Model DNA for investigation of mechanism of nucleotide excision repair

The living cell DNA is under permanent attack of a variety of exogenous and endogenous damaging factors. Nucleotide excision repair (NER) is main pathway which removes a wide variety of bulky DNA adducts formed by UV light, electrophilic environmental mutagens, and chemotherapeutic agents. NER proce...

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Datum:2014
Hauptverfasser: Evdokimov, A.N., Lavrik, O.I., Petruseva, I.O.
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Veröffentlicht: Інститут молекулярної біології і генетики НАН України 2014
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spelling nasplib_isofts_kiev_ua-123456789-1543012025-02-23T17:51:16Z Model DNA for investigation of mechanism of nucleotide excision repair Модельні ДНК для дослідження механизму нуклеотидної ексцизійної репарації Модельные ДНК для исследования механизма нуклеотидной эксцизионной репарации Evdokimov, A.N. Lavrik, O.I. Petruseva, I.O. Reviews The living cell DNA is under permanent attack of a variety of exogenous and endogenous damaging factors. Nucleotide excision repair (NER) is main pathway which removes a wide variety of bulky DNA adducts formed by UV light, electrophilic environmental mutagens, and chemotherapeutic agents. NER process in mammalian cells consistently leads to the very specific excision of damaged DNA fragments 24–32 nucleotides in length. The following DNA repair synthesis and DNA ligation restore intact DNA helix. The main set of the genes inactivated in NER-deficient higher eukaryotic cells was identified; about 30 proteins are involved in the specific multi-subunit complexes responsible for NER process. The specific NER feature is wide substrate specificity and great difference of damages elimination efficiencies. A key limiting step in NER is damage recognition and verification. One of the advanced and upcoming approaches to NER process investigation is based on the application of model DNAs – artificial DNA structures, which are analogs of substrate or intermediates of the repair process. This article reviews our current knowledge concerning the model DNA design, synthesis and application as a tool for the NER process comprehensive study. ДНК живих клітин перебуває під постійним впливом різноманітних пошкоджуючих факторів екзо- і ендогенного походження. Нуклеотидна ексцизійна репарація (NER) видаляє з ДНК широкий набір об’ємних адуктів, які утворилися в результаті дії УФ опромінення, а також електрофільних речовин – забруднювачів довкілля, що чинять мутагенний вплив, та хіміопрепаратів. У процесі репарації, яку виконує система NER ссавців, відбувається специфічне вищеплювання з ДНК фрагментів розміром 24––32 нуклеотиди, що містять пошкодження. Подальший репаративний синтез і лігування ДНК відновлюють інтактність спіралі ДНК. Ідентифіковано гени, інактивовані в NER-дефіцитних клітинах вищих евкаріотів. В репарації беруть участь приблизно 30 білків, які формують специфічні мультисубодиничні комплекси. Система NER характеризується широкою субстратною специфичністю і при цьому великими розбіжностями в ефективності видалення пошкоджень. Ключовою лімітуючою стадією процесу є упізнавання та верифікація пошкоджен. До ефективних і таких, що розвиваються, підходів до вивчення процесу NER належить метод, заснований на використанні модельних ДНК – синтетичних структур, які є аналогами субстрата або інтермедіатів цього процесу. Розглянуто існуючі дані щодо способів конструювання модельних ДНК та застосування їх як інструмента для всебічного дослідження процесу NER. ДНК живых клеток находится под постоянным воздействием различных повреждающих факторов экзо- и эндогенного происхождения. Нуклеотидная эксцизионная репарация (NER) удаляет из ДНК широкий набор объемных аддуктов, образовавшихся в результате воздействия УФ облучения, а также электрофильных веществ – загрязнителей окружающей среды, оказывающих мутагенное действие, и химиопрепаратов. В процессе репарации, проводимой системой NER млекопитающих, происходит специфическое выщепление из ДНК фрагментов размером 24–32 нуклеотида, содержащих повреждения. Последующий репаративный синтез и лигирование ДНК восстанавливают интактность спирали ДНК. Идентифицированы гены, инактивированные в NER- дефицитных клетках высших эукариотов. В репарации участвуют примерно 30 белков, формирующих специфические многосубъединичные комплексы. Система NER характеризуется широкой субстратной специфичностью и при этом большими различиями в эффективности удаления повреждений. Ключевой лимитирующей стадией процесса является узнавание и верификация повреждения. К эффективным и развивающимся подходам к исследованию процесса NER принадлежит метод, основанный на использовании модельных ДНК –синтетических структур, являющихся аналогами субстрата или интермедиатов этого процесса. Рассмотрены существующие данные о способах конструирования модельных ДНК и применении их в качестве инструмента для всестороннего изучения процесса NER. 2014 Article Model DNA for investigation of mechanism of nucleotide excision repair / A.N. Evdokimov, O.I. Lavrik, I.O. Petruseva // Вiopolymers and Cell. — 2014. — Т. 30, № 3. — С. 167-183. — Бібліогр.: 107 назв. — англ. 0233-7657 DOI: http://dx.doi.org/10.7124/bc.000893 https://nasplib.isofts.kiev.ua/handle/123456789/154301 577.21 en Вiopolymers and Cell application/pdf Інститут молекулярної біології і генетики НАН України
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
language English
topic Reviews
Reviews
spellingShingle Reviews
Reviews
Evdokimov, A.N.
Lavrik, O.I.
Petruseva, I.O.
Model DNA for investigation of mechanism of nucleotide excision repair
Вiopolymers and Cell
description The living cell DNA is under permanent attack of a variety of exogenous and endogenous damaging factors. Nucleotide excision repair (NER) is main pathway which removes a wide variety of bulky DNA adducts formed by UV light, electrophilic environmental mutagens, and chemotherapeutic agents. NER process in mammalian cells consistently leads to the very specific excision of damaged DNA fragments 24–32 nucleotides in length. The following DNA repair synthesis and DNA ligation restore intact DNA helix. The main set of the genes inactivated in NER-deficient higher eukaryotic cells was identified; about 30 proteins are involved in the specific multi-subunit complexes responsible for NER process. The specific NER feature is wide substrate specificity and great difference of damages elimination efficiencies. A key limiting step in NER is damage recognition and verification. One of the advanced and upcoming approaches to NER process investigation is based on the application of model DNAs – artificial DNA structures, which are analogs of substrate or intermediates of the repair process. This article reviews our current knowledge concerning the model DNA design, synthesis and application as a tool for the NER process comprehensive study.
format Article
author Evdokimov, A.N.
Lavrik, O.I.
Petruseva, I.O.
author_facet Evdokimov, A.N.
Lavrik, O.I.
Petruseva, I.O.
author_sort Evdokimov, A.N.
title Model DNA for investigation of mechanism of nucleotide excision repair
title_short Model DNA for investigation of mechanism of nucleotide excision repair
title_full Model DNA for investigation of mechanism of nucleotide excision repair
title_fullStr Model DNA for investigation of mechanism of nucleotide excision repair
title_full_unstemmed Model DNA for investigation of mechanism of nucleotide excision repair
title_sort model dna for investigation of mechanism of nucleotide excision repair
publisher Інститут молекулярної біології і генетики НАН України
publishDate 2014
topic_facet Reviews
url https://nasplib.isofts.kiev.ua/handle/123456789/154301
citation_txt Model DNA for investigation of mechanism of nucleotide excision repair / A.N. Evdokimov, O.I. Lavrik, I.O. Petruseva // Вiopolymers and Cell. — 2014. — Т. 30, № 3. — С. 167-183. — Бібліогр.: 107 назв. — англ.
