Different evolutionary fates of recently integrated human and chimpanzee LINE-1 retrotransposons

Identification of epigenetically active L1 promoters in the human brain and their relationship with psychiatric disorders

Long interspersed nuclear element-1 (LINE-1, L1) affects the transcriptome landscape in multiple ways. Promoter activity within its 5'UTR plays a critical role in regulating diverse L1 activities. However, the epigenetic status of L1 promoters in adult brain cells and their relationship with psychiatric disorders remain poorly understood. Here, we examined DNA methylation and hydroxymethylation of the full-length L1s in neurons and nonneurons and identified "epigenetically active" L1s. Notably, some of epigenetically active L1s were retrotransposition competent, which even had chimeric transcripts from the antisense promoters at their 5'UTRs. We also identified differentially methylated L1s in the prefrontal cortices of patients with psychiatric disorders. In nonneurons of bipolar disorder patients, one L1 was significantly hypomethylated and showed an inverse correlation with the expression level of the overlapping gene NREP. Finally, we observed that altered DNA methylation levels of L1 in patients with psychiatric disorders were not affected by the surrounding genomic regions but originated from the L1 sequences. These results suggested that altered epigenetic regulation of the L1 5'UTR in the brain was involved in the pathophysiology of psychiatric disorders.

From telomere to telomere: The transcriptional and epigenetic state of human repeat elements

Mobile elements and repetitive genomic regions are sources of lineage-specific genomic innovation and uniquely fingerprint individual genomes. Comprehensive analyses of such repeat elements, including those found in more complex regions of the genome, require a complete, linear genome assembly. We present a de novo repeat discovery and annotation of the T2T-CHM13 human reference genome. We identified previously unknown satellite arrays, expanded the catalog of variants and families for repeats and mobile elements, characterized classes of complex composite repeats, and located retroelement transduction events. We detected nascent transcription and delineated CpG methylation profiles to define the structure of transcriptionally active retroelements in humans, including those in centromeres. These data expand our insight into the diversity, distribution, and evolution of repetitive regions that have shaped the human genome.

Factors Regulating the Activity of LINE1 Retrotransposons

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

A comprehensive analysis of gorilla-specific LINE-1 retrotransposons

BACKGROUND

Long interspersed element-1 (LINE-1 or L1) is the most abundant retrotransposons in the primate genome. They have approximately 520,000 copies and make up ~ 17% of the primate genome. Full-length L1s can mobilize to a new genomic location using their enzymatic machinery. Gorilla is the second closest species to humans after the chimpanzee, and human-gorilla split 7-12 million years ago. The gorilla genome provides an opportunity to explore primate origins and evolution.

OBJECTIVE

L1s have contributed to genome diversity and variations during primate evolution. This study aimed to identify gorilla-specific L1s using a more recent version of the gorilla reference genome (Mar. 2016 GSMRT3/gorGor5).

METHODS

We collected gorilla-specific L1 candidates through computational analysis and manual inspection. L1Xplorer was used to identify whether full-length gorilla-specific L1s were intact. In addition, to determine the level of sequence conservation between intact fulllength gorilla-specific L1s, two ORFs of intact L1s were aligned with the L1PA2 consensus sequence.

RESULTS

2002 gorilla-specific L1 candidates were identified through computational analysis. Among them, we manually inspected 1,883 gorilla-specific L1s, among which most of them belong to the L1PA2 subfamily and 12 were intact L1s that could influence genomic variations in the gorilla genome. Interestingly, the 12 intact full-length gorilla-specific L1s have 14 highly conserved nonsynonymous mutations, including 6 mutations and 8 mutations in ORF1 and ORF2, respectively. In comparison to the intact full-length chimpanzee-specific L1s and human-specific hot-L1s, two of these in ORF1 (L256F and E293G) were shown as gorilla-specific nonsynonymous mutations.

CONCLUSION

The gorilla-specific L1s may have had significantly affected the gorilla genome to compose a genome different form that of other primates during primate evolution.

High levels of LINE-1 transposable elements expressed in Kaposi's sarcoma-associated herpesvirus-related primary effusion lymphoma

Kaposi's sarcoma-associated herpesvirus (KSHV, HHV-8) is a gamma herpesvirus associated with several human malignancies. Transposable elements (TEs) are ubiquitous in eukaryotic genomes, occupying about 45% of the human genome. TEs have been linked with a variety of disorders and malignancies, though the precise nature of their contribution to many of them has yet to be elucidated. Global transcriptome analysis for differentially expressed TEs in KSHV-associated primary effusion lymphoma (PEL) cells (BCBL1 and BC3) revealed large number of differentially expressed TEs. These differentially expressed TEs include LTR transposons, long interspersed nuclear elements (LINEs), and short interspersed nuclear elements (SINEs). Further analysis of LINE-1 (L1) elements revealed expression upregulation, hypo-methylation, and transition into open chromatin in PEL. In agreement with high L1 expression, PEL cells express ORF1 protein and possess high reverse transcriptase (RT)-activity. Interestingly, inhibition of this RT-activity suppressed PEL cell growth. Collectively, we identified high expression of TEs, and specifically of L1 as a critical component in the proliferation of PEL cells. This observation is relevant for the treatment of KSHV-associated malignancies since they often develop in AIDS patients that are treated with RT inhibitors with potent inhibition for both HIV and L1 RT activity.

Differences between human and chimpanzee genomes and their implications in gene expression, protein functions and biochemical properties of the two species

Chimpanzees are the closest living relatives of humans. The divergence between human and chimpanzee ancestors dates to approximately 6,5-7,5 million years ago. Genetic features distinguishing us from chimpanzees and making us humans are still of a great interest. After divergence of their ancestor lineages, human and chimpanzee genomes underwent multiple changes including single nucleotide substitutions, deletions and duplications of DNA fragments of different size, insertion of transposable elements and chromosomal rearrangements. Human-specific single nucleotide alterations constituted 1.23% of human DNA, whereas more extended deletions and insertions cover ~ 3% of our genome. Moreover, much higher proportion is made by differential chromosomal inversions and translocations comprising several megabase-long regions or even whole chromosomes. However, despite of extensive knowledge of structural genomic changes accompanying human evolution we still cannot identify with certainty the causative genes of human identity. Most structural gene-influential changes happened at the level of expression regulation, which in turn provoked larger alterations of interactome gene regulation networks. In this review, we summarized the available information about genetic differences between humans and chimpanzees and their potential functional impacts on differential molecular, anatomical, physiological and cognitive peculiarities of these species.

