Specific Transposable Elements Research Articles

Genome-wide profiling the epigenetic landscape of histone variant TH2B in murine oocytes and pre-implantation embryos.

The histone variant TH2B, enriched in oocytes, sperm, and early embryos, decreases as embryos differentiate into pre-gastrula stages. Despite its presence, the role of TH2B in epigenetic reprogramming during early embryonic development remains largely under-researched. Our study employed ultra-low-input ChIP-seq (ULI-ChIP) to analyze the genome-wide distribution of TH2B in MII oocytes and early embryos. We found that TH2B is enriched in the chromatin of oocytes and 2-cell stage embryos but becomes less prevalent after the 2-cell stage. Correlation analysis revealed that the TH2B chromatin patterns in sperm and preimplantation embryos are more similar to each other than to those in MII oocytes. Gene ontology (GO) analysis of TH2B-occupied loci linked them to various developmental processes, including oogenesis, fertilization, chromatin modification, and transcription regulation. The study also identified a strong association of TH2B with specific transposable elements (TEs), particularly long terminal repeats (LTRs), which are known to regulate preimplantation development. Additionally, early embryos showed H3K9me3 marks at TH2B-bound loci. TH2B exhibited strong correlations with H2A.Z and H3.3 in the 2-cell and 8-cell stages, a positive association with H3K27Ac and H3K4me3, and a negative correlation with H3K27me3. Allelic reprogramming analysis of TH2B in embryos from C57BL/6J and DBA/2J crosses revealed differential dynamics between maternal and paternal alleles, with a notable paternal bias at the promoter in 2-cell embryos. Thus, TH2B's enrichment in early embryonic stages and its association with key regulatory regions and histone modifications underscore its importance in ZGA and subsequent developmental processes.

Transposable elements contribute to tissue-specific gene regulation in humans.

Transposable elements (TEs) contribute to approximately half of the human genome, and along with many other functions, they have been known to play a role in gene regulation in the genome. With TEs' active/repressed states varying across tissue and cell types, they have the potential to regulate gene expression in a tissue-specific manner. To provide a systematic analysis of TEs' contribution in tissue-specific gene regulation, we examined the regulatory elements and genes in association with TE-derived regulatory sequences in 14 human cell lines belonging to 10 different tissue types using the functional genomics data from the ENCODE project. Specifically, we separately analyzed regulatory regions identified by three different approaches (DNase hypersensitive sites (DHS), histone active sites (HA), and histone repressive sites (HR)). These regulatory regions showed to be distinct from each other by sharing less than 2.5% among all three types and more than 95% showed to be cell line-specific. Despite a lower total TE content overall than the genome average, each regulatory sequence type showed enrichment for one or two specific TE type(s): DHS for long terminal repeats (LTRs) and DNA transposons, HA for short interspersed nucleotide elements (SINEs), and HR for LTRs. In contrast, SINE was shown to be overrepresented in all three types of regulatory sequences located in gene-neighboring regions. TE-regulated genes were mostly shown to have cell line specific pattern, and tissue-specific genes (TSGs) showed higher usage of TE regulatory sequences in the tissue of their expression. While TEs in the regulatory sequences showed to be older than their genome-wide counterparts, younger TEs were shown to be more likely used in cell line specific regulatory sequences. Collectively, our study provided further evidence enforcing an important contribution of TEs to tissue-specific gene regulation in humans.

