Cockayne syndrome complementation group B associated with xeroderma pigmentosum phenotype

Two siblings have been reported whose clinical manifestations (cutaneous photosensitivity and central nervous system dysfunction) are strongly reminiscent of the DeSanctis-Cacchione syndrome (DCS) variant of xeroderma pigmentosum (XP), a severe form of XP. Fibroblasts from the siblings showed UV sensitivity, a failure of recovery of RNA synthesis (RRS) after UV irradiation, and a normal level of unscheduled DNA synthesis (UDS), which were, unexpectedly, the biochemical characteristics usually associated with Cockayne syndrome (CS). However, no complementation group assignment in these cells has yet been performed. We here report that these patients can be assigned to CS complementation group B (CSB) by cell fusion complementation analysis. To our knowledge, these are the first patients with defects in the CSB gene to be associated with an XP phenotype. The results imply that the gene product from the CSB gene must interact with the gene products involved in excision repair and associated with XP.     

Xeroderma pigmentosum and Cockayne syndrome: overlapping clinical and biochemical phenotypes.

Two siblings are described whose clinical presentation of cutaneous photosensitivity and central nervous system dysfunction is strongly reminiscent of the DeSanctis-Cacchione syndrome (DCS) variant of xeroderma pigmentosum. An extensive clinical evaluation supported a diagnosis of DCS and documented previously unreported findings. In vitro fibroblast studies showed UV sensitivity that was two to three times that of normal controls. However, neither a post-UV-irradiation DNA excision-repair defect indicative of XP nor a semiconservative DNA replication defect indicative of XP variant was found. Rather, a failure of RNA synthesis to recover to normal levels after UV exposure was observed, a biochemical abnormality seen in Cockayne syndrome (CS), one of the premature-aging syndromes with clinical UV sensitivity. These patients, therefore, clinically have XP, but their biochemical characteristics suggest CS. The reason(s) for the severe neurologic disease, in light of the relatively mild cutaneous abnormalities, is unclear. Other cases with unusual fibroblast responses to irradiation have been noted in the literature and, along with the data from our patients, reinforce the notion of the complexity of DNA maintenance and repair.     

Xeroderma pigmentosum complementation group G associated with Cockayne syndrome.

Xeroderma pigmentosum (XP) and Cockayne syndrome (CS) are two rare inherited disorders with a clinical and cellular hypersensitivity to the UV component of the sunlight spectrum. Although the two traits are generally considered as clinically and genetically distinct entities, on the biochemical level a defect in the nucleotide excision-repair (NER) pathway is involved in both. Classical CS patients are primarily deficient in the preferential repair of DNA damage in actively transcribed genes, whereas in most XP patients the genetic defect affects both "preferential" and "overall" NER modalities. Here we report a genetic study of two unrelated, severely affected patients with the clinical characteristics of CS but with a biochemical defect typical of XP. By complementation analysis, using somatic cell fusion and nuclear microinjection of cloned repair genes, we assign these two patients to XP complementation group G, which previously was not associated with CS. This observation extends the earlier identification of two patients with a rare combined XP/CS phenotype within XP complementation groups B and D, respectively. It indicates that some mutations in at least three of the seven genes known to be involved in XP also can result in a picture of partial or even full-blown CS. We conclude that the syndromes XP and CS are biochemically closely related and may be part of a broader clinical disease spectrum. We suggest, as a possible molecular mechanism underlying this relation, that the XPGC repair gene has an additional vital function, as shown for some other NER genes.     

Clinical and biochemical studies in three patients with severe early infantile Cockayne syndrome

Summary We present clinical and biochemical data from three patients with severe Cockayne syndrome (CS) of very early onset. Unlike in classic CS, signs became evident in the first weeks of life and led to unusually early death. Fibroblasts from two of the patients showed a complete defect of the repair of UV-induced thymine dimer lesions. They were unable to remove thymine dimer lesions from their DNA, had a severe reduction of the RNA synthesis rates after UV irradiation, and showed no reactivation of an UV-inactivated indicator gene and no DNA recondensation after UV irradiation. DNA repair investigated in these two fibroblast cell strains resembled that of xeroderma pigmentosum cells of complementation group A. In contrast, fibroblasts from the third patient showed the same in vitro repair characteristics as classic CS cells.     

