Reciprocal monoallelic expression of ASAR lncRNA genes controls replication timing of human chromosome 6

Master transcription factor binding sites constitute the core of early replication control elements

Eukaryotic genomes replicate in a defined temporal order called the replication timing (RT) program. RT is developmentally regulated with potential to drive cell fate transitions, but mechanisms controlling RT remain elusive. We previously identified "Early Replication Control Elements" (ERCEs) necessary for early RT, domain-wide transcription, 3D chromatin architecture and compartmentalization in mouse embryonic stem cells (mESCs) but, deletions identifying ERCEs were large and encompassed many putative regulatory elements. Here, we show that ERCEs are compound elements whose RT activity can largely be accounted for by multiple sites of diverse master transcription factor binding (subERCEs), distinguished from other such sites by their long-range interactions. While deletion of subERCEs had large effects on both transcription and RT, deleting transcription start sites eliminated nearly all transcription with moderate effects on RT. Our results suggest a model in which subERCEs respond to diverse master transcription factors by functioning both as transcription enhancers and as elements that organize chromatin domains structurally and support early RT, potentially providing a feed-forward loop to drive robust epigenomic change during cell fate transitions.

R-loops acted on by RNase H1 influence DNA replication timing and genome stability in Leishmania

Genomes in eukaryotes normally undergo DNA replication in a choreographed temporal order, resulting in early and late replicating chromosome compartments. Leishmania, a human protozoan parasite, displays an unconventional DNA replication program in which the timing of DNA replication completion is chromosome size-dependent: larger chromosomes complete replication later then smaller ones. Here we show that both R-loops and RNase H1, a ribonuclease that resolves RNA-DNA hybrids, accumulate in Leishmania major chromosomes in a pattern that reflects their replication timing. Furthermore, we demonstrate that such differential organisation of R-loops, RNase H1 and DNA replication timing across the parasite's chromosomes correlates with size-dependent differences in chromatin accessibility, G quadruplex distribution and sequence content. Using conditional gene excision, we show that loss of RNase H1 leads to transient growth perturbation and permanently abrogates the differences in DNA replication timing across chromosomes, as well as altering levels of aneuploidy and increasing chromosome instability in a size-dependent manner. This work provides a link between R-loop homeostasis and DNA replication timing in a eukaryotic parasite and demonstrates that orchestration of DNA replication dictates levels of genome plasticity in Leishmania.

Parent-of-origin-specific DNA replication timing is confined to large imprinted regions

Genomic imprinting involves differential DNA methylation and gene expression between homologous paternal and maternal loci. It remains unclear, however, whether DNA replication also shows parent-of-origin-specific patterns at imprinted or other genomic regions. Here, we investigate genome-wide asynchronous DNA replication utilizing uniparental human embryonic stem cells containing either maternal-only (parthenogenetic) or paternal-only (androgenetic) DNA. Four clusters of imprinted genes exhibited differential replication timing based on parent of origin, while the remainder of the genome, 99.82%, showed no significant replication asynchrony between parental origins. Active alleles in imprinted gene clusters replicated earlier than their inactive counterparts. At the Prader-Willi syndrome locus, replication asynchrony spanned virtually the entirety of S phase. Replication asynchrony was carried through differentiation to neuronal precursor cells in a manner consistent with gene expression. This study establishes asynchronous DNA replication as a hallmark of large imprinted gene clusters.

ASAR lncRNAs control DNA replication timing through interactions with multiple hnRNP/RNA binding proteins

ASARs are a family of very-long noncoding RNAs that control replication timing on individual human autosomes, and are essential for chromosome stability. The eight known ASAR lncRNAs remain closely associated with their parent chromosomes. Analysis of RNA-protein interaction data (from ENCODE) revealed numerous RBPs with significant interactions with multiple ASAR lncRNAs, with several hnRNPs as abundant interactors. An ~7 kb domain within the ASAR6-141 lncRNA shows a striking density of RBP interaction sites. Genetic deletion and ectopic integration assays indicate that this ~7 kb RNA binding protein domain contains functional sequences for controlling replication timing of entire chromosomes in cis. shRNA-mediated depletion of 10 different RNA binding proteins, including HNRNPA1, HNRNPC, HNRNPL, HNRNPM, HNRNPU, or HNRNPUL1, results in dissociation of ASAR lncRNAs from their chromosome territories, and disrupts the synchronous replication that occurs on all autosome pairs, recapitulating the effect of individual ASAR knockouts on a genome-wide scale. Our results further demonstrate the role that ASARs play during the temporal order of genome-wide replication, and we propose that ASARs function as essential RNA scaffolds for the assembly of hnRNP complexes that help maintain the structural integrity of each mammalian chromosome.

