PROFILE PROFILE Profile of Tomas Lindahl, Paul Modrich, and Aziz Sancar, 2015 Nobel Laureates in Chemistry James E. Cleavera,1 In 1994, Science magazine heralded “the DNA repair Academy of Sciences (NAS), also recognized DNA re- enzyme” as the molecule of the year, on the basis of pair, now often called the DNA damage response to several significant advances made in mismatch repair encompass its increased breadth. and nucleotide excision repair, both recognized by This year’s awards are to Tomas Lindahl for pio- this year’s Chemistry Nobel awards, that heralds an neering work deriving from his prediction that the in- even better year for the field. This year’s Nobel Prize herent instability of DNA in an aqueous, oxygenated for Chemistry recognizes the importance of DNA re- environment required mechanisms for repair (2); to pair as a major player in maintenance of our genomes Paul Modrich for the mechanism of mismatch correc- and its recognition as a significant contribution to the tions caused by replication errors (3); and to Aziz San- biochemistry of DNA. Although the structure of DNA car for detailed mechanistic study of nucleotide provided a failsafe mechanism by which a damaged excision repair (NER) (4) and photolyase (5, 6). The strand could be mended using the sequence informa- award recognizes their contributions to understanding tion on the complementary strand, Frances Crick fa- the chemistry of DNA repair processes (see Fig. 1). mously admitted, “We totally missed the possible role Lindahl started his career as a medical student in of enzymes in repair...” (1). By a remarkable coinci- the Karolinska Institute (Stockholm), and after work in dence, the 2015 Lasker awards to Stephen J. Elledge Princeton and the Rockefeller University, joined the and Evelyn Witkin, both members of the National faculty at the Karolinska Institute. He then moved to direct the Mutagenesis Laboratory at the Imperial Cancer Research Fund Mill Hill and eventually became pyr=pyr director of the Imperial Cancer Research Fund’s Clare Hall laboratories that became part of Cancer Research UK. He was elected Fellow of the Royal Society in 1988 and is currently emeritus group leader at the NER photolyase CRY1,2 Mismatch Francis Crick Institute (London). repair Paul Modrich received a PhD degree in 1973 from O6tase ALKB oxidase Stanford University and an SB degree in 1968 from GGR TCR Massachusetts Institute of Technology. He joined BER Duke University’s faculty in 1976 and has been a Howard Hughes Investigator since 1994, pioneering Sancar Lindhal and Sancar Modrich studies on mismatch repair in both bacteria and mam- malian cells. He was elected to the NAS in 1993 and to Fig. 1. Schematic diagram denoting the various pathways of DNA repair for which the National Academy of Medicine in 2003. Lindahl, Modrich, and Sancar were honored. Pyr = pyr, cyclobutane pyrimidine dimers that are substrates for photolyase in most organisms except placental Aziz Sancar was a physician in Turkey who came mammals; CRY1, 2, homologous proteins in placental mammals that are involved to the United States to complete his PhD on the in diurnal regulation. Red lozenge, class of large adducts including cyclobutane photoreactivating enzyme of Escherichia coli in 1977 pyrimidine dimers that are substrates for NER; dashed arrows, excision occurs by at the University of Texas at Dallas in the laboratory of cleavage of the phosphodiester bonds in DNA on either side of an adduct. GGR Dr. C Stanley Rupert and subsequently worked with and TCR are two branches of NER that act on nontranscribed and transcribed strands of DNA, respectively; TCR involves cofactors for RNA pol II. Purple disk, Dean Rupp at Yale. He is Professor of Biochemistry at class of single base modifications (alkylation) that are substrates for BER; dashed the University of North Carolina at Chapel Hill. With arrow, excision occurs by a family of glycosylases that cleaves the sugar his wife, Gwen B. Sancar, who is also a Professor of 6 base bond. Alternative repair of specific single base lesions include the O Biochemistry and Biophysics at UNC at Chapel Hill, alkyltransferase and the ALKB oxidase. Green wedge, mismatch in a newly synthesized strand repaired by mismatch repair; dashed arrows, excision they continue to support Turkish graduate students and resynthesis can be initiated from single-strand breaks distal from the and promote Turkish–American relations. Sancar was mismatch. elected to the NAS in 2005. aDepartment of Dermatology, University of California, San Francisco, CA 94143 Author contributions: J.E.C. wrote the paper. The author declares no conflict of interest. 1Email: [email protected]. 