Commentary

DNA repair on the brain

R. R. Laposa and J. E. Cleaver*

Comprehensive Center, University of California, San Francisco, CA 94143

eroderma pigmentosum (XP), Cock- Xayne syndrome (CS), and trichothio- dystrophy (TTD) constitute a family of sun-sensitive human diseases (1). They are caused by in the components of the nucleotide excision repair (NER) sys- tem and in the sys- tem involving the low-fidelity polymerase H (also known as XP variant or XPV). XP was initially recognized as a disease in- volving a genetic predisposition to ex- tremely high levels of skin carcinogenesis from solar exposure. The relationship be- tween DNA damage from solar UVB exposure and deficiencies in repair, lead- ing to mutagenesis, genetic instability, and eventual carcinogenesis, provides a decep- tively simple explanation for the major clinical features of XP and has provided a paradigm for environmentally induced human cancer. These diseases are, how- ever, much more complex at all levels (1, 2), as the new report in this issue by Murai et al. (3) indicates (Fig. 1). NER has two main pathways: GGR and TCR. Global repair involves the XPC and XPE and is generally slower and modulated by p53 (4), which also modulates the pol H pathway (5, 6). TCR results in a more rapid repair of the transcribed strand of expressed genes, and involves the two CS genes (CSA, CSB), several of the XP genes (especially XPA, -B, and -D), and the mismatch repair system (1). Some of the genes are involved in both pathways, es- pecially XPA. The XPG endonuclease is Fig. 1. The DNA-repair pathway of NER involves two subpathways: global genome repair (GGR) and also a cofactor for a glycosylase (endonu- transcription-coupled repair (TCR). The A (XPA) plays a role in both clease III, encoded by nth) that acts on pathways. The B (CSB) protein is involved in TCR and also has additional effects related to transcription. Murai et al. (in this issue) demonstrate neurodegeneration in Xpa͞Csb double-knockout oxidative damage (7). Mutations in XPB mice. and XPD also can give rise to diseases that combine one or more of the three main disorders, XP, CS, and TTD, depending in groups also show a spectrum of neurode- addressed by developing mouse models part on the precise site of the mutations in generation involving neuronal loss and with the various genes knocked out (KOs; the genes. mental deterioration that can be evident ref. 9), but initial results were disappoint- Clinically, the situation is exceedingly from earliest ages or that sets in slowly ing. The first KOs of Xpa and a mouse complex, because cancer is only part of the symptomology and is not even found in CS over time. In general, those genes that are model of a TTD in Xpd did not and TTD that are primarily disorders of involved only in GGR and postreplication demonstrate neurological disorders, development and neural functions (1). Pa- repair (XPC, XPE, XPV) are not associ- whereas a KO of Csb demonstrated only a tients with CS and TTD suffer from pro- ated with neurodegeneration, hinting that mild neurological phenotype in older gressive mental deterioration, dysmeyeli- the failure of TCR is more closely corre- nation, growth defects, wizened facies, lated with neurodegeneration. Fibroblasts See companion article on page 13379. and hypogonadism. TTD also has charac- from patients with neurological disorders are generally the more UV-sensitive of the *To whom reprint requests should be addressed at: Com- teristic sulfur deficiencies in hair and nails, prehensive Cancer Center, 2340 Sutter Street, Box 0808, resulting in a banding pattern identifiable XP groups (8). An understanding of the Room N-424, University of California, San Francisco, CA in polarized light. Certain of the XP clinical situation could, it was hoped, be 94143. E-mail: [email protected].