A comprehensive analysis of chimpanzee (Pan Troglodytes)-specific AluYb8 element

BACKGROUND

Alu elements are most abundant retrotransposons with > 1.2 million copies in the primate genome. AluYb8 subfamily was diverged from AluY lineage, and has accumulated eight diagnostic mutations and 7-bp duplication during primate evolution. A total of 1851 AluYb copies are present in the human genome, and most of them are human-specific. On the other hand, only a few AluYb8 copies were identified in the chimpanzee genome by previous studies on AluYb8. The significantly different number of species-specific AluYb8 elements between human and chimpanzee might result from the incompletion of chimpanzee reference genome sequences at the time of the previous study.

OBJECTIVE

AluYb8 elements could generate genomic structural variations in the chimpanzee genome. This study aimed to identify and characterize chimpanzee-specific AluYb elements using the most updated chimpanzee reference genome sequences (Jan. 2018, panTro6).

METHODS

To identify chimpanzee-specific AluYb8, we carried out genomic comparison with non-chimpanzee primate genome using the UCSC table browser. In addition, chimpanzee-specific AluYb8 candidates were manually inspected and experimentally verified using PCR and Sanger sequencing.

RESULTS

Among a total of 231 chimpanzee-specific AluYb8 candidates, 11 of the candidates are chimpanzee-specific AluYb8, and 29 elements are shared between the chimpanzee and non-chimpanzee primate genomes. Through the sequence analysis of AluYb8 and other Alu subfamilies, we were able to observe various diagnostic mutations and variable length duplications in 7-bp duplication region of AluYb8 element. In addition, we further validated two of the chimpanzee-specific AluYb8 elements (CS8 and CS20) that were not previously discovered by display PCR and Sanger sequencing. Interestingly, we identified a AluYb8 insertion-mediated deletion (CS8 locus) in the chimpanzee genome.

CONCLUSION

Our study found that AluYb8 elements are much more abundant in the human genome than chimpanzee genome, and that it could be due to the absence of hyperactive "master" AluYb8 elements in the chimpanzee genome.

Cryptic genetic variation enhances primate L1 retrotransposon survival by enlarging the functional coiled coil sequence space of ORF1p

Accounting for continual evolution of deleterious L1 retrotransposon families, which can contain hundreds to thousands of members remains a major issue in mammalian biology. L1 activity generated upwards of 40% of some mammalian genomes, including humans where they remain active, causing genetic defects and rearrangements. L1 encodes a coiled coil-containing protein that is essential for retrotransposition, and the emergence of novel primate L1 families has been correlated with episodes of extensive amino acid substitutions in the coiled coil. These results were interpreted as an adaptive response to maintain L1 activity, however its mechanism remained unknown. Although an adventitious mutation can inactivate coiled coil function, its effect could be buffered by epistatic interactions within the coiled coil, made more likely if the family contains a diverse set of coiled coil sequences-collectively referred to as the coiled coil sequence space. Amino acid substitutions that do not affect coiled coil function (i.e., its phenotype) could be "hidden" from (not subject to) purifying selection. The accumulation of such substitutions, often referred to as cryptic genetic variation, has been documented in various proteins. Here we report that this phenomenon was in effect during the latest episode of primate coiled coil evolution, which occurred 30-10 MYA during the emergence of primate L1Pa7-L1Pa3 families. First, we experimentally demonstrated that while coiled coil function (measured by retrotransposition) can be eliminated by single epistatic mutations, it nonetheless can also withstand extensive amino acid substitutions. Second, principal component and cluster analysis showed that the coiled coil sequence space of each of the L1Pa7-3 families was notably increased by the presence of distinct, coexisting coiled coil sequences. Thus, sampling related networks of functional sequences rather than traversing discrete adaptive states characterized the persistence L1 activity during this evolutionary event.

Differential expressions of L1-chimeric transcripts in normal and matched-cancer tissues

L1s are a cis-regulatory elements and contain bidirectional internal promoters within the 5' untranslated region (UTR). L1s provide bidirectional promoters that generate alternative transcripts and affect differential expressions in the human genome. In particular, L1 antisense promoters (L1ASPs) could produce aberrant transcripts in cancer tissues compared to normal tissues. In this study, we identified the L1-chimeric transcripts derived from L1ASPs and analyzed relative expression of L1-chimeric transcripts between normal and matched-cancer tissues. First, we collected 425 L1-chimeric transcripts by referring to previous studies. Through the manual inspection, we identified 144 L1-chimeric transcripts derived from 44 L1 antisense promoters, suggesting that the antisense promoter acted as an alternative promoter. We analyzed relative gene expression levels of 16 L1-chimeric transcripts between matched cancer-normal tissue pair (lung, liver, gastric, kidney, thyroid, breast, ovary, uterus, and prostate) using real-time quantitative PCR (RT-qPCR) and investigated putative transcription factor binding motifs to determine activity of L1ASPs. Taken together, we propose that L1ASPs could contribute to the differential gene expression between normal and cancer tissues.

SQuIRE reveals locus-specific regulation of interspersed repeat expression

Transposable elements (TEs) are interspersed repeat sequences that make up much of the human genome. Their expression has been implicated in development and disease. However, TE-derived RNA-seq reads are difficult to quantify. Past approaches have excluded these reads or aggregated RNA expression to subfamilies shared by similar TE copies, sacrificing quantitative accuracy or the genomic context necessary to understand the basis of TE transcription. As a result, the effects of TEs on gene expression and associated phenotypes are not well understood. Here, we present Software for Quantifying Interspersed Repeat Expression (SQuIRE), the first RNA-seq analysis pipeline that provides a quantitative and locus-specific picture of TE expression (https://github.com/wyang17/SQuIRE). SQuIRE is an accurate and user-friendly tool that can be used for a variety of species. We applied SQuIRE to RNA-seq from normal mouse tissues and a Drosophila model of amyotrophic lateral sclerosis. In both model organisms, we recapitulated previously reported TE subfamily expression levels and revealed locus-specific TE expression. We also identified differences in TE transcription patterns relating to transcript type, gene expression and RNA splicing that would be lost with other approaches using subfamily-level analyses. Altogether, our findings illustrate the importance of studying TE transcription with locus-level resolution.

Computational Methods for Mapping, Assembly and Quantification for Coding and Non-coding Transcripts

The measurement of gene expression has long provided significant insight into biological functions. The development of high-throughput short-read sequencing technology has revealed transcriptional complexity at an unprecedented scale, and informed almost all areas of biology. However, as researchers have sought to gather more insights from the data, these new technologies have also increased the computational analysis burden. In this review, we describe typical computational pipelines for RNA-Seq analysis and discuss their strengths and weaknesses for the assembly, quantification and analysis of coding and non-coding RNAs. We also discuss the assembly of transposable elements into transcripts, and the difficulty these repetitive elements pose. In summary, RNA-Seq is a powerful technology that is likely to remain a key asset in the biologist's toolkit.

