Adult-onset neurological degeneration in a patient with Cockayne syndrome and a null mutation in the CSB gene

Next-generation sequencing through multi-gene panel testing for the diagnosis of a Chinese patient with atypical Cockayne syndrome

BACKGROUND

Cockayne syndrome (CS, OMIM #133540, #216400) is a rare autosomal recessive disease involving multiple systems, typically characterized by microcephaly, premature aging, growth retardation, neurosensory abnormalities, and photosensitivity. The age of onset is related to the severity of the clinical phenotype, which may lead to fatal outcomes.

METHODS

We report a 3-year-old girl who presented with photosensitivity, gait abnormalities, stunting, and microcephaly and showed atypical clinical classification due to mild clinical manifestations at an early onset age.

RESULTS

Next-generation sequencing reveals the frameshift mutation (c.394_398del, p.Leu132Asnfs*6) and a novel microdeletion of ERCC8 (exon4del, p.Arg92fs).

CONCLUSION

Therefore, it is still necessary to carry out next-generation sequencing for CS patients with atypical clinical manifestations, which is essential for diagnosis and accurate genetic counseling.

Siblings with Cockayne Syndrome B Type III Presenting with Slowly Progressive Cerebellar Ataxia

Two patients, 48- and 50-year-old sisters, presented with a characteristic facial appearance with slowly progressive deafness and cerebellar ataxia starting in their 30s. Genetic testing identified compound heterozygous pathogenic variants in the ERCC6 gene: c.1583G>A (p.G528E) and c.1873T>G (p.Y625D). A diagnosis of Cockayne syndrome (CS) B type III was made. CS is usually diagnosed in childhood with well-defined facial characteristics and photosensitivity. This case report describes rare cases of adulthood CS with a primary presentation of slowly progressing deafness and cerebellar ataxia. CS should be considered in adults with characteristic facial and skin findings, deafness, and cerebellar ataxia.

A case of Cockayne syndrome with unusually mild clinical manifestations

We present a mild case of Cockayne syndrome that was referred to us with an extreme sunburn at the age of 3. In early teens, although her cutaneous symptoms alleviated without any medications, she developed tremor and dysarthria. Neurological examination and brain imaging suggested demyelination disorders. The patient's cells indicated a reduced recovery of RNA synthesis, which was partially restored by the introduction of CSB (Cockayne Syndrome B)-cDNA. In addition, her cells indicated a substantially reduced level of CSB protein. Despite the insidious progression of neurological symptoms, she gave birth to a child. Such mild cases of Cockayne syndrome may be misdiagnosed.

Reactive Species in Progeroid Syndromes and Aging-Related Processes

Significance: Reactive species have been classically considered causative of age-related degenerative processes, but the scenario appears considerably more complex and to some extent counterintuitive than originally anticipated. The impact of reactive species in precocious aging syndromes is revealing new clues to understand and perhaps challenge the resulting degenerative processes. Recent Advances: Our understanding of reactive species has considerably evolved, including their hormetic effect (beneficial at a certain level, harmful beyond this level), the occurrence of diverse hormetic peaks in different cell types and organisms, and the extended type of reactive species that are relevant in biological processes. Our understanding of the impact of reactive species has also expanded from the dichotomic damaging/signaling role to modulation of gene expression. Critical Issues: These new concepts are affecting the study of aging and diseases where aging is greatly accelerated. We discuss how notions arising from the study of the underlying mechanisms of a progeroid disease, Cockayne syndrome, represent a paradigm shift that may shed a new light in understanding the role of reactive species in age-related degenerative processes. Future Issues: Future investigations urge to explore established and emerging notions to elucidate the multiple contributions of reactive species in degenerative processes linked to pathophysiological aging and their possible amelioration. Antioxid. Redox Signal. 37, 208-228.