Consistent accumulation of transposable elements in species of the Hawaiian Tetragnatha spiny-leg adaptive radiation across the archipelago chronosequence

Abstract The ecological and phenotypic diversity observed in oceanic island radiations presents an evolutionary paradox: a high level of genetic variation is typically required for diversification, but species colonizing a new island commonly suffer from founder effects. This reduction in population size leads to lower genetic diversity, which ultimately results in a reduction in the efficiency of natural selection. Then, what is the source of genetic variation which acts as the raw material for ecological and phenotypic diversification in oceanic archipelagos? Transposable elements (TEs) are mobile genetic elements that have been linked to the generation of genetic diversity, and evidence suggests that TE activity and accumulation along the genome can result from reductions in population size. Here, we use the Hawaiian spiny-leg spider radiation (Tetragnatha) to test whether TE accumulation increases due to demographic processes associated with island colonization. We sequenced and quantified TEs in 23 individuals representing 16 species from the spiny-leg radiation and 4 individuals from its sister radiation, the Hawaiian web-building Tetragnatha. Our results show that founder effects resulting from colonization of new islands have not resulted in TE accumulation over evolutionary time. Specifically, we found no evidence for increase in abundance of specific TE superfamilies, nor an accumulation of ‘young TEs’ in lineages which have recently colonized a new island or are present in islands with active volcanoes. We also found that the DNA/hAT transposon superfamily is by far the most abundant TE superfamily in the Tetragnatha radiation. This work shows that there is no clear trend of increasing TE abundance for the spiny-leg radiation across the archipelago chronosequence, and TE accumulation is not affected by population oscillations associated with island colonization events. Therefore, despite their known role in the generation of genetic diversity, TE activity does not appear to be the mechanism explaining the evolutionary paradox of insular diversification in the Tetragnatha spiny-leg radiation.

Single-cell transcriptome profiling reveals the spatiotemporal distribution of triterpenoid saponin biosynthesis and transposable element activity in Gynostemma pentaphyllum shoot apexes and leaves.

Gynostemma pentaphyllum (Thunb.) Makino is an important producer of dammarene-type triterpenoid saponins. These saponins (gypenosides) exhibit diverse pharmacological benefits such as anticancer, antidiabetic, and immunomodulatory effects, and have major potential in the pharmaceutical and health care industries. Here, we employed single-cell RNA sequencing (scRNA-seq) to profile the transcriptomes of more than 50,000 cells derived from G. pentaphyllum shoot apexes and leaves. Following cell clustering and annotation, we identified five major cell types in shoot apexes and four in leaves. Each cell type displayed substantial transcriptomic heterogeneity both within and between tissues. Examining gene expression patterns across various cell types revealed that gypenoside biosynthesis predominantly occurred in mesophyll cells, with heightened activity observed in shoot apexes compared to leaves. Furthermore, we explored the impact of transposable elements (TEs) on G. pentaphyllum transcriptomic landscapes. Our findings the highlighted the unbalanced expression of certain TE families across different cell types in shoot apexes and leaves, marking the first investigation of TE expression at the single-cell level in plants. Additionally, we observed dynamic expression of genes involved in gypenoside biosynthesis and specific TE families during epidermal and vascular cell development. The involvement of TE expression in regulating cell differentiation and gypenoside biosynthesis warrant further exploration. Overall, this study not only provides new insights into the spatiotemporal organization of gypenoside biosynthesis and TE activity in G. pentaphyllum shoot apexes and leaves but also offers valuable cellular and genetic resources for a deeper understanding of developmental and physiological processes at single-cell resolution in this species.

Transposable element-derived double-stranded RNAs stimulate pro-inflammatory activation in human astrocytes

