Retrotransposon Silencing by DNA Methylation Can Drive Mammalian Genomic Imprinting

1Genomic imprinting, or parent-of-origin-specific gene silencing, has been observed in both eutherian and marsupial, but not monotreme mammals. In eutherians, more than 80 imprinted genes have been found and differential DNA methylation plays a crucial role in regulating their imprinted expression patterns. In marsupials, three genes—IGF2, IGF2R, and PEG1/MEST—are imprinted, as they are in eutherians, but no differentially methylated regions (DMRs) have been found in marsupials. Consequently, the regulatory mechanisms of genomic imprinting were thought to have evolved differently between marsupials and eutherians.

2It has been hypothesized that genomic imprinting arose as a by-product of a DNA methylation mechanism that silences foreign DNAs, such as retrotransposons. Similarly, transgenes can also become methylated, depending on parent of origin, further supporting a link between genomic imprinting and silencing of foreign DNAs. PEG10 is an imprinted gene sharing homology with the sushi-ichi retrotransposon, and in humans and mice it has a clear DMR in its promoter region. Interestingly, PEG10 is conserved in eutherian mammals but not in nonmammalian vertebrates, such as birds and fish. Therefore, investigating the origin of the retrotransposon-derived PEG10 locus would clarify the relationship between retrotransposon (or exogenous DNA sequence) insertion and genomic imprinting.

3PEG10 is an essential placental gene in eutherians, since knock-out mice have severe placental defects with loss of spongiotrophoblast and labyrinth layers leading to early embryonic lethality. The origin of PEG10 is therefore of interest in view of its possible contribution to the evolution of mammalian placentation. Sequence identified as PEG10 has recently been reported in the South American marsupial the grey short-tailed opossum (Monodelphis domestica) but its precise location, genetic structure, and imprint status remains unknown. Here, we examined the PEG10 locus in two Australian mammals by isolating bacterial artificial chromosome (BAC) clones from a marsupial, the tammar wallaby, and from a monotreme, the platypus.

4We used the tammar wallaby and the platypus as the representative species of marsupials and monotremes, respectively, to compare with representative eutherians, the human and the mouse. Several BAC clones were isolated from tammar and platypus containing SGCE (sarcoglycan epsilon), the neighboring gene of PEG10 in eutherians. DNA sequencing of one tammar BAC clone demonstrated the existence of PEG10 and its conserved location adjacent to SGCE with transcription occurring in a head-to-head manner (Figure 1A). The genetic structure of tammar PEG10 was also the same as that of eutherians, with two open reading frames (ORFs) related to the sushi-ichi retrotransposon GAG and POL proteins, respectively. The CX2CX4HX4C RNA-binding motif of GAG and the DSG sequences of the proteinase activation site of POL were also conserved (Figure S1). Tammar PEG10 was localised close to the telomere of Chromosome 3q, consistent with its autosomal location on proximal mouse Chromosome 6 (Figure 1B). However, in the platypus, there were no PEG10 homologous sequences between SGCE and PPP1R9A (also called NEURABIN1) that flank PEG10 in other mammals (Figure 1A).

5Using our tammar sequence and the published opossum genome sequence, we compared the entire region between SGCE and PPP1R9A with the equivalent region in several vertebrates from fish (fugu) to mammals. The size of this region in the platypus was similar to that of the chicken and was smaller than in other mammals. There were numerous long interspersed nuclear elements (LINEs) and short interspersed nuclear elements (SINEs) (grey bars in Figure 1A) in all mammalian groups. A previous report suggests that there are less long terminal repeat (LTR)-type retrotransposon-derived sequences in the opossum and wallaby genomes, but a large number of these sequences were found in this region as well as in mouse and human (blue bars in Figure 1A). Consistent with the previous report, these were absent in the platypus, as was PEG10. Most of the LTR-type retrotransposon-derived sequences observed in these regions are species specific. This suggests that most of these insertions occurred after species diversification. The presence of PEG10 sharing homology with one of the LTR-type retrotransposons, sushi-ichi, in both marsupials and eutherians suggests that the original PEG10 sequence insertion in the common therian ancestor was an early event in the therian-specific expansion of LTR-type retrotransposons. These results indicate that the original PEG10 was inserted into the genome of the therian ancestor and evolved to its present function as an essential placental gene after divergence from the monotremes. The acquisition of a new function for an existing character during evolution, a process termed “exaptation” by Gould and colleagues, would be enhanced by the provision of novel genetic materials, such as retrotransposons. Thus, the requirement for PEG10 in placental function is a clear example of “exaptation.” The fossil record shows that there was extensive radiation of therian mammals after their split from the Prototheria. New LTR-type retrotranposon-derived sequences might therefore have contributed novel genetic resources to this radiation.