Statistical Approach of the Role of the Conserved CSB-PiggyBac Transposase Fusion Protein (CSB-PGBD3) in Genotype-Phenotype Correlation in Cockayne Syndrome Type B

Cockayne syndrome is a rare condition that encompasses a very wide spectrum of clinical severity. Mutations upstream of a transposon called PiggyBac Transposable Element Derived 3 in intron 5 of the CSB/ERCC6 gene could bring about less severe forms than mutations located downstream of that transposon insertion. Our aim was to study genotype-phenotype correlation by determining whether the position of each mutation of the CSB/ERCC6 gene has an impact on the phenotype. A hundred and forty-seven Cockayne patients, who had two pathogenic mutations in the CSB/ERCC6 gene and for whom clinical data was available, were retrospectively selected and included in the study. Data analysis was performed under the Bayesian paradigm. Analysis of the proportion of the different subtypes of Cockayne syndrome according to the position of the mutations was done using an ordinal logistic regression model. Using a vague prior, the risk of developing a more severe subtype when exposed to 2 mutations downstream compared to 2 mutations upstream was 2.0 [0.9–4.5]. Estimations varied through the sensitivity analysis. We could reasonably conclude that a relationship between the number of downstream mutations and the Cockayne syndrome clinical expression exists but it is still difficult to give a precise estimate of this relationship. The real effect could be more complex that the one described in the initial model and other genetic factors might be taken into consideration together with the mutation site to better explain clinical variability.

Novel frame shift mutation in ERCC6 leads to a severe form of Cockayne syndrome with postnatal growth failure and early death: A case report and brief literature review

INTRODUCTION

Cockayne syndrome (CS) is a rare multisystemic autosomal recessive disease. The primary manifestations of which are developmental delay, neurological impairment, abnormal skin sensitivity to sunlight and unique facial appearance as sunken eyes, large ears, and thin large nose. The disorders of the nucleotide excision repair system significantly are caused by mutations of Excision repair cross-complementing group 6 (ERCC6) and Excision repair cross-complementing group 8 (ERCC8) genes, and the ERCC6 gene mutations are present in approximately 65% of cases.

CASE PRESENTATION

Here we described a girl in a consanguineous Jordanian family with abnormal facial appearance and postnatal growth delay. She was not able to gain weight. Her condition deteriorated progressively and she developed difficulty of swallowing even to water. The patient was diagnosed as CS based on her facial appearance and neurologic dysfunction. The patient was examined at 3 years old, and died at 4 years old.

CONCLUSION

Genetic analysis and sequencing revealed homozygosity for a novel frame shift mutation c.2911_2915del5ins9 (p.Lys971TryfsX14) in the ERCC6. The mutation is predicted to delete 5 nucleotides and add 9 nucleotides with a premature termination, resulting in approximately 34% length reduction of the wild-type transcript. The multisystem malformations of CS are clinically heterogeneous. The frame shift mutation of ERCC6 found in this patient is a novel one, which caused postnatal growth failure and early death. Our findings indicate truncated mutation in CS lead to more severe CS phenotype and add to the genotype-phenotype correlations in CS.

Functional and clinical relevance of novel mutations in a large cohort of patients with Cockayne syndrome

BACKGROUND

Cockayne syndrome (CS) is a rare, autosomal recessive multisystem disorder characterised by prenatal or postnatal growth failure, progressive neurological dysfunction, ocular and skeletal abnormalities and premature ageing. About half of the patients with symptoms diagnostic for CS show cutaneous photosensitivity and an abnormal cellular response to UV light due to mutations in either the ERCC8/CSA or ERCC6/CSB gene. Studies performed thus far have failed to delineate clear genotype-phenotype relationships. We have carried out a four-centre clinical, molecular and cellular analysis of 124 patients with CS.

METHODS AND RESULTS

We assigned 39 patients to the ERCC8/CSA and 85 to the ERCC6/CSB genes. Most of the genetic variants were truncations. The missense variants were distributed non-randomly with concentrations in relatively short regions of the respective proteins. Our analyses revealed several hotspots and founder mutations in ERCC6/CSB. Although no unequivocal genotype-phenotype relationships could be made, patients were more likely to have severe clinical features if the mutation was downstream of the PiggyBac insertion in intron 5 of ERCC6/CSB than if it was upstream. Also a higher proportion of severely affected patients was found with mutations in ERCC6/CSB than in ERCC8/CSA.

CONCLUSION

By identifying >70 novel homozygous or compound heterozygous genetic variants in 124 patients with CS with different disease severity and ethnic backgrounds, we considerably broaden the CSA and CSB mutation spectrum responsible for CS. Besides providing information relevant for diagnosis of and genetic counselling for this devastating disorder, this study improves the definition of the puzzling genotype-phenotype relationships in patients with CS.

