The 3' --> 5' exonuclease of T4 DNA polymerase removes premutagenic alkyl mispairs and contributes to futile cycling at O6-methylguanine lesions

Mutagenic mechanisms of cancer-associated DNA polymerase ϵ alleles

A single amino acid residue change in the exonuclease domain of human DNA polymerase ϵ, P286R, is associated with the development of colorectal cancers, and has been shown to impart a mutator phenotype. The corresponding Pol ϵ allele in the yeast Saccharomyces cerevisiae (pol2-P301R), was found to drive greater mutagenesis than an entirely exonuclease-deficient Pol ϵ (pol2-4), an unexpected phenotype of ultra-mutagenesis. By studying the impact on mutation frequency, type, replication-strand bias, and sequence context, we show that ultra-mutagenesis is commonly observed in yeast cells carrying a range of cancer-associated Pol ϵ exonuclease domain alleles. Similarities between mutations generated by these alleles and those generated in pol2-4 cells indicate a shared mechanism of mutagenesis that yields a mutation pattern similar to cancer Signature 14. Comparison of POL2 ultra-mutator with pol2-M644G, a mutant in the polymerase domain decreasing Pol ϵ fidelity, revealed unexpected analogies in the sequence context and strand bias of mutations. Analysis of mutational patterns unique to exonuclease domain mutant cells suggests that backtracking of the polymerase, when the mismatched primer end cannot be accommodated in the proofreading domain, results in the observed insertions and T>A mutations in specific sequence contexts.

Mechanism of ganciclovir-induced chain termination revealed by resistant viral polymerase mutants with reduced exonuclease activity

Many antiviral and anticancer drugs are nucleoside analogs that target polymerases and cause DNA chain termination. Interestingly, ganciclovir (GCV), the first line of therapy for human cytomegalovirus (HCMV) infections, induces chain termination despite containing the equivalent of a 3'-hydroxyl group. Certain HCMV GCV resistance (GCV(r)) mutations, including ones associated with treatment failures, result in substitutions in the 3'-5' exonuclease (Exo) domain of the catalytic subunit of the viral DNA polymerase (Pol). To investigate how these mutations confer resistance, we overexpressed and purified wild-type (WT) HCMV Pol and three GCV(r) Exo mutants. Kinetic studies provided little support for resistance being due to effects on Pol binding or incorporation of GCV-triphosphate. The mutants were defective for Exo activity on all primer templates tested, including those with primers terminating with GCV, arguing against the mutations increasing excision of the incorporated drug. However, although the WT enzyme terminated DNA synthesis after incorporation of GCV-triphosphate and an additional nucleotide (N+1), the Exo mutants could efficiently synthesize DNA to the end of such primer templates. Notably, the Exo activity of WT Pol rapidly and efficiently degraded N+2 primer templates to N+1 products that were not further degraded. On N+1 primer templates, WT Pol, much more than the Exo mutants, converted the incoming deoxynucleoside triphosphate to its monophosphate, indicative of rapid addition and removal of incorporated nucleotides ("idling"). These results explain how GCV induces chain termination and elucidate a previously unidentified mechanism of antiviral drug resistance.

Capecitabine and temozolomide (CAPTEM) for metastatic, well-differentiated neuroendocrine cancers: The Pancreas Center at Columbia University experience

PURPOSE

We evaluated the efficacy and safety of capecitabine and temozolomide (CAPTEM) in patients with metastatic neuroendocrine tumors (NETs) to the liver. This regimen was based on our studies with carcinoid cell lines that showed synergistic cytotoxicity with sequence-specific dosing of 5-fluorouracil preceding temozolomide (TMZ).

METHODS

A retrospective review was conducted of 18 patients with NETs metastatic to the liver who had failed 60 mg/month of Sandostatin LAR™ (100%), chemotherapy (61%), and hepatic chemoembolization (50%). Patients received capecitabine at 600 mg/m(2) orally twice daily on days 1-14 (maximum 1,000 mg orally twice daily) and TMZ 150-200 mg/m(2) divided into two doses daily on days 10-14 of a 28-day cycle. Imaging was performed every 2 cycles, and serum tumor markers were measured every cycle.

