LINE-1 Cultured Cell Retrotransposition Assay

The domesticated transposon protein L1TD1 associates with its ancestor L1 ORF1p to promote LINE-1 retrotransposition

Repression of retrotransposition is crucial for the successful fitness of a mammalian organism. The domesticated transposon protein L1TD1, derived from LINE-1 (L1) ORF1p, is an RNA-binding protein that is expressed only in some cancers and early embryogenesis. In human embryonic stem cells, it is found to be essential for maintaining pluripotency. In cancer, L1TD1 expression is highly correlative with malignancy progression and as such considered a potential prognostic factor for tumors. However, its molecular role in cancer remains largely unknown. Our findings reveal that DNA hypomethylation induces the expression of L1TD1 in HAP1 human tumor cells. L1TD1 depletion significantly modulates both the proteome and transcriptome and thereby reduces cell viability. Notably, L1TD1 associates with L1 transcripts and interacts with L1 ORF1p protein, thereby facilitating L1 retrotransposition. Our data suggest that L1TD1 collaborates with its ancestral L1 ORF1p as an RNA chaperone, ensuring the efficient retrotransposition of L1 retrotransposons, rather than directly impacting the abundance of L1TD1 targets. In this way, L1TD1 might have an important role not only during early development but also in tumorigenesis.

p53-mediated regulation of LINE1 retrotransposon-derived R-loops

Long interspersed nuclear element 1 (LINE1/L1) retrotransposons, which comprise 17% of the human genome, typically remain inactive in healthy somatic cells but are reactivated in several cancers. We previously demonstrated that p53 silences L1 transposons in human somatic cells, potentially acting as a tumor-suppressive mechanism. However, the precise molecular mechanisms underlying p53-mediated repression of L1 and its life cycle intermediates remain unclear. In this study, we used DNA-RNA immunoprecipitation-sequencing experiments to investigate RNA-DNA hybrids, which are key intermediates formed during L1 retrotransposition. Our findings reveal that L1 mRNA-genomic DNA (cis L1 R-loops) and L1 mRNA-complementary DNA (trans L1 R-loops) hybrids are upregulated in p53-/- cells. This increase is synergistic with L1 activation by histone deacetylase (HDAC) inhibitors (HDACi). However, treatment with a reverse transcriptase inhibitor reduces this accumulation, indicating that retrotransposition activity plays a significant role in R-loop accumulation. Interestingly, in WT cells, hyperactivated L1 transposons are suppressed upon HDACi withdrawal. L1 suppression in WT cells coincided with the recruitment of repressive marks, specifically H3K9me3 and H3K27me3, simultaneously preventing the addition of activating marks like H3K4me3, and H3K9ac at the L1 5'UTR. Mechanistically, we demonstrate that p53 cooperates with histone methyltransferases SETDB1 and G9A to deposit H3K9me3 marks at the L1 promoter, thereby silencing transposons. This study is the first to reveal novel roles of p53 in preventing the formation of L1-derived RNA-DNA hybrids (R-loops) and suppression of hyperactivated L1 elements by cooperating with histone methyltransferases, underscoring its critical role in maintaining genomic stability.

Identification of a minimal Alu domain required for retrotransposition

Alu elements are primate-specific retrotransposon sequences that comprise ~11% of human genomic DNA. Alu sequences contain an internal RNA polymerase III promoter and the resultant Alu RNA transcripts mobilize by a replicative process termed retrotransposition. Alu retrotransposition requires the Long INterspersed Element-1 (LINE-1) open reading frame 2-encoded protein (ORF2p). Current models propose that Alu RNA binds to signal recognition particle proteins 9 and 14 (SRP9/14) and localizes to ribosomes, which allows Alu to 'hijack' L1 ORF2p. Here, we used HeLa cell-based retrotransposition assays to define a minimal Alu domain necessary for retrotransposition. We demonstrate that Alu transcripts expressed from a cytomegalovirus (CMV) RNA polymerase II promoter can efficiently undergo retrotransposition. The use of an external CMV promoter to express Alu RNA allowed us to construct separation-of-function mutations to examine the effects of large deletions within the Alu sequence on retrotransposition. Deletion mutagenesis demonstrated that a 46 nucleotide (nt) domain located at the 5' end of the Alu RNA transcript is necessary for Alu retrotransposition. Consistent with current models, the 46 nt 5' Alu domain associates with SRP9/14 in HeLa-HA cell extracts and can promote a single round of retrotransposition in HeLa-HA cells. We propose that the 46 nt 5' Alu domain forms a discrete structure that allows for SRP 9/14 binding and ribosomal association, thereby allowing the Alu poly(A) tract to compete with the L1 poly(A) tail for ORF2p RNA binding to mediate its retrotransposition.

Variable patterns of retrotransposition in different HeLa strains provide mechanistic insights into SINE RNA mobilization processes

Alu elements are non-autonomous Short INterspersed Elements (SINEs) derived from the 7SL RNA gene that are present at over one million copies in human genomic DNA. Alu mobilizes by a mechanism known as retrotransposition, which requires the Long INterspersed Element-1 (LINE-1) ORF2-encoded protein (ORF2p). Here, we demonstrate that HeLa strains differ in their capacity to support Alu retrotransposition. Human Alu elements retrotranspose efficiently in HeLa-HA and HeLa-CCL2 (Alu-permissive) strains, but not in HeLa-JVM or HeLa-H1 (Alu-nonpermissive) strains. A similar pattern of retrotransposition was observed for other 7SL RNA-derived SINEs and tRNA-derived SINEs. In contrast, mammalian LINE-1s, a zebrafish LINE, a human SINE-VNTR-Alu (SVA) element, and an L1 ORF1-containing mRNA can retrotranspose in all four HeLa strains. Using an in vitro reverse transcriptase-based assay, we show that Alu RNAs associate with ORF2p and are converted into cDNAs in both Alu-permissive and Alu-nonpermissive HeLa strains, suggesting that 7SL- and tRNA-derived SINEs use strategies to 'hijack' L1 ORF2p that are distinct from those used by SVA elements and ORF1-containing mRNAs. These data further suggest ORF2p associates with the Alu RNA poly(A) tract in both Alu-permissive and Alu-nonpermissive HeLa strains, but that Alu retrotransposition is blocked after this critical step in Alu-nonpermissive HeLa strains.

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.