Xeroderma pigmentosum-Cockayne syndrome complex

Xeroderma pigmentosum-Cockayne syndrome complex is a very rare multisystem degenerative disorder (Orpha: 220295; OMIM: 278730, 278760, 278780, 610651). Published information on XP-CS is mostly scattered throughout the literature. We compiled statistics related to symptom prevalence in XP-CS and have written a clinical description of the syndrome. We also drew on clinical practices used in XP and in Cockayne syndrome without XP to aid management of XP-CS.

Extensive searches of the literature identified 43 XP-CS patients. The diagnosis had been confirmed with molecular or biochemical methods in 42 of them. Clinical features of each patient were summarized in spreadsheets and summary statistics were generated from this data. XP patients are classified into complementation groups according to the gene that is mutated. There are four groups in XP-CS, and classification was available for 42 patients. Twenty-one were in the XP-G complementation group, 13 in XP-D, 5 in XP-B, and 3 in XP-F. Overall, the clinical features of XP-CS are very similar to those of CS without XP, with the exception of skin cancers in XP-CS. However, one intriguing finding was that cancer incidence was lower in XP-CS compared to XP alone or XP-neurological disorder. The cancer rate in XP-CS was higher than in CS without XP, an unsurprising finding. There is preliminary evidence for the existence of severity groups in XP-CS, as is the case in CS.

Although health problems in XP-CS vary both in severity and in when they the first occur, there was overall homogeneity between all complementation groups and putative severity groups. Severely affected patients met fewer milestones and died at younger ages compared to more mildly affected patients.

Uncommon nucleotide excision repair phenotypes revealed by targeted high-throughput sequencing

BACKGROUND

Deficient nucleotide excision repair (NER) activity causes a variety of autosomal recessive diseases including xeroderma pigmentosum (XP) a disorder which pre-disposes to skin cancer, and the severe multisystem condition known as Cockayne syndrome (CS). In view of the clinical overlap between NER-related disorders, as well as the existence of multiple phenotypes and the numerous genes involved, we developed a new diagnostic approach based on the enrichment of 16 NER-related genes by multiplex amplification coupled with next-generation sequencing (NGS).

METHODS

Our test cohort consisted of 11 DNA samples, all with known mutations and/or non pathogenic SNPs in two of the tested genes. We then used the same technique to analyse samples from a prospective cohort of 40 patients. Multiplex amplification and sequencing were performed using AmpliSeq protocol on the Ion Torrent PGM (Life Technologies).

RESULTS

We identified causative mutations in 17 out of the 40 patients (43%). Four patients showed biallelic mutations in the ERCC6(CSB) gene, five in the ERCC8(CSA) gene: most of them had classical CS features but some had very mild and incomplete phenotypes. A small cohort of 4 unrelated classic XP patients from the Basque country (Northern Spain) revealed a common splicing mutation in POLH (XP-variant), demonstrating a new founder effect in this population. Interestingly, our results also found ERCC2(XPD), ERCC3(XPB) or ERCC5(XPG) mutations in two cases of UV-sensitive syndrome and in two cases with mixed XP/CS phenotypes.

CONCLUSIONS

Our study confirms that NGS is an efficient technique for the analysis of NER-related disorders on a molecular level. It is particularly useful for phenotypes with combined features or unusually mild symptoms. Targeted NGS used in conjunction with DNA repair functional tests and precise clinical evaluation permits rapid and cost-effective diagnosis in patients with NER-defects.

Cockayne syndrome group B protein regulates DNA double-strand break repair and checkpoint activation

Mutations of CSB account for the majority of Cockayne syndrome (CS), a devastating hereditary disorder characterized by physical impairment, neurological degeneration and segmental premature aging. Here we report the generation of a human CSB-knockout cell line. We find that CSB facilitates HR and represses NHEJ. Loss of CSB or a CS-associated CSB mutation abrogating its ATPase activity impairs the recruitment of BRCA1, RPA and Rad51 proteins to damaged chromatin but promotes the formation of 53BP1-Rif1 damage foci in S and G2 cells. Depletion of 53BP1 rescues the formation of BRCA1 damage foci in CSB-knockout cells. In addition, knockout of CSB impairs the ATM- and Chk2-mediated DNA damage responses, promoting a premature entry into mitosis. Furthermore, we show that CSB accumulates at sites of DNA double-strand breaks (DSBs) in a transcription-dependent manner. The kinetics of DSB-induced chromatin association of CSB is distinct from that of its UV-induced chromatin association. These results reveal novel, important functions of CSB in regulating the DNA DSB repair pathway choice as well as G2/M checkpoint activation.

