Structure–function studies of DNA polymerase lambda

DNA Repair Protein XRCC1 Stimulates Activity of DNA Polymerase λ under Conditions of Microphase Separation

Non-membrane compartments or biomolecular condensates play an important role in the regulation of cellular processes including DNA repair. Here, an ability of XRCC1, a scaffold protein involved in DNA base excision repair (BER) and single-strand break repair, to form protein-rich microphases in the presence of DNA duplexes was discovered. We also showed that the gap-filling activity of BER-related DNA polymerase λ (Pol λ) is significantly increased by the presence of XRCC1. The stimulation of the Pol λ activity was observed only at micromolar XRCC1 concentrations, which were well above the nanomolar dissociation constant determined for the XRCC1-Pol λ complex and pointed to the presence of an auxiliary stimulatory factor in addition to protein-protein interactions. Indeed, according to dynamic light scattering measurements, the stimulation of the Pol λ activity by XRCC1 was coupled with microphase separation in a protein-DNA mixture. Fluorescence microscopy revealed colocalization of Pol λ, XRCC1, and gapped DNA within the microphases. Thus, stimulation of Pol λ activity is caused both by its interaction with XRCC1 and by specific conditions of microphase separation; this phenomenon is shown for the first time.

Combining Evolutionary Conservation and Quantum Topological Analyses To Determine Quantum Mechanics Subsystems for Biomolecular Quantum Mechanics/Molecular Mechanics Simulations

Selection of residues and other molecular fragments for inclusion in the quantum mechanics (QM) region for QM/molecular mechanics (MM) simulations is an important step for these calculations. Here, we present an approach that combines protein sequence/structure evolution and electron localization function (ELF) analyses. The combination of these two analyses allows the determination of whether a residue needs to be included in the QM subsystem or can be represented by the MM environment. We have applied this approach on two systems previously investigated by QM/MM simulations, 4-oxalocrotonate tautomerase (4OT) and ten-eleven translocation-2 (TET2), that provide examples where fragments may or may not need to be included in the QM subsystem. Subsequently, we present the use of this approach to determine the appropriate QM subsystem to calculate the minimum energy path (MEP) for the reaction catalyzed by human DNA polymerase λ (Polλ) with a third cation in the active site. Our results suggest that the combination of protein evolutionary and ELF analyses provides insights into residue/molecular fragment selection for QM/MM simulations.

A novel role of DNA polymerase λ in translesion synthesis in conjunction with DNA polymerase ζ

As an integral scaffolding component of DNA polymerase (Pol) zeta, Pol lambda adapts Pol zeta–dependent translesion synthesis to operate in a predominantly error-free manner in human cells.

By extending synthesis opposite from a diverse array of DNA lesions, DNA polymerase (Pol) ζ performs a crucial role in translesion synthesis (TLS). In yeast and cancer cells, Rev1 functions as an indispensable scaffolding component of Polζ and it imposes highly error-prone TLS upon Polζ. However, for TLS that occurs during replication in normal human cells, Rev1 functions instead as a scaffolding component of Pols η, ι, and κ and Rev1-dependent TLS by these Pols operates in a predominantly error-free manner. The lack of Rev1 requirement for Polζ function in TLS in normal cells suggested that some other protein substitutes for this Rev1 role. Here, we identify a novel role of Polλ as an indispensable scaffolding component of Polζ. TLS studies opposite a number of DNA lesions support the conclusion that as an integral component, Polλ adapts Polζ-dependent TLS to operate in a predominantly error-free manner in human cells, essential for genome integrity and cellular homeostasis.

Bacterial Ligase D preternary-precatalytic complex performs efficient abasic sites processing at double strand breaks during nonhomologous end joining

Abasic (AP) sites, the most common DNA lesions are frequently associated with double strand breaks (DSBs) and can pose a block to the final ligation. In many prokaryotes, nonhomologous end joining (NHEJ) repair of DSBs relies on a two-component machinery constituted by the ring-shaped DNA-binding Ku that recruits the multicatalytic protein Ligase D (LigD) to the ends. By using its polymerization and ligase activities, LigD fills the gaps that arise after realignment of the ends and seals the resulting nicks. Here, we show the presence of a robust AP lyase activity in the polymerization domain of Bacillus subtilis LigD (BsuLigD) that cleaves AP sites preferentially when they are proximal to recessive 5'-ends. Such a reaction depends on both, metal ions and the formation of a Watson-Crick base pair between the incoming nucleotide and the templating one opposite the AP site. Only after processing the AP site, and in the presence of the Ku protein, BsuLigD catalyzes both, the in-trans addition of the nucleotide to the 3'-end of an incoming primer and the ligation of both ends. These results imply that formation of a preternary-precatalytic complex ensures the coupling of AP sites cleavage to the end-joining reaction by the bacterial LigD.

Deployment of DNA polymerases beta and lambda in single-nucleotide and multinucleotide pathways of mammalian base excision DNA repair

There exist two major base excision DNA repair (BER) pathways, namely single-nucleotide or "short-patch" (SP-BER), and "long-patch" BER (LP-BER). Both pathways appear to be involved in the repair of small base lesions such as uracil, abasic sites and oxidized bases. In addition to DNA polymerase β (Polβ) as the main BER enzyme for repair synthesis, there is evidence for a minor role for DNA polymerase lambda (Polλ) in BER. In this study we explore the potential contribution of Polλ to both SP- and LP-BER in cell-free extracts. We measured BER activity in extracts of mouse embryonic fibroblasts using substrates with either a single uracil or the chemically stable abasic site analog tetrahydrofuran residue. The addition of purified Polλ complemented the pronounced BER deficiency of POLB-null cell extracts as efficiently as did Polβ itself. We have developed a new approach for determining the relative contributions of SP- and LP-BER pathways, exploiting mass-labeled nucleotides to distinguish single- and multinucleotide repair patches. Using this method, we found that uracil repair in wild-type and in Polβ-deficient cell extracts supplemented with Polλ was ∼80% SP-BER. The results show that recombinant Polλ can contribute to both SP- and LP-BER. However, endogenous Polλ, which is present at a level ˜50% that of Polβ in mouse embryonic fibroblasts, appears to make little contribution to BER in extracts. Thus Polλ in cells appears to be under some constraint, perhaps sequestered in a complex with other proteins, or post-translationally modified in a way that limits its ability to participate effectively in BER.

DNA polymerase beta and other gap-filling enzymes in mammalian base excision repair

DNA polymerase β plays a central role in the base excision DNA repair pathway that cleanses the genome of apurinic/apyrimidinic (AP) sites. AP sites arise in DNA from spontaneous base loss and DNA damage-specific glycosylases that hydrolyze the N-glycosidic bond between the deoxyribose and damaged base. AP sites are deleterious lesions because they can be mutagenic and/or cytotoxic. DNA polymerase β contributes two enzymatic activities, DNA synthesis and lyase, during the repair of AP sites; these activities reside on carboxyl- and amino-terminal domains, respectively. Accordingly, its cellular, structural, and kinetic attributes have been extensively characterized and it serves as model enzyme for the nucleotidyl transferase reaction utilized by other replicative, repair, and trans-lesion DNA polymerases.

