Correction: Correction: CDK1 Is a Synthetic Lethal Target for KRAS Mutant Tumours.

[This corrects the article DOI: 10.1371/journal.pone.0176578.].

Correction: CDK1 Is a Synthetic Lethal Target for KRAS Mutant Tumours.

[This corrects the article DOI: 10.1371/journal.pone.0149099.].

CDK1 Is a Synthetic Lethal Target for KRAS Mutant Tumours.

Activating KRAS mutations are found in approximately 20% of human cancers but no RAS-directed therapies are currently available. Here we describe a novel, robust, KRAS synthetic lethal interaction with the cyclin dependent kinase, CDK1. This was discovered using parallel siRNA screens in KRAS mutant and wild type colorectal isogenic tumour cells and subsequently validated in a genetically diverse panel of 26 colorectal and pancreatic tumour cell models. This established that the KRAS/CDK1 synthetic lethality applies in tumour cells with either amino acid position 12 (p.G12V, pG12D, p.G12S) or amino acid position 13 (p.G13D) KRAS mutations and can also be replicated in vivo in a xenograft model using a small molecule CDK1 inhibitor. Mechanistically, CDK1 inhibition caused a reduction in the S-phase fraction of KRAS mutant cells, an effect also characterised by modulation of Rb, a master control of the G1/S checkpoint. Taken together, these observations suggest that the KRAS/CDK1 interaction is a robust synthetic lethal effect worthy of further investigation.

Tankyrase-targeted therapeutics: expanding opportunities in the PARP family.

The poly(ADP-ribose) polymerase (PARP) protein superfamily has wide-ranging roles in cellular processes such as DNA repair and WNT signalling. Efforts to pharmacologically target PARP enzymes have largely focused on PARP1 and the closely related PARP2, but recent work highlighting the role of another family member, tankyrase 1 (TANK1; also known as PARP5A and ARTD5), in the control of WNT signalling has fuelled interest in the development of additional inhibitors to target this enzyme class. Tankyrase function is also implicated in other processes such as the regulation of telomere length, lung fibrogenesis and myelination, suggesting that tankyrase inhibitors could have broad clinical utility. Here, we discuss the biology of tankyrases and the discovery of tankyrase-specific inhibitors. We also consider the challenges that lie ahead for the clinical development of PARP family inhibitors in general.

Caspase-3-dependent mitotic checkpoint inactivation by the small-molecule inducers of mitotic slippage SU6656 and geraldol.

Microtubule-targeting cancer drugs such as paclitaxel block cell-cycle progression at mitosis by prolonged activation of the mitotic checkpoint. Cells can spontaneously escape mitotic arrest and enter interphase without chromosome segregation by a process termed mitotic slippage that involves the degradation of cyclin B1 without mitotic checkpoint inactivation. Inducing mitotic slippage with chemicals causes cells to die after multiple rounds of DNA replication without cell division, which may enhance the antitumor activity of microtubule-targeting drugs. Here, we explore pathways leading to mitotic slippage by using SU6656 and geraldol, two recently identified chemical inducers of mitotic slippage. Mitotic slippage induced by SU6656 or geraldol was blocked by the proteasome inhibitor MG-132 and involved proteasome-dependent degradation of cyclin B1 and the mitotic checkpoint proteins budding uninhibited by benzimidazole related 1 (BubR1) and cell division cycle 20 (Cdc20) in T98G cells. Mitotic slippage and the degradation of BubR1 and Cdc20 were also inhibited by the caspase-3 and -7 inhibitor DEVD-CHO. MCF-7 cells lacking caspase-3 expression could not degrade BubR1 or undergo mitotic slippage in response to SU6656 or geraldol. Introduction of caspase-3 completely restored the ability of MCF-7 cells to degrade BubR1 and undergo mitotic slippage. However, lack of expression of caspase-3 did not affect cell death after exposure to paclitaxel, with or without mitotic slippage induction. The requirement for caspase-3 for chemically induced mitotic slippage reveals a new mechanism for mitotic exit and a link between mitosis and apoptosis that has implications for the outcome of cancer chemotherapy.