A novel ERCC6 splicing variant associated with a mild Cockayne syndrome phenotype

BACKGROUND

Cockayne syndrome is an autosomal recessive, heterogeneous syndrome with classical features, including short stature, microcephaly, developmental delay, neuropathy, and photosensitivity. New genomic approaches offer improved molecular diagnostic potential.

METHODS

Whole-exome sequencing was employed to study a consanguineous extended family with severe short stature and variable presentations of peripheral neuropathy, lipoatrophy, photosensitivity, webbed neck, and hirsutism.

RESULTS

We identified a novel homozygous ERCC6 variant at the donor splice site of intron 9 (c.1992 + 3A>G), which was predicted to only slightly perturb splicing efficiencies. Assessment of primary fibroblast-derived mRNAs, however, revealed a dominant splicing species that utilized an unsuspected putative donor splice site within exon 9, resulting in predicted early protein termination (p.Arg637Serfs*34).

CONCLUSIONS

We describe a new splicing ERCC6 defect causal of Cockayne syndrome. The application of exome sequence analysis was integral to diagnosis, given the complexity of phenotypic presentation in the affected family members. The novel splicing defect, furthermore, illustrates how a seemingly minor change in the relative strength of a splice site can have significant biological consequences.

Tracking the Cognitive, Social, and Neuroanatomical Profile in Early Neurodegeneration: Type III Cockayne Syndrome

Cockayne syndrome (CS) is an autosomal recessive disease associated with premature aging, progressive multiorgan degeneration, and nervous system abnormalities including cerebral and cerebellar atrophy, brain calcifications, and white matter abnormalities. Although several clinical descriptions of CS patients have reported developmental delay and cognitive impairment with relative preservation of social skills, no previous studies have carried out a comprehensive neuropsychological and social cognition assessment. Furthermore, no previous research in individuals with CS has examined the relationship between brain atrophy and performance on neuropsychological and social cognition tests. This study describes the case of an atypical late-onset type III CS patient who exceeds the mean life expectancy of individuals with this pathology. The patient and a group of healthy controls underwent a comprehensive assessment that included multiple neuropsychological and social cognition (emotion recognition, theory of mind, and empathy) tasks. In addition, we compared the pattern of atrophy in the patient to controls and to its concordance with ERCC8 gene expression in a healthy brain. The results showed memory, language, and executive deficits that contrast with the relative preservation of social cognition skills. The cognitive profile of the patient was consistent with his pattern of global cerebral and cerebellar loss of gray matter volume (frontal structures, bilateral cerebellum, basal ganglia, temporal lobe, and occipito-temporal/occipito-parietal regions), which in turn was anatomically consistent with the ERCC8 gene expression level in a healthy donor’s brain. The study of exceptional cases, such as the one described here, is fundamental to elucidating the processes that affect the brain in premature aging diseases, and such studies provide an important source of information for understanding the problems associated with normal and pathological aging.

Blinded by the UV light: how the focus on transcription-coupled NER has distracted from understanding the mechanisms of Cockayne syndrome neurologic disease

Cockayne syndrome (CS) is a devastating neurodevelopmental disorder, with growth abnormalities, progeriod features, and sun sensitivity. CS is typically considered to be a DNA repair disorder, since cells from CS patients have a defect in transcription-coupled nucleotide excision repair (TC-NER). However, cells from UV-sensitive syndrome patients also lack TC-NER, but these patients do not suffer from the neurologic and other abnormalities that CS patients do. Also, the neurologic abnormalities that affect CS patients (CS neurologic disease) are qualitatively different from those seen in NER-deficient XP patients. Therefore, the TC-NER defect explains the sun sensitive phenotype common to both CS and UVsS, but cannot explain CS neurologic disease. However, as CS neurologic disease is of much greater clinical significance than the sun sensitivity, there is a pressing need to understand its molecular basis. While there is evidence for defective repair of oxidative DNA damage and mitochondrial abnormalities in CS cells, here I propose that the defects in transcription by both RNA polymerases I and II that have been documented in CS cells provide a better explanation for many of the severe growth and neurodevelopmental defects in CS patients than defective DNA repair. The implications of these ideas for interpreting results from mouse models of CS, and for the development of treatments and therapies for CS patients are discussed.

