UV damage causes uncontrolled DNA breakage in cells from patients with combined features of XP-D and Cockayne syndrome

Abstract

Nucleotide excision repair (NER) removes damage from DNA in a tightly regulated multiprotein process. Defects in NER result in three different human disorders, xeroderma pigmentosum (XP), trichothiodystrophy (TTD) and Cockayne syndrome (CS). Two cases with the combined features of XP and CS have been assigned to the XP-D complementation group. Despite their extreme UV sensitivity, these cells appeared to incise their DNA as efficiently as normal cells in response to UV damage. These incisions were, however, uncoupled from the rest of the repair process. Using cell-free extracts, we were unable to detect any incision activity in the neighbourhood of the damage. When irradiated plasmids were introduced into unirradiated XP-D/CS cells, the ectopically introduced damage triggered the induction of breaks in the undamaged genomic DNA. XP-D/CS cells thus have a unique response to sensing UV damage, which results in the introduction of breaks into the DNA at sites distant from the damage. We propose that it is these spurious breaks that are responsible for the extreme UV sensitivity of these cells.

Nucleotide Excision Repair Disorders and the Balance Between Cancer and Aging

“…Specifically, UV damage induces incisions in the genome in the vicinity of damage without actually committing to repair, likely leading to increased genomic instability. 13,31,32 Thus in the XPCS genome, damage may accumulate more quickly than in TTD or CS, and in spite of elevated gatekeeper activation lead to cancer formation. However, if the problem of cancer formation in XPD-XPCS is increased genomic instability due to an increase in DNA breaks, this is unlikely to explain the cancer predisposition in XPCS cases associated with mutations in XPG and XPG genes.…”

Restoring DNA repair capacity of cells from three distinct diseases by XPD gene-recombinant adenovirus

“…6,7,11 Mutations in the XPD gene can lead to three different diseases: XP, XP/CS and TTD. 5,14,45 There is no extended treatment for these patients, but the techniques based on gene complementation have been largely studied in the hope of prolonging survival and relieve the carcinogenic effects. One efficient way of delivering genes into mammalian cells is the use of a recombinant adenovirus vector, particularly when a high-level expression of transgene products in cultured cells is required.…”

“…48,49 Nevertheless, a spurious DNA degradation, not related to DNA repair, has been described for XP/CS cells, after UVirradiation. 5 The reason how these specific mutations in the XPD gene result in the peculiar properties of the XP/ CS cell lines remains obscure. 5 Anyway both cells, XP and XP/CS, show similar recovery of survival and UDS after virus complementation.…”

“…2,3 NER is a highly conserved mechanism 4 that involves more than 30 proteins acting in a tightly regulated manner. 5 Briefly, the NER pathway consists mainly of six steps: recognition of the lesion carried out basically by the XPC-hHR23B complex (for GGR) or RNA pol II stalling (for TCR); opening of the double helix at the lesion site by the concerted action of the two DNA helicases XPB and XPD; demarcation of the lesion requiring the activity of the XPA and RPA proteins; dual incision of the damaged strand by the XPF and XPG endonucleases; synthesis of DNA in the gap left by the removal of a 24mer-32mer oligonucleotide by the replicative DNA polymerases and PCNA; and ligation to the parental strand by DNA ligase I. 6,7 The XPD protein is a subunit of the TFIIH complex (transcription factor IIH) with a molecular weight of 86.9 kDa and comprising 760 amino acids.…”

“…12,13 Mutations in a single gene involved in NER may lead to three different human disorders: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD) that show distinct clinical phenotypes. 5,14 XP is an autosomal recessive syndrome in which patients have severe sunlight sensitivity that leads to progressive degeneration of sun-exposed regions of the skin and eyes, usually leading to various forms of cutaneous malignancy. 15,16 A significant number of patients present progressive neurological degeneration.…”