Monoallelically expressed noncoding RNAs form nucleolar territories on NOR-containing chromosomes and regulate rRNA expression

Out of the several hundred copies of rRNA genes arranged in the nucleolar organizing regions (NOR) of the five human acrocentric chromosomes, ~50% remain transcriptionally inactive. NOR-associated sequences and epigenetic modifications contribute to the differential expression of rRNAs. However, the mechanism(s) controlling the dosage of active versus inactive rRNA genes within each NOR in mammals is yet to be determined. We have discovered a family of ncRNAs, SNULs (Single NUcleolus Localized RNA), which form constrained sub-nucleolar territories on individual NORs and influence rRNA expression. Individual members of the SNULs monoallelically associate with specific NOR-containing chromosomes. SNULs share sequence similarity to pre-rRNA and localize in the sub-nucleolar compartment with pre-rRNA. Finally, SNULs control rRNA expression by influencing pre-rRNA sorting to the DFC compartment and pre-rRNA processing. Our study discovered a novel class of ncRNAs influencing rRNA expression by forming constrained nucleolar territories on individual NORs.

Replication timing and transcriptional control: beyond cause and effect - part IV

Decades of work on the spatiotemporal organization of mammalian DNA replication timing (RT) continues to unveil novel correlations with aspects of transcription and chromatin organization but, until recently, mechanisms regulating RT and the biological significance of the RT program had been indistinct. We now know that the RT program is both influenced by and necessary to maintain chromatin structure, forming an epigenetic positive feedback loop. Moreover, the discovery of specific cis-acting elements regulating mammalian RT at both the domain and the whole-chromosome level has revealed multiple cell-type-specific and developmentally regulated mechanisms of RT control. We review recent evidence for diverse mechanisms employed by different cell types to regulate their RT programs and the biological significance of RT regulation during development.

Epigenetic control of chromosome-associated lncRNA genes essential for replication and stability

ASARs are long noncoding RNA genes that control replication timing of entire human chromosomes in cis. The three known ASAR genes are located on human chromosomes 6 and 15, and are essential for chromosome integrity. To identify ASARs on all human chromosomes we utilize a set of distinctive ASAR characteristics that allow for the identification of hundreds of autosomal loci with epigenetically controlled, allele-restricted behavior in expression and replication timing of coding and noncoding genes, and is distinct from genomic imprinting. Disruption of noncoding RNA genes at five of five tested loci result in chromosome-wide delayed replication and chromosomal instability, validating their ASAR activity. In addition to the three known essential cis-acting chromosomal loci, origins, centromeres, and telomeres, we propose that all mammalian chromosomes also contain "Inactivation/Stability Centers" that display allele-restricted epigenetic regulation of protein coding and noncoding ASAR genes that are essential for replication and stability of each chromosome.

In Vivo Clonal Analysis Reveals Random Monoallelic Expression in Lymphocytes That Traces Back to Hematopoietic Stem Cells

Evaluating the epigenetic landscape in the stem cell compartment at the single-cell level is essential to assess the cells’ heterogeneity and predict their fate. Here, using a genome-wide transcriptomics approach in vivo, we evaluated the allelic expression imbalance in the progeny of single hematopoietic cells (HSCs) as a read-out of epigenetic marking. After 4 months of extensive proliferation and differentiation, we found that X-chromosome inactivation (XCI) is tightly maintained in all single-HSC derived hematopoietic cells. In contrast, the vast majority of the autosomal genes did not show clonal patterns of random monoallelic expression (RME). However, a persistent allele-specific autosomal transcription in HSCs and their progeny was found in a rare number of cases, none of which has been previously reported. These data show that: 1) XCI and RME in the autosomal chromosomes are driven by different mechanisms; 2) the previously reported high frequency of genes under RME in clones expanded in vitro (up to 15%) is not found in clones undergoing multiple differentiation steps in vivo; 3) prior to differentiation, HSCs have stable patterns of autosomal RME. We propose that most RME patterns in autosomal chromosomes are erased and established de novo during cell lineage differentiation.

