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Photoimmunology and nucleotide excision repair: impact of transcription coupled and global genome excision repair
Journal of Photochemistry and Photobiology B: Biology 56 (2001) 97–100 www.elsevier.com / locate / jphotobiol
Photoimmunology and nucleotide excision repair: impact of transcription coupled and global genome excision repair a, b Leon H.F. Mullenders *, Mark Berneburg a
Department of Radiation Genetics and Chemical Mutagenesis–Medical Genetics Center, Leiden University Medical Center, Wassenaarseweg 72, 2333 AL, Leiden, The Netherlands b ¨ , Germany Clinical and Experimental Photodermatology, Dept of Dermatology, Heinrich-Heine-University, Moorenstr. 5, D-40225 Dusseldorf
Abstract Ultraviolet (UV) light generates damage to DNA which is removed by a versatile mechanism called nucleotide excision repair (NER). There are two subpathways for NER: the transcription coupled repair (TCR) pathway which removes DNA damage from actively transcribed genes and the global genome repair pathway which removes damage throughout the genome. Most types of DNA lesions are processed more rapidly by TCR than by GGR. It is widely accepted that immunological processes play a pivotal role in the generation of skin tumours induced by exposure to ultraviolet light and first evidence is emerging that GGR and TCR play different roles in skin reactions such as erythema and delayed type hypersensitivity. The relationship between UV-induced responses of the skin and the two NER subpathways is discussed. 2001 Elsevier Science B.V. All rights reserved. Keywords: Photoimmunology; Nucleotide excision repair; Skin reactions; Transcription coupled repair; UV sensitive disorders
1. Introduction Exposure of cells or organisms to ultraviolet (UV) light leads to the formation of DNA damage. The most frequent types of DNA damage inflicted by UV-light are cyclobutyl–pyrimidone dimers (CPD), 6-4 cyclobutyl pyrimidone photoproducts (6-4 PP) and their Dewar isomers. UV-induced DNA damage is removed from the genome by a versatile mechanism called nucleotide excision repair (NER), a highly conserved and tightly regulated sequential repair pathway involving the interaction of at least 25 proteins [1–3]. NER is a complex process in which basically the following steps can be distinguished: (i) recognition of a DNA lesion; (ii) single strand incision at both sides of the lesion; (iii) excision of the lesion containing single stranded DNA fragment; (iv) DNA repair synthesis to replace the excised nucleotides and (v) ligation of the remaining single stranded nick. The heterodimer protein complex XPC-hHR23B is the primary damage recognition factor [3,4] and upon infliction of DNA damage, this complex will be recruited to sites of DNA *Corresponding author. Tel.: 131-71-527-6126; fax: 131-71-5276173. E-mail address: [email protected] (L.H.F. Mullenders).
lesions; in the case of UV exposure, the kinetics of preincision complex formation strongly suggest that XPChHR23B binds preferentially to 6-4PP. Subsequently the incision complex is formed by recruitment of the basic transcription factor TFIIH, the XPA–RPA protein complex and the two structure specific endonucleases XPG and XPF-ERCC1 capable to incise the DNA 39 and 59 respectively of the lesion. NER comprises at least two subpathways: the fast transcription coupled repair (TCR) pathway which removes DNA damage from actively transcribed genes and the global genome repair pathway which removes damage from the rest of the genome in a slower fashion than TCR [5]. Transcription-coupled repair (TCR) first described by Mellon and Hanawalt for cultured mammalian cells [6], specifically removes DNA lesions from the transcribed strand of an active gene. All data indicate that TCR is directly coupled to active transcription and it is generally assumed that a stalled transcript provides a strong signal to attract the repair machinery. In contrast to GGR, TCR does not require a functional XPC-HR23B complex for recognition of DNA lesion in the transcribed strand. Until now it is not clear how repair is coupled to transcription. Genetic analysis has put some light on specific factors that play a role in TCR. Among the various UV sensitive human cells, Cockayne syndrome cells appear to be unable to perform transcription-coupled repair, whereas the global repair
1011-1344 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S1011-1344( 01 )00244-5
L.H.F. Mullenders, M. Berneburg / Journal of Photochemistry and Photobiology B: Biology 65 (2001) 97 – 100
