Mechanisms of Ageing and Development 134 (2013) 284–290 Contents lists available at SciVerse ScienceDirect Mechanisms of Ageing and Development jo urnal homepage: www.elsevier.com/locate/mechagedev Conceptual developments in the causes of Cockayne syndrome a, b a c,1 James E. Cleaver *, Vladimir Bezrookove , Ingrid Revet , Eric J. Huang a Department of Dermatology, University of California San Francisco, 2340 Sutter Street, San Francisco, CA 94143, United States b Center for Melanoma Research & Treatment, California Pacific Medical Center & Research Institute, San Francisco, CA, United States c Department of Pathology, University of California San Francisco, San Francisco, CA 94143, United States A R T I C L E I N F O A B S T R A C T Article history: Cockayne syndrome is an autosomal recessive disease that covers a wide range of symptoms, from mild Available online 18 February 2013 photosensitivity to severe neonatal lethal disorder. The pathology of Cockayne syndrome may be caused by several mechanisms such as a DNA repair deficiency, transcription dysregulation, altered redox Keywords: balance and mitochondrial dysfunction. Conceivably each of these mechanisms participates during a Ultraviolet light different stage in life of a Cockayne syndrome patient. Endogenous reactive oxygen is considered as an Reactive oxygen ultimate cause of DNA damage that contributes to Cockayne syndrome pathology. Here we demonstrate Mitochondria that mitochondrial reactive oxygen does not cause detectable nuclear DNA damage. This observation Transcription coupled repair implies that a significant component of Cockayne syndrome pathology may be due to abnormal Neurodegeneration mitochondrial function independent of nuclear DNA damage. The source of nuclear DNA damage to central nervous system tissue most likely occurs from extrinsic neurotransmitter signaling. ß 2013 Elsevier Ireland Ltd. All rights reserved. 1. Cockayne syndrome: genes and transcription coupled repair recover has been less well investigated and may be secondary to the impact of CS mutations on RNA transcription. Cockayne syndrome (CS) is a neurocutaneous disorder named The discovery that repair was more rapid in transcribed genes, in the 1950s after a dermatologist who practiced in the early and regulated by CS genes, was based on a series of direct decades of the 20th century (Marie et al., 1958). Cellular studies in measurements of photoproduct excision in selected genes (Mans- the 1970s discovered that CS fibroblasts were sensitive to killing by bridge and Hanawalt, 1983; Venema et al., 1990). Repair in the ultraviolet (UV) light (Schmickel et al., 1977), ushering the alpha satellite sequence, a major repeat sequence, was found to be subsequent avalanche of studies on the molecular characterization less efficient than in the rest of the genome (Zolan et al., 1982). of this disorder (Table 1). The UV sensitivity was shown to be Highly expressed genes (dhfr, p53) were more rapidly repaired associated with a failure of CS cells to recover RNA and DNA than the remainder of the genome (Bohr et al., 1985; Mellon et al., synthesis after UV damage (Lehmann et al., 1979; Cleaver, 1982). 1986; Evans et al., 1993). Repair was faster in the transcribed This property was used to demonstrate that there were two genes strand than in the nontranscribed strand (Bohr et al., 1985; Mellon uniquely involved in the disease, named CSA and CSB (also ERCC8, et al., 1987). Detailed studies of photoproduct repair in one gene, ERCC6 respectively) and that there was overlap with another UV PGK1, showed wide variation in repair rates between the sensitive disease xeroderma pigmentosum (XP) (Lehmann, 1982). transcribed and nontranscribed strands, as well as additional The failure of RNA synthesis to recover after irradiation was found variations depending on nucleotide position, transcription start to correlate with loss of rapid excision repair subsequently site and the binding of transcription factors (Pfeifer et al., 1991; discovered in actively transcribed genes (transcription coupled Gao et al., 1994). repair, TCR (Mansbridge and Hanawalt, 1983; Venema et al., 1990; The CSB protein has a number of properties that can be Sancar, 1996; Wood, 1997)). The arrest of RNA Pol II at a damaged demonstrated in vitro. It has a nucleotide binding site and acts as a site is now recognized as the initiating signal for TCR (Fig. 1) DNA-dependent ATPase (Selby and Sancar, 1997; Citterio et al., (Lindsey-Boltz and Sancar, 2007). The failure of DNA synthesis to 1998). The CSB protein can actively wrap the DNA (Beerens et al., 2005) and has strand-annealing capacity (Muftuoglu et al., 2006). CSB associates with PCNA that may be the mechanism by which CS-B cells have reduced recovery of DNA synthesis (Balajee et al., * Corresponding author at: Dermatology and Pharmaceutical Chemistry, Department of Dermatology, Box 0808, Room N431, University of California San 1999). How many of these properties are directly related to the Francisco, San Francisco, CA 94143-0808, United States. Tel.: +1 415 476 4563; clinical disorders remains to be established. Proteins in vitro, may fax: +1 415 476 8218. reveal properties that are normally suppressed in vivo or lack E-mail addresses: [email protected] (J.E. Cleaver), [email protected] necessary modifications such as phosphorylation, polyADP-ribo- (V. Bezrookove), [email protected] (E.J. Huang). 