Transposable elements are often referred to as ‘jumping genes’ because they can move between different locations within a genome. These sequences of DNA are found in many organisms and can make up a significant proportion of the genetic material: almost 50% of the DNA in the case of the human genome.

Transposable elements are grouped by how they move to new locations in a genome. Some move by a cut-and-paste mechanism—whereby the transposable element DNA is removed from one location and inserted back at a new genomic location. Others, termed retrotransposons, move by a copy-and-paste mechanism: the DNA sequence is transcribed into an RNA intermediate, and then copied back into DNA before being inserted into a new location. Retrotransposons can accumulate to great numbers in genomes: and one retrotransposon, called LINE-1, is present at an estimated 500,000 copies in the human genome.

Although most copies of LINE-1 are inactive, the average human genome contains about 80–100 that are predicted to be able to ‘jump’ to new locations. Given that these retrotransposons could insert into, and disrupt, vital genes, it follows that our cells would have evolved ways to limit their movement. An enzyme named APOBEC3A is known to limit the movement of LINE-1 retrotransposons in cells. APOBEC3A can alter the letters, or bases, that make up the genetic code. This enzyme acts on single-strand DNA to change ‘C’ bases to ‘U’ bases, which could explain how APOBEC3A combats LINE-1. However, no evidence for such mutation of LINE-1 sequences by APOBEC3A had been found to date.

Now, Richardson et al. recreate the copying of LINE-1 RNA back into DNA in a test tube—and reveal that APOBEC3A can mutate single-strand LINE-1 DNA. Critically, as long as the RNA intermediate and DNA copy remain together, the LINE-1 DNA is protected. However, when LINE-1 inserts into a new location the temporarily exposed single strand of LINE-1 DNA becomes susceptible to mutation by APOBEC3A. Human cells can detect and destroy ‘U’ bases in DNA—and only by inhibiting this ability were Richardson et al. able to observe APOBEC3A mutations in new LINE-1 copies within the genomes of living cells.

Richardson et al. speculate that the activity of APOBEC3 enzymes must strike a balance between limiting the spread of retrotransposons and minimizing the mutation of the cell's own DNA. Future work could address important questions, such as: do APOBEC enzymes affect the ‘jumping’ of LINE-1 retrotransposons in human reproductive cells and the early embryo, where new LINE-1 insertions could be passed on to subsequent generations? Also, does a loss of APOBEC3 activity lead to new LINE-1 insertions in cancerous cells? And does this effect how tumors form and/or progress? Since APOBEC3 enzymes can cause mutations in cancers, they have been proposed as new targets for anti-cancer drugs—therefore, it is crucial to uncover any harmful effects of inhibiting APOBEC3 enzymes that might limit the effectiveness of such treatments.

Long INterspersed Element-1 (LINE-1 or L1) derived sequences comprise roughly 17% of human genomic DNA (Lander et al., 2001). The vast majority of L1 sequences are 5′ truncated, internally rearranged, or are inactivated by mutations and are unable to mobilize (i.e., retrotranspose) to new genomic locations (Grimaldi et al., 1984; Lander et al., 2001). However, it is estimated that the average human genome contains 80–100 retrotransposition-competent (i.e., active) L1s (Sassaman et al., 1997; Brouha et al., 2003). Active human L1s are approximately 6 kb in length and contain a 5′ untranslated region (UTR), two open reading frames (ORF1 and ORF2), and a 3′ UTR that terminates in a poly (A) tail (Scott et al., 1987; Dombroski et al., 1991). ORF1 encodes an ∼40 kDa protein (ORF1p) that binds L1 RNA as a trimer (Holmes et al., 1992; Hohjoh and Singer, 1996; Martin et al., 2003; Khazina and Weichenrieder, 2009). ORF1p also has nucleic acid chaperone activity (Martin and Bushman, 2001; Khazina and Weichenrieder, 2009). ORF2 encodes an ∼150 kDa protein (ORF2p) (Ergun et al., 2004; Doucet et al., 2010; Taylor et al., 2013b) with DNA endonuclease (Feng et al., 1996) and reverse transcriptase (Mathias et al., 1991) activities. The development of a cultured cell assay definitively demonstrated that both ORF1p and ORF2p are required for efficient L1 retrotransposition (Moran et al., 1996). Subsequent biochemical studies revealed that L1 retrotransposition occurs by a mechanism termed target-site primed reverse transcription (TPRT) (Luan et al., 1993; Feng et al., 1996; Cost et al., 2002; Kulpa and Moran, 2006).

