Values are the mean SD of duplicate experiments

Values are the mean SD of duplicate experiments. The loss of uracil excision activity coincides with a significant accumulation of uracilated bases in the genome of infected (-)-Catechin gallate cells without changes in cell division. Although UNG2 expression and uracilCDNA glycosylase activity are (-)-Catechin gallate recovered after the peak of retroviral replication, the mutagenic effect of transient DNA uracilation in cycling cells should be taken into account. Therefore, the possible consequences of Vpr-mediated temporary depletion of endogenous nuclear UNG2 and subsequent alteration of the genomic integrity of infected cells need to be evaluated in the physiopathogenesis of HIV infection. INTRODUCTION Genome uracilation is generated either by misincorporation of deoxyuridine triphosphate (dUTP) during DNA polymerization or repair or by cytosine deamination either by spontaneous non-enzymatic processes (e.g. base alteration by chemicals or ionizing radiations) or through the action of a cytidine deaminase [reviewed in (1)]. The presence of uracil in DNA presents a potential threat for living organisms from yeast and bacteria to humans. When left unrepaired, uracil residues in U:G mismatches are 100% mutagenic. Owing to the DNA polymerase inability to discriminate between U and T in the template, unrepaired uracil bases result in the accumulation of G-to-A mutations on the complementary strand of DNA after the next round of replication. Cytosine spontaneous deamination together with hydrolytic deamination is estimated to account for the accumulation of 100 mutations per genome per round of replication (2,3). Repair of uracil in DNA is ensured by the base excision repair (BER) pathway. The initial step is accomplished by a DNA glycosylase that catalyzes the hydrolysis of the N-glycosyl bond between uracil and the deoxyribose moiety. Then, an apyrimidinic/apurinic (AP) endonuclease creates a nick on the abasic site. Finally, the gap is repaired by the sequential action of DNA polymerase and DNA ligase activities (4). Five mammalian uracilCDNA glycosylases have been identified. Excision of uracil from U:A or U:G pairs in single- and double-stranded (-)-Catechin gallate DNA is essentially supported by the nuclear uracilCDNA glycosylase UNG2. UNG1, an UNG2 isoform generated by the same unique gene through the use of differentially regulated promoters and alternative splicing, is exclusively expressed in mitochondria and retains the Rabbit polyclonal to FTH1 same properties as UNG2 to ensure integrity of the mitochondrial genome (5). Besides UNG2, SMUG1 initially described as a single strand selective mono-functional UDG that excises uracil in U:A and U:G pairs (6), has recently been reported to exhibit a preferential activity towards double stranded genomic DNA in physiological conditions (7). SMUG1 also can remove some oxidized pyrimidines, suggesting a role in the repair of DNA oxidation damage (8,9). Finally, uracil from U:G can be removed by the thymineCDNA glycosylase (TDG) and the methyl-binding domain protein 4 (MBD4) that also excise thymine from T:G mismatches, preferentially in CpG sequences (3). The function of the apparently redundant uracilCDNA glycosylases is tightly regulated and they are differentially expressed during the cell cycle (3,10). Indeed, UNG2 appears as the sole contributor to post-replicative repair of U:A lesions during S-phase through specific interaction with proliferating cell nuclear antigen and replication (-)-Catechin gallate protein A at replication foci (11). Then, UNG2 is phosphorylated (11) and degraded by the proteasome to undetectable levels during the late S and G2 phases of the cell cycle. Conversely, SMUG1 and TDG are eliminated in cells entering the S-phase (11,12). UNG2 function in maintaining genomic integrity is common (-)-Catechin gallate to all cell types. However, its role is much more complex in activated B lymphocytes, in which UNG2 also facilitates mutagenic processing of AID-induced uracil in the switch (S) and V(D)J regions of immunoglobulin loci. Accordingly, UNG2 favors class-switch DNA recombination (CSR) and somatic hypermutation (SHM) and is critical for the maturation of the antibody response [for review see (2)]. UNG2 functional importance has specifically been highlighted by studies in mice and humans harboring mutations. In both situations, absence of UNG2 expression is associated with a 5-fold increase in genomic mutation frequency (10), hyper-IgM syndrome and a significant perturbation of the acquired immune response caused by failure in class-switch recombination and altered somatic hypermutation (2,13,14). UNG2 deficiency also correlates with a global immunological imbalance with reduction of T-helper and NK-cells in spleen and deregulation of interferon , interleukin (IL)-2 and IL-6 levels (15). Finally, in aged mice, it results in an increased risk of developing follicular and diffuse large B-cell lymphoma (13). A variety of viral proteins have the capacity to disturb DNA repair in the host cell. The mechanisms of.