Characterization of a synthetic human LINE-1 retrotransposon ORFeus-Hs

All-RNA-mediated targeted gene integration in mammalian cells with rationally engineered R2 retrotransposons

All-RNA-mediated targeted gene integration methods, rendering reduced immunogenicity, effective deliverability with non-viral vehicles, and a low risk of random mutagenesis, are urgently needed for next-generation gene addition technologies. Naturally occurring R2 retrotransposons hold promise in this context due to their site-specific integration profile. Here, we systematically analyzed the biodiversity of R2 elements and screened several R2 orthologs capable of full-length gene insertion in mammalian cells. Robust R2 system gene integration efficiency was attained using combined donor RNA and protein engineering. Importantly, the all-RNA-delivered engineered R2 system showed effective integration activity, with efficiency over 60% in mouse embryos. Unbiased high-throughput sequencing demonstrated that the engineered R2 system exhibited high on-target integration specificity (99%). In conclusion, our study provides engineered R2 tools for applications based on hit-and-run targeted DNA integration and insights for further optimization of retrotransposon systems.

Structures, functions and adaptations of the human LINE-1 ORF2 protein

The LINE-1 (L1) retrotransposon is an ancient genetic parasite that has written around one-third of the human genome through a 'copy and paste' mechanism catalysed by its multifunctional enzyme, open reading frame 2 protein (ORF2p)1. ORF2p reverse transcriptase (RT) and endonuclease activities have been implicated in the pathophysiology of cancer2,3, autoimmunity4,5 and ageing6,7, making ORF2p a potential therapeutic target. However, a lack of structural and mechanistic knowledge has hampered efforts to rationally exploit it. We report structures of the human ORF2p 'core' (residues 238-1061, including the RT domain) by X-ray crystallography and cryo-electron microscopy in several conformational states. Our analyses identified two previously undescribed folded domains, extensive contacts to RNA templates and associated adaptations that contribute to unique aspects of the L1 replication cycle. Computed integrative structural models of full-length ORF2p show a dynamic closed-ring conformation that appears to open during retrotransposition. We characterize ORF2p RT inhibition and reveal its underlying structural basis. Imaging and biochemistry show that non-canonical cytosolic ORF2p RT activity can produce RNA:DNA hybrids, activating innate immune signalling through cGAS/STING and resulting in interferon production6-8. In contrast to retroviral RTs, L1 RT is efficiently primed by short RNAs and hairpins, which probably explains cytosolic priming. Other biochemical activities including processivity, DNA-directed polymerization, non-templated base addition and template switching together allow us to propose a revised L1 insertion model. Finally, our evolutionary analysis demonstrates structural conservation between ORF2p and other RNA- and DNA-dependent polymerases. We therefore provide key mechanistic insights into L1 polymerization and insertion, shed light on the evolutionary history of L1 and enable rational drug development targeting L1.

Condensation of LINE-1 is critical for retrotransposition

LINE-1 (L1) is the only autonomously active retrotransposon in the human genome, and accounts for 17% of the human genome. The L1 mRNA encodes two proteins, ORF1p and ORF2p, both essential for retrotransposition. ORF2p has reverse transcriptase and endonuclease activities, while ORF1p is a homotrimeric RNA-binding protein with poorly understood function. Here, we show that condensation of ORF1p is critical for L1 retrotransposition. Using a combination of biochemical reconstitution and live-cell imaging, we demonstrate that electrostatic interactions and trimer conformational dynamics together tune the properties of ORF1p assemblies to allow for efficient L1 ribonucleoprotein (RNP) complex formation in cells. Furthermore, we relate the dynamics of ORF1p assembly and RNP condensate material properties to the ability to complete the entire retrotransposon life-cycle. Mutations that prevented ORF1p condensation led to loss of retrotransposition activity, while orthogonal restoration of coiled-coil conformational flexibility rescued both condensation and retrotransposition. Based on these observations, we propose that dynamic ORF1p oligomerization on L1 RNA drives the formation of an L1 RNP condensate that is essential for retrotransposition.

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.

Frequency and mechanisms of LINE-1 retrotransposon insertions at CRISPR/Cas9 sites

CRISPR/Cas9-based genome editing has revolutionized experimental molecular biology and entered the clinical world for targeted gene therapy. Identifying DNA modifications occurring at CRISPR/Cas9 target sites is critical to determine efficiency and safety of editing tools. Here we show that insertions of LINE-1 (L1) retrotransposons can occur frequently at CRISPR/Cas9 editing sites. Together with PolyA-seq and an improved amplicon sequencing, we characterize more than 2500 de novo L1 insertions at multiple CRISPR/Cas9 editing sites in HEK293T, HeLa and U2OS cells. These L1 retrotransposition events exploit CRISPR/Cas9-induced DSB formation and require L1 RT activity. Importantly, de novo L1 insertions are rare during genome editing by prime editors (PE), cytidine or adenine base editors (CBE or ABE), consistent with their reduced DSB formation. These data demonstrate that insertions of retrotransposons might be a potential outcome of CRISPR/Cas9 genome editing and provide further evidence on the safety of different CRISPR-based editing tools.

A Variety of Nucleic Acid Species Are Sensed by cGAS, Implications for Its Diverse Functions

Cyclic GMP-AMP synthase (cGAS) recognizes double-stranded DNA (dsDNA) derived from invading pathogens and induces an interferon response via activation of the key downstream adaptor protein stimulator of interferon genes (STING). This is the most classic biological function of the cGAS-STING signaling pathway and is critical for preventing pathogenic microorganism invasion. In addition, cGAS can interact with various types of nucleic acids, including cDNA, DNA : RNA hybrids, and circular RNA, to contribute to a diverse set of biological functions. An increasing number of studies have revealed an important relationship between the cGAS-STING signaling pathway and autophagy, cellular senescence, antitumor immunity, inflammation, and autoimmune diseases. This review details the mechanism of action of cGAS as it interacts with different types of nucleic acids, its rich biological functions, and the potential for targeting this pathway to treat various diseases.

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.

Human L1 Transposition Dynamics Unraveled with Functional Data Analysis

Long INterspersed Elements-1 (L1s) constitute >17% of the human genome and still actively transpose in it. Characterizing L1 transposition across the genome is critical for understanding genome evolution and somatic mutations. However, to date, L1 insertion and fixation patterns have not been studied comprehensively. To fill this gap, we investigated three genome-wide data sets of L1s that integrated at different evolutionary times: 17,037 de novo L1s (from an L1 insertion cell-line experiment conducted in-house), and 1,212 polymorphic and 1,205 human-specific L1s (from public databases). We characterized 49 genomic features-proxying chromatin accessibility, transcriptional activity, replication, recombination, etc.-in the ±50 kb flanks of these elements. These features were contrasted between the three L1 data sets and L1-free regions using state-of-the-art Functional Data Analysis statistical methods, which treat high-resolution data as mathematical functions. Our results indicate that de novo, polymorphic, and human-specific L1s are surrounded by different genomic features acting at specific locations and scales. This led to an integrative model of L1 transposition, according to which L1s preferentially integrate into open-chromatin regions enriched in non-B DNA motifs, whereas they are fixed in regions largely free of purifying selection-depleted of genes and noncoding most conserved elements. Intriguingly, our results suggest that L1 insertions modify local genomic landscape by extending CpG methylation and increasing mononucleotide microsatellite density. Altogether, our findings substantially facilitate understanding of L1 integration and fixation preferences, pave the way for uncovering their role in aging and cancer, and inform their use as mutagenesis tools in genetic studies.