Variable patterns of retrotransposition in different HeLa strains provide mechanistic insights into SINE RNA mobilization processes

Alu elements are non-autonomous Short INterspersed Elements (SINEs) derived from the 7SL RNA gene that are present at over one million copies in human genomic DNA. Alu mobilizes by a mechanism known as retrotransposition, which requires the Long INterspersed Element-1 (LINE-1 or L1) ORF2 -encoded protein (ORF2p). Here, we demonstrate that HeLa strains differ in their capacity to support Alu retrotransposition. Human Alu elements retrotranspose efficiently in HeLa-HA and HeLa-CCL2 ( Alu -permissive) strains, but not in HeLa-JVM or HeLa-H1 ( Alu -nonpermissive) strains. A similar pattern of retrotransposition was observed for other 7SL RNA -derived SINEs and tRNA -derived SINEs. In contrast, mammalian LINE-1s, a zebrafish LINE, a human SINE-VNTR - Alu ( SVA ) element, and an L1 ORF1 -containing messenger RNA can retrotranspose in all four HeLa strains. Using an in vitro reverse transcriptase-based assay, we show that Alu RNAs associate with ORF2p and are converted into cDNAs in both Alu -permissive and Alu -nonpermissive HeLa strains, suggesting that 7SL - and tRNA -derived SINE RNAs use strategies to 'hijack' L1 ORF2p that are distinct from those used by SVA elements and ORF1 -containing mRNAs. These data further suggest ORF2p associates with the Alu RNA poly(A) tract in both Alu -permissive and Alu -nonpermissive HeLa strains, but that Alu retrotransposition is blocked after this critical step in Alu -nonpermissive HeLa strains.

LINE-1 mRNA 3' end dynamics shape its biology and retrotransposition potential

LINE-1 (L1) retrotransposons are mobile genetic elements that create new genomic insertions by a copy-paste mechanism involving L1 RNA/RNP intermediates. L1 encodes two ORFs, of which L1-ORF2p nicks genomic DNA and reverse transcribes L1 mRNA using the nicked DNA as a primer which base-pairs with poly(A) tail of L1 mRNA. To better understand the importance of non-templated L1 3' ends' dynamics and the interplay between L1 3' and 5' ends, we investigated the effects of genomic knock-outs and temporal knock-downs of XRN1, DCP2, and other factors. We hypothesized that in the absence of XRN1, the major 5'→3' exoribonuclease, there would be more L1 mRNA and retrotransposition. Conversely, we observed that loss of XRN1 decreased L1 retrotransposition. This occurred despite slight stabilization of L1 mRNA, but with decreased L1 RNP formation. Similarly, loss of DCP2, the catalytic subunit of the decapping complex, lowered retrotransposition despite increased steady-state levels of L1 proteins. In both XRN1 and DCP2 depletions we observed shortening of L1 3' poly(A) tails and their increased uridylation by TUT4/7. We explain the observed reduction of L1 retrotransposition by the changed qualities of non-templated L1 mRNA 3' ends demonstrating the important role of L1 3' end dynamics in L1 biology.

LINE-1 retrotransposition and its deregulation in cancers: implications for therapeutic opportunities

Long interspersed element 1 (LINE-1) is the only protein-coding transposon that is active in humans. LINE-1 propagates in the genome using RNA intermediates via retrotransposition. This activity has resulted in LINE-1 sequences occupying approximately one-fifth of our genome. Although most copies of LINE-1 are immobile, ∼100 copies are retrotransposition-competent. Retrotransposition is normally limited via epigenetic silencing, DNA repair, and other host defense mechanisms. In contrast, LINE-1 overexpression and retrotransposition are hallmarks of cancers. Here, we review mechanisms of LINE-1 regulation and how LINE-1 may promote genetic heterogeneity in tumors. Finally, we discuss therapeutic strategies to exploit LINE-1 biology in cancers.

Nuclear cGAS restricts L1 retrotransposition by promoting TRIM41-mediated ORF2p ubiquitination and degradation

Cyclic GMP-AMP synthase (cGAS), initially identified as a cytosolic DNA sensor, detects DNA fragments to trigger an innate immune response. Recently, accumulating evidence reveals the presence of cGAS within the nucleus. However, the biological functions of nuclear cGAS are not fully understood. Here, we demonstrate that nuclear cGAS represses LINE-1 (L1) retrotransposition to preserve genome integrity in human cells. Mechanistically, the E3 ligase TRIM41 interacts with and ubiquitinates ORF2p to influence its stability, and cGAS enhances the association of ORF2p with TRIM41, thereby promoting TRIM41-mediated ORF2p degradation and the suppression of L1 retrotransposition. In response to DNA damage, cGAS is phosphorylated at serine residues 120 and 305 by CHK2, which promotes cGAS-TRIM41 association, facilitating TRIM41-mediated ORF2p degradation. Moreover, we show that nuclear cGAS mediates the repression of L1 retrotransposition in senescent cells induced by DNA damage agents. We also identify several cancer-associated cGAS mutations that abolish the suppressive effect on L1 retrotransposition by disrupting the CHK2-cGAS-TRIM41-ORF2p regulatory axis. Together, these findings indicate that nuclear cGAS exhibits an inhibitory function in L1 retrotransposition which could provide avenues for future interventions in both aging and tumorigenesis.

Retrotransposon LINE-1 bodies in the cytoplasm of piRNA-deficient mouse spermatocytes: Ribonucleoproteins overcoming the integrated stress response

Transposable elements (TE) are mobile DNA sequences whose excessive proliferation endangers the host. Although animals have evolved robust TE-targeting defenses, including Piwi-interacting (pi)RNAs, retrotransposon LINE-1 (L1) still thrives in humans and mice. To gain insights into L1 endurance, we characterized L1 Bodies (LBs) and ORF1p complexes in germ cells of piRNA-deficient Maelstrom null mice. We report that ORF1p interacts with TE RNAs, genic mRNAs, and stress granule proteins, consistent with earlier studies. We also show that ORF1p associates with the CCR4-NOT deadenylation complex and PRKRA, a Protein Kinase R factor. Despite ORF1p interactions with these negative regulators of RNA expression, the stability and translation of LB-localized mRNAs remain unchanged. To scrutinize these findings, we studied the effects of PRKRA on L1 in cultured cells and showed that it elevates ORF1p levels and L1 retrotransposition. These results suggest that ORF1p-driven condensates promote L1 propagation, without affecting the metabolism of endogenous RNAs.

