Activation of neuronal genes via LINE-1 elements upon global DNA demethylation in human neural progenitors

DNA methylation governs the sensitivity of repeats to restriction by the HUSH-MORC2 corepressor

The human silencing hub (HUSH) complex binds to transcripts of LINE-1 retrotransposons (L1s) and other genomic repeats, recruiting MORC2 and other effectors to remodel chromatin. How HUSH and MORC2 operate alongside DNA methylation, a central epigenetic regulator of repeat transcription, remains largely unknown. Here we interrogate this relationship in human neural progenitor cells (hNPCs), a somatic model of brain development that tolerates removal of DNA methyltransferase DNMT1. Upon loss of MORC2 or HUSH subunit TASOR in hNPCs, L1s remain silenced by robust promoter methylation. However, genome demethylation and activation of evolutionarily-young L1s attracts MORC2 binding, and simultaneous depletion of DNMT1 and MORC2 causes massive accumulation of L1 transcripts. We identify the same mechanistic hierarchy at pericentromeric α-satellites and clustered protocadherin genes, repetitive elements important for chromosome structure and neurodevelopment respectively. Our data delineate the epigenetic control of repeats in somatic cells, with implications for understanding the vital functions of HUSH-MORC2 in hypomethylated contexts throughout human development.

LINE-1 retrotransposons contribute to mouse PV interneuron development

Retrotransposons are mobile DNA sequences duplicated via transcription and reverse transcription of an RNA intermediate. Cis-regulatory elements encoded by retrotransposons can also promote the transcription of adjacent genes. Somatic LINE-1 (L1) retrotransposon insertions have been detected in mammalian neurons. It is, however, unclear whether L1 sequences are mobile in only some neuronal lineages or therein promote neurodevelopmental gene expression. Here we report programmed L1 activation by SOX6, a transcription factor critical for parvalbumin (PV) interneuron development. Mouse PV interneurons permit L1 mobilization in vitro and in vivo, harbor unmethylated L1 promoters and express full-length L1 mRNAs and proteins. Using nanopore long-read sequencing, we identify unmethylated L1s proximal to PV interneuron genes, including a novel L1 promoter-driven Caps2 transcript isoform that enhances neuron morphological complexity in vitro. These data highlight the contribution made by L1 cis-regulatory elements to PV interneuron development and transcriptome diversity, uncovered due to L1 mobility in this milieu.

Mini-heterochromatin domains constrain the cis-regulatory impact of SVA transposons in human brain development and disease

SVA (SINE (short interspersed nuclear element)-VNTR (variable number of tandem repeats)-Alu) retrotransposons remain active in humans and contribute to individual genetic variation. Polymorphic SVA alleles harbor gene regulatory potential and can cause genetic disease. However, how SVA insertions are controlled and functionally impact human disease is unknown. Here we dissect the epigenetic regulation and influence of SVAs in cellular models of X-linked dystonia parkinsonism (XDP), a neurodegenerative disorder caused by an SVA insertion at the TAF1 locus. We demonstrate that the KRAB zinc finger protein ZNF91 establishes H3K9me3 and DNA methylation over SVAs, including polymorphic alleles, in human neural progenitor cells. The resulting mini-heterochromatin domains attenuate the cis-regulatory impact of SVAs. This is critical for XDP pathology; removal of local heterochromatin severely aggravates the XDP molecular phenotype, resulting in increased TAF1 intron retention and reduced expression. Our results provide unique mechanistic insights into how human polymorphic transposon insertions are recognized and how their regulatory impact is constrained by an innate epigenetic defense system.

Navigating the brain and aging: exploring the impact of transposable elements from health to disease

Transposable elements (TEs) are mobile genetic elements that constitute on average 45% of mammalian genomes. Their presence and activity in genomes represent a major source of genetic variability. While this is an important driver of genome evolution, TEs can also have deleterious effects on their hosts. A growing number of studies have focused on the role of TEs in the brain, both in physiological and pathological contexts. In the brain, their activity is believed to be important for neuronal plasticity. In neurological and age-related disorders, aberrant activity of TEs may contribute to disease etiology, although this remains unclear. After providing a comprehensive overview of transposable elements and their interactions with the host, this review summarizes the current understanding of TE activity within the brain, during the aging process, and in the context of neurological and age-related conditions.

Long interspersed nuclear elements safeguard neural progenitors from precocious differentiation

Long interspersed nuclear element-1 (L1 or LINE-1) is a highly abundant mobile genetic element in both humans and mice, comprising almost 20% of each genome. L1s are silenced by several mechanisms, as their uncontrolled expression has the potential to induce genomic instability. However, L1s are paradoxically expressed at high levels in differentiating neural progenitor cells. Using in vitro and in vivo techniques to modulate L1 expression, we report that L1s play a critical role in both human and mouse brain development by regulating the rate of neural differentiation in a reverse-transcription-independent manner.

Locus-level L1 DNA methylation profiling reveals the epigenetic and transcriptional interplay between L1s and their integration sites

Long interspersed element 1 (L1) retrotransposons are implicated in human disease and evolution. Their global activity is repressed by DNA methylation, but deciphering the regulation of individual copies has been challenging. Here, we combine short- and long-read sequencing to unveil L1 methylation heterogeneity across cell types, families, and individual loci and elucidate key principles involved. We find that the youngest primate L1 families are specifically hypomethylated in pluripotent stem cells and the placenta but not in most tumors. Locally, intronic L1 methylation is intimately associated with gene transcription. Conversely, the L1 methylation state can propagate to the proximal region up to 300 bp. This phenomenon is accompanied by the binding of specific transcription factors, which drive the expression of L1 and chimeric transcripts. Finally, L1 hypomethylation alone is typically insufficient to trigger L1 expression due to redundant silencing pathways. Our results illuminate the epigenetic and transcriptional interplay between retrotransposons and their host genome.

Transposable Elements: Emerging Therapeutic Targets in Neurodegenerative Diseases

Neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS), are characterized by the progressive loss of neuronal function and structure. While several genetic and environmental factors have been implicated in the pathogenesis of these disorders, emerging evidence suggests that transposable elements (TEs), once considered "junk DNA," play a significant role in their development and progression. TEs are mobile genetic elements capable of moving within the genome, and their dysregulation has been associated with genomic instability, altered gene expression, and neuroinflammation. This review provides an overview of TEs, including long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and endogenous retroviruses (ERVs), mechanisms of repression and derepression, and their potential impact on neurodegeneration. The evidence linking TEs to AD, PD, and ALS by shedding light on the complex interactions between TEs and neurodegeneration has been discussed. Furthermore, the therapeutic potential of targeting TEs in neurodegenerative diseases has been explored. Understanding the role of TEs in neurodegeneration holds promise for developing novel therapeutic strategies aimed at mitigating disease progression and preserving neuronal health.

