Date Published: December 26, 2013
Publisher: Public Library of Science
Author(s): Michal Lieberman-Lazarovich, Cathy Melamed-Bessudo, Sylvia de Pater, Avraham A. Levy, Xiaoyu Zhang.
Gene Targeting (GT) is the integration of an introduced vector into a specific chromosomal site, via homologous recombination. It is considered an effective tool for precise genome editing, with far-reaching implications in biological research and biotechnology, and is widely used in mice, with the potential of becoming routine in many species. Nevertheless, the epigenetic status of the targeted allele remains largely unexplored. Using GT-modified lines of the model plant Arabidopsis thaliana, we show that the DNA methylation profile of the targeted locus is changed following GT. This effect is non-directional as methylation can be either completely lost, maintained with minor alterations or show instability in the generations subsequent to GT. As DNA methylation is known to be involved in several cellular processes, GT-related alterations may result in unexpected or even unnoticed perturbations. Our analysis shows that GT may be used as a new tool for generating epialleles, for example, to study the role of gene body methylation. In addition, the analysis of DNA methylation at the targeted locus may be utilized to investigate the mechanism of GT, many aspects of which are still unknown.
The ability to modify genomes in a precise manner provides a powerful tool for both basic and applied research [1,2]. It can be used to study gene function, to perform gene therapy or to improve crop plants. Several methods have been developed to allow for precise modifications of genomes. These include gene targeting (GT), DNA cleavage and mutagenesis by custom-designed endonucleases, site-specific recombination (e.g. Cre-Lox recombination), and oligo-mediated mutagenesis. These techniques enable targeted modifications, such as gene knock-outs, gene replacement, mutations or insertions and are routinely implemented in model organisms such as yeast  and mice .
For the GT experiments, Arabidopsis thaliana plants (ecotype Columbia) were grown in controlled chambers at 21°C, with 16/8 hours of light/dark, respectively. The GT vector was transformed into inflorescences using the floral-dip method. Mature seeds were collected and screened for successful GT events. Genetic characterization of the GT lines was performed by PCR and Southern-blot assays. Methylation analysis of targeted lines was performed using the EpiTect kit (QIAGEN) for bisulfite conversion  of genomic DNA, according to the manufacturer’s instructions with one modification: the number of incubation cycles was increased in order to ensure full conversion. Genomic DNA was extracted with the DNeasy Plant Mini Kit (QIAGEN), according to the manufacturer’s protocol. Converted DNA was used for amplification of the target and PCR products were cloned and Sanger-sequenced. The number of clones that were sequenced in each experiment is indicated in the relevant supplementary table. For the PPOX locus analysis, pools of 4-8 young Arabidopsis plantlets were used in each sample. For the CRUCIFERIN3 and PROHIBITIN1 methylation analyses mature plant tissue were sampled, by pooling 2-6 plants from each line. Conversion rate was estimated at 99 to100%, by sequencing a fragment of the non-methylated PHAVOLUTA gene (At1g30490). Sequence analysis for calculating methylation fractions and for generating methylation plots was performed by the Kismeth software . In order to independently determine the methylation patterns of the endogenous PROHIBITIN1and the copied PROHIBITIN1, we took an approach of size separation; the genomic DNA was digested with PvuII, which generates a fragment of around 8300bp at the endogenous PROHIBITIN1 locus, and a 4000bp fragment at the duplicated gene. Digested DNA was extracted from the gel in two separate reactions, corresponding to the 8300bp and 4000bp fragments, and then used for bisulfite conversion and sequencing.
