Cockayne syndrome exhibits dysregulation of p21 and other gene products that may be independent of transcription-coupled repair

Abstract

Cockayne syndrome (CS) is a progressive childhood neurodegenerative disorder associated with a DNA repair defect caused by mutations in either of two genes, CSA and CSB. These genes are involved in nucleotide excision repair (NER) of DNA damage from ultraviolet (UV) light, other bulky chemical adducts and reactive oxygen in transcriptionally active genes (transcription coupled repair, TCR). For a long period it has been assumed that the symptoms of CS patients are all due to reduced TCR of endogenous DNA damage in the brain, together with unexplained unique sensitivity of specific neural cells in the cerebellum. Not all the symptoms of CS patients are however easily related to repair deficiencies, so we hypothesize that there are additional pathways relevant to the disease, particularly those that are downstream consequences of a common defect in the E3 ubiquitin ligase associated with the CSA and CSB gene products. We have found that the CSB defect results in altered expression of anti-angiogenic and cell cycle genes and proteins at the level of both gene expression and protein lifetime. We find an over-abundance of p21 due to reduced protein turnover, possibly due to the loss of activity of the CSA/CSB E3 ubiquitylation pathway. Increased levels of p21 can result in growth inhibition, reduced repair from the p21-PCNA interaction, and increased generation of reactive oxygen. Consistent with increased reactive ozygen levels we find that CS-A and -B cells grown under ambient oxygen show increased DNA breakage, as compared to xeroderma pigmentosum cells. Thus the complex symptoms of CS may be due to multiple, independent downstream targets of the E3 ubiquitylation system that results in increased DNA damage, reduced transcription coupled repair, and inhibition of cell cycle progression and growth.

Increased apoptosis, p53 up-regulation, and cerebellar neuronal degeneration in repair-deficient Cockayne syndrome mice

“…Overexpressed neural genes included myelin basic protein, Synaptogyrin 3 (which plays a role in learning and memory), neogenin (which plays a role in axonal guidance in the nervous system), and the potassium voltage-gated channel, shaker-related subfamily, ␤ member 1. Previous results have also shown increased levels of the p21 cell cycle regulatory protein and underexpression of collagen 15a1 in human fibroblasts from CS patients (27).…”

“…Mutations in the XPD component of the transcription factor TFIIH, with which CS proteins interact, also result in many transcriptional defects, including reduced responses to nuclear receptors (60,61), dysregulation of peroxisome proliferator-activated target genes (62), vitamin D receptor-responsive genes (63), and thyroid hormone-responsive genes leading to hypomyelination (64). Both CSA and CSB cells also exhibit down-regulation of collagen 15a1 expression and dysregulation of p21 (27), and CSB plays a role in chromatin structure (13).…”

DNA polymerase η reduces the γ-H2AX response to psoralen interstrand crosslinks in human cells

“…Thus, an alternative interpretation of the data is that the observed γ-H2AX induction, particularly outside of S-phase, is not entirely in response to DSB formation. Recently, it has been reported that following ultraviolet radiation γ-H2AX does not necessarily reflect DSB during the G1 phase of the cell cycle [42], as well as in growtharrested cells in G0 [24]. Interestingly, ICL-forming agents have been reported to generate ssDNA foci that depend on NER, and ssDNA resulting from impaired gap-filling during nucleotide excision repair has recently been reported to be associated with γ-H2AX induction [24,43].…”

Sirt1 suppresses RNA synthesis after UV irradiation in combined xeroderma pigmentosum group D/Cockayne syndrome (XP-D/CS) cells

“…Furthermore, XPG was recently identified as a protein required for maintaining the integrity of the TFIIH complex, and therefore also engaged in the transcription process (20). Even though the contribution of the DNA repair deficiency to the clinical features of patients with XP, CS, or XP/ CS is irrefutable, studies have shown a clear dysregulation of a variety of transcriptional pathways, which may also contribute to the clinical phenotype of these patients (21)(22)(23)(24)(25)(26)(27). Interestingly, at the cellular level, XP/CS cells share with CS cells a sustained global transcriptional arrest after UV irradiation, which has been always explained by the inability of these cells to perform TCR (17,28).…”