L1 retrotransposons exploit RNA m6A modification as an evolutionary driving force

L1 retrotransposons can pose a threat to genome integrity. The host has evolved to restrict L1 replication. However, mechanisms underlying L1 propagation out of the host surveillance remains unclear. Here, we propose an evolutionary survival strategy of L1, which exploits RNA m⁶A modification. We discover that m⁶A 'writer' METTL3 facilitates L1 retrotransposition, whereas m⁶A 'eraser' ALKBH5 suppresses it. The essential m⁶A cluster that is located on L1 5' UTR serves as a docking site for eukaryotic initiation factor 3 (eIF3), enhances translational efficiency and promotes the formation of L1 ribonucleoprotein. Furthermore, through the comparative analysis of human- and primate-specific L1 lineages, we find that the most functional m⁶A motif-containing L1s have been positively selected and became a distinctive feature of evolutionarily young L1s. Thus, our findings demonstrate that L1 retrotransposons hijack the RNA m⁶A modification system for their successful replication.

Cytoplasmic synthesis of endogenous Alu complementary DNA via reverse transcription and implications in age-related macular degeneration

Alu retroelements propagate via retrotransposition by hijacking long interspersed nuclear element-1 (L1) reverse transcriptase (RT) and endonuclease activities. Reverse transcription of Alu RNA into complementary DNA (cDNA) is presumed to occur exclusively in the nucleus at the genomic integration site. Whether Alu cDNA is synthesized independently of genomic integration is unknown. Alu RNA promotes retinal pigmented epithelium (RPE) death in geographic atrophy, an untreatable type of age-related macular degeneration. We report that Alu RNA-induced RPE degeneration is mediated via cytoplasmic L1-reverse-transcribed Alu cDNA independently of retrotransposition. Alu RNA did not induce cDNA production or RPE degeneration in L1-inhibited animals or human cells. Alu reverse transcription can be initiated in the cytoplasm via self-priming of Alu RNA. In four health insurance databases, use of nucleoside RT inhibitors was associated with reduced risk of developing atrophic macular degeneration (pooled adjusted hazard ratio, 0.616; 95% confidence interval, 0.493-0.770), thus identifying inhibitors of this Alu replication cycle shunt as potential therapies for a major cause of blindness.

BRCA1 and S phase DNA repair pathways restrict LINE-1 retrotransposition in human cells

Long interspersed element-1 (LINE-1 or L1) is the only autonomous retrotransposon active in human cells. Different host factors have been shown to influence L1 mobility however, systematic analyses of these factors are limited. Here, we developed a high-throughput microscopy-based retrotransposition assay that identified the Double-Stranded Break (DSB) repair and Fanconi Anemia factors active in the S/G2 phase as potent inhibitors and regulators of L1 activity. In particular BRCA1, an E3 ubiquitin ligase with a key role in several DNA repair pathways, directly affects L1 retrotransposition frequency and structure and also plays a distinct role in controlling L1 ORF2 protein translation through L1 mRNA binding. These results suggest the existence of a “battleground” at the DNA replication fork between HR factors and L1 retrotransposons, and revealing a potential role for L1 in the genotypic evolution of tumors characterized by BRCA1 and HR repair deficiencies.

Cell fitness screens reveal a conflict between LINE-1 retrotransposition and DNA replication

LINE-1 retrotransposon overexpression is a hallmark of human cancers. We identified a colorectal cancer wherein a fast-growing tumor subclone downregulated LINE-1, prompting us to examine how LINE-1 expression affects cell growth. We find that nontransformed cells undergo a TP53-dependent growth arrest and activate interferon signaling in response to LINE-1. TP53 inhibition allows LINE-1+ cells to grow, and genome-wide-knockout screens show that these cells require replication-coupled DNA-repair pathways, replication-stress signaling and replication-fork restart factors. Our findings demonstrate that LINE-1 expression creates specific molecular vulnerabilities and reveal a retrotransposition-replication conflict that may be an important determinant of cancer growth.

Long interspersed element-1 ribonucleoprotein particles protect telomeric ends in alternative lengthening of telomeres dependent cells

Malignant cells ensure telomere maintenance by the alternative lengthening of telomeres (ALT) in the absence of telomerase activity (TA). The retrotransposons "long interspersed nuclear element-1" (LINE-1, L1) are expressed in malignant cells and are primarily known to contribute to complex karyotypes. Here we demonstrate that LINE-1 ribonucleoprotein particles (L1-RNPs) expression is significantly higher in ALT⁺- versus in TA⁺-human glioma. Analyzing a role of L1-RNP in ALT, we show that L1-RNPs bind to telomeric repeat containing RNA (TERRA), which is critical for telomere stabilization and which is overexpressed in ALT⁺ cells. In turn, L1-RNP knockdown (KD) abrogated the nuclear retention of TERRA, resulted in increased telomeric DNA damage, decreased cell growth and reduced expression of ALT characteristics such as c-circles and PML-bodies. L1-RNP KD also decreased the expression of Shelterin- and the ALT-regulating protein Topoisomerase IIIα (TopoIIIα) indicating a more general role of L1-RNPs in supporting telomeric integrity in ALT. Our findings suggest an impact of L1-RNP on telomere stability in ALT⁺ dependent tumor cells. As L1-RNPs are rarely expressed in normal adult human tissue those elements might serve as a novel target for tumor ablative therapy.

Tracing the history of LINE and SINE extinction in sigmodontine rodents

Background

L1 retrotransposons have co-evolved with their mammalian hosts for the entire history of mammals and currently compose ~ 20% of a mammalian genome. B1 retrotransposons are dependent on L1 for retrotransposition and span the evolutionary history of rodents since their radiation. L1s were found to have lost their activity in a group of South American rodents, the Sigmodontinae, and B1 inactivation preceded the extinction of L1 in the same group. Consequently, a basal group of sigmodontines have active L1s but inactive B1s and a derived clade have both inactive L1s and B1s. It has been suggested that B1s became extinct during a long period of L1 quiescence and that L1s subsequently reemerged in the basal group.

Results

Here we investigate the evolutionary histories of L1 and B1 in the sigmodontine rodents and show that L1 activity continued until after the L1-extinct clade and the basal group diverged. After the split, L1 had a small burst of activity in the former group, followed by extinction. In the basal group, activity was initially low but was followed by a dramatic increase in L1 activity. We found the last wave of B1 retrotransposition was large and probably preceded the split between the two rodent clades.

Conclusions

Given that L1s had been steadily retrotransposing during the time corresponding to B1 extinction and that the burst of B1 activity preceding B1 extinction was large, we conclude that B1 extinction was not a result of L1 quiescence. Rather, the burst of B1 activity may have contributed to L1 extinction both by competition with L1 and by putting strong selective pressure on the host to control retrotransposition.

Electronic supplementary material

The online version of this article (10.1186/s13100-019-0164-5) contains supplementary material, which is available to authorized users.

Identification of RNase-sensitive LINE-1 Ribonucleoprotein Interactions by Differential Affinity Immobilization

Long Interspersed Nuclear Element-1 (LINE-1, L1) constitutes a family of autonomous, self-replicating genetic elements known as retrotransposons. Although most are inactive, copious L1 sequences populate the human genome. L1s proliferate in a 'copy-and-paste' fashion through an RNA intermediate; a full-length L1 transcript is ~6,000 nucleotides long and functions as a bicistronic mRNA that encodes and assembles in cis with two main polypeptides, ORF1p and ORF2p, forming a ribonucleoprotein (RNP); L1 RNPs also interact with a wide range of host factors in positive and negative regulatory capacities. The following protocol describes an approach to affinity enrich ectopically expressed L1 RNPs and, using RNases, release the fraction of protein that depends upon the presence of intact RNA for retention in the immobilized macromolecules.