Gene replacement of the endogenous PPOX locus, by a vector presenting one or two SNPs (Single Nucleotide Polymorphisms) that modify specific amino acids, confers herbicide resistance . We analyzed three independent GT-derived lines by Southern blotting and sequencing, to confirm that gene targeting resulted in the precise replacement of the endogenous PPOX locus. These targeted lines did not contain randomly integrated copies of the vector in the genome (Figure S1), and were thus considered True Gene Targeting (TGT) events and consequently named TGT-1 , TGT-2, and TGT-3 (originally named C1 and T2, respectively ). Bisulfite-sequencing was performed on the CG-methylated region, encompassing the second SNP introduced by the GT vector (Figure 1), to enable comparison of the post-targeting methylation state of the targeted allele to that of the WT. In addition, methylation status in the progeny of the initial GT events was also evaluated. Two types of DNA methylation modifications were identified, that may have occurred at the targeted PPOX locus. The TGT-1 line, generated in the wild-type (WT) background (ecotype Wassilewskija, Ws), lost half of the total CG methylation in the examined region, reaching a level of 35.2%, compared to 69.7% in the wild-type Ws plant (Table S1). Methylation was virtually obliterated in the central region, encompassing six of the eleven CG sites as well as the second SNP introduced by the GT vector (Figure 2). This prominent pattern may possibly reflect the location and length of the conversion tract in the process of homology-mediated GT. The last CG site in this sequence, at position 320 (Figure 2), was methylated in TGT-1, but not in the WT (Ws). We found this specific site to be particularly unstable, as even in WT isolates of the same ecotype (Columbia), both full and lack of methylation was seen (Table S2). Since other CG sites in this region showed a consistent methylation pattern in different WT isolates, position 320 can be considered an outlier. To address the question of whether the observed loss of methylation has an influence on gene expression, we carried out expression analysis of the PPOX gene using two different primer pairs. As presented in Figure S2, the PPOX transcript level did not differ between TGT-1 and the WS line used as control. In contrast to the local depletion of methylation in the PPOX allele of the TGT-1 line, the overall methylation level was not dramatically changed in TGT-2 and TGT-3 , both in the Columbia background (Figure 3a). While the mean WT (Columbia) methylation level was 68.1%, the rates of CG methylation in the targeted allele of TGT-2 and TGT-3 were 66.4% and 74.4%, respectively (Table S1). Most CG sites (7 out of 11) presented methylation levels similar to WT, however, methylation at three sites (at positions 64, 155 and 194) was altered (Figure 3b) (Table S2). The CG dinucleotide at position 64 differed from the other two sites, presenting sharp increase in methylation only in the T3 generation of the TGT-2 line (Figure 3b). The CG sites at positions 155 and 194 exhibited altered methylation levels already in the T2 generation. Interestingly, this trend of change was further enhanced in the following generation (T3) (Figure 3b). At position 155, the WT methylation level was moderate (65%, Table S2). In the TGT-2 line, this position had lost some methylation in the first generation after GT, and completely lost all methylation in the T3 generation. In contrast, the TGT-3 line had gained methylation at this site, which rose to 90-100% in T3. In position 194, methylation was very low in WT (7%) but increased in the T2 generation of both TGT-2 and TGT-3 lines, and showed a further increase in T3, reaching methylation levels as high as 78-80% (Figure 3b). Non-CG methylation (i.e. CHG, CHH) was very low in this region (0 to 2.55% in WT, Table S1), a status that was maintained in the targeted lines, with the exception of the targeted allele of TGT-3, where CHG methylation was higher (Table S1). In order to confirm that changes in DNA methylation did not occur globally due to the transformation process, we have tested the methylation status of a non-target locus, which, similarly to the PPOX gene, presents CG body methylation in the WT background. As shown in Figure S3, no changes in CG body-methylation were observed for this locus (At2g33860) in all GT lines used in this study. In the TGT-1 line, a single CG site differs in methylation from the Ws WT line, however, it is unlikely that this is the result of GT, as the TGT-1 pattern is in agreement with the published methylome of the Ws ecotype . In addition, this specific site is found to be unstable among various Arabidopsis ecotypes . Gene targeting events at the CRUCIFERIN3 locus, which lacks cytosine methylation, can be identified using a seed-fluorescent assay previously described [24,25] and shown in Figure 1. We analyzed methylation in plants derived from fluorescent seeds corresponding to two GT events. The first event gave rise to a precise gene