12860–12862 ͉ PNAS ͉ November 6, 2001 ͉ vol. 98 ͉ no. 23 www.pnas.org͞cgi͞doi͞10.1073͞pnas.241519498 Downloaded by guest on October 2, 2021 mice. Instead, these animals were more repair of oxidative damage to mitochon- damage checkpoint Atm (32). In the cur- cancer-prone than the corresponding hu- drial DNA (15). At least three hypotheses, rent study, Murai et al. observed reduced man disorder (10–13). which are not mutually exclusive, can be cell proliferation, as measured by BrdUrd Murai et al. have made significant put forward linking persistent DNA dam- labeling, in the developing cerebel- progress in developing a neurological age to neurodegeneration. lum, providing further support for this model by crossing Xpa and Csb KO mice First, decreased DNA repair and per- hypothesis. to produce double-KO mice lacking GGR sistent DNA damage may cause neurode- The work of Murai et al. shows that and TCR with major neurological defects generation by impairing the transcription GGR and TCR have important nonover- that are similar in many respects to the of undefined critical neural genes. Several lapping functions in rodents. These results human situation. This cross combined a candidate lesions that are produced by are important because much early work on total block in both GGR and TCR by oxidative stress and are processed at least rodent cells in culture showed that they elimination of XPA, which is involved in a in part by the NER pathway (5Ј,8-pur- were often innately low in DNA repair, common damage recognition and binding ine cyclodeoxynucleosides, purine because of cell culture-derived GGR de- step for both GGR and TCR, with a block dimers, 8-oxoguanine, thymine glycol; ficiencies. A cross between Xpc and Csb to the additional, TCR-specific CSB refs. 16–19) can block or delay transcrip- KO animals would be exceedingly inter- branch (Fig. 1). These double-KO mice tion (20, 21). esting, because cross-breeding would de- showed ataxia and abnormal locomotor Second, unrepaired DNA damage can termine whether more limited deficiencies activity, as well as postnatal growth retar- be a trigger for apoptosis, and the conse- in GGR combined with TCR also could dation and lethality at Ϸ3 weeks of quent loss of neurons by apoptosis could give rise to neurological deficiencies. age. There seemed to be a selective re- result in the neurodegeneration seen in Several additional considerations may duction in size of XP and related help explain the difference between hu- the cerebella of patients. There man and mouse phenotypes resulting these mice, which is a contrasting from single- deficiencies or KOs. One The work of Murai et al. shows that the authors at- role for apopto- possibility is that the actual level of oxi- tributed both to global genome repair and transcription sis in cancer and dative DNA damage is higher in humans. decreased neuro- coupled repair have important neurodegenera- Accordingly, the levels of DNA damage genesis and in- tion: whereas an may need to be increased in mice before a creased apopto- nonoverlappng functions in rodents. efficientapopto- phenotype becomes apparent. Apoptosis sis. In addition, tic pathway can is dramatically increased in cultured Xpa- Purkinje cells dis- remove geneti- null neurons by exposure to UV light played abnormal dendritic morphology. cally unstable cells from a pool of poten- relative to wild-type controls (33), sug- To appreciate the significance of the cur- tially malignant proliferating cells, such gesting that, in vivo Xpa-null mice might rent study of Murai et al., we should losses from nondividing neuronal popula- exhibit a neurological phenotype under consider the general problem of correlat- tion may result in functional deficits. In- conditions of exogenous genotoxic stress. ing DNA-repair deficiency with develop- creased apoptosis has been observed in Mice deficient in nonhomologous end- mental and neurological defects, and the several mouse models of DNA-repair de- joining (DNA-PKcs) exhibit no endoge- COMMENTARY unique problems of the differences be- ficiency. Mice deficient in homologous nous phenotype, but do show an increased tween human and mouse NER. recombination repair (Xrcc2), nonho- susceptibility to exogenous agents in vitro To understand the cause of neurode- mologous end-joining ( 70, Ku 80, and in vivo (34). generation in the TCR-deficient disorders Xrcc4, DNA-ligase IV gene) or base- One factor that might contribute to requires answering questions such as: is it excision repair (DNA polymerase ␤ gene) differences between levels of endoge- a direct consequence of the deficiency and all show increased apoptosis under condi- nous DNA damage between mice and not an alternative purely transcription tions of endogenous oxidative stress (22– humans could be the activity of antioxi- function; what is the origin and nature of 26). The DNA-damage checkpoints Atax- dative enzymes. Cultured cells from the damage; and what confers the speci- ia-telangiectasia mutated (Atm) and p53 some XP patients are also deficient in ficity to the symptoms. Much of the work also play a role. Atm is required for central catalase (35), although activities of cata- on XP has related the cancer aspect of this nervous system (CNS) apoptosis after ion- lase or other antioxidative enzymes in disease to the effects of NER deficiencies izing radiation (27). Atm and p53 are NER-deficient mice have yet to be mea- on mutation frequencies. However, muta- required for the neuronal apoptosis in at sured. In human patients, the increased tions may be irrelevant for the neurolog- least two of the DNA-repair KO mice endogenous DNA damage in the CNS ical aspects of these syndromes. Although listed above, because crossbreeding to could result from dysregulated gluta- mice deficient in Xpa have an increased Atm-null or p53-null mice can rescue the mate, because reduced expression of the mutation frequency in the liver, mutation embryolethality of those strains (28–30). glial glutamate transporters EATT1 and frequency is unaltered in brain (14), prob- In the current study, Murai et al. found GLT-1 and an increase in oxidatively ably because most cells there are postmi- increased TUNEL labeling (a marker for modified were observed to a totic. An alternate explanation, therefore, DNA breaks generated during apoptosis) greater extent in brains of CS than XPA must be put forward for the involvement in the cerebellum, a result similar to that patients (36), and brains from these pa- of NER in neurodegeneration. recently observed in Xpg-null mice (31) tients differed in the regions most af- Endogenous oxidative stress, resulting that are deficient in both NER (common fected by cell loss (37). In support of such from leakage of electron transport in mi- pathway) and TCR of the oxidative lesion a theory, knockdown of these glutamate tochondria, is an obvious culprit as a thymine glycol (7). transporters by antisense techniques can source of both bulky and structurally mi- A third hypothesis is that persistent exacerbate neuronal loss in vivo in ani- nor DNA lesions that involve, at least in DNA damage results in defective neuro- mal models (38, 39). part, the NER pathway. The brain is also genesis. Abnormal neurogenesis has been Another possibility may be that the a tissue with one of the highest levels of reported for mice deficient in double- existing mouse models of NER and TCR oxidative metabolism. Although NER is strand break repair (Xrcc2, Ku 70, Ku 80) deficiencies have not been examined in defective in general for mitochondria in (22, 25), (DNA poly- sufficient detail in the optimal mouse normal cells, XPA is required for the merase ␤ gene) (26) and the DNA- strains. Use of strains with a known pro-