Mobile elements contribute to the uniqueness of human genome with 15,000 human-specific insertions and 14 Mbp sequence increase

Mobile elements (MEs) collectively contribute to at least 50% of the human genome. Due to their past incremental accumulation and ongoing DNA transposition, MEs serve as a significant source for both inter- and intra-species genetic and phenotypic diversity during primate and human evolution. By making use of the most recent genome sequences for human and many other closely related primates and robust multi-way comparative genomic approach, we identified a total of 14,870 human-specific MEs (HS-MEs) with more than 8,000 being newly identified. Collectively, these HS-MEs contribute to a total of 14.2 Mbp net genome sequence increase. Several new observations were made based on these HS-MEs, including the finding of Y chromosome as a strikingly hot target for HS-MEs and a strong mutual preference for SINE-R/VNTR/Alu (SVAs). Furthermore, ∼8,000 of these HS-MEs were found to locate in the vicinity of ∼4,900 genes, and collectively they contribute to ∼84 kb sequences in the human reference transcriptome in association with over 300 genes, including protein-coding sequences for 40 genes. In conclusion, our results demonstrate that MEs made a significant contribution to the evolution of human genome by participating in gene function in a human-specific fashion.

Sequence analysis of cell-free DNA derived from cultured human bone osteosarcoma (143B) cells

The true importance of cell-free DNA in human biology, together with the potential scale of its clinical utility, is tarnished by a lack of understanding of its composition and origin. In investigating the cell-free DNA present in the growth medium of cultured 143B cells, we previously demonstrated that the majority of cell-free DNA is neither a product of apoptosis nor necrosis. In the present study, we investigated the composition and origin of this cell-free DNA population using next-generation sequencing. We found that the cell-free DNA comprises mainly of repetitive DNA, including α-satellite DNA, mini satellites, and transposons that are currently active or exhibit the capacity to become reactivated. A significant portion of these cell-free DNA fragments originates from specific chromosomes, especially chromosomes 1 and 9. In healthy adult somatic cells, the centromeric and pericentromeric regions of these chromosomes are normally densely methylated. However, in many cancer types, these regions are preferentially hypomethylated. This can lead to double-stranded DNA breaks or it can directly impair the formation of proper kinetochore structures. This type of chromosomal instability is a precursor to the formation of nuclear anomalies, including lagging chromosomes and anaphase bridges. DNA fragments derived from these structures can recruit their own nuclear envelope and form secondary nuclear structures known as micronuclei, which can localize to the nuclear periphery and bud out from the membrane. We postulate that the majority of cell-free DNA present in the growth medium of cultured 143B cells originates from these micronuclei.

Contrasting Rates of LINE-1 Amplification among New World Primates of the Atelidae Family

LINE-1 (L1) retrotransposons constitute the dominant category of transposons in mammalian genomes. L1 elements are active in the vast majority of mammals, and only a few cases of L1 extinction have been documented. The only possible case of extinction in primates was suggested for South American spider monkeys. However, these previous studies were based on a single species. We revisited this question with a larger phylogenetic sample, covering all 4 genera of Atelidae and 3 species of spider monkeys. We used an enrichment method to clone recently inserted L1 elements and performed an evolutionary analysis of the sequences. We were able to identify young L1 elements in all taxa, suggesting that L1 is probably still active in all Atelidae examined. However, we also detected considerable variations in the proportion of recent elements indicating that the rate of L1 amplification varies among Atelidae by a 3-fold factor. The extent of L1 amplification in Atelidae remains overall lower than in other New World monkeys. Multiple factors can affect the amplification of L1, such as the demography of the host and the control of transposition. These factors are discussed in the context of host life history.

Repression of Stress-Induced LINE-1 Expression Protects Cancer Cell Subpopulations from Lethal Drug Exposure

Maintenance of phenotypic heterogeneity within cell populations is an evolutionarily conserved mechanism that underlies population survival upon stressful exposures. We show that the genomes of a cancer cell subpopulation that survives treatment with otherwise lethal drugs, the drug-tolerant persisters (DTPs), exhibit a repressed chromatin state characterized by increased methylation of histone H3 lysines 9 and 27 (H3K9 and H3K27). We also show that survival of DTPs is, in part, maintained by regulators of H3K9me3-mediated heterochromatin formation and that the observed increase in H3K9me3 in DTPs is most prominent over long interspersed repeat element 1 (LINE-1). Disruption of the repressive chromatin over LINE-1 elements in DTPs results in DTP ablation, which is partially rescued by reducing LINE-1 expression or function.

LINE-1 Hypermethylation in Serum Cell-Free DNA of Relapsing Remitting Multiple Sclerosis Patients

Concentrations of cell-free DNA (cfDNA) circulating in blood and its epigenetic variation, such as DNA methylation, may provide useful diagnostic or prognostic information. Long interspersed nuclear element-1 (LINE-1) constitutes approximately 20% of the human genome and its 5'UTR region is CpG rich. Due to its wide distribution, the methylation level of the 5'UTR of LINE-1 can serve as a surrogate marker of global genomic DNA methylation. The aim of the current study was to investigate whether the methylation status of LINE-1 elements in serum cell-free DNA differs between relapsing remitting multiple sclerosis (RRMS) patients and healthy control subjects (CTR). Serum DNA samples of 6 patients and 6 controls were subjected to bisulfite sequencing. The results showed that the methylation level varies among distinct CpG sites in the 5'UTR of LINE-1 repeats and revealed differences in the methylation state of specific sites in this element between patients and controls. The latter differences were largely due to CpG sites in the L1PA2 subfamily, which were more frequently methylated in the RRMS patients than in the CTR group, whereas such differences were not observed in the L1HS subfamily. These data were verified by quantitative PCR using material from 18 patients and 18 control subjects. The results confirmed that the methylation level of a subset of the CpG sites within the LINE-1 promoter is elevated in DNA from RRMS patients in comparison with CTR. The present data suggest that the methylation status of CpG sites of LINE repeats could be a basis for development of diagnostic or prognostic tests.

Single genome retrieval of context-dependent variability in mutation rates for human germline

BACKGROUND

Accurate knowledge of the core components of substitution rates is of vital importance to understand genome evolution and dynamics. By performing a single-genome and direct analysis of 39,894 retrotransposon remnants, we reveal sequence context-dependent germline nucleotide substitution rates for the human genome.

RESULTS

The rates are characterised through rate constants in a time-domain, and are made available through a dedicated program (Trek) and a stand-alone database. Due to the nature of the method design and the imposed stringency criteria, we expect our rate constants to be good estimates for the rates of spontaneous mutations. Benefiting from such data, we study the short-range nucleotide (up to 7-mer) organisation and the germline basal substitution propensity (BSP) profile of the human genome; characterise novel, CpG-independent, substitution prone and resistant motifs; confirm a decreased tendency of moieties with low BSP to undergo somatic mutations in a number of cancer types; and, produce a Trek-based estimate of the overall mutation rate in human.

