Liane Gagnier

Retroviral Elements and Their Hosts: Insertional Mutagenesis in the Mouse Germ Line

The inbred mouse is an invaluable model for human biology and disease. Nevertheless, when considering genetic mechanisms of variation and disease, it is important to appreciate the significant differences in the spectra of spontaneous mutations that distinguish these species. While insertions of transposable elements are responsible for only ~0.1% of de novo mutations in humans, the figure is 100-fold higher in the laboratory mouse. This striking difference is largely due to the ongoing activity of mouse endogenous retroviral elements. Here we briefly review mouse endogenous retroviruses (ERVs) and their influence on gene expression, analyze mechanisms of interaction between ERVs and the host cell, and summarize the variety of mutations caused by ERV insertions. The prevalence of mouse ERV activity indicates that the genome of the laboratory mouse is presently behind in the “arms race” against invasion.

Genome-Wide Assessments Reveal Extremely High Levels of Polymorphism of Two Active Families of Mouse Endogenous Retroviral Elements

Endogenous retroviral elements (ERVs) in mice are significant genomic mutagens, causing ∼10% of all reported spontaneous germ line mutations in laboratory strains. The majority of these mutations are due to insertions of two high copy ERV families, the IAP and ETn/MusD elements. This significant level of ongoing retrotranspositional activity suggests that inbred mice are highly variable in content of these two ERV groups. However, no comprehensive genome-wide studies have been performed to assess their level of polymorphism. Here we compared three test strains, for which sufficient genomic sequence is available, to each other and to the reference C57BL/6J genome and detected very high levels of insertional polymorphism for both ERV families, with an estimated false discovery rate of only 0.4%. Specifically, we found that at least 60% of IAP and 25% of ETn/MusD elements detected in any strain are absent in one or more of the other three strains. The polymorphic nature of a set of 40 ETn/MusD elements found within gene introns was confirmed using genomic PCR on DNA from a panel of mouse strains. For some cases, we detected gene-splicing abnormalities involving the ERV and obtained additional evidence for decreased gene expression in strains carrying the insertion. In total, we identified nearly 700 polymorphic IAP or ETn/MusD ERVs or solitary LTRs that reside in gene introns, providing potential candidates that may contribute to gene expression differences among strains. These extreme levels of polymorphism suggest that ERV insertions play a significant role in genetic drift of mouse lines.

Retrotransposon-Induced Heterochromatin Spreading in the Mouse Revealed by Insertional Polymorphisms

The “arms race” relationship between transposable elements (TEs) and their host has promoted a series of epigenetic silencing mechanisms directed against TEs. Retrotransposons, a class of TEs, are often located in repressed regions and are thought to induce heterochromatin formation and spreading. However, direct evidence for TE–induced local heterochromatin in mammals is surprisingly scarce. To examine this phenomenon, we chose two mouse embryonic stem (ES) cell lines that possess insertionally polymorphic retrotransposons (IAP, ETn/MusD, and LINE elements) at specific loci in one cell line but not the other. Employing ChIP-seq data for these cell lines, we show that IAP elements robustly induce H3K9me3 and H4K20me3 marks in flanking genomic DNA. In contrast, such heterochromatin is not induced by LINE copies and only by a minority of polymorphic ETn/MusD copies. DNA methylation is independent of the presence of IAP copies, since it is present in flanking regions of both full and empty sites. Finally, such spreading into genes appears to be rare, since the transcriptional start sites of very few genes are less than one Kb from an IAP. However, the B3galtl gene is subject to transcriptional silencing via IAP-induced heterochromatin. Hence, although rare, IAP-induced local heterochromatin spreading into nearby genes may influence expression and, in turn, host fitness.

Ly49 genes in non-rodent mammals

The Ly49 family of natural killer (NK) receptors is encoded by a highly polymorphic multigene family in the mouse and is also present in multiple copies in the rat. However, this gene exists as a single copy in primates and is mutated to non-function in humans. We recently showed that the cow also likely has only one Ly49 gene, but it is unclear what the Ly49 gene content is for other mammals. We have now isolated Ly49 cDNAs from the domestic cat, dog and pig and show that the corresponding gene appears to be single copy in these three species. The open reading frame is intact in all the genes and the putative proteins contain an immune tyrosine-based inhibition motif (ITIM), suggesting a role as an inhibitory receptor. In contrast to the other mammals, several Ly49-like genes appear to exist in the horse, indicating that amplification of this locus has occurred in a non-rodent lineage. Finally, phylogenetic analysis suggests that the rodent Ly49 genes have evolved more rapidly than their counterparts in mammals where the gene has remained as a single copy.

