Activation of endogenous retroviruses and induction of viral mimicry by MEK1/2 inhibition in pancreatic cancer.

While pancreatic ductal adenocarcinomas (PDACs) are addicted to KRAS-activating mutations, inhibitors of downstream KRAS effectors, such as the MEK1/2 kinase inhibitor trametinib, are devoid of therapeutic effects. However, the extensive rewiring of regulatory circuits driven by the attenuation of the KRAS pathway may induce vulnerabilities of therapeutic relevance. An in-depth molecular analysis of the transcriptional and epigenomic alterations occurring in PDAC cells in the initial hours after MEK1/2 inhibition by trametinib unveiled the induction of endogenous retroviruses (ERVs) escaping epigenetic silencing, leading to the production of double-stranded RNAs and the increased expression of interferon (IFN) genes. We tracked ERV activation to the early induction of the transcription factor ELF3, which extensively bound and activated nonsilenced retroelements and synergized with IRF1 (interferon regulatory factor 1) in the activation of IFNs and IFN-stimulated genes. Trametinib-induced viral mimicry in PDAC may be exploited in the rational design of combination therapies in immuno-oncology.

3D COMBO chrRNA-DNA-ImmunoFISH.

Epigenetic mechanisms govern the quality, the stability, and the responsiveness of transcriptional programs to the environment. This regulation is ensured via the concerted action of different players (transcription factors, "reader" and "writer" enzymes, histone marks, structural proteins, noncoding regulatory RNAs) that flow in the 3D organization of the genome. Indeed, nuclear architecture participates in the punctual and cell-type-specific regulation of transcription. Hence, the fine dissection of these mechanisms will allow a deeper understanding of the gene expression machinery. In this chapter, we propose a challenging imaging-based method to study the reciprocal interactions between chromatin-associated RNAs, genomic loci, and chromatin compartment with a procedure of 3D COMBO chrRNA-DNA-ImmunoFISH, specifically developed to preserve the nuclear integrity and topology of human primary T cells. We believe that our protocol will contribute to the improvement of epigenetic studies on the 3D nuclear structure of T cell subsets, possibly shedding light on the still hidden epigenetic players responsible for the great plasticity and functional diversification exerted by T cells.

An Algorithm for the Analysis of the 3D Spatial Organization of the Genome.

We present an algorithm, and its MATLAB implementation, based on mathematical methods to detect and localize 3D multicolor DNA FISH spots in fluorescence cell image z-stacks. This algorithm provides a method to measure the relative positioning of spots in the nucleus and inter-spot distances with the aim to enrich our understanding of the 3D spatial organization of the genome within the cell nucleus.

Profound alterations of the chromatin architecture at chromosome 11p15.5 in cells from Beckwith-Wiedemann and Silver-Russell syndromes patients.

Beckwith-Wiedemann syndrome (BWS) and Silver-Russell syndrome (SRS) are imprinting-related disorders associated with genetic/epigenetic alterations of the 11p15.5 region, which harbours two clusters of imprinted genes (IGs). 11p15.5 IGs are regulated by the methylation status of imprinting control regions ICR1 and ICR2. 3D chromatin structure is thought to play a pivotal role in gene expression control; however, chromatin architecture models are still poorly defined in most cases, particularly for IGs. Our study aimed at elucidating 11p15.5 3D structure, via 3C and 3D FISH analyses of cell lines derived from healthy, BWS or SRS children. We found that, in healthy cells, IGF2/H19 and CDKN1C/KCNQ1OT1 domains fold in complex chromatin conformations, that facilitate the control of IGs mediated by distant enhancers. In patient-derived cell lines, we observed a profound impairment of such a chromatin architecture. Specifically, we identified a cross-talk between IGF2/H19 and CDKN1C/KCNQ1OT1 domains, consisting in in cis, monoallelic interactions, that are present in healthy cells but lost in patient cell lines: an inter-domain association that sees ICR2 move close to IGF2 on one allele, and to H19 on the other. Moreover, an intra-domain association within the CDKN1C/KCNQ1OT1 locus seems to be crucial for maintaining the 3D organization of the region.