Activation of Young LINE-1 Elements by CRISPRa

Long interspersed element-1 (LINE-1; L1s) are mobile genetic elements that comprise nearly 20% of the human genome. L1s have been shown to have important functions in various biological processes, and their dysfunction is thought to be linked with diseases and cancers. However, the roles of the repetitive elements are largely not understood. While the CRISPR activation (CRISPRa) system based on catalytically deadCas9 (dCas9) is widely used for genome-wide interrogation of gene function and genetic interaction, few studies have been conducted on L1s. Here, we report using the CRISPRa method to efficiently activate L1s in human L02 cells, a derivative of the HeLa cancer cell line. After CRISPRa, the young L1 subfamilies such as L1HS/L1PA1 and L1PA2 are found to be expressed at higher levels than the older L1s. The L1s with high levels of transcription are closer to full-length and are more densely occupied by the YY1 transcription factor. The activated L1s can either be mis-spliced to form chimeric transcripts or act as alternative promoters or enhancers to facilitate the expression of neighboring genes. The method described here can be used for studying the functional roles of young L1s in cultured cells of interest.

An intronic LINE-1 regulates IFNAR1 expression in human immune cells

BACKGROUND

Despite their origins as selfish parasitic sequences, some transposons in the human genome have been co-opted to serve as regulatory elements, contributing to the evolution of transcriptional networks. Most well-characterized examples of transposon-derived regulatory elements derive from endogenous retroviruses (ERVs), due to the intrinsic regulatory activity of proviral long terminal repeat regions. However, one subclass of transposable elements, the Long Interspersed Nuclear Elements (LINEs), have been largely overlooked in the search for functional regulatory transposons, and considered to be broadly epigenetically repressed.

RESULTS

We examined the chromatin state of LINEs by analyzing epigenomic data from human immune cells. Many LINEs are marked by the repressive H3K9me3 modification, but a subset exhibits evidence of enhancer activity in human immune cells despite also showing evidence of epigenetic repression. We hypothesized that these competing forces of repressive and activating epigenetic marks might lead to inducible enhancer activity. We investigated a specific L1M2a element located within the first intron of Interferon Alpha/Beta Receptor 1 (IFNAR1). This element shows epigenetic signatures of B cell-specific enhancer activity, despite being repressed by the Human Silencing Hub (HUSH) complex. CRISPR deletion of the element in B lymphoblastoid cells revealed that the element acts as an enhancer that regulates both steady state and interferon-inducible expression of IFNAR1.

CONCLUSIONS

Our study experimentally demonstrates that an L1M2a element was co-opted to function as an interferon-inducible enhancer of IFNAR1, creating a feedback loop wherein IFNAR1 is transcriptionally upregulated by interferon signaling. This finding suggests that other LINEs may exhibit cryptic cell type-specific or context-dependent enhancer activity. LINEs have received less attention than ERVs in the effort to understand the contribution of transposons to the regulatory landscape of cellular genomes, but these are likely important, lineage-specific players in the rapid evolution of immune system regulatory networks and deserve further study.

The Molecular Impacts of Retrotransposons in Development and Diseases

Retrotransposons are invasive genetic elements that constitute substantial portions of mammalian genomes. They have the potential to influence nearby gene expression through their cis-regulatory sequences, reverse transcription machinery, and the ability to mold higher-order chromatin structures. Due to their multifaceted functions, it is crucial for host fitness to maintain strict regulation of these parasitic sequences to ensure proper growth and development. This review explores how subsets of retrotransposons have undergone evolutionary exaptation to enhance the complexity of mammalian genomes. It also highlights the significance of regulating these elements, drawing on recent studies conducted in human and murine systems.

LINE-1 retrotransposons drive human neuronal transcriptome complexity and functional diversification

The genetic mechanisms underlying the expansion in size and complexity of the human brain remain poorly understood. Long interspersed nuclear element-1 (L1) retrotransposons are a source of divergent genetic information in hominoid genomes, but their importance in physiological functions and their contribution to human brain evolution are largely unknown. Using multiomics profiling, we here demonstrate that L1 promoters are dynamically active in the developing and the adult human brain. L1s generate hundreds of developmentally regulated and cell type-specific transcripts, many that are co-opted as chimeric transcripts or regulatory RNAs. One L1-derived long noncoding RNA, LINC01876, is a human-specific transcript expressed exclusively during brain development. CRISPR interference silencing of LINC01876 results in reduced size of cerebral organoids and premature differentiation of neural progenitors, implicating L1s in human-specific developmental processes. In summary, our results demonstrate that L1-derived transcripts provide a previously undescribed layer of primate- and human-specific transcriptome complexity that contributes to the functional diversification of the human brain.

Research progress of LINE-1 in the diagnosis, prognosis, and treatment of gynecologic tumors

The retrotransposon known as long interspersed nuclear element-1 (LINE-1), which is currently the sole autonomously mobile transposon in the human genome, can result in insertional mutations, chromosomal rearrangements, and genomic instability. In recent years, numerous studies have shown that LINE-1 is involved in the development of various diseases and also plays an important role in the immune regulation of the organism. The expression of LINE-1 in gynecologic tumors suggests that it is expected to be an independent indicator for early diagnosis and prognosis, and also, as a therapeutic target, LINE-1 is closely associated with gynecologic tumor prognosis. This article discusses the function of LINE-1 in the diagnosis, treatment, and prognosis of ovarian, cervical, and endometrial malignancies, as well as other gynecologic malignancies. It offers fresh perspectives on the early detection of tumors and the creation of novel anti-tumor medications.

LINE-1 repression in Epstein-Barr virus-associated gastric cancer through viral-host genome interaction

Long INterspersed Element 1 (LINE-1 or L1) acts as a major remodeling force in genome regulation and evolution. Accumulating evidence shows that virus infection impacts L1 expression, potentially impacting host antiviral response and diseases. The underlying regulation mechanism is unclear. Epstein-Barr virus (EBV), a double-stranded DNA virus linked to B-cell and epithelial malignancies, is known to have viral-host genome interaction, resulting in transcriptional rewiring in EBV-associated gastric cancer (EBVaGC). By analyzing publicly available datasets from the Gene Expression Omnibus (GEO), we found that EBVaGC has L1 transcriptional repression compared with EBV-negative gastric cancer (EBVnGC). More specifically, retrotransposition-associated young and full-length L1s (FL-L1s) were among the most repressed L1s. Epigenetic alterations, especially increased H3K9me3, were observed on FL-L1s. H3K9me3 deposition was potentially attributed to increased TASOR expression, a key component of the human silencing hub (HUSH) complex for H3K9 trimethylation. The 4C- and HiC-seq data indicated that the viral DNA interacted in the proximity of the TASOR enhancer, strengthening the loop formation between the TASOR enhancer and its promoter. These results indicated that EBV infection is associated with increased H3K9me3 deposition, leading to L1 repression. This study uncovers a regulation mechanism of L1 expression by chromatin topology remodeling associated with viral-host genome interaction in EBVaGC.