1 sylation or ubiquitylation that occur in vivo. Tel.: +1 415 476 8525. 0047-6374/$ – see front matter ß 2013 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mad.2013.02.005 J.E. Cleaver et al. / Mechanisms of Ageing and Development 134 (2013) 284–290 285 Table 1 Cellular and in vivo consequences of Cockayne mutations. CSA (ERCC8) CSB (ERCC6) UVSSA Clinical features Growth failure Yes Yes No Aging Yes Yes No Neurodegeneration Yes Yes No Skin photosensitivity Yes Yes Yes Cellular features UV sensitivity Yes Yes Yes a a ROS sensitivity Yes Yes No Transcription coupled repair Reduced Reduced Reduced RNA synthesis recovery Reduced Reduced Reduced a Although most reports describe CS-A cells as being sensitive to exogenous Fig. 1. Nuclear and mitochondrial functions of the RNA Pol II cofactors CSA, CSB, reactive oxygen, two reports found human and mouse CS-A cells less sensitive than UVSSA and USP7 that regulate transcription of damaged DNA and facilitate CS-B cells (de Waard et al., 2004; D’Errico et al., 2007) and one had normal cleavage enhanced repair of the transcribed strand. CSA and CSB are also located in the of 8-OH-Gua in vitro whereas CS-B cells lacked this activity (D’Errico et al., 2007). mitochondria where they play a role in autophagy and electron transport. Nuclear s UVSSA cells have been less intensively investigated but two UV patients with either functions regulate gene transcription and responses to exogenous stresses. a CSA or a UVSSA mutation showed a normal response to ROS (Nardo et al., 2009; Mutations in mitochondrial functions may cause accumulation of damaged Spivak and Hanawalt, 2006). mitochondria from endogenous oxidative stress that may lead to neurodegeneration (Wallace, 2005; Cleaver, 2012). s Once the association of transcription with repair was estab- (Graham et al., 2001), and the mild photosensitive UV syndrome lished by direct measurements in a small number of genes, (Fujiwara et al., 1981; Itoh et al., 1995). followed by the discovery of the central role of the transcription COFS syndrome is also an autosomal, recessively inherited and factor TFIIH in nucleotide excision repair (NER) (Schaeffer et al., rapidly progressive, neurologic disorder. The disease leads to brain 1993), little attention was devoted to further direct measurement microcephaly and atrophy with calcifications, cataracts, micro- of rates of repair in specific genes and different functional states of cornea, optic atrophy, progressive joint contractures, and growth the genome. There has yet to be a whole genome assessment of failure. COFS appears to be a particularly severe developmental and rates of repair. Whether additional levels of regulation beyond neurological disease with mutations in CSB, XPG, XPD or ERCC1 transcription can affect repair remains unanswered. Recent whole (Graham et al., 2001; Niedernhofer et al., 2006; Jaspers et al., 2007; genome sequencing studies have demonstrated that most of the Laugel et al., 2008). All of these genes are associated with genome is transcribed into RNA at some stage, raising questions of transcription regulation either as cofactors of RNA pol II or the definition of TCR and its restriction to protein-coding regions of components of transcription factor TFIIH. s the genome. Some UV-sensitive patients, defined as the UV syndrome, lack the neurological pathology of CS but their cells are UV sensitive and defective in TCR (Table 1) (Fujiwara et al., 1981; Cleaver et al., 1992; 2. Clinical complexity of CS I, II, UVS, COFS Itoh et al., 1995). Some of these represent mild manifestations of mutations in CSA (Nardo et al., 2009; Fei and Che, 2012) and CSB, CS is an autosomal recessive disease in which the clinical remarkably one resulting in no detectable CSB protein (Horibata disorders can be classified in three general areas of growth failure, et al., 2004). Others appear to be genuinely different (Itoh et al., 1994, s pathological aging and neurodegeneration (Nance and Berry, 1992; 1995). A gene identified as mutated in several UV patients encodes Weidenheim et al., 2009). Symptoms involve many organ systems another RNA pol II