242–245 | PNAS | January 12, 2016 | vol. 113 | no. 2 www.pnas.org/cgi/doi/10.1073/pnas.1521829112 Downloaded by guest on September 26, 2021 Much of the work honored by this year’s Nobel via glycosylases and prizes harks back to early observations in 1950s and apurinic endonucle- 1960s of radiosensitive mutants in E. coli showing that ase (21). Lindahl was radiation damage was more than a hit and miss event but then able to recon- that cellular responses are under genetic and hence bio- stitute BER in vitro chemical control (7, 8). Much of the essential groundwork (22). Lindhal’s labo- was carried out in bacteria and only later in human ratory went on to cells. A molecular approach to DNA repair began with discover a family of Richard B Setlow’s observation that UV light-induced glycosylases with dif- damage (cyclobutane pyrimidine dimers) in DNA could ferent substrate spec- be biochemically characterized. He showed that dimers ificity and identify were actively removed from bacterial DNA over time numerous other en- Tomas Lindahl. Image courtesy of Cancer Research UK. (9). Pettijohn and Hanawalt showed that UV irradiated zymatic components bacteria carried out a form of DNA synthesis that was and novel mechanisms of repair including multiple li- distinct from normal semiconservative replication (10). gases, exonucleases, demethylation via alkyltransferase, In parallel, Robert B. Painter, a coinventor of tritiated and oxidation of cytosine methylation by AlkB (23). thymidine, the radioactive precursor of DNA, discov- Despite the versatility of NER and BER, small ered that UV-irradiated human cells also carried out mismatches that arise during replication errors and in novel DNA synthesis outside of the normal DNA syn- microsatellite repeat sequences appear to escape thesis period (11). These processes were later shown to their detection. These are repaired by the mismatch represent replacement synthesis of the excised dimers. repair system for which Paul Modrich was honored (3). Aziz Sancar, while in Rupp’s laboratory, developed Wagner and Meselson (24), among others, had pre- the maxi-cell method that enabled him to produce viously shown that mismatches were corrected in long purified repair proteins, UVRA, B, and C, from E. coli tracts involving the bacterial gene dam. Modrich was plasmids and reconstitute the bacterial NER process the first to develop a method for detecting mismatch in vitro (12). Unraveling the biochemical details of NER repair in vitro using plasmids with synthetic mis- in human cells began with the development in Lin- matches of various kinds (25). In E. coli, a mismatch dahl’s laboratory, with his colleague Richard Wood, is corrected by excision and resynthesis of the newly of a method for in vitro analysis (13). By an ingenious replicated DNA strand that contains the mismatched use of a pair of plasmids of differing sizes, only one of base. In bacteria, the mismatch in the newly replicated which contained damage, they could discriminate strand is marked by transiently unmethylated adenine specific repair from the background of nonspecific nu- (3, 24), but in mammalian cells may involve transient cleases that had bedeviled previous work in mam- nicks or remnants of RNA. Single-strand breaks in malian cells. Subsequent work in both Lindahl’sand DNA are made many nucleotides distant, 3′ or 5′ from Sancar’s laboratories eventually led to defining the the mismatch, in the new strand, initiate repair numerous components of human NER and recon- through coordinated action of proteins: MutS and structing the complete process in vitro (14, 15). The MutL that act as homo-oligomers and MutH, which is bacterial and mammalian systems are similar in prin- a nuclease. In human cells, a similar mechanism oper- ciple, both being a “cut and patch” process, but use ates, but using heterodimers. The human homolog to different panoplies of proteins and excise different MutS representing 80–90% of the repair activity is sized damaged oligonucleotides. The excised fragment MSH2:MSH6 (MutSα); the human homolog of MutL is 12–13 nt in bacteria and 24–32 nt in humans. NER is MUTLH1:PMS2 (MUTLα), representing 90% of the was found to have wide substrate specificity, possibly activity. Modrich demonstrated that a purified system due to a recognition mechanism using dedicated damage- that could carry out mismatch repair in vitro required specific binding proteins based on distortions to DNA MutSα, MUTLα, exonucease1, RPA, and ATP (3). No rather than precise enzymatic specificity (16). Mellon human homolog of MutH has yet been identified. Ge- (17) and Lindsey-Boltz and Sancar (18) later proposed netic inactivation of the human MutSα or MutLα that arrest of transcription by RNA pol II was an alter- proteins makes cells native and more sensitive damage sensor. NER by resistant to chemo- then had been discriminated into global genome re- therapeutic agents pair (GGR) and the more rapid transcription coupled by blocking apopto- repair (TCR), according to the transcriptional activity sis (26). of the gene regions repaired (19). This revealed a Sancar’sinitial close relationship between NER and transcription reg- work in the United ulation in which the transcription factor TFIIH con- States was with Stanley tained two helicases that unwound DNA around Rupert, who was the damaged site (20).