The interferon stimulated gene-encoded protein HELZ2 inhibits human LINE-1 retrotransposition and LINE-1 RNA-mediated type I interferon induction

Some interferon stimulated genes (ISGs) encode proteins that inhibit LINE-1 (L1) retrotransposition. Here, we use immunoprecipitation followed by liquid chromatography-tandem mass spectrometry to identify proteins that associate with the L1 ORF1-encoded protein (ORF1p) in ribonucleoprotein particles. Three ISG proteins that interact with ORF1p inhibit retrotransposition: HECT and RLD domain containing E3 ubiquitin-protein ligase 5 (HERC5); 2'-5'-oligoadenylate synthetase-like (OASL); and helicase with zinc finger 2 (HELZ2). HERC5 destabilizes ORF1p, but does not affect its cellular localization. OASL impairs ORF1p cytoplasmic foci formation. HELZ2 recognizes sequences and/or structures within the L1 5'UTR to reduce L1 RNA, ORF1p, and ORF1p cytoplasmic foci levels. Overexpression of WT or reverse transcriptase-deficient L1s lead to a modest induction of IFN-α expression, which is abrogated upon HELZ2 overexpression. Notably, IFN-α expression is enhanced upon overexpression of an ORF1p RNA binding mutant, suggesting ORF1p binding might protect L1 RNA from "triggering" IFN-α induction. Thus, ISG proteins can inhibit retrotransposition by different mechanisms.

Ribonucleoprotein condensation driven by retrotransposon LINE-1 sustains RNA integrity and translation in mouse spermatocytes

Transposable elements (TE) are mobile DNA sequences whose excessive proliferation endangers the host. Although animals have evolved robust TE-targeting defenses, including Piwi-interacting (pi)RNAs, retrotransposon LINE-1 (L1) still thrives in humans and mice. To gain insights into L1 endurance, we characterized L1 Bodies (LBs) and ORF1p complexes in germ cells of piRNA-deficient Maelstrom null mice. We report that ORF1p interacts with TE RNAs, genic mRNAs, and stress granule proteins, consistent with earlier studies. We also show that ORF1p associates with the CCR4-NOT deadenylation complex and PRKRA, a Protein Kinase R factor. Despite ORF1p interactions with these negative regulators of RNA expression, the stability and translation of LB-localized mRNAs remain unchanged. To scrutinize these findings, we studied the effects of PRKRA on L1 in cultured cells and showed that it elevates ORF1p levels and L1 retrotransposition. These results suggest that ORF1p-driven condensates promote L1 propagation, without affecting the metabolism of endogenous RNAs.

LINE-1 Retrotransposition Assays in Embryonic Stem Cells

The ongoing mobilization of active non-long terminal repeat (LTR) retrotransposons continues to impact the genomes of most mammals, including humans and rodents. Non-LTR retrotransposons mobilize using an intermediary RNA and a copy-and-paste mechanism termed retrotransposition. Non-LTR retrotransposons are subdivided into long and short interspersed elements (LINEs and SINEs, respectively), depending on their size and autonomy; while active class 1 LINEs (LINE-1s or L1s) encode the enzymatic machinery required to mobilize in cis, active SINEs use the enzymatic machinery of active LINE-1s to mobilize in trans. The mobilization mechanism used by LINE-1s/SINEs was exploited to develop ingenious plasmid-based retrotransposition assays in cultured cells, which typically exploit a reporter gene that can only be activated after a round of retrotransposition. Retrotransposition assays, in cis or in trans, are instrumental tools to study the biology of mammalian LINE-1s and SINEs. In fact, these and other biochemical/genetic assays were used to uncover that endogenous mammalian LINE-1s/SINEs naturally retrotranspose during early embryonic development. However, embryonic stem cells (ESCs) are typically used as a cellular model in these and other studies interrogating LINE-1/SINE expression/regulation during early embryogenesis. Thus, human and mouse ESCs represent an excellent model to understand how active retrotransposons are regulated and how their activity impacts the germline. Here, we describe robust and quantitative protocols to study human/mouse LINE-1 (in cis) and SINE (in trans) retrotransposition using (human and mice) ESCs. These protocols are designed to study the mobilization of active non-LTR retrotransposons in a cellular physiologically relevant context.

Retrotransposon instability dominates the acquired mutation landscape of mouse induced pluripotent stem cells

Induced pluripotent stem cells (iPSCs) can in principle differentiate into any cell of the body, and have revolutionized biomedical research and regenerative medicine. Unlike their human counterparts, mouse iPSCs (miPSCs) are reported to silence transposable elements and prevent transposable element-mediated mutagenesis. Here we apply short-read or Oxford Nanopore Technologies long-read genome sequencing to 38 bulk miPSC lines reprogrammed from 10 parental cell types, and 18 single-cell miPSC clones. While single nucleotide variants and structural variants restricted to miPSCs are rare, we find 83 de novo transposable element insertions, including examples intronic to Brca1 and Dmd. LINE-1 retrotransposons are profoundly hypomethylated in miPSCs, beyond other transposable elements and the genome overall, and harbor alternative protein-coding gene promoters. We show that treatment with the LINE-1 inhibitor lamivudine does not hinder reprogramming and efficiently blocks endogenous retrotransposition, as detected by long-read genome sequencing. These experiments reveal the complete spectrum and potential significance of mutations acquired by miPSCs.

Condensin I and condensin II proteins form a LINE-1 dependent super condensin complex and cooperate to repress LINE-1

Condensin I and condensin II are multi-subunit complexes that are known for their individual roles in genome organization and preventing genomic instability. However, interactions between condensin I and condensin II subunits and cooperative roles for condensin I and condensin II, outside of their genome organizing functions, have not been reported. We previously discovered that condensin II cooperates with Gamma Interferon Activated Inhibitor of Translation (GAIT) proteins to associate with Long INterspersed Element-1 (LINE-1 or L1) RNA and repress L1 protein expression and the retrotransposition of engineered L1 retrotransposition in cultured human cells. Here, we report that the L1 3'UTR is required for condensin II and GAIT association with L1 RNA, and deletion of the L1 RNA 3'UTR results in increased L1 protein expression and retrotransposition. Interestingly, like condensin II, we report that condensin I also binds GAIT proteins, associates with the L1 RNA 3'UTR, and represses L1 retrotransposition. We provide evidence that the condensin I protein, NCAPD2, is required for condensin II and GAIT protein association with L1 RNA. Furthermore, condensin I and condensin II subunits interact to form a L1-dependent super condensin complex (SCC) which is located primarily within the cytoplasm of both transformed and primary epithelial cells. These data suggest that increases in L1 expression in epithelial cells promote cytoplasmic condensin protein associations that facilitate a feedback loop in which condensins may cooperate to mediate L1 repression.