Methods to Analyze the Non-Coding RNA Interactome—Recent Advances and Challenges

Most of the human genome is transcribed to generate a multitude of non-coding RNAs. However, while these transcripts have generated an immense amount of scientific interest, their biological function remains a subject of an intense debate. Understanding mechanisms of action of non-coding RNAs is a key to addressing the issue of biological relevance of these transcripts. Based on some well-understood non-coding RNAs that function inside the cell by interacting with other molecules, it is generally believed many other non-coding transcripts could also function in a similar fashion. Therefore, development of methods that can map RNA interactome is the key to understanding functionality of the extensive cellular non-coding transcriptome. Here, we review the vast progress that has been made in the past decade in technologies that can map RNA interactions with different sites in DNA, proteins or other RNA molecules; the general approaches used to validate the existence of novel interactions; and the challenges posed by interpreting the data obtained using the interactome mapping methods.

Non-coding RNAs in Necrotizing Enterocolitis- A New Frontier?

With the recognition that only 2% of the human genome encodes for a protein, a large part of the "non-coding" portion is now being evaluated for a regulatory role in cellular processes. These non-coding RNAs (ncRNAs) are subdivided based on the size of the nucleotide transcript into microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), but most of our attention has been focused on the role of microRNAs (miRNAs) in human health and disease. Necrotizing enterocolitis (NEC), an inflammatory bowel necrosis affecting preterm infants, has a multifactorial, unclear etiopathogenesis, and we have no specific biomarkers for diagnosis or the impact of directed therapies. The information on ncRNAs, in general, and particularly in NEC, is limited. Increasing information from other inflammatory bowel disorders suggests that these transcripts may play an important role in intestinal inflammation. Here, we review ncRNAs for definitions, classifications, and possible roles in prematurity and NEC using some preliminary information from our studies and from an extensive literature search in multiple databases including PubMed, EMBASE, and Science Direct. miRNAs will be described in another manuscript in this series, hence in this manuscript we mainly focus on lncRNAs.

Differential Allelic Expression among Long Non-Coding RNAs

Long non-coding RNAs (lncRNA) comprise a diverse group of non-protein-coding RNAs >200 bp in length that are involved in various normal cellular processes and disease states, and can affect coding gene expression through mechanisms in cis or in trans. Since the discovery of the first functional lncRNAs transcribed by RNA Polymerase II, H19 and Xist, many others have been identified and noted for their unusual transcriptional pattern, whereby expression from one chromosome homolog is strongly favored over the other, also known as mono-allelic or differential allelic expression. lncRNAs with differential allelic expression have been observed to play critical roles in developmental gene regulation, chromosome structure, and disease. Here, we will focus on known examples of differential allelic expression of lncRNAs and highlight recent research describing functional lncRNAs expressed from both imprinted and random mono-allelic expression domains.

Complex Age- and Cancer-Related Changes in Human Blood Transcriptome-Implications for Pan-Cancer Diagnostics

Early cancer detection is the key to a positive clinical outcome. While a number of early diagnostics methods exist in clinics today, they tend to be invasive and limited to a few cancer types. Thus, a clear need exists for non-invasive diagnostics methods that can be used to detect the presence of cancer of any type. Liquid biopsy based on analysis of molecular components of peripheral blood has shown significant promise in such pan-cancer diagnostics; however, existing methods based on this approach require improvements, especially in sensitivity of early-stage cancer detection. The improvement would likely require diagnostics assays based on multiple different types of biomarkers and, thus, calls for identification of novel types of cancer-related biomarkers that can be used in liquid biopsy. Whole-blood transcriptome, especially its non-coding component, represents an obvious yet under-explored biomarker for pan-cancer detection. In this study, we show that whole transcriptome analysis using RNA-seq could indeed serve as a viable biomarker for pan-cancer detection. Furthermore, a class of long non-coding (lnc) RNAs, very long intergenic non-coding (vlinc) RNAs, demonstrated superior performance compared with protein-coding mRNAs. Finally, we show that age and presence of non-blood cancers change transcriptome in similar, yet not identical, directions and explore implications of this observation for pan-cancer diagnostics.