Cockayne syndrome: the expanding clinical and mutational spectrum

Cockayne syndrome is a progressive multisystem disorder characterized by a specific cellular defect in transcription-coupled repair. Typical features include developmental delay, failure to thrive, microcephaly, cutaneous photosensitivity, dental anomalies, progressive hearing loss, pigmentary retinopathy, cataracts and enophthalmia. Various levels of severity have been described including the "classical" or moderate type I CS, the early-onset or severe type II and the mild or late-onset type III. Adult-onset cases with prolonged survival and normal initial development have also been identified. At the opposite end of the scale, the most severely affected patients, showing a prenatal onset of the symptoms, are overlapping with the cerebro-oculo-facio-skeletal (COFS) syndrome. These overlapping subtypes build a continuous spectrum without clear thresholds. Revised diagnostic criteria are proposed to improve the recognition of the disease. Two thirds of the patients are linked to mutations in the CSB (ERCC6) gene, one third to mutations in the CSA (ERCC8) gene. At least 78 different mutations are known in the CSB gene and 30 in the CSA gene to date, in more than 120 genetically confirmed patients. Large clinical and molecular databases are needed to unravel genotype-phenotype correlations and to gain more insight into the underlying molecular mechanisms.

What role (if any) does the highly conserved CSB-PGBD3 fusion protein play in Cockayne syndrome?

The PGBD3 piggyBac transposon inserted into CSB intron 5 early in the primate lineage. As a result of alternative splicing, the human CSB gene now encodes three proteins: CSB, a CSB-PGBD3 fusion protein that joins the N-terminal CSB domain to the C-terminal PGBD3 transposase domain, and PGBD3 transposase. The fusion protein is as highly conserved as CSB, suggesting that it is advantageous in health; however, expression of the fusion protein in CSB-null cells induces a constitutive interferon (IFN) response. The fusion protein binds in vivo to PGBD3-related MER85 elements, but is also tethered to c-Jun, TEAD1, and CTCF motifs by interactions with the cognate transcription factors. The fusion protein regulates nearby genes from the c-Jun (and to a lesser extent TEAD1 and CTCF) motifs, but not from MER85 elements. We speculate that the fusion protein interferes with CSB-dependent chromatin remodeling, generating double-stranded RNA (dsRNA) that induces an IFN response through endosomal TLR or cytoplasmic RIG-I and/or MDA5 RNA sensors. We suggest that the fusion protein was fixed in primates because an elevated IFN response may help to fight viral infection. We also speculate that an inappropriate IFN response may contribute to the clinical presentation of CS.

The conserved Cockayne syndrome B-piggyBac fusion protein (CSB-PGBD3) affects DNA repair and induces both interferon-like and innate antiviral responses in CSB-null cells

Cockayne syndrome is a segmental progeria most often caused by mutations in the CSB gene encoding a SWI/SNF-like ATPase required for transcription-coupled DNA repair (TCR). Over 43Mya before marmosets diverged from humans, a piggyBac3 (PGBD3) transposable element integrated into intron 5 of the CSB gene. As a result, primate CSB genes now generate both CSB protein and a conserved CSB-PGBD3 fusion protein in which the first 5 exons of CSB are alternatively spliced to the PGBD3 transposase. Using a host cell reactivation assay, we show that the fusion protein inhibits TCR of oxidative damage but facilitates TCR of UV damage. We also show by microarray analysis that expression of the fusion protein alone in CSB-null UV-sensitive syndrome (UVSS) cells induces an interferon-like response that resembles both the innate antiviral response and the prolonged interferon response normally maintained by unphosphorylated STAT1 (U-STAT1); moreover, as might be expected based on conservation of the fusion protein, this potentially cytotoxic interferon-like response is largely reversed by coexpression of functional CSB protein. Interestingly, expression of CSB and the CSB-PGBD3 fusion protein together, but neither alone, upregulates the insulin growth factor binding protein IGFBP5 and downregulates IGFBP7, suggesting that the fusion protein may also confer a metabolic advantage, perhaps in the presence of DNA damage. Finally, we show that the fusion protein binds in vitro to members of a dispersed family of 900 internally deleted piggyBac elements known as MER85s, providing a potential mechanism by which the fusion protein could exert widespread effects on gene expression. Our data suggest that the CSB-PGBD3 fusion protein is important in both health and disease, and could play a role in Cockayne syndrome.