CONCLUSIONS

The extended set of rate constants we report may enrich our resources and help advance our understanding of genome dynamics and evolution, with possible implications for the role of spontaneous mutations in the emergence of pathological genotypes and neutral evolution of proteomes.

LINEs between Species: Evolutionary Dynamics of LINE-1 Retrotransposons across the Eukaryotic Tree of Life

LINE-1 (L1) retrotransposons are dynamic elements. They have the potential to cause great genomic change because of their ability to ‘jump’ around the genome and amplify themselves, resulting in the duplication and rearrangement of regulatory DNA. Active L1, in particular, are often thought of as tightly constrained, homologous and ubiquitous elements with well-characterized domain organization. For the past 30 years, model organisms have been used to define L1s as 6–8 kb sequences containing a 5′-UTR, two open reading frames working harmoniously in cis, and a 3′-UTR with a polyA tail. In this study, we demonstrate the remarkable and overlooked diversity of L1s via a comprehensive phylogenetic analysis of elements from over 500 species from widely divergent branches of the tree of life. The rapid and recent growth of L1 elements in mammalian species is juxtaposed against the diverse lineages found in other metazoans and plants. In fact, some of these previously unexplored mammalian species (e.g. snub-nosed monkey, minke whale) exhibit L1 retrotranspositional ‘hyperactivity’ far surpassing that of human or mouse. In contrast, non-mammalian L1s have become so varied that the current classification system seems to inadequately capture their structural characteristics. Our findings illustrate how both long-term inherited evolutionary patterns and random bursts of activity in individual species can significantly alter genomes, highlighting the importance of L1 dynamics in eukaryotes.

Genome-wide characterization of human L1 antisense promoter-driven transcripts

Background

Long INterspersed Element-1 (LINE-1 or L1) is the only autonomously active, transposable element in the human genome. L1 sequences comprise approximately 17 % of the human genome, but only the evolutionarily recent, human-specific subfamily is retrotransposition competent. The L1 promoter has a bidirectional orientation containing a sense promoter that drives the transcription of two proteins required for retrotransposition and an antisense promoter. The L1 antisense promoter can drive transcription of chimeric transcripts: 5’ L1 antisense sequences spliced to the exons of neighboring genes.

Results

The impact of L1 antisense promoter activity on cellular transcriptomes is poorly understood. To investigate this, we analyzed GenBank ESTs for messenger RNAs that initiate in the L1 antisense promoter. We identified 988 putative L1 antisense chimeric transcripts, 911 of which have not been previously reported. These appear to be alternative genic transcripts, sense-oriented with respect to gene and initiating near, but typically downstream of, the gene transcriptional start site. In multiple cell lines, L1 antisense promoters display enrichment for YY1 transcription factor and histone modifications associated with active promoters. Global run-on sequencing data support the activity of the L1 antisense promoter. We independently detected 124 L1 antisense chimeric transcripts using long read Pacific Biosciences RNA-seq data. Furthermore, we validated four chimeric transcripts by quantitative RT-PCR and Sanger sequencing and demonstrated that they are readily detectable in many normal human tissues.

Conclusions

We present a comprehensive characterization of human L1 antisense promoter-driven transcripts and provide substantial evidence that they are transcribed in a variety of human cell-types. Our findings reveal a new wide-reaching aspect of L1 biology by identifying antisense transcripts affecting as many as 4 % of all human genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2800-5) contains supplementary material, which is available to authorized users.

Composite non-LTR retrotransposons in hominoid primates

Composite retrotransposons are widely distributed in the plant and animal kingdoms. Some of the most complex of these are found in hominoid primates. SVA, LAVA, PVA and FVA combine simple repeats, Alu fragments, a VNTR (Variable Number of Tandem Repeats) and variable 3' domains, which are, except for PVA, derived from other retrotransposons. Although a likely precursor of SVA-a "tailed VNTR" named SVA2-had been identified in the Rhesus genome, the exact sequence and mechanism of the assembly of this type of composite retrotransposon had been elusive. The discovery of LAVA, PVA and FVA in gibbons provided the opportunity to delineate the order of assembly of the components of VNTR-containing retrotransposons. Our recent analysis suggests that an extinct "Alu-SVA2" acquired variant 3' ends by splicing. In this commentary I will discuss the mode of assembly of VNTR composites in the context of their capacity to engage in alternative splicing to co-mobilize host RNA sequences and to become exonized. The second part will focus on structural determinants of VNTR composite retrotransposon mobilization in the context of lineage-specific expansion of particular families/subfamilies of these elements.

Profiling of extensively diversified plant LINEs reveals distinct plant-specific subclades

A large fraction of eukaryotic genomes is made up of long interspersed nuclear elements (LINEs). Due to their capability to create novel copies via error-prone reverse transcription, they generate multiple families and reach high copy numbers. Although mammalian LINEs have been well described, plant LINEs have been only poorly investigated. Here, we present a systematic cross-species survey of LINEs in higher plant genomes shedding light on plant LINE evolution as well as diversity, and facilitating their annotation in genome projects. Applying a Hidden Markov Model (HMM)-based analysis, 59 390 intact LINE reverse transcriptases (RTs) were extracted from 23 plant genomes. These fall in only two out of 28 LINE clades (L1 and RTE) known in eukaryotes. While plant RTE LINEs are highly homogenous and mostly constitute only a single family per genome, plant L1 LINEs are extremely diverse and form numerous families. Despite their heterogeneity, all members across the 23 species fall into only seven L1 subclades, some of them defined here. Exemplarily focusing on the L1 LINEs of a basal reference plant genome (Beta vulgaris), we show that the subclade classification level does not only reflect RT sequence similarity, but also mirrors structural aspects of complete LINE retrotransposons, like element size, position and type of encoded enzymatic domains. Our comprehensive catalogue of plant LINE RTs serves the classification of highly diverse plant LINEs, while the provided subclade-specific HMMs facilitate their annotation.

BLAT-based comparative analysis for transposable elements: BLATCAT

The availability of several whole genome sequences makes comparative analyses possible. In primate genomes, the priority of transposable elements (TEs) is significantly increased because they account for ~45% of the primate genomes, they can regulate the gene expression level, and they are associated with genomic fluidity in their host genomes. Here, we developed the BLAST-like alignment tool (BLAT) based comparative analysis for transposable elements (BLATCAT) program. The BLATCAT program can compare specific regions of six representative primate genome sequences (human, chimpanzee, gorilla, orangutan, gibbon, and rhesus macaque) on the basis of BLAT and simultaneously carry out RepeatMasker and/or Censor functions, which are widely used Windows-based web-server functions to detect TEs. All results can be stored as a HTML file for manual inspection of a specific locus. BLATCAT will be very convenient and efficient for comparative analyses of TEs in various primate genomes.