Ongoing endeavors to detect mobilization of transposable elements

Transposable elements (TEs) are DNA sequences capable of mobilization from one location to another in the genome. Since the discovery of ‘Dissociation (Dc) locus’ by Barbara McClintock in maize (1), mounting evidence in the era of genomics indicates that a significant fraction of most eukaryotic genomes is composed of TE sequences, involving in various aspects of biological processes such as development, physiology, diseases and evolution. Although technical advances in genomics have discovered numerous functional impacts of TE across species, our understanding of TEs is still ongoing process due to challenges resulted from complexity and abundance of TEs in the genome. In this mini-review, we briefly summarize biology of TEs and their impacts on the host genome, emphasizing importance of understanding TE landscape in the genome. Then, we introduce recent endeavors especially in vivo retrotransposition assays and long read sequencing technology for identifying de novo insertions/TE polymorphism, which will broaden our knowledge of extraordinary relationship between genomic cohabitants and their host.

Somatic retrotransposition in the developing rhesus macaque brain

The retrotransposon LINE-1 (L1) is central to the recent evolutionary history of the human genome and continues to drive genetic diversity and germline pathogenesis. However, the spatiotemporal extent and biological significance of somatic L1 activity are poorly defined and are virtually unexplored in other primates. From a single L1 lineage active at the divergence of apes and Old World monkeys, successive L1 subfamilies have emerged in each descendant primate germline. As revealed by case studies, the presently active human L1 subfamily can also mobilize during embryonic and brain development in vivo. It is unknown whether nonhuman primate L1s can similarly generate somatic insertions in the brain. Here we applied approximately 40× single-cell whole-genome sequencing (scWGS), as well as retrotransposon capture sequencing (RC-seq), to 20 hippocampal neurons from two rhesus macaques (Macaca mulatta). In one animal, we detected and PCR-validated a somatic L1 insertion that generated target site duplications, carried a short 5' transduction, and was present in ∼7% of hippocampal neurons but absent from cerebellum and nonbrain tissues. The corresponding donor L1 allele was exceptionally mobile in vitro and was embedded in PRDM4, a gene expressed throughout development and in neural stem cells. Nanopore long-read methylome and RNA-seq transcriptome analyses indicated young retrotransposon subfamily activation in the early embryo, followed by repression in adult tissues. These data highlight endogenous macaque L1 retrotransposition potential, provide prototypical evidence of L1-mediated somatic mosaicism in a nonhuman primate, and allude to L1 mobility in the brain over the past 30 million years of human evolution.

Ongoing endeavors to detect mobilization of transposable elements

Transposable elements (TEs) are DNA sequences capable of mobilization from one location to another in the genome. Since the discovery of 'Dissociation (Dc) locus' by Barbara McClintock in maize (1), mounting evidence in the era of genomics indicates that a significant fraction of most eukaryotic genomes is composed of TE sequences, involving in various aspects of biological processes such as development, physiology, diseases and evolution. Although technical advances in genomics have discovered numerous functional impacts of TE across species, our understanding of TEs is still ongoing process due to challenges resulted from complexity and abundance of TEs in the genome. In this mini-review, we briefly summarize biology of TEs and their impacts on the host genome, emphasizing importance of understanding TE landscape in the genome. Then, we introduce recent endeavors especially in vivo retrotransposition assays and long read sequencing technology for identifying de novo insertions/TE polymorphism, which will broaden our knowledge of extraordinary relationship between genomic cohabitants and their host. [BMB Reports 2022; 55(7): 305-315].

RNA m6A modification orchestrates a LINE-1-host interaction that facilitates retrotransposition and contributes to long gene vulnerability

The molecular basis underlying the interaction between retrotransposable elements (RTEs) and the human genome remains poorly understood. Here, we profiled N6-methyladenosine (m6A) deposition on nascent RNAs in human cells by developing a new method MINT-Seq, which revealed that many classes of RTE RNAs, particularly intronic LINE-1s (L1s), are strongly methylated. These m6A-marked intronic L1s (MILs) are evolutionarily young, sense-oriented to hosting genes, and are bound by a dozen RNA binding proteins (RBPs) that are putative novel readers of m6A-modified RNAs, including a nuclear matrix protein SAFB. Notably, m6A positively controls the expression of both autonomous L1s and co-transcribed L1 relics, promoting L1 retrotransposition. We showed that MILs preferentially reside in long genes with critical roles in DNA damage repair and sometimes in L1 suppression per se, where they act as transcriptional "roadblocks" to impede the hosting gene expression, revealing a novel host-weakening strategy by the L1s. In counteraction, the host uses the SAFB reader complex to bind m6A-L1s to reduce their levels, and to safeguard hosting gene transcription. Remarkably, our analysis identified thousands of MILs in multiple human fetal tissues, enlisting them as a novel category of cell-type-specific regulatory elements that often compromise transcription of long genes and confer their vulnerability in neurodevelopmental disorders. We propose that this m6A-orchestrated L1-host interaction plays widespread roles in gene regulation, genome integrity, human development and diseases.

Long-read assembly of a Great Dane genome highlights the contribution of GC-rich sequence and mobile elements to canine genomes

Technological advances have allowed improvements in genome reference sequence assemblies. Here, we combined long- and short-read sequence resources to assemble the genome of a female Great Dane dog. This assembly has improved continuity compared to the existing Boxer-derived (CanFam3.1) reference genome. Annotation of the Great Dane assembly identified 22,182 protein-coding gene models and 7,049 long noncoding RNAs, including 49 protein-coding genes not present in the CanFam3.1 reference. The Great Dane assembly spans the majority of sequence gaps in the CanFam3.1 reference and illustrates that 2,151 gaps overlap the transcription start site of a predicted protein-coding gene. Moreover, a subset of the resolved gaps, which have an 80.95% median GC content, localize to transcription start sites and recombination hotspots more often than expected by chance, suggesting the stable canine recombinational landscape has shaped genome architecture. Alignment of the Great Dane and CanFam3.1 assemblies identified 16,834 deletions and 15,621 insertions, as well as 2,665 deletions and 3,493 insertions located on secondary contigs. These structural variants are dominated by retrotransposon insertion/deletion polymorphisms and include 16,221 dimorphic canine short interspersed elements (SINECs) and 1,121 dimorphic long interspersed element-1 sequences (LINE-1_Cfs). Analysis of sequences flanking the 3' end of LINE-1_Cfs (i.e., LINE-1_Cf 3'-transductions) suggests multiple retrotransposition-competent LINE-1_Cfs segregate among dog populations. Consistent with this conclusion, we demonstrate that a canine LINE-1_Cf element with intact open reading frames can retrotranspose its own RNA and that of a SINEC_Cf consensus sequence in cultured human cells, implicating ongoing retrotransposon activity as a driver of canine genetic variation.

p53 directly represses human LINE1 transposons

p53 is a potent tumor suppressor and commonly mutated in human cancers. Recently, we demonstrated that p53 genes act to restrict retrotransposons in germline tissues of flies and fish but whether this activity is conserved in somatic human cells is not known. Here we show that p53 constitutively restrains human LINE1s by cooperatively engaging sites in the 5'UTR and stimulating local deposition of repressive histone marks at these transposons. Consistent with this, the elimination of p53 or the removal of corresponding binding sites in LINE1s, prompted these retroelements to become hyperactive. Concurrently, p53 loss instigated chromosomal rearrangements linked to LINE sequences and also provoked inflammatory programs that were dependent on reverse transcriptase produced from LINE1s. Taken together, our observations establish that p53 continuously operates at the LINE1 promoter to restrict autonomous copies of these mobile elements in human cells. Our results further suggest that constitutive restriction of these retroelements may help to explain tumor suppression encoded by p53, since erupting LINE1s produced acute oncogenic threats when p53 was absent.