6In mice, there is a large imprinting cluster near Peg10 which includes the paternally expressed Sgce gene Ppp1r9a, which is maternally biased in extraembryonic tissues and Asb4, which is completely maternally expressed in both embryos and the extraembryonic tissues. Tammar PEG10 showed almost complete monoallelic expression in all individuals. Paternal expression was confirmed in two embryos and one yolk sac placenta sample (Figure 2). Unexpectedly, tammar SGCE showed predominantly biallelic expression with only a small paternal bias, despite a short 200-bp distance between the transcription start sites of PEG10 and SGCE (Figure 2). PPP1R9A and ASB4 showed biallelic expression without parental bias (Figure 2). These results clearly demonstrate that imprinting in this region is restricted to the PEG10 gene in the tammar. As described above, the eutherian PEG10 imprinted region includes several neighboring genes, suggesting that the imprinted region expanded in the eutherians while in marsupials imprinting was restricted to PEG10.

7A CpG island is present in the promoter regions of SGCE and PEG10 in tammar as well as in mouse. To determine why SGCE did not show imprinted expression we examined its methylation status. Surprisingly, we found a DMR with a clear boundary of DNA methylation between the PEG10 side and the SGCE side of the CpG island in both embryos and yolk sac placentas (Figure 3). Furthermore, selective DNA methylation of the maternal allele was confirmed using a DNA polymorphism in this region as predicted by the paternal expression of PEG10. The DNA methylation started about 60 bp downstream from the transcription start site of PEG10, suggesting that maternal transcription is inhibited by methylation of downstream regulatory elements and not by the typical mechanism of promoter methylation (Figure 3). Both LTR and non-LTR retrotransposons are known to have internal transcriptional regulatory elements for their transcription. As PEG10 is a retrotransposon-derived gene, these elements may exist within the DMR not, as is usual, upstream of the transcription start site. A part of the CpG island was possibly derived from LTRs in the original PEG10 sequence and the methylation that originated from a host–defense mechanism may be restricted to the ancient repetitive-element homologous region. Alternatively, a boundary function for DNA methylation spreading and/or transcription regulation may be included in the marsupial CpG island. The retrotransposon LTR sequences are CpG-rich and have such a boundary function in Saccharomyces cerevisiae and Drosophila melanogaster. In Drosophila, the boundary function has been attributed to the binding of the SU(HW) protein. A consensus SU(HW) binding site was not found around the methylation boundary in the tammar PEG10. However, the CTCF protein, well known to have a similar insulator/boundary function in mammals, may bind to the possible boundary elements containing CT-rich sequences in this region.

8Even with the presence of a DMR, it is possible that the maternal copy of PEG10 in the tammar is silenced by another mechanism and is only secondarily methylated. We therefore examined whether the imprinted expression of tammar PEG10 was regulated by DNA methylation. A reduced level of DNA methylation was observed in three sites of the CpG island in cells cultured with 5-aza-2′-deoxycytidine, a DNA methylation inhibitor (Figure 4A). Repetitive experiments performed for the most 3′ site using three independent cell lines established from fetal lung and endometrium also showed statistically significant reductions in DNA methylation levels (Figure 4B), and increased PEG10 expression from normally repressed alleles was observed in each case (Figure 4C, black and grey bars), although the expression levels were still much lower than active alleles (Figure 4C, white bars). These results demonstrate the association between imprinted expression of PEG10 and DNA methylation in a marsupial, although it still remains unknown if the differential methylation originates in the germline as does a typical primary DMR in eutherians.

9The DNA methylation status of retrotransposons can differ between male and female germ cells. For example, IAP and LINE1 are more highly methylated in sperm than oocytes, while Alu is less methylated. Mice and humans with paternal disomy that express PEG10 and SGCE biallelically have normal phenotypes, so monoallelic expression of these genes is not essential for development. Therefore, although PEG10 is essential for placental development in the mouse, the imprinting of this locus may be a functionally unimportant inheritance derived from the nature of the original retrotransposition of PEG10.

10There were CpG islands in the putative promoter region of SGCE of the chicken, platypus, tammar, mouse, and human (Figure 5A). We hypothesize that insertion of PEG10 after the divergence of therian from prototherian mammals expanded the CpG islands (Figure 5). In the tammar, DNA methylation is restricted to PEG10, but in the mouse and human, the entire region is differentially methylated. These differences may be explained by the presence or absence of a boundary function of the CpG island in these groups as discussed above. However, in both cases, insertion of PEG10, which must have occurred in the therian ancestor, is clearly sufficient to establish imprinting of this region (Figure 5B). Our study confirms that silencing of exogenous DNA after retrotransposon insertion can drive the evolution of genomic imprinting in mammals.

11Tammar wallabies of Kangaroo Island origin were maintained in our breeding colony in grassy, outdoor enclosures. Lucerne cubes, grass and water were provided ad libitum and supplemented with fresh vegetables. Fetuses and yolk sac placenta tissue were collected between days 22 and 25 of the 26.5-d gestation. Experimental procedures conformed to Australian National Health and Medical Research Council (1990) guidelines and were approved by the Animal Experimentation Ethics Committees of the University of Melbourne.

12Each BAC clone in the tammar and platypus BAC libraries was stored separately and was spotted onto nylon membranes correspondently. These membranes were hybridized with the partial SGCE probes of each species, and the positive clones were identified according to the locus information of the signals on the membranes. The tammar PEG10 sequence was determined by the primer walking method from a partial fragment amplified using cross-species degenerate primers. The platypus sequence between SGCE and PPP1R9A was determined by the shotgun sequencing method, and it was completed using a published database of whole genome shotgun sequences and direct sequencing of PCR products.