RESULTS

Using RECIST parameters, 1 patient (5.5%) with midgut carcinoid achieved a surgically proven complete pathological response (CR), 10 patients (55.5%) achieved a partial response (PR), and 4 patients (22.2%) had stable disease (SD). Total response rate was 61%, and clinical benefit (responders and SD) was 83.2%. Of four carcinoid cases treated with CAPTEM, there was 1 CR, 1 PR, 1 SD, and 1 progressive disease. Median progression-free survival was 14.0 months (11.3-18.0 months). Median overall survival from diagnosis of liver metastases was 83 months (28-140 months). The only grade 3 toxicity was thrombocytopenia (11%). There were no grade 4 toxicities, hospitalizations, opportunistic infections, febrile neutropenias, or deaths.

CONCLUSIONS

CAPTEM is highly active, well tolerated and may prolong survival in patients with well-differentiated, metastatic NET who have progressed on previous therapies.

Kinetic basis for the differing response to an oxidative lesion by a replicative and a lesion bypass DNA polymerase from Sulfolobus solfataricus

8-Oxo-7,8-dihydro-2'-deoxyguanine (8-oxoG), a major oxidative DNA lesion, exhibits ambiguous coding potential and can lead to genomic mutations. Tight control of 8-oxoG bypass during DNA replication is therefore extremely important in hyperthermophiles as the rate of oxidative damage to DNA is significantly increased at high temperatures. Here we employed pre-steady state kinetics to compare the kinetic responses to an 8-oxoG lesion of the main replicative and lesion bypass DNA polymerases of Sulfolobus solfataricus, a hyperthermophilic crenarchaeon. Upon encountering 8-oxoG, PolB1, the replicative DNA polymerase, was completely stalled by the lesion, as its 3' → 5' exonuclease activity increased significantly and outcompeted its slowed polymerase activity at and near the lesion site. In contrast, our results show that Dpo4, the lone Y-family DNA polymerase in S. solfataricus, can faithfully and efficiently incorporate nucleotides opposite 8-oxoG and extend from an 8-oxoG:C base pair with a mechanism similar to that observed for the replication of undamaged DNA. Furthermore, we show that the stalling of PolB1 at the lesion site can be relieved by Dpo4. Finally, the 3' → 5' exonuclease activity of PolB1 was the highest when 8-oxoG was mispaired with an incorrect nucleotide and could therefore correct rare mistakes made by Dpo4 during 8-oxoG bypass. These results provide a kinetic basis for a potential polymerase switching mechanism during 8-oxoG bypass whereby Dpo4 can switch with the stalled PolB1 at the replication fork to bypass and extend the damaged DNA and then switch off of the DNA substrate to allow continued replication of undamaged DNA by the more faithful PolB1.

The 3'-5' proofreading exonuclease of archaeal family-B DNA polymerase hinders the copying of template strand deaminated bases

Archaeal family B polymerases bind tightly to the deaminated bases uracil and hypoxanthine in single-stranded DNA, stalling replication on encountering these pro-mutagenic deoxynucleosides four steps ahead of the primer-template junction. When uracil is specifically bound, the polymerase-DNA complex exists in the editing rather than the polymerization conformation, despite the duplex region of the primer-template being perfectly base-paired. In this article, the interplay between the 3'-5' proofreading exonuclease activity and binding of uracil/hypoxanthine is addressed, using the family-B DNA polymerase from Pyrococcus furiosus. When uracil/hypoxanthine is bound four bases ahead of the primer-template junction (+4 position), both the polymerase and the exonuclease are inhibited, profoundly for the polymerase activity. However, if the polymerase approaches closer to the deaminated bases, locating it at +3, +2, +1 or even 0 (paired with the extreme 3' base in the primer), the exonuclease activity is strongly stimulated. In these situations, the exonuclease activity is actually stronger than that seen with mismatched primer-templates, even though the deaminated base-containing primer-templates are correctly base-paired. The resulting exonucleolytic degradation of the primer serves to move the uracil/hypoxanthine away from the primer-template junction, restoring the stalling position to +4. Thus the 3'-5' proofreading exonuclease contributes to the inability of the polymerase to replicate beyond deaminated bases.

