Firing of Replication Origins Is Disturbed by a CDK4/6 Inhibitor in a pRb-Independent Manner.

Over the last decade, CDK4/6 inhibitors (palbociclib, ribociclib and abemaciclib) have emerged as promising anticancer drugs. Numerous studies have demonstrated that CDK4/6 inhibitors efficiently block the pRb-E2F pathway and induce cell cycle arrest in pRb-proficient cells. Based on these studies, the inhibitors have been approved by the FDA for treatment of advanced hormonal receptor (HR) positive breast cancers in combination with hormonal therapy. However, some evidence has recently shown unexpected effects of the inhibitors, underlining a need to characterize the effects of CDK4/6 inhibitors beyond pRb. Our study demonstrates how palbociclib impairs origin firing in the DNA replication process in pRb-deficient cell lines. Strikingly, despite the absence of pRb, cells treated with palbociclib synthesize less DNA while showing no cell cycle arrest. Furthermore, this CDK4/6 inhibitor treatment disturbs the temporal program of DNA replication and reduces the density of replication forks. Cells treated with palbociclib show a defect in the loading of the Pre-initiation complex (Pre-IC) proteins on chromatin, indicating a reduced initiation of DNA replication. Our findings highlight hidden effects of palbociclib on the dynamics of DNA replication and of its cytotoxic consequences on cell viability in the absence of pRb. This study provides a potential therapeutic application of palbociclib in combination with other drugs to target genomic instability in pRB-deficient cancers.

Low Replicative Stress Triggers Cell-Type Specific Inheritable Advanced Replication Timing.

DNA replication timing (RT), reflecting the temporal order of origin activation, is known as a robust and conserved cell-type specific process. Upon low replication stress, the slowing of replication forks induces well-documented RT delays associated to genetic instability, but it can also generate RT advances that are still uncharacterized. In order to characterize these advanced initiation events, we monitored the whole genome RT from six independent human cell lines treated with low doses of aphidicolin. We report that RT advances are cell-type-specific and involve large heterochromatin domains. Importantly, we found that some major late to early RT advances can be inherited by the unstressed next-cellular generation, which is a unique process that correlates with enhanced chromatin accessibility, as well as modified replication origin landscape and gene expression in daughter cells. Collectively, this work highlights how low replication stress may impact cellular identity by RT advances events at a subset of chromosomal domains.

Replication Stress, Genomic Instability, and Replication Timing: A Complex Relationship.

The replication-timing program constitutes a key element of the organization and coordination of numerous nuclear processes in eukaryotes. This program is established at a crucial moment in the cell cycle and occurs simultaneously with the organization of the genome, thus indicating the vital significance of this process. With recent technological achievements of high-throughput approaches, a very strong link has been confirmed between replication timing, transcriptional activity, the epigenetic and mutational landscape, and the 3D organization of the genome. There is also a clear relationship between replication stress, replication timing, and genomic instability, but the extent to which they are mutually linked to each other is unclear. Recent evidence has shown that replication timing is affected in cancer cells, although the cause and consequence of this effect remain unknown. However, in-depth studies remain to be performed to characterize the molecular mechanisms of replication-timing regulation and clearly identify different cis- and trans-acting factors. The results of these studies will potentially facilitate the discovery of new therapeutic pathways, particularly for personalized medicine, or new biomarkers. This review focuses on the complex relationship between replication timing, replication stress, and genomic instability.

Efficient, quick and easy-to-use DNA replication timing analysis with START-R suite.

DNA replication must be faithful and follow a well-defined spatiotemporal program closely linked to transcriptional activity, epigenomic marks, intranuclear structures, mutation rate and cell fate determination. Among the readouts of the spatiotemporal program of DNA replication, replication timing analyses require not only complex and time-consuming experimental procedures, but also skills in bioinformatics. We developed a dedicated Shiny interactive web application, the START-R (Simple Tool for the Analysis of the Replication Timing based on R) suite, which analyzes DNA replication timing in a given organism with high-throughput data. It reduces the time required for generating and analyzing simultaneously data from several samples. It automatically detects different types of timing regions and identifies significant differences between two experimental conditions in ∼15 min. In conclusion, START-R suite allows quick, efficient and easier analyses of DNA replication timing for all organisms. This novel approach can be used by every biologist. It is now simpler to use this method in order to understand, for example, whether 'a favorite gene or protein' has an impact on replication process or, indirectly, on genomic organization (as Hi-C experiments), by comparing the replication timing profiles between wild-type and mutant cell lines.

A hypothesis-driven approach identifies CDK4 and CDK6 inhibitors as candidate drugs for treatments of adrenocortical carcinomas.