Age-related neuronal degeneration: complementary roles of nucleotide excision repair and transcription-coupled repair in preventing neuropathology

Neuronal degeneration is a hallmark of many DNA repair syndromes. Yet, how DNA damage causes neuronal degeneration and whether defects in different repair systems affect the brain differently is largely unknown. Here, we performed a systematic detailed analysis of neurodegenerative changes in mouse models deficient in nucleotide excision repair (NER) and transcription-coupled repair (TCR), two partially overlapping DNA repair systems that remove helix-distorting and transcription-blocking lesions, respectively, and that are associated with the UV-sensitive syndromes xeroderma pigmentosum (XP) and Cockayne syndrome (CS). TCR–deficient Csa^−/− and Csb^−/− CS mice showed activated microglia cells surrounding oligodendrocytes in regions with myelinated axons throughout the nervous system. This white matter microglia activation was not observed in NER–deficient Xpa^−/− and Xpc^−/− XP mice, but also occurred in Xpd^XPCS mice carrying a point mutation (G602D) in the Xpd gene that is associated with a combined XPCS disorder and causes a partial NER and TCR defect. The white matter abnormalities in TCR–deficient mice are compatible with focal dysmyelination in CS patients. Both TCR–deficient and NER–deficient mice showed no evidence for neuronal degeneration apart from p53 activation in sporadic (Csa^−/−, Csb^−/−) or highly sporadic (Xpa^−/−, Xpc^−/−) neurons and astrocytes. To examine to what extent overlap occurs between both repair systems, we generated TCR–deficient mice with selective inactivation of NER in postnatal neurons. These mice develop dramatic age-related cumulative neuronal loss indicating DNA damage substrate overlap and synergism between TCR and NER pathways in neurons, and they uncover the occurrence of spontaneous DNA injury that may trigger neuronal degeneration. We propose that, while Csa^−/− and Csb^−/− TCR–deficient mice represent powerful animal models to study the mechanisms underlying myelin abnormalities in CS, neuron-specific inactivation of NER in TCR–deficient mice represents a valuable model for the role of NER in neuronal maintenance and survival.

Metabolism produces reactive oxygen species that damage our DNA and other cellular components, and as such it contributes to the aging process, including neuronal degeneration. Accordingly, genetic disorders associated with impaired DNA damage repair are frequently associated with premature onset of aging pathology in a variety of tissues, including the brain. This is well-illustrated by the progeroid DNA repair syndromes xeroderma pigmentosum (XP) and Cockayne syndrome (CS), in which patients suffer from defects in nucleotide excision repair (NER) and transcription-coupled repair (TCR), two partially overlapping DNA repair systems that remove helix-distorting and transcription-blocking lesions, respectively. We have used a panel of XP and CS mice (including conditional double-mutant animals) to systematically investigate the impact of NER and TCR defects on neuronal degeneration. We have shown that, whereas a TCR defect causes white matter pathology, a NER defect can result in age related cumulative loss of neurons. These findings well match the neuropathology observed in CS and XP patients, underscoring the impact of spontaneous DNA damage in the onset of neuronal aging. Therefore, the XP and CS mouse models serve as valuable tools to delineate intervention strategies that combat age-associated pathology of the brain.

A comprehensive description of the severity groups in Cockayne syndrome

Cockayne syndrome (CS) is a rare degenerative disorder with a common set of symptoms but a very wide variation in phenotype. The variation is sufficiently wide that CS patients have traditionally been described in three different severity groups. Unfortunately, there is no single source for information about the different severity groups. This problem can complicate not only diagnosis, but accurate prognosis as well. The goal of this study was to describe the phenotypic variation in CS as completely as possible. This article provides extensive descriptions of traits common to each group and their degree of severity in each group. Forty-five people with CS were surveyed and information from the published literature was used to increase the sample sizes for calculations. This study provides new information, including statistical data for each of the three severity groups (mean age at death, average head circumference, and average length or stature). The study includes cerebro-oculo-facial syndrome (COFS) as a severe form of CS, based on results of recently published genetic studies performed by other authors. This study proposes revised names for CS severity groups: severe, moderate, and mild. The groups were formerly called Type II/early onset CS, Type I/classical CS, and Type III/atypical/mild/late-onset CS, respectively. A fourth newly documented group, UV sensitivity only/adult onset, is also described. Average ages of death were calculated as 5.0 years (severe), 16.1 years (moderate), and 30.3 years (mild).

