This year's Nobel Prize in Chemistry went to a trio of researchers who all worked on how DNA sequences are maintained by minimizing the propagation of mutations. These processes were described by Dr. Aziz Sancar (University of North Carolina, Chapel Hill, NC, USA), Dr. Tomas Lindahl (Francis Crick Institute, Hertfordshire, United Kingdom, Clare Hall Laboratory, Hertfordshire, United Kingdom), and Dr. Paul Modrich (Howard Hughes Medical Institute, Durham, NC, USA, Duke University School of Medicine, Durham, NC, USA). Each of these Nobel Laureates isolated the enzymes for the machinery that carries out Nucleotide Excision Repair (NER), Base Excision Repair (BER), and Mismatch repair in the cell, respectively, and described the mechanisms by which these enzymes work. As such, their work is fundamental to cell biology, and provides the basis for studying a variety of processes, including cancer formation and progression, and for determining potential therapeutic targets.
Dr. Aziz Sancar exhibited that bacteria are able to repair UV induced damage, specifically thymine dimers, through the actions of the products of the genes uvrA, uvrB, uvrC, and uvrD, along with DNA polymerase I, and DNA ligase (Sancar and Rupp, 1983; Husain et al., 1985). Thymine dimers, in which two adjacent thymines are fused, if left unrepaired often lead to improper base insertion during replication, generating a mutation. Dr. Sancar showed that a complex of UvrA, UvrB, and UvrC scans the DNA molecule and recognizes the UV adduct, later shown to be recognized by UvrA specifically (Petit and Sancar, 1999). UvrB then works as a helicase to unwind the DNA locally, UvrC is swapped for UvrA, and together UvrB and C make incisions on either side of the abduct. UvrD is then recruited to the nicked DNA strand and displaces the small single stranded DNA molecule present between the nicks, releasing it. DNA polymerase I then synthesizes a new strand in the gap, which is ligated by DNA ligase (Husain et al., 1985). This mechanism outlined how external damage to DNA is dealt with to limit mutations, and was the basis for determining how eukaryotes deal with the same type of damage.
Dr. Tomas Lindahl showed that DNA was an inherently unstable molecule, and could undergo a large variety of modifications, such as cytosine deamination (Lindahl and Nyberg, 1974). This type of damage leads to the replacement of a cytosine with a uracil, the thymine equivalent in RNA that will pair with adenine instead of guanine in subsequent replication cycles. Dr. Lindahl investigated this event, leading to the discovery of the Uracil-DNA glycosylase (UNG) in E.coli, a discovery followed up by the discovery of other glycosylase enzymes (Lindahl, 1974; Lindahl, 1976). These enzymes recognize improper nucleotides in the DNA molecule and cleaves the aberrant nucleotide, leaving an abasic site in the first step of BER. An apurinc/apyrimidinic endonuclease then removes the abasic deooxyribose from the DNA strand, leaving an empty site which is filled by DNA polymerase and ligated by DNA ligase. This work showed the cellular machinery that is necessary for BER, to help the cell ensure that mutations caused by base modifications do not progress through replication events.
Dr. Paul Modrich helped determine the roles that various enzymes play, and how they are directed to the proper site, during Mismatch repair. Mismatch repair occurs when the DNA replication machinery incorporates an improper base, leading to non-Watson-Crick base pairs, and the cell works to repair it. Dr. Modrich showed that this type of repair is dependent on the methylation status of the DNA, specifically requiring a hemimethylated state, when the existing strand is methylated and the newly synthesized strand is not (Lu et al, 1983). He found that MutH bound to non-methylated GATC methylation sites on the new strand, and through interactions with MutL, recruited MutS (Su and Modrich, 1986; Welsh et al., 1987; Grilley et al., 1989). MutS recognizes the non-Watson-Crick base pair and activates MutH, which nicks the DNA strand where it is bound. UvrD is then recruited and, being a helicase, unwinds the DNA strand, and a portion of the new strand is removed. DNA polymerase III then fills in the new gap, resulting in repair of the mismatch.
The work of the three Nobel Laureates in Chemistry for 2015 outlines how cells maintain the integrity of DNA, processes that are necessary to limit mutations that can cause disease. Through their discovery of BER, NER, and Mismatch repair, Drs. Sancar, Lindahl, and Modrich helped us understand how cells protect themselves from DNA damage and replication error, and gave insight into the machinery that, when it goes wrong, can lead to disease states such as cancer. Their work laid the foundation for a vast amount of cellular biology work that has come after them.
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