Astrocyte-related inflammation contributes to neuroinflammation with brain aging and Alzheimer’s disease (AD), but the causes of pro-inflammatory astrocyte activation in this context are incompletely understood. One driver of pro-inflammatory signaling in astrocytes may be endogenous double-stranded RNA (dsRNA) derived from transposable elements (TEs). TEs make up more than 50% of the human genome, and recent evidence suggests a role for TEs in pro-inflammatory signaling with aging and neurodegenerative diseases. However, the role of TEs and/or TE-derived dsRNA in astrocyte-related inflammation, and the specific TEs involved, have yet to be investigated. Here, we increased the expression of TE transcripts in human astrocytes by suppressing ADAR1 (an enzyme involved in the suppression/regulation of endogenous dsRNA) and performed RNA-seq to broadly characterize resulting biological changes. Using a computational pipeline (SPRINT) to detect potential dsRNA transcripts through adenosine-to-inosine (A-to-I; a marker of ADAR1 activity) edits, we found that dsRNA transcripts were more likely to come from TEs compared to coding genes. Of these TEs, highly edited and increased transcripts consisted of the TE families Alu and LINE-1 (L1) elements. We confirmed the increase in L1 transcripts in vitro by characterizing colocalization of J2 (antibody for dsRNA) and an L1 element via fluorescence in situ hybridization. Increases in these TEs were associated with increases in pro-inflammatory signaling and decreases in expression of genes relating to astrocyte identity/function (e.g., synapse modifying and cholesterol-related genes). Lastly, we found similar increases in Alu and L1 transcripts in brain tissue of AD patients and older subjects. Together, these data suggest a role for TE-derived dsRNA in pro-inflammatory activation in astrocytes, and possibly in aging and AD. To expand on these findings, we have performed RNA immunoprecipitation sequencing to identify the specific TEs that form dsRNA with ADAR1 suppression in astrocytes, and we are developing an approach to mitigate their effects by increasing ADAR1 expression in the brains of old mice. NIA: F31AG084330, R01AG078859. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Identification of key TE associated with myocarditis based on RNA and single-cell sequencing data mining

Cardiomyopathy is a severe cardiac condition characterized by complex immune regulatory mechanisms. While the role of immune genes is recognized, the specifics of their regulation in cardiomyopathy are not fully understood. Recent studies highlight the significance of transposable elements (TEs) in various diseases, particularly their potential to modulate immune responses. This paper utilizes publicly available databases to explore the role of TEs in myocarditis: RNA Seq data and single-cell sequencing data were analyzed, with a focus on the mouse model of experimental autoimmune myocarditis (EAM). The RNA-Seq analysis revealed substantial upregulation of a range of immune genes in cardiac tissue. Further investigation using single-cell sequencing of cardiac immune cells identified specific expression of certain transposable elements (TEs) across different types of immune cells in the heart. Additionally, there was an overall increase in the expression of the ERVB7-1. LTR-MM transposon across various cells in the EAM model, suggesting a widespread impact of this transposon on the immune response in this disease context. The findings of this study highlight the intricate interplay between transposable elements and the immune system in cardiomyopathy, providing new insights into the molecular mechanisms underlying this condition. The discovery of specific TEs expression in cardiac immune cells and the overall increase in ERVB7-1. LTR-MM expression across the EAM model underscore the potential of these elements in modulating immune responses and contribute to our understanding of cardiomyopathy's pathogenesis. These observations open avenues for further research into the role of TEs in cardiac disases and may lead to novel therapeutic strategies.

Transposable elements acquire time- and sex-specific transcriptional and epigenetic signatures along mouse fetal gonad development.

Gonadal sex determination in mice is a complex and dynamic process, which is crucial for the development of functional reproductive organs. The expression of genes involved in this process is regulated by a variety of genetic and epigenetic mechanisms. Recently, there has been increasing evidence that transposable elements (TEs), which are a class of mobile genetic elements, play a significant role in regulating gene expression during embryogenesis and organ development. In this study, we aimed to investigate the involvement of TEs in the regulation of gene expression during mouse embryonic gonadal development. Through bioinformatics analysis, we aimed to identify and characterize specific TEs that operate as regulatory elements for sex-specific genes, as well as their potential mechanisms of regulation. We identified TE loci expressed in a time- and sex-specific manner along fetal gonad development that correlate positively and negatively with nearby gene expression, suggesting that their expression is integrated to the gonadal regulatory network. Moreover, chromatin accessibility and histone post-transcriptional modification analyses in differentiating supporting cells revealed that TEs are acquiring a sex-specific signature for promoter-, enhancer-, and silencer-like elements, with some of them being proximal to critical sex-determining genes. Altogether, our study introduces TEs as the new potential players in the gene regulatory network that controls gonadal development in mammals.

