these miRNAs, suggesting that they are downstream effectors of cMyc promoted pluripotency. However, unlike exogenous cMyc, these miRNAs induced a homogeneous population of reprogrammed colonies suggesting overlapping and independent functions of cMyc and the miRNAs.The miR-290 cluster constitutes over 70% of the entire miRNA population in mouse ES cells1. Its expression is rapidly down-regulated upon ES cell differentiation2. A subset of the miR-290 cluster, called the embryonic stem cell cycle (ESCC) regulating miRNAs, enhances the unique stem cell cell cycle3. This subset includes miR-291-3p, miR-294, and miR-295. To test whether ESCC miRNAs could promote the induction of pluripotency, we introduced these miRNAs along with retroviruses4 expressing Oct4, Sox2, and Klf4 (OSK) into mouse embryonic fibroblasts (MEFs). The MEFs carried two reporters: an Oct4-GFP reporter that activates GFP with the induction of pluripotency and ubiquitous expression of a β-galactosidase/neo fusion from the Rosa26 locus5. MiRNAs were introduced on days 0 and 6 post-infection by transfection of synthesized double-stranded RNAs that mimic their mature endogenous counterparts. This method transiently recapitulates ES-like levels of the miR-290 cluster miRNAs ( Supplementary Fig. 1).OSK plus miR-291-3p, miR-294, or miR-295 consistently increased the number of Oct4-GFP+ colonies as compared to controls transduced with OSK plus transfection reagent (Fig. 1a). The miR-294 mimic showed the greatest effects, increasing efficiency from 0.01-0.05% to 0.1-0.3% of transduced MEFs. Introduction of a chemically synthesized miR-294 premiRNA similarly enhanced reprogramming (Supplementary Figure 2). Two other members of the miR-290 cluster that are not ESCC miRNAs, miR-292-3p and miR-293, did not increase colony number (Fig. 1a). The ESCC miRNAs share a conserved seed sequence, which largely specifies target mRNAs (Fig. 1b). MiR-302d, a member of another miRNA cluster that has the same seed sequence also enhanced reprogramming (Fig. 1b&c). Mutation of the seed sequence in miR-294 blocked the increase in colony number (Fig. 1b&c). In summary, together with Oct4, Sox2, and Klf4, the ESCC miRNAs and related

53BP1 is a conserved nuclear protein that is implicated in the DNA damage response. After irradiation, 53BP1 localizes rapidly to nuclear foci, which represent sites of DNA double strand breaks, but its precise function is unclear. Using small interference RNA (siRNA), we demonstrate that 53BP1 functions as a DNA damage checkpoint protein. 53BP1 is required for at least a subset of ataxia telangiectasia-mutated (ATM)-dependent phosphorylation events at sites of DNA breaks and for cell cycle arrest at the G2-M interphase after exposure to irradiation. Interestingly, in cancer cell lines expressing mutant p53, 53BP1 was localized to distinct nuclear foci and ATM-dependent phosphorylation of Chk2 at Thr 68 was detected, even in the absence of irradiation. In addition, Chk2 was phosphorylated at Thr 68 in more than 50% of surgically resected lung and breast tumour specimens from otherwise untreated patients [corrected]. We conclude that the constitutive activation of the DNA damage checkpoint pathway may be linked to the high frequency of p53 mutations in human cancer, as p53 is a downstream target of Chk2 and ATM.

During replicative stress, Claspin mediates the phosphorylation and consequent activation of Chk1 by ATR. We found that during recovery from the DNA replication checkpoint response, Claspin is degraded in a betaTrCP-dependent manner. In vivo, Claspin is phosphorylated in a canonical DSGxxS degron sequence, which is typical of betaTrCP substrates. Phosphorylation of Claspin is mediated by Plk1 and is essential for binding to betaTrCP. In vitro ubiquitylation of Claspin requires betaTrCP, Plk1, and an intact DSGxxS degron. Significantly, expression of a stable Claspin mutant unable to bind betaTrCP prolongs the activation of Chk1, thereby attenuating the recovery from the DNA replication stress response and significantly delaying entry into mitosis. Thus, the SCFbetaTrCP-dependent degradation of Claspin is necessary for the efficient and timely termination of the DNA replication checkpoint. Importantly, in response to DNA damage in G2, Claspin proteolysis is inhibited to allow the prompt reestablishment of the checkpoint.

Neural cancers display cellular hierarchies with self-renewing tumorigenic cancer stem cells (CSCs) at the apex. Instructive cues to maintain CSCs are generated by both intrinsic networks and the niche microenvironment. The CSC-microenvironment relationship is complex as CSCs can modify their environment and extrinsic forces induce plasticity in the cellular hierarchy.

No abstract

With the goal to remove the roots of cancer, eliminate metastatic seeds, and overcome therapy resistance, the 2014 inaugural International Cancer Stem Cell (CSC) Conference at Cleveland, OH, convened together over 320 investigators, including 55 invited world-class speakers, 25 short oral presenters, and 100 poster presenters, to gain an in-depth understanding of CSCs and explore therapeutic opportunities targeting CSCs. The meeting enabled intriguing discussions on several topics including: genetics and epigenetics; cancer origin and evolution; microenvironment and exosomes; metabolism and inflammation; metastasis and therapy resistance; single cell and heterogeneity; plasticity and reprogramming; as well as other new concepts. Reports of clinical trials targeting CSCs emphasized the urgent need for strategically designing combinational CSC-targeting therapies against cancer.

No abstract