Loss of Nuclear TDP-43 Is Associated with Decondensation of LINE Retrotransposons

Loss of the nuclear RNA binding protein TAR DNA binding protein-43 (TDP-43) into cytoplasmic aggregates is the strongest correlate to neurodegeneration in amyotrophic lateral sclerosis and frontotemporal degeneration. The molecular changes associated with the loss of nuclear TDP-43 in human tissues are not entirely known. Using subcellular fractionation andfluorescent-activated cell sorting to enrich for diseased neuronal nuclei without TDP-43 from post-mortem frontotemporal degeneration-amyotro-phic lateral sclerosis (FTD-ALS) human brain, we characterized the effects of TDP-43 loss on the transcriptome and chromatin accessibility. Nuclear TDP-43 loss is associated with gene expression changes that affect RNA processing, nucleocytoplas-mic transport, histone processing, and DNA damage. Loss of nuclear TDP-43 is also associated with chromatin decondensation around long interspersed nuclear elements (LINEs) and increased LINE1 DNA content. Moreover, loss of TDP-43 leads to increased retrotransposition that can be inhibited with antiretro-viral drugs, suggesting that TDP-43 neuropathology is associated with altered chromatin structure including decondensation of LINEs.

Liu et al. fractionated and sorted for diseased neuronal nuclei from post-mortem FTD-ALS human brains and showed that loss of an RNA-binding protein, TDP-43, altered the transcriptome and chromatin accessibility. Their results suggest that loss of nuclear TDP-43 is associated with decondensation of LINE retrotransposons.

Our Conflict with Transposable Elements and Its Implications for Human Disease

Our genome is a historic record of successive invasions of mobile genetic elements. Like other eukaryotes, we have evolved mechanisms to limit their propagation and minimize the functional impact of new insertions. Although these mechanisms are vitally important, they are imperfect, and a handful of retroelement families remain active in modern humans. This review introduces the intrinsic functions of transposons, the tactics employed in their restraint, and the relevance of this conflict to human pathology. The most straightforward examples of disease-causing transposable elements are germline insertions that disrupt a gene and result in a monogenic disease allele. More enigmatic are the abnormal patterns of transposable element expression in disease states. Changes in transposon regulation and cellular responses to their expression have implicated these sequences in diseases as diverse as cancer, autoimmunity, and neurodegeneration. Distinguishing their epiphenomenal from their pathogenic effects may provide wholly new perspectives on our understanding of disease.

Long INterspersed element-1 mobility as a sensor of environmental stresses

Long INterspersed element (LINE-1, L1) retrotransposons are the most abundant transposable elements in the human genome, constituting approximately 17%. They move by a "copy-paste" mechanism, involving reverse transcription of an RNA intermediate and insertion of its cDNA copy at a new site in the genome. L1 retrotransposition (L1-RTP) can cause insertional mutations, alter gene expression, transduce exons, and induce epigenetic dysregulation. L1-RTP is generally repressed; however, a number of observations collected over about 15 years revealed that it can occur in response to environmental stresses. Moreover, emerging evidence indicates that L1-RTP can play a role in the onset of several neurological and oncological diseases in humans. In recent years, great attention has been paid to the exposome paradigm, which proposes that health effects of an environmental factor should be evaluated considering both cumulative environmental exposures and the endogenous processes resulting from the biological response. L1-RTP could be an endogenous process considered for this application. Here, we summarize the current understanding of environmental factors that can affect the retrotransposition of human L1 elements. Evidence indicates that L1-RTP alteration is triggered by numerous and various environmental stressors, such as chemical agents (heavy metals, carcinogens, oxidants, and drugs), physical agents (ionizing and non-ionizing radiations), and experiential factors (voluntary exercise, social isolation, maternal care, and environmental light/dark cycles). These data come from in vitro studies on cell lines and in vivo studies on transgenic animals: future investigations should be focused on physiologically relevant models to gain a better understanding of this topic.

sRNA/L1 retrotransposition: using siRNAs and miRNAs to expand the applications of the cell culture-based LINE-1 retrotransposition assay

The cell culture-based retrotransposition reporter assay has been (and is) an essential tool for the study of vertebrate Long INterspersed Elements (LINEs). Developed more than 20 years ago, this assay has been instrumental in characterizing the role of LINE-encoded proteins in retrotransposition, understanding how ribonucleoprotein particles are formed, how host factors regulate LINE mobilization, etc. Moreover, variations of the conventional assay have been developed to investigate the biology of other currently active human retrotransposons, such as Alu and SVA. Here, we describe a protocol that allows combination of the conventional cell culture-based LINE-1 retrotransposition reporter assay with short interfering RNAs (siRNAs) and microRNA (miRNAs) mimics or inhibitors, which has allowed us to uncover specific miRNAs and host factors that regulate retrotransposition. The protocol described here is highly reproducible, quantitative, robust and flexible, and allows the study of several small RNA classes and various retrotransposons. To illustrate its utility, here we show that siRNAs to Fanconi anaemia proteins (FANC-A and FANC-C) and an inhibitor of miRNA-20 upregulate and downregulate human L1 retrotransposition, respectively. This article is part of a discussion meeting issue 'Crossroads between transposons and gene regulation'.