Molecular identification of polymorphic transposable elements in populations of the invasive ant Cardiocondyla obscurior.

Transposable elements (TEs) are found in virtually every eukaryotic genome and are important for generating de novo genetic variation. However, outside of costly and time-consuming whole-genome sequencing approaches, the set of available methods to study TE polymorphisms in non-model species is very limited. The Transposon Display (TD) is a simple yet effective technique to characterize polymorphisms across samples by identifying amplified fragment length polymorphisms using primers targeting specific TE families. So far, this technique has almost exclusively been used in plants. Here, we present an optimized TD protocol for insect species with small genomes such as ants (ca. 200-600 Mb). We characterized TE polymorphisms between two distinct genetic lineages of the invasive ant Cardiocondyla obscurior, as well as between neighboring populations of the New World lineage. We found active LTR/Ty3 retrotransposons, that contributed to the genetic diversification of populations in this species.

Abstract LB_B21: Extracellular RNA signatures of mutant KRAS-driven lung cancer

Abstract RAS genes are the most frequently mutated oncogenes in cancer, yet the effects of oncogenic RAS signaling on the noncoding transcriptome remain unclear. We analyzed the transcriptomes of human airway and bronchial epithelial cells transformed with mutant KRAS to define the landscape of KRAS-regulated noncoding RNAs. We find that oncogenic KRAS signaling upregulates noncoding transcripts throughout the genome, many of which arise from transposable elements (TEs). These TE RNAs exhibit differential expression and are regulated by KRAB zinc-finger (KZNF) genes, which are broadly downregulated in mutant KRAS cells and lung adenocarcinomas in vivo. Using both affinity- and nanofiltration-based EV isolation approaches, we show that specific TE RNAs, lncRNAs, and mRNAs, some of which are prognostic for lung adenocarcinoma (LUAD) patient survival, are preferentially released in extracellular vesicles (EVs). Moreover, we find that inhibition of oncogenic KRAS signaling broadly reprograms the noncoding transcriptome, as evidenced by a substantial increase in TE RNA secretion. Mutant KRAS inhibition also increases the abundance of secreted lncRNAs and retained intron-containing transcripts, while decreasing the mRNA content of EVs. Notably, oncogenic KRAS signaling is required for the secretion of mRNAs from a set of 20 genes that are significantly associated with unfavorable clinical outcomes in LUAD. Our results indicate that mutant KRAS remodels the repetitive noncoding transcriptome, demonstrating the broad scope of intracellular and extracellular RNAs regulated by this oncogenic signaling pathway. Our study also suggests that both coding and noncoding RNAs that are secreted in EVs may serve as mutant KRAS-specific signatures for diagnosing lung cancer via minimally invasive RNA liquid biopsy technology, which we are currently developing as a companion diagnostic for mutant KRAS-targeting therapies. Citation Format: Sree Lakshmi Velandi Maroli, Roman E Reggiardo, Reem Khojah, Daniel H Kim. Extracellular RNA signatures of mutant KRAS-driven lung cancer [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr LB_B21.

The blackcap (Sylvia atricapilla) genome reveals a recent accumulation of LTR retrotransposons

Transposable elements (TEs) are mobile genetic elements that can move around the genome, and as such are a source of genomic variability. Based on their characteristics we can annotate TEs within the host genome and classify them into specific TE types and families. The increasing number of available high-quality genome references in recent years provides an excellent resource that will enhance the understanding of the role of recently active TEs on genetic variation and phenotypic evolution. Here we showcase the use of a high-quality TE annotation to understand the distinct effect of recent and ancient TE insertions on the evolution of genomic variation, within our study species the Eurasian blackcap (Sylvia atricapilla). We investigate how these distinct TE categories are distributed along the genome and evaluate how their coverage across the genome is correlated with four genomic features: recombination rate, gene coverage, CpG island coverage and GC content. We found within the recent TE insertions an accumulation of LTRs previously not seen in birds. While the coverage of recent TE insertions was negatively correlated with both GC content and recombination rate, the correlation with recombination rate disappeared and turned positive for GC content when considering ancient TE insertions.