LINE1 Derepression in Aged Wild-Type and SIRT6-Deficient Mice Drives Inflammation

Mice deficient for SIRT6 exhibit a severely shortened lifespan, growth retardation, and highly elevated LINE1 (L1) activity. Here we report that SIRT6-deficient cells and tissues accumulate abundant cytoplasmic L1 cDNA, which triggers strong type I interferon response via activation of cGAS. Remarkably, nucleoside reverse-transcriptase inhibitors (NRTIs), which inhibit L1 retrotransposition, significantly improved health and lifespan of SIRT6 knockout mice and completely rescued type I interferon response. In tissue culture, inhibition of L1 with siRNA or NRTIs abrogated type I interferon response, in addition to a significant reduction of DNA damage markers. These results indicate that L1 activation contributes to the pathologies of SIRT6 knockout mice. Similarly, L1 transcription, cytoplasmic cDNA copy number, and type I interferons were elevated in the wild-type aged mice. As sterile inflammation is a hallmark of aging, we propose that modulating L1 activity may be an important strategy for attenuating age-related pathologies.

L1 drives IFN in senescent cells and promotes age-associated inflammation

Retrotransposable elements are deleterious at many levels, and the failure of host surveillance systems for these elements can thus have negative consequences. However, the contribution of retrotransposon activity to ageing and age-associated diseases is not known. Here we show that during cellular senescence, L1 (also known as LINE-1) retrotransposable elements become transcriptionally derepressed and activate a type-I interferon (IFN-I) response. The IFN-I response is a phenotype of late senescence and contributes to the maintenance of the senescence-associated secretory phenotype. The IFN-I response is triggered by cytoplasmic L1 cDNA, and is antagonized by inhibitors of the L1 reverse transcriptase. Treatment of aged mice with the nucleoside reverse transcriptase inhibitor lamivudine downregulated IFN-I activation and age-associated inflammation (inflammaging) in several tissues. We propose that the activation of retrotransposons is an important component of sterile inflammation that is a hallmark of ageing, and that L1 reverse transcriptase is a relevant target for the treatment of age-associated disorders.

Prostate-specific loss of UXT promotes cancer progression

Ubiquitously-expressed, prefoldin-like chaperone (UXT) also called Androgen Receptor Trapped clone-27 (ART-27) is widely expressed in human tissues. Our previous studies showed that UXT regulates transcription repression including androgen receptor (AR) signaling in prostate cancer. Here we analyzed a tissue microarray consisting of normal prostate, benign prostatic hyperplasia, high grade prostatic intraepithelial neoplasia (HGPIN) and primary prostate cancer cases for UXT protein expression. We found that HGPIN and malignant tumors have significantly decreased UXT expression compared to the normal prostate. Loss of UXT expression in primary prostate cancer is positively associated with high Gleason grade and poor relapse-free survival. We engineered prostate-specific Uxt^KO mice that developed a hyperplastic phenotype with apparent prostate secretion fluid blockage as well as PIN by 4-6 months. Doubly mutant Uxt^KO/Pten^KO mice developed a more aggressive PIN phenotype. UXT depletion in prostate cancer cells also increased retroelements expression, including LINE-1 and Alu. Consistent with this finding Uxt^KO mice have increased LINE-1 protein levels in the prostate compared to control mice. In addition, cancer cells with UXT depletion have increased retrotransposition activity and accumulated DNA damage. Our findings demonstrate that loss of UXT is an early event during prostate cancer progression, which may contribute to genome instability.

APOBEC3B Activity Is Prevalent in Urothelial Carcinoma Cells and Only Slightly Affected by LINE-1 Expression

The most common mutational signature in urothelial carcinoma (UC), the most common type of urinary bladder cancer is assumed to be caused by the misdirected activity of APOBEC3 (A3) cytidine deaminases, especially A3A or A3B, which are known to normally restrict the propagation of exogenous viruses and endogenous retroelements such as LINE-1 (L1). The involvement of A3 proteins in urothelial carcinogenesis is unexpected because, to date, UC is thought to be caused by chemical carcinogens rather than viral activity. Therefore, we explored the relationship between A3 expression and L1 activity, which is generally upregulated in UC. We found that UC cell lines highly express A3B and in some cases A3G, but not A3A, and exhibit corresponding cytidine deamination activity in vitro. While we observed evidence suggesting that L1 expression has a weak positive effect on A3B and A3G expression and A3B promoter activity, neither efficient siRNA-mediated knockdown nor overexpression of functional L1 elements affected catalytic activity of A3 proteins consistently. However, L1 knockdown diminished proliferation of a UC cell line exhibiting robust endogenous L1 expression, but had little impact on a cell line with low L1 expression levels. Our results indicate that UC cells express A3B at levels exceeding A3A levels by far, making A3B the prime candidate for causing genomic mutations. Our data provide evidence that L1 activation constitutes only a minor and negligible factor involved in induction or upregulation of endogenous A3 expression in UC.

L1 retrotransposition in the soma: a field jumping ahead

Retrotransposons are transposable elements (TEs) capable of "jumping" in germ, embryonic and tumor cells and, as is now clearly established, in the neuronal lineage. Mosaic TE insertions form part of a broader landscape of somatic genome variation and hold significant potential to generate phenotypic diversity, in the brain and elsewhere. At present, the LINE-1 (L1) retrotransposon family appears to be the most active autonomous TE in most mammals, based on experimental data obtained from disease-causing L1 mutations, engineered L1 reporter systems tested in cultured cells and transgenic rodents, and single-cell genomic analyses. However, the biological consequences of almost all somatic L1 insertions identified thus far remain unknown. In this review, we briefly summarize the current state-of-the-art in the field, including estimates of L1 retrotransposition rate in neurons. We bring forward the hypothesis that an extensive subset of retrotransposition-competent L1s may be de-repressed and mobile in the soma but largely inactive in the germline. We discuss recent reports of non-canonical L1-associated sequence variants in the brain and propose that the elevated L1 DNA content reported in several neurological disorders may predominantly comprise accumulated, unintegrated L1 nucleic acids, rather than somatic L1 insertions. Finally, we consider the main objectives and obstacles going forward in elucidating the biological impact of somatic retrotransposition.

LINE1 contributes to autoimmunity through both RIG-I- and MDA5-mediated RNA sensing pathways

Improper host immune activation leads to the development of the autoimmune disease Aicardi-Goutières syndrome (AGS), which is attributed to defined genetic mutations in such proteins as TREX1 and ADAR1. The mechanism of immune activation in AGS patients has not been thoroughly elucidated to date. In this study, we report that endogenous LINE1 components trigger IFNβ production in multiple human cell types, including those defective for cGAS/STING-mediated DNA sensing. In these cells, LINE1 DNA synthesis and retrotransposition were not required for LINE1-triggered immune activation, but RNA sensing pathways were essential. LINE1-triggered immune activation could be suppressed by diverse LINE1 inhibitors, including AGS-associated proteins targeting LINE1 RNA or proteins. However, AGS-associated ADAR1 or TREX1 mutants were defective in suppressing LINE1 retrotransposition or LINE1-triggered immune activation. Therefore, we have revealed a new function for LINE1 as an endogenous trigger of innate immune activation, which is important for understanding the molecular basis of IFN-based autoimmune diseases and may offer new intervention strategies.

