Downloaded from genesdev.cshlp.org on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press PERSPECTIVE LINEing germ and embryonic stem cells’ silencing of retrotransposons Takashi Ishiuchi and Maria-Elena Torres-Padilla1 Institut de Ge´ne´tique et de Biologie Mole´culaire et Cellulaire, CNRS/INSERM U964, Universite´ de Strasbourg, F-67404 Illkirch, C.U. de Strasbourg, France Almost half of our genome is occupied by transposable silencing mechanism must be in place to control their elements. Although most of them are inactive, one type transcriptional activity; indeed, cells have evolved path- of non-long terminal repeat (LTR) retrotransposon, long ways to tightly regulate L1 expression. For example, in interspersed nuclear element 1 (LINE1), is capable of mouse embryonic stem cells (mESCs), loss of Suv39 en- retrotransposition. Two studies in this issue, Pezic and zymes or DNA methyltransferases (DNMTs) that catalyze colleagues (pp. 1410–1428) and Castro-Diaz and col- the H3K9me3 mark and DNA methylation, respectively, leagues (pp. 1397–1409), provide novel insight into leads to the derepression of L1, suggesting that histone the regulation of LINE1s in human embryonic stem cells modifications and DNA methylation have an important and mouse germ cells and shed new light on the con- role in L1 silencing in mESCs (Matsui et al. 2010). It has not servation of complex mechanisms to ensure silencing of yet been established, however, whether the various L1 transposable elements in mammals. subfamilies are controlled by a common mechanism or whether different silencing mechanisms have evolved over time. In this issue of Genes & Development, Castro-Diaz et al. (2014) and Pezic et al. (2014) provide new insights into DNA transposons and retrotransposons occupy roughly the regulation of L1 subfamilies in human ESCs (hESCs) half of our genome. Retrotransposons are classified in two and mouse germ cells. main classes: long terminal repeat (LTR) and non-LTR Castro-Diaz et al. (2014) examined the role of KAP1 retrotransposons. LTR retrotransposons include endoge- binding on L1s in hESCs. KAP1, also known as TRIM28 nous retroviruses and compose ;8% of the human genome. (tripartite motif protein 28) , is a cofactor of the DNA- Non-LTR retrotransposons, which contribute to >30% of binding KRAB [Kru¨ ppel-associated box domain]-containing the human genome, include long interspersed nuclear zinc finger proteins (KRAB-ZFPs), which are thought to elements (LINEs) and short interspersed nuclear elements recruit KAP1 (KRAB-associated protein 1) to specific geno- (SINEs) (Cordaux and Batzer 2009). Although this view is mic sites. In addition, KAP1 can bind to HP1a (heterochro- changing as new regulatory roles have been ascribed to matin protein 1a) and the SETDB1/ESET histone methyl- them, transposable elements (TEs) have long been con- transferase to catalyze the H3K9me3 mark found at sidered to be junk DNA. Indeed, over the course of constitutive heterochromatin, suggesting that KAP1 has evolution, most TEs have accumulated mutations that an important role in creating a heterochromatin environ- interfered with their ability to retrotranspose and thus ment at specific genomic loci (Ryan et al. 1999; Schultz rendered them inactive. Nevertheless, some classes of et al. 2002). In support of this, KAP1 depletion in mESCs TEs, like the non-LTR retrotransposon LINE-1 (L1), are induces a strong derepression of LTR retrotransposons still active and capable of retrotransposition in mammals. such as IAP while only modestly affecting L1 expression L1s are divided into subfamilies according to their evo- (Rowe et al. 2010). Castro-Diaz et al. (2014) performed lutionary history; L1s seem to have evolved as a single chromatin immunoprecipitation (ChIP) combined with lineage, but only young L1 subfamilies have the ability to deep sequencing (ChIP-seq) for KAP1 in hESCs and found retrotranspose. The youngest and human-specific L1 sub- that KAP1 is enriched at the 59 UTR of L1s, where family, L1Hs, is capable of retrotransposition, although it H3K9me3 is also enriched. Surprisingly, analysis of seems that other L1 subfamilies can also be transcribed KAP1 enrichment for each L1 subfamily revealed that because they have maintained promoter activity at their KAP1 binding was enriched on relatively young, but not 59 untranslated regions (UTRs) (Brouha et al. 2003). The the youngest, L1 subfamilies. This pattern was also found mutagenic potential of L1s implies that an effective Ó 2014 Ishiuchi and Torres-Padilla This article is distributed exclusively [Keywords: DNA methylation; evolution; KAP1; LINE1; piRNA; by Cold Spring Harbor Laboratory Press for the first six months after the transposon] full-issue publication date (see