Substrate binding and active site residues in RNases E and G: role of the 5'-sensor.

The paralogous endoribonucleases, RNase E and RNase G, play major roles in intracellular RNA metabolism in Escherichia coli and related organisms. To assay the relative importance of the principal RNA binding sites identified by crystallographic analysis, we introduced mutations into the 5'-sensor, the S1 domain, and the Mg(+2)/Mn(+2) binding sites. The effect of such mutations has been measured by assays of activity on several substrates as well as by an assay of RNA binding. RNase E R169Q and the equivalent mutation in RNase G (R171Q) exhibit the strongest reductions in both activity (the k(cat) decrease approximately 40- to 100-fold) and RNA binding consistent with a key role for the 5'-sensor. Our analysis also supports a model in which the binding of substrate results in an increase in catalytic efficiency. Although the phosphate sensor plays a key role in vitro, it is unexpectedly dispensable in vivo. A strain expressing only RNase E R169Q as the sole source of RNase E activity is viable, exhibits a modest reduction in doubling time and colony size, and accumulates immature 5 S rRNA. Our results point to the importance of alternative RNA binding sites in RNase E and to alternative pathways of RNA recognition.

Effects of chemical manipulation of mitotic arrest and slippage on cancer cell survival and proliferation.

Microtubule-targeting cancer therapies interfere with mitotic spindle dynamics and block cells in mitosis by activating the mitotic checkpoint. Cells arrested in mitosis may remain arrested for extended periods of time or undergo mitotic slippage and enter interphase without having separated their chromosomes. How extended mitotic arrest and mitotic slippage contribute to subsequent cell death or survival is incompletely understood. To address this question, automated fluorescence microscopy assays were designed and used to screen chemical libraries for modulators of mitotic slippage. Chlorpromazine and triflupromazine were identified as drugs that inhibit mitotic slippage and SU6656 and geraldol as chemicals that stimulate mitotic slippage. Using the drugs to extend mitotic arrest imposed by low concentrations of paclitaxel led to increased cell survival and proliferation after drug removal. Cells arrested at mitosis with paclitaxel or vinblastine and chemically induced to undergo mitotic slippage underwent several rounds of DNA replication without cell division and exhibited signs of senescence but eventually all died. By contrast, cells arrested at mitosis with the KSP/Eg5 inhibitor S-trityl-L-cysteine and induced to undergo mitotic slippage were able to successfully divide and continued to proliferate after drug removal. These results show that reinforcing mitotic arrest with drugs that inhibit mitotic slippage can lead to increased cell survival and proliferation, while inducing mitotic slippage in cells treated with microtubule-targeting drugs seems to lead to protracted cell death.

Synthetic approaches to the microtubule-stabilizing sponge alkaloid ceratamine A and desbromo analogues.

Two synthetic approaches to the microtubule-stabilizing ceratamine alkaloids are described. The first approach involved attempts to graft an aminoimidazole moiety onto an azepine ring to form partially hydrogenated versions of the unprecedented aromatic imidazo[4,5-d]azepine core of the ceratamines. This route ultimately failed because it was not possible to aromatize the partially hydrogenated ceratamine intermediates. A second approach started with tribromoimidazole that was sequentially metalated and functionalized to efficiently generate a key imidazole intermediate containing vinyl bromide and amide functionalities. An intramolecular Buchwald vinyl amidation reaction converted this key intermediate into a bicyclic imidazo[4,5-d]azepine that was at the same oxidation state as the aromatic core of the ceratamines. The 2-amino functionality present on the imidazole ring of the ceratamines was installed using a Buchwald/Hartwig amination reaction on a 2-chloroimidazole precursor. Deprotection and aromatization resulted in the first synthesis of desbromoceratamine A (55) and desmethyldesbromoceratamine A (60). An unanticipated addition of atmospheric oxygen was encountered during deprotection of the imidazole ring in the last step of the synthesis leading to C-11 oxygenated ceratamine analogues as byproducts. Evaluation of the synthetic ceratamines in a TG3 cell-based assay for mitotic arrest revealed that the C-14 and C-16 bromine substituents in ceratamine A (1) play a major role in the antimitotic potency of the natural product. The synthetic route to ceratamine analogues has provided sufficient quantities of desbromoceratamine A (55) for testing in mouse models of cancer.