Detection of reduced RNA synthesis in UV-irradiated Cockayne syndrome group B cells using an isolated nuclear system

In aqueous methyl linoleate emulsions (pH 7.4, 25 degrees C, air-saturated), nitrosylmyoglobin and saturated fatty acid anions (palmitate and stearate investigated) each showed antioxidant effect on metmyoglobin-induced peroxidation as measured by oxygen depletion rate. For equimolar concentration of nitrosylmyoglobin and metmyoglobin and for metmyoglobin in moderate excess, a reduction in oxygen consumption rate of approximately 70% was observed. Fatty acid anions reduced oxygen consumption rate most significantly for palmitate (up to 60% for a fatty acid:heme protein ratio of 90:1). No further antioxidative effect was seen for fatty acid anions in the presence of nitrosylmyoglobin, whereas nitrosylmyoglobin showed a further antioxidant effect in presence of fatty acid anions in the metmyoglobin-catalyzed process. The antioxidative mechanism of nitrosylmyoglobin and fatty acid anions is different, and while the fatty acid anions seem active in inhibiting initiation of oxidation through protection against metmyoglobin activation into perferrylmyoglobin, as shown by freeze-quench Electron Spin Resonance (ESR) spectroscopy, nitrosylmyoglobin is rather active in the oxygen consuming (propagation) phase.     

Oxidative damage-induced PCNA complex formation is efficient in xeroderma pigmentosum group A but reduced in Cockayne syndrome group B cells.

Proliferating cell nuclear antigen (PCNA), a processivity factor for DNA polymerases delta and epsilon, is essential for both DNA replication and repair. PCNA is required in the resynthesis step of nucleotide excision repair (NER). After UV irradiation, PCNA translocates into an insoluble protein complex, most likely associated with the nuclear matrix. It has not previously been investigated in vivo whether PCNA complex formation also takes place after oxidative stress. In this study, we have examined the involvement of PCNA in the repair of oxidative DNA damage. PCNA complex formation was studied in normal human cells after treatment with hydrogen peroxide, which generates a variety of oxidative DNA lesions. PCNA was detected by two assays, immunofluorescence and western blot analyses. We observed that PCNA redistributes from a soluble to a DNA-bound form during the repair of oxidative DNA damage. PCNA complex formation was analyzed in two human natural mutant cell lines defective in DNA repair: xeroderma pigmentosum group A (XP-A) and Cockayne syndrome group B (CS-B). XP-A cells are defective in overall genome NER while CS-B cells are defective only in the preferential repair of active genes. Immunofluorescent detection of PCNA complex formation was similar in normal and XP-A cells, but was reduced in CS-B cells. Consistent with this observation, western blot analysis in CS-B cells showed a reduction in the ratio of PCNA relocated as compared to normal and XP-A cells. The efficient PCNA complex formation observed in XP-A cells following oxidative damage suggests that formation of PCNA-dependent repair foci may not require the XPA gene product. The reduced PCNA complex formation observed in CS-B cells suggests that these cells are defective in the processing of oxidative DNA damage.     

Manitoba aboriginal kindred with original cerebro-oculo- facio-skeletal syndrome has a mutation in the Cockayne syndrome group B (CSB) gene

Cerebro-oculo-facio-skeletal (COFS) syndrome is a rapidly progressive neurological disorder leading to brain atrophy with calcification, cataracts, microcornea, optic atrophy, progressive joint contractures, and growth failure. Cockayne syndrome (CS) is a recessively inherited neurodegenerative disorder characterized by low-to-normal birth weight; growth failure; brain dysmyelination with calcium deposits; cutaneous photosensitivity; pigmentary retinopathy, cataracts, or both; and sensorineural hearing loss. CS cells are hypersensitive to UV radiation because of impaired nucleotide excision repair of UV radiation-induced damage in actively transcribed DNA. The abnormalities in CS are associated with mutations in the CSA or CSB genes. In this report, we present evidence that two probands related to the Manitoba Aboriginal population group within which COFS syndrome was originally reported have cellular phenotypes indistinguishable from those in CS cells. The identical mutation was detected in the CSB gene from both children with COFS syndrome and in both parents of one of the patients. This mutation was also detected in three other patients with COFS syndrome from the Manitoba Aboriginal population group. These results suggest that CS and COFS syndrome share a common pathogenesis.     

A C. elegans homolog of the Cockayne syndrome complementation group A gene

Cockayne syndrome (CS) is a debilitating and complex disorder that results from inherited mutations in the CS complementation genes A and B, CSA and CSB. The links between the molecular functions of the CS genes and the complex pathophysiology of CS are as of yet poorly understood and are the subject of intense debate. While mouse models reflect the complexity of CS, studies on simpler genetic models might shed new light on the consequences of CS mutations. Here we describe a functional homolog of the human CSA gene in Caenorhabditis elegans. Similar to its human counterpart, mutations in the nematode csa-1 gene lead to developmental growth defects as a consequence of DNA lesions.