The conserved Cockayne syndrome B-piggyBac fusion protein (CSB-PGBD3) affects DNA repair and induces both interferon-like and innate antiviral responses in CSB-null cells

Abstract

Cockayne syndrome is a segmental progeria most often caused by mutations in the CSB gene encoding a SWI/SNF-like ATPase required for transcription-coupled DNA repair (TCR). Over 43 Mya before marmosets diverged from humans, a piggyBac3 (PGBD3) transposable element integrated into intron 5 of the CSB gene. As a result, primate CSB genes now generate both CSB protein and a conserved CSB-PGBD3 fusion protein in which the first 5 exons of CSB are alternatively spliced to the PGBD3 transposase. Using a host cell reactivation assay, we show that the fusion protein inhibits TCR of oxidative damage but facilitates TCR of UV damage. We also show by microarray analysis that expression of the fusion protein alone in CSB-null UV-sensitive syndrome (UVSS) cells induces an interferon-like response that resembles both the innate antiviral response and the prolonged interferon response normally maintained by unphosphorylated STAT1 (U-STAT1); moreover, as might be expected based on conservation of the fusion protein, this potentially cytotoxic interferon-like response is largely reversed by coexpression of functional CSB protein. Interestingly, expression of CSB and the CSB-PGBD3 fusion protein together, but neither alone, upregulates the insulin growth factor binding protein IGFBP5 and downregulates IGFBP7, suggesting that the fusion protein may also confer a metabolic advantage, perhaps in the presence of DNA damage. Finally, we show that the fusion protein binds in vitro to members of a dispersed family of 900 internally deleted piggyBac elements known as MER85s, providing a potential mechanism by which the fusion protein could exert widespread effects on gene expression. Our data suggest that the CSB-PGBD3 fusion protein is important in both health and disease, and could play a role in Cockayne syndrome.

Cockayne syndrome pathogenesis: Lessons from mouse models

“…The chimeric CSB-PGBD3 protein is expressed in wild type human cells, partially cofractionates with chromatin, does not show increased association with chromatin after UV irradiation, and may have a role in transcription regulation Lake and Fan, 2013;Lake et al, 2010;Newman et al, 2008). It has been proposed that unbalanced expression of CSB-PGBD3 in patients with truncation mutations downstream of codon 466 at the exon 5/6 boundary, which results in expression of CSB-PGBD3 in the absence of functional CSB, could aggravate the CS phenotype (Bailey et al, 2012;Gray et al, 2012;Newman et al, 2008;Weiner and Gray, 2013). However, this hypothesis is not fully compatible with known genotype-phenotype relationships in CS-B patients (Laugel et al, 2010).…”

“…In addition, it has been proposed that an alternative splice product encoded by the CSB locus that is specific for primates plays a role in CS. This protein consists of the N-terminus of CSB fused to a domesticated transposon (termed PGBD3), encoded by intron 5 (Bailey et al, 2012;Gray et al, 2012;Newman et al, 2008). The chimeric CSB-PGBD3 protein is expressed in wild type human cells, partially cofractionates with chromatin, does not show increased association with chromatin after UV irradiation, and may have a role in transcription regulation Lake and Fan, 2013;Lake et al, 2010;Newman et al, 2008).…”

Blinded by the UV light: How the focus on transcription-coupled NER has distracted from understanding the mechanisms of Cockayne syndrome neurologic disease

“…As a result of alternative splicing, a fusion protein containing the N-terminal portion of CSB and the PGBD3 is produced that can regulate both DNA repair and transcription. A recent publication [125] provides evidence that in the absence of CSB, the fusion protein can induce an interferon and innate immune-like transcriptional response, which may have implications for CS neurologic disease and other phenotypes. This paper also discusses the potential role of genetic background and homozygosity on the expression of the CS phenotype.…”

“…While much more work needs to be done on this hypothesis, an example in this regard are the interferon-α regulated genes that are activated in CSB deficient cells [125], and could be involved in some aspects of CS neurologic disease [12] (see Figure 2). If one or more transcription factors could be identified that mediate the abnormal transcriptional response (e.g.…”

A Novel <b><i>ERCC6</i></b> Splicing Variant Associated with a Mild Cockayne Syndrome Phenotype

“…Indeed, phenotypic correlation could not be definitively established between missense mutations, nonsense mutations, and severity of disease [19]. Interestingly, alternative splicing of ERCC6 involving exon 5 and a domesticated “PiggyBac” transposable element PGBD3 (encoding for a transposase) residing in intron 5 of ERCC6 , generated a CSB-PGBD3 fusion protein, that, in the absence of the full length CBS protein, was proposed to be causal of Cockayne syndrome [20,21]. The CSB-PGBD3 fusion protein, however, was not detected in all severe cases of Cockayne syndrome [4].…”