ZFP92, a KRAB domain zinc finger protein enriched in pancreatic islets, binds to B1/Alu SINE transposable elements and regulates retroelements and genes.

Repressive KRAB domain-containing zinc-finger proteins (KRAB-ZFPs) are abundant in mammalian genomes and contribute both to the silencing of transposable elements (TEs) and to the regulation of developmental stage- and cell type-specific gene expression. Here we describe studies of zinc finger protein 92 (Zfp92), an X-linked KRAB-ZFP that is highly expressed in pancreatic islets of adult mice, by analyzing global Zfp92 knockout (KO) mice. Physiological, transcriptomic and genome-wide chromatin binding studies indicate that the principal function of ZFP92 in mice is to bind to and suppress the activity of B1/Alu type of SINE elements and modulate the activity of surrounding genomic entities. Deletion of Zfp92 leads to changes in expression of select LINE and LTR retroelements and genes located in the vicinity of ZFP92-bound chromatin. The absence of Zfp92 leads to altered expression of specific genes in islets, adipose and muscle that result in modest sex-specific alterations in blood glucose homeostasis, body mass and fat accumulation. In islets, Zfp92 influences blood glucose concentration in postnatal mice via transcriptional effects on Mafb, whereas in adipose and muscle, it regulates Acacb, a rate-limiting enzyme in fatty acid metabolism. In the absence of Zfp92, a novel TE-Capn11 fusion transcript is overexpressed in islets and several other tissues due to de-repression of an IAPez TE adjacent to ZFP92-bound SINE elements in intron 3 of the Capn11 gene. Together, these studies show that ZFP92 functions both to repress specific TEs and to regulate the transcription of specific genes in discrete tissues.

Phenotype stability and dynamics of transposable elements in a strain of the microalga Tisochrysis lutea with improved lipid traits.

Microalgal domestication is an expanding research field that aims to multiply and accelerate the potential of microalgae for various biotechnological purposes. We investigated the stability of improved lipid traits and genetic changes of a domesticated strain of the haptophyte Tisochrysis lutea, TisoS2M2, previously obtained by a mutation-selection improvement program. After 7 years of maintenance, TisoS2M2 still displayed improved lipid traits compared with the native strain, demonstrating that a mutation-selection improvement program is suitable for obtaining a domesticated strain with stable, improved phenotype over time. We identified specific genetic variations between the native and domesticated strains and focused on the dynamics of transposable elements (TEs). DNA transposons mainly caused specific TE indels of the domesticated strain TisoS2M2, and some specific TE indels may have impacted genes associated to the neutral lipid pathway. We revealed transposition events for TEs in T. lutea and discussed on the potential role of the improvement program on their activity.

Environmental response in gene expression and DNA methylation reveals factors influencing the adaptive potential of Arabidopsis lyrata.

Understanding what factors influence plastic and genetic variation is valuable for predicting how organisms respond to changes in the selective environment. Here, using gene expression and DNA methylation as molecular phenotypes, we study environmentally induced variation among Arabidopsis lyrata plants grown at lowland and alpine field sites. Our results show that gene expression is highly plastic, as many more genes are differentially expressed between the field sites than between populations. These environmentally responsive genes evolve under strong selective constraint - the strength of purifying selection on the coding sequence is high, while the rate of adaptive evolution is low. We find, however, that positive selection on cis-regulatory variants has likely contributed to the maintenance of genetically variable environmental responses, but such variants segregate only between distantly related populations. In contrast to gene expression, DNA methylation at genic regions is largely insensitive to the environment, and plastic methylation changes are not associated with differential gene expression. Besides genes, we detect environmental effects at transposable elements (TEs): TEs at the high-altitude field site have higher expression and methylation levels, suggestive of a broad-scale TE activation. Compared to the lowland population, plants native to the alpine environment harbor an excess of recent TE insertions, and we observe that specific TE families are enriched within environmentally responsive genes. Our findings provide insight into selective forces shaping plastic and genetic variation. We also highlight how plastic responses at TEs can rapidly create novel heritable variation in stressful conditions.