series Вiopolymers and Cell
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fulltext REVIEWS UDC 577.21 Model DNA for investigation of mechanism of nucleotide excision repair A. N. Evdokimov1, 2, O. I. Lavrik1, 2, 3, I. O. Petruseva1 1Institute of Chemical Biology and Fundamental Medicine, Siberian Branch of the Russian Academy of Sciences 8, Akad. Lavrent’eva Ave., Novosibirsk, Russian Federation, 630090 2Altai State University, Ministry of Education and Science of the Russian Federation 61, Lenina Ave., Barnaul, Russian Federation, 656049 3Novosibirsk State University, Ministry of Education and Science of the Russian Federation 2, Pirogova Str., Novosibirsk, Russian Federation, 630090 irapetru@niboch.nsc.ru The living cell DNA is under permanent attack of a variety of exogenous and endogenous damaging factors. Nuc- leotide excision repair (NER) is main pathway which removes a wide variety of bulky DNA adducts formed by UV light, electrophilic environmental mutagens, and chemotherapeutic agents. NER process in mammalian cells consistently leads to the very specific excision of damaged DNA fragments 24–32 nucleotides in length. The fol- lowing DNA repair synthesis and DNA ligation restore intact DNA helix. The main set of the genes inactivated in NER-deficient higher eukaryotic cells was identified; about 30 proteins are involved in the specific multi-subunit complexes responsible for NER process. The specific NER feature is wide substrate specificity and great diffe- rence of damages elimination efficiencies. A key limiting step in NER is damage recognition and verification. One of the advanced and upcoming approaches to NER process investigation is based on the application of mo- del DNAs – artificial DNA structures, which are analogs of substrate or intermediates of the repair process. This article reviews our current knowledge concerning the model DNA design, synthesis and application as a tool for the NER process comprehensive study. Keywords: nucleotide excision repair, bulky DNA adducts, model DNA. Introduction. The DNA of the living cells is subject of various modifications due to the impact of both exoge- nous and endogenous factors. There are several ways of damaged DNA repair in order to prevent the accu- mulation of modifications and to protect the genetic information in cells. The nucleotide excision repair (NER) is one of the most important ways to achieve ge- nome safety. NER protects the cells from the broadest range of structurally and chemically different bulky DNA damages. The damages are present in the form of modifica tions of nitrogenous bases, occurring due to the effect of UV light, ionizing radiation, environmen- tal electrophilic chemical mutagens, some medical drugs as well as chemically active endogenous metabolites, including reactive derivatives of oxygen and nitrogen. In the cells of higher eukaryotes the NER system with high precision removes oligonucleotide fragments 24– 32 nucleotides in length, containing the damage [1]. The subsequent reparative synthesis using the non-da- maged strand as a template, followed by the ligation of the single-strand break, complete the process of DNA repair [2] (Fig. 1). The main genes, inactivated in NER- deficient cells of higher eukaryotes, have been identi- fied and the protein factors and enzymes have been de- termined. This process is known to be performed by co- ordinated activity of about 30 proteins, subsequently forming on DNA complexes of variable composition [3]. The mutations in the genes, encoding these pro- teins, lead to the down regulation of the NER process. Such mutations result in some diseases, including xe- 167 ISSN 0233–7657. Biopolymers and Cell. 2014. Vol. 30. N 3. P. 167–183 doi: http://dx.doi.org/10.7124/bc.000893 � Institute of Molecular Biology and Genetics, NAS of Ukraine, 2014 roderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD), characterized by UV- sensitivity, high risk of cancer and a number of neuro- degenerative symptoms [4]. There are two variants of NER process, differing at the damage recognition step. The signal for the start of the transcription-coupled repair (TCR) is the stop of RNA-polymerase, when the damage in the transcribed DNA strand arrests the transcription activity [5, 6]. Glo- bal genome nucleotide excision repair (GG-NER) per- forms the searching and removal of the bulky damages regardless of the functional state of the genome, inclu- ding the non-transcribed parts of chromatin. The specia- lized sensor factors – XPC (protein complex XPC/ HR23B/Cen2) and XPE (DDB1/DDB2 heterodimer, promoting the recognition of UV-damage) are respon- sible for the primary recognition of the damage in the GG-NER process [1, 7–10]. A broad range of biochemical and molecular bio- logy methods, including the methods, based on the ap- plication of artificial model DNAs, are used for the de- tailed study of the mechanism of the interaction bet- ween NER protein factors among themselves and with the DNA in vitro. Generally the model DNAs are either the analogues of the substrate (of the damaged DNA) or the analogues 168 EVDOKIMOV A. N. ET AL. DNA damage XPC-HR23B TFIIH RPA XPG XPC-HR23B XPA ERCC1-XPF ERCC1-XPF RFC PCNA Pol ä Pol ä RFC PCNA TFIIH XPG XPA RPA Ligase I or Ligase III Undamaged DNA Ligase I or Ligase III Fig. 1. Scheme of global genome nucleotide excision repair of the intermediates of the NER process. In particular, these intermediates are modified duplexes, containing artificially introduced non-complementary parts («bub- bles» of different length), «fork»-like structures, as well as some structures, containing 5' or 3'-overhangs [11–14]. The main types of damages, repaired by the NER system and the modifications, used for model DNA synthesis. Regardless of the fact that the model DNA has been used to study the NER process for many years, the elaboration and improvement of the methods for the model DNA synthesis still remains actual. Different bulky lesions obtained due to the modification of nitro- genous bases are used as a damage-mimicking adducts, repaired by the NER system. Such adducts may imitate the structure of damages that appear in cellular DNA un- der exogenous or endogenous damaging factors. On the other hand, these lesions may be artificial construct- ions, the introduction of which into dsDNA also leads to the distortion in its structure and the change in the ther- modynamic stability. The products of UV irradiation. One of the main DNA-damaging agents is UV radiation. It was determi- ned that UV light is the most dangerous for the organism part of the sunlight, reaching the surface of the Earth. The UV spectrum is composed of three parts: UV A (300–400 nm), UV B (290–320 nm) and UV C (200– 290 nm) with the highest energy, but it is mainly filte- red out by the ozone layer of the Earth. The maximal formation of the photoproducts is observed when DNA is exposed to UV light at the wavelength lesser than 300 nm. It is in good agreement with the adsorption spectrum of the main DNA chromophores – thymine and cytosine. Prokaryotes and lower eukaryotes are ca- pable of remove UV-damages using the NER system. Additionally, they can repair UV-lesions by the specific enzymes – photolyases, transforming pyrimidine dimers into the intact monomers [15]. In contrast, in higher eukaryotes cells UV-damages only can be removed by NER system. Pyrimidine dimers (Fig. 2), the UV-radiation pro- ducts, are the most common model of pyrimidine da- mage. Cyclobutane pyrimidine dimers (CPD) (Fig. 2, b) are formed due to (2 + 2) cyclo attaching along double bonds C5–C6 of adjacent pyrimidine bases. The majo- rity of CPD are formed by adjacent TT bases, but the formation of these adducts is also possible due to the in- teraction of CC, CT or TC depending on the wavelength, radiation dose and nucleotide sequence. The formation of CPD is reversible after the impact of UV-light at the wavelength of 250 nm [16]. 