LINE-1 protein localization and functional dynamics during the cell cycle

LINE-1/L1 retrotransposon sequences comprise 17% of the human genome. Among the many classes of mobile genetic elements, L1 is the only autonomous retrotransposon that still drives human genomic plasticity today. Through its co-evolution with the human genome, L1 has intertwined itself with host cell biology. However, a clear understanding of L1’s lifecycle and the processes involved in restricting its insertion and intragenomic spread remains elusive. Here we identify modes of L1 proteins’ entrance into the nucleus, a necessary step for L1 proliferation. Using functional, biochemical, and imaging approaches, we also show a clear cell cycle bias for L1 retrotransposition that peaks during the S phase. Our observations provide a basis for novel interpretations about the nature of nuclear and cytoplasmic L1 ribonucleoproteins (RNPs) and the potential role of DNA replication in L1 retrotransposition.

Only two percent of our genetic material or genome are occupied by genes, while between 60-70 percent are made up of hundreds of thousands of copies of very similar DNA sequences. These repetitive sequences evolved from genetic elements called transposons.

Transposons are often referred to as ‘jumping genes’, as they can randomly move within the genome and thereby create dangerous mutations that may lead to cancer or other genetic diseases. LINE-1 is the only remaining active transposon in humans, and it expands by copying and pasting itself to new locations via a process called 'retrotransposition'. To do so, it is first transcribed into RNA – the molecules that help to make proteins – and then converted back into identical DNA sequences.

Previous research has shown that LINE-1 can form complexes with a series of proteins, including the two encoded by LINE-1 RNA itself: ORF1p and ORF2p. The LINE-1 complexes can enter the nucleus of the cell and insert a new copy of LINE-1 into the genome. However, until now it was not known how they do this. To investigate this further, Mita et al. used human cancer cells grown in the lab and tracked LINE-1 during the different stages of the cell cycle.

The results showed that LINE-1 enters the nucleus as the cell starts to divide and the membrane of the nucleus breaks down. The LINE-1 complexes are then retained in the nucleus while the membrane of the nucleus reforms. Later, as the cell duplicates its genetic material, LINE-1 starts to copy and paste itself. Mita et al., together with another group of researchers, also found that during this process, only LINE-1 RNA and ORF2p were found in the nucleus.

This shows that the cell cycle dictates both where the LINE-1 complexes gather and when LINE-1 is active. A next step will be to further investigate how the ‘copy and paste’ mechanisms of LINE-1 and the two LINE-1 proteins are regulated during the cell cycle. In future, this may help to identify LINE-1’s role in processes like aging or in diseases such as cancer.

Dissection of affinity captured LINE-1 macromolecular complexes

Long Interspersed Nuclear Element-1 (LINE-1, L1) is a mobile genetic element active in human genomes. L1-encoded ORF1 and ORF2 proteins bind L1 RNAs, forming ribonucleoproteins (RNPs). These RNPs interact with diverse host proteins, some repressive and others required for the L1 lifecycle. Using differential affinity purifications, quantitative mass spectrometry, and next generation RNA sequencing, we have characterized the proteins and nucleic acids associated with distinctive, enzymatically active L1 macromolecular complexes. Among them, we describe a cytoplasmic intermediate that we hypothesize to be the canonical ORF1p/ORF2p/L1-RNA-containing RNP, and we describe a nuclear population containing ORF2p, but lacking ORF1p, which likely contains host factors participating in target-primed reverse transcription.

Truncated ORF1 proteins can suppress LINE-1 retrotransposition in trans

Long interspersed element 1 (L1) is an autonomous non-LTR retroelement that is active in mammalian genomes. Although retrotranspositionally incompetent and functional L1 loci are present in the same genomes, it remains unknown whether non-functional L1s have any trans effect on mobilization of active elements. Using bioinformatic analysis, we identified over a thousand of human L1 loci containing at least one stop codon in their ORF1 sequence. RNAseq analysis confirmed that many of these loci are expressed. We demonstrate that introduction of equivalent stop codons in the full-length human L1 sequence leads to the expression of truncated ORF1 proteins. When supplied in trans some truncated human ORF1 proteins suppress human L1 retrotransposition. This effect requires the N-terminus and coiled-coil domain (C-C) as mutations within the ORF1p C-C domain abolish the suppressive effect of truncated proteins on L1 retrotransposition. We demonstrate that the expression levels and length of truncated ORF1 proteins influence their ability to suppress L1 retrotransposition. Taken together these findings suggest that L1 retrotransposition may be influenced by coexpression of defective L1 loci and that these L1 loci may reduce accumulation of de novo L1 integration events.

Aicardi-Goutières syndrome protein TREX1 suppresses L1 and maintains genome integrity through exonuclease-independent ORF1p depletion

Maintaining genome integrity is important for cells and damaged DNA triggers autoimmunity. Previous studies have reported that Three-prime repair exonuclease 1(TREX1), an endogenous DNA exonuclease, prevents immune activation by depleting damaged DNA, thus preventing the development of certain autoimmune diseases. Consistently, mutations in TREX1 are linked with autoimmune diseases such as systemic lupus erythematosus, Aicardi–Goutières syndrome (AGS) and familial chilblain lupus. However, TREX1 mutants competent for DNA exonuclease activity are also linked to AGS. Here, we report a nuclease-independent involvement of TREX1 in preventing the L1 retrotransposon-induced DNA damage response. TREX1 interacted with ORF1p and altered its intracellular localization. Furthermore, TREX1 triggered ORF1p depletion and reduced the L1-mediated nicking of genomic DNA. TREX1 mutants related to AGS were deficient in inducing ORF1p depletion and could not prevent L1-mediated DNA damage. Therefore, our findings not only reveal a new mechanism for TREX1-mediated L1 suppression and uncover a new function for TREX1 in protein destabilization, but they also suggest a novel mechanism for TREX1-mediated suppression of innate immune activation through maintaining genome integrity.

Intact piRNA pathway prevents L1 mobilization in male meiosis

The PIWI-interacting RNA (piRNA) pathway is essential for retrotransposon silencing. In piRNA-deficient mice, L1-overexpressing male germ cells exhibit excessive DNA damage and meiotic defects. It remains unknown whether L1 expression simply highlights piRNA deficiency or actually drives the germ-cell demise. Specifically, the sheer abundance of genomic L1 copies prevents reliable quantification of new insertions. Here, we developed a codon-optimized L1 transgene that is controlled by an endogenous mouse L1 promoter. Importantly, DNA methylation dynamics of a single-copy transgene were indistinguishable from those of endogenous L1s. Analysis of Mov10l1^-/- testes established that de novo methylation of the L1 transgene required the intact piRNA pathway. Consistent with loss of DNA methylation and programmed reduction of H3K9me2 at meiotic onset, the transgene showed 1,400-fold increase in RNA expression and consequently 70-fold increase in retrotransposition in postnatal day 14 Mov10l1^-/- germ cells compared with the wild-type. Analysis of adult Mov10l1^-/- germ-cell fractions indicated a stage-specific increase of retrotransposition in the early meiotic prophase. However, extrapolation of the transgene data to endogenous L1s suggests that it is unlikely insertional mutagenesis alone accounts for the Mov10l1^-/- phenotype. Indeed, pharmacological inhibition of reverse transcription did not rescue the meiotic defect. Cumulatively, these results establish the occurrence of productive L1 mobilization in the absence of an intact piRNA pathway but leave open the possibility of processes preceding L1 integration in triggering meiotic checkpoints and germ-cell death. Additionally, our data suggest that many heritable L1 insertions originate from individuals with partially compromised piRNA defense.