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pathway is functioning normally. This defect in transcription-coupled repair has been related to the inability of CS cells to restore UV-inhibited RNA synthesis [7]. Deficiencies in proteins involved in NER can lead to three clinically distinct autosomal recessive syndromes: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD) [8]. Patients with mutations in the NER gene XPA are deficient in both pathways TCR and GGR, whereas patients defective in the XPC gene and the CS genes are deficient in GGR and TCR respectively. For these three clinically distinct phenotypes (XPA, XPC and CSB) knockout mice have been generated allowing to carry out investigations being infeasible in the human background ethically [9]. Although transgenic mice display striking similarities in UV response compared to humans, repair of photolesions is different between mouse and human cells. Most notably, rodent cells are deficient in repair of UV-induced CPD in the bulk DNA and hence exhibit a defect in GGR of CPD [10]. In contrast TCR of CPD in these cells appears to be very efficient, resulting in selective repair of CPD in the transcribed strand of expressed genes. Human cells exhibit both GGR and TCR of CPD resulting in the preferential repair of CPD in the transcribed strand of expressed genes. There has been substantial evidence that defects in NER genes are not only associated with defective repair of DNA damage but also with alterations of immunological responses [11–13]. While general immunological parameters such as leucocyte cell number and natural killer cell activity appear to be unchanged, there are observations indicating that NER-deficient cells demonstrate abnormal post-UV reactions such as erythema and suppression of contact hypersensitivity [14,15]. Recent reports employing NER-deficient mice indicate that NER deficiencies do not influence immunological responses homogeneously but that the two NER pathways TCR and GGR may contribute differentially to photoimmunological processes occurring after exposure to ultraviolet light. In this brief review the different influences of TCR and GGR on acute responses / immunological reactions following exposure to ultraviolet B light in humans and DNA repair deficient mice will be discussed.
2. NER pathways and acute skin response after UVB irradiation The hairless mouse model has been employed to study acute skin reactions upon infliction of DNA damage by UVB light [14,16]. To study the influence of various NER pathways, transgenic mice carrying defined mutations in XP or CS genes, have been generated and crossed into the hairless background. To estimate the minimal erythema dose (MED), the various XP and CS genotypes were exposed for increasing time periods to UVB light emitted by a Kromayer lamp. All mice showed a clear UV inflammatory response (redness and swelling of the exposed skin) but the dose required to induce this response differed dramatically for the various genotypes: when compared to wild-type mice the MED was not reduced in XPC mice, but appeared to be strongly reduced in XPA and CS (see Table 1). Hence, mice lacking TCR (XPA, CS) exhibit a MED value that is about a factor 10 lower than that of their wild-type littermates, whereas mice lacking GGR only (e.g., XPC mice) have an MED that is not lower than that of their wild-type littermates [16–18]. TTD mice exhibiting 25% residual repair capacity displayed only a very mild increase in MED when compared to wild-type animals [18], suggesting that TTD mice are capable to perform TCR. Indeed, human fibroblasts from TTD patients are capable to perform TCR albeit at a reduced level (Mullenders, unpublished results). The data obtained with the mice are in line with the observations made with XP patients in Japanese studies. XPA patients had a low MED [19,20] whereas XPC patients showed MED within the range of healthy humans [21]. Although there are reports of XPC patients that have ‘sun sensitive’ skin, this condition may well have resulted from accumulated effects of a series of successive (suberythemal) exposures, and not from an acute sunburn. In contrast to XPC patients, XPA patients are clearly skin cancer prone and have a low MED [19,21]. The majority of XPD and XPE patients appear to have a low MED and are mildly skin cancer prone, i.e., most of them get skin cancer but at a much later age than XPA patients [22]. Most XPF patients have an MED within the range of healthy humans,
Table 1 NER pathways and acute responses to UVB light Strain
GGR
TCR
MED a,b
Hyperplasia a,b
Apoptosis a,b
