Identical mutations in the CSB gene associated with either Cockayne syndrome or the DeSanctis-cacchione variant of xeroderma pigmentosum

Xeroderma pigmentosum (XP) and Cockayne syndrome (CS) are two hereditary disorders in which photosensitivity is associated with distinct clinical and cellular phenotypes and results from genetically different defects. We have identified the primary molecular alteration in two patients in whom clinical manifestations strongly reminiscent of a severe form of XP were unexpectedly associated with the CS cellular phenotype and with a defect in the CSB gene. Sequencing of the CSB -coding region in both cDNA and genomic DNA showed that these patients had identical alterations to those in a patient with the clinical features of the classical form of CS. These data, together with fluorescence in situ hybridization analysis, demonstrated that the two siblings with XP as well as the CS patient were homozygous for the same CSB mutated allele, containing a silent C2830T change and a nonsense mutation C2282T converting Arg735 to a stop codon. The finding that the same inactivating mutation underlies different pathological phenotypes indicates that there is no simple correlation between the molecular defect and the clinical features. Therefore, alterations in the CSB gene give rise to the same repair defect at the cellular level but other genetic and/or environmental factors determine the pathological phenotype.     

Novel XPG (ERCC5) Mutations Affect DNA Repair and Cell Survival after Ultraviolet but not Oxidative Stress

Nucleotide excision repair (NER) is the most flexible of all known DNA‐repair mechanisms, and XPG is a 3′‐endonuclease that participates in NER. Mutations in this gene (ERCC5) may result in the human syndrome xeroderma pigmentosum (XP) and, in some cases, in the complex phenotype of Cockayne syndrome (CS). Two Brazilian XP siblings, who were mildly affected, were investigated and classified into the XP‐G group. The cells from these patients were highly ultraviolet (UV) sensitive but not sensitive to photosensitized methylene blue, an agent that causes oxidative stress. This phenotype is in contrast to XP‐G/CS cells, which are highly sensitive to this oxidative agent. Sequencing revealed a compound heterozygous genotype with two novel missense mutations: c.83C>A (p.Ala28Asp) and c.2904G>C (p.Trp968Cys). The first mutation maps to the catalytic site of the XPG protein, whereas the second may compromise binding to DNA. Functional assays indicated that the mutated alleles were unable to perform the complete repair of UV‐irradiated plasmids; however, full correction was observed for oxidatively damaged plasmids. Therefore, the XP phenotype of these patients is caused by novel missense mutations that specifically affect DNA repair for UV‐ but not oxidative‐stress‐induced DNA damage, and implications for XP versus XP/CS phenotype are discussed.     

Radiation-induced chromatid aberrations in Cockayne syndrome and xeroderma pigmentosum group C fibroblasts in relation to cancer predisposition.

We showed previously that the persistence of chromatid breaks and gaps after G2 phase irradiation with X-rays or near-UV visible light characterizes skin fibroblasts from individuals with cancer-prone genetic diseases. This abnormal response appears to result from deficient DNA repair during G2 and to be associated with cancer proneness. We have, therefore, compared the responses of cells from two genetic disorders, Cockayne syndrome (CS) and xeroderma pigmentosum complementation group C(XP-C), both of which exhibit cellular hypersensitivity to sunlight, but only one of which, XP, manifests a high rate of sunlight-induced cancer. CS cells, in contrast to XP cells, showed a normal G2 response to irradiation with either X-rays or near-UV visible light. However, CS cells showed a deficiency in repair of DNA damage inflicted by light during S and G1 phases of the cell cycle. The present results support the concept that deficient DNA repair during G2 phase plays a role in carcinogenesis. This deficient repair in the presence of DNA damage and continuous cell cycling from activation of proto-oncogenes or loss of suppressor genes may be necessary and sufficient for cancer development.     

New areas of focus at workshop on human diseases involving DNA repair deficiency and premature aging

Researchers and clinicians interested in human diseases of DNA repair deficiency and premature aging gathered at the National Conference Center in Lansdowne, Virginia on 5-8 September 2006 to attend a workshop co-organized by Vilhelm Bohr (National Institute of Aging) and Kenneth Kraemer (National Cancer Institute). An important feature of this workshop was the participation of representatives from xeroderma pigmentosum (XP), Cockayne Syndrome (CS) and trichothiodystrophy (TTD) family support groups. Studies presented at the workshop described important new insights into the phenotypic complexity of XP, CS and TTD, renewed focus on the neurological manifestations of each of these diseases, as well as keen interest in the role of oxidative stress and mitochondrial dysfunction in neurodegenerative processes and normal and/or premature aging. This workshop report summarizes some of the presentations and outcomes of the workshop.     

Different regulation of p53 stability in UV-irradiated normal and DNA repair deficient human cells

The stabilization of p53 protein was studied after UV exposure of normal human skin fibroblasts and cells derived from patients suffering from xeroderma pigmentosum (XP) and trichothiodystrophy (TTD). The data show that p53 is transiently stabilized both in UV-irradiated normal and repair deficient cells. However, particularly at later times after UV irradiation, stabilization of p53 persists much longer in repair deficient XP and TTD cells than in normal cells. The stabilization of p53 was found to be dose-dependent in normal and XP cells. These results indicate that unremoved DNA damage could possibly be responsible for the induction of transient stabilization of p53.     

Woman with UV hypersensitivity and a de novo unbalanced chromosome translocation

We report a Japanese woman with de novo 6p monosomy and 10q trisomy [46,XX,der(6)t(6;10)(p25.1;q25.2)] whose clinical manifestations resemble those of xeroderma pigmentosum (XP) and Cockayne syndrome (CS), known as premature aging syndromes. She had a history of easy sunburning and presented a number of freckles and hypopigmented spots on her face as those of XP. Magnetic resonance imaging and computed tomography scanning demonstrated intracranial abnormalities like those seen in CS. DNA repair studies using the patient's fibroblasts demonstrated hypersensitive responses to ultraviolet (UV). XP, CS, and UV-sensitive syndromes with photosensitivity disturbances have been known as DNA repair abnormalities. However, an association of 6p monosomy with these diseases has not been reported so far. Molecular analysis of the patient we described may contribute to the identification of novel DNA-repair-related gene(s) and/or to the senile mechanism.