Ongoing L1 retrotransposition events contribute to intra- and inter-individual genetic variation and, on occasion, can disrupt gene function, leading to sporadic genetic diseases (Beck et al., 2011; Hancks and Kazazian, 2012). Given the mutagenic potential of active L1s, it is not surprising that host cells have evolved multiple mechanisms to restrict L1 retrotransposition (Levin and Moran, 2011). Previous studies revealed that over-expression of the wild-type (WT) APOBEC3A cytidine deaminase protein (A3A), but not deaminase-defective A3A mutants, could potently inhibit L1 retrotransposition in cultured human cells (Chen et al., 2006; Muckenfuss et al., 2006; Bogerd et al., 2006b; Kinomoto et al., 2007; Niewiadomska et al., 2007). However, deamination events were not detected in L1 retrotransposition events generated in the presence of WT A3A (Chen et al., 2006; Muckenfuss et al., 2006; Bogerd et al., 2006b; Kinomoto et al., 2007). Thus, the molecular mechanism by which WT A3A inhibits L1 retrotransposition has remained a mystery.

Here, we employ an in vitro assay (L1 Element Amplification Protocol or LEAP) that recapitulates the first strand cDNA synthesis step of the L1 integration to elucidate a mechanism for A3A-mediated inhibition of L1 retrotransposition. In LEAP reactions that include RNase H, an enzyme that specifically degrades the RNA strand of an RNA/DNA heteroduplex, we report that single-strand L1 cDNAs are rendered susceptible to A3A-mediated deamination. These data suggest that newly synthesized L1 cDNAs remain annealed to their mRNA templates after reverse transcription, protecting the L1 cDNA from deamination by A3A. We further demonstrate that engineered L1 retrotransposition events derived from cultured human cells that co-express A3A and the uracil DNA glycosylase inhibitor protein (UGI) contain strand-specific, A3A-mediated C-to-U deamination events. Together, these data indicate that A3A can inhibit L1 retrotransposition by deaminating transiently exposed single-strand DNA that arises during the process of L1 integration. Moreover, our results provide direct evidence that A3A can deaminate single-strand genomic DNA in human cells.

We used a cultured cell assay (Moran et al., 1996; Wei et al., 2000) to determine the extent to which A3A inhibits retrotransposition of LINE elements that differ significantly in their nucleotide sequence and structure (Figure 1A). The retrotransposition indicator cassette mneoI (Freeman et al., 1994; Moran et al., 1996) was used to tag the 3′ untranslated regions (UTRs) of a human L1 (pJM101/L1.3) (Sassaman et al., 1997), a natural mouse L1 element (TGF21) (Goodier et al., 2001), a synthetic mouse L1 element (L1SM) (Han and Boeke, 2004), and a LINE-2 element from zebrafish (ZfL2-2) (Sugano et al., 2006; Figure 1A). The mneoI cassette consists of an antisense neomycin phosphotransferase gene (NEO) disrupted by an intron, which is in the same transcriptional orientation as the LINE element (Figure 1A). This arrangement ensures that G418-resistant foci arise only if the tagged LINE element undergoes successful retrotransposition (Freeman et al., 1994; Moran et al., 1996). HeLa cells do not express endogenous A3A (Wissing et al., 2011), and therefore provide a system to measure directly the effects of ectopically expressed A3A on L1 retrotransposition.