Transposable elements in cancer as a by-product of stress-induced evolvability

Transposable elements (TEs) are ubiquitous in eukaryotic genomes. Barbara McClintock's famous notion of TEs acting as controlling elements modifying the genetic response of an organism upon exposure to stressful environments has since been solidly supported in a series of model organisms. This requires the TE activity response to possess an element of specificity and be targeted toward certain parts of the genome. We propose that a similar TE response is present in human cells, and that this stress response may drive the onset of human cancers. As such, TE-driven cancers may be viewed as an evolutionary by-product of organisms' abilities to genetically adapt to environmental stress.

Differential L1 regulation in pluripotent stem cells of humans and apes

Identifying cellular and molecular differences between human and non-human primates (NHPs) is essential to the basic understanding of the evolution and diversity of our own species. Until now, preserved tissues have been the main source for most comparative studies between humans, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)^1,2. However, these tissue samples do not fairly represent the distinctive traits of live cell behavior and are not amenable to genetic manipulation. We hypothesized that induced pluripotent stem cells (iPSCs) could be a unique biological resource to elucidate relevant phenotypical differences between human and NHPs and that those differences could have potential adaptation and speciation value. Here, we describe the generation and initial characterization of iPSCs from chimpanzees and bonobos as novel tools to explore factors that have contributed to great ape evolution. Comparative gene expression analysis of human and NHP iPSCs revealed differences in the regulation of Long Interspersed Nuclear Element-1 (LINE-1 or L1) transposons. A force of change in mammalian evolution, L1 elements are retrotransposons that have remained active during primate evolution^3-5. Decreased levels of L1 restricting factors APOBEC3B (A3B)⁶ and PIWIL2⁷ in NHP iPSCs correlated with increased L1 mobility and endogenous L1 mRNA levels. Moreover, results from manipulation of A3B and PIWIL2 levels in iPSCs supported a causal inverse relationship between levels of these proteins and L1 retrotransposition. Finally, we found increased copy numbers of species-specific L1 elements in the genome of chimpanzees compared to humans, supporting the idea that increased L1 mobility in NHPs is not limited to iPSCs in culture and may have also occurred in the germline or embryonic cells developmentally upstream to germline specification during primate evolution. We propose that differences in L1 mobility may have differentially shaped the genomes of humans and NHPs and could have ongoing adaptive significance.

Evolutionary age of repetitive element subfamilies and sensitivity of DNA methylation to airborne pollutants

Background

Repetitive elements take up >40% of the human genome and can change distribution through transposition, thus generating subfamilies. Repetitive element DNA methylation has associated with several diseases and environmental exposures, including exposure to airborne pollutants. No systematic analysis has yet been conducted to examine the effects of exposures across different repetitive element subfamilies. The purpose of the study is to evaluate sensitivity of DNA methylation in differentially‒evolved LINE, Alu, and HERV subfamilies to different types of airborne pollutants.

Methods

We sampled a total of 120 male participants from three studies (20 high-, 20 low-exposure in each study) of steel workers exposed to metal-rich particulate matter (measured as PM₁₀) (Study 1); gas-station attendants exposed to air benzene (Study 2); and truck drivers exposed to traffic-derived elemental carbon (Study 3). We measured methylation by bisulfite-PCR-pyrosequencing in 10 differentially‒evolved repetitive element subfamilies.

Results

High-exposure groups exhibited subfamily-specific methylation differences compared to low-exposure groups: L1PA2 showed lower DNA methylation in steel workers (P=0.04) and gas station attendants (P=0.03); L1Ta showed lower DNA methylation in steel workers (P=0.02); AluYb8 showed higher DNA methylation in truck drivers (P=0.05). Within each study, dose–response analyses showed subfamily-specific correlations of methylation with exposure levels. Interaction models showed that the effects of the exposures on DNA methylation were dependent on the subfamily evolutionary age, with stronger effects on older LINEs from PM₁₀ (p‒interaction=0.003) and benzene (p‒interaction=0.04), and on younger Alus from PM₁₀ (p-interaction=0.02).

Conclusions

The evolutionary age of repetitive element subfamilies determines differential susceptibility of DNA methylation to airborne pollutants.

Human-specific HERV-K insertion causes genomic variations in the human genome

Human endogenous retroviruses (HERV) sequences account for about 8% of the human genome. Through comparative genomics and literature mining, we identified a total of 29 human-specific HERV-K insertions. We characterized them focusing on their structure and flanking sequence. The results showed that four of the human-specific HERV-K insertions deleted human genomic sequences via non-classical insertion mechanisms. Interestingly, two of the human-specific HERV-K insertion loci contained two HERV-K internals and three LTR elements, a pattern which could be explained by LTR-LTR ectopic recombination or template switching. In addition, we conducted a polymorphic test and observed that twelve out of the 29 elements are polymorphic in the human population. In conclusion, human-specific HERV-K elements have inserted into human genome since the divergence of human and chimpanzee, causing human genomic changes. Thus, we believe that human-specific HERV-K activity has contributed to the genomic divergence between humans and chimpanzees, as well as within the human population.

LINE-1 retrotransposition in the nervous system

Long interspersed element-1 (LINE-1 or L1) is a repetitive DNA retrotransposon capable of duplication by a copy-and-paste genetic mechanism. Scattered throughout mammalian genomes, L1 is typically quiescent in most somatic cell types. In developing neurons, however, L1 can express and retrotranspose at high frequency. The L1 element can insert into various genomic locations including intragenic regions. These insertions can alter the dynamic of the neuronal transcriptome by changing the expression pattern of several nearby genes. The consequences of L1 genomic alterations in somatic cells are still under investigation, but the high level of mutagenesis within neurons suggests that each neuron is genetically unique. Furthermore, some neurological diseases, such as Rett syndrome and ataxia telangiectasia, misregulate L1 retrotransposition, which could contribute to some pathological aspects. In this review, we survey the literature related to neurodevelopmental retrotransposition and discuss possible relevance to neuronal function, evolution, and neurological disease.

Evolution of genetic and genomic features unique to the human lineage

Given the unprecedented tools that are now available for rapidly comparing genomes, the identification and study of genetic and genomic changes that are unique to our species have accelerated, and we are entering a golden age of human evolutionary genomics. Here we provide an overview of these efforts, highlighting important recent discoveries, examples of the different types of human-specific genomic and genetic changes identified, and salient trends, such as the localization of evolutionary adaptive changes to complex loci that are highly enriched for disease associations. Finally, we discuss the remaining challenges, such as the incomplete nature of current genome sequence assemblies and difficulties in linking human-specific genomic changes to human-specific phenotypic traits.