The Newly Sequenced Genome of Pisum sativum Is Replete with Potential G-Quadruplex-Forming Sequences-Implications for Evolution and Biological Regulation.

G-quadruplexes (G4s) have been long considered rare and physiologically unimportant in vitro curiosities, but recent methodological advances have proved their presence and functions in vivo. Moreover, in addition to their functional relevance in bacteria and animals, including humans, their importance has been recently demonstrated in evolutionarily distinct plant species. In this study, we analyzed the genome of Pisum sativum (garden pea, or the so-called green pea), a unique member of the Fabaceae family. Our results showed that this genome contained putative G4 sequences (PQSs). Interestingly, these PQSs were located nonrandomly in the nuclear genome. We also found PQSs in mitochondrial (mt) and chloroplast (cp) DNA, and we experimentally confirmed G4 formation for sequences found in these two organelles. The frequency of PQSs for nuclear DNA was 0.42 PQSs per thousand base pairs (kbp), in the same range as for cpDNA (0.53/kbp), but significantly lower than what was found for mitochondrial DNA (1.58/kbp). In the nuclear genome, PQSs were mainly associated with regulatory regions, including 5′UTRs, and upstream of the rRNA region. In contrast to genomic DNA, PQSs were located around RNA genes in cpDNA and mtDNA. Interestingly, PQSs were also associated with specific transposable elements such as TIR and LTR and around them, pointing to their role in their spreading in nuclear DNA. The nonrandom localization of PQSs uncovered their evolutionary and functional significance in the Pisum sativum genome.

Do Ty3/Gypsy Transposable Elements Play Preferential Roles in Sex Chromosome Differentiation?

Transposable elements (TEs) comprise a substantial portion of eukaryotic genomes. They have the unique ability to integrate into new locations and serve as the main source of genomic novelties by mediating chromosomal rearrangements and regulating portions of functional genes. Recent studies have revealed that TEs are abundant in sex chromosomes. In this review, we propose evolutionary relationships between specific TEs, such as Ty3/Gypsy, and sex chromosomes in different lineages based on the hypothesis that these elements contributed to sex chromosome differentiation processes. We highlight how TEs can drive the dynamics of sex-determining regions via suppression recombination under a selective force to affect the organization and structural evolution of sex chromosomes. The abundance of TEs in the sex-determining regions originates from TE-poor genomic regions, suggesting a link between TE accumulation and the emergence of the sex-determining regions. TEs are generally considered to be a hallmark of chromosome degeneration. Finally, we outline recent approaches to identify TEs and study their sex-related roles and effects in the differentiation and evolution of sex chromosomes.

High Stability of the Epigenome in Drosophila Interspecific Hybrids.

Interspecific hybridization is often seen as a genomic stress that may lead to new gene expression patterns and deregulation of transposable elements (TEs). The understanding of expression changes in hybrids compared with parental species is essential to disentangle their putative role in speciation processes. However, to date we ignore the detailed mechanisms involved in genomic deregulation in hybrids. We studied the ovarian transcriptome and epigenome of the Drosophila buzzatii and Drosophila koepferae species together with their F1 hybrid females. We found a trend toward underexpression of genes and TE families in hybrids. The epigenome in hybrids was highly similar to the parental epigenomes and showed intermediate histone enrichments between parental species in most cases. Differential gene expression in hybrids was often associated only with changes in H3K4me3 enrichments, whereas differential TE family expression in hybrids may be associated with changes in H3K4me3, H3K9me3, or H3K27me3 enrichments. We identified specific genes and TE families, which their differential expression in comparison with the parental species was explained by their differential chromatin mark combination enrichment. Finally, cis–trans compensatory regulation could also contribute in some way to the hybrid deregulation. This work provides the first study of histone content in Drosophila interspecific hybrids and their effect on gene and TE expression deregulation.