A conserved role for the ESCRT membrane budding complex in LINE retrotransposition

Long interspersed nuclear element-1s (LINE-1s, or L1s) are an active family of retrotransposable elements that continue to mutate mammalian genomes. Despite the large contribution of L1 to mammalian genome evolution, we do not know where active L1 particles (particles in the process of retrotransposition) are located in the cell, or how they move towards the nucleus, the site of L1 reverse transcription. Using a yeast model of LINE retrotransposition, we identified ESCRT (endosomal sorting complex required for transport) as a critical complex for LINE retrotransposition, and verified that this interaction is conserved for human L1. ESCRT interacts with L1 via a late domain motif, and this interaction facilitates L1 replication. Loss of the L1/ESCRT interaction does not impair RNP formation or enzymatic activity, but leads to loss of retrotransposition and reduced L1 endonuclease activity in the nucleus. This study highlights the importance of the ESCRT complex in the L1 life cycle and suggests an unusual mode for L1 RNP trafficking.

Long interspersed nuclear elements (LINEs) are a class of retrotransposable elements that mutate mammalian genomes. LINEs have been highly successful in the human genome, multiplying to over 800,000 copies. The LINE-encoded replication machinery is also used by other retrotransposons, and in total, has been responsible for the generation of over 1/3 of human DNA sequence. To replicate, a LINE mRNA forms a ribonucleoprotein particle (RNP) with its proteins. This RNP eventually enters the nucleus to integrate a cDNA copy of itself into chromosomes. The events between RNP formation and successful integration are difficult to study and largely unknown. Here we show that the ESCRT complex plays a conserved role in LINE retrotransposition in both yeast and humans. ESCRT is a membrane budding complex involved in cellular trafficking and membrane budding/fusion. Our results imply that membranes play an integral part of LINE replication, and ESCRT may be required for RNP trafficking towards the nucleus.

Conservation and Innovation of APOBEC3A Restriction Functions during Primate Evolution

LINE-1 (long interspersed element-1) retroelements are the only active autonomous endogenous retroelements in human genomes. Their retrotransposition activity has created close to 50% of the current human genome. Due to the apparent costs of this proliferation, host genomes have evolved multiple mechanisms to curb LINE-1 retrotransposition. Here, we investigate the evolution and function of the LINE-1 restriction factor APOBEC3A, a member of the APOBEC3 cytidine deaminase gene family. We find that APOBEC3A genes have evolved rapidly under diversifying selection in primates, suggesting changes in APOBEC3A have been recurrently selected in a host-pathogen "arms race." Nonetheless, in contrast to previous reports, we find that the LINE-1 restriction activity of APOBEC3A proteins has been strictly conserved throughout simian primate evolution in spite of its pervasive diversifying selection. Based on these results, we conclude that LINE-1s have not driven the rapid evolution of APOBEC3A in primates. In contrast to this conserved LINE-1 restriction, we find that a subset of primate APOBEC3A genes have enhanced antiviral restriction. We trace this gain of antiviral restriction in APOBEC3A to the common ancestor of a subset of Old World monkeys. Thus, APOBEC3A has not only maintained its LINE-1 restriction ability, but also evolved a gain of antiviral specificity against other pathogens. Our findings suggest that while APOBEC3A has evolved to restrict additional pathogens, only those adaptive amino acid changes that leave LINE-1 restriction unperturbed have been tolerated.

The Influence of LINE-1 and SINE Retrotransposons on Mammalian Genomes

Transposable elements have had a profound impact on the structure and function of mammalian genomes. The retrotransposon Long INterspersed Element-1 (LINE-1 or L1), by virtue of its replicative mobilization mechanism, comprises ∼17% of the human genome. Although the vast majority of human LINE-1 sequences are inactive molecular fossils, an estimated 80-100 copies per individual retain the ability to mobilize by a process termed retrotransposition. Indeed, LINE-1 is the only active, autonomous retrotransposon in humans and its retrotransposition continues to generate both intra-individual and inter-individual genetic diversity. Here, we briefly review the types of transposable elements that reside in mammalian genomes. We will focus our discussion on LINE-1 retrotransposons and the non-autonomous Short INterspersed Elements (SINEs) that rely on the proteins encoded by LINE-1 for their mobilization. We review cases where LINE-1-mediated retrotransposition events have resulted in genetic disease and discuss how the characterization of these mutagenic insertions led to the identification of retrotransposition-competent LINE-1s in the human and mouse genomes. We then discuss how the integration of molecular genetic, biochemical, and modern genomic technologies have yielded insight into the mechanism of LINE-1 retrotransposition, the impact of LINE-1-mediated retrotransposition events on mammalian genomes, and the host cellular mechanisms that protect the genome from unabated LINE-1-mediated retrotransposition events. Throughout this review, we highlight unanswered questions in LINE-1 biology that provide exciting opportunities for future research. Clearly, much has been learned about LINE-1 and SINE biology since the publication of Mobile DNA II thirteen years ago. Future studies should continue to yield exciting discoveries about how these retrotransposons contribute to genetic diversity in mammalian genomes.

How retrotransposons shape genome regulation

Retrotransposons are mutagenic units able to move within the genome. Despite many defenses deployed by the host to suppress potentially harmful activities of retrotransposons, these genetic units have found ways to meld with normal cellular functions through processes of exaptation and domestication. The same host mechanisms targeting transposon mobility allow for expansion and rewiring of gene regulatory networks on an evolutionary time scale. Recent works demonstrating retrotransposon activity during development, cell differentiation and neurogenesis shed new light on unexpected activities of transposable elements. Moreover, new technological advances illuminated subtler nuances of the complex relationship between retrotransposons and the host genome, clarifying the role of retroelements in evolution, development and impact on human disease.

Characterization of L1-Ribonucleoprotein Particles

The LINE-1 retrotransposon (L1) encodes two proteins, ORF1p and ORF2p, which bind to the L1 RNA in cis, forming a ribonucleoprotein (RNP) complex that is critical for retrotransposition. Interactions with both permissive and repressive host factors pervade every step of the L1 life cycle. Until recently, limitations in detection and production precluded in-depth characterization of L1 RNPs. Inducible expression and recombinant engineering of epitope tags have made detection of both L1 ORFs routine. Here, we describe large-scale production of L1-expressing HEK-293T cells in suspension cell culture, cryomilling and affinity capture of L1 RNP complexes, sample preparation for analysis by mass spectrometry, and assay using the L1 element amplification protocol (LEAP) and qRT-PCR.

Biochemical Approaches to Study LINE-1 Reverse Transcriptase Activity In Vitro

In vitro reverse transcriptase assays have been developed to monitor the presence and activity of ORF2p, an essential protein product of the LINE-1 retrotransposon (L1), in cellular fractions. We describe methods for expression and isolation of L1 ribonucleoprotein particles, and identification of ORF2p reverse transcriptase activity. Two independent methods are described: L1 element amplification protocol (LEAP) and direct L1 extension assay (DLEA). The first method involves cDNA synthesis by primer extension using dNTPs followed by a step of PCR amplification. The second method involves primer extension by incorporation of radiolabeled dTMPs followed by dot-blot or gel separation detection. Finally, we discuss the output and benefits of the two methods.

