Cell Science at a Glance 515 DNA repair with its consequences (e.g. tolerance and pathways each require a number of apoptosis) as well as direct correction of proteins. By contrast, O-alkylated bases, Oliver Fleck* and Olaf Nielsen* the damage by DNA repair mechanisms, such as O6-methylguanine can be Department of Genetics, Institute of Molecular which may require activation of repaired by the action of a single protein, Biology, University of Copenhagen, Øster checkpoint pathways. There are various O6-methylguanine-DNA Farimagsgade 2A, DK-1353 Copenhagen K, Denmark forms of DNA damage, such as base methyltransferase (MGMT). MGMT *Authors for correspondence (e-mail: modifications, strand breaks, crosslinks removes the alkyl group in a suicide fl[email protected]; [email protected]) and mismatches. There are also reaction by transfer to one of its cysteine numerous DNA repair pathways. Each residues. Photolyases are able to split Journal of Cell Science 117, 515-517 repair pathway is directed to specific Published by The Company of Biologists 2004 covalent bonds of pyrimidine dimers doi:10.1242/jcs.00952 types of damage, and a given type of produced by UV radiation. They bind to damage can be targeted by several a UV lesion in a light-independent Organisms are permanently exposed to pathways. Major DNA repair pathways process, but require light (350-450 nm) endogenous and exogenous agents that are mismatch repair (MMR), nucleotide as an energy source for repair. Another damage DNA. If not repaired, such excision repair (NER), base excision NER-independent pathway that can damage can result in mutations, diseases repair (BER), homologous recombi- remove UV-induced damage, UVER, is and cell death. The cellular responses to national repair (HR), and non-homolo- present in only a few organisms, such DNA damage include processes that deal gous end joining (NHEJ). These as the yeast Schizosaccharomyces pombe. A key factor in UVER is the endonuclease Uve1/UVDE, which cuts 5′ of various types of damage. Recent work has uncovered novel pathways, Oliver Fleck and Olaf Nielsen such as transcription-coupled BER, break-induced replication, and nucleotide incision repair as well as Crosslinking agents Repair Tolerance Carcinogenic agents Alkylation interconnections between known Replication UV light Alkylation Oxidation Crosslinking Ionizing errors Deamination agents radiation pathways. For simplicity, we do not DNA damage consider these here. Although most me me O G TT<> G G A G = U G Checkpoint Apoptosis repair proteins are usually homologous T AA C C T A G G activation between organisms, their designations Mismatches Pyrimidine OmeG6 8oxoG Crosslinks Cell cycle dimers Loops Bulky adducts 3meA Mismatches Double-strand are often different. Here we generally arrest Crosslinks breaks use the names of human proteins. MMR Photolyases NER MGMT BER HR NHEJ UVER jcs.biologists.org Mismatch repair Mismatch repair (MMR) Nucleotide excision repair (NER) Non-homologous end joining (NHEJ) MSH2 MSH2 MSH2 Global genome Transcription-coupled The main task of MMR is to remove DNA-PKcs MSH6 MSH6 MSH3 repair (GGR) repair (TCR) Ku70 Ku80 G Ku80 Ku70 DNA-PKcs base mismatches and small insertion/ DDB1 XPC T hHR23B DDB2 TT<> AA MRE11 NBS1 AA ol IITT<<> deletion loops (IDLs) introduced during M P RAD50 H2 SH6 A MS RN CSB DNA-P G Kcs CSA Ku70 Ku80 MLH1 PMS2 Ku80 Ku70 replication. In Escherichia coli, the DNA-PKcs Strand discrimination TT<> Mediated by PCNA? TFIIH XPB XPD Pol ? Ligase IV ′ ′ main players in MMR are MutS, MutL 3 -5 exonuclease? XRCC4 Exo1? and MutH. MutH nicks the non- Exo1 Polδε , Pol ERCC1 XPA XPG PCNA XPF RFC, RPA TFIIH XPB RPA XPD methylated strand and thereby enables Ligase I O6-methylguanine-DNA methyltransferase (MGMT) discrimination between the newly e MGMT G m Polδε , Pol RFC, RPA G synthesized strand and the template. C Homologous recombinational repair (HR) PCNA Ligase I C e m MGMT MMR is bidirectional, i.e. nicking and MRE11 NBS1 degradation can occur from either the 5′ RAD50 RPA Base excision repair (BER) or 3′ side of the mismatch. In eukaryotes, RAD52 RAD51 paralogs O G = U several MutS and MutL homologues are RAD51 C G C AP endo involved in MMR; MutH homologues DNA glycosylase DNA glycosylase (bifunctional) (monofunctional) RAD54 C G C appear to be absent. Inactivation of AP endo Pol β AP lyase PCNA human MMR causes hereditary RFC Pol δε/ C G DNA synthesis AP endo Pol β Pol β nonpolyposis colorectal cancer Phosphodiesterase G C C (HNPCC) and some types of sporadic C G Pol β Pol β FEN1 Ligase III dRPase tumor. In the course of human MMR, XRCC1 Ligation Holliday junction Branch migration resolution C G G C base mismatches are bound by the Ligase III Ligase I XRCC1 MutS-homologous heterodimer MSH2- MSH6, while small IDLs can be bound by MSH2-MSH6 and MSH2-MSH3. Journal of Cell Science 2004 (117, pp. 515-517) Subsequently, the MutL-homologous (See poster insert) heterodimer MLH1-PMS2 is recruited. 