DNA damage alters DNA polymerase delta to a form that exhibits increased discrimination against modified template bases and mismatched primers

Human DNA polymerase δ (Pol δ4), a key enzyme in chromosomal replication, is a heterotetramer composed of the p125, p50, p68 and p12 subunits. Genotoxic agents such as UV and alkylating chemicals trigger a DNA damage response in which Pol δ4 is converted to a trimer (Pol δ3) by degradation of p12. We show that Pol δ3 has altered enzymatic properties: it is less able to perform translesion synthesis on templates containing base lesions (O⁶-MeG, 8-oxoG, an abasic site or a thymine-thymine dimer); a greater proofreading activity; an increased exonuclease/polymerase activity ratio; a decreased tendency for the insertion of wrong nucleotides, and for the extension of mismatched primers. Overall, our findings indicate that Pol δ3 exhibits an enhanced ability for the detection of errors in both primers and templates over its parent enzyme. These alterations in Pol δ3 show that p12 plays a major role in Pol δ4 catalytic functions, and provides significant insights into the rationale for the conversion of Pol δ4 to Pol δ3 in the cellular response to DNA damage.

Mismatch repair-dependent iterative excision at irreparable O6-methylguanine lesions in human nuclear extracts

The response of mammalian cells to Sn1 DNA methylators depends on functional MutSalpha and MutLalpha. Cells deficient in either of these activities are resistant to the cytotoxic effects of this class of chemotherapeutic drug. Because killing by Sn1 methylators has been attributed to O6-methylguanine (MeG), we have constructed nicked circular heteroduplexes that contain a single MeG-T mispair, and we have examined processing of these molecules by mismatch repair in nuclear extracts of human cells. Excision provoked by MeG-T is restricted to the incised heteroduplex strand, leading to removal of the MeG when it resides on this strand. However, when the MeG is located on the continuous strand, the heteroduplex is irreparable. MeG-T-dependent repair DNA synthesis is observed on both reparable and irreparable 3' and 5' heteroduplexes as judged by [32P]dAMP incorporation. Labeling with [alpha-32P]dATP followed by a cold dATP chase has demonstrated that newly synthesized DNA on irreparable molecules is subject to re-excision in a reaction that is MutLalpha-dependent, an effect attributable to the presence of MeG on the template strand. Processing of the irreparable 3' heteroduplex is also associated with incision of the discontinuous strand of a few percent of molecules near the thymidylate of the MeG-T base pair. These results provide the first direct evidence for mismatch repair-mediated iterative processing of DNA methylator damage, an effect that may be relevant to damage signaling events triggered by this class of chemotherapeutic agent.

Effect of DNA polymerase beta loop variants on discrimination of O6-methyldeoxyguanosine modification present in the nucleotide versus template substrate

We have examined the mechanism of DNA polymerase beta (pol beta) lesion discrimination using alkylated dNTP versus alkylated DNA template substrates and the pol beta variants R253M and E249K. Both of these amino acid variants are located in the loop region of the palm domain and are known to play a role in pol beta fidelity and discrimination of 3'-azido-3'-deoxythymidine triphosphate substrates. We observed that these variants affect O(6)-methyldeoxyguanosine- (m6G-) modified dNTP discrimination without affecting m6G template translesion synthesis. Under steady-state conditions, the ratio of inherent reactivity values for the m6dGTP substrate relative to the dGTP substrate was greater for both variant polymerases than for wild-type (WT) pol beta. Biochemical assays of translesion synthesis using m6G lesion-containing templates demonstrated no significant differences between the variants and WT. Using N-methyl-N-nitrosourea- (MNU-) modified DNA templates in the HSV-tk in vitro assay, no difference among the enzymes in the frequency of alkylation-induced G to A transition mutations was observed. However, differences among the polymerases in the frequency of alkylation-induced C to A transversions were observed, consistent with a mutator tendency for E249K and an antimutator tendency for R253M. We conclude that a specific interaction at the loop of the palm domain is involved in pol beta discrimination of the m6G lesion when present on the incoming dNTP substrate but not when present in the DNA template. Our data support a role for the flexible loop in pol beta error discrimination.