Poly(ADP-Ribose) Polymerase 2 Recruits Replication Protein A to Sites of LINE-1 Integration to Facilitate Retrotransposition

Long interspersed element-1 (LINE-1 or L1) retrotransposition poses a threat to genome integrity, and cells have evolved mechanisms to restrict retrotransposition. However, how cellular proteins facilitate L1 retrotransposition requires elucidation. Here, we demonstrate that single-strand DNA breaks induced by the L1 endonuclease trigger the recruitment of poly(ADP-ribose) polymerase 2 (PARP2) to L1 integration sites and that PARP2 activation leads to the subsequent recruitment of the replication protein A (RPA) complex to facilitate retrotransposition. We further demonstrate that RPA directly binds activated PARP2 through poly(ADP-ribosyl)ation and can protect single-strand L1 integration intermediates from APOBEC3-mediated cytidine deamination in vitro. Paradoxically, we provide evidence that RPA can guide APOBEC3A, and perhaps other APOBEC3 proteins, to sites of L1 integration. Thus, the interplay of L1-encoded and evolutionarily conserved cellular proteins is required for efficient retrotransposition; however, these interactions also may be exploited to restrict L1 retrotransposition in the human genome.

LINE-1 Evasion of Epigenetic Repression in Humans

Epigenetic silencing defends against LINE-1 (L1) retrotransposition in mammalian cells. However, the mechanisms that repress young L1 families and how L1 escapes to cause somatic genome mosaicism in the brain remain unclear. Here we report that a conserved Yin Yang 1 (YY1) transcription factor binding site mediates L1 promoter DNA methylation in pluripotent and differentiated cells. By analyzing 24 hippocampal neurons with three distinct single-cell genomic approaches, we characterized and validated a somatic L1 insertion bearing a 3' transduction. The source (donor) L1 for this insertion was slightly 5' truncated, lacked the YY1 binding site, and was highly mobile when tested in vitro. Locus-specific bisulfite sequencing revealed that the donor L1 and other young L1s with mutated YY1 binding sites were hypomethylated in embryonic stem cells, during neurodifferentiation, and in liver and brain tissue. These results explain how L1 can evade repression and retrotranspose in the human body.

Dynamic Methylation of an L1 Transduction Family during Reprogramming and Neurodifferentiation

The retrotransposon LINE-1 (L1) is a significant source of endogenous mutagenesis in humans. In each individual genome, a few retrotransposition-competent L1s (RC-L1s) can generate new heritable L1 insertions in the early embryo, primordial germ line, and germ cells. L1 retrotransposition can also occur in the neuronal lineage and cause somatic mosaicism. Although DNA methylation mediates L1 promoter repression, the temporal pattern of methylation applied to individual RC-L1s during neurogenesis is unclear. Here, we identified a de novo L1 insertion in a human induced pluripotent stem cell (hiPSC) line via retrotransposon capture sequencing (RC-seq). The L1 insertion was full-length and carried 5' and 3' transductions. The corresponding donor RC-L1 was part of a large and recently active L1 transduction family and was highly mobile in a cultured-cell L1 retrotransposition reporter assay. Notably, we observed distinct and dynamic DNA methylation profiles for the de novo L1 and members of its extended transduction family during neuronal differentiation. These experiments reveal how a de novo L1 insertion in a pluripotent stem cell is rapidly recognized and repressed, albeit incompletely, by the host genome during neurodifferentiation, while retaining potential for further retrotransposition.

A study of meiomitosis and novel pathways of genomic instability in cutaneous T-cell lymphomas (CTCL)

Genomic instability is a hallmark of cancer and an enabling factor for genetic alterations that drive cancer development and progression. The clashing of mitosis and aberrantly expressed meiosis machineries, which may contribute to genomic instability, has been coined cancer “meiomitosis”. LINE-1 retrotransposition, a process active in germ cells, acts outside of the meiotic machinery to create DNA double strand breaks (DNA DSBs) and has played an important role in the evolution of the human genome. We have previously demonstrated that in CTCL several cancer testis/meiotic genes are expressed. Furthermore, this cancer exhibits extensive and ongoing chromosomal/microsatellite instability. In this study we analyzed immortalized patient-derived cells and primary CTCL patient samples using RT-PCR, western blotting and confocal microscopy and found that proteins critically involved in meiosis and LINE-1 retrotransposition are expressed and are associated with chromosomal instability and DNA DSB formation. Using cell cycle synchronization, we show G1/S phase-transition-specific expression of meiosis proteins. Using the Alu retrotransposition assay, we demonstrate the functional activity of LINE-1 retrotransposon in CTCL. Histone acetyltransferase inhibition results in downregulation of the ectopic germ cell programs and concomitant decrease in DNA DSBs foci formation. Notably, LINE-1 and meiosis genes were expressed across a panel of other solid tumor cell lines. Taken together, our results indicate that malignant cells in culture undergo “cancer meiomitosis” rather than the classic mitosis division. The ectopic expression of meiosis genes and reactivation of LINE-1 may be contributing to genomic instability and represent novel targets for immunotherapy in this and other cancers.

Uridylation by TUT4/7 Restricts Retrotransposition of Human LINE-1s

LINE-1 retrotransposition is tightly restricted by layers of regulatory control, with epigenetic pathways being the best characterized. Looking at post-transcriptional regulation, we now show that LINE-1 mRNA 3' ends are pervasively uridylated in various human cellular models and in mouse testes. TUT4 and TUT7 uridyltransferases catalyze the modification and function in cooperation with the helicase/RNPase MOV10 to counteract the RNA chaperone activity of the L1-ORF1p retrotransposon protein. Uridylation potently restricts LINE-1 retrotransposition by a multilayer mechanism depending on differential subcellular localization of the uridyltransferases. We propose that uridine residues added by TUT7 in the cytoplasm inhibit initiation of reverse transcription of LINE-1 mRNAs once they are reimported to the nucleus, whereas uridylation by TUT4, which is enriched in cytoplasmic foci, destabilizes mRNAs. These results provide a model for the post-transcriptional restriction of LINE-1, revealing a key physiological role for TUT4/7-mediated uridylation in maintaining genome stability.

RNase H2, mutated in Aicardi-Goutières syndrome, promotes LINE-1 retrotransposition

Long INterspersed Element class 1 (LINE-1) elements are a type of abundant retrotransposons active in mammalian genomes. An average human genome contains ~100 retrotransposition-competent LINE-1s, whose activity is influenced by the combined action of cellular repressors and activators. TREX1, SAMHD1 and ADAR1 are known LINE-1 repressors and when mutated cause the autoinflammatory disorder Aicardi-Goutières syndrome (AGS). Mutations in RNase H2 are the most common cause of AGS, and its activity was proposed to similarly control LINE-1 retrotransposition. It has therefore been suggested that increased LINE-1 activity may be the cause of aberrant innate immune activation in AGS Here, we establish that, contrary to expectations, RNase H2 is required for efficient LINE-1 retrotransposition. As RNase H1 overexpression partially rescues the defect in RNase H2 null cells, we propose a model in which RNase H2 degrades the LINE-1 RNA after reverse transcription, allowing retrotransposition to be completed. This also explains how LINE-1 elements can retrotranspose efficiently without their own RNase H activity. Our findings appear to be at odds with LINE-1-derived nucleic acids driving autoinflammation in AGS.