Mutation update for the CSB/ERCC6 and CSA/ERCC8 genes involved in Cockayne syndrome

Cockayne syndrome is an autosomal recessive multisystem disorder characterized principally by neurological and sensory impairment, cachectic dwarfism, and photosensitivity. This rare disease is linked to mutations in the CSB/ERCC6 and CSA/ERCC8 genes encoding proteins involved in the transcription-coupled DNA repair pathway. The clinical spectrum of Cockayne syndrome encompasses a wide range of severity from severe prenatal forms to mild and late-onset presentations. We have reviewed the 45 published mutations in CSA and CSB to date and we report 43 new mutations in these genes together with the corresponding clinical data. Among the 84 reported kindreds, 52 (62%) have mutations in the CSB gene. Many types of mutations are scattered along the whole coding sequence of both genes, but clusters of missense mutations can be recognized and highlight the role of particular motifs in the proteins. Genotype-phenotype correlation hypotheses are considered with regard to these new molecular and clinical data. Additional cases of molecular prenatal diagnosis are reported and the strategy for prenatal testing is discussed. Two web-based locus-specific databases have been created to list all identified variants and to allow the inclusion of future reports (www.umd.be/CSA/ and www.umd.be/CSB/).

Hereditary photodermatoses

Photodermatoses are defined as the abnormal reactions of the skin to photons, usually those of wavelengths found in sunlight. These reactions can be caused by a wide variety of reasons, including defects in repair oflight-induced DNA lesions, the interaction of certain chemicals or medications with sunlight to produce toxic mediators and photo-induced immune reactions. In this chapter we will describe photodermatoses that are associated with hereditary conditions. These can be subdivided into several groups: dermatoses caused by abnormal metabolic conditions, idiopathic photodermatoses, defects in cancer suppressor genes not directly involved in DNA repair but that predispose to photodistributed tumors and photosensitivity due to abnormalities in DNA repair pathways. Special emphasis will be placed on the relatively recently described UV-sensitive syndrome.

A UV-sensitive syndrome patient with a specific CSA mutation reveals separable roles for CSA in response to UV and oxidative DNA damage

UV-sensitive syndrome (UV(S)S) is a recently-identified autosomal recessive disorder characterized by mild cutaneous symptoms and defective transcription-coupled repair (TC-NER), the subpathway of nucleotide excision repair (NER) that rapidly removes damage that can block progression of the transcription machinery in actively-transcribed regions of DNA. Cockayne syndrome (CS) is another genetic disorder with sun sensitivity and defective TC-NER, caused by mutations in the CSA or CSB genes. The clinical hallmarks of CS include neurological/developmental abnormalities and premature aging. UV(S)S is genetically heterogeneous, in that it appears in individuals with mutations in CSB or in a still-unidentified gene. We report the identification of a UV(S)S patient (UV(S)S1VI) with a novel mutation in the CSA gene (p.trp361cys) that confers hypersensitivity to UV light, but not to inducers of oxidative damage that are notably cytotoxic in cells from CS patients. The defect in UV(S)S1VI cells is corrected by expression of the WT CSA gene. Expression of the p.trp361cys-mutated CSA cDNA increases the resistance of cells from a CS-A patient to oxidative stress, but does not correct their UV hypersensitivity. These findings imply that some mutations in the CSA gene may interfere with the TC-NER-dependent removal of UV-induced damage without affecting its role in the oxidative stress response. The differential sensitivity toward oxidative stress might explain the difference between the range and severity of symptoms in CS and the mild manifestations in UV(s)S patients that are limited to skin photosensitivity without precocious aging or neurodegeneration.