6–4-photoproducts (Fig. 2, a) are included in the number of the most common UV-damages in the cell [17]. The intermediate, obtained due to (2 + 2) attaching, is regrouped with the formation of 6–4-photoproduct which with further radiation may be reversed into the Dewar isomer. In contrast with CPD, 6–4-photopro- ducts cause higher distortion of the DNA structure with the disturbance of nucleotide coupling at the site of the damage, which is reflected in higher efficiency of their repair by the NER system [18, 19]. The products of environmental chemical agents. Po- lycyclic aromatic hydrocarbons (PAH) are widespread environmental pollutants, which are the products of in- complete combustion of different materials. Benzo[a]- pyrene and benzo[c]-anthracene are inert non-polar compounds. However in the process of metabolic detoxification in the mammals, involving cytochrome C and epoxyhydrolases action, potentially toxic lipophilic PAH molecules transform not only into excreted soluble derivatives, phenols and dihydrodioles, but also into electrophilic, chemically active diolepoxides, capable to stereoselective reacting with nucleobases with the for- mation of bulky adducts [20]. The repair of benzo[a]- pyrene damages has been studied in details. The me- thods of synthesis of the DNA, containing dG- and dA- benzo[a]-pyrene adducts with different structure and stereochemistry, are developed. Analysis of repair pro- cess of such model DNA, along with the results of in- strumental methods of analysis and computer simula- 169 MODEL DNA FOR INVESTIGATION OF MECHANISM OF NUCLEOTIDE EXCISION REPAIR UV a b O HN N O O NH N O O HN N O O H N N O HN N O O NH N O OH Fig. 2. DNA UV irradiation products – pyrimidines dimers: a – 6–4-photoproducts; b – cyclobutane pyrimidines dimers tion led to an understanding of the principles of the bulky damages recognizing by the NER system. The im- pact of the stereochemistry, thermodynamic properties of such DNAs, sequence of damaged DNAs as well as the adduct topology, on the efficiency of lesion recog- nition and removal from DNA have been considered in a fine details [20–24]. The metabolic transformation of the polycyclic di- hydrofuranes of aflatoxin B1 (AFB1), aflatoxin G1 and sterigmatomycin, which are produced by some kinds of fungi, polluting food products, also involves the forma- tion of the corresponding epoxides [25], which interact with DNA. Another class of compounds, damaging mammali- an DNA, is arylamines (Fig. 3). Arylamines are also pre- sented in the tobacco smoke, hair dye and some other sources. Tobacco smoke contains some compounds of the arylamine class, which are known carcinogens, in particular, 2-naphthylamine, 4-aminobiphenyl and ben- zidine [26] (Fig. 3, a) as well as aminofluorene and ace- tylaminofluorene. Arylamines may be activated in the process of metabolism in N-hydroxy-, N-acetoxy- or N-sulfoxy compound. Further heterolytic breakage of N-O bond leads to the formation of highly active inter- mediates, forming the extensive adducts during the in- teraction with DNA [27] (Fig. 3, b). Many works, dedicated to the investigation of the NER mechanism and mutagenesis processes, describe the application of the model DNA, containing N-(deoxyguanosin-8-yl)- 2-acetylaminofluorene (dGAAF) [28–30]. Another threat to the mammalians is presented by the nitro compounds of the polycyclic aromatic hydrocarbons. These compounds released into the environment from different sources; mainly these are the products of in- complete combustion, but some are also present in food and drinks. 6-nitrochrysene (6-NC) is not the most common compound of this class, but it is also known as a carcinogen, capable to induce breast cancer. The car- cinogenic effect of 6-NC exceeds not only that of ben- zo[a]-pyrene, but also such a powerful carcinogen as he- terocyclic aromatic amine 2-amino-1-methyl-6-phenyl- imidazol[4,5-b]pyrimidine [31]. It is commonly known that the metabolic activation of 6-nitrochrysene may occur in one of two ways (Fig. 4). The first way sug- gests the ordinary reduction of the nitrogroup with the formation of 6-hydroxylaminochrysene, the interaction of which with deoxyguanosin in DNA leads to the for- mation of N-(deoxyguanosin-8-yl)-6-aminochrysene and 5-(deoxyguanosin-N(2)-yl)-6-aminochrysene [32, 33]. The second way is the oxidation of the fourth ring of chrysene, the reduction of a nitrogroup with the forma- tion of high-reactive electrophilic intermediate trans-1, 2-dihydroxy-1,2-dihydro-6-hydroxylaminochrysene.The interaction of this intermediate with DNA also leads to 170 EVDOKIMOV A. N. ET AL. N H N NH N N O HN O NH2N NH N N O N O N O O O NH2N NH N N O NH DNA NH2 NH2 NH2H2N a b Fig. 3. a – the most common arylamines (left to right): naph- thylamine, 4-aminobi- phenyl and benzidine; b – interaction of N- acetoxyacetylamino- fluorene with DNA le- ads to bulky adducts formation: 3-(deoxy- guanosine-N2-yl)-2- acetylaminofluorene (left), N-(deoxyguano- sine-8-yl)-2-acetyl- aminofluorene (mid- dle) or N-(deoxygua- nosine-8-yl)-2-amino- fluorene (right) the formation of 5-(deoxyguanosin-N(2)-yl)-6-amino- chrysene [34, 35]. It has been recently demon- strated that the repair of these damages occurs with low effici- ency (approximately the same with repair of N2-deoxy- guanosin-benzo[a]-pyrenyl adducts) [36], i. e. one order less efficiently than the repair of efficient NER subst- rates – intrastrand crosslinks, introduced into dsDNA due to the effect of platinum derivatives [37, 38]. The effects of the medical preparations on DNA components. Cis-diaminodichloroplatinum (II) (or cis- platin) is one of commonly used chemotherapeutic drug, performing its cytotoxic function via the DNA dama- ging. Cisplatin is a non-charged complex of bivalent platinum with the configuration of a flat square. Once intravenously injected into the organism, this complex compound remains stable in the blood plasma until it penetrates the cell cytoplasm, where low concentration of chloride ions leads to their replacement in the inner sphere of the complex with water or hydroxy-ions with the formation of the active electrophilic agent. Its in- teraction with DNA leads to the formation of a monoad- duct as well as intrastrand or interstrand crosslinks. Si- milar to the UV-lesions, the efficiency of repair of the cisplatin-induced damages is affected by the differen- ces in their structure [39,40]. An example of the inter- action between the pharmacological activity of the sub- stance and its structure is bifunctional platinum anti- cancer drugs. As stated above, pharmacologically acti- ve substances are platinum complexes with the cis-struc- ture [39, 40], whereas the trans-isomer (transplatin) has no clinical efficiency. However the complexes with the trans-structure, demonstrating higher cytotoxicity in the tumor cells, have been obtained. The replacement of an amino group in transplatin for an iminoester resulted in a considerable increase in the complex cytotoxicity even in comparison with the cytotoxicity of the correspon- ding cis-isomer. The trans-(PtCl2(E-iminoester)2 de- monstrates high anticancer activity regarding cancer cells, resistant to the effect of commonly used platinum compounds [41]. There have been attempts of elabora- ting a new generation of antineoplastic preparations, based on platinum II, in order to solve the problem of drug resistance and decreasing the side effects of che- motherapy [42]. The monofunctional complexes of pla- tinum II of the general formula cis-[Pt(NH3)2(N-hetero- cycle)Cl]Cl form a single covalent bond with DNA, in- troduce minor distortions into the regular structure of dsDNA and differ in their properties from classic rea- gents like cisplatin. Pyriplatin (a monofunctional comp- lex with pyridine as a ligand) may serve as an example of such drug. The variation of heterocyclic replace- ments allowed revealing the compounds with high an- titumor activity [43]. Phenanthriplatin, containing an extensive ligand phenanthridin (cis-[Pt(NH3)2(phenan- tridin) Cl]NO3), has a considerably higher activity than commonly used cisplatin and oxaliplatin. There are al- so differences in the spectrum of antitumor activity, de- monstrated by this compound, which means that phe- nanthriplatin may be active regarding the types of can- cer cells, demonstrating resistance to common platinum chemotherapy [44]. In recent years the attention of researchers was at- tracted to bulky adducts, introduced in DNA due to inter- action of exocyclic amino groups of DNA purine bases with reactive metabolites of aristolochic acids (MAA). 