High proliferation rate and high mutation density are both indicators of poor prognosis in adrenocortical carcinomas. We performed a hypothesis-driven association study between clinical features in adrenocortical carcinomas and the expression levels of 136 genes involved in DNA metabolism and G1/S phase transition. In 79 samples downloaded from The Cancer Genome Atlas portal, high Cyclin Dependent Kinase 6 (CDK6) mRNA levels gave the most significant association with shorter time to relapse and poorer survival of patients. A hierarchical clustering approach assembled most tumors with high levels of CDK6 mRNA into one group. These tumors tend to cumulate mutations activating the Wnt/ß-catenin pathway and show reduced MIR506 expression. Actually, the level of MIR506 RNA is inversely correlated with the levels of both CDK6 and CTNNB1 (encoding ß-catenin). Together these results indicate that high CDK6 expression is found in aggressive tumors with activated Wnt/ß-catenin pathway. Thus we tested the impact of Food and Drug Administration-approved CDK4 and CDK6 inhibitors, namely palbociclib and ribociclib, on SW-13 and NCI-H295R cells. While both drugs reduced viability and induced senescence in SW-13 cells, only palbociclib was effective on the retinoblastoma protein (pRB)-negative NCI-H295R cells, by inducing apoptosis. In NCI-H295R cells, palbociclib induced an increase of the active form of Glycogen Synthase Kinase 3ß (GSK3ß) responsible for the reduced amount of active ß-catenin, and altered the amount of AXIN2 mRNA. Taken together, these data underline the impact of CDK4 and CDK6 inhibitors in treating adrenocortical carcinomas.

Characterization of the replication timing program of 6 human model cell lines.

During the S-phase, the DNA replication process is finely orchestrated and regulated by two programs: the spatial program that determines where replication will start in the genome (Cadoret et al. (2008 Oct 14), Cayrou et al. (2011 Sep), Picard et al. (2014 May 1) [1], [2], [3]), and the temporal program that determines when during the S phase different parts of the genome are replicated and when origins are activated. The temporal program is so well conserved for each cell type from independent individuals [4] that it is possible to identify a cell type from an unknown sample just by determining its replication timing program. Moreover, replicative domains are strongly correlated with the partition of the genome into topological domains (determined by the Hi-C method, Lieberman-Aiden et al. (2009 Oct 9), Pope et al. (2014 Nov 20) [5], [6]). On the one hand, replicative areas are well defined and participate in shaping the spatial organization of the genome for a given cell type. On the other hand, studies on the timing program during cell differentiation showed a certain plasticity of this program according to the stage of cell differentiation Hiratani et al. (2008 Oct 7, 2010 Feb) [7], [8]. Domains where a replication timing change was observed went through a nuclear re-localization. Thus the temporal program of replication can be considered as an epigenetic mark Hiratani and Gilbert (2009 Feb 16) [9]. We present the genomic data of replication timing in 6 human model cell lines: U2OS (GSM2111308), RKO (GSM2111309), HEK 293T (GSM2111310), HeLa (GSM2111311), MRC5-SV (GSM2111312) and K562 (GSM2111313). A short comparative analysis was performed that allowed us to define regions common to the 6 cell lines. These replication timing data can be taken into account when performing studies that use these model cell lines.

The Physarum polycephalum Genome Reveals Extensive Use of Prokaryotic Two-Component and Metazoan-Type Tyrosine Kinase Signaling.

Physarum polycephalum is a well-studied microbial eukaryote with unique experimental attributes relative to other experimental model organisms. It has a sophisticated life cycle with several distinct stages including amoebal, flagellated, and plasmodial cells. It is unusual in switching between open and closed mitosis according to specific life-cycle stages. Here we present the analysis of the genome of this enigmatic and important model organism and compare it with closely related species. The genome is littered with simple and complex repeats and the coding regions are frequently interrupted by introns with a mean size of 100 bases. Complemented with extensive transcriptome data, we define approximately 31,000 gene loci, providing unexpected insights into early eukaryote evolution. We describe extensive use of histidine kinase-based two-component systems and tyrosine kinase signaling, the presence of bacterial and plant type photoreceptors (phytochromes, cryptochrome, and phototropin) and of plant-type pentatricopeptide repeat proteins, as well as metabolic pathways, and a cell cycle control system typically found in more complex eukaryotes. Our analysis characterizes P. polycephalum as a prototypical eukaryote with features attributed to the last common ancestor of Amorphea, that is, the Amoebozoa and Opisthokonts. Specifically, the presence of tyrosine kinases in Acanthamoeba and Physarum as representatives of two distantly related subdivisions of Amoebozoa argues against the later emergence of tyrosine kinase signaling in the opisthokont lineage and also against the acquisition by horizontal gene transfer.

Replication forks reverse at high frequency upon replication stress in Physarum polycephalum.

The addition of hydroxyurea after the onset of S phase allows replication to start and permits the successive detecting of replication-dependent joint DNA molecules and chicken foot structures in the synchronous nuclei of Physarum polycephalum. We find evidence for a very high frequency of reversed replication forks upon replication stress. The formation of these reversed forks is dependent on the presence of joint DNA molecules, the impediment of the replication fork progression by hydroxyurea, and likely on the propensity of some replication origins to reinitiate replication to counteract the action of this compound. As hydroxyurea treatment enables us to successively detect the appearance of joint DNA molecules and then of reversed replication forks, we propose that chicken foot structures are formed both from the regression of hydroxyurea-frozen joint DNA molecules and from hydroxyurea-stalled replication forks. These experiments underscore the transient nature of replication fork regression, which becomes detectable due to the hydroxyurea-induced slowing down of replication fork progression.