Repetitive elements in aging and neurodegeneration

Repetitive elements (REs), such as transposable elements (TEs) and satellites, comprise much of the genome. Here, we review how TEs and (peri)centromeric satellite DNA may contribute to aging and neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS). Alterations in RE expression, retrotransposition, and chromatin microenvironment may shorten lifespan, elicit neurodegeneration, and impair memory and movement. REs may cause these phenotypes via DNA damage, protein sequestration, insertional mutagenesis, and inflammation. We discuss several TE families, including gypsy, HERV-K, and HERV-W, and how TEs interact with various factors, including transactive response (TAR) DNA-binding protein 43 kDa (TDP-43) and the siRNA and piwi-interacting (pi)RNA systems. Studies of TEs in neurodegeneration have focused on Drosophila and, thus, further examination in mammals is needed. We suggest that therapeutic silencing of REs could help mitigate neurodegenerative disorders.

The genome-wide mutational consequences of DNA hypomethylation

DNA methylation is important for establishing and maintaining cell identity and for genomic stability. This is achieved by regulating the accessibility of regulatory and transcriptional elements and the compaction of subtelomeric, centromeric, and other inactive genomic regions. Carcinogenesis is accompanied by a global loss in DNA methylation, which facilitates the transformation of cells. Cancer hypomethylation may also cause genomic instability, for example through interference with the protective function of telomeres and centromeres. However, understanding the role(s) of hypomethylation in tumor evolution is incomplete because the precise mutational consequences of global hypomethylation have thus far not been systematically assessed. Here we made genome-wide inventories of all possible genetic variation that accumulates in single cells upon the long-term global hypomethylation by CRISPR interference-mediated conditional knockdown of DNMT1. Depletion of DNMT1 resulted in a genomewide reduction in DNA methylation. The degree of DNA methylation loss was similar to that observed in many cancer types. Hypomethylated cells showed reduced proliferation rates, increased transcription of genes, reactivation of the inactive X-chromosome and abnormal nuclear morphologies. Prolonged hypomethylation was accompanied by increased chromosomal instability. However, there was no increase in mutational burden, enrichment for certain mutational signatures or accumulation of structural variation to the genome. In conclusion, the primary consequence of hypomethylation is genomic instability, which in cancer leads to increased tumor heterogeneity and thereby fuels cancer evolution.

Transposons Acting as Competitive Endogenous RNAs: In-Silico Evidence from Datasets Characterised by L1 Overexpression

LINE L1 are transposable elements that can replicate within the genome by passing through RNA intermediates. The vast majority of these element copies in the human genome are inactive and just between 100 and 150 copies are still able to mobilize. During evolution, they could have been positively selected for beneficial cellular functions. Nonetheless, L1 deregulation can be detrimental to the cell, causing diseases such as cancer. The activity of miRNAs represents a fundamental mechanism for controlling transcript levels in somatic cells. These are a class of small non-coding RNAs that cause degradation or translational inhibition of their target transcripts. Beyond this, competitive endogenous RNAs (ceRNAs), mostly made by circular and non-coding RNAs, have been seen to compete for the binding of the same set of miRNAs targeting protein coding genes. In this study, we have investigated whether autonomously transcribed L1s may act as ceRNAs by analyzing public dataset in-silico. We observed that genes sharing miRNA target sites with L1 have a tendency to be upregulated when L1 are overexpressed, suggesting the possibility that L1 might act as ceRNAs. This finding will help in the interpretation of transcriptomic responses in contexts characterized by the specific activation of transposons.

Epigenetic targeting of transposon relics: beating the dead horses of the genome?

Transposable elements (TEs) have been seen as selfish genetic elements that can propagate in a host genome. Their propagation success is however hindered by a combination of mechanisms such as mutations, selection, and their epigenetic silencing by the host genome. As a result, most copies of TEs in a given genome are dead relics: their sequence is too degenerated to allow any transposition. Nevertheless, these TE relics often, but not always, remain epigenetically silenced, and if not to prevent transposition anymore, one can wonder the reason for this phenomenon. The mere self-perpetuating loop inherent to epigenetic silencing could alone explain that even when inactive, TE copies remain silenced. Beyond this process, nevertheless, antagonistic selective forces are likely to act on TE relic silencing. Especially, without the benefit of preventing transposition, TE relic silencing may prove deleterious to the host fitness, suggesting that the maintenance of TE relic silencing is the result of a fine, and perhaps case-by-case, evolutionary trade-off between beneficial and deleterious effects. Ultimately, the release of TE relics silencing may provide a 'safe' ground for adaptive epimutations to arise. In this review, we provide an overview of these questions in both plants and animals.

LINE-1 activation in the cerebellum drives ataxia

Dysregulation of long interspersed nuclear element 1 (LINE-1, L1), a dominant class of transposable elements in the human genome, has been linked to neurodegenerative diseases, but whether elevated L1 expression is sufficient to cause neurodegeneration has not been directly tested. Here, we show that the cerebellar expression of L1 is significantly elevated in ataxia telangiectasia patients and strongly anti-correlated with the expression of epigenetic silencers. To examine the role of L1 in the disease etiology, we developed an approach for direct targeting of the L1 promoter for overexpression in mice. We demonstrated that L1 activation in the cerebellum led to Purkinje cell dysfunctions and degeneration and was sufficient to cause ataxia. Treatment with a nucleoside reverse transcriptase inhibitor blunted ataxia progression by reducing DNA damage, attenuating gliosis, and reversing deficits of molecular regulators for calcium homeostasis in Purkinje cells. Our study provides the first direct evidence that L1 activation can drive neurodegeneration.

Young transposable elements rewired gene regulatory networks in human and chimpanzee hippocampal intermediate progenitors

The hippocampus is associated with essential brain functions, such as learning and memory. Human hippocampal volume is significantly greater than expected compared with that of non-human apes, suggesting a recent expansion. Intermediate progenitors, which are able to undergo multiple rounds of proliferative division before a final neurogenic division, may have played a role in evolutionary hippocampal expansion. To investigate the evolution of gene regulatory networks underpinning hippocampal neurogenesis in apes, we leveraged the differentiation of human and chimpanzee induced pluripotent stem cells into TBR2 (or EOMES)-positive hippocampal intermediate progenitor cells (hpIPCs). We found that the gene networks active in hpIPCs are significantly different between humans and chimpanzees, with ∼2500 genes being differentially expressed. We demonstrate that species-specific transposon-derived enhancers contribute to these transcriptomic differences. Young transposons, predominantly endogenous retroviruses and SINE-Vntr-Alus (SVAs), were co-opted as enhancers in a species-specific manner. Human-specific SVAs provided substrates for thousands of novel TBR2-binding sites, and CRISPR-mediated repression of these SVAs attenuated the expression of ∼25% of the genes that are upregulated in human intermediate progenitors relative to the same cell population in the chimpanzee.

