International Journal of Molecular Sciences Review Focus on UV-Induced DNA Damage and Repair—Disease Relevance and Protective Strategies Mateusz Kciuk 1,2,* , Beata Marciniak 2 , Mariusz Mojzych 3 and Renata Kontek 2 1 Doctoral School of Exact and Natural Sciences, University of Lodz, Banacha Street 12/16, 90-237 Lodz, Poland 2 Department of Molecular Biotechnology and Genetics, Faculty of Biology and Environmental Protection, University of Lodz, 12/16 Banacha St., 90-237 Lodz, Poland; [email protected] (B.M.); [email protected] (R.K.) 3 Department of Chemistry, Siedlce University of Natural Sciences and Humanities, 3 Maja 54, 08-110 Siedlce, Poland; [email protected] * Correspondence: [email protected] Received: 24 August 2020; Accepted: 29 September 2020; Published: 1 October 2020 Abstract: The protective ozone layer is continually depleting due to the release of deteriorating environmental pollutants. The diminished ozone layer contributes to excessive exposure of cells to ultraviolet (UV) radiation. This leads to various cellular responses utilized to restore the homeostasis of exposed cells. DNA is the primary chromophore of the cells that absorbs sunlight energy. Exposure of genomic DNA to UV light leads to the formation of multitude of types of damage (depending on wavelength and exposure time) that are removed by effectively working repair pathways. The aim of this review is to summarize current knowledge considering cellular response to UV radiation with special focus on DNA damage and repair and to give a comprehensive insight for new researchers in this field. We also highlight most important future prospects considering application of the progressing knowledge of UV response for the clinical control of diverse pathologies. Keywords: UV radiation; DNA damage; DNA repair; photoproducts; ROS 1. Introduction The ozone layer in the upper atmosphere is continually diminished due to release of pollutants that damage the protective barrier. As a consequence, exposure to ultraviolet (UV) light is increasing. DNA is the main target of UV radiation both in lower organisms such as bacteria and higher such as placental mammals including humans [1,2]. Deteriorating effects of UV radiation on DNA include formation of various photoproducts, generation of free radicals and subsequent introduction of strand breaks [3]. In response to these threats, cells have evolved multiple mechanisms that counter the damage. The DNA damage response (DDR) includes multitude damage signaling pathways and repair mechanisms that allow the preservation of genomic stability of cells [4,5]. 2. UV-Induced DNA Damage The two main photoproducts that arise during exposure to UV light include cyclobutane- pyrimidine dimers (CPDs), and 6-4 photoproducts (6-4PPs), but less prevalent Dewar valence isomers also may arise (Figure1). These less common photoproducts emerge when an additional photon is absorbed by (6-4PPs) [6]. DNA is damaged predominantly by longer wavelength radiation—UV-B (280–315 nm) and UV-A (315–400 nm)—while UV-C (<280 nm) is considered less harmful as it is absorbed by oxygen and the ozone layer of the atmosphere. UV-A, on the other hand, is a much less deleterious damaging factor than UV-B because it is not so effectively absorbed by the DNA. However, it can act as a photosensitizer and can promote the formation of secondary photoproducts [1,7]. Int. J. Mol. Sci. 2020, 21, 7264; doi:10.3390/ijms21197264 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2020, 21, x FOR PEER REVIEW 2 of 32 Int. J. Mol. Sci. 2020, 21, 7264 2 of 33 DNA. However, it can act as a photosensitizer and can promote the formation of secondary photoproducts [1,7]. The energy content of UV-radiation decreases with increase in wavelength with Theorder energy of: UV-C, content UV-B of and UV-radiation UV-A. The aromatic decreases ring with structures increase of in DNA wavelength bases absorb with short order wavelength of: UV-C, UV-BUV more and UV-A.efficiently, The aromaticthus UV-C ring and structures UV-B is of effectively DNA bases captured absorb short[8]. Moreover, wavelength longer UV more UV ewavelengthsfficiently, thus may UV-C also and induce UV-B small is eff ectivelyamounts captured of DNA [breaks8]. Moreover, and DNA-protein longer UV wavelengthsand interstrand may cross- also inducelinks [9–13]. small amounts of DNA breaks and DNA-protein and interstrand cross-links [9–13]. FigureFigure 1.1. Common ultraviolet (UV)-induced photoproducts.photoproducts. Cyclobutane-pyrimidine dimers (CPDs) comprisecomprise 75%75% ofof photodamage,photodamage, whilewhile 6-46-4 photoproductsphotoproducts (6-4PPs) constitute 25% of photodamage. DerivativesDerivatives ofof twotwo thyminethymine basesbases werewere shownshown inin thethe figure.figure. DespiteDespite thethe factfact thatthat 6-4PPs6-4PPs causecause moremore prominentprominent perturbations in the structure of DNA helix, CPDsCPDs areare the the main main cytotoxic cytotoxic lesions lesions responsible responsible for cellfor cell death death following following UV exposure. UV exposure. However, However, some studiessome studies indicate indicate that, duethat, to due inability to inability of cells of cells to bypass to bypass 6-4PPs, 6-4PPs, this this photolesion photolesion may may constitute constitute an importantan important contributor contributor to cell to cell death death [14]. [14]. In contrast, In contrast, some some studies studies indicate indicate that thethat formation the formation of 6-4PPs of 6- may4PPs havemay nohave impact no impact on mutagenesis on mutagenesis [15]. Also, [15]. 