Transposable Elements: No More 'Junk DNA'

Since the advent of whole-genome sequencing, transposable elements (TEs), just thought to be 'junk' DNA, have been noticed because of their numerous copies in various eukaryotic genomes. Many studies about TEs have been conducted to discover their functions in their host genomes. Based on the results of those studies, it has been generally accepted that they have a function to cause genomic and genetic variations. However, their infinite functions are not fully elucidated. Through various mechanisms, including de novo TE insertions, TE insertion-mediated deletions, and recombination events, they manipulate their host genomes. In this review, we focus on Alu, L1, human endogenous retrovirus, and short interspersed element/variable number of tandem repeats/Alu (SVA) elements and discuss how they have affected primate genomes, especially the human and chimpanzee genomes, since their divergence.

Modes of retrotransposition of long interspersed element-1 by environmental factors

Approximately 42% of the human genome is composed of endogenous retroelements, and the major retroelement component, long interspersed element-1 (L1), comprises ∼17% of the total genome. A single human cell has more than 5 × 10(5) copies of L1, 80∼100 copies of which are competent for retrotransposition (RTP). Notably, L1 can induce RTP of other retroelements, such as Alu and SVA, and is believed to function as a driving force of evolution. Although L1-RTP during early embryogenesis has been highlighted in the literature, recent observations revealed that L1-RTP also occurs in somatic cells. However, little is known about how environmental factors induce L1-RTP. Here, we summarize our current understanding of the mechanism of L1-RTP in somatic cells. We have focused on the mode of L1-RTP that is dependent on the basic helix-loop-helix/per-arnt-sim (bHLH/PAS) family of transcription factors. Along with the proposed function of bHLH/PAS proteins in environmental adaptation, we discuss the functional linking of L1-RTP and bHLH/PAS proteins for environmental adaptation and evolution.

UniPrimer: A Web-Based Primer Design Tool for Comparative Analyses of Primate Genomes

Whole genome sequences of various primates have been released due to advanced DNA-sequencing technology. A combination of computational data mining and the polymerase chain reaction (PCR) assay to validate the data is an excellent method for conducting comparative genomics. Thus, designing primers for PCR is an essential procedure for a comparative analysis of primate genomes. Here, we developed and introduced UniPrimer for use in those studies. UniPrimer is a web-based tool that designs PCR- and DNA-sequencing primers. It compares the sequences from six different primates (human, chimpanzee, gorilla, orangutan, gibbon, and rhesus macaque) and designs primers on the conserved region across species. UniPrimer is linked to RepeatMasker, Primer3Plus, and OligoCalc softwares to produce primers with high accuracy and UCSC In-Silico PCR to confirm whether the designed primers work. To test the performance of UniPrimer, we designed primers on sample sequences using UniPrimer and manually designed primers for the same sequences. The comparison of the two processes showed that UniPrimer was more effective than manual work in terms of saving time and reducing errors.

High Levels of Sequence Diversity in the 5' UTRs of Human-Specific L1 Elements

Approximately 80 long interspersed element (LINE-1 or L1) copies are able to retrotranspose actively in the human genome, and these are termed retrotransposition-competent L1s. The 5′ untranslated region (UTR) of the human-specific L1 contains an internal promoter and several transcription factor binding sites. To better understand the effect of the L1 5′ UTR on the evolution of human-specific L1s, we examined this population of elements, focusing on the sequence diversity and accumulated substitutions within their 5′ UTRs. Using network analysis, we estimated the age of each L1 component (the 5′ UTR, ORF1, ORF2, and 3′ UTR). Through the comparison of the L1 components based on their estimated ages, we found that the 5′ UTR of human-specific L1s accumulates mutations at a faster rate than the other components. To further investigate the L1 5′ UTR, we examined the substitution frequency per nucleotide position among them. The results showed that the L1 5′ UTRs shared relatively conserved transcription factor binding sites, despite their high sequence diversity. Thus, we suggest that the high level of sequence diversity in the 5′ UTRs could be one of the factors controlling the number of retrotransposition-competent L1s in the human genome during the evolutionary battle between L1s and their host genomes.

Pathogenesis of chagas' disease: parasite persistence and autoimmunity

Acute Trypanosoma cruzi infections can be asymptomatic, but chronically infected individuals can die of Chagas' disease. The transfer of the parasite mitochondrial kinetoplast DNA (kDNA) minicircle to the genome of chagasic patients can explain the pathogenesis of the disease; in cases of Chagas' disease with evident cardiomyopathy, the kDNA minicircles integrate mainly into retrotransposons at several chromosomes, but the minicircles are also detected in coding regions of genes that regulate cell growth, differentiation, and immune responses. An accurate evaluation of the role played by the genotype alterations in the autoimmune rejection of self-tissues in Chagas' disease is achieved with the cross-kingdom chicken model system, which is refractory to T. cruzi infections. The inoculation of T. cruzi into embryonated eggs prior to incubation generates parasite-free chicks, which retain the kDNA minicircle sequence mainly in the macrochromosome coding genes. Crossbreeding transfers the kDNA mutations to the chicken progeny. The kDNA-mutated chickens develop severe cardiomyopathy in adult life and die of heart failure. The phenotyping of the lesions revealed that cytotoxic CD45, CD8(+) γδ, and CD8α(+) T lymphocytes carry out the rejection of the chicken heart. These results suggest that the inflammatory cardiomyopathy of Chagas' disease is a genetically driven autoimmune disease.

Comparative and demographic analysis of orang-utan genomes

“Orangutan” is derived from the Malay term “man of the forest” and aptly describes the Southeast Asian great apes native to Sumatra and Borneo. The orangutan species, Pongo abelii (Sumatran) and Pongo pygmaeus (Bornean), are the most phylogenetically distant great apes from humans, thereby providing an informative perspective on hominid evolution. Here we present a Sumatran orangutan draft genome assembly and short read sequence data from five Sumatran and five Bornean orangutan genomes. Our analyses reveal that, compared to other primates, the orangutan genome has many unique features. Structural evolution of the orangutan genome has proceeded much more slowly than other great apes, evidenced by fewer rearrangements, less segmental duplication, a lower rate of gene family turnover and surprisingly quiescent Alu repeats, which have played a major role in restructuring other primate genomes. We also describe the first primate polymorphic neocentromere, found in both Pongo species, emphasizing the gradual evolution of orangutan genome structure. Orangutans have extremely low energy usage for a eutherian mammal1, far lower than their hominid relatives. Adding their genome to the repertoire of sequenced primates illuminates new signals of positive selection in several pathways including glycolipid metabolism. From the population perspective, both Pongo species are deeply diverse; however, Sumatran individuals possess greater diversity than their Bornean counterparts, and more species-specific variation. Our estimate of Bornean/Sumatran speciation time, 400k years ago (ya), is more recent than most previous studies and underscores the complexity of the orangutan speciation process. Despite a smaller modern census population size, the Sumatran effective population size (N_e) expanded exponentially relative to the ancestral N_e after the split, while Bornean N_e declined over the same period. Overall, the resources and analyses presented here offer new opportunities in evolutionary genomics, insights into hominid biology, and an extensive database of variation for conservation efforts.