The landscape of aging

Aging is characterized by a progressive deterioration of physiological integrity, leading to impaired functional ability and ultimately increased susceptibility to death. It is a major risk factor for chronic human diseases, including cardiovascular disease, diabetes, neurological degeneration, and cancer. Therefore, the growing emphasis on “healthy aging” raises a series of important questions in life and social sciences. In recent years, there has been unprecedented progress in aging research, particularly the discovery that the rate of aging is at least partly controlled by evolutionarily conserved genetic pathways and biological processes. In an attempt to bring full-fledged understanding to both the aging process and age-associated diseases, we review the descriptive, conceptual, and interventive aspects of the landscape of aging composed of a number of layers at the cellular, tissue, organ, organ system, and organismal levels.

Towards bridging the translational gap by improved modeling of human nociception in health and disease

Despite numerous studies which have explored the pathogenesis of pain disorders in preclinical models, there is a pronounced translational gap, which is at least partially caused by differences between the human and rodent nociceptive system. An elegant way to bridge this divide is the exploitation of human-induced pluripotent stem cell (iPSC) reprogramming into human iPSC-derived nociceptors (iDNs). Several protocols were developed and optimized to model nociceptive processes in health and disease. Here we provide an overview of the different approaches and summarize the knowledge obtained from such models on pain pathologies associated with monogenetic sensory disorders so far. In addition, novel perspectives offered by increasing the complexity of the model systems further to better reflect the natural environment of nociceptive neurons by involving other cell types in 3D model systems are described.

LINE-1 and Alu methylation signatures in autism spectrum disorder and their associations with the expression of autism-related genes

Long interspersed nucleotide element-1 (LINE-1) and Alu elements are retrotransposons whose abilities cause abnormal gene expression and genomic instability. Several studies have focused on DNA methylation profiling of gene regions, but the locus-specific methylation of LINE-1 and Alu elements has not been identified in autism spectrum disorder (ASD). Here we interrogated locus- and family-specific methylation profiles of LINE-1 and Alu elements in ASD whole blood using publicly-available Illumina Infinium 450 K methylation datasets from heterogeneous ASD and ASD variants (Chromodomain Helicase DNA-binding 8 (CHD8) and 16p11.2del). Total DNA methylation of repetitive elements were notably hypomethylated exclusively in ASD with CHD8 variants. Methylation alteration in a family-specific manner including L1P, L1H, HAL, AluJ, and AluS families were observed in the heterogeneous ASD and ASD with CHD8 variants. Moreover, LINE-1 and Alu methylation within target genes is inversely related to the expression level in each ASD variant. The DNA methylation signatures of the LINE-1 and Alu elements in ASD whole blood, as well as their associations with the expression of ASD-related genes, have been identified. If confirmed in future larger studies, these findings may contribute to the identification of epigenomic biomarkers of ASD.

TEspeX: consensus-specific quantification of transposable element expression preventing biases from exonized fragments

SUMMARY

Transposable elements (TEs) play key roles in crucial biological pathways. Therefore, several tools enabling the quantification of their expression were recently developed. However, many of the existing tools lack the capability to distinguish between the transcription of autonomously expressed TEs and TE fragments embedded in canonical coding/non-coding non-TE transcripts. Consequently, an apparent change in the expression of a given TE may simply reflect the variation in the expression of the transcripts containing TE-derived sequences. To overcome this issue, we have developed TEspeX, a pipeline for the quantification of TE expression at the consensus level. TEspeX uses Illumina RNA-seq short reads to quantify TE expression avoiding counting reads deriving from inactive TE fragments embedded in canonical transcripts.

AVAILABILITY AND IMPLEMENTATION

The tool is implemented in python3, distributed under the GNU General Public License (GPL) and available on Github at https://github.com/fansalon/TEspeX (Zenodo URL: https://doi.org/10.5281/zenodo.6800331).

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

The Role of Transposable Elements in Sexual Development

Up to 50% of most mammalian genomes are made up of transposable elements (TEs) that have the potential to mobilize around the genome. Despite this prevalence, research on TEs is only beginning to gain traction within the field of neuroscience. While TEs have long been regarded as "junk" or parasitic DNA, it has become evident that they are adaptive DNA and RNA regulatory elements. In addition to their vital role in normal development, TEs can also interact with steroid receptors, which are key elements to sexual development. In this review, we provide an overview of the involvement of TEs in processes related to sexual development- from TE activity in the germline to TE accumulation in sex chromosomes. Moreover, we highlight sex differences in TE activity and their regulation of genes related to sexual development. Finally, we speculate on the epigenetic mechanisms that may govern TEs' role in sexual development. In this context, we emphasize the need to further the understanding of sexual development through the lens of TEs including in a variety of organs at different developmental stages, their molecular networks, and evolution.

L1 Retrotransposons: A Potential Endogenous Regulator for Schizophrenia

The long interspersed nuclear elements 1 (LINE-1/L1s) are the only active autonomous retrotransposons found in humans which can integrate anywhere in the human genome. They can expand the genome and thus bring good or bad effects to the host cells which really depends on their integration site and associated polymorphism. LINE-1 retrotransposition has been found participating in various neurological disorders such as autism spectrum disorder, Alzheimer’s disease, major depression disorder, post-traumatic stress disorder and schizophrenia. Despite the recent progress, the roles and pathological mechanism of LINE-1 retrotransposition in schizophrenia and its heritable risks, particularly, contribution to “missing heritability” are yet to be determined. Therefore, this review focuses on the potentially etiological roles of L1s in the development of schizophrenia, possible therapeutic choices and unaddressed questions in order to shed lights on the future research.

Meta-Analysis Suggests That Intron Retention Can Affect Quantification of Transposable Elements from RNA-Seq Data

Simple Summary

Transposable elements (TEs) are repetitive sequences comprising more than one third of the human genome with the original ability to change their location within the genome. Owing to their repetitive nature, the quantification of TEs results often challenging. RNA-seq is a useful tool for genome-wide TEs quantification, nevertheless it also presents technical issues, including low reads mappability and erroneous quantification derived from the transcription of TEs fragments embedded in canonical transcripts. Fragments derived from TEs are found within the introns of most genes, which led to the hypothesis that intron retention (IR) can affect the unbiased quantification of TEs expression. Performing meta-analysis of public RNA-seq datasets, here we observe that IR can indeed impact the quantification of TEs by increasing the number of reads mapped on intronic TE copies. Our work highlights a correlation between IR and TEs expression measurement by RNA-seq that should be taken into account to achieve reliable TEs quantification, especially in samples characterized by extensive IR, because differential IR might be confused with differential TEs expression.