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We co-transfected HeLa cells with retroelement expression constructs and plasmids encoding WT A3A or a deaminase-deficient mutant, A3A-C106S (Chen et al., 2006). As a control we included β-arrestin, which does not significantly affect L1 retrotransposition (Bogerd et al., 2006b). A3A expression inhibited human L1.3 retrotransposition to ∼28% of control levels (Figure 1B), in agreement with previous reports (Chen et al., 2006; Muckenfuss et al., 2006; Bogerd et al., 2006b). In comparison to control levels, A3A also inhibited retrotransposition of TGF21 (to ∼20%), ZfL2-2 (to ∼17%), and L1SM (to ∼61%) (Figure 1B). The milder inhibition of A3A on L1SM retrotransposition may be due to the GC-rich nature of the L1SM transcript, the elevated steady state levels of L1SM mRNA, and/or an increased number of L1SM retrotransposition events per cell (Han and Boeke, 2004). Additional controls revealed that the reduction of G418-resistant HeLa cell foci observed in the presence of A3A reflects specific inhibition of L1 retrotransposition, rather than non-specific cytotoxicity (Figure 1—figure supplement 1A–C; see Methods and Materials). A deaminase-deficient mutant, A3A-C106S, did not significantly restrict retroelement mobility (Figure 1B), in agreement with previous studies (Chen et al., 2006; Muckenfuss et al., 2006; Bogerd et al., 2006b). Moreover, A3A inhibited L1.3 retrotransposition when it was expressed from its native 5′UTR on a non-episomal vector (Figure 1—figure supplement 1D: pKS101/L1.3/sv+). Together, these experiments show that A3A inhibits retrotransposition of diverse LINE elements in cultured cells.

Since LINE element retrotransposition proceeds by target-site primed reverse transcription (TPRT), which requires element-encoded endonuclease (EN) and reverse transcriptase (RT) activities (Luan et al., 1993; Feng et al., 1996), we investigated whether A3A specifically interferes with L1 EN or L1 RT activity. In addition to canonical TPRT, human L1s can mobilize by an alternative, endonuclease-independent retrotransposition mechanism (ENi) in cells that lack p53 activity and are defective for components of the non-homologous end joining (NHEJ) DNA repair machinery (Morrish et al., 2002, 2007; Coufal et al., 2011). We reasoned that if A3A specifically inhibits L1 EN activity, ENi L1 retrotransposition events should escape A3A inhibition. To test this hypothesis, we co-transfected a human L1 (either WT pAD2TE1 or EN-deficient pAD136 [Doucet et al., 2010]) and a WT A3A expression construct into Chinese Hamster Ovary (CHO) cell lines that are NHEJ-proficient (4364a) or NHEJ-deficient (XR-1) (Morrish et al., 2002, 2007). A3A expression inhibited WT L1 retrotransposition in both cell lines, and also inhibited ENi retrotransposition in XR-1 cells (Figure 1C). These data demonstrate A3A can inhibit L1 retrotransposition in a hamster cell line and that the A3A-mediated reduction of L1 retrotransposition is not due to inhibition of L1 EN activity.

To test whether A3A inhibits L1 RT activity, we performed in vitro LEAP assays (Kulpa and Moran, 2006) with ribonucleoprotein particle (RNP) preparations derived from cells transfected with an engineered L1 construct (Figure 2A). We purified recombinant WT A3A (rA3A) (Figure 2—figure supplement 1A,B) and deaminase-deficient (rA3A_C106S) (data not shown) proteins from Escherichia coli. As expected rA3A had cytidine deaminase activity on single-strand (Figure 2B), but not on double-stranded DNA (Figure 2—figure supplement 1C), whereas rA3A_C106S lacked appreciable deaminase activity (Figure 2B). Incorporating increasing amounts of rA3A or rA3A_C106S into LEAP or Moloney murine leukemia virus (MMLV) RT reactions did not significantly affect the synthesis of L1 or MMLV-derived cDNA products (Figure 2C, Figure 2—figure supplement 1D). Thus, A3A does not specifically block L1 RT activity.