6-4PPs Also, 6-4PPs may undergo may undergo photoisomerization photoisomerization to Dewar to isomersDewar isomers upon exposure upon exposure to UV-B to or UV UV-A.-B or ThisUV-A. reaction This reaction is thought is th toought be effi tociently be efficiently catalyzed catalyzed by the wavelengthsby the wavelengths longer thanlonger 290 than nm [2901]. Moreover,nm [1]. Moreover, CPDs may CPDs also may be reverted also be reverted upon exposure upon exposure to UV-light to ofUV-light 230 nm of wavelength 230 nm wavelength [15]. [15]. 3. Pyrimidine Photoproducts 3. Pyrimidine Photoproducts In CPDs adjacent pyrimidine bases are covalently linked in [2 + 2] cycloaddition reaction involving In CPDs adjacent pyrimidine bases are covalently linked in [2 + 2] cycloaddition reaction C5 and C6 carbon atoms, while in 6-4PPs, a noncyclic bond is formed between the C6 and C4 atoms involving C5 and C6 carbon atoms, while in 6-4PPs, a noncyclic bond is formed between the C6 and of pyrimidine residues. The formation of 6-4PPs is a two-step process in which two intermediates C4 atoms of pyrimidine residues. The formation of 6-4PPs is a two-step process in which two can be formed. The oxetane intermediate is formed when the 3 -end is thymine. On the other hand, intermediates can be formed. The oxetane intermediate is formed0 when the 3′-end is thymine. On the an azetidine intermediate is formed when the 3 -end is cytosine. For CPDs two bases can be on the other hand, an azetidine intermediate is formed0 when the 3′-end is cytosine. For CPDs two bases can same or on the opposite sides of cyclobutane ring. These stereoisomers are referred to as cis and trans be on the same or on the opposite sides of cyclobutane ring. These stereoisomers are referred to as cis stereoisomers. Additionally, bounding of C5 (or C6) atom of one pyrimidine with the C5 (or C6) atom and trans stereoisomers. Additionally, bounding of C5 (or C6) atom of one pyrimidine with the C5 of the second pyrimidine leads to syn configuration of the bases. The anti-configuration is achieved (or C6) atom of the second pyrimidine leads to syn configuration of the bases. The anti-configuration when the C5 atom of one pyrimidine is linked with the C6 atom of other pyrimidine base. It has been is achieved when the C5 atom of one pyrimidine is linked with the C6 atom of other pyrimidine base. found that cis-syn-configured CPDs are formed much more frequently than trans-syn-configured CPD. It has been found that cis-syn-configured CPDs are formed much more frequently than trans-syn- CPDs may also be reverted to original bases following exposure to UV-C light [2,7,16]. On the other configured CPD. CPDs may also be reverted to original bases following exposure to UV-C light hand, 6-4PPs exist only in two stereoisomers after irradiation. Moreover, 6-4PPs may be converted [2,7,16]. On the other hand, 6-4PPs exist only in two stereoisomers after irradiation. Moreover, 6-4PPs into Dewar valence isomers. The new covalent bond is then formed between the N3 and C6 atoms may be converted into Dewar valence isomers. The new covalent bond is then formed between the of the pyrimidone following absorption at 315/325 nm. For both photoproducts containing cytosine N3 and C6 atoms of the pyrimidone following absorption at 315/325 nm. For both photoproducts bases, a deamination reaction may occur. In this reaction, an exocyclic amine group in a cytosine is lost, containing cytosine bases, a deamination reaction may occur. In this reaction, an exocyclic amine and the base is converted into uracil [16]. The formed uracil can be further converted to thymine after group in a cytosine is lost, and the base is converted into uracil [16]. The formed uracil can be further two rounds of DNA replication. Additionally, cytosine residues in CpG sequences can be modified to converted to thymine after two rounds of DNA replication. Additionally, cytosine residues in CpG 5-methylcytosine and deaminated to thymine residues [17]. sequences can be modified to 5-methylcytosine and deaminated to thymine residues [17]. Additionally, several other minor photoproducts were identified. These include cytosine Additionally, several other minor photoproducts were identified. These include cytosine photohydrate (6-hydroxy-5,6-dihydrocytosine) formed during addition of water molecule to a C5–C6 photohydrate (6-hydroxy-5,6-dihydrocytosine) formed during addition of water molecule to a C5– double bond of the cytosine (photohydration reaction) (Figure2).