Evolutionary diversity and novelty of DNA repair genes in asexual Bdelloid rotifers

Background

Bdelloid rotifers are the oldest, most diverse and successful animal taxon for which males, hermaphrodites, and traditional meiosis are unknown. Their degenerate tetraploid genome, with 2–4 copies of most loci, includes thousands of genes acquired from all domains of life by horizontal transfer. Many bdelloid species thrive in ephemerally aquatic habitats by surviving desiccation at any life stage with no loss of fecundity or lifespan. Their unique genomic diversity and the intense selective pressure of desiccation provide an exceptional opportunity to study the evolution of diversity and novelty in genes involved in DNA repair.

Results

We used genomic data and RNA-Seq of the desiccation process in the bdelloid Adineta vaga to characterize DNA damage reversal, translesion synthesis, and the major DNA repair pathways: base, nucleotide, and alternate excision repair, mismatch repair (MMR), and double strand break repair by homologous recombination (HR) and classical non-homologous end joining (NHEJ). We identify multiple horizontally transferred DNA damage response genes otherwise unknown in animals (AlkD, Fpg, LigK UVDE), and the presence of genes often considered vertebrate specific, particularly in the NHEJ complex and X family polymerases. While 75–100% of genes involved in MMR and HR are present in 0–2 copies, genes involved in NHEJ, which are present in only a single copy in nearly all other animals, are retained in 3–8 copies. We present structural predictions and expression evidence of neo- or sub-functionalization of multiple copy genes involved in NHEJ and other repair processes.

Conclusion

The horizontally-acquired genes and duplicated genes in BER and NHEJ suggest resilience to oxidative damage is conferred in part by increased DNA damage recognition and efficient end repair capabilities. The pattern of gene loss and retention in MMR and HR may facilitate recombination and gene conversion between divergent sequences, thus providing at least some of the benefits of sex. The unique retention and divergence of duplicates genes in NHEJ may be facilitated by the lack of efficient selection in the absence of meiotic recombination and independent assortment, and may contribute to the evolutionary success of bdelloids.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1288-9) contains supplementary material, which is available to authorized users.

Variable termination sites of DNA polymerases encountering a DNA-protein cross-link

DNA-protein cross-links (DPCs) are important DNA lesions induced by endogenous crosslinking agents such as formaldehyde or acetaldehyde, as well as ionizing radiation, cancer chemotherapeutic drugs, and abortive action of some enzymes. Due to their very bulky nature, they are expected to interfere with DNA and RNA synthesis and DNA repair. DPCs are highly genotoxic and the ability of cells to deal with them is relevant for many chemotherapeutic interventions. However, interactions of DNA polymerases with DPCs have been poorly studied due to the lack of a convenient experimental model. We have used NaBH4-induced trapping of E. coli formamidopyrimidine-DNA glycosylase with DNA to construct model DNA polymerase substrates containing a DPC in single-stranded template, or in the template strand of double-stranded DNA, or in the non-template (displaced) strand of double-stranded DNA. Nine DNA polymerases belonging to families A, B, X, and Y were studied with respect to their behavior upon encountering a DPC: Klenow fragment of E. coli DNA polymerase I, Thermus aquaticus DNA polymerase I, Pyrococcus furiosus DNA polymerase, Sulfolobus solfataricus DNA polymerase IV, human DNA polymerases β, κ and λ, and DNA polymerases from bacteriophages T4 and RB69. Although none were able to fully bypass DPCs in any context, Family B DNA polymerases (T4, RB69) and Family Y DNA polymerase IV were able to elongate the primer up to the site of the cross-link if a DPC was located in single-stranded template or in the displaced strand. In other cases, DNA synthesis stopped 4-5 nucleotides before the site of the cross-link in single-stranded template or in double-stranded DNA if the polymerases could displace the downstream strand. We suggest that termination of DNA polymerases on a DPC is mostly due to the unrelieved conformational strain experienced by the enzyme when pressing against the cross-linked protein molecule.

Regulation of human polλ by ATM-mediated phosphorylation during non-homologous end joining

DNA double strand breaks (DSBs) trigger a variety of cellular signaling processes, collectively termed the DNA-damage response (DDR), that are primarily regulated by protein kinase ataxia-telangiectasia mutated (ATM). Among DDR activated processes, the repair of DSBs by non-homologous end joining (NHEJ) is essential. The proper coordination of NHEJ factors is mainly achieved through phosphorylation by an ATM-related kinase, the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), although the molecular basis for this regulation has yet to be fully elucidated. In this study we identify the major NHEJ DNA polymerase, DNA polymerase lambda (Polλ), as a target for both ATM and DNA-PKcs in human cells. We show that Polλ is efficiently phosphorylated by DNA-PKcs in vitro and predominantly by ATM after DSB induction with ionizing radiation (IR) in vivo. We identify threonine 204 (T204) as a main target for ATM/DNA-PKcs phosphorylation on human Polλ, and establish that its phosphorylation may facilitate the repair of a subset of IR-induced DSBs and the efficient Polλ-mediated gap-filling during NHEJ. Molecular evidence suggests that Polλ phosphorylation might favor Polλ interaction with the DNA-PK complex at DSBs. Altogether, our work provides the first demonstration of how Polλ is regulated by phosphorylation to connect with the NHEJ core machinery during DSB repair in human cells.

The use of non-natural nucleotides to probe template-independent DNA synthesis

The vast majority of DNA polymerases use the complementary templating strand of DNA to guide each nucleotide incorporation. There are instances, however, in which polymerases can efficiently incorporate nucleotides in the absence of templating information. This process, known as translesion DNA synthesis, can alter the proper genetic code of an organism. To further elucidate the mechanism of template-independent DNA synthesis, we monitored the incorporation of various nucleotides at the "blunt-end" of duplex DNA by the high-fidelity bacteriophage T4 DNA polymerase. Although natural nucleotides are not incorporated at the blunt-end, a limited subset of non-natural indolyl analogues containing extensive pi-electron surface areas are efficiently utilized by the T4 DNA polymerase. These analogues possess high binding affinities that are remarkably similar to those measured during incorporation opposite an abasic site. In contrast, the k(pol) values are significantly lower during blunt-end extension when compared to incorporation opposite an abasic site. These kinetic differences suggest that the single-stranded region of the DNA template plays an important role during polymerization through stacking interactions with downstream bases, interactions with key amino acid residues, or both. In addition, we demonstrate that terminal deoxynucleotide transferase, a template-independent enzyme, can efficiently incorporate many of these non-natural nucleotides. However, that this unique polymerase cannot extend large, bulky non-natural nucleotides suggests that elongation is limited by steric constraints imposed by structural features present within the polymerase. Regardless, the kinetic data obtained from using either DNA polymerase indicate that template-independent synthesis can occur without the contributions of hydrogen-bonding interactions and suggest that pi-electron interactions play an important role in polymerization efficiency when templating information is not present.