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To determine whether A3A can deaminate L1 cDNAs in vitro, we sequenced 100 L1 cDNA products from LEAP reactions conducted in the absence of A3A (no rA3A) or in the presence of rA3A or deaminase-deficient rA3A_C106S. We did not detect significant numbers of C-to-U deamination events in L1 cDNAs derived from LEAP reactions in the absence of rA3A or the presence of rA3A_C106S (Figure 2D). In contrast, LEAP products generated in the presence of rA3A contained abundant C-to-U editing that was most frequent at the single 5′-TCA-3′ consensus A3A deamination site (Chen et al., 2006) present in the single-strand oligonucleotide adapter used to prime L1 RT reactions (Figure 2D, Figure 2—figure supplement 2, top chart in the middle panel, 91 of 100 (91%) total 5′-TCA-3′ sites within the 100 single-strand oligonucleotide adapters showed evidence of deamination). However, far less deamination was detected within L1 cDNA LEAP products at the six internal 5′-TCA-3′ sites (Figure 2D, Figure 2—figure supplement 2, top chart in the middle panel, only 25 of 600 (4.2%) 5′-TCA-3′ sites within the 100 L1 cDNA products showed evidence of deamination).

The non-uniform pattern of deamination events observed above suggested that the L1 cDNA may remain annealed to L1 mRNA, generating L1 mRNA/cDNA heteroduplexes after reverse transcription, thereby leaving only the single-strand oligonucleotide LEAP adapter susceptible to deamination. Thus, we tested whether inclusion of RNase H into LEAP reactions renders L1 cDNA susceptible to deamination. In the presence of rA3A and RNase H, 228 of 600 (38%) 5′-TCA-3′ substrates within the L1 cDNAs showed evidence of deamination (Figure 2D, Figure 2—figure supplement 2, bottom chart in the middle panel). Importantly, sequencing revealed differing poly (A) tail lengths and unique deamination patterns in each of the 100 LEAP products, indicating that they were derived from independent L1 cDNAs (Figure 2—figure supplement 3, panel e; Figure 2—figure supplement 4). In contrast, in the presence of rA3A_C106S and RNase H, only 6.7% of 5′-TCA-3′ substrates within 100 L1 cDNAs demonstrated evidence of deamination (Figure 2D, Figure 2—figure supplement 2, bottom chart in the right panel). Similarly, in reactions conducted with RNase H alone, 4.2% of 5′-TCA-3′ substrates within 100 L1 cDNAs contained signatures of deamination (Figure 2D, Figure 2—figure supplement 2, bottom chart in the left panel). The slightly increased number of deamination events observed with RNase H alone may reflect endogenous cytidine deaminase activity present in HeLa RNP preparations. APOBEC3B (A3B) might be responsible for these events as it is expressed in HeLa cells and can suppress L1 retrotransposition (Chen et al., 2006; Bogerd et al., 2006b; Wissing et al., 2011). These results suggest that the L1 cDNA, when annealed to the L1 mRNA in an mRNA/cDNA heteroduplex, is protected from mutation by cellular deaminases.

Our results demonstrated that rA3A could deaminate single-strand L1 cDNAs in vitro. We therefore hypothesized that A3A may deaminate transiently exposed single-strand DNA intermediates that arise during L1 TPRT in cultured cells. Uridine residues are efficiently removed from DNA through combined actions of uracil DNA glycosylase (UNG) and apurinic/apyrimidinic endonuclease (APE) (Krokan et al., 2002). In principle, these activities could repair and/or degrade deaminated single-strand DNAs that arise during TPRT. Degradation of deaminated retrotransposition intermediates would reduce L1 retrotransposition levels in cultured cell assays and could, in principle, eliminate evidence of A3A-mediated deamination events in the resultant retrotransposed L1 sequences. Such a scenario could explain why previous studies failed to detect A3A-mediated deamination events in engineered L1 retrotransposition events (Chen et al., 2006; Muckenfuss et al., 2006; Bogerd et al., 2006b; Kinomoto et al., 2007).