Abstract

Transposable elements (TEs), also known as “jumping genes”, are repetitive sequences with the capability of changing their location within the genome. They are key players in many different biological processes in health and disease. Therefore, a reliable quantification of their expression as transcriptional units is crucial to distinguish between their independent expression and the transcription of their sequences as part of canonical transcripts. TEs quantification faces difficulties of different types, the most important one being low reads mappability due to their repetitive nature preventing an unambiguous mapping of reads originating from their sequences. A large fraction of TEs fragments localizes within introns, which led to the hypothesis that intron retention (IR) can be an additional source of bias, potentially affecting accurate TEs quantification. IR occurs when introns, normally removed from the mature transcript by the splicing machinery, are maintained in mature transcripts. IR is a widespread mechanism affecting many different genes with cell type-specific patterns. We hypothesized that, in an RNA-seq experiment, reads derived from retained introns can introduce a bias in the detection of overlapping, independent TEs RNA expression. In this study we performed meta-analysis using public RNA-seq data from lymphoblastoid cell lines and show that IR can impact TEs quantification using established tools with default parameters. Reads mapped on intronic TEs were indeed associated to the expression of TEs and influence their correct quantification as independent transcriptional units. We confirmed these results using additional independent datasets, demonstrating that this bias does not appear in samples where IR is not present and that differential TEs expression does not impact on IR quantification. We concluded that IR causes the over-quantification of intronic TEs and differential IR might be confused with differential TEs expression. Our results should be taken into account for a correct quantification of TEs expression from RNA-seq data, especially in samples in which IR is abundant.

The Role of Transposable Elements of the Human Genome in Neuronal Function and Pathology

Transposable elements (TEs) have been extensively studied for decades. In recent years, the introduction of whole-genome and whole-transcriptome approaches, as well as single-cell resolution techniques, provided a breakthrough that uncovered TE involvement in host gene expression regulation underlying multiple normal and pathological processes. Of particular interest is increased TE activity in neuronal tissue, and specifically in the hippocampus, that was repeatedly demonstrated in multiple experiments. On the other hand, numerous neuropathologies are associated with TE dysregulation. Here, we provide a comprehensive review of literature about the role of TEs in neurons published over the last three decades. The first chapter of the present review describes known mechanisms of TE interaction with host genomes in general, with the focus on mammalian and human TEs; the second chapter provides examples of TE exaptation in normal neuronal tissue, including TE involvement in neuronal differentiation and plasticity; and the last chapter lists TE-related neuropathologies. We sought to provide specific molecular mechanisms of TE involvement in neuron-specific processes whenever possible; however, in many cases, only phenomenological reports were available. This underscores the importance of further studies in this area.

Analysis of epigenetic features characteristic of L1 loci expressed in human cells

Only a select few L1 loci in the human genome are expressed in any given cell line or organ, likely to minimize damage done to the genome. The epigenetic features and requirements of expressed L1 loci are currently unknown. Using human cells and comprehensive epigenetic analysis of individual expressed and unexpressed L1 loci, we determined that endogenous L1 transcription depends on a combination of epigenetic factors, including open chromatin, activating histone modifications, and hypomethylation at the L1 promoter. We demonstrate that the L1 promoter seems to require interaction with enhancer elements for optimal function. We utilize epigenetic context to predict the expression status of L1Hs loci that are poorly mappable with RNA-Seq. Our analysis identified a population of ‘transitional’ L1 loci that likely have greater potential to be activated during the epigenetic dysregulation seen in tumors and during aging because they are the most responsive to targeted CRISPR-mediated delivery of trans-activating domains. We demonstrate that an engineered increase in endogenous L1 mRNA expression increases Alu mobilization. Overall, our findings present the first global and comprehensive analysis of epigenetic status of individual L1 loci based on their expression status and demonstrate the importance of epigenetic context for L1 expression heterogeneity.

Mammalian antiviral systems directed by small RNA

There are strong incentives for human populations to develop antiviral systems. Similarly, genomes that encode antiviral systems have had strong selective advantages. Protein-guided immune systems, which have been well studied in mammals, are necessary for survival in our virus-laden environments. Small RNA–directed antiviral immune systems suppress invasion of cells by non-self genetic material via complementary base pairing with target sequences. These RNA silencing-dependent systems operate in diverse organisms. In mammals, there is strong evidence that microRNAs (miRNAs) regulate endogenous genes important for antiviral immunity, and emerging evidence that virus-derived nucleic acids can be directly targeted by small interfering RNAs (siRNAs), PIWI-interacting RNAs (piRNAs), and transfer RNAs (tRNAs) for protection in some contexts. In this review, we summarize current knowledge of the antiviral functions of each of these small RNA types and consider their conceptual and mechanistic overlap with innate and adaptive protein-guided immunity, including mammalian antiviral cytokines, as well as the prokaryotic RNA-guided immune system, CRISPR. In light of recent successes in delivery of RNA for antiviral purposes, most notably for vaccination, we discuss the potential for development of small noncoding RNA–directed antiviral therapeutics and prophylactics.

Viruses are all around us and are likely inside some of the reader’s cells at this moment. Organisms are accommodated to this reality and encode various immune systems to limit virus replication. In mammals, the best studied immune systems are directed by proteins that specifically recognize viruses. These include diverse antibodies and T cell receptors, which recognize viral proteins, and pattern recognition receptors, some of which can recognize viral nucleic acids. In other organisms, including bacteria, immune systems directed by small RNAs are also well known; spacer-derived guide RNAs in CRISPR/Cas immune systems are one prominent example. The small RNAs directing these systems derive their specificity via complementary base pairing with their targets, which include both host and viral nucleic acids. Rather than having “traded in” these systems for more advanced protein-directed systems, increasing evidence supports the perspective that small RNA–directed immune systems remain active in mammalian antiviral immunity in some contexts. Here, we review what is known so far about the emerging roles of mammalian siRNAs, miRNAs, piRNAs, and tRNAs in directing immunity to viruses.

The roles of DNA methylation and hydroxymethylation at short interspersed nuclear elements in the hypothalamic arcuate nucleus during puberty

Puberty is the gateway to adult reproductive competence, encompassing a suite of complex, integrative, and coordinated changes in neuroendocrine functions. However, the regulatory mechanisms of transcriptional reprogramming in the arcuate nucleus (ARC) during onset of puberty are still not fully understood. To understand the role of epigenetics in regulating gene expression, mouse hypothalamic ARCs were isolated at 4 and 8 weeks, and the transcriptome, DNA hydroxymethylation, DNA methylation, and chromatin accessibility were assessed via RNA sequencing (RNA-seq), reduced representation bisulfite sequencing (RRBS-seq), reduced representation hydroxymethylation profiling (RRHP)-seq, and assay for transposase-accessible chromatin (ATAC-seq), respectively. The overall DNA hydroxymethylation and DNA methylation changes in retroelements (REs) were associated with gene expression modeling for puberty in the ARC. We focused on analyzing DNA hydroxymethylation and DNA methylation at two short interspersed nuclear elements (SINEs) located on the promoter of the 5-hydroxytryptamine receptor 6 (Htr6) gene and the enhancer of the KISS-1 metastasis suppressor (Kiss1) gene and investigated their regulatory roles in gene expression. Our data uncovered a novel epigenetic mechanism by which SINEs regulate gene expression during puberty.