MID a,c
Wild-type XPA XPC CSB
11 22 22 11
11 22 11 22
1000 100 1000 100
500 40 500 40
2000 250 2000 250
900 150 900 900
a
Values in J / m 2 . Data from van Oosten et al. [16]; Berg et al. [14]. c Data from Garssen et al. [18]. b
L.H.F. Mullenders, M. Berneburg / Journal of Photochemistry and Photobiology B: Biology 65 (2001) 97 – 100
and the ones that get skin cancers also get them at a relatively late age [20]. In addition to the profound protecting effect of expression of TCR against UVB induced erythema it has become evident that like in cultured cells, expression of the TCR pathway enables damaged cells to progress through Sphase, but prevents the induction of apoptosis and hyperplasia in the mouse epidermis [16]. Thus, at moderate UVB dose (5-fold the MED values for XPA and CS mice) XPA and CSB mice display an accumulation of apoptotic cells in the epidermis whereas no such apoptotic reaction is seen in XPC and wild-type mice. Only at much higher dose apoptosis is seen in the latter two genotypes. Hence, the hyperplasia response seems to be coupled to the apoptotic response as a massive renewal of dead cells. Since TCR is the principal pathway that can restore stalled transcription at sites of UV photolesions in the transcribed strand of transcriptionally active genes, we speculate that the capability to restore UVB inhibited transcription, thereby counteracting apoptosis, prevents hypersensitivity towards acute effects of sunlight (erythema, hyperplasia) in a yet unknown way. One possible mechanism would be the expression of specific genes after UVB radiation; such a response might depend on active TCR. The augmented production of cytokines in UVB irradiated CSB and XPA mice, but not in XPC mice [23], might be consistent with altered RNA synthesis after UVB exposure.
3. NER pathways and systemic immunosuppression after UVB irradiation It is well documented that UV-B exposure also impairs specific and nonspecific immune responses [24] and that this impairment exerts deleterious effects on human health. To evaluate the role of immunological processes in photocarcinogenesis the allergic delayed type hypersensitivity reaction has been employed as a model for presentation of tumour associated antigens (TAA) in which haptens are used as a surrogate for TAAs. In this background elegant studies by Kripke et al. demonstrated that in previously UVB-irradiated mice tumours did continue to grow unlike in unirradiated animals where the tumours were rejected through a sufficiently mounted immune response [25]. In line with the notion that UVB plays a role in photoimmunology is the finding that it leads to the expression of an array of cytokines such as IL-1, IL-6 and TNF-a [26] and that TNF-a and IL-10 are capable of suppressing delayed type hypersensitivity [27,28]. In addition, UV-B exposure has been demonstrated to impair resistance to bacterial, viral, parasitic and fungal infections. It is important to note that the effects of UV are not restricted to skin-associated infections, but also to systemic (non-skin-associated) infections. Part of these effects might be directly induced by UVB
99
induced DNA damage. This is demonstrated by the observations that photoreactivation of UVB induced CPD or enhanced excision repair of CPD by introduction of repair enzymes (by liposomes containing a CPD specific glycosylase [15,29]) counteracted suppression of contact hypersensitivity (CHS) locally as well as systemically. Additional evidence for a significant role of DNA damage in UV-B-induced immunosuppression was provided by studies exploring NER-deficient mice. The sensitivity of four transgenic mouse models (i.e, defective in either the XPA, XPC, CSB, and TTD gene) was studied for UVinduced systemic immunomodulation (delayed type of hypersensitivity (DTH) and contact hypersensitivity (CHS)). In contrast to the results seen for the induction of erythema and edema, the UVB induced immune effects are much more pronounced in XPA mice than in the CSB mice. XPC and TTD mice were not significantly sensitive to the UVB induced immunomodulation [18,22].
4. Conclusion Taken together, the published reports point at different roles of GGR and TCR in skin reactions such as UVB induced erythema or delayed type hypersensitivity. More data are needed to clarify the precise roles of NER subpathways in photoimmunological processes but the present data indicates that GGR and TCR are essential for the prevention of immunomodulation by UVB irradiation, whereas functional TCR is sufficient to protect against UVB induced erythema / edema.