To test whether UNG contributed to the inhibition of L1 retrotransposition downstream of A3A activity, we conducted L1 retrotransposition assays in the presence of codon optimized bacteriophage-derived uracil DNA glycosylase inhibitor (UGI) that selectively inhibits UNG activity (Kaiser and Emerman, 2006; Figure 3A). Remarkably, UGI expression from a transfected plasmid (pLGCX_UGI) alleviated A3A-mediated L1 inhibition by ∼twofold (Figure 3B: from ∼25% to ∼57% of control levels). In contrast, UGI expression had little effect on L1 retrotransposition activity in assays conducted in the presence of A3A deaminase mutant (A3A-C106S) or β-arrestin control (Figure 3B). Similarly, UGI expression did not affect the ability of deaminase-deficient A3B mutants (Bogerd et al., 2006b) to inhibit L1 retrotransposition (Figure 3—figure supplement 1A: see A3B_N-term and A3B_CS data). Thus, UGI expression specifically alleviates a deaminase-dependent mechanism of L1 retrotransposition inhibition.

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We hypothesized that UGI expression should allow the detection of A3A-mediated C-to-U deamination events in retrotransposed L1s. Thus, we co-transfected HeLa cells with A3A, UGI, and pJM140/L1.3/Δ2/k7, an engineered L1 vector that contains both an mneoI retrotransposition cassette and a ColE1 bacterial origin of replication (Gilbert et al., 2002, 2005) (Figure 3A). This modified cassette facilitates recovery of engineered L1 retrotransposition events and their associated flanking genomic DNA sequences as autonomously replicating, kanamycin-resistant plasmids in bacteria (Gilbert et al., 2002, 2005). Characterization of 12 independent retrotransposition events revealed four G-to-A mutations on the L1 coding strand, which correspond to C-to-U deamination events on the (−) strand L1 cDNA (Figure 3C). Notably, the 5′ target-site duplication (TSD) of one insertion contained a C-to-T change within a 5′-TCA-3′ consensus A3A deamination site in flanking genomic DNA. This putative A3A-mediated deamination event led to an inexact TSD flanking a newly retrotransposed L1 (Figure 3D).

To gain further evidence for A3A-mediated deamination of L1 retrotransposition events, we employed a stable UGI-expressing U2OS cell line (Landry et al., 2011) (Figure 3A: right side). This strategy ensured that UNG activity is inhibited before co-transfection of L1 and A3A expression plasmids (Landry et al., 2011). In control U2OS cells, transfection of a low amount of A3A plasmid (25 ng) inhibited retrotransposition to ∼28.0% of control levels without substantial cytotoxicity (Figure 3E, Figure 3—figure supplement 1B). By comparison, in U2OS_UGI-expressing cells, A3A only slightly inhibited L1 retrotransposition (Figure 3E: to ∼83% of control levels; Figure 3—figure supplement 1C). Thus, consistent with experiments performed in HeLa cells, stable UGI expression relieves A3A-mediated inhibition of L1 retrotransposition in U2OS cells.