516 Journal of Cell Science 117 (4) In some eukaryotes additional MutL damaged strand. After binding of XPF- processed by BER, has no biological homologues exist. These form ERCC1, dual incision occurs by XPG consequence. In fact, knockout mice heterodimers with MLH1 and may play and XPF-ERCC1, which cut 3′ and 5′ to lacking factors acting downstream of a minor role in MMR. It is not yet the damage, respectively. In this way, DNA glycosylases exhibit an embryonic understood how eukaryotes distinguish the damage is released in a 24-32 lethal phenotype, while a defect in a between the new and the old strand. nucleotide long oligonucleotide. Repair single DNA glycosylase does not cause Strand discrimination may be either is completed by DNA synthesis and any phenotypic abnormality. mediated by the replication accessory ligation. The typical disorder caused by factor PCNA or could be simply a defect in NER is xeroderma achieved by recognition of nicks, gaps or pigmentosum (XP), while Cockayne Homologous recombinational free 3′ ends that are present in the syndrome (CS) and trichothiodystrophy repair nascent strand during replication. In a (TTD) are due to impaired TCR and in Double-strand breaks (DSBs) can be downstream step, the newly synthesized the latter case eventually also to affected repaired by either HR or NHEJ. HR uses strand is degraded, which removes the transcription. a homologous DNA template and is mismatch. MMR patches are ~100 to highly accurate, whereas NHEJ rejoins >1000 nucleotides in length. EXO1 is the broken ends without using a template involved in 5′ to 3′ excision. It is not yet Base excision repair and is often accompanied by loss of clear which factors participate in 3′ to 5′ BER mainly repairs non-bulky lesions some nucleotides. The relative excision, but DNA Pol δ and ε and produced by alkylation, oxidation or contribution of each pathway depends on EXO1 may be involved. MMR is deamination of bases. Cells contain the cell-cycle stage, with NHEJ being completed after DNA synthesis by the several DNA glycosylases, each of them more active in G1 and HR dominating replication machinery and ligation of the exhibiting a specific substrate spectrum. during S and G2 phases. During HR remaining nick. After cleavage of the N-glycosylic bond DSBs are converted to 3′ single-stranded by a DNA glycosylase, the damaged DNA (ssDNA) tails, which are bound by base is released and an apurinic/ RPA. Processing of DSBs probably Nucleotide excision repair apyrimidinic (AP site) is created. An AP requires MRE11-RAD50-NBS1. NER removes a variety of forms of DNA site can also occur spontaneously and RAD52 interacts with RPA and damage, including photoproducts represents damage itself. Bifunctional promotes binding of RAD51 to the induced by UV and other bulky lesions. glycosylases have an intrinsic AP lyase ssDNA, which may be stabilized by NER consists of two subpathways: activity, which cleaves the sugar- RAD51 paralogues (RAD51B, global genome repair (GGR), which phosphate backbone 3′ to the AP site. RAD51C, RAD51D, XRCC2 and removes damage in the genome overall The resulting fragmented sugar residue XRCC3 in human, RAD55 and RAD57 and transcription-coupled repair (TCR), is removed by a phosphodiesterase in yeast). Subsequently, the RAD51- which specifically repairs the transcribed activity, contributed by either an AP bound ssDNA invades a homologous strand of active genes. The main endonuclease or by DNA polymerase β. molecule in a reaction stimulated by difference between GGR and TCR is the The one-nucleotide gap is filled by Pol β RAD54. After DNA synthesis and requirement for different factors during and ligated. Processing of AP sites ligation, two Holliday junctions are the initial recognition steps. UV-DDB, produced by a monofunctional DNA formed and branch migration can occur. consisting of DDB1 and DDB2, and glycosylase requires 5′ incision by an AP The Holliday junctions are finally XPC-hHR23B are involved in the endonuclease (the major human AP resolved by resolvases, which in recognition step of GGR, while TCR is endonuclease is APE1). Pol β eukaryotes are not yet identified. thought to be initiated by RNA incorporates a nucleotide and its polymerase II stalled at a lesion. deoxyribophosphodiesterase (dRPase) HR also represents an error-free Additional factors required for TCR are activity removes the 5′ moiety. The subpathway of damage tolerance, CSA and CSB. The proteins acting remaining nick is sealed by ligation. allowing replicational bypass of lesions further downstream in GGR and TCR During a minor, long-patch BER through a template switch. Alternatively, are likely to be identical. First, pathway, 2-8 nucleotides are removed damage tolerance can be achieved by transcription factor IIH (TFIIH), a together with the damaged nucleotide. error-free and error-prone translesion complex consisting of nine subunits, is Long-patch BER may be required in the synthesis carried out by specialized recruited to the damaged site.