DNA polymerases δ and λ cooperate in repairing double-strand breaks by microhomology-mediated end-joining in Saccharomyces cerevisiae

Maintenance of genome stability is carried out by a suite of DNA repair pathways that ensure the repair of damaged DNA and faithful replication of the genome. Of particular importance are the repair pathways, which respond to DNA double-strand breaks (DSBs), and how the efficiency of repair is influenced by sequence homology. In this study, we developed a genetic assay in diploid Saccharomyces cerevisiae cells to analyze DSBs requiring microhomologies for repair, known as microhomology-mediated end-joining (MMEJ). MMEJ repair efficiency increased concomitant with microhomology length and decreased upon introduction of mismatches. The central proteins in homologous recombination (HR), Rad52 and Rad51, suppressed MMEJ in this system, suggesting a competition between HR and MMEJ for the repair of a DSB. Importantly, we found that DNA polymerase delta (Pol δ) is critical for MMEJ, independent of microhomology length and base-pairing continuity. MMEJ recombinants showed evidence that Pol δ proofreading function is active during MMEJ-mediated DSB repair. Furthermore, mutations in Pol δ and DNA polymerase 4 (Pol λ), the DNA polymerase previously implicated in MMEJ, cause a synergistic decrease in MMEJ repair. Pol λ showed faster kinetics associating with MMEJ substrates following DSB induction than Pol δ. The association of Pol δ depended on RAD1, which encodes the flap endonuclease needed to cleave MMEJ intermediates before DNA synthesis. Moreover, Pol δ recruitment was diminished in cells lacking Pol λ. These data suggest cooperative involvement of both polymerases in MMEJ.

Arabidopsis DNA polymerase lambda mutant is mildly sensitive to DNA double strand breaks but defective in integration of a transgene

The DNA double-strand break (DSB) is a critical type of damage, and can be induced by both endogenous sources (e.g., errors of oxidative metabolism, transposable elements, programmed meiotic breaks, or perturbation of the DNA replication fork) and exogenous sources (e.g., ionizing radiation or radiomimetic chemicals). Although higher plants, like mammals, are thought to preferentially repair DSBs via nonhomologous end joining (NHEJ), much remains unclear about plant DSB repair pathways. Our reverse genetic approach suggests that DNA polymerase λ is involved in DSB repair in Arabidopsis. The Arabidopsis T-DNA insertion mutant (atpolλ-1) displayed sensitivity to both gamma-irradiation and treatment with radiomimetic reagents, but not to other DNA damaging treatments. The atpolλ-1 mutant showed a moderate sensitivity to DSBs, while Arabidopsis Ku70 and DNA ligase 4 mutants (atku70-3 and atlig4-2), both of which play critical roles in NHEJ, exhibited a hypersensitivity to these treatments. The atpolλ-1/atlig4-2 double mutant exhibited a higher sensitivity to DSBs than each single mutant, but the atku70/atpolλ-1 showed similar sensitivity to the atku70-3 mutant. We showed that transcription of the DNA ligase 1, DNA ligase 6, and Wee1 genes was quickly induced by BLM in several NHEJ deficient mutants in contrast to wild-type. Finally, the T-DNA transformation efficiency dropped in NHEJ deficient mutants and the lowest transformation efficiency was scored in the atpolλ-1/atlig4-2 double mutant. These results imply that AtPolλ is involved in both DSB repair and DNA damage response pathway.

Phylogenetic analysis and evolutionary origins of DNA polymerase X-family members

Mammalian DNA polymerase (pol) β is the founding member of a large group of DNA polymerases now termed the X-family. DNA polymerase β has been kinetically, structurally, and biologically well characterized and can serve as a phylogenetic reference. Accordingly, we have performed a phylogenetic analysis to understand the relationship between pol β and other members of the X-family of DNA polymerases. The bacterial X-family DNA polymerases, Saccharomyces cerevisiae pol IV, and four mammalian X-family polymerases appear to be directly related. These enzymes originated from an ancient common ancestor characterized in two Bacillus species. Understanding distinct functions for each of the X-family polymerases, evolving from a common bacterial ancestor is of significant interest in light of the specialized roles of these enzymes in DNA metabolism.

N-terminal domains of human DNA polymerase lambda promote primer realignment during translesion DNA synthesis

The X-family DNA polymerases λ (Polλ) and β (Polβ) possess similar 5'-2-deoxyribose-5-phosphate lyase (dRPase) and polymerase domains. Besides these domains, Polλ also possesses a BRCA1 C-terminal (BRCT) domain and a proline-rich domain at its N terminus. However, it is unclear how these non-enzymatic domains contribute to the unique biological functions of Polλ. Here, we used primer extension assays and a newly developed high-throughput short oligonucleotide sequencing assay (HT-SOSA) to compare the efficiency of lesion bypass and fidelity of human Polβ, Polλ and two N-terminal deletion constructs of Polλ during the bypass of either an abasic site or an 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) lesion. We demonstrate that the BRCT domain of Polλ enhances the efficiency of abasic site bypass by approximately 1.6-fold. In contrast, deletion of the N-terminal domains of Polλ did not affect the efficiency of 8-oxodG bypass relative to nucleotide incorporations opposite undamaged dG. HT-SOSA analysis demonstrated that Polλ and Polβ preferentially generated -1 or -2 frameshift mutations when bypassing an abasic site and the single or double base deletion frequency was highly sequence dependent. Interestingly, the BRCT and proline-rich domains of Polλ cooperatively promoted the generation of -2 frameshift mutations when the abasic site was situated within a sequence context that was susceptible to homology-driven primer realignment. Furthermore, both N-terminal domains of Polλ increased the generation of -1 frameshift mutations during 8-oxodG bypass and influenced the frequency of substitution mutations produced by Polλ opposite the 8-oxodG lesion. Overall, our data support a model wherein the BRCT and proline-rich domains of Polλ act cooperatively to promote primer/template realignment between DNA strands of limited sequence homology. This function of the N-terminal domains may facilitate the role of Polλ as a gap-filling polymerase within the non-homologous end joining pathway.

A specific N-terminal extension of the 8 kDa domain is required for DNA end-bridging by human Polμ and Polλ

Human DNA polymerases mu (Polµ) and lambda (Polλ) are X family members involved in the repair of double-strand breaks in DNA during non-homologous end joining. Crucial abilities of these enzymes include bridging of the two 3′ single-stranded overhangs and trans-polymerization using one 3′ end as primer and the other as template, to minimize sequence loss. In this context, we have studied the importance of a previously uncharacterised sequence (‘brooch’), located at the N-terminal boundary of the Polß-like polymerase core, and formed by Tyr¹⁴¹, Ala¹⁴², Cys¹⁴³, Gln¹⁴⁴ and Arg¹⁴⁵ in Polµ, and by Trp²³⁹, Val²⁴⁰, Cys²⁴¹, Ala²⁴² and Gln²⁴³ in Polλ. The brooch is potentially implicated in the maintenance of a closed conformation throughout the catalytic cycle, and our studies indicate that it could be a target of Cdk phosphorylation in Polµ. The brooch is irrelevant for 1 nt gap filling, but of specific importance during end joining: single mutations in the conserved residues reduced the formation of two ended synapses and strongly diminished the ability of Polµ and polymerase lambda to perform non-homologous end joining reactions in vitro.