LEAP: L1 Element Amplification Protocol

Long INterspersed Element-1 (LINE-1 or L1) retrotransposons encode two proteins (ORF1p and ORF2p) that are required for retrotransposition. The L1 element amplification protocol (LEAP) assays the ability of L1 ORF2p to reverse transcribe L1 RNA in vitro. Ultracentrifugation or immunoprecipitation is used to isolate L1 ribonucleoprotein particle (RNP) complexes from cultured human cells transfected with an engineered L1 expression construct. The isolated RNPs are incubated with an oligonucleotide that contains a unique sequence at its 5' end and a thymidine-rich sequence at its 3' end. The addition of dNTPs to the reaction allows L1 ORF2p bound to L1 RNA to generate L1 cDNA. The resultant L1 cDNAs then are amplified using polymerase chain reaction (PCR) and the products are visualized by gel electrophoresis. Sequencing the resultant PCR products then allows product verification. The LEAP assay has been instrumental in determining how mutations in L1 ORF1p and ORF2p affect L1 reverse transcriptase (RT) activity. Furthermore, the LEAP assay has revealed that the L1 ORF2p RT can extend a DNA primer with mismatched 3' terminal bases when it is annealed to an L1 RNA template. As the LINE-1 biology field gravitates toward studying cellular proteins that regulate LINE-1, molecular genetic and biochemical approaches such as LEAP, in conjunction with the LINE-1-cultured cell retrotransposition assay, are essential to dissect the molecular mechanism of L1 retrotransposition.

The Broad-Spectrum Antiviral Protein ZAP Restricts Human Retrotransposition

Intrinsic immunity describes the set of recently discovered but poorly understood cellular mechanisms that specifically target viral pathogens. Their discovery derives in large part from intensive studies of HIV and SIV that revealed restriction factors acting at various stages of the retroviral life cycle. Recent studies indicate that some factors restrict both retroviruses and retrotransposons but surprisingly in ways that may differ. We screened known interferon-stimulated antiviral proteins previously untested for their effects on cell culture retrotransposition. Several factors, including BST2, ISG20, MAVS, MX2, and ZAP, showed strong L1 inhibition. We focused on ZAP (PARP13/ZC3HAV1), a zinc-finger protein that targets viruses of several families, including Retroviridae, Tiloviridae, and Togaviridae, and show that ZAP expression also strongly restricts retrotransposition in cell culture through loss of L1 RNA and ribonucleoprotein particle integrity. Association of ZAP with the L1 ribonucleoprotein particle is supported by co-immunoprecipitation and co-localization with ORF1p in cytoplasmic stress granules. We also used mass spectrometry to determine the protein components of the ZAP interactome, and identified many proteins that directly interact and colocalize with ZAP, including MOV10, an RNA helicase previously shown to suppress retrotransposons. The detection of a chaperonin complex, RNA degradation proteins, helicases, post-translational modifiers, and components of chromatin modifying complexes suggest mechanisms of ZAP anti-retroelement activity that function in the cytoplasm and perhaps also in the nucleus. The association of the ZAP ribonucleoprotein particle with many interferon-stimulated gene products indicates it may be a key player in the interferon response.

LINE-1 retrotransposons: from 'parasite' sequences to functional elements

Long interspersed nuclear elements-1 (LINE-1) are the most abundant and active retrotransposons in the mammalian genomes. Traditionally, the occurrence of LINE-1 sequences in the genome of mammals has been explained by the selfish DNA hypothesis. Nevertheless, recently, it has also been argued that these sequences could play important roles in these genomes, as in the regulation of gene expression, genome modelling and X-chromosome inactivation. The non-random chromosomal distribution is a striking feature of these retroelements that somehow reflects its functionality. In the present study, we have isolated and analysed a fraction of the open reading frame 2 (ORF2) LINE-1 sequence from three rodent species, Cricetus cricetus, Peromyscus eremicus and Praomys tullbergi. Physical mapping of the isolated sequences revealed an interspersed longitudinal AT pattern of distribution along all the chromosomes of the complement in the three genomes. A detailed analysis shows that these sequences are preferentially located in the euchromatic regions, although some signals could be detected in the heterochromatin. In addition, a coincidence between the location of imprinted gene regions (as Xist and Tsix gene regions) and the LINE-1 retroelements was also observed. According to these results, we propose an involvement of LINE-1 sequences in different genomic events as gene imprinting, X-chromosome inactivation and evolution of repetitive sequences located at the heterochromatic regions (e.g. satellite DNA sequences) of the rodents' genomes analysed.

Reviving the dead: history and reactivation of an extinct l1

Although L1 sequences are present in the genomes of all placental mammals and marsupials examined to date, their activity was lost in the megabat family, Pteropodidae, ∼24 million years ago. To examine the characteristics of L1s prior to their extinction, we analyzed the evolutionary history of L1s in the genome of a megabat, Pteropus vampyrus, and found a pattern of periodic L1 expansion and quiescence. In contrast to the well-characterized L1s in human and mouse, megabat genomes have accommodated two or more simultaneously active L1 families throughout their evolutionary history, and major peaks of L1 deposition into the genome always involved multiple families. We compared the consensus sequences of the two major megabat L1 families at the time of their extinction to consensus L1s of a variety of mammalian species. Megabat L1s are comparable to the other mammalian L1s in terms of adenosine content and conserved amino acids in the open reading frames (ORFs). However, the intergenic region (IGR) of the reconstructed element from the more active family is dramatically longer than the IGR of well-characterized human and mouse L1s. We synthesized the reconstructed element from this L1 family and tested the ability of its components to support retrotransposition in a tissue culture assay. Both ORFs are capable of supporting retrotransposition, while the IGR is inhibitory to retrotransposition, especially when combined with either of the reconstructed ORFs. We dissected the inhibitory effect of the IGR by testing truncated and shuffled versions and found that length is a key factor, but not the only one affecting inhibition of retrotransposition. Although the IGR is inhibitory to retrotransposition, this inhibition does not account for the extinction of L1s in megabats. Overall, the evolution of the L1 sequence or the quiescence of L1 is unlikely the reason of L1 extinction.

Characterisation of cytoplasmic DNA complementary to non-retroviral RNA viruses in human cells

The synthesis and subsequent genomic integration of DNA that is complementary to the genomes of non-retroviral RNA viruses are rarely observed. However, upon infection of various human cell lines and primary fibroblasts with the vesicular stomatitis virus (VSV), we detected DNA complementary to the VSV RNA. The VSV DNA was detected in the cytoplasm as single-stranded DNA fully complementary to the viral mRNA from the poly(A) region to the 7-methyl guanosine cap. The formation of this DNA was cell-dependent. Experimentally, we found that the transduction of cells that do not produce VSV DNA with the long interspersed nuclear element 1 and their infection with VSV could lead to the formation of VSV DNA. Viral DNA complementary to other RNA viruses was also detected in the respective infected human cells. Thus, the genetic information of the non-retroviral RNA virus genome can flow into the DNA of mammalian cells expressing LINE-1-like elements.

Expression and detection of LINE-1 ORF-encoded proteins

LINE-1 (L1) elements are endogenous retrotransposons active in mammalian genomes. The L1 RNA is bicistronic, encoding two non-overlapping open reading frames, ORF1 and ORF2, whose protein products (ORF1p and ORF2p) bind the L1 RNA to form a ribonucleoprotein (RNP) complex that is presumed to be a critical retrotransposition intermediate. However, ORF2p is expressed at a significantly lower level than ORF1p; these differences are thought to be controlled at the level of translation, due to a low frequency ribosome reinitiation mechanism controlling ORF2 expression. As a result, while ORF1p is readily detectable, ORF2p has previously been very challenging to detect in vitro and in vivo. To address this, we recently tested several epitope tags fused to the N- or C-termini of the ORF proteins in an effort to enable robust detection and affinity purification from native (L1RP) and synthetic (ORFeus-Hs) L1 constructs. An analysis of tagged RNPs from both L1RP and ORFeus-Hs showed similar host-cell-derived protein interactors. Our observations also revealed that the tag sequences affected the retrotransposition competency of native and synthetic L1s differently although they encode identical ORF proteins. Unexpectedly, we observed apparently stochastic expression of ORF2p within seemingly homogenous L1-expressing cell populations.