171 MODEL DNA FOR INVESTIGATION OF MECHANISM OF NUCLEOTIDE EXCISION REPAIR NO2 NHOH NHOH HO OH NHN NH N N O NHOH OH HO NH2N NH N N O NH DNADNA trans dihydroxy ihydro hydroxylamine chrysene -1,2- -1,2- d -6- 5-( -N(2)- ) - deoxyguanosine yl 6-aminochrysene N-( -8- )- 6- deoxyguanosine yl aminochrysene 6-hydroxylamine chrysene Fig. 4. 6-nitrochrysene activation leads to bulky adducts It has been recently revealed that these components of herbal medical drugs, used for centuries, have serious toxic effect and cause nephropathy, in 50 % leading to cancer [45]. The results of the research, performed using the mo- del DNA, demonstrated that the newly formed MAA derivatives of deoxyguanosine are removed from DNA, whereas the adducts of deoxyadenosine are accumula- ted in the organism which leads to the progress of di- seases [46]. Also has been described the application of the mo- del structures, containing modifications, which introdu- ced into DNA due to the effect of hormonal drugs like premarin. The metabolites of the conjugated estrogens, 4-hydroxyequilenin and 4-hydroxyequilin, are widely used as a components of drugs for the hormone-repla- cement therapy. They are capable of autoxidation with the formation of o-quinones, actively interacting with the nucleobases of DNA, mainly with cytosine, which results in the formation of bulky adducts that are hard to repair [47–49]. The artificial analogues of damages. An example of artificial nucleotide injury is deoxyuridine or deoxy- thymidine, containing the fluorescein residue, attached to C5 position via the linker fragment (Fig. 5, a). The monomer moiety for the synthesis of such DNA is a commercially available amidite for the standard solid- phase synthesis. It is also possible to introduce this mo- dification using the DNA-polymerase reaction with the modified dNTP as a substrate. The category of artificial analogues of damages in- cludes recently suggested derivatives of dCMP and dUMP, containing the photoactivated arylazide groups, linked to the nitrogenous bases via extended and flexi- ble linker fragments (Fig. 5, b, c) [50]. It was demon- strated that such arylazide derivatives of nucleotides in- troduced into model DNA are recognized as substrates by the bacterial NER system [51] as well as the NER sys- tem of the mammalians [52]. The photoactivated ana- logues of damages expand the capabilities of such re- search instrument since they allow to perform the study of a multicomponent system NER using the [53]. Du- ring repair NER proteins form unstable specific comple- xes of various compositions and architecture on the da- maged DNA. The main idea of affinity modification ap- proach is a covalent fixation of such protein complexes. 172 EVDOKIMOV A. N. ET AL. O O O HN O H N O HN N O O O O P O O- O PO O O- NH O O NH O O P O O- O PO O O- NH OHHO O O O O NH O N N H O HN N N O F N3 F Cl H N N N H OO HN N F N3 F Cl H N O a b c d e Fig. 5. Synthetic analogous of damage: a – fluoresceine-5(6)-carboxy- aminocaproyl-[5-(3-amino allyl)-2'-deoxyuridine; b – 5-{N-[N-(4- azido-2,5-difluoro-3-chloropyridine-6-yl)-3-aminopropionyl]-trans-3- aminopropenyl-1}-2'-deoxyuridine; c – exo-N-[2-N-(N-(4-azido-2,5- difluoro-3-chloropyridine-6-yl)-3-aminopropionyl)-aminoethyl]-2'- deoxycytidine; d – non-nucleoside fragments of modified DNA strand, containing N-[6-(9-antracenylcarbamoyl)hexanoyl]-3-amino- 1,2-propandiol; e – non-nucleoside fragments of modified DNA strand, containing N-[6-(5(6)-fluoresceinylcarbamoyl)hexanoyl]-3- amino-1,2-propandiol Recently a novel artificial non-nucleoside damages on the basis of N-[6-(9-antracenylcarbomoyl)hexanoyl]- 3-amino-1,2-propandiol (nAnt) and N-[6-(5(6)-fluo- resceinylcarbomoyl)hexanoyl]-3-amino-1,2-propandi- ol (nFlu) (Fig. 5, d, e) have been suggested, which imi- tate NER-recognizable adducts [54]. The example of the synthetic analogues of the dama- ges to be repaired by NER can be found in the oligopep- tide-DNA adducts, the products of proteolytic degra- dation of larger protein-DNA adducts, formed in the cell under the impact of ionizing radiation, a number of chemical reagents as well as the processes of DNA me- tabolism [55]. The advantages of artificial damages are the possibility of their introduction into the target posi- tion of a DNA molecule and the possibility of varia- tions in their structure, which is often required for the experiments investigating the NER system in vitro. The model DNA, used to investigate the NER process. In order to form the functionally active prein- cision NER complex, the damage should either be intro- duced into a circular DNA [3], or into extended (� 120 bp) linear duplex [56]. The circular DNA with multiple damages. A number of works, investigating the substrate properties of a mo- del damages, have been performed using model struc- tures created on the basis of phage or plasmid DNA, which contain damages, statistically distributed along the molecule. The damages are introduced into DNA using either chemical modification or dosed UV-radia- tion [57]. The subsequent product purification and its analysis allow to estimate the amount and composition of the introduced damages [58]. The characteristics of the obtained model DNA depend on the type of the da- maging reagent and the conditions, in which the treat- ment of the DNA was performed. The context of DNA sequence surrounding the target nucleobase is also im- portant [59, 60]. The circular DNA, affected by high do- ses of UV-radiation, was suggested as one of the first model system for the research on the nucleotide exci- sion repair. The irradiation of the plasmid DNA with UV-light at the wavelength of 254 nm resulted in the formation of both types of photoproducts. The obtained substrate contained the mixture of cyclobutane pyrimi- dine adducts and 6–4-photoproducts in random positi- 173 MODEL DNA FOR INVESTIGATION OF MECHANISM OF NUCLEOTIDE EXCISION REPAIR a elongation ligation b introduction of modified dNTPs hybridization with flanking oligonucleotide ligation hybridization with complementary strand c ligation hybridization with complementary strand Fig. 6. Model DNA synthe- sis methods: a – circle DNA; b – synthesis of linear model DNA using enzymatic ap- proach; c – synthesis of li- near model DNA using pre- synthetized oligonucleotides ons of DNA strands [58, 61]. There are evident prob- lems and restrictions of such artificial NER substrates use. Nevertheless, such model DNAs was applied to es- timate the level of radioactive nucleotides incorpora- tion during the reparative synthesis. This approach was used for investigation of status of NER system [62, 63]. The circular DNA with the damage at the defined po- sition of the strand. The fill-in method based on DNA- polymerase-catalyzed reaction is widely used for the synthesis of NER substrates, containing the damage (da- mages) in the defined position (positions) of the DNA strand. The modified oligonucleotide is used