Toward a Deeper Understanding of Enzyme Reactions Using the Coupled ELF/NCI Analysis: Application to DNA Repair Enzymes

The combined Electron Localization Funtion (ELF)/ Noncovalent Interaction (NCI) topological analysis (Gillet et al. J. Chem. Theory Comput.2012, 8, 3993) has been extended to enzymatic reaction paths. We applied ELF/NCI to the reactions of DNA polymerase λ and the ε subunit of DNA polymerase III. ELF/NCI is shown to provide insights on the interactions during the evolution of enzymatic reactions including predicting the location of TS from structures located earlier along the reaction coordinate, differential metal coordination, and on barrier differences with two different cations.

DNA polymerase λ inactivation by oxidized abasic sites

Base excision repair (BER) plays a vital role in maintaining genomic integrity in mammalian cells. DNA polymerase λ (Pol λ) is believed to play a backup role to DNA polymerase β (Pol β) in base excision repair. Two oxidized abasic lesions that are produced by a variety of DNA-damaging agents, including several antitumor antibiotics, the C4'-oxidized abasic site following Ape1 incision (pC4-AP), and 5'-(2-phosphoryl-1,4-dioxobutane) (DOB), irreversibly inactivate Pol β and Pol λ. The interactions of DOB and pC4-AP with Pol λ are examined in detail using DNA substrates containing these lesions at defined sites. Single-turnover kinetic experiments show that Pol λ excises DOB almost 13 times more slowly than a 5'-phosphorylated 2-deoxyribose (dRP). pC4-AP is excised approximately twice as fast as DOB. The absolute rate constants are considerably slower than those reported for Pol β for the respective reactions, suggesting that Pol λ may be an inefficient backup in BER. DOB inactivates Pol λ approximately 3-fold less efficiently than it does Pol β, and the difference can be attributed to a higher K(I) (33 ± 7 nM). Inactivation of Pol λ's lyase activity by DOB also prevents the enzyme from conducting polymerization following preincubation of the protein and DNA. Mass spectral analysis of GluC-digested Pol λ inactivated by DOB shows that Lys324 is modified. There is inferential support for the idea that Lys312 may also be modified. Both residues are within the Pol λ lyase active site. When acting on pC4-AP, Pol λ achieves approximately four turnovers on average before being inactivated. Lyase inactivation by pC4-AP is also accompanied by loss of polymerase activity, and mass spectrometry indicates that Lys312 and Lys324 are modified by the lesion. The ability of DOB and pC4-AP to inactivate Pol λ provides additional evidence that these lesions are significant sources of the cytotoxicity of DNA-damaging agents that produce them.

Microhomology-mediated DNA strand annealing and elongation by human DNA polymerases λ and β on normal and repetitive DNA sequences

'Classical' non-homologous end joining (NHEJ), dependent on the Ku70/80 and the DNA ligase IV/XRCC4 complexes, is essential for the repair of DNA double-strand breaks. Eukaryotic cells possess also an alternative microhomology-mediated end-joining (MMEJ) mechanism, which is independent from Ku and DNA ligase 4/XRCC4. The components of the MMEJ machinery are still largely unknown. Family X DNA polymerases (pols) are involved in the classical NHEJ pathway. We have compared in this work, the ability of human family X DNA pols β, λ and μ, to promote the MMEJ of different model templates with terminal microhomology regions. Our results reveal that DNA pol λ and DNA ligase I are sufficient to promote efficient MMEJ repair of broken DNA ends in vitro, and this in the absence of auxiliary factors. However, DNA pol β, not λ, was more efficient in promoting MMEJ of DNA ends containing the (CAG)n triplet repeat sequence of the human Huntingtin gene, leading to triplet expansion. The checkpoint complex Rad9/Hus1/Rad1 promoted end joining by DNA pol λ on non-repetitive sequences, while it limited triplet expansion by DNA pol β. We propose a possible novel role of DNA pol β in MMEJ, promoting (CAG)n triplet repeats instability.

A new proofreading mechanism for lesion bypass by DNA polymerase-λ

Replicative DNA polymerases (DNA pols) increase their fidelity by removing misincorporated nucleotides with their 3' → 5' exonuclease activity. Exonuclease activity reduces translesion synthesis (TLS) efficiency and TLS DNA pols lack 3' → 5' exonuclease activity. Here we show that physiological concentrations of pyrophosphate (PP(i)) activate the pyrophosphorolytic activity by DNA pol-λ, allowing the preferential excision of the incorrectly incorporated A opposite a 7,8-dihydro-8-oxoguanine lesion, or T opposite a 6-methyl-guanine, with respect to the correct C. This is the first example of an alternative proofreading mechanism used during TLS.

In vitro gap-directed translesion DNA synthesis of an abasic site involving human DNA polymerases epsilon, lambda, and beta

DNA polymerase (pol) ε is thought to be the leading strand replicase in eukaryotes, whereas pols λ and β are believed to be mainly involved in re-synthesis steps of DNA repair. DNA elongation by the human pol ε is halted by an abasic site (apurinic/apyrimidinic (AP) site). In this study, we present in vitro evidence that human pols λ, β, and η can perform translesion synthesis (TLS) of an AP site in the presence of pol ε, likely by initiating the 3'OHs created at the lesion by the arrested pol ε. However, in the case of pols λ and β, this TLS requires the presence of a DNA gap downstream from the product synthesized by the pol ε, and the optimal gap for efficient TLS is different for the two polymerases. The presence of gaps did not affect the TLS capacity of human pol η. Characterization of the reaction products showed that pol β inserted dAMP opposite the AP site, whereas gap filling synthesis by pol λ resulted in single or double deletions opposite the lesion. The synthesis up to the AP site by pol ε and the subsequent TLS by pols λ and β are not influenced by human processivity factor proliferating cell nuclear antigen and human single-stranded DNA-binding protein replication protein A. The bypass capacity of pol λ at the AP site is greatly reduced when a truncated form of the enzyme, which has lost the BRCA1 C-terminal and proline-rich domains, is used. Collectively, our in vitro results support the existence of a mechanism of gap-directed TLS at an AP site involving a switch between the replicative pol ε and the repair pols λ and β.

Ubiquitylation of DNA polymerase λ

DNA polymerase (pol) λ, one of the 15 cellular pols, belongs to the X family. It is a small 575 amino-acid protein containing a polymerase, a dRP-lyase, a proline/serine rich and a BRCT domain. Pol λ shows various enzymatic activities including DNA polymerization, terminal transferase and dRP-lyase. It has been implicated to play a role in several DNA repair pathways, particularly base excision repair (BER), non-homologous end-joining (NHEJ) and translesion DNA synthesis (TLS). Similarly to other DNA repair enzymes, pol λ undergoes posttranslational modifications during the cell cycle that regulate its stability and possibly its subcellular localization. Here we describe our knowledge about ubiquitylation of pol λ and the impact of this modification on its regulation.