L1 retrotransposition is a common feature of mammalian hepatocarcinogenesis

The retrotransposon Long Interspersed Element 1 (LINE-1 or L1) is a continuing source of germline and somatic mutagenesis in mammals. Deregulated L1 activity is a hallmark of cancer, and L1 mutagenesis has been described in numerous human malignancies. We previously employed retrotransposon capture sequencing (RC-seq) to analyze hepatocellular carcinoma (HCC) samples from patients infected with hepatitis B or hepatitis C virus and identified L1 variants responsible for activating oncogenic pathways. Here, we have applied RC-seq and whole-genome sequencing (WGS) to an Abcb4 (Mdr2)-/- mouse model of hepatic carcinogenesis and demonstrated for the first time that L1 mobilization occurs in murine tumors. In 12 HCC nodules obtained from 10 animals, we validated four somatic L1 insertions by PCR and capillary sequencing, including TF subfamily elements, and one GF subfamily example. One of the TF insertions carried a 3' transduction, allowing us to identify its donor L1 and to demonstrate that this full-length TF element retained retrotransposition capacity in cultured cancer cells. Using RC-seq, we also identified eight tumor-specific L1 insertions from 25 HCC patients with a history of alcohol abuse. Finally, we used RC-seq and WGS to identify three tumor-specific L1 insertions among 10 intra-hepatic cholangiocarcinoma (ICC) patients, including one insertion traced to a donor L1 on Chromosome 22 known to be highly active in other cancers. This study reveals L1 mobilization as a common feature of hepatocarcinogenesis in mammals, demonstrating that the phenomenon is not restricted to human viral HCC etiologies and is encountered in murine liver tumors.

Selective elimination of long INterspersed element-1 expressing tumour cells by targeted expression of the HSV-TK suicide gene

In gene therapy, effective and selective suicide gene expression is crucial. We exploited the endogenous Long INterspersed Element-1 (L1) machinery often reactivated in human cancers to integrate the Herpes Simplex Virus Thymidine Kinase (HSV-TK) suicide gene selectively into the genome of cancer cells. We developed a plasmid-based system directing HSV-TK expression only when reverse transcribed and integrated in the host genome via the endogenous L1 ORF1/2 proteins and an Alu element. Delivery of these new constructs into cells followed by Ganciclovir (GCV) treatment selectively induced mortality of L1 ORF1/2 protein expressing cancer cells, but had no effect on primary cells that do not express L1 ORF1/2. This novel strategy for selective targeting of tumour cells provides high tolerability as the HSV-TK gene cannot be expressed without reverse transcription and integration, and high selectivity as these processes take place only in cancer cells expressing high levels of functional L1 ORF1/2.

Spliced integrated retrotransposed element (SpIRE) formation in the human genome

Human Long interspersed element-1 (L1) retrotransposons contain an internal RNA polymerase II promoter within their 5′ untranslated region (UTR) and encode two proteins, (ORF1p and ORF2p) required for their mobilization (i.e., retrotransposition). The evolutionary success of L1 relies on the continuous retrotransposition of full-length L1 mRNAs. Previous studies identified functional splice donor (SD), splice acceptor (SA), and polyadenylation sequences in L1 mRNA and provided evidence that a small number of spliced L1 mRNAs retrotransposed in the human genome. Here, we demonstrate that the retrotransposition of intra-5′UTR or 5′UTR/ORF1 spliced L1 mRNAs leads to the generation of spliced integrated retrotransposed elements (SpIREs). We identified a new intra-5′UTR SpIRE that is ten times more abundant than previously identified SpIREs. Functional analyses demonstrated that both intra-5′UTR and 5′UTR/ORF1 SpIREs lack Cis-acting transcription factor binding sites and exhibit reduced promoter activity. The 5′UTR/ORF1 SpIREs also produce nonfunctional ORF1p variants. Finally, we demonstrate that sequence changes within the L1 5′UTR over evolutionary time, which permitted L1 to evade the repressive effects of a host protein, can lead to the generation of new L1 splicing events, which, upon retrotransposition, generates a new SpIRE subfamily. We conclude that splicing inhibits L1 retrotransposition, SpIREs generally represent evolutionary “dead-ends” in the L1 retrotransposition process, mutations within the L1 5′UTR alter L1 splicing dynamics, and that retrotransposition of the resultant spliced transcripts can generate interindividual genomic variation.

Long interspersed element-1 (L1) sequences comprise about 17% of the human genome reference sequence. The average human genome contains about 100 active L1s that mobilize throughout the genome by a “copy and paste” process termed retrotransposition. Active L1s encode two proteins (ORF1p and ORF2p). ORF1p and ORF2p preferentially bind to their encoding RNA, forming a ribonucleoprotein particle (RNP). During retrotransposition, the L1 RNP translocates to the nucleus, where the ORF2p endonuclease makes a single-strand nick in target site DNA that exposes a 3′ hydroxyl group in genomic DNA. The 3′ hydroxyl group then is used as a primer by the ORF2p reverse transcriptase to copy the L1 RNA into cDNA, leading to the integration of an L1 copy at a new genomic location. The evolutionary success of L1 requires the faithful retrotransposition of full-length L1 mRNAs; thus, it was surprising to find that a small number of L1 retrotransposition events are derived from spliced L1 mRNAs. By using genetic, biochemical, and computational approaches, we demonstrate that spliced L1 mRNAs can undergo an initial round of retrotransposition, leading to the generation of spliced integrated retrotransposed elements (SpIREs). SpIREs represent about 2% of previously annotated full-length primate-specific L1s in the human genome reference sequence. However, because splicing leads to intra-L1 deletions that remove critical sequences required for L1 expression, SpIREs generally cannot undergo subsequent rounds of retrotransposition and can be considered “dead on arrival” insertions. Our data further highlight how genetic conflict between L1 and its host has influenced L1 expression, L1 retrotransposition, and L1 splicing dynamics over evolutionary time.