Affinity proteomics reveals human host factors implicated in discrete stages of LINE-1 retrotransposition

LINE-1s are active human DNA parasites that are agents of genome dynamics in evolution and disease. These streamlined elements require host factors to complete their life cycles, whereas hosts have developed mechanisms to combat retrotransposition's mutagenic effects. As such, endogenous L1 expression levels are extremely low, creating a roadblock for detailed interactomic analyses. Here, we describe a system to express and purify highly active L1 RNP complexes from human suspension cell culture and characterize the copurified proteome, identifying 37 high-confidence candidate interactors. These data sets include known interactors PABPC1 and MOV10 and, with in-cell imaging studies, suggest existence of at least three types of compositionally and functionally distinct L1 RNPs. Among the findings, UPF1, a key nonsense-mediated decay factor, and PCNA, the polymerase-delta-associated sliding DNA clamp, were identified and validated. PCNA interacts with ORF2p via a PIP box motif; mechanistic studies suggest that this occurs during or immediately after target-primed reverse transcription.

An antisense promoter in mouse L1 retrotransposon open reading frame-1 initiates expression of diverse fusion transcripts and limits retrotransposition

Between 6 and 30% of human and mouse transcripts are initiated from transposable elements. However, the promoters driving such transcriptional activity are mostly unknown. We experimentally characterized an antisense (AS) promoter in mouse L1 retrotransposons for the first time, oriented antiparallel to the coding strand of L1 open reading frame-1. We found that AS transcription is mediated by RNA polymerase II. Rapid amplification of cDNA ends cloning mapped transcription start sites adjacent to the AS promoter. We identified >100 novel fusion transcripts, of which many were conserved across divergent mouse lineages, suggesting conservation of potential functions. To evaluate whether AS L1 transcription could regulate L1 retrotransposition, we replaced portions of native open reading frame-1 in donor elements by synonymously recoded sequences. The resulting L1 elements lacked AS promoter activity and retrotransposed more frequently than endogenous L1s. Overexpression of AS L1 transcripts also reduced L1 retrotransposition. This suppression of retrotransposition was largely independent of Dicer. Our experiments shed new light on how AS fusion transcripts are initiated from endogenous L1 elements across the mouse genome. Such AS transcription can contribute substantially both to natural transcriptional variation and to endogenous regulation of L1 retrotransposition.

Lost in translation: The biogenesis of non-LTR retrotransposon proteins

“Young” APE-type non-LTR retrotransposons (non-LTRs) typically encode two open reading frames (ORFs 1 and 2). The shorter ORF1 translation product (ORF1p) comprises an RNA binding activity, thought to bind to non-LTR transcript RNA, protect against nuclease degradation and specify nuclear import of the ribonuclear protein complex (RNP). ORF2 encodes a multifunctional protein (ORF2p) comprising apurinic/apyrimidinic endonuclease (APE) and reverse-transcriptase (RT) activities, responsible for genome replication and re-integration into chromosomal DNA. However, some clades of APE-type non-LTRs only encode a single ORF—corresponding to the multifunctional ORF2p outlined above (and for simplicity referred-to as ORF2 below). The absence of an ORF1 correlates with the acquisition of a 2A oligopeptide translational recoding element (some 18–30 amino acids) into the N-terminal region of ORF2p. In the case of non-LTRs encoding two ORFs, the presence of ORF1 would necessarily downregulate the translation of ORF2. We argue that in the absence of an ORF1, 2A could provide the corresponding translational downregulation of ORF2. While multiple molecules of ORF1p are required to decorate the non-LTR transcript RNA in the cytoplasm, conceivably only a single molecule of ORF2p is required for target-primed reverse transcription/integration in the nucleus. Why would the translation of ORF2 need to be controlled by such mechanisms? An “excess” of ORF2p could result in disadvantageous levels of genome instability by, for example, enhancing short, interspersed, element (SINE) retrotransposition and the generation of processed pseudogenes. If so, the acquisition of mechanisms—such as 2A—to control ORF2p biogenesis would be advantageous.

TE-array--a high throughput tool to study transposon transcription

Background

Although transposable element (TE) derived DNA accounts for more than half of mammalian genomes and initiates a significant proportion of RNA transcripts, high throughput methods are rarely leveraged specifically to detect expression from interspersed repeats.

Results

To characterize the contribution of transposons to mammalian transcriptomes, we developed a custom microarray platform with probes covering known human and mouse transposons in both sense and antisense orientations. We termed this platform the “TE-array” and profiled TE repeat expression in a panel of normal mouse tissues. Validation with nanoString^® and RNAseq technologies demonstrated that TE-array is an effective method. Our data show that TE transcription occurs preferentially from the sense strand and is regulated in highly tissue-specific patterns.

Conclusions

Our results are consistent with the hypothesis that transposon RNAs frequently originate within genomic TE units and do not primarily accumulate as a consequence of random ‘read-through’ from gene promoters. Moreover, we find TE expression is highly dependent on the tissue context. This suggests that TE expression may be related to tissue-specific chromatin states or cellular phenotypes. We anticipate that TE-array will provide a scalable method to characterize transposable element RNAs.

Modulation of LINE-1 and Alu/SVA retrotransposition by Aicardi-Goutières syndrome-related SAMHD1

Long interspersed elements 1 (LINE-1) occupy at least 17% of the human genome and are its only active autonomous retrotransposons. However, the host factors that regulate LINE-1 retrotransposition are not fully understood. Here, we demonstrate that the Aicardi-Goutières syndrome gene product SAMHD1, recently revealed to be an inhibitor of HIV/simian immunodeficiency virus (SIV) infectivity and neutralized by the viral Vpx protein, is also a potent regulator of LINE-1 and LINE-1-mediated Alu/SVA retrotransposition. We also found that mutant SAMHD1s of Aicardi-Goutières syndrome patients are defective in LINE-1 inhibition. Several domains of SAMHD1 are critical for LINE-1 regulation. SAMHD1 inhibits LINE-1 retrotransposition in dividing cells. An enzymatic active site mutant SAMHD1 maintained substantial anti-LINE-1 activity. SAMHD1 inhibits ORF2p-mediated LINE-1 reverse transcription in isolated LINE-1 ribonucleoproteins by reducing ORF2p level. Thus, SAMHD1 may be a cellular regulator of LINE-1 activity that is conserved in mammals.