Effect of 6-thioguanine on the stability of duplex DNA

The incorporation of 6-thioguanine (S6G) into DNA is a prerequisite for its cytotoxic action, but duplex structure is not significantly perturbed by the presence of the lesion [J. Bohon and C. R. de los Santos (2003) Nucleic Acids Res., 31, 1331–1338]. It is therefore possible that the mechanism of cytotoxicity relies on a loss of stability rather than a pathway involving direct structural recognition. The research described here focuses on the changes in thermodynamic properties of duplex DNA owing to the introduction of S6G as well as the kinetic properties of base pairs involving S6G. Replacement of a guanine in a G•C pair by S6G results in ∼1 kcal/mol less favorable Gibbs free energy of duplex formation at 37°C. S6G•T and G•T mismatch-containing duplexes have almost identical Gibbs free energy at 37°C, with values ∼3 kcal/mol less favorable than that of the control. Base pair stability is affected by S6G. The lifetime of the normal G•C base pair is ∼125 ms, whereas that of the G•T mismatch is below the detection limit. The lifetimes of S6G•C and S6G•T pairs are ∼7 and 2 ms, respectively, demonstrating that, although S6G significantly decreases the stability of the pairing with cytosine, it slightly increases that of a mismatch.

Mismatch repair-dependent G2 checkpoint induced by low doses of SN1 type methylating agents requires the ATR kinase

S(N)1-type alkylating agents represent an important class of chemotherapeutics, but the molecular mechanisms underlying their cytotoxicity are unknown. Thus, although these substances modify predominantly purine nitrogen atoms, their toxicity appears to result from the processing of O(6)-methylguanine ((6Me)G)-containing mispairs by the mismatch repair (MMR) system, because cells with defective MMR are highly resistant to killing by these agents. In an attempt to understand the role of the MMR system in the molecular transactions underlying the toxicity of alkylating agents, we studied the response of human MMR-proficient and MMR-deficient cells to low concentrations of the prototypic methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). We now show that MNNG treatment induced a cell cycle arrest that was absolutely dependent on functional MMR. Unusually, the cells arrested only in the second G(2) phase after treatment. Downstream targets of both ATM (Ataxia telangiectasia mutated) and ATR (ATM and Rad3-related) kinases were modified, but only the ablation of ATR, or the inhibition of CHK1, attenuated the arrest. The checkpoint activation was accompanied by the formation of nuclear foci containing the signaling and repair proteins ATR, the S(*)/T(*)Q substrate, gamma-H2AX, and replication protein A (RPA). The persistence of these foci implied that they may represent sites of irreparable damage.

Lys691 and Asp714 of the Na+/K+-ATPase α Subunit Are Essential for Phosphorylation, Dephosphorylation, and Enzyme Turnover

P-type ATPases such as the sodium pump appear to be members of a superfamily of hydrolases structurally typified by the L-2-haloacid dehalogenases. In the dehalogenase L-DEX-ps, Lys151 serves to stabilize the excess negative charge in the substrate/reaction intermediates and Asp180 coordinates a water molecule that is directly involved in ester intermediate hydrolysis. To investigate the importance of the corresponding Lys691 and Asp714 of the sodium pump alpha subunit, sodium pump mutants were expressed in yeast and analyzed for their properties. Lys691Ala, Lys691Asp, Asp714Ala, and Asp714Arg mutants were inactive, not only with respect to ATPase activity but also to interaction with the highly sodium pump-specific inhibitors ouabain or palytoxin (PTX). In contrast, conservative mutants Lys691Arg and Asp714Glu retained some of the partial activities of the wild-type enzyme, although they completely failed to display any ATPase activity. Yeast cells expressing Lys691Arg and Asp714Glu mutants are sensitive to the sodium pump-specific inhibitor PTX and lose intracellular K+. Their sensitivity to PTX, with EC50 values of 118 +/- 24 and 76.5 +/- 3.6 nM, respectively, was clearly reduced by almost 7- or 4-fold below that of the native sodium pump (17.8 +/- 2.7 nM). Ouabain was recognized under these conditions with low affinity by the mutants and inhibited the PTX-induced K+ efflux from the yeast cells. The EC50 for the ouabain effect was 183 +/- 20 microM for Lys691Arg and 2.3 +/- 0.08 mM for the Asp714Glu mutant. The corresponding value obtained with cells expressing the native sodium pump was 69 +/- 18 microM. In the presence of Pi and Mg2+, none of the mutant sodium pumps were able to bind ouabain. When Mg2+ was omitted, however, both Lys691Asp and Asp714Glu mutants displayed ouabain binding that was reduced by Mg2+ with an EC50 of 0.76 +/- 0.11 and 2.3 +/- 0.2 mM, respectively. In the absence of Mg2+, ouabain binding was also reduced by K+. The EC50 values were 1.33 +/- 0.23 mM for the wild-type enzyme, 0.93 +/- 0.2 mM for the Lys691Arg mutant, and 1.02 +/- 0.24 mM for the Asp714Glu enzyme. None of the neutral or nonconservative mutants displayed any ouabain-sensitive ATPase activity. Ouabain-sensitive phosphatase activity, however, was present in membranes containing either the wild-type (1105 +/- 100 micromol of p-nitrophenol phosphate hydrolyzed min(-1) mg of protein(-1)) or the Asp714Glu mutant (575 +/- 75 micromol min(-1) mg(-1)) sodium pump. Some phosphatase activity was also associated with the Lys691Arg mutant (195 +/- 63 micromol min(-1) mg(-1)). The results are consistent with Lys691 and Asp714 being essential for the phosphorylation/dephosphorylation process that allows the sodium pump to accomplish the catalytic cycle.