Condensin II and GAIT complexes cooperate to restrict LINE-1 retrotransposition in epithelial cells

LINE-1 (L1) retrotransposons can mobilize (retrotranspose) within the human genome, and mutagenic de novo L1 insertions can lead to human diseases, including cancers. As a result, cells are actively engaged in preventing L1 retrotransposition. This work reveals that the human Condensin II complex restricts L1 retrotransposition in both non-transformed and transformed cell lines through inhibition of L1 transcription and translation. Condensin II subunits, CAP-D3 and CAP-H2, interact with members of the Gamma-Interferon Activated Inhibitor of Translation (GAIT) complex including the glutamyl-prolyl-tRNA synthetase (EPRS), the ribosomal protein L13a, Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), and NS1 associated protein 1 (NSAP1). GAIT has been shown to inhibit translation of mRNAs encoding inflammatory proteins in myeloid cells by preventing the binding of the translation initiation complex, in response to Interferon gamma (IFN-γ). Excitingly, our data show that Condensin II promotes complexation of GAIT subunits. Furthermore, RNA-Immunoprecipitation experiments in epithelial cells demonstrate that Condensin II and GAIT subunits associate with L1 RNA in a co-dependent manner, independent of IFN-γ. These findings suggest that cooperation between the Condensin II and GAIT complexes may facilitate a novel mechanism of L1 repression, thus contributing to the maintenance of genome stability in somatic cells.

Deamination-independent restriction of LINE-1 retrotransposition by APOBEC3H

The APOBEC3 family of cytosine deaminase enzymes are able to restrict replication of retroelements, such as LINE-1. However, each of the seven APOBEC3 enzymes have been reported to act differentially to prevent LINE-1 retrotransposition and the mechanisms of APOBEC3-mediated LINE-1 inhibition has not been well understood. The prevailing view for many years was that APOBEC3-mediated LINE-1 inhibition was deamination-independent and relied on APOBEC3s blocking the LINE-1 reverse transcriptase DNA polymerization or transport of the LINE-1 RNA into the nucleus. However, recently it was shown that APOBEC3A can deaminate cytosine, to form uracil, on transiently exposed single-stranded LINE-1 cDNA and this leads to LINE-1 cDNA degradation. In this study, we confirmed that APOBEC3A is a potent deamination-dependent inhibitor of LINE-1 retrotransposition, but show that in contrast, A3H haplotype II and haplotype V restrict LINE-1 activity using a deamination-independent mechanism. Our study supports the model that different APOBEC3 proteins have evolved to inhibit LINE-1 retrotransposition through distinct mechanisms.

Mobilization of LINE-1 retrotransposons is restricted by Tex19.1 in mouse embryonic stem cells

Mobilization of retrotransposons to new genomic locations is a significant driver of mammalian genome evolution, but these mutagenic events can also cause genetic disorders. In humans, retrotransposon mobilization is mediated primarily by proteins encoded by LINE-1 (L1) retrotransposons, which mobilize in pluripotent cells early in development. Here we show that TEX19.1, which is induced by developmentally programmed DNA hypomethylation, can directly interact with the L1-encoded protein L1-ORF1p, stimulate its polyubiquitylation and degradation, and restrict L1 mobilization. We also show that TEX19.1 likely acts, at least in part, through promoting the activity of the E3 ubiquitin ligase UBR2 towards L1-ORF1p. Moreover, loss of Tex19.1 increases L1-ORF1p levels and L1 mobilization in pluripotent mouse embryonic stem cells, implying that Tex19.1 prevents de novo retrotransposition in the pluripotent phase of the germline cycle. These data show that post-translational regulation of L1 retrotransposons plays a key role in maintaining trans-generational genome stability in mammals.

Transposable elements in cancer

Transposable elements give rise to interspersed repeats, sequences that comprise most of our genomes. These mobile DNAs have been historically underappreciated - both because they have been presumed to be unimportant, and because their high copy number and variability pose unique technical challenges. Neither impediment now seems steadfast. Interest in the human mobilome has never been greater, and methods enabling its study are maturing at a fast pace. This Review describes the activity of transposable elements in human cancers, particularly long interspersed element-1 (LINE-1). LINE-1 sequences are self-propagating, protein-coding retrotransposons, and their activity results in somatically acquired insertions in cancer genomes. Altered expression of transposable elements and animation of genomic LINE-1 sequences appear to be hallmarks of cancer, and can be responsible for driving mutations in tumorigenesis.

Transposable elements in cancer

Transposable elements give rise to interspersed repeats, sequences that comprise most of our genomes. These mobile DNAs have been historically underappreciated - both because they have been presumed to be unimportant, and because their high copy number and variability pose unique technical challenges. Neither impediment now seems steadfast. Interest in the human mobilome has never been greater, and methods enabling its study are maturing at a fast pace. This Review describes the activity of transposable elements in human cancers, particularly long interspersed element-1 (LINE-1). LINE-1 sequences are self-propagating, protein-coding retrotransposons, and their activity results in somatically acquired insertions in cancer genomes. Altered expression of transposable elements and animation of genomic LINE-1 sequences appear to be hallmarks of cancer, and can be responsible for driving mutations in tumorigenesis.

Heritable L1 retrotransposition in the mouse primordial germline and early embryo

LINE-1 (L1) retrotransposons are a noted source of genetic diversity and disease in mammals. To expand its genomic footprint, L1 must mobilize in cells that will contribute their genetic material to subsequent generations. Heritable L1 insertions may therefore arise in germ cells and in pluripotent embryonic cells, prior to germline specification, yet the frequency and predominant developmental timing of such events remain unclear. Here, we applied mouse retrotransposon capture sequencing (mRC-seq) and whole-genome sequencing (WGS) to pedigrees of C57BL/6J animals, and uncovered an L1 insertion rate of ≥1 event per eight births. We traced heritable L1 insertions to pluripotent embryonic cells and, strikingly, to early primordial germ cells (PGCs). New L1 insertions bore structural hallmarks of target-site primed reverse transcription (TPRT) and mobilized efficiently in a cultured cell retrotransposition assay. Together, our results highlight the rate and evolutionary impact of heritable L1 retrotransposition and reveal retrotransposition-mediated genomic diversification as a fundamental property of pluripotent embryonic cells in vivo.

Restricting retrotransposons: a review

Retrotransposons have generated about 40 % of the human genome. This review examines the strategies the cell has evolved to coexist with these genomic "parasites", focussing on the non-long terminal repeat retrotransposons of humans and mice. Some of the restriction factors for retrotransposition, including the APOBECs, MOV10, RNASEL, SAMHD1, TREX1, and ZAP, also limit replication of retroviruses, including HIV, and are part of the intrinsic immune system of the cell. Many of these proteins act in the cytoplasm to degrade retroelement RNA or inhibit its translation. Some factors act in the nucleus and involve DNA repair enzymes or epigenetic processes of DNA methylation and histone modification. RISC and piRNA pathway proteins protect the germline. Retrotransposon control is relaxed in some cell types, such as neurons in the brain, stem cells, and in certain types of disease and cancer, with implications for human health and disease. This review also considers potential pitfalls in interpreting retrotransposon-related data, as well as issues to consider for future research.