From a DNA helicase to brittle hair

The XPD subunit of TFIIH is required for transcription-associated but not DNA double-strand break-induced recombination in mammalian cells

Mutations in the XPD gene can give rise to three phenotypically distinct disorders: xeroderma pigmentosum (XP), trichothiodystrophy (TTD) or combined XP and Cockayne syndrome (CS) (XP/CS). The role of Xeroderma Pigmentosum group D protein (XPD) in nucleotide excision repair explains the increased risk of skin cancer in XP patients but not all the clinical phenotypes found in XP/CS or TTD patients. Here, we describe that the XPD-defective UV5 cell line is impaired in transcription-associated recombination (TAR), which can be reverted by the introduction of the wild-type XPD gene expressed from a vector. UV5 cells are defective in TAR, despite having intact transcription and homologous recombination (HR) repair of DNA double-strand breaks (DSBs). Interestingly, we find reduced spontaneous HR in XPD-defective cells, suggesting that transcription underlies a portion of spontaneous HR events. We also report that transcription-coupled repair (TCR)-defective cells, mutated in the Cockayne syndrome B (CSB) protein, have a defect in TAR, but not in DSB-induced HR. However, the TAR defect may be associated with a general transcription defect in CSB-deficient cells. In conclusion, we show a novel role for the XPD protein in TAR, linking TAR with TCR.

Ocular manifestations in the inherited DNA repair disorders

Deoxyribonucleic acid (DNA) repair is a fundamental process designed to keep the integrity of genomic DNA that is continuously challenged by intrinsic or environmental induced alterations. Numerous genes involved in DNA repair have been cloned and are involved in different DNA repair pathways: base excision repair, nucleotide excision repair, mismatch repair, DNA recombination. Inherited conditions due to mutations in DNA repair genes include mainly: xeroderma pigmentosum, Cockayne syndrome, Trichothiodystrophy, Bloom syndrome, Rothmund-Thomson syndrome, and Werner syndrome. Minor to major ocular manifestations occur in these syndromes. For example, eyelid skin cancers in xeroderma pigmentosum and retinal dystrophy in Cockayne syndrome are major features of these syndromes. This review focuses on the DNA repair pathways, the general and ocular features of the related syndromes, the laboratory tests useful for diagnosis, and the general processes implied with DNA repair (ultraviolet sensitivity, carcinogenesis, apoptosis, oxydative stress, and premature aging).

Effects of XPD mutations on ultraviolet-induced apoptosis in relation to skin cancer-proneness in repair-deficient syndromes

To understand the relationship between DNA repair, apoptosis, transcription, and cancer-proneness, we have studied the apoptotic response and the recovery of RNA synthesis following ultraviolet C and ultraviolet B irradiation in nucleotide excision repair deficient diploid fibroblasts from the cancer-prone xeroderma pigmentosum (XP) syndrome patients and the non-cancer-prone trichothiodystrophy (TTD) patients. Analysis of four XPD and four TTD/XPD fibroblast strains presenting different mutations on the XPD gene has shown that XPD cells are more sensitive to ultraviolet-induced apoptosis than TTD/XPD cells, and this response seems to be modulated by the type and the location of the mutation on the XPD gene. Moreover, the other xeroderma pigmentosum fibroblast strains analyzed (groups A and C) are more sensitive to undergo apoptosis after ultraviolet irradiation than normal human fibroblasts, showing that the cancer-proneness of xeroderma pigmentosum patients is not due to a deficiency in the ultraviolet-induced apoptotic response. We have also found that cells from transcription-coupled repair deficient XPA, XPD, TTD/XPD, and Cockayne's syndrome patients undergo apoptosis at lower ultraviolet doses than transcription-coupled repair proficient cells (normal human fibroblasts and XPC), indicating that blockage of RNA polymerase II at unrepaired lesions on the transcribed strand is the trigger. Moreover, XPD and XPA cells are more sensitive to ultraviolet-induced apoptosis than trichothiodystrophy and Cockayne's syndrome fibroblasts, suggesting that both cyclobutane pyrimidine dimers and pyrimidine 6-4 pyrimidone on the transcribed strand trigger apoptosis. Finally, we show that apoptosis is directly proportional to the level of inhibition of transcription, which depends on the density of ultraviolet-induced lesions occurring on transcribed sequences.

Ectodermal dysplasias: not only 'skin' deep

The ectodermal dysplasias (EDs) are a large and complex nosologic group of diseases; more than 170 different pathologic clinical conditions have been identified. Despite the great number of EDs described so far, few causative genes have been identified. We review EDs in the light of the most recent molecular findings and propose a new classification of EDs integrating both molecular-genetic data and corresponding clinical findings of related diseases.

Nucleotide excision repair and human syndromes

DNA damage is implicated in cancer and aging, and several DNA repair mechanisms exist that safeguard the genome from these deleterious consequences. Nucleotide excision repair (NER) removes a wide diversity of lesions, the main of which include UV-induced lesions, bulky chemical adducts and some forms of oxidative damage. The NER process involves the action of at least 30 proteins in a 'cut-and-paste'-like mechanism. The consequences of a defect in one of the NER proteins are apparent from three rare recessive syndromes: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and the photosensitive form of the brittle hair disorder trichothiodystrophy (TTD). Sun-sensitive skin is associated with skin cancer predisposition in the case of XP, but remarkably not in CS and TTD. Moreover, the spectrum of clinical symptoms differs considerably between the three syndromes. CS and TTD patients exhibit a spectrum of neurodevelopmental abnormalities and, in addition, TTD is associated with ichthyosis and brittle hair. These typical CS and TTD abnormalities are difficult to comprehend as a consequence of defective NER. This review briefly describes the biochemistry of the NER process, summarizes the clinical features of the NER disorders and speculates on the molecular basis underlying these pleitropic syndromes.