A model for the dynamics of mammalian family X DNA polymerases

Based on available structural studies, a model is presented for polymerization dynamics of mammalian family X DNA polymerases, including polymerases β, λ, μ, and terminal deoxynucleotidyl transferase (TdT). Using the model, distinct polymerization activities and processivities of the four polymerases acting on different forms of DNA substrate are analyzed and studied theoretically. A "gradient" of template dependence of polymerases β, λ, μ, and TdT is well explained. The much higher occurrence frequencies of the -1 frameshift DNA synthesis by pols λ and μ than that by pol β are well explained. The theoretical results on the polymerization processivities are also in agreement with the available experimental data.

Oxygen as a friend and enemy: How to combat the mutational potential of 8-oxo-guanine

The maintenance of genetic stability is of crucial importance for any form of life. Prior to cell division in each mammalian cell, the process of DNA replication must faithfully duplicate the three billion bases with an absolute minimum of mistakes. Various environmental and endogenous agents, such as reactive oxygen species (ROS), can modify the structural properties of DNA bases and thus damage the DNA. Upon exposure of cells to oxidative stress, an often generated and highly mutagenic DNA damage is 7,8-dihydro-8-oxo-guanine (8-oxo-G). The estimated steady-state level of 8-oxo-G lesions is about 10(3) per cell/per day in normal tissues and up to 10(5) lesions per cell/per day in cancer tissues. The presence of 8-oxo-G on the replicating strand leads to frequent (10-75%) misincorporations of adenine opposite the lesion (formation of A:8-oxo-G mispairs), subsequently resulting in C:G to A:T transversion mutations. These mutations are among the most predominant somatic mutations in lung, breast, ovarian, gastric and colorectal cancers. Thus, in order to reduce the mutational burden of ROS, human cells have evolved base excision repair (BER) pathways ensuring (i) the correct and efficient repair of A:8-oxo-G mispairs and (ii) the removal of 8-oxo-G lesions from the genome. Very recently it was shown that MutY glycosylase homologue (MUTYH) and DNA polymerase lambda play a crucial role in the accurate repair of A:8-oxo-G mispairs. Here we review the importance of accurate BER of 8-oxo-G damage and its regulation in prevention of cancer.

Ancient and recent adaptive evolution of primate non-homologous end joining genes

In human cells, DNA double-strand breaks are repaired primarily by the non-homologous end joining (NHEJ) pathway. Given their critical nature, we expected NHEJ proteins to be evolutionarily conserved, with relatively little sequence change over time. Here, we report that while critical domains of these proteins are conserved as expected, the sequence of NHEJ proteins has also been shaped by recurrent positive selection, leading to rapid sequence evolution in other protein domains. In order to characterize the molecular evolution of the human NHEJ pathway, we generated large simian primate sequence datasets for NHEJ genes. Codon-based models of gene evolution yielded statistical support for the recurrent positive selection of five NHEJ genes during primate evolution: XRCC4, NBS1, Artemis, POLλ, and CtIP. Analysis of human polymorphism data using the composite of multiple signals (CMS) test revealed that XRCC4 has also been subjected to positive selection in modern humans. Crystal structures are available for XRCC4, Nbs1, and Polλ; and residues under positive selection fall exclusively on the surfaces of these proteins. Despite the positive selection of such residues, biochemical experiments with variants of one positively selected site in Nbs1 confirm that functions necessary for DNA repair and checkpoint signaling have been conserved. However, many viruses interact with the proteins of the NHEJ pathway as part of their infectious lifecycle. We propose that an ongoing evolutionary arms race between viruses and NHEJ genes may be driving the surprisingly rapid evolution of these critical genes.

Because all cells experience DNA damage, they must also have mechanisms for repairing DNA. When the proteins that repair DNA malfunction, mutation and disease often result. Based on their fundamental importance, DNA repair proteins would be expected to be well preserved over evolutionary time in order to ensure optimal DNA repair function. However, a previous genome-wide study of molecular evolution in Saccharomyces yeast identified the non-homologous end joining (NHEJ) DNA repair pathway as one of the two most rapidly evolving pathways in the yeast genome. In order to analyze the evolution of this pathway in humans, we have generated large evolutionary sequence sets of NHEJ genes from our primate relatives. Similar to the scenario in yeast, several genes in this pathway are evolving rapidly in primate genomes and in modern human populations. Thus, complex and seemingly opposite selective forces are shaping the evolution of these important DNA repair genes. The finding that NHEJ genes are rapidly evolving in species groups as diverse as yeasts and primates indicates a systematic perturbation of the NHEJ pathway, one that is potentially important to human health.

Efficiency and fidelity of human DNA polymerases λ and β during gap-filling DNA synthesis

The base excision repair (BER) pathway coordinates the replacement of 1-10 nucleotides at sites of single-base lesions. This process generates DNA substrates with various gap sizes which can alter the catalytic efficiency and fidelity of a DNA polymerase during gap-filling DNA synthesis. Here, we quantitatively determined the substrate specificity and base substitution fidelity of human DNA polymerase λ (Pol λ), an enzyme proposed to support the known BER DNA polymerase β (Pol β), as it filled 1-10-nucleotide gaps at 1-nucleotide intervals. Pol λ incorporated a correct nucleotide with relatively high efficiency until the gap size exceeded 9 nucleotides. Unlike Pol λ, Pol β did not have an absolute threshold on gap size as the catalytic efficiency for a correct dNTP gradually decreased as the gap size increased from 2 to 10 nucleotides and then recovered for non-gapped DNA. Surprisingly, an increase in gap size resulted in lower polymerase fidelity for Pol λ, and this downregulation of fidelity was controlled by its non-enzymatic N-terminal domains. Overall, Pol λ was up to 160-fold more error-prone than Pol β, thereby suggesting Pol λ would be more mutagenic during long gap-filling DNA synthesis. In addition, dCTP was the preferred misincorporation for Pol λ and its N-terminal domain truncation mutants. This nucleotide preference was shown to be dependent upon the identity of the adjacent 5'-template base. Our results suggested that both Pol λ and Pol β would catalyze nucleotide incorporation with the highest combination of efficiency and accuracy when the DNA substrate contains a single-nucleotide gap. Thus, Pol λ, like Pol β, is better suited to catalyze gap-filling DNA synthesis during short-patch BER in vivo, although, Pol λ may play a role in long-patch BER.

Identification of critical residues for the tight binding of both correct and incorrect nucleotides to human DNA polymerase λ

DNA polymerase λ (Pol λ) is a novel X-family DNA polymerase that shares 34% sequence identity with DNA polymerase β. Pre-steady-state kinetic studies have shown that the Pol λ-DNA complex binds both correct and incorrect nucleotides 130-fold tighter, on average, than the DNA polymerase β-DNA complex, although the base substitution fidelity of both polymerases is 10(-)(4) to 10(-5). To better understand Pol λ's tight nucleotide binding affinity, we created single-substitution and double-substitution mutants of Pol λ to disrupt the interactions between active-site residues and an incoming nucleotide or a template base. Single-turnover kinetic assays showed that Pol λ binds to an incoming nucleotide via cooperative interactions with active-site residues (R386, R420, K422, Y505, F506, A510, and R514). Disrupting protein interactions with an incoming correct or incorrect nucleotide impacted binding to each of the common structural moieties in the following order: triphosphate≫base>ribose. In addition, the loss of Watson-Crick hydrogen bonding between the nucleotide and the template base led to a moderate increase in K(d). The fidelity of Pol λ was maintained predominantly by a single residue, R517, which has minor groove interactions with the DNA template.