Differential regulation of transposable elements (TEs) during the murine submandibular gland development

The submandibular gland (SG) is a relatively simple organ formed by three cell types: acinar, myoepithelial, and an intricate network of duct-forming epithelial cells, that together fulfills several physiological functions from assisting food digestion to acting as an immune barrier against pathogens. Successful SG organogenesis is the product of highly controlled and orchestrated genetic and transcriptional programs. Mounting evidence links Transposable Elements (TEs), originally thought to be selfish genetic elements, to different aspects of gene regulation in mammalian development and disease. To our knowledge, the role of TEs during murine SG organogenesis has not been studied. Using novel bioinformatic tools and publicly available RNA-Seq datasets, our results indicate that a significant number of genic and intergenic TEs are differentially expressed during the SG development. Furthermore, changes in expression of specific TEs correlated with that of genes involved in cellular division and differentiation, critical aspects for SG maturation. Altogether, we propose that TEs modulate gene networks that operate during SG development.

A population-level invasion by transposable elements triggers genome expansion in a fungal pathogen.

Genome evolution is driven by the activity of transposable elements (TEs). The spread of TEs can have deleterious effects including the destabilization of genome integrity and expansions. However, the precise triggers of genome expansions remain poorly understood because genome size evolution is typically investigated only among deeply divergent lineages. Here, we use a large population genomics dataset of 284 individuals from populations across the globe of Zymoseptoria tritici, a major fungal wheat pathogen. We built a robust map of genome-wide TE insertions and deletions to track a total of 2456 polymorphic loci within the species. We show that purifying selection substantially depressed TE frequencies in most populations, but some rare TEs have recently risen in frequency and likely confer benefits. We found that specific TE families have undergone a substantial genome-wide expansion from the pathogen's center of origin to more recently founded populations. The most dramatic increase in TE insertions occurred between a pair of North American populations collected in the same field at an interval of 25 years. We find that both genome-wide counts of TE insertions and genome size have increased with colonization bottlenecks. Hence, the demographic history likely played a major role in shaping genome evolution within the species. We show that both the activation of specific TEs and relaxed purifying selection underpin this incipient expansion of the genome. Our study establishes a model to recapitulate TE-driven genome evolution over deeper evolutionary timescales.

Comprehensive Characterization of Transposable Elements in the Acute Myeloid Leukemia Transcriptome