http://genesdev.cshlp.org/site/misc/ 1Corresponding author terms.xhtml). After six months, it is available under a Creative Com- E-mail [email protected] mons License (Attribution-NonCommercial 4.0 International), as de- Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.246462.114. scribed at http://creativecommons.org/licenses/by-nc/4.0/. GENES & DEVELOPMENT 28:1381–1383 Published by Cold Spring Harbor Laboratory Press; ISSN 0890-9369/14; www.genesdev.org 1381 Downloaded from genesdev.cshlp.org on October 1, 2021 - Published by Cold Spring Harbor Laboratory Press Ishiuchi and Torres-Padilla in mESCs, suggesting that KAP1 ‘‘discrimination’’ of nascent transcripts to alter the chromatin state through subfamilies is conserved. How does KAP1 distinguish deposition of H3K9me3. between discrete L1 subfamilies? The investigators pro- It was recently demonstrated that PIWI is responsible vide evidence that a precise KRAB-ZFP recognizes spe- for the repression of L1Hs retrotransposition in human cific L1 subfamilies through a putative DNA-binding motif. induced pluripotent stem cells (iPSCs) but that this L1MdF2 and L1MdF3, two relatively young L1 subfamilies regulatory pathway is not functional in iPSCs from non- in mice, were found to be associated with GM6871, a KRAB- human primates (Marchetto et al. 2013). Together with ZFP. GM6871 depletion caused the specific derepression of the results from Castro-Diaz et al. (2014), it seems that L1s to which it binds. The investigators also depleted the youngest L1Hs are suppressed by piRNA-dependent KAP1 in hESCs and found that KAP1-enriched L1s were mechanisms, while KAP1 suppresses older L1s in human also specifically derepressed, while the expression of pluripotent cell types. On the other hand, Pezic et al. the youngest L1Hs (on which KAP1 is not enriched) was (2014) showed that the piRNA pathway is important for not affected. The investigators then examined the role H3K9me3 acquisition on youngest L1s, which might be of DNA methylation on the regulation of L1 expression responsible for their silencing in mouse germ cells. It may by knocking down the DNMTs DNMT1, DNMT3A, be possible that piRNA-dependent H3K9me3 acquisition and DNMT3B in hESCs. Interestingly, the youngest L1s on young L1s in mouse germ cells is independent of were significantly derepressed by the knockdown of KAP1, considering that Castro-Diaz et al. (2014) showed DNMTs, suggesting that cells have adopted different that KAP1 is not enriched at those young L1s in mESCs. mechanisms to silence distinct L1 subfamilies during Indeed, it is already known that Suv39s are involved in evolution. the suppression of L1 expression (Matsui et al. 2010). The maintenance of genomic integrity from one gen- Taken together, this suggests that a combination of eration to the next requires the strict inhibition of retro- different mechanisms acts to silence transcriptionally transposition in the germline. It has been clearly demon- active, full-length intact copies of L1s as opposed to strated that PIWI-interacting RNAs (piRNAs) have a older or truncated L1s that are less prone to activation. crucial role in silencing retrotransposons in germ cells. Future efforts will be directed at the identification of Mouse PIWI family proteins, such as MILI and MIWI2, are enzymes responsible for H3K9me3 acquisition in germ responsible for the activity of piRNAs via RNA-directed cells in conjunction with the piRNA pathway as well as mechanisms that are based on some degree of sequence the determination of whether the H3K9me3 mark is similarity. Many piRNAs derive from retrotransposon indeed responsible for L1 silencing in germ cells. In addi- elements, suggesting that piRNAs might be important tion, it might be intriguing to see whether the H3K9me3 for their targeted silencing. piRNA synthesis is required mark is deposited onto L1Hs via a KAP1-independent and for DNA methylation on retrotransposon elements and piRNA-dependent mechanism in hESCs, as has been ob- for their transcriptional repression (Aravin et al. 2007; served in mouse germ cells. Kuramochi-Miyagawa et al. 2008). However, the involve- ment of histone modifications in this pathway in the mammalian germline has been unclear. Pezic et al. (2014) Acknowledgments used ChIP to examine the enrichment of H3K9me3 on Work in the Torres-Padilla laboratory is funded through retrotransposon elements in germ cells. They found that EpiGeneSys Network of Excellence (NoE), a European Research H3K9me3 was enriched on the 59 end of L1s and tended to Council Starting Grant, ‘‘NuclearPotency;’’ the EMBO Young spread to upstream flanking regions. RNA sequencing Investigator Programme; and the Fondation Schlumberger pour (RNA-seq) profiling of piRNAs showed that piRNAs were l’Education et la Recherche.