Structure and Expression of Mobile ETnII Retroelements and Their Coding-Competent MusD Relatives in the Mouse

ABSTRACT ETnII elements are mobile members of the repetitive early transposon family of mouse long terminal repeat (LTR) retroelements and have caused a number of mutations by inserting into genes. ETnII sequences lack retroviral genes, but the recent discovery of related MusD retroviral elements with regions similar to gag, pro, and pol suggests that MusD provides the proteins necessary for ETnII transposition in trans. For this study, we analyzed all ETnII elements in the draft sequence of the C57BL/6J genome and classified them into three subtypes (α, β, and γ) based on structural differences. We then used database searches and quantitative real-time PCR to determine the copy number and expression of ETnII and MusD elements in various mouse strains. In 7.5-day-old embryos of a mouse strain in which two mutations due to ETnII-β insertions have been identified (SELH/Bc), we detected a three- to sixfold higher level of ETnII-β and MusD transcripts than in control strains (C57BL/6J and LM/Bc). The increased ETnII transcription level can in part be attributed to a higher number of ETnII-β elements, but 70% of the MusD transcripts appear to have been derived from one or a few MusD elements that are not detectable in C57BL/6J mice. This element belongs to a young MusD subgroup with intact open reading frames and identical LTRs, suggesting that the overexpressed element(s) in SELH/Bc mice might provide the proteins for the retrotransposition of ETnII and MusD elements. We also show that ETnII is expressed up to 30-fold more than MusD, which could explain why only ETnII, but not MusD, elements have been positively identified as new insertions.

Evidence for Epigenetic Maintenance of Ly49a Monoallelic Gene Expression

Although structurally unrelated, the human killer cell Ig-like (KIR) genes and the rodent lectin-like Ly49 genes serve similar functional roles in NK cells. Moreover, both gene families display variegated, monoallelic expression patterns established at the transcriptional level. DNA methylation has been shown to play an important role in maintenance of expression patterns of KIR genes, which have CpG island promoters. The potential role of DNA methylation in expression of Ly49 genes, which have CpG-poor promoters, is unknown. In this study, we show that hypomethylation of the region encompassing the Pro-2 promoter of Ly49a and Ly49c in primary C57BL/6 NK cells correlates with expression of the gene. Using C57BL/6 × BALB/c F1 hybrid mice, we demonstrate that the expressed allele of Ly49a is hypomethylated while the nonexpressed allele is heavily methylated, indicating a role for epigenetics in maintaining monoallelic Ly49 gene expression. Furthermore, the Ly49a Pro-2 region is heavily methylated in fetal NK cells but variably methylated in nonlymphoid tissues. Finally, in apparent contrast to the KIR genes, we show that DNA methylation and the histone acetylation state of the Pro-2 region are strictly linked with Ly49a expression status.

Epigenetic interplay between mouse endogenous retroviruses and host genes

BackgroundTransposable elements are often the targets of repressive epigenetic modifications such as DNA methylation that, in theory, have the potential to spread toward nearby genes and induce epigenetic silencing. To better understand the role of DNA methylation in the relationship between transposable elements and genes, we assessed the methylation state of mouse endogenous retroviruses (ERVs) located near genes.ResultsWe found that ERVs of the ETn/MusD family show decreased DNA methylation when near transcription start sites in tissues where the nearby gene is expressed. ERVs belonging to the IAP family, however, are generally heavily methylated, regardless of the genomic environment and the tissue studied. Furthermore, we found full-length ETn and IAP copies that display differential DNA methylation between their two long terminal repeats (LTRs), suggesting that the environment surrounding gene promoters can prevent methylation of the nearby LTR. Spreading from methylated ERV copies to nearby genes was rarely observed, with the regions between the ERVs and genes apparently acting as a boundary, enriched in H3K4me3 and CTCF, which possibly protects the unmethylated gene promoter. Furthermore, the flanking regions of unmethylated ERV copies harbor H3K4me3, consistent with spreading of euchromatin from the host gene toward ERV insertions.ConclusionsWe have shown that spreading of DNA methylation from ERV copies toward active gene promoters is rare. We provide evidence that genes can be protected from ERV-induced heterochromatin spreading by either blocking the invasion of repressive marks or by spreading euchromatin toward the ERV copy.