RNase-dependent discontinuities associated with the crossovers of spontaneously formed joint DNA molecules in Physarum polycephalum.

Transient four stranded joint DNA molecules bridging sister chromatids constitute an intriguing feature of replicating genomes. Here, we studied their structure and frequency of formation in Physarum polycephalum. By "3D gels", we evidenced that they are not made of four continuous DNA strands. Discontinuities, which do not interfere with the unique propensity of the joint DNA molecules to branch migrate in vitro, are linked to the crossover, enhanced by RNaseA, and affect at most half of the DNA strands. We propose a structural model of joint DNA molecules containing ribonucleotides inserted within one strand, a gapped strand, and two continuous DNA strands. We further show that spontaneous joint DNA molecules are short-lived and are as abundant as replication forks. Our results emphasize the highly frequent formation of joint DNA molecules involving newly replicated DNA in an untreated cell and uncover a transitory mechanism connecting the sister chromatids during S phase.

Interplay between DNA replication and gene expression: a harmonious coexistence.

Multicellular organisms have evolved highly sophisticated machinery to that their genomes are accurately duplicated and that the various gene expression programs are established correctly. Recent large-scale studies have shed light on how these fundamental processes interact. Although the machinery mediating these processes share similar cis-regulatory elements, they are not strictly coregulated. Furthermore, studies of the replisome show that highly transcribed genes present a major obstacle to its operation. Further studies will be needed to identify key regulators of the spatio-temporal program of DNA replication, for the elucidation of the complex interplay between replication and transcription.

Low rate of replication fork progression lengthens the replication timing of a locus containing an early firing origin.

Invariance of temporal order of genome replication in eukaryotic cells and its correlation with gene activity has been well-documented. However, recent data suggest a relax control of replication timing. To evaluate replication schedule accuracy, we detailed the replicational organization of the developmentally regulated php locus that we previously found to be lately replicated, even though php gene is highly transcribed in naturally synchronous plasmodia of Physarum. Unexpectedly, bi-dimensional agarose gel electrophoreses of DNA samples prepared at specific time points of S phase showed that replication of the locus actually begins at the onset of S phase but it proceeds through the first half of S phase, so that complete replication of php-containing DNA fragments occurs in late S phase. Origin mapping located replication initiation upstream php coding region. This proximity and rapid fork progression through the coding region result in an early replication of php gene. We demonstrated that afterwards an unusually low fork rate and unidirectional fork pausing prolong complete replication of php locus, and we excluded random replication timing. Importantly, we evidenced that the origin linked to php gene in plasmodium is not fired in amoebae when php expression dramatically reduced, further illustrating replication-transcription coupling in Physarum.

Developmentally regulated usage of Physarum DNA replication origins.

To determine the extent to which eukaryotic replication origins are developmentally regulated in transcriptionally competent cells, we compared origin use in untreated growing amoebae and plasmodia of Physarum polycephalum. At loci that contain genes transcribed in both developmental stages, such as the ribosomal RNA genes and two unlinked actin genes, we show that there is a similar replicational organization, with the same origins used with comparable efficiencies, as shown by two-dimensional agarose-gel electrophoresis. By contrast, we found cell-type-specific replication patterns for the homologous, unlinked profilin A (proA) and profilin P (proP) genes. proA is replicated from a promoter-proximal origin in amoebae, in which it is highly expressed, and is replicated passively in the plasmodium, in which it is not expressed. Conversely, proP is replicated passively and is not expressed in amoebae, but coincides with an efficient origin when highly expressed in the plasmodium. Our results show a reprogramming of S phase that is linked to the reprogramming of transcription during Physarum cell differentiation. This is achieved by the use of two classes of promoter-associated replication origins: those that are constitutively active and those that are developmentally regulated. This suggests that replication origins, like genes, are under epigenetic control associated with cellular differentiation.

Replicational organization of three weakly expressed loci in Physarum polycephalum.

We previously mapped early-activated replication origins in the promoter regions of five abundantly transcribed genes in the slime mold Physarum polycephalum. This physical linkage between origins and genes is congruent with the preferential early replication of the active genes in mammalian cells. To determine how general this replicational organization is in the synchronous plasmodium of Physarum, we analyzed the replication of three weakly expressed genes. Bromodeoxyuridine (BrdUrd) density-shift and gene dosage experiments indicated that the redB (regulated in development) and redE genes replicate early, whereas redA replicates in mid-S phase. Bi-dimensional gel electrophoresis revealed that redA coincides with an origin that appears to be activated within a large temporal window in S phase so that the replication of the gene is not well defined temporally. The early replication of the redB and redE genes is due to the simultaneous activation of flanking origins at the onset of S phase. As a result, these two genes correspond to termination sites of DNA replication. Our data demonstrate that not all the Physarum promoters are preferred sites of initiation but, so far, all the expressed genes analyzed in detail either coincide with a replication origin or are embedded into a cluster of early firing replicons.