Identification of human-specific transcript variants induced by DNA insertions in the human genome

MOTIVATION

Many genes in the human genome produce a wide variety of transcript variants resulting from alternative exon splicing, differential promoter usage, or altered polyadenylation site utilization that may function differently in human cells. Here, we present a bioinformatics method for the systematic identification of human-specific novel transcript variants that might have arisen after the human-chimpanzee divergence.

RESULTS

The procedure involved collecting genomic insertions that are unique to the human genome when compared with orthologous chimpanzee and rhesus macaque genomic regions, and that are expressed in the transcriptome as exons evidenced by mRNAs and/or expressed sequence tags (ESTs). Using this procedure, we identified 112 transcript variants that are specific to humans; 74 were associated with known genes and the remaining transcripts were located in unannotated genomic loci. The original source of inserts was mostly transposable elements including L1, Alu, SVA, and human endogenous retroviruses (HERVs). Interestingly, some non-repetitive genomic segments were also involved in the generation of novel transcript variants. Insert contributions to the transcripts included promoters, terminal exons and insertions in exons, splice donors and acceptors and complete exon cassettes. Comparison of personal genomes revealed that at least seven loci were polymorphic in humans. The exaptation of human-specific genomic inserts as novel transcript variants may have increased human gene versatility or affected gene regulation.

Different integration site structures between L1 protein-mediated retrotransposition in cis and retrotransposition in trans

Background

Long interspersed nuclear element-1 (LINE-1 or L1) is a dominant repetitive sequence in the human genome. Besides mediating its own retrotransposition, L1 can mobilize Alu and messenger RNA (mRNA) in trans, and probably also SVA and non-coding RNA. The structures of L1 copies and trans-mobilized retrocopies are variable and can be classified into three categories: full-length; 5'-truncated; and 5'-inverted insertions. These structures may be generated by different 5' integration mechanisms.

Results

In this study, a method to correctly characterize insertions with short target site duplications (TSDs) is developed and extranucleotides, TSDs and microhomologies (MHs) at junctions were analysed for the three types of insertions. Only 5'-truncated L1 insertions were found to be associated with short TSDs. Both full-length and 5'-truncated retrotransposed sequences in trans, including Alu, SVA and mRNA retrocopies and also full-length and 5'-inverted L1, were not associated with short TSDs, indicating the difference of 5' attachment between retrotransposition in cis and retrotransposition in trans. Target sequence analysis suggested that short TSDs were generated in an L1 endonuclease-dependent manner. The MHs were longer for 5'-inverted L1 than for 5'-truncated L1, indicating less dependence on annealing in 5'-truncated L1 insertions.

Conclusions

The results suggest that insertions flanked by short TSDs occur more often coupled with the insertion of 5'-truncated L1 than with those of other types of insertions in vivo. The method used in this study can be used to characterize elements without any apparent boundary structures.

The structure, organization and radiation of Sadhu non-long terminal repeat retroelements in Arabidopsis species

Background

Sadhu elements are non-autonomous retroposons first recognized in Arabidopsis thaliana. There is a wide degree of divergence among different elements, suggesting that these sequences are ancient in origin. Here we report the results of several lines of investigation into the genomic organization and evolutionary history of this element family.

Results

We present a classification scheme for Sadhu elements in A. thaliana, describing derivative elements related to the full-length elements we reported previously. We characterized Sadhu5 elements in a set of A. thaliana strains in order to trace the history of radiation in this subfamily. Sequences surrounding the target sites of different Sadhu insertions are consistent with mobilization by LINE retroelements. Finally, we identified Sadhu elements grouping into distinct subfamilies in two related species, Arabidopsis arenosa and Arabidopsis lyrata.

Conclusions

Our analyses suggest that the Sadhu retroelement family has undergone target primed reverse transcription-driven retrotransposition during the divergence of different A. thaliana strains. In addition, Sadhu elements can be found at moderate copy number in three distinct Arabidopsis species, indicating that the evolutionary history of these sequences can be traced back at least several millions of years.

Laboratory methods for the analysis of primate mobile elements

Mobile elements represent a unique and powerful set of tools for understanding the variation in a genome. Methods exist not only to utilize the polymorphisms among and within taxa to various ends but also to investigate the mechanism through which mobilization occurs. The number of methods to accomplish these ends is ever growing. Here, we present several protocols designed to assay mobile element-based variation within and among individual genomes.

Chromosomal inversions between human and chimpanzee lineages caused by retrotransposons

The long interspersed element-1 (LINE-1 or L1) and Alu elements are the most abundant mobile elements comprising 21% and 11% of the human genome, respectively. Since the divergence of human and chimpanzee lineages, these elements have vigorously created chromosomal rearrangements causing genomic difference between humans and chimpanzees by either increasing or decreasing the size of genome. Here, we report an exotic mechanism, retrotransposon recombination-mediated inversion (RRMI), that usually does not alter the amount of genomic material present. Through the comparison of the human and chimpanzee draft genome sequences, we identified 252 inversions whose respective inversion junctions can clearly be characterized. Our results suggest that L1 and Alu elements cause chromosomal inversions by either forming a secondary structure or providing a fragile site for double-strand breaks. The detailed analysis of the inversion breakpoints showed that L1 and Alu elements are responsible for at least 44% of the 252 inversion loci between human and chimpanzee lineages, including 49 RRMI loci. Among them, three RRMI loci inverted exonic regions in known genes, which implicates this mechanism in generating the genomic and phenotypic differences between human and chimpanzee lineages. This study is the first comprehensive analysis of mobile element bases inversion breakpoints between human and chimpanzee lineages, and highlights their role in primate genome evolution.

L1 recombination-associated deletions generate human genomic variation

Mobile elements have created structural variation in the human genome through their de novo insertions and post-insertional genomic rearrangements. L1 elements are a type of long interspersed element (LINE) that is dispersed at high copy numbers within most mammalian genomes. To determine the magnitude of L1 recombination-associated deletions (L1RADs), we computationally extracted L1RAD candidates by comparing the human and chimpanzee genomes and verified each of the L1RAD events by using wet-bench analyses. Through these analyses, we identified 73 human-specific L1RAD events that occurred subsequent to the divergence of the human and chimpanzee lineages. Despite their low frequency, the L1RAD events deleted approximately 450 kb of the human genome. One L1RAD event generated a large deletion of approximately 64 kb. Multiple alignments of prerecombination and postrecombination L1 elements suggested that two different deletion mechanisms generated the L1RADs: nonallelic homologous recombination (55 events) and nonhomologous end joining between two L1s (18 events). In addition, the position of L1RADs throughout the genome does not correlate with local chromosomal recombination rates. This process may be implicated in the partial regulation of L1 copy numbers by the finding that approximately 60% of the DNA sequences deleted by the L1RADs consist of L1 sequences that were either directly involved in the recombination events or located in the intervening sequence between recombining L1s. Overall, there is increasing evidence that L1RADs have played an important role in creating structural variation.