as a pri- mer and the single-stranded form of the circular DNA (plasmid or phage) is used as a template. Then the enzy- matic ligation is used to restore the strand integrity (Fig. 6, a) [64, 65]. The method of the modified primer creation is defined by the type of the damage [19, 66– 68]. This approach to the synthesis of the NER system substrates is rather labor-consuming and does not provi- de a high yield, but it allows produce modified DNA of certain structure and properties. The circular DNAs with the damage in the defined position were used to estimate the efficiency of the da- mages elimination as well as to analyze the NER mecha- nism [69]. The use of the circular DNA, containing the regions of several uncoupled bases at the distance of 60 bp from the damage at 3'- or 5'-side, allowed confir- ming the hypothesis of a bipartite mechanism of the da- mage recognition. It was shown that the efficiency of re- pair of model DNA, containing the cyclobutane pyrimi- dine dimers that are hard to repair, increases conside- rably with the introduction of 3nt bubble at 5'-side of the damage. The further research, performed with recom- binant dimer XPC-HR23B and factor TFIIH – the sys- tem of recognition and control of the damage, reconsti- tuted from the purified proteins – demonstrated that connecting to the bubble, XPC promotes the assembly of the protein complex, containing TFIIH. This complex starts to melt the duplex and moves towards the 3'-side from the initial binding place in the search for the da- mage, containing the bulky chemical modification. Du- ring this scanning the complex is translocated along the DNA for the distance, measured with hundreds of base pairs [70]. The linear DNA with the damage at the defined po- sition of the strand. One of the most widely used inst- ruments of in vitro NER investigation are linear dsDNA with the damage at the defined position. Depending on the suggested tasks, these model DNA may be of diffe- rent length, structure and sequence. The DNA-duplexes, not exceeding 50 bp, were used to study the interaction of specific proteins of the NER system with the damaged DNA by methods of affinity modification, gel-retardation, equilibrium titration as well as immunological techniques. Generally the obtaining of these model DNAs does not cause any complications, especially if the corresponding monomers were availab- le for the automatic standard solid-phase synthesis. The extended linear DNA-duplexes with the bulky modification in the middle of the strand may be conside- red as universal models, suitable for the experiments of all the mentioned types [71–74]. Also such DNAs may be used for the investigation of the excision of the da- mage, as well as the changes of the nucleoprotein comp- lexes, occurring during the NER process, which are as- sociated with each stage [11, 75], including the ones in the composition of artificial nucleosomes [76, 77]. These model DNAs may be also obtained via enzy- matic ligation of the structure, composed of several short overlapping oligonucleotide fragments. This ap- proach is not complicated and allows varying the type of the damage by replacing the modified oligonucleoti- de, but the yield of the target extended DNA is not high – 5–6 % of the amount of oligonucleotide with the modifi- cation [78]. The improved method, providing for considerably higher yield, involves the ligation of the structure, com- posed of the modified oligonucleotide, which is 12–16 nucleotides long, and two flanking rather extended (60– 70 nucleotides) oligonucleotides on the template of the optimal length (30–35 nucleotides). The hybridization of the obtained extended modified DNA and the comp- lementary strand is performed to obtain the target dup- lex (Fig. 6, c) [54]. Recently the method of creation of NER substrate analogues, involving the use of several enzymes, inclu- ding DNA-polymerases and DNA-ligase, has also be- en elaborated (Fig. 6, b) [79]. A number of modified dNTP, containing photoactive and fluorescent bulky sub- stitutes introduced via long and flexible spacers along the nitrogenous base, were used as substrates of DNA- polymerase [50, 80, 81]. Good substrate properties of 174 EVDOKIMOV A. N. ET AL. these dNTP and obtained modified oligonucleotides in the reactions, catalyzed by the DNA-polymerase and DNA-ligase, provided a high yield and wide variety of the model DNAs for the NER system investigation [82]. The DNA-analogues of NER substrates as pro- bes for the affinity modification. The method of pho- toaffinity modification – covalent crosslinking of pro- teins with DNA-substrate analogues, containing photo- active groups – proved itself as informative approach to the investigation of multicomponent nucleoprotein com- plexes. The reaction of a covalent attachment may be initiated via UV-irradiation at any suitable time after the formation of a specific non-covalent protein-DNA complex. The activation of photoreagents may be per- formed in very short time. The adduction may be very efficient in a wide pH range, ionic strength and the tem- perature, whereas the reaction conditions often have to be optimized for different types of reactive groups. UV- induced adduction allows analyze even unstable comp- lexes of variable composition. This is especially impor- tant for the investigation on the multicomponent dyna- mic systems of DNA metabolism including the NER system. The variation of the structure of the model DNA allows to investigate interaction between the NER pro- tein factors and the DNA-substrate and their mutual im- pact as well as the structure and mechanism of functio- ning of the reparative complexes [51–53, 83]. The DNA probes successfully used to study protein-DNA inter- action contain the aryl azide group, introduced as a sub- stitute into the pyrimidine base at C5 position of dU (dT) or the exocyclic atom of nitrogen in dC [81]. After UV-radiation the arylazide derivatives generate the short- term singlet and long-term triplet nitrene, capable to ef- ficiently react with a number of amino acid residues [80, 84, 85]. Being bulky substitutes, these photoreagents distort the structure of the DNA duplex [86], while the halogen- and tio-derivatives such as 5-J-dU and 4S-dU (dT) introduce minimal changes into the DNA structu- re [87]. Such derivatives do not provide high yield of the covalent adducts, when the source of coherent radia- tion of high radiation power (laser) was replaced with more common sources of ultraviolet (high pressure mer- cury lamp, etc.). However, the DNAs containing these derivatives were applied for investigation of NER sys- tem [88–91]. In addition, the reaction of p-azidophena- cylbromide for the inter-nucleotide phosphorothioate may be used to introduce photoactive groups into the DNA-analogues of the NER substrates. The DNAs, con- taining this modification, were used as photoactive ana- logues of the undamaged chain [51]. The variation of the type of reagents and damages and their mutual location provides possibilities to design the efficient probes for the NER the mechanism investigation using the affinity modification method. The experiments using the photoactive model DNA resulted in a considerable advancement in the under- standing of the mechanism of key stages of the NER eu- karyotic system – damage recognition and preincision complex formation. The pyrimidine nucleoside-5'-tri- phosphates containing bulky fluorochloroazidobenzo- yl (Fab) and fluorochloroazidopyridyl (Fap) groups ha- ve good substrate properties in the DNA-polymerase re- action and the efficiency as photoreagents. That allow- ed to synthesize the set of DNA-probes with photoreac- tive damages [50]. The analysis of the interaction of human recombi- nant RPA and XPA with the modified DNA-duplexes of different structure as well as