Transposable elements shape the evolution of mammalian development

Transposable elements (TEs) promote genetic innovation but also threaten genome stability. Despite multiple layers of host defence, TEs actively shape mammalian-specific developmental processes, particularly during pre-implantation and extra-embryonic development and at the maternal-fetal interface. Here, we review how TEs influence mammalian genomes both directly by providing the raw material for genetic change and indirectly via co-evolving TE-binding Krüppel-associated box zinc finger proteins (KRAB-ZFPs). Throughout mammalian evolution, individual activities of ancient TEs were co-opted to enable invasive placentation that characterizes live-born mammals. By contrast, the widespread activity of evolutionarily young TEs may reflect an ongoing co-evolution that continues to impact mammalian development.

Transposable elements as new players in neurodegenerative diseases

Neurodegenerative diseases (NDs), including the most prevalent Alzheimer's disease and Parkinson disease, share common pathological features. Despite decades of gene-centric approaches, the molecular mechanisms underlying these diseases remain widely elusive. In recent years, transposable elements (TEs), long considered 'junk' DNA, have gained growing interest as pathogenic players in NDs. Age is the major risk factor for most NDs, and several repressive mechanisms of TEs, such as heterochromatinization, fail with age. Indeed, heterochromatin relaxation leading to TE derepression has been reported in various models of neurodegeneration and NDs. There is also evidence that certain pathogenic proteins involved in NDs (e.g., tau, TDP-43) may control the expression of TEs. The deleterious consequences of TE activation are not well known but they could include DNA damage and genomic instability, altered host gene expression, and/or neuroinflammation, which are common hallmarks of neurodegeneration and aging. TEs might thus represent an overlooked pathogenic culprit for both brain aging and neurodegeneration. Certain pathological effects of TEs might be prevented by inhibiting their activity, pointing to TEs as novel targets for neuroprotection.

Factors Regulating the Activity of LINE1 Retrotransposons

LINE-1 (L1) is a class of autonomous mobile genetic elements that form somatic mosaicisms in various tissues of the organism. The activity of L1 retrotransposons is strictly controlled by many factors in somatic and germ cells at all stages of ontogenesis. Alteration of L1 activity was noted in a number of diseases: in neuropsychiatric and autoimmune diseases, as well as in various forms of cancer. Altered activity of L1 retrotransposons for some pathologies is associated with epigenetic changes and defects in the genes involved in their repression. This review discusses the molecular genetic mechanisms of the retrotransposition and regulation of the activity of L1 elements. The contribution of various factors controlling the expression and distribution of L1 elements in the genome occurs at all stages of the retrotransposition. The regulation of L1 elements at the transcriptional, post-transcriptional and integration into the genome stages is described in detail. Finally, this review also focuses on the evolutionary aspects of L1 accumulation and their interplay with the host regulation system.

Transposable elements and their KZFP controllers are drivers of transcriptional innovation in the developing human brain

Transposable elements (TEs) account for more than 50% of the human genome and many have been co-opted throughout evolution to provide regulatory functions for gene expression networks. Several lines of evidence suggest that these networks are fine-tuned by the largest family of TE controllers, the KRAB-containing zinc finger proteins (KZFPs). One tissue permissive for TE transcriptional activation (termed "transposcription") is the adult human brain, however comprehensive studies on the extent of this process and its potential contribution to human brain development are lacking. To elucidate the spatiotemporal transposcriptome of the developing human brain, we have analyzed two independent RNA-seq data sets encompassing 16 brain regions from eight weeks postconception into adulthood. We reveal a distinct KZFP:TE transcriptional profile defining the late prenatal to early postnatal transition, and the spatiotemporal and cell type-specific activation of TE-derived alternative promoters driving the expression of neurogenesis-associated genes. Long-read sequencing confirmed these TE-driven isoforms as significant contributors to neurogenic transcripts. We also show experimentally that a co-opted antisense L2 element drives temporal protein relocalization away from the endoplasmic reticulum, suggestive of novel TE dependent protein function in primate evolution. This work highlights the widespread dynamic nature of the spatiotemporal KZFP:TE transcriptome and its importance throughout TE mediated genome innovation and neurotypical human brain development. To facilitate interactive exploration of these spatiotemporal gene and TE expression dynamics, we provide the "Brain TExplorer" web application freely accessible for the community.

Unbiased proteomic mapping of the LINE-1 promoter using CRISPR Cas9

BACKGROUND

The autonomous retroelement Long Interspersed Element-1 (LINE-1) mobilizes though a copy and paste mechanism using an RNA intermediate (retrotransposition). Throughout human evolution, around 500,000 LINE-1 sequences have accumulated in the genome. Most of these sequences belong to ancestral LINE-1 subfamilies, including L1PA2-L1PA7, and can no longer mobilize. Only a small fraction of LINE-1 sequences, approximately 80 to 100 copies belonging to the L1Hs subfamily, are complete and still capable of retrotransposition. While silenced in most cells, many questions remain regarding LINE-1 dysregulation in cancer cells.

RESULTS

Here, we optimized CRISPR Cas9 gRNAs to specifically target the regulatory sequence of the L1Hs 5'UTR promoter. We identified three gRNAs that were more specific to L1Hs, with limited binding to older LINE-1 sequences (L1PA2-L1PA7). We also adapted the C-BERST method (dCas9-APEX2 Biotinylation at genomic Elements by Restricted Spatial Tagging) to identify LINE-1 transcriptional regulators in cancer cells. Our LINE-1 C-BERST screen revealed both known and novel LINE-1 transcriptional regulators, including CTCF, YY1 and DUSP1.

CONCLUSION

Our optimization and evaluation of gRNA specificity and application of the C-BERST method creates a tool for studying the regulatory mechanisms of LINE-1 in cancer. Further, we identified the dual specificity protein phosphatase, DUSP1, as a novel regulator of LINE-1 transcription.

Non-codon Optimized PiggyBac Transposase Induces Developmental Brain Aberrations: A Call for in vivo Analysis

In this perspective article, we briefly review tools for stable gain-of-function expression to explore key fate determinants in embryonic brain development. As the piggyBac transposon system has the highest insert size, a seamless integration of the transposed sequence into the host genome, and can be delivered by transfection avoiding viral vectors causing an immune response, we explored its use in the murine developing forebrain. The original piggyBac transposase PBase or the mouse codon-optimized version mPB and the construct to insert, contained in the piggyBac transposon, were introduced by in utero electroporation at embryonic day 13 into radial glia, the neural stem cells, in the developing dorsal telencephalon, and analyzed 3 or 5 days later. When using PBase, we observed an increase in basal progenitor cells, often accompanied by folding aberrations. These effects were considerably ameliorated when using the piggyBac plasmid together with mPB. While size and strength of the electroporated region was not correlated to the aberrations, integration was essential and the positive correlation to the insert size implicates the frequency of transposition as a possible mechanism. We discuss this in light of the increase in transposing endogenous viral vectors during mammalian phylogeny and their role in neurogenesis and radial glial cells. Most importantly, we aim to alert the users of this system to the phenotypes caused by non-codon optimized PBase application in vivo.