Synthesis of antimitotic analogs of the microtubule stabilizing sponge alkaloid ceratamine A.

Antimitotic analogs of the microtubule stabilizing sponge alkaloid ceratamine A (1) have been synthesized starting from tribromoimidazole. A key step in the synthesis is the formation of the azepine ring via an intramolecular Buchwald coupling between a vinyl bromide and a N-methyl amide. This represents the first synthesis of a fully unsaturated imidazo[4,5,d]azepine. NMR data obtained for the synthetic ceratamine analogs has provided support for the structure assigned to the natural product.

Spirastrellolides C to G: macrolides obtained from the marine sponge Spirastrella coccinea.

Five new macrolides, spirastrellolides C (3) to G (7), have been isolated from extracts of the marine sponge Spirastrella coccinea collected in Dominica. Their structures have been elucidated by a combination of spectroscopic analysis and chemical transformations.

Crystallization of aqueous inorganic-malonic acid particles: nucleation rates, dependence on size, and dependence on the ammonium-to-sulfate ratio.

Using an electrodynamic balance, we determined the relative humidity (RH) at which aqueous inorganic-malonic acid particles crystallized, with ammonium sulfate ((NH(4))(2)SO(4)), letovicite ((NH(4))(3)H(SO(4))(2)), or ammonium bisulfate (NH(4)HSO(4)) as the inorganic component. The results for (NH(4))(2)SO(4)-malonic acid particles and (NH(4))(3)H(SO(4))(2)-malonic acid particles show that malonic acid decreases the crystallization RH of the inorganic particles by less than 7% RH when the dry malonic acid mole fraction is less than 0.25. At a dry malonic acid mole fraction of about 0.5, the presence of malonic acid can decrease the crystallization RH of the inorganic particles by up to 35% RH. For the NH(4)HSO(4)-malonic acid particles, the presence of malonic acid does not significantly modify the crystallization RH of the inorganic particles for the entire range of dry malonic acid mole fractions studied; in all cases, either the particles did not crystallize or the crystallization RH was close to 0% RH. Size dependent measurements show that the crystallization RH of aqueous (NH(4))(2)SO(4) particles is not a strong function of particle volume. However, for aqueous (NH(4))(2)SO(4)-malonic acid particles (with dry malonic acid mole fraction = 0.36), the crystallization RH is a stronger function of particle volume, with the crystallization RH decreasing by 6 +/- 3% RH when the particle volume decreases by an order of magnitude. To our knowledge, these are the first size dependent measurements of the crystallization RH of atmospherically relevant inorganic-organic particles. These results suggest that for certain organic mole fractions the particle size and observation time need to be considered when extrapolating laboratory crystallization results to atmospheric scenarios. For aqueous (NH(4))(2)SO(4) particles, the homogeneous nucleation rate data are a strong function of RH, but for aqueous (NH(4))(2)SO(4)-malonic acid particles (with dry organic mole fraction = 0.36), the rates are not as dependent on RH. The homogeneous nucleation rates for aqueous (NH(4))(2)SO(4) particles were parametrized using classical nucleation theory, and from this analysis we determined that the interfacial surface tension between the crystalline ammonium sulfate critical nucleus and an aqueous ammonium sulfate solution is between 0.053 and 0.070 J m(-2).