Role of mammalian long non-coding RNAs in normal and neuro oncological disorders

Long non-coding RNAs (lncRNAs) are expressed at lower levels than protein-coding genes but have a crucial role in gene regulation. LncRNA is distinct, they are being transcribed using RNA polymerase II, and their functionality depends on subcellular localization. Depending on their niche, they specifically interact with DNA, RNA, and proteins and modify chromatin function, regulate transcription at various stages, forms nuclear condensation bodies and nucleolar organization. lncRNAs may also change the stability and translation of cytoplasmic mRNAs and hamper signaling pathways. Thus, lncRNAs affect the physio-pathological states and lead to the development of various disorders, immune responses, and cancer. To date, ~40% of lncRNAs have been reported in the nervous system (NS) and are involved in the early development/differentiation of the NS to synaptogenesis. LncRNA expression patterns in the most common adult and pediatric tumor suggest them as potential biomarkers and provide a rationale for targeting them pharmaceutically. Here, we discuss the mechanisms of lncRNA synthesis, localization, and functions in transcriptional, post-transcriptional, and other forms of gene regulation, methods of lncRNA identification, and their potential therapeutic applications in neuro oncological disorders as explained by molecular mechanisms in other malignant disorders.

Mammalian DNA Replication Timing

Immediately following the discovery of the structure of DNA and the semi-conservative replication of the parental DNA sequence into two new DNA strands, it became apparent that DNA replication is organized in a temporal and spatial fashion during the S phase of the cell cycle, correlated with the large-scale organization of chromatin in the nucleus. After many decades of limited progress, technological advances in genomics, genome engineering, and imaging have finally positioned the field to tackle mechanisms underpinning the temporal and spatial regulation of DNA replication and the causal relationships between DNA replication and other features of large-scale chromosome structure and function. In this review, we discuss these major recent discoveries as well as expectations for the coming decade.

Regulatory and Functional Involvement of Long Non-Coding RNAs in DNA Double-Strand Break Repair Mechanisms

Protection of genome integrity is vital for all living organisms, particularly when DNA double-strand breaks (DSBs) occur. Eukaryotes have developed two main pathways, namely Non-Homologous End Joining (NHEJ) and Homologous Recombination (HR), to repair DSBs. While most of the current research is focused on the role of key protein players in the functional regulation of DSB repair pathways, accumulating evidence has uncovered a novel class of regulating factors termed non-coding RNAs. Non-coding RNAs have been found to hold a pivotal role in the activation of DSB repair mechanisms, thereby safeguarding genomic stability. In particular, long non-coding RNAs (lncRNAs) have begun to emerge as new players with vast therapeutic potential. This review summarizes important advances in the field of lncRNAs, including characterization of recently identified lncRNAs, and their implication in DSB repair pathways in the context of tumorigenesis.

Very long intergenic non-coding (vlinc) RNAs directly regulate multiple genes in cis and trans

BACKGROUND

The majority of the human genome is transcribed in the form of long non-coding (lnc) RNAs. While these transcripts have attracted considerable interest, their molecular mechanisms of function and biological significance remain controversial. One of the main reasons behind this lies in the significant challenges posed by lncRNAs requiring the development of novel methods and concepts to unravel their functionality. Existing methods often lack cross-validation and independent confirmation by different methodologies and therefore leave significant ambiguity as to the authenticity of the outcomes. Nonetheless, despite all the caveats, it appears that lncRNAs may function, at least in part, by regulating other genes via chromatin interactions. Therefore, the function of a lncRNA could be inferred from the function of genes it regulates. In this work, we present a genome-wide functional annotation strategy for lncRNAs based on identification of their regulatory networks via the integration of three distinct types of approaches: co-expression analysis, mapping of lncRNA-chromatin interactions, and assaying molecular effects of lncRNA knockdowns obtained using an inducible and highly specific CRISPR/Cas13 system.

RESULTS

We applied the strategy to annotate 407 very long intergenic non-coding (vlinc) RNAs belonging to a novel widespread subclass of lncRNAs. We show that vlincRNAs indeed appear to regulate multiple genes encoding proteins predominantly involved in RNA- and development-related functions, cell cycle, and cellular adhesion via a mechanism involving proximity between vlincRNAs and their targets in the nucleus. A typical vlincRNAs can be both a positive and negative regulator and regulate multiple genes both in trans and cis. Finally, we show vlincRNAs and their regulatory networks potentially represent novel components of DNA damage response and are functionally important for the ability of cancer cells to survive genotoxic stress.

CONCLUSIONS

This study provides strong evidence for the regulatory role of the vlincRNA class of lncRNAs and a potentially important role played by these transcripts in the hidden layer of RNA-based regulation in complex biological systems.