Transposable element sequence fragments incorporated into coding and noncoding transcripts modulate the transcriptome of human pluripotent stem cells

Transposable elements (TEs) occupy nearly 40% of mammalian genomes and, whilst most are fragmentary and no longer capable of transposition, they can nevertheless contribute to cell function. TEs within genes transcribed by RNA polymerase II can be copied as parts of primary transcripts; however, their full contribution to mature transcript sequences remains unresolved. Here, using long and short read (LR and SR) RNA sequencing data, we show that 26% of coding and 65% of noncoding transcripts in human pluripotent stem cells (hPSCs) contain TE-derived sequences. Different TE families are incorporated into RNAs in unique patterns, with consequences to transcript structure and function. The presence of TE sequences within a transcript is correlated with TE-type specific changes in its subcellular distribution, alterations in steady-state levels and half-life, and differential association with RNA Binding Proteins (RBPs). We identify hPSC-specific incorporation of endogenous retroviruses (ERVs) and LINE:L1 into protein-coding mRNAs, which generate TE sequence-derived peptides. Finally, single cell RNA-seq reveals that hPSCs express ERV-containing transcripts, whilst differentiating subpopulations lack ERVs and express SINE and LINE-containing transcripts. Overall, our comprehensive analysis demonstrates that the incorporation of TE sequences into the RNAs of hPSCs is more widespread and has a greater impact than previously appreciated.

Expression of Retroelements in Cervical Cancer and Their Interplay with HPV Infection and Host Gene Expression

Retroelements are expressed in diverse types of cancer and are related to tumorigenesis and to cancer progression. We characterized the expression of retroelements in cervical cancer and explored their interplay with HPV infection and their association with expression of neighboring genes. Forty biopsies of invasive cervical carcinoma (squamous cell carcinomas and adenocarcinomas) with genotyped HPV were selected and analyzed for human endogenous retrovirus (HERV) and long interspersed nuclear element 1 (L1) expression through RNA-seq data. We found 8060 retroelements expressed in the samples and a negative correlation of DNA methyltransferase 1 expression with the two most expressed L1 elements. A total of 103 retroelements were found differentially expressed between tumor histological types and between HPV types, including several HERV families (HERV-K, HERV-H, HERV-E, HERV-I and HERV-L). The comparison between HPV mono- and co-infections showed the highest proportion of differentially expressed L1 elements. The location of retroelements affected neighboring gene expression, such as shown for the interleukin-20 gene family. Three HERVs and seven L1 were located close to this gene family and two L1 showed a positive association with IL20RB expression. This study describes the expression of retroelements in cervical cancer and shows their association with HPV status and host gene expression.

Losing DNA methylation at repetitive elements and breaking bad

Background

DNA methylation is an epigenetic chromatin mark that allows heterochromatin formation and gene silencing. It has a fundamental role in preserving genome stability (including chromosome stability) by controlling both gene expression and chromatin structure. Therefore, the onset of an incorrect pattern of DNA methylation is potentially dangerous for the cells. This is particularly important with respect to repetitive elements, which constitute the third of the human genome.

Main body

Repetitive sequences are involved in several cell processes, however, due to their intrinsic nature, they can be a source of genome instability. Thus, most repetitive elements are usually methylated to maintain a heterochromatic, repressed state. Notably, there is increasing evidence showing that repetitive elements (satellites, long interspersed nuclear elements (LINEs), Alus) are frequently hypomethylated in various of human pathologies, from cancer to psychiatric disorders. Repetitive sequences’ hypomethylation correlates with chromatin relaxation and unscheduled transcription. If these alterations are directly involved in human diseases aetiology and how, is still under investigation.

Conclusions

Hypomethylation of different families of repetitive sequences is recurrent in many different human diseases, suggesting that the methylation status of these elements can be involved in preservation of human health. This provides a promising point of view towards the research of therapeutic strategies focused on specifically tuning DNA methylation of DNA repeats.

New insights into the functional role of retrotransposon dynamics in mammalian somatic cells

Retrotransposons are genetic elements present across all eukaryotic genomes. While their role in evolution is considered as a potentially beneficial natural source of genetic variation, their activity is classically considered detrimental due to their potentially harmful effects on genome stability. However, studies are increasingly shedding light on the regulatory function and beneficial role of somatic retroelement reactivation in non-pathological contexts. Here, we review recent findings unveiling the regulatory potential of retrotransposons, including their role in noncoding RNA transcription, as modulators of mammalian transcriptional and epigenome landscapes. We also discuss technical challenges in deciphering the multifaceted activity of retrotransposable elements, highlighting an unforeseen central role of this neglected portion of the genome both in early development and in adult life.

Retrotransposons as Drivers of Mammalian Brain Evolution

Retrotransposons, a large and diverse class of transposable elements that are still active in humans, represent a remarkable force of genomic innovation underlying mammalian evolution. Among the features distinguishing mammals from all other vertebrates, the presence of a neocortex with a peculiar neuronal organization, composition and connectivity is perhaps the one that, by affecting the cognitive abilities of mammals, contributed mostly to their evolutionary success. Among mammals, hominids and especially humans display an extraordinarily expanded cortical volume, an enrichment of the repertoire of neural cell types and more elaborate patterns of neuronal connectivity. Retrotransposon-derived sequences have recently been implicated in multiple layers of gene regulation in the brain, from transcriptional and post-transcriptional control to both local and large-scale three-dimensional chromatin organization. Accordingly, an increasing variety of neurodevelopmental and neurodegenerative conditions are being recognized to be associated with retrotransposon dysregulation. We review here a large body of recent studies lending support to the idea that retrotransposon-dependent evolutionary novelties were crucial for the emergence of mammalian, primate and human peculiarities of brain morphology and function.

Activation of endogenous retroviruses during brain development causes an inflammatory response

Endogenous retroviruses (ERVs) make up a large fraction of mammalian genomes and are thought to contribute to human disease, including brain disorders. In the brain, aberrant activation of ERVs is a potential trigger for an inflammatory response, but mechanistic insight into this phenomenon remains lacking. Using CRISPR/Cas9‐based gene disruption of the epigenetic co‐repressor protein Trim28, we found a dynamic H3K9me3‐dependent regulation of ERVs in proliferating neural progenitor cells (NPCs), but not in adult neurons. In vivo deletion of Trim28 in cortical NPCs during mouse brain development resulted in viable offspring expressing high levels of ERVs in excitatory neurons in the adult brain. Neuronal ERV expression was linked to activated microglia and the presence of ERV‐derived proteins in aggregate‐like structures. This study demonstrates that brain development is a critical period for the silencing of ERVs and provides causal in vivo evidence demonstrating that transcriptional activation of ERV in neurons results in an inflammatory response.