An abundant evolutionarily conserved CSB-PiggyBac fusion protein expressed in Cockayne syndrome

Cockayne syndrome (CS) is a devastating progeria most often caused by mutations in the CSB gene encoding a SWI/SNF family chromatin remodeling protein. Although all CSB mutations that cause CS are recessive, the complete absence of CSB protein does not cause CS. In addition, most CSB mutations are located beyond exon 5 and are thought to generate only C-terminally truncated protein fragments. We now show that a domesticated PiggyBac-like transposon PGBD3, residing within intron 5 of the CSB gene, functions as an alternative 3′ terminal exon. The alternatively spliced mRNA encodes a novel chimeric protein in which CSB exons 1–5 are joined in frame to the PiggyBac transposase. The resulting CSB-transposase fusion protein is as abundant as CSB protein itself in a variety of human cell lines, and continues to be expressed by primary CS cells in which functional CSB is lost due to mutations beyond exon 5. The CSB-transposase fusion protein has been highly conserved for at least 43 Myr since the divergence of humans and marmoset, and appears to be subject to selective pressure. The human genome contains over 600 nonautonomous PGBD3-related MER85 elements that were dispersed when the PGBD3 transposase was last active at least 37 Mya. Many of these MER85 elements are associated with genes which are involved in neuronal development, and are known to be regulated by CSB. We speculate that the CSB-transposase fusion protein has been conserved for host antitransposon defense, or to modulate gene regulation by MER85 elements, but may cause CS in the absence of functional CSB protein.

For reasons that are still unclear, genetic defects in DNA repair can cause diseases that resemble aspects of premature ageing (“segmental progerias”). Cockayne syndrome (CS) is a particularly devastating progeria most commonly caused by mutations in the CSB chromatin remodeling gene. About 43 million years ago, before humans diverged from marmosets, one of the last PiggyBac transposable elements to invade the human lineage landed within intron 5 of the 21 exon CSB gene. As a result, the CSB locus now encodes two equally abundant proteins generated by alternative mRNA splicing: the original full length CSB protein, and a novel CSB-PiggyBac fusion protein in which the N-terminus of CSB is fused to the complete PiggyBac transposase. Conservation of the CSB-PiggyBac fusion protein since marmoset suggests that it is normally beneficial, demonstrating once again that “selfish” transposable elements can be exploited or “domesticated” by the host. More importantly, almost all CSB mutations that cause CS continue to make the CSB-PiggyBac fusion protein, whereas a mutation that compromises both does not cause CS. Thus the fusion protein which is beneficial in the presence of functional CSB may be harmful in its absence. This may help clarify the cause of CS and other progerias.

Do all of the neurologic diseases in patients with DNA repair gene mutations result from the accumulation of DNA damage?

The classic model for neurodegeneration due to mutations in DNA repair genes holds that DNA damage accumulates in the absence of repair, resulting in the death of neurons. This model was originally put forth to explain the dramatic loss of neurons observed in patients with xeroderma pigmentosum neurologic disease, and is likely to be valid for other neurodegenerative diseases due to mutations in DNA repair genes. However, in trichiothiodystrophy (TTD), Aicardi-Goutières syndrome (AGS), and Cockayne syndrome (CS), abnormal myelin is the most prominent neuropathological feature. Myelin is synthesized by specific types of glial cells called oligodendrocytes. In this review, we focus on new studies that illustrate two disease mechanisms for myelin defects resulting from mutations in DNA repair genes, both of which are fundamentally different than the classic model described above. First, studies using the TTD mouse model indicate that TFIIH acts as a co-activator for thyroid hormone-dependent gene expression in the brain, and that a causative XPD mutation in TTD results in reduction of this co-activator function and a dysregulation of myelin-related gene expression. Second, in AGS, which is caused by mutations in either TREX1 or RNASEH2, recent evidence indicates that failure to degrade nucleic acids produced during S-phase triggers activation of the innate immune system, resulting in myelin defects and calcification of the brain. Strikingly, both myelin defects and brain calcification are both prominent features of CS neurologic disease. The similar neuropathology in CS and AGS seems unlikely to be due to the loss of a common DNA repair function, and based on the evidence in the literature, we propose that vascular abnormalities may be part of the mechanism that is common to both diseases. In summary, while the classic DNA damage accumulation model is applicable to the neuronal death due to defective DNA repair, the myelination defects and brain calcification seem to be better explained by quite different mechanisms. We discuss the implications of these different disease mechanisms for the rational development of treatments and therapies.