13RepeatMasker (http://www.repeatmasker.org) was used for the detection of LINEs, SINEs, and LTR elements in the genomic region between SGCE and PPP1R9A.

14BAC DNA was labeled by nick translation with digoxygenin-11-dUTP. Hybridization was operated with the labeled BAC DNA and C0t-1 DNA at 37 °C overnight. Anti-digoxygenin-Cy3 and DAPI were used for the detection of the signals and for the counterstain, respectively.

15The details have been described in our previously published paper. The 3′ UTRs of PEG10 and ASB4 including the polymorphisms were amplified by 30–35 cycles of RT-PCR using the following pairs of primers: PEG10-F1, 5′-CAAATGCCATTGCCGTCT-3′ and PEG10-R1, 5′-GTTAGACGGTCAGCTCCACG-3′; PEG10-F2, 5′-CAACCAGGGGAGCTAGGATT-3′ and PEG10-R2, 5′-GAACATCCATGCACCGTAGA-3′; PEG10-F3, 5′-CTCTCTGGAGCGGTATCCAG-3′ and PEG10-R3, 5′-TGTGAGATTTGGCAATCATACA-3′; and ASB4-F, 5′-AACACCCCGAGGTCTCTCAT-3′ and ASB4-R, 5′-GAGGACCATGGCATTTATTCA-3′. The following correspondent SNuPE primers were used for the single nucleotide extension: PEG10-SN1, 5′-ATTCATTTCCCTTCCCAACAT-3′; PEG10-SN2, 5′-CCCTGGCTGCGAGACCA-3′; PEG10-SN3, 5′-CCCGGGGAGCTCCGAGC-3′; and ASB4-SN, 5′-CAAGAAGCAAGTAGTTCTTCAAAGG-3′

16The labeling of the primers for the last hot cycle was operated using [γ-32P]ATP and T4 DNA kinase. The final PCR products of SGCE and PPP1R9A were digested by Alul and Mspl, respectively. The 3′ UTRs of SGCE and PPP1R9A including the polymorphisms on the recognition sequences of these restriction enzymes were amplified by 30–35 cycles of PCR using the following pairs of primers. Asterisks indicate the labeled primers: SGCE-F, 5′-CAGTGATGGCGTTCTGTACG-3′; *SGCE-R, 5′-GTTGATGACCAGGTTGTGCC-3′; *PPP1R9A-F, 5′-CCAGGAGAAGATGGAGAAGC-3′; and PPP1R9A-R, 5′-GTTGGGGATGAAGGAGTGTG-3′. Ten percent polyacrylamide gels were used for the gel electrophoresis of the digested samples.

17After the bisulphite treatment for the genomic DNA of tammar, the region corresponding to the CpG islands over the promoter regions of SGCE and PEG10 was amplified by 35 cycles of PCR using the following pair of primers: CGI-F, 5′-GGAGTGATTGTGGAAATGGAGGTG-3′ and CGI-R, 5′-ATACAAAATCCCCCCCCTAAACCTC-3′. The PCR products were cloned and the clones were analyzed by sequencing.

18Primary culture of tammar fetal lung cells from day 25 of gestation and adult endometrium cells were used in this study. Control cells were cultured in 50% AmnioMAX (Invitrogen, http://www.invitrogen.com) and 50% DMEM supplemented with 10% fetal calf serum and penicillin/streptomycin at 37 °C/5% CO2. Cells for 5-aza-2′-deoxycytidine treatment were cultured in the same media but containing 10 μM of 5-aza-2′-deoxycytidine (Sigma). Fresh media with 5-aza-2′-deoxycytidine were added every 24 h for 6 d.

19Three regions in the PEG10 DMR were amplified by 35 (for the middle and right side in Figure 4A) and 40 (for the left side in Figure 4A) cycles of PCR from the bisulphite treated genomic DNA of cells using the following pair of primers: LEFT-F, 5′-GTATTAGTTTTTTTGTAGTT-3′ and LEFT-R, 5′-CCTAAAAAACTACCCTACTCC-3′; MID-F, 5′- GAGATGGGGAGATTGATATTT-3′ and MID-R, 5′- CCCTATAACTAAACTACAATCTCTCC-3′; and RIGHT-F, 5′- CCTCCCATTAACTTTAAAATCACC-3′ and RIGHT-R, 5′- ATTGTAGTAATGGGGTAGGTTATG-3′.

20PCR products were digested by RsaI (for the left side) or Aci I (for the middle and right side) for analyses.

21The 3′ UTR of PEG10, including the polymorphism on the TaqI or BceAI recognition sequences, was amplified by 30 cycles of RT-PCR using the PEG10-F1R1 or PEG10-F3R3 primer pairs. PCR products were digested by TaqI or BceAI for analyses.

22The National Center for Biotechnology Information GenBank (http://www.ncbi.nlm.nih.gov/Genbank) sequence accession numbers for tammar and platypus BACs are AB260975 and AB260976, respectively.