targeting allele (CRU-TGT), where the WT locus was replaced by the GT vector and with no additional non-homologous integration in the background (Figure S4). Three regions were analyzed in WT, non-targeted sibling, and the homozygous CRU-TGT line, in T2 and/or T3 generations. One region was located upstream to the CRUCIFERIN3 promoter, beyond the region of homology between the GT cassette and the genomic target, the second, was around the CRUCIFERIN3 translation start site, a region of homology between the GT vector and the target, and the third, was at the junction between the CRUCIFERIN3 gene and the introduced mRFP reporter gene (yellow bars in Figure 1). For all 3 regions, no alteration in GT-related cytosine methylation was observed, indicating that the occurrence of a precise GT did not change the methylation state of a target which was formerly un-methylated (Table S3). The second GT event was not precise; as frequently happens in GT experiments, the vector invades its homologous target, but instead of replacing the target, it dissociates and integrates at an ectopic location, often capturing target sequences, while the endogenous target is left intact . Such events are thus referred to as Ectopic Gene Targeting (EGT). Detailed genetic characterization of this EGT event is presented in Figure S5. The methylation patterns of the PROHIBITIN1 gene (PHB1, AT4G28510) that was duplicated upon this EGT event (Figure 4) were analyzed. In contrast to the unmethylated CRUCIFERIN3 target, the upstream gene – PHB1 – is naturally enriched with body CG methylation. Thus, in the EGT line, we could test the methylation state of the newly duplicated PHB1 gene as well as that of its original template locus. To this end, we separately isolated the endogenous PHB1 and the duplicated copy for bisulfite-sequencing analysis (17). T2 and T3 generations of this EGT line were also analyzed, in order to test whether methylation of the duplicated copy is stable throughout generations. Interestingly, we found that cytosine methylation was altered following the GT-derived gene duplication at both the endogenous and the duplicated alleles, and further changed from T2 to T3 generation, in both CG and non-CG sites (Figure 4, Figure S6 and Tables S4 and S5). Increased methylation was observed at all CG dinucleotides analyzed in T2, with a most profound increase at position 18, which was unmethylated prior to targeting. In the subsequent generation (T3), methylation levels at position 18 reverted back to WT levels (0%). At the other CG sites, a decrease in methylation in the duplicated copy was observed, although not reconstructing the WT state as at position 18 (Figure 4, Figure S6 and Table S4). Although non-CG methylation is absent at the PHB1 locus in WT, it appeared to be highly induced upon GT-derived duplication, as most (68.4%) non-CG sites were found to be 90-100% methylated in both the endogenous and the duplicated allele in the T2 generation. Interestingly, this methylation profile reverted back to the WT state already in T3, when only traces of non-CG methylation could be detected (Table S5).
Taken together, our results demonstrate the possible impact of GT on the epigenetic landscape of the targeted locus. We found that several types of epigenetic alterations may occur at the targeted locus, even when the alteration is genetically precise. GT can induce complete removal of methylation, in a limited region of the target, or cause loss of stability at certain methyl-CG sites, an effect that can be further enhanced in the following generation. The changes observed suggest that the effect of GT on the methylation status of the targeted locus is not uniform or directional. The possible causes for the different effects on epigenetic alterations are not clear. One possibility is the different Arabidopsis ecotypes used for the initial GT experiments (i.e. Ws in the case of the TGT-1 line, and Col-0 for TGT-2 and 3 lines), both performed via floral-dip Agrobacterium-mediated transformation. It is known that different Arabidopsis ecotypes differ in their genome-wide methylation patterns, raising the question of whether differences in the mechanism maintaining DNA methylation exist as well, an issue that was never tested. If so, that could explain the different response to GT. It has been shown that different types of biotic stresses may cause epigenetic alterations , raising the possibility that the transformation event itself, carried out using bacterial infection, would trigger the observed changes in DNA methylation. To address that, we tested the methylation status of a non-target locus, where we found no changes in methylation patterns (Figure S3), suggesting that the loss of methylation at the PPOX locus is indeed linked and caused by the GT process. In the case of the EGT line (Figure 4, Figure S6, Table S4 and Table S5), it is likely that the changes in DNA methylation were triggered by the duplication of the PHB1 gene, similarly to methylation changes observed following genome duplication