Chromatin and Epigenetic Rearrangements in Embryonic Stem Cell Fate Transitions

A major event in embryonic development is the rearrangement of epigenetic information as the somatic genome is reprogrammed for a new round of organismal development. Epigenetic data are held in chemical modifications on DNA and histones, and there are dramatic and dynamic changes in these marks during embryogenesis. However, the mechanisms behind this intricate process and how it is regulating and responding to embryonic development remain unclear. As embryos develop from totipotency to pluripotency, they pass through several distinct stages that can be captured permanently or transiently in vitro. Pluripotent naïve cells resemble the early epiblast, primed cells resemble the late epiblast, and blastomere-like cells have been isolated, although fully totipotent cells remain elusive. Experiments using these in vitro model systems have led to insights into chromatin changes in embryonic development, which has informed exploration of pre-implantation embryos. Intriguingly, human and mouse cells rely on different signaling and epigenetic pathways, and it remains a mystery why this variation exists. In this review, we will summarize the chromatin rearrangements in early embryonic development, drawing from genomic data from in vitro cell lines, and human and mouse embryos.

Control of LINE-1 Expression Maintains Genome Integrity in Germline and Early Embryo Development

Maintenance of genome integrity in the germline and in preimplantation embryos is crucial for mammalian development. Epigenetic remodeling during primordial germ cell (PGC) and preimplantation embryo development may contribute to genomic instability in these cells, since DNA methylation is an important mechanism to silence retrotransposons. Long interspersed elements 1 (LINE-1 or L1) are the most common autonomous retrotransposons in mammals, corresponding to approximately 17% of the human genome. Retrotransposition events are more frequent in germ cells and in early stages of embryo development compared with somatic cells. It has been shown that L1 activation and expression occurs in germline and is essential for preimplantation development. In this review, we focus on the role of L1 retrotransposon in mouse and human germline and early embryo development and discuss the possible relationship between L1 expression and genomic instability during these stages. Although several studies have addressed L1 expression at different stages of development, the developmental consequences of this expression remain poorly understood. Future research is still needed to highlight the relationship between L1 retrotransposition events and genomic instability during germline and early embryo development.

LINE-1 retrotransposon methylation in chorionic villi of first trimester miscarriages with aneuploidy

PURPOSE

High frequency of aneuploidy in meiosis and cleavage stage coincides with waves of epigenetic genome reprogramming that may indicate a possible association between epigenetic mechanisms and aneuploidy occurrence. This study aimed to assess the methylation level of the long interspersed repeat element 1 (LINE-1) retrotransposon in chorionic villi of first trimester miscarriages with a normal karyotype and aneuploidy.

METHODS

The methylation level was assessed at 19 LINE-1 promoter CpG sites in chorionic villi of 141 miscarriages with trisomy of chromosomes 2, 6, 8-10, 13-15, 16, 18, 20-22, and monosomy X using massive parallel sequencing.

RESULTS

The LINE-1 methylation level was elevated statistically significant in chorionic villi of miscarriages with both trisomy (45.2 ± 4.3%) and monosomy X (46.9 ± 4.2%) compared with that in induced abortions (40.0 ± 2.4%) (p < 0.00001). The LINE-1 methylation levels were specific for miscarriages with different aneuploidies and significantly increased in miscarriages with trisomies 8, 14, and 18 and monosomy X (p < 0.05). The LINE-1 methylation level increased with gestational age both for group of miscarriages regardless of karyotype (R = 0.21, p = 0.012) and specifically for miscarriages with trisomy 16 (R = 0.48, p = 0.007). LINE-1 methylation decreased with maternal age in miscarriages with a normal karyotype (R = - 0.31, p = 0.029) and with trisomy 21 (R = - 0.64, p = 0.024) and increased with paternal age for miscarriages with trisomy 16 (R = 0.38, p = 0.048) and monosomy X (R = 0.73, p = 0.003).

CONCLUSION

Our results indicate that the pathogenic effects of aneuploidy in human embryogenesis can be supplemented with significant epigenetic changes in the repetitive sequences.

Many Different LINE-1 Retroelements Are Activated in Bladder Cancer

Human genomes contain about 100,000 LINE-1 (L1) retroelements, of which more than 100 are intact. L1s are normally tightly controlled by epigenetic mechanisms, which often fail in cancer. In bladder urothelial carcinoma (UC), particularly, L1s become DNA-hypomethylated, expressed and contribute to genomic instability and tumor growth. It is, however, unknown which individual L1s are activated. Following RNA-immunoprecipitation with a L1-specific antibody, third generation nanopore sequencing detected transcripts of 90 individual elements in the VM-Cub-1 UC line with high overall L1 expression. In total, 10 L1s accounted for >60% of the reads. Analysis of five specific L1s by RT-qPCR revealed generally increased expression in UC tissues and cell lines over normal controls, but variable expression among tumor cell lines from bladder, prostate and testicular cancer. Chromatin immunoprecipitation demonstrated active histone marks at L1 sequences with increased expression in VM-Cub-1, but not in a different UC cell line with low L1 expression. We conclude that many L1 elements are epigenetically activated in bladder cancer in a varied pattern. Our findings indicate that expression of individual L1s is highly heterogeneous between and among cancer types.

Retroelement-derived RNA and its role in the brain

Comprising ~40% of the human genome, retroelements are mobile genetic elements which are transcribed into RNA, then reverse-transcribed into DNA and inserted into a new site in the genome. Retroelements are referred to as "genetic parasites", residing among host genes and relying on host machinery for transcription and evolutionary propagation. The healthy brain has the highest expression of retroelement-derived sequences compared to other somatic tissue, which leads to the question: how does retroelement-derived RNA influence human traits and cellular states? While the functional importance of upregulating retroelement expression in the brain is an active area of research, RNA species derived from retroelements influence both self- and host gene expression by contributing to chromatin remodeling, alternative splicing, somatic mosaicism and translational repression. Here, we review the emerging evidence that the functional importance of RNA derived from retroelements is multifaceted. Retroelements can influence organismal states through the seeding of epigenetic states in chromatin, the production of structured RNA and even catalytically active ribozymes, the generation of cytoplasmic ssDNA and RNA/DNA hybrids, the production of viral-like proteins, and the generation of somatic mutations. Comparative sequencing suggests that retroelements can contribute to intraspecies variation through these mechanisms to alter transcript identity and abundance. In humans, an increasing number of neurodevelopmental and neurodegenerative conditions are associated with dysregulated retroelements, including Aicardi-Goutieres syndrome (AGS), Rett syndrome (RTT), Amyotrophic Lateral Sclerosis (ALS), Alzheimer's disease (AD), multiple sclerosis (MS), schizophrenia (SZ), and aging. Taken together, these concepts suggest a larger functional role for RNA derived from retroelements. This review aims to define retroelement-derived RNA, discuss how it impacts the mammalian genome, as well as summarize data supporting phenotypic consequences of this unique RNA subset in the brain.