Acknowledgements This work was supported by the Dutch Cancer Society grant no. UU 97-1531 and CA-44247 from the USPHS (ECF). M. Berneburg is supported by the Deutsche Forschungsgemeinschaft Emmy Noether Programm grant no. Be 2005 / 2-1.
References [1] A. Aboussekhra, M. Biggerstaf, M.K.K. Shivji, J.A. Vilpo, V. ¨ Moncollin, V.N. Podust, M. Protic, U. Hubscher, J.-M. Egly, R.D. Wood, Mammalian DNA nucleotide excision repair reconstituted with purified protein components, Cell 80 (1995) 859–868. [2] W. de Laat, N.G.J. Jaspers, J.H.J. Hoeijmakers, Molecular mechanism of nucleotide excision repair, Genes Dev. 13 (1999) 768–785. ´ P. Karmakar, W. Schul, W. Vermeulen, J.H.J. [3] M. Volker, M.J. Mone, Hoeijmakers, R. van Driel, A.A. van Zeeland, L.H.F. Mullenders, Sequential assembly of the nucleotide excision repair factors in vivo, Mol. Cell 8 (2001) 1–20. [4] K. Sugasawa, J.M.Y. Ng, C. Masutani, S. Iwai, P.J. van der Spek, A.P.M. Eker, F. Hanaoka, D. Bootsma, J.H.J. Hoeijmakers, Xeroderma pigmentosum group C protein complex is the initiator of
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L.H.F. Mullenders, M. Berneburg / Journal of Photochemistry and Photobiology B: Biology 65 (2001) 97 – 100 global genome nucleotide excision repair, Mol. Cell 2 (1998) 223–232. L.H.F. Mullenders, Transcription response and nucleotide excision repair, Mutat Res. 409 (1998) 59–64. I. Mellon, G. Spivak, P.C. Hanawalt, Selective removal of transcription-blocking DNA damage from the transcribed strand of the mammalian DHFR gene, Cell 51 (1987) 241–249. L.V. Mayne, A.R. Lehmann, Failure of RNA synthesis to recover after UV irradiation: an early defect in cells from individuals with Cockayne’s syndrome and xeroderma pigmentosum, Cancer Res. 42 (1982) 1473–1478. M. Berneburg, A.R. Lehmann, Xeroderma pigmentosum and related disorders: defects in DNA repair and transcription (In Process Citation), Adv. Genet. 43 (2001) 71–102. J. de Boer, J.H. Hoeijmakers, Cancer from the outside, aging from the inside: mouse models to study the consequences of defective nucleotide excision repair, Biochimie 81 (1999) 127–137. R.J.T. Hendrik, C.M.J. Seelen, P.H.M. Lohman, H. van Kranen, A.A. van Zeeland, L.H.F. Mullenders, Strand-specific removal of cyclobutane pyrimidine dimers from the p53 gene in the epidermis of UVB-irradiated hairless mice, Oncogene 9 (1994) 3427–3432. C. Ahrens, M. Grewe, M. Berneburg, S. Grether-Beck, X. Quilliet, M. Mezzina, A. Sarasin, A.R. Lehmann, C.F. Arlett, J. Krutmann, Photocarcinogenesis and inhibition of intercellular adhesion molecule 1 expression in cells of DNA-repair-defective individuals, Proc. Natl. Acad. Sci. USA 94 (1997) 6837–6841. P.G. Norris, G.A. Limb, A.S. Hamblin, J.L. Hawk, Impairment of natural-killer-cell activity in xeroderma pigmentosum [letter], N. Engl. J. Med. 319 (1988) 1668–1669. A.A. Gaspari, T.A. Fleisher, K.H. Kraemer, Impaired interferon production and natural killer cell activation in patients with the skin cancer-prone disorder, xeroderma pigmentosum, J. Clin. Invest. 92 (1993) 1135–1142. R.J.W. Berg, H.J.T. Ruven, A.T. Sands, F.R. de Gruijl, L.H.F. Mullenders, Defective global genome repair in XPC mice is associated with skin cancer susceptibility but not with sensitivity to UVB-induced erythema and edema, J. Invest. Dermatol. 110 (1998) 405–409. A.A. Vink, A.M. Moodycliffe, V. Shreedhar, S.E. Ullrich, L. Roza, D.B. Yarosh, M.L. Kripke, The inhibition of antigen-presenting activity of dendritic cells resulting from UV irradiation of murine skin is restored by in vitro photorepair of cyclobutane pyrimidine dimers, Proc. Natl. Acad. Sci. USA 94 (1997) 5255–5260. M. van Oosten, H. Rebel, E.C. Friedberg, H. van Steeg, G.T. van der Horst, H.J. Kranen, A. Westermann, A.A. van Zeeland, L.H.F. Mullenders, F. de Gruijl, Differential role of transcription coupled repair in UVB-induced G2 arrest and apoptosis in mouse epidermis, Proc. Natl. Acad. Sci. USA 97 (2000) 11268–11273.