and polyploidization, through the involvement of siRNAs and the RdDM (RNA directed DNA Methylation) pathway. Transgene silencing is yet another well studied phenomenon where the presence of repeated sequences drives hypermethylation and silencing of both copies, mediated by RdDM. It is probable that GT-mediated demethylation, as shown here in plants, can also occur in mammalian systems, due to the conserved nature of homologous recombination mechanisms. Considering the routine use of gene knockout in mammalian research this possibility should receive more attention. Indeed, earlier works have shown an alteration in allele imprinting as a result of targeted deletions or insertions [27,28], however, a detailed analysis of patterns of cytosine methylation was lacking. Note that the fate of the unmethylated DNA might differ between plants and mammals. While in mammals CG methylation can be regained de-novo, according to sequence determinants [29,30], in plants, de-novo methylation is usually of a non-CG type. Hence, loss of CG methylation in plants may result in accumulation of de-novo non-CG methylation [17,18] or, if the signal for de-novo methylation is lacking, then there is no compensation for the lost methylation and the region remains unmethylated. Epialleles have been shown to have potentially important phenotypic effects on development [31,32], in contributing to biodiversity  and in human disease, such as cancer . In addition to its impact on gene expression and chromatin organization [13-16,35], DNA methylation was shown to be involved in essential cellular processes, such as meiotic recombination [36-38] and nucleosome positioning . Therefore, the epigenetic status of a targeted locus should be taken into consideration in GT experiments, particularly when the target is methylated in the WT. In our case, we did not detect a change in expression of the PPOX gene, in seedlings of the TGT-1, 2, 3 lines (Figure S2). Nevertheless, since the exact role of gene body methylation is unclear, we cannot exclude an effect on another process and/or in a specific cell-type, developmental stage, or environmental condition. Targeted demethylation presents an ideal system to address the role of gene body methylation. The advances in technologies enhancing the precision and efficacy of gene targeting, such as Zinc Finger Nucleases , TALENs [11,41], and the more recent CRISPR [42-46], suggest that GT will become routine in several species in a near future, including plants [47-51]. In turn, the prospects of targeting genes for the sole purpose of changing their methylation status will rise. This may also provide a new tool for research or for applications in medicine and agriculture. Our results show that the changes in methylation provide new mechanistic insight into the GT process. For example, obliteration of methylation patterns during precise GT, as with the PPOX TGT-1 allele, provides support for the double strand break (DSB) repair model , whereby the DSB in the target is followed by gap enlargement and repaired via strand invasion and copying of the unmethylated vector (Figure 5). The EGT event at the CRUCIFERIN3 locus provides support for the GT model involving vector invasion into the target and initiation of DNA synthesis, as in the synthesis-dependent strand annealing model . In this model, the vector is released from the target template, and inserted into a new location, thereby generating duplication of the target sequence as well as of its original methylation imprints (Figure 5). Remarkably, this type of duplication can induce epigenetic instability in both the genetically unaltered endogenous target region and in the new duplicated locus. The case of TGT-2 and TGT-3, where original methylation was maintained at most positions, can be explained by two different models, namely the model of strand assimilation  or a DSB repair model, where protruding ends are generated but without gap enlargement (Figure 5). In both cases, a template is present for methylation maintenance. Interestingly, we observed methylation instability at few CG sites of the region examined. Although the mechanism of this epigenetic instability is yet to be elucidated, it is reminiscent of the genetic instability reported during mitotic gene conversion in yeast  where mutations resulting from errors made by DNA polymerases seemed to be affected by the local sequence context. It is possible that the observed epigenetic instability reported here results from perturbations of the maintenance methyltransferase activity. Finally, we show (in TGT-1) that DNA methylation analysis of GT events provides novel insight into the analysis of the conversion tract predicted from the DSB repair model. Namely, by using methylation polymorphism, gap enlargement can be measured at a higher resolution than previously achieved, even in the absence of DNA polymorphism. We therefore propose that DNA methylation may be exploited in future experiments to better understand the mechanism(s) of GT in higher eukaryotes.