Article

Tethering of the Conserved piggyBac Transposase Fusion Protein CSB-PGBD3 to Chromosomal AP-1 Proteins Regulates Expression of Nearby Genes in Humans

Department of Biochemistry, School of Medicine, University of Washington, Seattle, Washington, United States of America.
PLoS Genetics (Impact Factor: 7.53). 09/2012; 8(9):e1002972. DOI: 10.1371/journal.pgen.1002972
Source: PubMed

ABSTRACT

The CSB-PGBD3 fusion protein arose more than 43 million years ago when a 2.5-kb piggyBac 3 (PGBD3) transposon inserted into intron 5 of the Cockayne syndrome Group B (CSB) gene in the common ancestor of all higher primates. As a result, full-length CSB is now coexpressed with an abundant CSB-PGBD3 fusion protein by alternative splicing of CSB exons 1-5 to the PGBD3 transposase. An internal deletion of the piggyBac transposase ORF also gave rise to 889 dispersed, 140-bp MER85 elements that were mobilized in trans by PGBD3 transposase. The CSB-PGBD3 fusion protein binds MER85s in vitro and induces a strong interferon-like innate antiviral immune response when expressed in CSB-null UVSS1KO cells. To explore the connection between DNA binding and gene expression changes induced by CSB-PGBD3, we investigated the genome-wide DNA binding profile of the fusion protein. CSB-PGBD3 binds to 363 MER85 elements in vivo, but these sites do not correlate with gene expression changes induced by the fusion protein. Instead, CSB-PGBD3 is enriched at AP-1, TEAD1, and CTCF motifs, presumably through protein-protein interactions with the cognate transcription factors; moreover, recruitment of CSB-PGBD3 to AP-1 and TEAD1 motifs correlates with nearby genes regulated by CSB-PGBD3 expression in UVSS1KO cells and downregulated by CSB rescue of mutant CS1AN cells. Consistent with these data, the N-terminal CSB domain of the CSB-PGBD3 fusion protein interacts with the AP-1 transcription factor c-Jun and with RNA polymerase II, and a chimeric CSB-LacI construct containing only the N-terminus of CSB upregulates many of the genes induced by CSB-PGBD3. We conclude that the CSB-PGBD3 fusion protein substantially reshapes the transcriptome in CS patient CS1AN and that continued expression of the CSB-PGBD3 fusion protein in the absence of functional CSB may affect the clinical presentation of CS patients by directly altering the transcriptional program.

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    • "Thus far, only the function of PGBD3 has been investigated. CSB-PGBD3 is capable of binding DNA, including endogenous piggyBac-like transposons in the human genome, but has no known catalytic activity, though biochemical and genetic evidence indicates that it may participate in DNA damage response (Bailey et al., 2012; Gray et al., 2012). PGBD5 is distinct from other human piggyBac-derived genes by having been domesticated much earlier in vertebrate evolution approximately 500 million years (My) ago, in the common ancestor of cephalochordates and vertebrates (Sarkar et al., 2003; Pavelitz et al., 2013). "
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    ABSTRACT: eLife digest Transposons are mobile genetic elements that can be cut out of and inserted into DNA. They are present in most living things and make up almost half of the human genome. Transposons help to rearrange and increase the variety of DNA sequences, which can drive evolution and regulate the expression of genes. Enzymes called transposases help to move transposons, but very few genes that encode these enzymes have been studied in humans. PiggyBac transposase—which was first discovered in the cabbage looper moth—helps to move transposons of the piggyBac family. Humans and many other animals have genes that encode similar enzymes. In particular, the gene that encodes the human PGBD5 transposase is expressed in the developing embryo and particular areas of the brain and is highly similar to genes found in other vertebrate animals. These intriguing features prompted Henssen et al. to investigate PGBD5. The experiments reveal that PGBD5 is able to move piggyBac-like transposons in human cells and insert them into sites that contain similar DNA sequences that are preferred by other PiggyBac transposases. Henssen et al. compared human PGBD5 to the piggyBac transposases from other organisms, including insects, bats, and frogs. They found that PGBD5 is deeply conserved among vertebrate organisms, and is distinct from other piggyBac transposases. These findings suggest that PGBD5 is indeed a fully working piggyBac transposase. Further work is needed to understand what portions of the human genome may be rearranged by PGBD5, and how this may contribute to human brain development or disease. DOI: http://dx.doi.org/10.7554/eLife.10565.002
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    • "Most likely, this process represents a mechanism of formation of pseudogenes (Pavli cek et al., 2002). In addition to the disruption of the gene, the insertion of TEs into a functional copy of a gene can cause exon skipping, alternative splicing or transcription alteration (Bernstein et al., 1995; Stewart et al., 1997; Musova et al., 2006; Gray et al., 2012). For example, 25% of human promoter regions contain TE-derived sequences, including experimentally verified cis-regulatory elements (reviewed in Jordan et al., 2003; Bi emont & Vieira, 2006); cryptic TEs might contribute to the regulatory regions of many genes. "
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    • "Humans have 5 substantially complete piggyBac elements, designated PGBD1, 2, 3, 4, and 5. PGBD1, 2, and 3 have multiple coding exons, but in each case the piggyBac transposase-related sequence is encoded by a single uninterrupted 3′ terminal exon. Thus, PGBD1 and 2 may resemble the PGBD3 transposon in which the transposase ORF is flanked upstream by a 3′ splice site and downstream by a polyadenylation site [13,15]. As a result, insertion of PGBD3 into intron 5 of the CSB host gene enables the transposon to take advantage of the CSB promoter, using transposon-encoded alternative mRNA splicing and polyadenylation signals to express transposase as a CSB-PGBD3 fusion protein. "
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