single-stranded DNA, con- taining Fab-dUMP (5-{[4-(4-azido-2,3,5,6-tetrafluoro- benzoylamino) butyl]-aminocarbonyl-carbamoyl-pro- pyl-oxymethyl}-2'-deoxyuridine-5'-monophosphate) or fluorochloroazidopyridyl modifications Fap-dUMP (5- {N-[N-(4-azido-2,5-difluoro-3-chloropyridyl-6-yl)-3- aminopropionyl]-trans-3-aminopropenyl-1}-2'-deoxy- uridine-5'-monophosphate) or Fap-dCMP (exo-N-{2- [N-(4-azido-2,5-difluoro-3-chlorpyridine-6-yl)-3-ami- nopropionyl]-aminoethyl}-2'-deoxycytidinine-5'-mono- phosphate), demonstrated that the efficiency of photo- affinity labeling depends on the structure of both DNA and the photoanalogue. The presence of extended sing- le-stranded fragments in the DNA-probe played a cru- cial role in the interaction with RPA. The exception was effective RPA-DNA adducts formation when the probe represents DNA duplex containing nick flanked by Fab- dNMP on 5'-side. This was likely to be related to the substantial struc- tural distortion in the DNA, induced by bulky modifi- cation at the 5'-end of the oligonucleotide, bearing high flexibility in the undamaged strand. XPA did not demon- strate preference to single-stranded or double-stranded DNA, and was interacted more efficiently with DNA- duplexes containing nick [12]. 175 MODEL DNA FOR INVESTIGATION OF MECHANISM OF NUCLEOTIDE EXCISION REPAIR The interaction of XPC-HR23B, RPA and XPA with the DNA-duplexes, containing the photoactive dama- ges Fap-dC and Fap-dU in the inner positions, was also studied. Fap-dC in the context of 3nt mismatch interac- ted with XPC more efficiently. This is in agreement with a higher level of specific excision of the damaged frag- ment from the corresponding plasmid DNA. The level of the photocrosslinking as well as the level of specific ex- cision for Fap-dU did not depend considerably on the complementarity of the pair, containing the damage [52]. To analyze NER factors capability to contact with the damaged and undamaged strands of the substrate, the set of DNA probes of different structure have been used. Photoactive strand of the ds DNA contained damage- imitating Fap-dNMP or not distorting 4-S-dUMP. The second DNA strand either did not contain any modifica- tions, or contained a bulky modified nucleotide (Antr- dCMP) opposite the photoactive group. During the pho- tocrosslinking of XPC-HR23B the covalent adducts with 48-bp-DNA probe were formed only by the XPC-subu- nit of the heterodimer. The XPC modification level was increased by the RPA presence. The amount of photo- crosslinked XPC was below the detection level while using the DNA with in oppositely located bulky modifi- cations in both strands. This indicated that the binding of XPC required the intact complementary strand oppo- site the damage. The direct evidence of the role of the undamaged DNA strand in this interaction was later ob- tained using the X-ray structural analysis of the yeast or- tholog of XPC complex and other experiments [92, 93]. The introduction of bulky modification into the second strand decreased the level of RPA modification conside- rably, which is also explained by preferential coupling of the undamaged chain that is the function, performed by RPA in the NER process [94, 95]. The presence of the second damage did not impact the XPA modifica- tion considerably. Therefore, the data, obtained due to the experiments of the photocrosslinking of RPA and XPA, indicates that the XPA factor prefer the damaged chain, and the RPA factor – the undamaged chain [96]. The equilibrium fluorescent titration was used to es- timate the parameters of the affinity of RPA to the seri- es of DNA-duplexes, containing bulky substitutes, intro- duced into the pyrimidine bases. Since this protein has twice higher affinity to the single-stranded DNA than to double-stranded DNA, it was assumed that in this case RPA may serve as a specific sensor of a single-strand character in the DNA-duplexes, caused by the distortion of the DNA structure via the introduction of bulky sub- stitutes. It was demonstrated that the affinity of the mo- dified DNA exceeds 3–20-fold the affinity of this pro- tein to the DNA-duplex, not containing any modifica- tions. Using 5-J-dU-containing duplexes it was demonst- rated that the level of RPA modification decreased ra- pidly when an bulky substitute introdused in the comp- lementary chain opposite photoactive nucleotide [79]. The stimulating effect of RPA on the formation of XPC-HR23B complexes with DNA was observed in the analysis of the interaction of proteins XPC-HR23B, XPA and RPA with the duplexes, containing the photoreac- tive residue 5-J-dUMP in one of the strands, and fluo- rescein-replaced derivative of dUMP as the damage – in the second one. The synergism in the interaction of proteins XPA and XPC-HR23B with DNA was obser- ved in case of their simultaneous presence in the reaction mixture. The amount of modification products of pro- teins XPA, RPA and XPC was depended on the mutual location of the damage and the photoactivated group and correlated with the affinity of these proteins to the damaged DNA, estimated by the gel retardation [91]. The detailed analysis of the interaction of XPA and RPA with the damaged and undamaged DNA strands was performed using a series of photoactive J-dU-DNA with different mutual location of the photoreagent and a damaged nucleotide, which contains a fluorescein resi- due [90]. For this experiments the model DNA mimi- cking the repair intermediate (a DNA-duplex contai- ning an noncomplementary fragment of 15 nucleoti- des), were used. The application of the photocross-lin- king methods, gel retardation and enzymatic footprin- ting allowed determining the site of the damaged DNA, with which these proteins contact more intensively. It was confirmed that XPA and RPA mutually stimulate their binding to DNA. It was also confirmed that RPA mainly interacts with the undamaged DNA strand that supported the conclusion that XPA has contacts with the damaged chain, located at the 5'-side of the damage. The asymmetry regarding the damaged and undamaged chains and the synergism in the interaction of XPA and RPA with the damaged DNA was also observed in some previous works. The cisplatin-induced intrastrand lin- king of DNA and cholesterol modifications were used 176 EVDOKIMOV A. N. ET AL. as damages in these works, while J- and Br-dUMP as well as J-dCMP were used as photoreagents [14, 88]. The photoaffinity labeling together with the estima- tion of the affinity to the DNA-substrate by the gel retar- dation and the equilibrium titration allowed comparing the properties of human recombinant XPC-HR23B and yeast Rad4/Rad23 [93]. The comparative analysis of the interaction of two eukaryotic sensors with bulky dama- ges and a set of model DNA-duplexes, containing a fluorescein residue and 5-J-dUMP as a photoactive link in different positions of the chain revealed crucial simi- larity in their interaction with the damaged DNA and so- me differences in the details of this interaction. The da- ta on the contacts of these proteins with the DNA contai- ning bulky lesion, supports model, created previously on the basis of the results of the X-ray structural analysis. In this analysis the crystals of the triple complex contai- ning Rad23, the truncated form of Rad4 and the DNA- duplex containing the UV-damage was used [92]. As for the duplexes, containing both Fap-dC and the platinum adduct in one chain [82], regardless of the mu- tual location of these bulky modifications in the DNA molecule, the target of the modification was only a large subunit of XPC-HR23B complex. The second adduct with lower electrophoretic mobility is the most likely product of photocrosslinking of the DNA-binding subu- nit of XPC along other amino acid residues [89]. The products of labeling of the 58 kDa regulatory subunit HR23B were not