[17] H. Miyauchi-Hashimoto, K. Tanaka, T. Horio, Enhanced inflammation and immunosuppression by ultraviolet radiation in xeroderma pigmentosum group A (XPA) model mice, J. Invest. Dermatol. 107 (1996) 343–348. [18] J. Garssen, H. van Steeg, F. de Gruijl, J. de Boer, G.T. van der Horst, H. van Kranen, H. van Loveren, M. van Dijk, A. Fluitman, G. Weeda, J.H. Hoeijmakers, Transcription-coupled and global genome repair differentially influence UV-B-induced acute skin effects and systemic immunosuppression, J. Immunol. Jun. 164 (2000) 6199– 6205. [19] M. Ichihashi, Y. Fujiwara, Clinical and photobiological characteristics of Japanese xeroderma pigmentosum variant, Br. J. Dermatol. 105 (1981) 1–12. [20] S. Kondo, A. Mamada, C. Miyamoto, C.H. Keong, Y. Satoh, Y. Fujiwara, Late onset of skin cancers in 2 xeroderma pigmentosum group F siblings and a review of 30 Japanese xeroderma pigmentosum patients in groups D, E and F, Photodermatol. 6 (1989) 89–95. [21] S. Kondo, S. Fukuro, K. Nishioka, Y. Satoh, Xeroderma pigmentosum: recent clinical and photobiological aspects, J. Dermatol. 19 (1992) 690–695. [22] A. Mamada, S. Kondo, A. Kawada, Y. Satoh, Y. Fujiwara, Delayed sensorineural deafness and skin carcinogenesis in a Japanese xeroderma pigmentosum group D patient, Photodermatol. 5 (1988) 83–91. [23] A. Boonstra, A. van Oudenaren, M. Baert, H. van Steeg, P.J. Leenen, G.T. van der Horst, J.H. Hoeijmakers, H.F. Savelkoul, J. Garssen, Differential ultraviolet-B-induced immunomodulation in XPA, XPC, and CSB DNA repair-deficient mice, J. Invest. Dermatol. 117 (2001) 141–146. [24] B.J. Vermeer, M. Hurks, The clinical relevance of immunosuppression by UV irradation, J. Photochem. Photobiol. B 24 (1994) 149–154. [25] M.L. Kripke, Immunologic mechanisms in UV radiation carcinogenesis, Adv. Cancer Res. 34 (1981) 69–106. [26] R.D. Granstein, Cytokines and photocarcinogenesis, Photochem. Photobiol. 63 (1996) 390–394. [27] T. Yoshikawa, J.W. Streilein, Tumor necrosis factor-alpha and ultraviolet B light have similar effects on contact hypersensitivity in mice, Reg. Immunol. 3 (1990) 139–144. [28] J.M. Rivas, S.E. Ullrich, The role of IL-4, IL-10 and TNFalpha in the immune suppression induced by ultraviolet radiation, J. Leukocyte Biol. 56 (1994) 769–775. [29] L.A. Applegate, R.D. Ley, J. Alcalay, M.L. Kripke, Identification of the molecular target for the suppression of contact hypersensitivity by ultraviolet radiation, J. Exp. Med. 170 (1989) 1117–1131.