Laposa and Cleaver PNAS ͉ November 6, 2001 ͉ vol. 98 ͉ no. 23 ͉ 12861 Downloaded by guest on October 2, 2021 pensity to develop neurological deficits XP, CS, and TTD are extremely rare of the mechanisms of neurodegeneration might increase our ability to detect subtle hereditary disorders, but their study by of the Xpa͞Csb double-KO mice, and ex- or overt neurological and behavioral de- numerous investigators has been exceed- tension to other crosses, will be greatly fects, and even identify modifier genes ingly informative about the causes of hu- assisted by the findings of Murai et al.in that influence neurological development. man cancer. Similar study of the neuro- the current issue of this journal. This work Examination of embryos and neonates degeneration in these disorders promises may help clarify the mechanisms of com- might also reveal defects no longer evi- to be equally informative about the causes mon human neurodegenerative condi- dent in adult animals. For example, sev- of major human neurodegenerative con- tions and provide model systems for the eral strains of KO mice deficient in non- ditions. A growing number of studies sup- development of treatment methods for homologous end-joining repair (Ku 70, Ku port a role for endogenous DNA damage, human patients. 80) were originally thought to have no including lesions handled by the NER neurological phenotype when adult ani- pathway, in the mechanism of several ma- We thank Mr. and Mrs. Dan Mahar of the XP mals were studied (40–42), but when em- jor human neurodegenerative conditions: Society and Mr. and Mrs. Kevin O’Brien of the bryos were analyzed, neural deficits be- Alzheimer’s, Parkinson’s, and amyotro- Luke O’Brien Foundation for their interest and came apparent (24). phic lateral sclerosis (ALS). Clarification support.

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