detected in any reaction mixtures af- ter the photocrosslinking. In addition, the product of cross-linking between XPC and HR23B subunits, re- vealed using the Western-blotting, was not have radio- active label and is formed regardless of the presence of the DNA-probe [82]. The absence of the products of photocross-linking of HR23B to the analogues of the da- maged DNA indicated that this subunit of the complex does not participate in any direct contacts with DNA. One might assume that the absence of the modification products for a small subunit was conditioned by the to- pology of the heterodimer coupling with DNA, steric complications, caused by the location of subunits. How- ever, it has been recently shown in the experiments in vivo that a small subunit is released from the complex while XPC binds to the damage [97]. To investigate the order of NER factors binding to the DNA-substrate, the model DNA, containing the 1,3- intrastrand link G*CG*, obtained in the reaction with cis-diaminodichloroplatinum was used. It should be no- ted that the structure of this model DNA is characteri- zed in details. For instance, it is known that the axis of dsDNA strand, containing 1,3-Pt-modification, forms an approximate 150° angle, and the complementary in- teractions of bases from the position «–3» (relative to the damage) to the position «+3» were distorted [98– 100]. The photoaffinity modification experiments were performed using DNA, containing the photoactive 5- (N-p-azidobenzoyl)-3-aminoallyl)-dUMP (Az-dUMP) in with such a Pt-damage in the same strand. The data, obtained in the permanganate foot-printing in the pre- sence of recombinant XPC-HR23B, RPA, XPA, XPF, XPG and TFIIH (in different combinations), together with the data of photocrosslinking allow to detect the conformational changes in the damaged DNA. These changes were induced by the binding of NER factors and led to the formation of an asymmetric bubble around the damage from the position «–20» to «+9». The order of binding and positioning of these proteins was sug- gested. However Az-dUMP can also introduce some distortions in the dsDNA structure, the modification patterns actually reflect the interaction of the NER fac- tors and DNA, containing two damages, so the inter- pretation of the photocross-linking results becomes complicated. The research, performed with the set of photoactive model DNA, also provided insight into the mechanism of functioning of the prokaryotic system NER (UvrABC). One of the key stages of the work of the NER proka- ryotic system, the mechanism of which has not been de- termined for a long time, is the transfer of the damaged DNA from UcrA to UvrB, preceding the UvrC-cata- lyzed incision. Two types of arylazide photoaffinity re- agents were used to determine this mechanism. The role of the damage in the first type of probes was played by the fluorescein residue, introduced as a substituent via a linker on deoxytimidine. In this case p-azidophenacyl derivative of deoxyoligonucleotide was used as a photo- active analogue of the undamaged chain. Other probes contained photoactive damages Fabc-dUMP or Fab- dCMP, modified nucleotides, where a fluoroaryloben- zoyl group was introduced via linkers on nitrogenous bases. The application of photoactive DNA-analogues of substrates revealed the principles of damage recog- 177 MODEL DNA FOR INVESTIGATION OF MECHANISM OF NUCLEOTIDE EXCISION REPAIR nizion and the details of interaction between factors UvrA, UrvB and UrvC and DNA in the NER process. It was demonstrated that the «transfer» of the damaged DNA from UvrA to UvrB consists of three stages. At the first stage UvrA and UvrB are linked to the place of the damage, and the direct contact with the damage is performed by UvrA. At the second stage there is a reac- tion of relocation of the damage containing fragment of DNA strand with the ATP consumption, where UvrB mainly contacts with the undamaged DNA, and at the third stage a special pocket-like structure of UvrB binds to the damage with the simultaneous release of UvrA. Then the complex ready for the excision is formed with the introduction of UvrC into this complex [51]. The photoaffinity modification technique was appli- ed to search for mammalian cells NER-competent ex- tracts proteins, specifically interacting with bulky sub- stituted DNA. The proteins of HeLa cell extract selecti- vely and efficiently interacted with Fap-dC- and Fap- dU-duplexes (48 bp). The major targets of the modifica- tion were several proteins with apparent molecular weight of 35–90 kDa. The application of specific antibodies al- lowed to identify a large RPA subunit (p70) as modifi- cation target [89]. The experiments with the purified re- combinant protein also demonstrated that RPA effici- ently crosslinked with dsDNA, containing bulky dama- ges. The major role in the interaction with damaged DNA was played by p70 [79]. Using the more extended DNA probe (137 bp Fap-dC-DNA) the relationship of modification and the ratio of the concentrations of DNA and protein extract was demonstrated. Decrease of the concentration of extract proteins was led to the broadening of the set of the targets forming covalent adducts to DNA. The most probable reason of the observed effect was the increase in the possibility for proteins with the high- er affinity to the damaged DNA to compete for the pho- toactive group with the abundant protein of cancer cell extracts – Ku70/80 antigen [101]. This assumption was confirmed by the differences in the patterns of the protein modification of the NER-competent extracts of HeLa and the Chinese hamster ovarian cells (CHO). The application of the functional test, based on the use of NAD+, allowed identifying the 115-kDa protein of the CHO extract, forming the adducts with the extended DNA-probe, as PARP1 [102]. The experiments with dsDNA probes of different length, containing photoactivated (benzophenone) deri- vatives of cisplantin allowed performing the mass- spectrometry identification of a number of proteins of the mammalian cell extracts, interacting with the bulky damage. It should be noted that there are differences in the modification patterns, determined using the probes of different length, containing similar modification. The main targets of photocrosslinking for 1,3 Pt(GCG)- DNA probe were Ku70/Ku80, DNA-dependent protein- kinase, DNA-ligase III, PARP1 and RPA (p70) [73]. The model DNA and the estimation of the exci- sion activity of the NER system. The recognition and excision of damage is a key stage of NER. The accurate and reliable evaluation of the specific excision activity is required both for medical and fundamental research. It would be useful for the investigation of the mecha- nisms of bulky damage repair in reconstituted systems. Also the estimation of the NER system activity in the tu- mor cells would be useful to select appropriate chemo- therapeutic drugs. It would probably help to correct the method of anticancer treatment during therapy. Not very sophisticated and convenient method of NER system functional status control has not yet developed. One of the important conditions for the development of this me- thod is the existence of easily accessible and effective substrate of excision. The long linear dsDNA appears most promising for the elaboration of a NER activity test system, despite some of the problems associated with their use in the real systems like extracts of tissues and cells [101, 103, 104]. The linear DNA substrates are more available, especially if for their preparation can be used the standard automatic solid-phase synthesis. The second important characteristic is good sub- strate properties of the model damage, high efficiency of its conversion. For instance, the occurrence of the products of the excision reaction was not registered for the