Photoimmunology and nucleotide excision repair: impact of transcription coupled and global genome excision repair a, b Leon H.F. Mullenders *, Mark Berneburg a
Department of Radiation Genetics and Chemical Mutagenesis–Medical Genetics Center, Leiden University Medical Center, Wassenaarseweg 72, 2333 AL, Leiden, The Netherlands b ¨ , Germany Clinical and Experimental Photodermatology, Dept of Dermatology, Heinrich-Heine-University, Moorenstr. 5, D-40225 Dusseldorf
Abstract Ultraviolet (UV) light generates damage to DNA which is removed by a versatile mechanism called nucleotide excision repair (NER). There are two subpathways for NER: the transcription coupled repair (TCR) pathway which removes DNA damage from actively transcribed genes and the global genome repair pathway which removes damage throughout the genome. Most types of DNA lesions are processed more rapidly by TCR than by GGR. It is widely accepted that immunological processes play a pivotal role in the generation of skin tumours induced by exposure to ultraviolet light and first evidence is emerging that GGR and TCR play different roles in skin reactions such as erythema and delayed type hypersensitivity. The relationship between UV-induced responses of the skin and the two NER subpathways is discussed. 2001 Elsevier Science B.V. All rights reserved. Keywords: Photoimmunology; Nucleotide excision repair; Skin reactions; Transcription coupled repair; UV sensitive disorders
1. Introduction Exposure of cells or organisms to ultraviolet (UV) light leads to the formation of DNA damage. The most frequent types of DNA damage inflicted by UV-light are cyclobutyl–pyrimidone dimers (CPD), 6-4 cyclobutyl pyrimidone photoproducts (6-4 PP) and their Dewar isomers. UV-induced DNA damage is removed from the genome by a versatile mechanism called nucleotide excision repair (NER), a highly conserved and tightly regulated sequential repair pathway involving the interaction of at least 25 proteins [1–3]. NER is a complex process in which basically the following steps can be distinguished: (i) recognition of a DNA lesion; (ii) single strand incision at both sides of the lesion; (iii) excision of the lesion containing single stranded DNA fragment; (iv) DNA repair synthesis to replace the excised nucleotides and (v) ligation of the remaining single stranded nick. The heterodimer protein complex XPC-hHR23B is the primary damage recognition factor [3,4] and upon infliction of DNA damage, this complex will be recruited to sites of DNA *Corresponding author. Tel.: 131-71-527-6126; fax: 131-71-5276173. E-mail address: [email protected] (L.H.F. Mullenders).
lesions; in the case of UV exposure, the kinetics of preincision complex formation strongly suggest that XPChHR23B binds preferentially to 6-4PP. Subsequently the incision complex is formed by recruitment of the basic transcription factor TFIIH, the XPA–RPA protein complex and the two structure specific endonucleases XPG and XPF-ERCC1 capable to incise the DNA 39 and 59 respectively of the lesion. NER comprises at least two subpathways: the fast transcription coupled repair (TCR) pathway which removes DNA damage from actively transcribed genes and the global genome repair pathway which removes damage from the rest of the genome in a slower fashion than TCR [5]. Transcription-coupled repair (TCR) first described by Mellon and Hanawalt for cultured mammalian cells [6], specifically removes DNA lesions from the transcribed strand of an active gene. All data indicate that TCR is directly coupled to active transcription and it is generally assumed that a stalled transcript provides a strong signal to attract the repair machinery. In contrast to GGR, TCR does not require a functional XPC-HR23B complex for recognition of DNA lesion in the transcribed strand. Until now it is not clear how repair is coupled to transcription. Genetic analysis has put some light on specific factors that play a role in TCR. Among the various UV sensitive human cells, Cockayne syndrome cells appear to be unable to perform transcription-coupled repair, whereas the global repair
1011-1344 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S1011-1344( 01 )00244-5
L.H.F. Mullenders, M. Berneburg / Journal of Photochemistry and Photobiology B: Biology 65 (2001) 97 – 100
98