3D Multicolor DNA FISH Tool to Study Nuclear Architecture in Human Primary Cells.

A major question in cell biology is genomic organization within the nuclear space and how chromatin architecture can influence processes such as gene expression, cell identity and differentiation. Many approaches developed to study the 3D architecture of the genome can be divided into two complementary categories: chromosome conformation capture based technologies (C-technologies) and imaging. While the former is based on capturing the chromosome conformation and proximal DNA interactions in a population of fixed cells, the latter, based on DNA fluorescence in situ hybridization (FISH) on 3D-preserved nuclei, allows contemporary visualization of multiple loci at a single cell level (multicolor), examining their interactions and distribution within the nucleus (3D multicolor DNA FISH). The technique of 3D multicolor DNA FISH has a limitation of visualizing only a few predetermined loci, not permitting a comprehensive analysis of the nuclear architecture. However, given the robustness of its results, 3D multicolor DNA FISH in combination with 3D-microscopy and image reconstruction is a possible method to validate C-technology based results and to unambiguously study the position and organization of specific loci at a single cell level. Here, we propose a step by step method of 3D multicolor DNA FISH suitable for a wide range of human primary cells and discuss all the practical actions, crucial steps, notions of 3D imaging and data analysis needed to obtained a successful and informative 3D multicolor DNA FISH within different biological contexts.

Dysfunctional polycomb transcriptional repression contributes to lamin A/C-dependent muscular dystrophy.

Lamin A is a component of the inner nuclear membrane that, together with epigenetic factors, organizes the genome in higher order structures required for transcriptional control. Mutations in the lamin A/C gene cause several diseases belonging to the class of laminopathies, including muscular dystrophies. Nevertheless, molecular mechanisms involved in the pathogenesis of lamin A-dependent dystrophies are still largely unknown. The polycomb group (PcG) of proteins are epigenetic repressors and lamin A interactors, primarily involved in the maintenance of cell identity. Using a murine model of Emery-Dreifuss muscular dystrophy (EDMD), we show here that lamin A loss deregulated PcG positioning in muscle satellite stem cells, leading to derepression of non-muscle-specific genes and p16INK4a, a senescence driver encoded in the Cdkn2a locus. This aberrant transcriptional program caused impairment in self-renewal, loss of cell identity, and premature exhaustion of the quiescent satellite cell pool. Genetic ablation of the Cdkn2a locus restored muscle stem cell properties in lamin A/C-null dystrophic mice. Our findings establish a direct link between lamin A and PcG epigenetic silencing and indicate that lamin A-dependent muscular dystrophy can be ascribed to intrinsic epigenetic dysfunctions of muscle stem cells.

4q-D4Z4 chromatin architecture regulates the transcription of muscle atrophic genes in facioscapulohumeral muscular dystrophy.

Despite increasing insights in genome structure organization, the role of DNA repetitive elements, accounting for more than two thirds of the human genome, remains elusive. Facioscapulohumeral muscular dystrophy (FSHD) is associated with deletion of D4Z4 repeat array below 11 units at 4q35.2. It is known that the deletion alters chromatin structure in cis, leading to gene up-regulation. Here we show a genome-wide role of 4q-D4Z4 array in modulating gene expression via 3D nuclear contacts. We have developed an integrated strategy of 4q-D4Z4-specific 4C-seq and chromatin segmentation analyses, showing that 4q-D4Z4 3D interactome and chromatin states of interacting genes are impaired in FSHD1 condition; in particular, genes that have lost the 4q-D4Z4 interaction and with a more active chromatin state are enriched for muscle atrophy transcriptional signature. Expression level of these genes is restored by the interaction with an ectopic 4q-D4Z4 array, suggesting that the repeat directly modulates the transcription of contacted targets. Of note, the up-regulation of atrophic genes is a common feature of several FSHD1 and FSHD2 patients, indicating that we have identified a core set of deregulated genes involved in FSHD pathophysiology.