DNA polymerases involved in the incorporation of oxidized nucleotides into DNA: their efficiency and template base preference

Genetic information must be duplicated with precision and accurately passed on to daughter cells and later generations. In order to achieve this goal, DNA polymerases (Pols) have to faithfully execute DNA synthesis during chromosome replication and repair. However, the conditions under which Pols synthesize DNA are not always optimal; the template DNA can be damaged by various endogenous and exogenous genotoxic agents including reactive oxygen species (ROS), and ROS oxidize dNTPs in the nucleotide pool from which Pols elongate DNA strands. Both damaged DNA and oxidized dNTPs interfere with faithful DNA synthesis by Pols, inducing various cellular abnormalities, such as mutations, cancer, neurological diseases, and cellular senescence. In this review, we focus on the process by which Pols incorporate oxidized dNTPs into DNA and compare the properties of Pols: efficiency, i.e., k(cat)/K(m), k(pol)/K(d) or V(max)/K(m), and template base preference for the incorporation of 8-oxo-dGTP, an oxidized form of dGTP. In general, Pols involved in chromosome replication, the A- and B-family Pols, are resistant to the incorporation of 8-oxo-dGTP, whereas Pols involved in repair and/or translesion synthesis, the X- and Y-family Pols, incorporate nucleotides in a relatively efficient manner and tend to incorporate it opposite template dA rather than template dC, though there are several exceptions. We discuss the molecular mechanisms by which Pols exhibit different template base preferences for the incorporation of 8-oxo-dGTP and how Pols are involved in the induction of mutations via the incorporation of oxidized nucleotides under oxidative stress.

Interaction between DNA Polymerase lambda and anticancer nucleoside analogs

The anticancer activity of cytarabine (AraC) and gemcitabine (dFdC) is thought to result from chain termination after incorporation into DNA. To investigate their incorporation into DNA at atomic level resolution, we present crystal structures of human DNA polymerase lambda (Pol lambda) bound to gapped DNA and containing either AraC or dFdC paired opposite template dG. These structures reveal that AraC and dFdC can bind within the nascent base pair binding pocket of Pol lambda. Although the conformation of the ribose of AraCTP is similar to that of normal dCTP, the conformation of dFdCTP is significantly different. Consistent with these structures, Pol lambda efficiently incorporates AraCTP but not dFdCTP. The data are consistent with the possibility that Pol lambda could modulate the cytotoxic effect of AraC.

Parp1 activation in mouse embryonic fibroblasts promotes Pol beta-dependent cellular hypersensitivity to alkylation damage

Alkylating agents induce cell death in wild-type (WT) mouse embryonic fibroblasts (MEFs) by multiple mechanisms, including apoptosis, autophagy and necrosis. DNA polymerase beta (Pol beta) knockout (KO) MEFs are hypersensitive to the cytotoxic effect of alkylating agents, as compared to WT MEFs. To test the hypothesis that Parp1 is preferentially activated by methyl methanesulfonate (MMS) exposure of Pol beta KO MEFs, we have examined the relationship between Pol beta expression, Parp1 activation and cell survival following MMS exposure in a series of WT and Pol beta deficient MEF cell lines. Consistent with our hypothesis, we observed elevated Parp1 activation in Pol beta KO MEFs as compared to matched WT MEFs. Both the MMS-induced activation of Parp1 and the MMS-induced cytotoxicity of Pol beta KO MEFs are attenuated by pre-treatment with the Parp1/Parp2 inhibitor PJ34. Further, elevated Parp1 activation is observed following knockdown (KD) of endogenous Pol beta, as compared to WT cells. Pol beta KD MEFs are hypersensitive to MMS and both the MMS-induced hypersensitivity and Parp1 activation is prevented by pre-treatment with PJ34. In addition, the MMS-induced cellular sensitivity of Pol beta KO MEFs is reversed when Parp1 is also deleted (Pol beta/Parp1 double KO MEFs) and we observe no MMS sensitivity differential between Pol beta/Parp1 double KO MEFs and those that express recombinant mouse Pol beta. These studies suggest that Parp1 may function as a sensor of BER to initiate cell death when BER is aborted or fails. Parp1 may therefore function in BER as a tumor suppressor by initiating cell death and preventing the accumulation of cells with chromosomal damage due to a BER defect.

DNA polymerase beta and PARP activities in base excision repair in living cells

To examine base excision repair (BER) capacity in the context of living cells, we developed and applied a plasmid-based reporter assay. Non-replicating plasmids containing unique DNA base lesions were designed to express luciferase only after lesion repair had occurred, and luciferase expression in transfected cells was measured continuously during a repair period of 14 h. Two types of DNA lesions were examined: uracil opposite T reflecting repair primarily by the single-nucleotide BER sub-pathway, and the abasic site analogue tetrahydrofuran (THF) opposite C reflecting repair by long-patch BER. We found that the repair capacity for uracil-DNA in wild type mouse fibroblasts was very strong, whereas the repair capacity for THF-DNA, although strong, was slightly weaker. Repair capacity in DNA polymerase beta (Pol beta) null cells for uracil-DNA and THF-DNA was reduced by approximately 15% and 20%, respectively, compared to that in wild type cells. In both cases, the repair deficiency was fully complemented in Pol beta null cells expressing recombinant Pol beta. The effect of inhibition of poly(ADP-ribose) polymerase (PARP) activity on repair capacity was examined by treatment of cells with the inhibitor 4-amino-1,8-naphthalimide (4-AN). PARP inhibition decreased the repair capacity for both lesions in wild type cells, and this reduction was to the same level as that seen in Pol beta null cells. In contrast, 4-AN had no effect on repair in Pol beta null cells. The results highlight that Pol beta and PARP function in the same repair pathway, but also suggest that there is repair independent of both Pol beta and PARP activities. Thus, before the BER capacity of a cell can be predicted or modulated, a better understanding of Pol beta and PARP activity-independent BER pathways is required.

SCO-ping out the mechanisms underlying the etiology of hydrocephalus

The heterogeneous nature of congenital hydrocephalus has hampered our understanding of the molecular basis of this common clinical problem. However, disease gene identification and characterization of multiple transgenic mouse models has highlighted the importance of the subcommissural organ (SCO) and the ventricular ependymal (vel) cells. Here, we review how altered development and function of the SCO and vel cells contributes to hydrocephalus.

The family X DNA polymerase from Deinococcus radiodurans adopts a non-standard extended conformation

Deinococcus radiodurans is an extraordinarily radioresistant bacterium that is able to repair hundreds of radiation-induced double-stranded DNA breaks. One of the players in this pathway is an X family DNA polymerase (PolX(Dr)). Deletion of PolX(Dr) has been shown to decrease the rate of repair of double-stranded DNA breaks and increase cell sensitivity to gamma-rays. A 3'-->5' exonuclease activity that stops cutting close to DNA loops has also been demonstrated. The present crystal structure of PolX(Dr) solved at 2.46-A resolution reveals that PolX(Dr) has a novel extended conformation in stark contrast to the closed "right hand" conformation commonly observed for DNA polymerases. This extended conformation is stabilized by the C-terminal PHP domain, whose putative nuclease active site is obstructed by its interaction with the polymerase domain. The overall conformation and the presence of non standard residues in the active site of the polymerase X domain makes PolX(Dr) the founding member of a novel class of polymerases involved in DNA repair but whose detailed mode of action still remains enigmatic.

