The Department of Biomedicine

BBB seminar: Hans Einar Krokan

DNA repair and repair proteins - mechanisms and functions

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Hans Einar Krokan
Department of Cancer Research and Molecular Medicine, Norwegian University of Science and Technology, Trondheim

DNA is damaged by exogenous agents such as radiation and reactive chemicals, but perhaps to a larger extent by normal cellular processes. Some types of lesions are commonly caused by external agents and cellular processes e.g. oxidative damage, alkylations and base loss. This talk will concentrate on endogenous types of damage to DNA and their consequences, as well as novel links between the acquired immune system and DNA repair proteins.

Uracil in DNA results from spontaneous deamination of cytosine, resulting in mutagenic U:G mispairs, and misincorporation of dUMP, which gives a less harmful U:A pair. Recent research has also indicated that uracil in DNA is formed by activation induced deaminase (AID) at the Ig locus in B-cells, as a trigger of antigen driven antibody diversification. At least four different human DNA glycosylases may remove uracil and thus generate an abasic site, which is itself cytotoxic and potentially mutagenic. These enzymes are mitochondrial UNG1 and nuclear UNG2, SMUG1, TDG and MBD4. The base excision repair (BER) process is completed either by a short patch- or long patch pathway, which largely use different proteins. UNG2 is a major nuclear uracil-DNA glycosylase, central in removal of misincorporated dUMP in replication foci, and UNG-deficient cells accumulate uracil in their genome (Nilsen et al., 2000). However, UNG2 also appears to have an important role in repair of U:G mispairs and possibly U in single-stranded DNA. SMUG1 has broader specificity than UNG2 and may serve as a relatively efficient backup for UNG in repair of U:G mismatches and single-stranded DNA (Kavli et al., 2002). TDG and MBD4 may have specialised roles in the repair of U and T in mismatches in CpG contexts.

An Ung-deficient mouse model has indicated that deficiency in uracil removal induces B-cell lymphomas and shortens the life span of the animal (Nilsen et al., 2003). Among the uracil-DNA glycosylases, only UNG2 has a proven role in somatic hypermutation and class switch in B-cells. In humans, inactivating UNG-gene mutations results in a profound defect in class switch recombination, while the frequency of somatic hypermutation is not decreased, but significantly skewed in the direction of GC to AT transitions (Imai et al., 2003). AID is a single strand specific DNA deaminase. Extracts of wild type B-cells contains UNG2 that efficiently removes uracil from both single stranded and double stranded DNA. Surprisingly, SMUG1in B-cell extracts is unable to remove uracil from single stranded DNA, but removes uracil from double stranded DNA relatively efficiently (Kavli et al., 2005). We have recently reported that UNG2 is apparently present in preassembled complexes proficient in BER (Akbari et al., 2004). It is possible that several different types of BER complexes may exist. In antigen activated B-cells a specialised type of UNG2-complex might be present. This may have a role in targeted mutagenesis, rather than repair.


Akbari M, Otterlei M, Pena-Diaz J, Aas PA, Kavli B, Liabakk NB, Hagen L, Imai K, Durandy A, Slupphaug G, Krokan HE. (2004) Repair of U/G and U/A in DNA by UNG2-associated repair complexes takes place predominantly by short-patch repair both in proliferating and growth-arrested cells. Nucleic Acids Res. 32:5486-5498.
Imai K, Slupphaug G, Lee WI, Revy P, Nonoyama S, Catalan N, Yel L, Forveille M, Kavli B, Krokan HE, Ochs HD, Fischer A, Durandy A. (2003) Human uracil-DNA glycosylase deficiency associated with profoundly impaired immunoglobulin class-switch recombination. Nat Immunol. 4:1023-1028.
Kavli B, Andersen S, Otterlei M, Liabakk NB, Imai K, Fischer A, Durandy A, Krokan HE, Slupphaug G.(2005) B-cells from hyper-IgM patients carrying UNG mutations lack ability to remove uracil from ssDNA and have elevated genomic uracil. J Exp Med. 201:2011-2021.
Kavli B, Sundheim O, Akbari M, Otterlei M, Nilsen H, Skorpen F, Aas PA, Hagen L, Krokan HE, Slupphaug G. (2002) hUNG2 is the major repair enzyme for removal of uracil from U:A matches, U:G mismatches, and U in single-stranded DNA, with hSMUG1 as a broad specificity backup. J Biol Chem. 277:39926-39936.
Nilsen H, Rosewell I, Robins P, Skjelbred CF, Andersen S, Slupphaug G, Daly G, Krokan HE, Lindahl T, Barnes DE. (2000) Uracil-DNA glycosylase (UNG)-deficient mice reveal a primary role of the enzyme during DNA replication. Mol Cell 5:1059-1065.
Nilsen H, Stamp G, Andersen S, Hrivnak G, Krokan HE, Lindahl T, Barnes DE. (2003) Gene-targeted mice lacking the Ung uracil-DNA glycosylase develop B-cell lymphomas. Oncogene 22:5381-5386.

Host: Beate Stern, Department of Biomedicine

Hans Einar Krokan is Professor at the Department of Cancer Research and Molecular Medicine at the Norwegian University of Science and Technology (NTNU), Trondheim. He and his team were the first to isolate and clone a DNA-repair gene and later studied thoroughly the structure and function of the encoded enzyme (uracil-DNA glycosylase). Hans Einar Krokan has been active in research concerning the molecular mechanism of DNA repair and its role in cellular defence for many years. In 2004 he received the Anders Jahre's Award for Medical Research in recognition of his outstanding scientific achievements.