CpG dinucleotides and the mutation rate of non-CpG DNA

The neutral mutation rate is equal to the base substitution rate when the latter is not affected by natural selection. Differences between these rates may reveal that factors such as natural selection, linkage, or a mutator locus are affecting a given sequence. We examined the neutral base substitution rate by measuring the sequence divergence of approximately 30,000 pairs of inactive orthologous L1 retrotransposon sequences interspersed throughout the human and chimpanzee genomes. In contrast to other studies, we related ortholog divergence to the time (age) that the L1 sequences resided in the genome prior to the chimpanzee and human speciation. As expected, the younger orthologs contained more hypermutable CpGs than the older ones because of their conversion to TpGs (and CpAs). Consequently, the younger orthologs accumulated more CpG mutations than the older ones during the approximately 5 million years since the human and chimpanzee lineages separated. But during this same time, the younger orthologs also accumulated more non-CpG mutations than the older ones. In fact, non-CpG and CpG mutations showed an almost perfect (R2 = 0.98) correlation for approximately 97% of the ortholog pairs. The correlation is independent of G + C content, recombination rate, and chromosomal location. Therefore, it likely reflects an intrinsic effect of CpGs, or mutations thereof, on non-CpG DNA rather than the joint manifestation of the chromosomal environment. The CpG effect is not uniform for all regions of non-CpG DNA. Therefore, the mutation rate of non-CpG DNA is contingent to varying extents on local CpG content. Aside from their implications for mutational mechanisms, these results indicate that a precise determination of a uniform genome-wide neutral mutation rate may not be attainable.

Alu recombination-mediated structural deletions in the chimpanzee genome

With more than 1.2 million copies, Alu elements are one of the most important sources of structural variation in primate genomes. Here, we compare the chimpanzee and human genomes to determine the extent of Alu recombination-mediated deletion (ARMD) in the chimpanzee genome since the divergence of the chimpanzee and human lineages (∼6 million y ago). Combining computational data analysis and experimental verification, we have identified 663 chimpanzee lineage-specific deletions (involving a total of ∼771 kb of genomic sequence) attributable to this process. The ARMD events essentially counteract the genomic expansion caused by chimpanzee-specific Alu inserts. The RefSeq databases indicate that 13 exons in six genes, annotated as either demonstrably or putatively functional in the human genome, and 299 intronic regions have been deleted through ARMDs in the chimpanzee lineage. Therefore, our data suggest that this process may contribute to the genomic and phenotypic diversity between chimpanzees and humans. In addition, we found four independent ARMD events at orthologous loci in the gorilla or orangutan genomes. This suggests that human orthologs of loci at which ARMD events have already occurred in other nonhuman primate genomes may be “at-risk” motifs for future deletions, which may subsequently contribute to human lineage-specific genetic rearrangements and disorders.

The recent sequencing of a number of primate genomes shows that small segments of DNA known as Alu elements are found repeatedly along all chromosomes, and indeed comprise ∼10% of the human genome. Although older Alu elements that have been in the genome for a long time accumulate some random mutations, overall these elements retain high levels of sequence identity among themselves. The presence of many near-identical Alu elements located close to each other makes primate genomes prone to DNA recombination events that generate genomic deletions of varying sizes. Here, by scanning the chimpanzee genome for such deletions, we determined the role of the Alu recombination-mediated deletion process in creating structural differences between the chimpanzee and human genomes. Using a combination of computational and experimental techniques, we identified 663 deletions, involving the removal of ∼771 kb of genomic sequence. Interestingly, about half of these deletions were located within known or predicted genes, and in several cases, the deletions removed coding exons from chimpanzee genes as compared to their human counterparts. Alu recombination-mediated deletion shows signs of being a major sculptor of primate genomes and may be responsible for generating some of the genetic differences between humans and chimpanzees.

Endonuclease-independent insertion provides an alternative pathway for L1 retrotransposition in the human genome

LINE-1 elements (L1s) are a family of highly successful retrotransposons comprising approximately 17% of the human genome, the majority of which have inserted through an endonuclease-dependent mechanism termed target-primed reverse transcription. Recent in vitro analyses suggest that in the absence of non-homologous end joining proteins, L1 elements may utilize an alternative, endonuclease-independent pathway for insertion. However, it remains unknown whether this pathway operates in vivo or in cell lines where all DNA repair mechanisms are functional. Here, we have analyzed the human genome to demonstrate that this alternative pathway for L1 insertion has been active in recent human evolution and characterized 21 loci where L1 elements have integrated without signs of endonuclease-related activity. The structural features of these loci suggest a role for this process in DNA double-strand break repair. We show that endonuclease-independent L1 insertions are structurally distinguishable from classical L1 insertion loci, and that they are associated with inter-chromosomal translocations and deletions of target genomic DNA.

Identification and characterization of novel polymorphic LINE-1 insertions through comparison of two human genome sequence assemblies

Mobile elements represent a relatively new class of markers for the study of human evolution. Long interspersed elements (LINEs) belong to a group of retrotransposons comprising approximately 21% of the human genome. Young LINE-1 (L1) elements that have integrated recently into the human genome can be polymorphic for insertion presence/absence in different human populations at particular chromosomal locations. To identify putative novel L1 insertion polymorphisms, we computationally compared two draft assemblies of the whole human genome (Public and Celera Human Genome assemblies). We identified a total of 148 potential polymorphic L1 insertion loci, among which 73 were candidates for novel polymorphic loci. Based on additional analyses we selected 34 loci for further experimental studies. PCR-based assays and DNA sequence analysis were performed for these 34 loci in 80 unrelated individuals from four diverse human populations: African-American, Asian, Caucasian, and South American. All but two of the selected loci were confirmed as polymorphic in our human population panel. Approximately 47% of the analyzed loci integrated into other repetitive elements, most commonly older L1s. One of the insertions was accompanied by a BC200 sequence. Collectively, these mobile elements represent a valuable source of genomic polymorphism for the study of human population genetics. Our results also suggest that the exhaustive identification of L1 insertion polymorphisms is far from complete, and new whole genome sequences are valuable sources for finding novel retrotransposon insertion polymorphisms.