Structural effect of the anticancer agent 6-thioguanine on duplex DNA

The incorporation of 6-thioguanine (S6G) into DNA is an essential step in the cytotoxic activity of thiopurines. However, the structural effects of this substitution on duplex DNA have not been fully characterized. Here, we present the solution structures of DNA duplexes containing S6G opposite thymine (S6G.T) and opposite cytosine (S6G.C), solved by high-resolution NMR spectroscopy and restrained molecular dynamics. The data indicate that both duplexes adopt right-handed helical conformations with all Watson-Crick hydrogen bonding in place. The S6G.T structures exhibit a wobble-type base pairing at the lesion site, with thymine shifted toward the major groove and S6G displaced toward the minor groove. Aside from the lesion site, the helices, including the flanking base pairs, are not highly perturbed by the presence of the lesion. Surprisingly, thermal dependence experiments suggest greater stability in the S6G-T mismatch than the S6G-C base pair.

The proofreading 3'-->5' exonuclease activity of DNA polymerases: a kinetic barrier to translesion DNA synthesis

The 3'-->5' exonuclease activity intrinsic to several DNA polymerases plays a primary role in genetic stability; it acts as a first line of defense in correcting DNA polymerase errors. A mismatched basepair at the primer terminus is the preferred substrate for the exonuclease activity over a correct basepair. The efficiency of the exonuclease as a proofreading activity for mispairs containing a DNA lesion varies, however, being dependent upon both the DNA polymerase/exonuclease and the type of DNA lesion. The exonuclease activities intrinsic to the T4 polymerase (family B) and DNA polymerase gamma (family A) proofread DNA mispairs opposite endogenous DNA lesions, including alkylation, oxidation, and abasic adducts. However, the exonuclease of the Klenow polymerase cannot discriminate between correct and incorrect bases opposite alkylation and oxidative lesions. DNA damage alters the dynamics of the intramolecular partitioning of DNA substrates between the 3'-->5' exonuclease and polymerase activities. Enzymatic idling at lesions occurs when an exonuclease activity efficiently removes the same base that is preferentially incorporated by the DNA polymerase activity. Thus, the exonuclease activity can also act as a kinetic barrier to translesion synthesis (TLS) by preventing the stable incorporation of bases opposite DNA lesions. Understanding the downstream consequences of exonuclease activity at DNA lesions is necessary for elucidating the mechanisms of translesion synthesis and damage-induced cytotoxicity.

Rapid gene cloning using terminator primers and modular vectors

We seek to create useful biological diversity by exploiting the modular nature of genetic information. In this report we describe experiments that focus on the modular nature of plasmid cloning vectors. Bacterial plasmids are modular entities composed of origins of replication, selectable markers and other components. We describe a new ligation-independent cloning method that allows for rapid and seamless assembly of vectors from component modules. We further demonstrate that gene cloning can be accomplished simultaneously with assembly of a modular vector. This approach provides considerable flexibility as it allows for 'menu driven' cloning of genes into custom assembled modular vectors.