linear DNA duplex of 120 bp, containing the acetyl- aminofluorene modification (AAF-dG). A well-de- tectable level of excision of the AAF-dG-containing fragment was observed only for the circular substrate [27]. The model DNA, containing nAnt and nFlu have be- en suggested as a substrates of NER system. The modi- fied deoxyoligonucleotides were synthesized using the standard solid-phase synthesis and the corresponding 178 EVDOKIMOV A. N. ET AL. non-nucleosidic phosphoramidites. The nFlu and nAnt containing extended (137 bp) linear DNA-duplexes de- monstrated the properties of efficient substrates of spe- cific excision reaction, catalyzed by the proteins of the NER-competent extracts of eukaryotic cells. It has been shown that incubation of these DNA with extracts of CHO cells as well as the extracts, obtained from HeLa, SiHa and C33A cells results in the formation of the frag- ments of specific sequence and size. The levels of a rela- tive activity of specific endonucleases of the extracts differed 1.5–2-fold, and the excision profiles matched for all the extracts. Therefore, the use of nFlu- and nAnt- DNA allows estimating the level of the activity of the NER system in the extracts of cells of different origin. The radioactive labeling of the reaction products was performed using the DNA-polymerase and �-32P-dCTP. The template for fill-in synthesis was complementary to the DNA fragment which contains the damage [105]. The 5'-end of the template contains the oligo-dG-frag- ment; the 3'-end is protected from elongation with phos- phorylation. This approach allows using the non-radio- active model DNA. At the same time the sensitivity of the method is considerably increased and one can reve- al the products of the specific hydrolysis even at low ex- cision levels. Using the model DNA, containing the radioactive label (32P) near the damage, it is possible to perform the direct detection of the excision products [64]. The ad- vantages of this method are the absence of the addi- tional stages and the possibility of a quantitative estima- tion of the excision efficiency. Considerable disadvan- tages of this method should be also noted. Firstly, the applied model DNA should have high relative radioac- tivity which requires additional special protection du- ring the synthesis and storage of these compounds. Se- condly, such highly radioactive molecules have an ex- tremely short lifetime which limits their application in the series of experiments. Conclusions. Using various methods including the target mutagenesis, X-ray structural research as well as fluorescent microscopy in the living cell, the conside- rable progress have been done in the NER process in- vestigation. The biochemical approaches based on the application of the model DNA though remain very ap- plicable to get insight of NER proteins interaction with DNA-substrate and their mutual interactions in the spe- cific nucleoprotein complexes [72, 106, 107]. The ac- tuality of these investigations is also supported by the broadening of the spectrum of the genotoxic factors. In addition, the majority of antitumor agents, used in the clinical practice, also realize their cytotoxicity via da- maging DNA. However due to a wide substrate specifi- city of NER, this repair system action interfere with the safety of the damages, thus the medicinal effect of the drugs is decreased. Therefore, the understanding of the mechanisms of functioning of protein complexes and the possibility of estimation of the functional status of the NER system using the model DNA are required for successful design of drugs for the anticancer chemotherapy. Funding. The work was supported by the Russian Foundation for Fundamental Research (Grant 12-04- 00487a), the Russian Academy of Sciences (the pro- grams of fundamental research in the field of molecular and cell biology), the Ministry of Education and Sci- ence of the Russian Federation (NSh-420.2014.4) and COMFI 13-04-40197. Ìîäåëüíûå ÄÍÊ äëÿ èññëåäîâàíèÿ ìåõàíèçìà íóêëåîòèäíîé ýêñöèçèîííîé ðåïàðàöèè À. Í. Åâäîêèìîâ, Î. È. Ëàâðèê, È. Î. Ïåòðóñåâà Ðåçþìå ÄÍÊ æèâûõ êëåòîê íàõîäèòñÿ ïîä ïîñòîÿííûì âîçäåéñòâèåì ðàçëè÷íûõ ïîâðåæäàþùèõ ôàêòîðîâ ýêçî- è ýíäîãåííîãî ïðîèñ- õîæäåíèÿ. Íóêëåîòèäíàÿ ýêñöèçèîííàÿ ðåïàðàöèÿ (NER) óäàëÿåò èç ÄÍÊ øèðîêèé íàáîð îáúåìíûõ àääóêòîâ, îáðàçîâàâøèõñÿ â ðå- çóëüòàòå âîçäåéñòâèÿ ÓÔ îáëó÷åíèÿ, à òàêæå ýëåêòðîôèëüíûõ âåùåñòâ – çàãðÿçíèòåëåé îêðóæàþùåé ñðåäû, îêàçûâàþùèõ ìó- òàãåííîå äåéñòâèå, è õèìèîïðåïàðàòîâ.  ïðîöåññå ðåïàðàöèè, ïðîâîäèìîé ñèñòåìîé NER ìëåêîïèòàþùèõ, ïðîèñõîäèò ñïåöè- ôè÷åñêîå âûùåïëåíèå èç ÄÍÊ ôðàãìåíòîâ ðàçìåðîì 24–32 íóêëåî- òèäà, ñîäåðæàùèõ ïîâðåæäåíèÿ. Ïîñëåäóþùèé ðåïàðàòèâíûé ñèíòåç è ëèãèðîâàíèå ÄÍÊ âîññòàíàâëèâàþò èíòàêòíîñòü ñïè- ðàëè ÄÍÊ. Èäåíòèôèöèðîâàíû ãåíû, èíàêòèâèðîâàííûå â NER- äåôèöèòíûõ êëåòêàõ âûñøèõ ýóêàðèîòîâ.  ðåïàðàöèè ó÷àñòâó- þò ïðèìåðíî 30 áåëêîâ, ôîðìèðóþùèõ ñïåöèôè÷åñêèå ìíîãîñóáú- åäèíè÷íûå êîìïëåêñû. Ñèñòåìà NER õàðàêòåðèçóåòñÿ øèðîêîé ñóáñòðàòíîé ñïåöèôè÷íîñòüþ è ïðè ýòîì áîëüøèìè ðàçëè÷èÿìè â ýôôåêòèâíîñòè óäàëåíèÿ ïîâðåæäåíèé. Êëþ÷åâîé ëèìèòèðóþ- ùåé ñòàäèåé ïðîöåññà ÿâëÿåòñÿ óçíàâàíèå è âåðèôèêàöèÿ ïîâðåæ- äåíèÿ. Ê ýôôåêòèâíûì è ðàçâèâàþùèìñÿ ïîäõîäàì ê èññëåäîâà- íèþ ïðîöåññà NER ïðèíàäëåæèò ìåòîä, îñíîâàííûé íà èñïîëü- çîâàíèè ìîäåëüíûõ ÄÍÊ – ñèíòåòè÷åñêèõ ñòðóêòóð, ÿâëÿþùèõ- ñÿ àíàëîãàìè ñóáñòðàòà èëè èíòåðìåäèàòîâ ýòîãî ïðîöåññà. Ðàññìîòðåíû ñóùåñòâóþùèå äàííûå î ñïîñîáàõ êîíñòðóèðîâà- íèÿ ìîäåëüíûõ ÄÍÊ è ïðèìåíåíèè èõ â êà÷åñòâå èíñòðóìåíòà äëÿ âñåñòîðîííåãî èçó÷åíèÿ ïðîöåññà NER. Êëþ÷åâûå ñëîâà: íóêëåîòèäíàÿ ýêñöèçèîííàÿ ðåïàðàöèÿ, îáú- åìíûå ÄÍÊ-àääóêòû, ìîäåëüíûå ÄÍÊ. 179 MODEL DNA FOR INVESTIGATION OF MECHANISM OF NUCLEOTIDE EXCISION REPAIR Ìîäåëüí³ ÄÍÊ äëÿ äîñë³äæåííÿ ìåõàíèçìó íóêëåîòèäíî¿ åêñöèç³éíî¿ ðåïàðàö³¿ À. Í. ªâäîêèìîâ, Î. ². Ëàâðèê, ². Î. Ïåòðóñåâà Ðåçþìå ÄÍÊ æèâèõ êë³òèí ïåðåáóâຠï³ä ïîñò³éíèì âïëèâîì ð³çíî- ìàí³òíèõ ïîøêîäæóþ÷èõ ôàêòîð³â åêçî- ³ åíäîãåííîãî ïîõîä- æåííÿ. Íóêëåîòèäíà åêñöèç³éíà ðåïàðàö³ÿ (NER) âèäàëÿº ç ÄÍÊ øèðîêèé íàá³ð îá’ºìíèõ àäóêò³â, ÿê³ óòâîðèëèñÿ â ðåçóëüòàò³ 䳿 ÓÔ îïðîì³íåííÿ, à òàêîæ åëåêòðîô³ëüíèõ ðå÷îâèí – çàáðóäíþâà- ÷³â äîâê³ëëÿ, ùî ÷èíÿòü ìóòàãåííèé âïëèâ, òà õ³ì³îïðåïàðàò³â. Ó ïðîöåñ³ ðåïàðàö³¿, ÿêó âèêîíóº ñèñòåìà NER ññàâö³â, â³äáóâàºòü- ñÿ ñïåöèô³÷íå âèùåïëþâàííÿ ç ÄÍÊ ôðàãìåíò³â ðîçì³ðîì 24–32 íóêëåîòèäè, ùî ì³ñòÿòü ïîøêîäæåííÿ. Ïîäàëüøèé ðåïàðàòèâ- íèé ñèíòåç ³ ë³ãóâàííÿ ÄÍÊ â³äíîâëþþòü ³íòàêòí³ñòü ñï³ðàë³ ÄÍÊ. ²äåíòèô³êîâàíî ãåíè, ³íàêòèâîâàí³ â NER-äåô³öèòíèõ êë³òèíàõ âèùèõ åâêàð³îò³â.  ðåïàðàö³¿ áåðóòü ó÷àñòü ïðèáëèçíî 30 á³ëê³â, ÿê³ ôîðìóþòü ñïåöèô³÷í³ ìóëüòèñóáîäèíè÷í³ êîìïëåêñè. Ñèñòå- ìà NER õàðàêòåðèçóºòüñÿ øèðîêîþ ñóáñòðàòíîþ ñïåöèôè÷í³- ñòþ ³ ïðè öüîìó âåëèêèìè ðîçá³æíîñòÿìè â åôåêòèâíîñò³ âèäà- ëåííÿ ïîøêîäæåíü. Êëþ÷îâîþ ë³ì³òóþ÷îþ ñòà䳺þ ïðîöåñó º óï³- çíàâàííÿ òà âåðèô³êàö³ÿ ïîøêîäæåí. Äî åôåêòèâíèõ ³ òàêèõ, ùî ðîçâèâàþòüñÿ, ï³äõîä³â äî âèâ÷åííÿ ïðîöåñó NER íàëåæèòü ìå- òîä, çàñíîâàíèé íà âèêîðèñòàíí³ ìîäåëüíèõ ÄÍÊ – ñèíòåòè÷íèõ ñòðóêòóð, ÿê³ º àíàëîãàìè ñóáñòðàòà àáî ³íòåðìåä³àò³â öüîãî ïðîöåñó. Ðîçãëÿíóòî ³ñíóþ÷³ äàí³ ùîäî ñïîñîá³â êîíñòðóþâàííÿ ìîäåëüíèõ ÄÍÊ òà çàñòîñóâàííÿ ¿õ ÿê ³íñòðóìåíòà äëÿ âñåá³÷íî- ãî äîñë³äæåííÿ ïðîöåñó NER. Êëþ÷îâ³ ñëîâà: íóêëåîòèäíà åêñöèç³éíà ðåïàðàö³ÿ, îá’ºìí³ ÄÍÊ-àäóêòè, ìîäåëüí³ ÄÍÊ. REFERENCES 1. Dip R, Camenisch U, Naegeli H. Mechanisms of DNA damage recognition and strand discrimination in human nucleotide ex- cision repair. DNA Repair (Amst). 2004; 3(11):1409–23. 2. Scharer OD. Chemistry and biology of DNA repair. Angew Chem Int Ed Engl. 2003; 42(26):2946–74. 3. Gillet LC, Scharer OD. Molecular mechanisms of mammalian global genome nucleotide excision repair. Chem Rev. 2006; 106 (2):253–76. 4. Lehmann AR. DNA repair-deficient diseases, xeroderma pig- mentosum, Cockayne syndrome and trichothiodystrophy. 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