We next analyzed sequences of engineered L1 retrotransposition events. We used pJM140/L1.3/Δ2/k7 to generate a panel of clonal G418-resistant U2OS_UGI cell lines, isolated the resultant L1 retrotransposition events, and examined them for evidence of A3A-mediated deamination. Characterization of 24 control L1 retrotransposition events did not reveal any G-to-A mutations (Figure 3F, top panel, Figure 3—figure supplement 2). Similarly, characterization of 30 L1 retrotransposition events generated in the presence of A3A-C106S revealed only five G-to-A mutations (Figure 3F, middle panel, Figure 3—figure supplement 3). In contrast, characterization of 33 L1 retrotransposition events generated in the presence of A3A revealed 39 G-to-A mutations (Figure 3F, bottom panel, Figure 3—figure supplement 4). The A3A-induced mutations occurred in a strand-specific manner (i.e., we only detected one C-to-T mutation on the L1 coding strand), and far outnumbered any other nucleotide changes in retrotransposed products. Notably, one mutation (pJM140/L1.3/Δ2/k7: G-5390-A) introduced a premature termination codon into L1 ORF2, which would create a 145 amino acid truncation in ORF2p. This mutation reduced L1 retrotransposition efficiency to ∼10% of control levels (Figure 3—figure supplement 5). Thus, coupled with identification of a deamination-induced mutation in genomic DNA flanking a newly retrotransposed L1 (Figure 3D), these data suggest that the detected A3A-mediated deamination events occur on single-strand DNA during TPRT rather than on L1 mRNA or single-strand episomal plasmid DNA prior to L1 retrotransposition.

We propose a straightforward model for how A3A inhibits L1 retrotransposition (Figure 4). In the presence of cellular RNase H, the (−) strand L1 cDNA becomes vulnerable to A3A deamination (Figure 4A). Normally, these events would be repaired by combined actions of UNG and APE, which we speculate could lead to the integration of a 5′ truncated L1 cDNA that lacks evidence of deamination. However, in the presence of ectopically expressed UGI, A3A-mediated deamination events become unveiled in the retrotransposed L1s (Figure 3D,F, 4A). We further propose that A3A can act on transiently exposed single-strand genomic DNA flanking L1 integration sites (Figure 4B,C). The deamination of 5′-flanking top-strand genomic DNA could account for generation of inexact target-site duplications (Figure 3D). Interestingly, the structure of the proposed L1 integration intermediate resembles R-loop intermediates, formed during immunoglobulin class switching, that are targeted by an A3A related enzyme, activation-induced cytidine deaminase (AID) (Goodman et al., 2007). The deamination of 3′ flanking bottom-strand genomic DNA, in principle, could also lead to the generation of inexact TSDs (Figure 4C). While we did not observe these events in cultured cells, we did observe deamination events in the single-strand DNA oligonucleotide primer used to simulate 3′ flanking genomic DNA in LEAP assays (Figure 2D). It is intriguing to speculate that, in the absence of UGI, A3A-mediated editing within 3′ flanking bottom-strand genomic DNA could ultimately lead to loss of the L1 cDNA. Indeed, the ERCC1/XPF endonuclease has been implicated in limiting L1 retrotransposition by cleaving the transiently exposed single-strand 3’ flanking bottom-strand genomic DNA during TPRT (Gasior et al., 2008). The DNA lesion resulting from cleavage of this region could then be repaired using the top-strand genomic DNA as a template, leaving no detectable evidence of thwarted L1 retrotransposition events.

Figure 4

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The over-expression of A3A does not abolish L1 retrotransposition completely, but reduces it to ∼30% of control levels. Moreover, some L1 retrotransposition events generated in the presence of A3A and UGI lacked evidence of deamination. Why and how some L1 retrotransposition events evade A3A-mediated deamination requires further study. However, it is possible that ectopic A3A expression is too low to efficiently target L1 TPRT intermediates in some cells or that L1 second-strand cDNA synthesis occasionally is completed before A3A can gain access to TPRT intermediates to deaminate single-strand L1 cDNAs.

In sum, we conclude that deamination of transiently exposed single-strand DNA arising during L1 TPRT is responsible, at least in part, for A3A-mediated inhibition of L1 retrotransposition. It is noteworthy that the mobility of an evolutionarily related bacterial group II intron requires RNase H1 activity (Smith et al., 2005). Similarly, the related Bombyx mori R2 retrotransposon RT can displace RNA from RNA/DNA hybrids during second-strand cDNA synthesis in vitro (Kurzynska-Kokorniak et al., 2007). We hypothesize that annealed L1 mRNA protects the first-strand L1 cDNA, and that cellular RNase H renders these L1 cDNAs vulnerable to A3A deamination during TPRT.