pathway is functioning normally. This defect in transcription-coupled repair has been related to the inability of CS cells to restore UV-inhibited RNA synthesis [7]. Deficiencies in proteins involved in NER can lead to three clinically distinct autosomal recessive syndromes: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD) [8]. Patients with mutations in the NER gene XPA are deficient in both pathways TCR and GGR, whereas patients defective in the XPC gene and the CS genes are deficient in GGR and TCR respectively. For these three clinically distinct phenotypes (XPA, XPC and CSB) knockout mice have been generated allowing to carry out investigations being infeasible in the human background ethically [9]. Although transgenic mice display striking similarities in UV response compared to humans, repair of photolesions is different between mouse and human cells. Most notably, rodent cells are deficient in repair of UV-induced CPD in the bulk DNA and hence exhibit a defect in GGR of CPD [10]. In contrast TCR of CPD in these cells appears to be very efficient, resulting in selective repair of CPD in the transcribed strand of expressed genes. Human cells exhibit both GGR and TCR of CPD resulting in the preferential repair of CPD in the transcribed strand of expressed genes. There has been substantial evidence that defects in NER genes are not only associated with defective repair of DNA damage but also with alterations of immunological responses [11–13]. While general immunological parameters such as leucocyte cell number and natural killer cell activity appear to be unchanged, there are observations indicating that NER-deficient cells demonstrate abnormal post-UV reactions such as erythema and suppression of contact hypersensitivity [14,15]. Recent reports employing NER-deficient mice indicate that NER deficiencies do not influence immunological responses homogeneously but that the two NER pathways TCR and GGR may contribute differentially to photoimmunological processes occurring after exposure to ultraviolet light. In this brief review the different influences of TCR and GGR on acute responses / immunological reactions following exposure to ultraviolet B light in humans and DNA repair deficient mice will be discussed.
2. NER pathways and acute skin response after UVB irradiation The hairless mouse model has been employed to study acute skin reactions upon infliction of DNA damage by UVB light [14,16]. To study the influence of various NER pathways, transgenic mice carrying defined mutations in XP or CS genes, have been generated and crossed into the hairless background. To estimate the minimal erythema dose (MED), the various XP and CS genotypes were exposed for increasing time periods to UVB light emitted by a Kromayer lamp. All mice showed a clear UV inflammatory response (redness and swelling of the exposed skin) but the dose required to induce this response differed dramatically for the various genotypes: when compared to wild-type mice the MED was not reduced in XPC mice, but appeared to be strongly reduced in XPA and CS (see Table 1). Hence, mice lacking TCR (XPA, CS) exhibit a MED value that is about a factor 10 lower than that of their wild-type littermates, whereas mice lacking GGR only (e.g., XPC mice) have an MED that is not lower than that of their wild-type littermates [16–18]. TTD mice exhibiting 25% residual repair capacity displayed only a very mild increase in MED when compared to wild-type animals [18], suggesting that TTD mice are capable to perform TCR. Indeed, human fibroblasts from TTD patients are capable to perform TCR albeit at a reduced level (Mullenders, unpublished results). The data obtained with the mice are in line with the observations made with XP patients in Japanese studies. XPA patients had a low MED [19,20] whereas XPC patients showed MED within the range of healthy humans [21]. Although there are reports of XPC patients that have ‘sun sensitive’ skin, this condition may well have resulted from accumulated effects of a series of successive (suberythemal) exposures, and not from an acute sunburn. In contrast to XPC patients, XPA patients are clearly skin cancer prone and have a low MED [19,21]. The majority of XPD and XPE patients appear to have a low MED and are mildly skin cancer prone, i.e., most of them get skin cancer but at a much later age than XPA patients [22]. Most XPF patients have an MED within the range of healthy humans,
Table 1 NER pathways and acute responses to UVB light Strain
GGR
TCR
MED a,b
Hyperplasia a,b
Apoptosis a,b
MID a,c
Wild-type XPA XPC CSB
11 22 22 11
11 22 11 22
1000 100 1000 100
500 40 500 40
2000 250 2000 250
900 150 900 900
a
Values in J / m 2 . Data from van Oosten et al. [16]; Berg et al. [14]. c Data from Garssen et al. [18]. b
L.H.F. Mullenders, M. Berneburg / Journal of Photochemistry and Photobiology B: Biology 65 (2001) 97 – 100