Repair of ionizing radiation-induced DNA double-strand breaks by non-homologous end-joining

DNA DSBs (double-strand breaks) are considered the most cytotoxic type of DNA lesion. They can be introduced by external sources such as IR (ionizing radiation), by chemotherapeutic drugs such as topoisomerase poisons and by normal biological processes such as V(D)J recombination. If left unrepaired, DSBs can cause cell death. If misrepaired, DSBs may lead to chromosomal translocations and genomic instability. One of the major pathways for the repair of IR-induced DSBs in mammalian cells is NHEJ (non-homologous end-joining). The main proteins required for NHEJ in mammalian cells are the Ku heterodimer (Ku70/80 heterodimer), DNA-PKcs [the catalytic subunit of DNA-PK (DNA-dependent protein kinase)], Artemis, XRCC4 (X-ray-complementing Chinese hamster gene 4), DNA ligase IV and XLF (XRCC4-like factor; also called Cernunnos). Additional proteins, including DNA polymerases mu and lambda, PNK (polynucleotide kinase) and WRN (Werner's Syndrome helicase), may also play a role. In the present review, we will discuss our current understanding of the mechanism of NHEJ in mammalian cells and discuss the roles of DNA-PKcs and DNA-PK-mediated phosphorylation in NHEJ.

Characterization of a Bacillus subtilis 64-kDa DNA polymerase X potentially involved in DNA repair

Bacillus subtilis gene yshC encodes a 64-kDa family X DNA polymerase (PolXBs), which contains all the critical residues involved in DNA and nucleotide binding as well as those responsible for catalysis of DNA polymerization, conserved in most family X members. Biochemical analyses of the purified enzyme indicate that PolXBs is a monomeric and strictly template-directed DNA polymerase, preferentially acting on DNA structures containing gaps from one to a few nucleotides and bearing a phosphate group at the 5' end of the downstream DNA. The fact that PolXBs is able to conduct filling of a single-nucleotide gap, allowing further sealing of the resulting nick by a DNA ligase, points to a putative role in base excision repair during the B. subtilis life cycle.

Biochemical studies with DNA polymerase beta and DNA polymerase beta-PAK of Trypanosoma cruzi suggest the involvement of these proteins in mitochondrial DNA maintenance

Mammalian DNA polymerase beta is a nuclear enzyme involved in the base excision and single-stranded DNA break repair pathways. In trypanosomatids, this protein does not have a defined cellular localization, and its function is poorly understood. We characterized two Trypanosoma cruzi proteins homologous to mammalian DNA polymerasebeta, TcPolbeta and TcPolbetaPAK, and showed that both enzymes localize to the parasite kinetoplast. In vitro assays with purified proteins showed that they have DNA polymerization and deoxyribose phosphate lyase activities. Optimal conditions for polymerization were different for each protein with respect to dNTP concentration and temperature, and TcPolbetaPAK, in comparison to TcPolbeta, conducted DNA synthesis over a much broader pH range. TcPolbeta was unable to carry out mismatch extension or DNA synthesis across 8-oxodG lesions, and was able to discriminate between dNTP and ddNTP. These specific abilities of TcPolbeta were not observed for TcPolbetaPAK or other X family members, and are not due to a phenylalanine residue at position 395 in the C-terminal region of TcPolbeta, as assessed by a site-directed mutagenesis experiment reversing this residue to a well conserved tyrosine. Our data suggest that both polymerases from T. cruzi could cooperate to maintain mitochondrial DNA integrity through their multiple roles in base excision repair, gap filling and translesion synthesis.

Catalytic mechanism of human DNA polymerase lambda with Mg2+ and Mn2+ from ab initio quantum mechanical/molecular mechanical studies

DNA polymerases play a crucial role in the cell cycle due to their involvement in genome replication and repair. Understanding the reaction mechanism by which these polymerases carry out their function can provide insights into these processes. Recently, the crystal structures of human DNA polymerase lambda (Pollambda) have been reported both for pre- and post-catalytic complexes [García-Díaz et al., DNA Repair 3 (2007), 1333]. Here we employ the pre-catalytic complex as a starting structure for the determination of the catalytic mechanism of Pollambda using ab initio quantum mechanical/molecular mechanical methods. The reaction path has been calculated using Mg(2+) and Mn(2+) as the catalytic metals. In both cases the reaction proceeds through a two-step mechanism where the 3'-OH of the primer sugar ring is deprotonated by one of the conserved Asp residues (D490) in the active site before the incorporation of the nucleotide to the nascent DNA chain. A significant charge transfer is observed between both metals and some residues in the active site as the reaction proceeds. The optimized reactant and product structures agree with the reported crystal structures. In addition, the calculated reaction barriers for both metals are close to experimentally estimated barriers. Energy decomposition analysis to explain individual residue contributions suggests that several amino acids surrounding the active site are important for catalysis. Some of these residues, including R420, R488 and E529, have been implicated in catalysis by previous mutagenesis experiments on the homologous residues on Polbeta. Furthermore, Pollambda residues R420 and E529 found to be important from the energy decomposition analysis, are homologous to residues R183 and E295 in Polbeta, both of which are linked to cancer. In addition, residues R386, E391, K422 and K472 appear to have an important role in catalysis and could be a potential target for mutagenesis experiments. There is partial conservation of these residues across the Pol X family of DNA polymerases.

Tolerance for 8-oxoguanine but not thymine glycol in alignment-based gap filling of partially complementary double-strand break ends by DNA polymerase lambda in human nuclear extracts

Ionizing radiation induces various clustered DNA lesions, including double-strand breaks (DSBs) accompanied by nearby oxidative base damage. Previous work showed that, in HeLa nuclear extracts, DSBs with partially complementary 3′ overhangs and a one-base gap in each strand are accurately rejoined, with the gaps being filled by DNA polymerase λ. To determine the possible effect of oxidative base damage on this process, plasmid substrates were constructed containing overhangs with 8-oxoguanine or thymine glycol in base-pairing positions of 3-base (-ACG or -GTA) 3′ overhangs. In this context, 8-oxoguanine was well tolerated by the end-joining machinery when present at one end of the break, but not when present at both ends. Thymine glycol was less well tolerated than 8-oxoguanine, reducing gap filling and accurate rejoining by at least 10-fold. The results suggest that complex DSBs can be accurately rejoined despite the presence of accompanying base damage, but that nonplanar bases constitute a major barrier to this process and promote error-prone joining. A chimeric DNA polymerase, in which the catalytic domain of polymerase λ was replaced with that of polymerase β, could not substitute for polymerase λ in these assays, suggesting that this domain is specifically adapted for gap filling on aligned DSB ends.