Distinct isoform of FABP7 revealed by screening for retroelement-activated genes in diffuse large B-cell lymphoma

Significance Sequences derived from transposable elements (TEs) are abundant in the human genome and can influence gene expression. In normal cells, most TEs are silenced by epigenetic mechanisms such as DNA methylation but, in cancer, normally dormant TEs can become active. We hypothesized that cancer-specific release of epigenetic suppression of TEs could result in gene expression perturbations, which could promote oncogenesis. Using a bioinformatics method, we identified many genes expressed in diffuse large B-cell lymphoma (DLBCL) via activation of TE promoters. Further analysis of one, FABP7, showed it was expressed in some DLBCL samples through use of a TE promoter. The TE-driven FABP7 transcript encodes a novel isoform of the protein, which is required for optimal DLBCL cell line proliferation. Remnants of ancient transposable elements (TEs) are abundant in mammalian genomes. These sequences harbor multiple regulatory motifs and hence are capable of influencing expression of host genes. In response to environmental changes, TEs are known to be released from epigenetic repression and to become transcriptionally active. Such activation could also lead to lineage-inappropriate activation of oncogenes, as one study described in Hodgkin lymphoma. However, little further evidence for this mechanism in other cancers has been reported. Here, we reanalyzed whole transcriptome data from a large cohort of patients with diffuse large B-cell lymphoma (DLBCL) compared with normal B-cell centroblasts to detect genes ectopically expressed through activation of TE promoters. We have identified 98 such TE-gene chimeric transcripts that were exclusively expressed in primary DLBCL cases and confirmed several in DLBCL-derived cell lines. We further characterized a TE-gene chimeric transcript involving a fatty acid-binding protein gene (LTR2-FABP7), normally expressed in brain, that was ectopically expressed in a subset of DLBCL patients through the use of an endogenous retroviral LTR promoter of the LTR2 family. The LTR2-FABP7 chimeric transcript encodes a novel chimeric isoform of the protein with characteristics distinct from native FABP7. In vitro studies reveal a dependency for DLBCL cell line proliferation and growth on LTR2-FABP7 chimeric protein expression. Taken together, these data demonstrate the significance of TEs as regulators of aberrant gene expression in cancer and suggest that LTR2-FABP7 may contribute to the pathogenesis of DLBCL in a subgroup of patients.

A protocol for detection of mitochondrial DNA deletions: characterization of a novel deletion

OBJECTIVES To develop a protocol capable of identifying deletions in mitochondrial DNA and use it to identify the breakpoints of a mtDNA deletion in a patient with chronic progressive external ophthalmoplegia (CPEO). DESIGN AND METHODS Deletions in mtDNA were identified by a combination of long range PCR and Southern blotting. The precise breakpoints were determined by automated DNA sequencing. RESULTS A series of DNA samples from patients with suspected mitochondrial disease was subjected to a protocol, which combines long range PCR and Southern blotting. We found a unique deletion in a patient with CPEO and we identified the precise location of this deletion through DNA sequencing. CONCLUSIONS Long range PCR has the advantages of speed, minimal samples requirements, and sensitivity. Southern blotting is better able to evaluate heteroplasmy and detect duplications. We suggest a protocol that enables us to identify precisely the breakpoints in a unique mutation of mtDNA in a patient with CPEO.

Spectrum of mutations in the arylsulfatase A gene in a Canadian DNA collection including two novel frameshift mutations, a new missense mutation (C488R) and an MLD mutation (R84Q) in cis with a pseudodeficiency allele.

We describe three novel mutations in the human arylsulfatase A gene in three patients with MLD, an autosomal recessive lysosomal storage disorder. An insertion, 2590_2591insCCCC in exon 8 and a deletion, 752_758delGCCGGCC, in exon 3 will both result in frameshifts. A mutation in exon 8, 2566T-->C, results in a missense mutation C488R, disrupting an unusual cysteine-knot at the C-terminal end of the protein. All three mutations are heterozygous with previously documented mutations. A previously reported mutation, R84Q was identified on a pseudodeficiency allele. These mutations are part of a heterogeneous spectrum of mutations found in a collection of DNA samples from MLD patients from across Canada and the USA.