and the ones that get skin cancers also get them at a relatively late age [20]. In addition to the profound protecting effect of expression of TCR against UVB induced erythema it has become evident that like in cultured cells, expression of the TCR pathway enables damaged cells to progress through Sphase, but prevents the induction of apoptosis and hyperplasia in the mouse epidermis [16]. Thus, at moderate UVB dose (5-fold the MED values for XPA and CS mice) XPA and CSB mice display an accumulation of apoptotic cells in the epidermis whereas no such apoptotic reaction is seen in XPC and wild-type mice. Only at much higher dose apoptosis is seen in the latter two genotypes. Hence, the hyperplasia response seems to be coupled to the apoptotic response as a massive renewal of dead cells. Since TCR is the principal pathway that can restore stalled transcription at sites of UV photolesions in the transcribed strand of transcriptionally active genes, we speculate that the capability to restore UVB inhibited transcription, thereby counteracting apoptosis, prevents hypersensitivity towards acute effects of sunlight (erythema, hyperplasia) in a yet unknown way. One possible mechanism would be the expression of specific genes after UVB radiation; such a response might depend on active TCR. The augmented production of cytokines in UVB irradiated CSB and XPA mice, but not in XPC mice [23], might be consistent with altered RNA synthesis after UVB exposure.
3. NER pathways and systemic immunosuppression after UVB irradiation It is well documented that UV-B exposure also impairs specific and nonspecific immune responses [24] and that this impairment exerts deleterious effects on human health. To evaluate the role of immunological processes in photocarcinogenesis the allergic delayed type hypersensitivity reaction has been employed as a model for presentation of tumour associated antigens (TAA) in which haptens are used as a surrogate for TAAs. In this background elegant studies by Kripke et al. demonstrated that in previously UVB-irradiated mice tumours did continue to grow unlike in unirradiated animals where the tumours were rejected through a sufficiently mounted immune response [25]. In line with the notion that UVB plays a role in photoimmunology is the finding that it leads to the expression of an array of cytokines such as IL-1, IL-6 and TNF-a [26] and that TNF-a and IL-10 are capable of suppressing delayed type hypersensitivity [27,28]. In addition, UV-B exposure has been demonstrated to impair resistance to bacterial, viral, parasitic and fungal infections. It is important to note that the effects of UV are not restricted to skin-associated infections, but also to systemic (non-skin-associated) infections. Part of these effects might be directly induced by UVB
99
induced DNA damage. This is demonstrated by the observations that photoreactivation of UVB induced CPD or enhanced excision repair of CPD by introduction of repair enzymes (by liposomes containing a CPD specific glycosylase [15,29]) counteracted suppression of contact hypersensitivity (CHS) locally as well as systemically. Additional evidence for a significant role of DNA damage in UV-B-induced immunosuppression was provided by studies exploring NER-deficient mice. The sensitivity of four transgenic mouse models (i.e, defective in either the XPA, XPC, CSB, and TTD gene) was studied for UVinduced systemic immunomodulation (delayed type of hypersensitivity (DTH) and contact hypersensitivity (CHS)). In contrast to the results seen for the induction of erythema and edema, the UVB induced immune effects are much more pronounced in XPA mice than in the CSB mice. XPC and TTD mice were not significantly sensitive to the UVB induced immunomodulation [18,22].
4. Conclusion Taken together, the published reports point at different roles of GGR and TCR in skin reactions such as UVB induced erythema or delayed type hypersensitivity. More data are needed to clarify the precise roles of NER subpathways in photoimmunological processes but the present data indicates that GGR and TCR are essential for the prevention of immunomodulation by UVB irradiation, whereas functional TCR is sufficient to protect against UVB induced erythema / edema.
Acknowledgements This work was supported by the Dutch Cancer Society grant no. UU 97-1531 and CA-44247 from the USPHS (ECF). M. Berneburg is supported by the Deutsche Forschungsgemeinschaft Emmy Noether Programm grant no. Be 2005 / 2-1.
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