Mechanistic insights gained from studying deaminase-dependent A3A-mediated L1 inhibition could be applicable to retroelement inhibition by other A3 proteins. A3A expression is restricted to peripheral blood lymphocytes (Chen et al., 2006; Refsland et al., 2010; Stenglein et al., 2010; Thielen et al., 2010; Berger et al., 2011; Koning et al., 2011), suggesting that it does not play a physiological role in restricting heritable L1 retrotransposition events. By comparison, A3B, which like A3A localizes to the nucleus, is expressed in human embryonic stem cells (hESCs) (Bogerd et al., 2006b) and can restrict engineered L1 retrotransposition in hESCs (Wissing et al., 2011). Since heritable L1-mediated retrotransposition events can occur in the early human embryo (de Boer et al., 2014; van den Hurk et al., 2007), A3B may restrict heritable L1 retrotransposition events. Consistent with this notion, recent reports suggest that A3B is expressed at higher levels in human induced pluripotent stem cells (iPSCs) when compared to non-human primate iPSCs, and that A3B expression may, in part, account for the lower number of recent L1 insertions in the human genome compared to the chimpanzee genome (Marchetto et al., 2013). Intriguingly, an A3B deletion allele also is present at a high allele frequency in some human geographic populations (Kidd et al., 2007); thus, it would be interesting to determine whether the loss of A3B correlates with increased frequency of de novo L1 insertions.

APOBEC3 activity has been implicated in genome hyper-mutation in numerous cancers (Roberts et al., 2013; Burns et al., 2013a, 2013b). It is interesting to speculate that APOBEC3 expression in cancers may restrict potentially deleterious L1 retrotransposition events, yet may exact a mutagenic cost by contributing to genome hyper-mutation (Narvaiza et al., 2012). Indeed, our results suggest that in addition to TPRT, A3A may deaminate any transiently exposed single-strand region of genomic DNA (e.g., those arising at replication forks or during transcription). This hypothesis is consistent with recent reports indicating that resected single-strand DNA, around DNA breaks, is susceptible to hyper-mutation by APOBEC deamination (Nik-Zainal et al., 2012; Roberts et al., 2012; Taylor et al., 2013a).

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Author details

1. Sandra R Richardson

   Department of Human Genetics, University of Michigan Medical School, Ann Arbor, United States

   Contribution

   SRR, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   risandra@umich.edu

   Competing interests

   No competing interests declared.

2. Iñigo Narvaiza

   Laboratory of Genetics, The Salk Institute for Biological Studies, La Jolla, United States

   Contribution

   IN, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   No competing interests declared.

3. Randy A Planegger

   Department of Human Genetics, University of Michigan Medical School, Ann Arbor, United States

   Contribution

   RAP, Acquisition of data, Analysis and interpretation of data

   Competing interests

   No competing interests declared.

4. Matthew D Weitzman

   Department of Pathology and Laboratory Medicine, University of Pennsylvania Perelman School of Medicine and the Children's Hospital of Philadelphia, Philadelphia, United States

   Contribution

   MDW, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   No competing interests declared.

5. John V Moran

   1. Department of Human Genetics, University of Michigan Medical School, Ann Arbor, United States
   2. Department of Internal Medicine, Howard Hughes Medical Institute, University of Michigan Medical School, Ann Arbor, United States

   Contribution

   JVM, Conception and design, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   moranj@umich.edu

   Competing interests

   JVM: is an inventor on patent 6,150,160. Kazazian, H.H., Boeke, J.D., Moran, J.V., and Dombroski, B.A.: Compositions and methods of use of mammalian retrotransposons.

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