Over half of the human genome is comprised of genomic transposable elements (TEs). TE expression has been implicated in cancer. It has been shown to induce genomic instability and more recently, a potential beneficial effect by causing cancer cell death by activating the viral recognition pathways. Recently we showed that the expression of distinct classes of TE such as SINEs and LTRs, but not LINEs, are suppressed in acute myeloid leukemia (AML) stem cells. Hence, it is likely that different classes and subtypes of TE perform diverse functions in cancer.Several large-scale studies characterized the transcriptome of numerous cancer types, and improved our understanding of oncogenesis in cancer. However, the expression of TEs has not been analyzed in these impactful studies. We developed a tool, Arkas, to comprehensively characterize the expression of TEs, accompanying coding gene expression, and gene pathways in large transcriptome databases. Using this we investigated the dysregulation of TEs in 178 AML transcriptome from The Cancer Genome Atlas.We first investigated the effect of recurrent AML mutations on TE dysregulation. In order to disentangle the effect of recurrent mutations on the TE expression, we combined mutation with TE expression in a multivariate linear model. Using a mutation-centric view, we observed that the number TEs significantly differentially expressed with respect to mutational drivers varied significantly with different mutations (Figure A). TE dysregulation was most affected by TP53 mutation, showing downregulation. MLL gene rearrangement was the second most, with mostly upregulated TE expression.Regulation of TE expression and its downstream effects is not fully understood. In the past, assumptions have been made generalizing the function of TE as a whole without understanding the diversity in TE. In addition to directly activating the interferon pathway and DNA damage, TEs are key regulators of coding gene expression, hence likely exhibiting diverse function. In order to gain insight into this, we correlated the expression of TE biotypes with coding gene networks. The coding genes were first clustered together (to form modules) based on their co-expression profile. 50 modules were formed using this method (Figure D). These modules are likely co-regulated and/or functionally related. These modules were then correlated with the expression of specific TE types. Distinct correlation pattern emerged with coding gene networks; positively associated modules with LINE1 were negatively associated with LTR elements including ERVs. Within the LTR class, distinct modules were associated with each ERV types. Though these findings do not prove direct functional relationship between TE types and the coding gene networks, they support diverse functions of exhibited by each TE type and subtype.Finally, we investigated the role of TE in predicting prognosis in AML. Using a penalized cox regression analysis, which included nested 10-fold cross validation, we examined the TE expression association to survival prediction. We identified 17 TE transcripts able to predict prognosis in AML (Figure C), classifying them into good-risk (N=77) and bad risk AML (N=101) (P=0.0011) (Figure B). Interestingly, this signature was able to further divide good-risk AML (N=87, identified based on gene mutations) into good-risk (N=41) and bad-risk AML (N=46) (P=0.026), suggesting TE predicts prognosis independently from gene mutations. 12 (9 of which were ERVs) of the 17 TEs predicting prognosis were associated with good prognosis. 5 TEs including L1M3B_5, which is LINE1, predicted worse prognosis. Interestingly, 3 of the good risk TEs were suppressed in TP53 mutation. In summary, this is the first study to comprehensively characterize TE expression in cancer. TE expression in AML shows distinct expression pattern associated with specific mutations and coding gene networks. TE expression offers independent prognostic information. These finding suggest the need for future mechanistic characterization of the role of distinct TEs in AML. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

YY2 in Mouse Preimplantation Embryos and in Embryonic Stem Cells.

Yin Yang 2 encodes a mammalian-specific transcription factor (YY2) that shares high homology in the zinc finger region with both YY1 and REX1/ZFP42, encoded by the Yin Yang 1 and Reduced Expression Protein 1/Zinc Finger Protein 42 gene, respectively. In contrast to the well-established roles of the latter two in gene regulation, X chromosome inactivation and binding to specific transposable elements (TEs), much less is known about YY2, and its presence during mouse preimplantation development has not been described. As it has been reported that mouse embryonic stem cells (mESC) cannot be propagated in the absence of Yy2, the mechanistic understanding of how Yy2 contributes to mESC maintenance remains only very partially characterized. We describe Yy2 expression studies using RT-PCR and staining with a high-affinity polyclonal serum in mouse embryos and mESC. Although YY2 is expressed during preimplantation development, its presence appears dispensable for developmental progress in vitro until formation of the blastocyst. Attenuation of Yy2 levels failed to alter either Zscan4 levels in two-cell embryos or IAP and MERVL levels at later preimplantation stages. In contrast to previous claims that constitutively expressed shRNA against Yy2 in mESC prohibited the propagation of mESC in culture, we obtained colonies generated from mESC with attenuated Yy2 levels. Concomitant with a decreased number of undifferentiated colonies, Yy2-depleted mESC expressed higher levels of Zscan4 but no differences in the expression of TEs or other pluripotency markers including Sox2, Oct4, Nanog and Esrrb were observed. These results confirm the contribution of Yy2 to the maintenance of mouse embryonic stem cells and show the preimplantation expression of YY2. These functions are discussed in relation to mammalian-specific functions of YY1 and REX1.