In vitro screening for compounds that enhance human L1 mobilization

The Long interspersed element 1 (LINE1 or L1) retrotransposon constitutes 17% of the human genome. There are currently 80-100 human L1 elements that are thought to be active in any diploid human genome. These elements can mobilize into new locations of the genome, resulting in changes in genomic information. Active L1s are thus considered to be a type of endogenous mutagen, and L1 insertions can cause disease. Certain stresses, such as gamma radiation, oxidative stress, and treatment with some agents, can induce transcription and/or mobilization of retrotransposons. In this study, we used a reporter gene assay in HepG2 cells to screen compounds for the potential to enhance the transcription of human L1. We assessed 95 compounds including genotoxic agents, substances that induce cellular stress, and commercially available drugs. Treatment with 15 compounds increased the L1 promoter activity by >1.5-fold (p<0.05) after 6 or 24 hours of treatment. In particular, genotoxic agents (benzo[a]pyrene, camptothecin, cytochalasin D, merbarone, and vinblastine), PPARα agonists (bezafibrate and fenofibrate), and non-steroidal anti-inflammatory drugs (diflunisal, flufenamic acid, salicylamide, and sulindac) induced L1 promoter activity. To examine their effects on L1 retrotransposition, we developed a high-throughput real-time retrotransposition assay using a novel secreted Gaussia luciferase reporter cassette. Three compounds (etomoxir, WY-14643, and salicylamide) produced a significant enhancement in L1 retrotransposition. This is the first study to report the effects of a wide variety of compounds on L1 transcription and retrotransposition. These results suggest that certain chemical- and drug-induced stresses might have the potential to cause genomic mutations by inducing L1 mobilization. Thus, the risk of induced L1 transcription and retrotransposition should be considered during drug safety evaluation and environmental risk assessments of chemicals.

Molecular reconstruction of extinct LINE-1 elements and their interaction with nonautonomous elements

Non-long terminal repeat retroelements continue to impact the human genome through cis-activity of long interspersed element-1 (LINE-1 or L1) and trans-mobilization of Alu. Current activity is dominated by modern subfamilies of these elements, leaving behind an evolutionary graveyard of extinct Alu and L1 subfamilies. Because Alu is a nonautonomous element that relies on L1 to retrotranspose, there is the possibility that competition between these elements has driven selection and antagonistic coevolution between Alu and L1. Through analysis of synonymous versus nonsynonymous codon evolution across L1 subfamilies, we find that the C-terminal ORF2 cys domain experienced a dramatic increase in amino acid substitution rate in the transition from L1PA5 to L1PA4 subfamilies. This observation coincides with the previously reported rapid evolution of ORF1 during the same transition period. Ancestral Alu sequences have been previously reconstructed, as their short size and ubiquity have made it relatively easy to retrieve consensus sequences from the human genome. In contrast, creating constructs of extinct L1 copies is a more laborious task. Here, we report our efforts to recreate and evaluate the retrotransposition capabilities of two ancestral L1 elements, L1PA4 and L1PA8 that were active ∼18 and ∼40 Ma, respectively. Relative to the modern L1PA1 subfamily, we find that both elements are similarly active in a cell culture retrotransposition assay in HeLa, and both are able to efficiently trans-mobilize Alu elements from several subfamilies. Although we observe some variation in Alu subfamily retrotransposition efficiency, any coevolution that may have occurred between LINEs and SINEs is not evident from these data. Population dynamics and stochastic variation in the number of active source elements likely play an important role in individual LINE or SINE subfamily amplification. If coevolution also contributes to changing retrotransposition rates and the progression of subfamilies, cell factors are likely to play an important mediating role in changing LINE-SINE interactions over evolutionary time.

Poly(A) binding protein C1 is essential for efficient L1 retrotransposition and affects L1 RNP formation

Poly(A) binding proteins (PABPs) specifically bind the polyadenosine tail of mRNA and have been shown to be important for RNA polyadenylation, translation initiation, and mRNA stability. Using a modified L1 retrotransposition vector, we examined the effects of two PABPs (encoded by PABPN1 and PABPC1) on the retrotransposition activity of the L1 non-long-terminal-repeat (non-LTR) retrotransposon in both HeLa and HEK293T cells. We demonstrated that knockdown of these two genes by RNA interference (RNAi) effectively reduced L1 retrotransposition by 70 to 80% without significantly changing L1 transcription or translation or the status of the poly(A) tail. We identified that both poly(A) binding proteins were associated with the L1 ribonucleoprotein complex, presumably through L1 mRNA. Depletion of PABPC1 caused a defect in L1 RNP formation. Knockdown of the PABPC1 inhibitor PAIP2 increased L1 retrotransposition up to 2-fold. Low levels of exogenous overexpression of PABPN1 and PABPC1 increased L1 retrotransposition, whereas unregulated overexpression of these two proteins caused pleiotropic effects, such as hypersensitivity to puromycin and decreased L1 activity. Our data suggest that PABPC1 is essential for the formation of L1 RNA-protein complexes and may play a role in L1 RNP translocation in the host cell.

Evolutionary conservation of the functional modularity of primate and murine LINE-1 elements

LINE-1 (L1) retroelements emerged in mammalian genomes over 80 million years ago with a few dominant subfamilies amplifying over discrete time periods that led to distinct human and mouse L1 lineages. We evaluated the functional conservation of L1 sequences by comparing retrotransposition rates of chimeric human-rodent L1 constructs to their parental L1 counterparts. Although amino acid conservation varies from ∼35% to 63% for the L1 ORF1p and ORF2p, most human and mouse L1 sequences can be functionally exchanged. Replacing either ORF1 or ORF2 to create chimeric human-mouse L1 elements did not adversely affect retrotransposition. The mouse ORF2p retains retrotransposition-competency to support both Alu and L1 mobilization when any of the domain sequences we evaluated were substituted with human counterparts. However, the substitution of portions of the mouse cys-domain into the human ORF2p reduces both L1 retrotransposition and Alu trans-mobilization by 200–1000 fold. The observed loss of ORF2p function is independent of the endonuclease or reverse transcriptase activities of ORF2p and RNA interaction required for reverse transcription. In addition, the loss of function is physically separate from the cysteine-rich motif sequence previously shown to be required for RNP formation. Our data suggest an additional role of the less characterized carboxy-terminus of the L1 ORF2 protein by demonstrating that this domain, in addition to mediating RNP interaction(s), provides an independent and required function for the retroelement amplification process. Our experiments show a functional modularity of most of the LINE sequences. However, divergent evolution of interactions within L1 has led to non-reciprocal incompatibilities between human and mouse ORF2 cys-domain sequences.

Effect of reverse transcriptase inhibitors on LINE-1 and Ty1 reverse transcriptase activities and on LINE-1 retrotransposition

BACKGROUND

LINE-1s (L1, Long Interspersed Element-1) are the most abundant autonomous non-LTR retrotransposons in the human genome and replicate by reverse transcription of an RNA intermediate. Full-length L1 encodes two open reading frames (ORF1, ORF2) and ORF2 has reverse transcriptase activity.

RESULTS

Here we expressed human L1 RT in E. coli and the purified protein displayed the same RT activity as that of ORF2p expressed in insect cells. We tested the effect of different reverse transcriptase inhibitors on L1 RT and found that all four tested nucleoside inhibitors efficiently inhibited L1 RT activity competitively. The Ki values of NRTIs were calculated (AZTTP, 16.4 ± 4.21 nM; d4TTP, 0.73 ± 0.22 nM; ddCTP, 0.72 ± 0.16 nM; 3TCTP, 12.9 ± 2.07 nM). L1 RT was less sensitive to non-nucleoside reverse transcriptase inhibitors, among these nevirapine had no effect, even at concentrations up to 500 μM. We also examined the effect of RT inhibitors on L1 retrotransposition efficiency in vivo using a cell-based retrotransposition assay. Similarly, all analog inhibitors decreased L1 retrotransposition frequency with different potencies whereas nevirapine had little or no effect on L1 retrotransposition. For comparison, we also tested the same inhibitors to highly purified RT of an LTR-retrotransposon (Ty1) and found it was less sensitive to NRTIs than L1 RT and has the same inhibition profile as L1 RT to NNRTIs.

CONCLUSIONS

These data indicate that bacterially expressed L1 RT is an active reverse transcriptase sensitive to nucleoside RT inhibitors but not to non-nucleoside inhibitors.