Substrate-induced DNA strand misalignment during catalytic cycling by DNA polymerase lambda

The simple deletion of nucleotides is common in many organisms. It can be advantageous when it activates genes beneficial to microbial survival in adverse environments, and deleterious when it mutates genes relevant to survival, cancer or degenerative diseases. The classical idea is that simple deletions arise by strand slippage. A prime opportunity for slippage occurs during DNA synthesis, but it remains unclear how slippage is controlled during a polymerization cycle. Here, we report crystal structures and molecular dynamics simulations of mutant derivatives of DNA polymerase λ bound to a primer–template during strand slippage. Relative to the primer strand, the template strand is in multiple conformations, indicating intermediates on the pathway to deletion mutagenesis. Consistent with these intermediates, the mutant polymerases generate single-base deletions at high rates. The results indicate that dNTP-induced template strand repositioning during conformational rearrangements in the catalytic cycle is crucial to controlling the rate of strand slippage.

The X family portrait: structural insights into biological functions of X family polymerases

The mammalian family X DNA polymerases (DNA polymerases beta, lambda, mu, and TdT) contribute to base excision repair and double-strand break repair by virtue of their ability to fill short gaps in DNA. Structural information now exists for all four of these enzymes, making this the first mammalian polymerase family whose structural portrait is complete. Here we consider how distinctive structural features of these enzymes contribute to their biological functions in vivo.

Role of the catalytic metal during polymerization by DNA polymerase lambda

The incorporation of dNMPs into DNA by polymerases involves a phosphoryl transfer reaction hypothesized to require two divalent metal ions. Here we investigate this hypothesis using as a model human DNA polymerase lambda (Pol lambda), an enzyme suggested to be activated in vivo by manganese. We report the crystal structures of four complexes of human Pol lambda. In a 1.9 A structure of Pol lambda containing a 3'-OH and the non-hydrolyzable analog dUpnpp, a non-catalytic Na+ ion occupies the site for metal A and the ribose of the primer-terminal nucleotide is found in a conformation that positions the acceptor 3'-OH out of line with the alpha-phosphate and the bridging oxygen of the pyrophosphate leaving group. Soaking this crystal in MnCl2 yielded a 2.0 A structure with Mn2+ occupying the site for metal A. In the presence of Mn2+, the conformation of the ribose is C3'-endo and the 3'-oxygen is in line with the leaving oxygen, at a distance from the phosphorus atom of the alpha-phosphate (3.69 A) consistent with and supporting a catalytic mechanism involving two divalent metal ions. Finally, soaking with MnCl2 converted a pre-catalytic Pol lambda/Na+ complex with unreacted dCTP in the active site into a product complex via catalysis in the crystal. These data provide pre- and post-transition state information and outline in a single crystal the pathway for the phosphoryl transfer reaction carried out by DNA polymerases.

Mechanisms of Disease: DNA repair defects and neurological disease

In this Review, familial and sporadic neurological disorders reported to have an etiological link with DNA repair defects are discussed, with special emphasis placed on the molecular link between the disease phenotype and the precise DNA repair defect. Of the 15 neurological disorders listed, some of which have symptoms of progeria, six--spinocerebellar ataxia with axonal neuropathy-1, Huntington's disease, Alzheimer's disease, Parkinson's disease, Down syndrome and amyotrophic lateral sclerosis--seem to result from increased oxidative stress, and the inability of the base excision repair pathway to handle the damage to DNA that this induces. Five of the conditions (xeroderma pigmentosum, Cockayne's syndrome, trichothiodystrophy, Down syndrome, and triple-A syndrome) display a defect in the nucleotide excision repair pathway, four (Huntington's disease, various spinocerebellar ataxias, Friedreich's ataxia and myotonic dystrophy types 1 and 2) exhibit an unusual expansion of repeat sequences in DNA, and four (ataxia-telangiectasia, ataxia-telangiectasia-like disorder, Nijmegen breakage syndrome and Alzheimer's disease) exhibit defects in genes involved in repairing double-strand breaks. The current overall picture indicates that oxidative stress is a major causative factor in genomic instability in the brain, and that the nature of the resulting neurological phenotype depends on the pathway through which the instability is normally repaired.

Multiple functions of DNA polymerases

The primary role of DNA polymerases is to accurately and efficiently replicate the genome in order to ensure the maintenance of the genetic information and its faithful transmission through generations. This is not a simple task considering the size of the genome and its constant exposure to endogenous and environmental DNA damaging agents. Thus, a number of DNA repair pathways operate in cells to protect the integrity of the genome. In addition to their role in replication, DNA polymerases play a central role in most of these pathways. Given the multitude and the complexity of DNA transactions that depend on DNA polymerase activity, it is not surprising that cells in all organisms contain multiple highly specialized DNA polymerases, the majority of which have only recently been discovered. Five DNA polymerases are now recognized in Escherichia coli, 8 in Saccharomyces cerevisiae, and at least 15 in humans. While polymerases in bacteria, yeast and mammalian cells have been extensively studied much less is known about their counterparts in plants. For example, the plant model organism Arabidopsis thaliana is thought to contain 12 DNA polymerases, whose functions are mostly unknown. Here we review the properties and functions of DNA polymerases focusing on yeast and mammalian cells but paying special attention to the plant enzymes and the special circumstances of replication and repair in plant cells.

Structural insight into the substrate specificity of DNA Polymerase mu

DNA polymerase mu (Pol mu) is a family X enzyme with unique substrate specificity that contributes to its specialized role in nonhomologous DNA end joining (NHEJ). To investigate Pol mu's unusual substrate specificity, we describe the 2.4 A crystal structure of the polymerase domain of murine Pol mu bound to gapped DNA with a correct dNTP at the active site. This structure reveals substrate interactions with side chains in Pol mu that differ from other family X members. For example, a single amino acid substitution, H329A, has little effect on template-dependent synthesis by Pol mu from a paired primer terminus, but it reduces both template-independent and template-dependent synthesis during NHEJ of intermediates whose 3' ends lack complementary template strand nucleotides. These results provide insight into the substrate specificity and differing functions of four closely related mammalian family X DNA polymerases.

DNA repair in aging rat neurons

This laboratory, using post-mitotic rat brain neurons as a model system, has been testing the hypothesis that the inherited DNA repair potential would have profound influence on the aging process of the individual. It has been found that both single and double strand breaks in DNA accumulate in neurons with age. Since base excision repair (BER) is the pathway to effect repair of the type of DNA damage that is likely to occur in neurons, model oligo duplexes were used to assess the BER pathway. Both extension of a primer and one or four nucleotide gap repair are markedly reduced in aging neurons as compared with the young. The extension activity could be restored by supplementing the neuronal extracts with pure DNA polymerase beta (pol beta) while the restoration of gap repair needed the addition of both pol beta and DNA ligase. It thus appears that both pol beta and DNA ligase are deficient in aging neurons. We have also established a system to study the non-homologous end joining (NHEJ) mode of DNA repair in neurons. The end joining of cohesive but not of blunt or non-matching ends, is reduced with age and attempts to identify the limiting factor(s) in this case have been unsuccessful so far. These results are reviewed vis-à-vis the existing literature.