Hovlinc is a recently evolved class of ribozyme found in human lncRNA

Although naturally occurring catalytic RNA molecules-ribozymes-have attracted a great deal of research interest, very few have been identified in humans. Here, we developed a genome-wide approach to discovering self-cleaving ribozymes and identified a naturally occurring ribozyme in humans. The secondary structure and biochemical properties of this ribozyme indicate that it belongs to an unidentified class of small, self-cleaving ribozymes. The sequence of the ribozyme exhibits a clear evolutionary path, from its appearance between ~130 and ~65 million years ago (Ma), to acquiring self-cleavage activity very recently, ~13-10 Ma, in the common ancestors of humans, chimpanzees and gorillas. The ribozyme appears to be functional in vivo and is embedded within a long noncoding RNA belonging to a class of very long intergenic noncoding RNAs. The presence of a catalytic RNA enzyme in lncRNA creates the possibility that these transcripts could function by carrying catalytic RNA domains.

Gene regulation by long non-coding RNAs and its biological functions

Evidence accumulated over the past decade shows that long non-coding RNAs (lncRNAs) are widely expressed and have key roles in gene regulation. Recent studies have begun to unravel how the biogenesis of lncRNAs is distinct from that of mRNAs and is linked with their specific subcellular localizations and functions. Depending on their localization and their specific interactions with DNA, RNA and proteins, lncRNAs can modulate chromatin function, regulate the assembly and function of membraneless nuclear bodies, alter the stability and translation of cytoplasmic mRNAs and interfere with signalling pathways. Many of these functions ultimately affect gene expression in diverse biological and physiopathological contexts, such as in neuronal disorders, immune responses and cancer. Tissue-specific and condition-specific expression patterns suggest that lncRNAs are potential biomarkers and provide a rationale to target them clinically. In this Review, we discuss the mechanisms of lncRNA biogenesis, localization and functions in transcriptional, post-transcriptional and other modes of gene regulation, and their potential therapeutic applications.

Interplay between DNA replication stress, chromatin dynamics and DNA-damage response for the maintenance of genome stability

DNA replication stress is a major source of DNA damage, including double-stranded breaks that promote DNA damage response (DDR) signaling. Inefficient repair of such lesions can affect genome integrity. During DNA replication different factors act on chromatin remodeling in a coordinated way. While recent studies have highlighted individual molecular mechanisms of interaction, less is known about the orchestration of chromatin changes under replication stress. In this review we attempt to explore the complex relationship between DNA replication stress, DDR and genome integrity in mammalian cells, taking into account the role of chromatin disposition as an important modulator of DNA repair. Recent data on chromatin restoration and epigenetic re-establishment after DNA replication stress are reviewed.

Exploring the Roles of lncRNAs in GBM Pathophysiology and Their Therapeutic Potential

Glioblastoma (GBM) remains the most devastating primary central nervous system malignancy with a median survival of around 15 months. The past decades of research have not yielded significant advancements in the treatment of GBM. In that same time, a novel class of molecules, long non-coding RNAs (lncRNAs), has been found to play a multitude of roles in cancer and normal biology. The increased accessibility of next generation sequencing technologies and the advent of lncRNA-specific microarrays have facilitated the study of lncRNA etiology. Molecular and computational methods can be applied to predict lncRNA function. LncRNAs can serve as molecular decoys, scaffolds, super-enhancers, or repressors. These molecules can serve as phenotypic switches for GBM cells at the expression and/or epigenetic levels. LncRNAs can affect stemness/differentiation, proliferation, invasion, survival, DNA damage response, and chromatin dynamics. Aberrant expression of these transcripts may facilitate therapy resistance, leading to tumor recurrence. LncRNAs could serve as novel theragnostic or prognostic biomarkers in GBM and other cancers. RNA-based therapeutics may also be employed to target lncRNAs as a novel route of treatment for primary or recurrent GBM. In this review, we explore the roles of lncRNAs in GBM pathophysiology and posit their novel therapeutic potential for GBM.

Reverse-genetics studies of lncRNAs-what we have learnt and paths forward

Long non-coding RNAs (lncRNAs) represent a major fraction of the transcriptome in multicellular organisms. Although a handful of well-studied lncRNAs are broadly recognized as biologically meaningful, the fraction of such transcripts out of the entire collection of lncRNAs remains a subject of vigorous debate. Here we review the evidence for and against biological functionalities of lncRNAs and attempt to arrive at potential modes of lncRNA functionality that would reconcile the contradictory conclusions. Finally, we discuss different strategies of phenotypic analyses that could be used to investigate such modes of lncRNA functionality.