MicroRNA-26a/cyclin-dependent kinase 5 axis controls proliferation, apoptosis and in vivo tumor growth of diffuse large B-cell lymphoma cell lines.

Diffuse large B-cell lymphoma (DLBCL) is the most frequent type of non-Hodgkin lymphoma. Despite a favorable therapeutic response to first-line chemo-immunotherapy, still 30-40% of patients is refractory, or relapse after this treatment. Thus, alternative strategies must be sought. Previous studies have indicated that cyclin-dependent kinase 5 (CDK5), a serine/threonine protein kinase, is involved in tumor development and progression, and it may represent a potential therapeutic target. However, its role in modulating DLBCL growth and progression remains largely unexplored. In this study, we show that CDK5 and its activator, cyclin-dependent kinase 5 activator 1 (CDK5R1 or p35), are overexpressed in DLBCL cell lines and that signal transducer and activator of transcription 3 (STAT3) phosphorylation and activity is dependent on CDK5 expression in DLBCL. Using public data sets, we also demonstrate that patients with DLBCL show a higher expression of CDK5 compared with healthy individuals. By using loss-of-function approaches, we demonstrate that CDK5's activity regulates proliferation and survival of DLBCL cells. MicroRNAs (miRNAs or miRs) are small noncoding RNAs that negatively regulating gene expression and are involved in cancer initiation and progression. We identify miR-26a as direct regulator of p35 expression and CDK5 activity. We show that miR-26a expression is lower in DLBCL cell lines compared to B lymphocytes and that its ectopic expression leads to a drastic reduction of DLBCL tumor growth in vivo and decreased proliferation, cell-cycle progression, and survival in vitro. Remarkably, concomitant overexpression of a 3'-UTR-truncated form of p35 promoted tumor growth in vivo and cell proliferation, cell-cycle progression, and cell survival in vitro. In conclusion, these results demonstrate an important role for miR-26a and CDK5 together in the survival and growth of DLBCL cells, suggesting the existence of potential novel therapeutic targets for the treatment of DLBCL.

Chromosome Conformation Capture in Primary Human Cells.

3D organization of the genome, its structural and regulatory function of cell identity, is acquiring prominent features in epigenetics studies; more efforts have been done to develop techniques that allow studying nuclear structure. Chromosome conformation capture (3C) has been set up in 2002 from Dekker and from that moment great investments were made to develop genomics variants of 3C technology (4C, 5C, Hi-C) providing new tools to investigate the shape of the genome in a more systematic and unbiased manner. 3C method allows scientists to fix dynamic and variable 3D interactions in nuclear space, and consequently to study which sequences interact, how a gene is regulated by different and distant enhancer, or how a set of enhancer could regulate transcriptional units; to follow the conformation that mediates regulation change in development; and to evaluate if this fine epigenetic mechanism is impaired in disease condition.

Lamin A/C sustains PcG protein architecture, maintaining transcriptional repression at target genes.

Beyond its role in providing structure to the nuclear envelope, lamin A/C is involved in transcriptional regulation. However, its cross talk with epigenetic factors--and how this cross talk influences physiological processes--is still unexplored. Key epigenetic regulators of development and differentiation are the Polycomb group (PcG) of proteins, organized in the nucleus as microscopically visible foci. Here, we show that lamin A/C is evolutionarily required for correct PcG protein nuclear compartmentalization. Confocal microscopy supported by new algorithms for image analysis reveals that lamin A/C knock-down leads to PcG protein foci disassembly and PcG protein dispersion. This causes detachment from chromatin and defects in PcG protein-mediated higher-order structures, thereby leading to impaired PcG protein repressive functions. Using myogenic differentiation as a model, we found that reduced levels of lamin A/C at the onset of differentiation led to an anticipation of the myogenic program because of an alteration of PcG protein-mediated transcriptional repression. Collectively, our results indicate that lamin A/C can modulate transcription through the regulation of PcG protein epigenetic factors.