Bacille Calmette-Guérin vaccine reprograms human neonatal lipid metabolism in vivo and in vitro.

Vaccines have generally been developed with limited insight into their molecular impact. While systems vaccinology enables characterization of mechanisms of action, these tools have yet to be applied to infants, who are at high risk of infection and receive the most vaccines. Bacille Calmette-Guérin (BCG) protects infants against disseminated tuberculosis (TB) and TB-unrelated infections via incompletely understood mechanisms. We employ mass-spectrometry-based metabolomics of blood plasma to profile BCG-induced infant responses in Guinea-Bissau in vivo and the US in vitro. BCG-induced lysophosphatidylcholines (LPCs) correlate with both TLR-agonist- and purified protein derivative (PPD, mycobacterial antigen)-induced blood cytokine production in vitro, raising the possibility that LPCs contribute to BCG immunogenicity. Analysis of an independent newborn cohort from The Gambia demonstrates shared vaccine-induced metabolites, such as phospholipids and sphingolipids. BCG-induced changes to the plasma lipidome and LPCs may contribute to its immunogenicity and inform the development of early life vaccines.

Apolipoprotein E genotype-dependent nutrigenetic effects to prebiotic inulin for modulating systemic metabolism and neuroprotection in mice via gut-brain axis.

OBJECTIVE: The goal of the study was to identify the potential nutrigenetic effects to inulin, a prebiotic fiber, in mice with different human apolipoprotein E (APOE) genetic variants. Specifically, we compared responses to inulin for the potential modulation of the systemic metabolism and neuroprotection via gut-brain axis in mice with human APOE ϵ3 and ϵ4 alleles. METHOD: We performed experiments with young mice expressing the human APOE3 (E3FAD mice and APOE4 gene (E4FAD mice). We fed mice with either inulin or control diet for 16 weeks starting from 3 months of age. We determined gut microbiome diversity and composition using16s rRNA sequencing, systemic metabolism using in vivo MRI and metabolomics, and blood-brain barrier (BBB) tight junction expression using Western blot. RESULTS: In both E3FAD and E4FAD mice, inulin altered the alpha and beta diversity of the gut microbiome, increased beneficial taxa of bacteria and elevated cecal short chain fatty acid and hippocampal scyllo-inositol. E3FAD mice had altered metabolism related to tryptophan and tyrosine, while E4FAD mice had changes in the tricarboxylic acid cycle, pentose phosphate pathway, and bile acids. Differences were found in levels of brain metabolites related to oxidative stress, and levels of Claudin-1 and Claudin-5 BBB tight junction expression. DISCUSSION: We found that inulin had many similar beneficial effects in the gut and brain for both E3FAD and E4FAD mice, which may be protective for brain functions and reduce risk for neurodegeneration. . E3FAD and E4FAD mice also had distinct responses in several metabolic pathways, suggesting an APOE-dependent nutrigenetic effects in modulating systemic metabolism and neuroprotection.

APOE genotype-dependent pharmacogenetic responses to rapamycin for preventing Alzheimer's disease.

The ε4 allele of Apolipoprotein (APOE4) is the strongest genetic risk factor for Alzheimer's disease (AD), the most common form of dementia. Cognitively normal APOE4 carriers have developed amyloid beta (Aß) plaques and cerebrovascular, metabolic and structural deficits decades before showing the cognitive impairment. Interventions that can inhibit Aß retention and restore the brain functions to normal would be critical to prevent AD for the asymptomatic APOE4 carriers. A major goal of the study was to identify the potential usefulness of rapamycin (Rapa), a pharmacological intervention for extending longevity, for preventing AD in the mice that express human APOE4 gene and overexpress Aß (the E4FAD mice). Another goal of the study was to identify the potential pharmacogenetic differences in response to rapamycin between the E4FAD and E3FAD mice, the mice with human APOE ε3 allele. We used multi-modal MRI to measure in vivo cerebral blood flow (CBF), neurotransmitter levels, white matter integrity, water content, cerebrovascular reactivity (CVR) and somatosensory response; used behavioral assessments to determine cognitive function; used biochemistry assays to determine Aß retention and blood-brain barrier (BBB) functions; and used metabolomics to identify brain metabolic changes. We found that in the E4FAD mice, rapamycin normalized bodyweight, restored CBF (especially in female), BBB activity for Aß transport, neurotransmitter levels, neuronal integrity and free fatty acid level, and reduced Aß retention, which were not observe in the E3FAD-Rapa mice. In contrast, E3FAD-Rapa mice had lower CVR responses, lower anxiety and reduced glycolysis in the brain, which were not seen in the E4FAD-Rapa mice. Further, rapamycin appeared to normalize lipid-associated metabolism in the E4FAD mice, while slowed overall glucose-associated metabolism in the E3FAD mice. Finally, rapamycin enhanced overall water content, water diffusion in white matter, and spatial memory in both E3FAD and E4FAD mice, but did not impact the somatosensory responses under hindpaw stimulation. Our findings indicated that rapamycin was able to restore brain functions and reduce AD risk for young, asymptomatic E4FAD mice, and there were pharmacogenetic differences between the E3FAD and E4FAD mice. As the multi-modal MRI methods used in the study are readily to be used in humans and rapamycin is FDA-approved, our results may pave a way for future clinical testing of the pharmacogenetic responses in humans with different APOE alleles, and potentially using rapamycin to prevent AD for asymptomatic APOE4 carriers.

Dietary inulin alters the gut microbiome, enhances systemic metabolism and reduces neuroinflammation in an APOE4 mouse model.

The apolipoprotein ε4 allele (APOE4) is the strongest genetic risk factor for Alzheimer's disease (AD). APOE4 carriers develop systemic metabolic dysfunction decades before showing AD symptoms. Accumulating evidence shows that the metabolic dysfunction accelerates AD development, including exacerbated amyloid-beta (Aß) retention, neuroinflammation and cognitive decline. Therefore, preserving metabolic function early on may be critical to reducing the risk for AD. Here, we show that inulin increases beneficial microbiota and decreases harmful microbiota in the feces of young, asymptomatic APOE4 transgenic (E4FAD) mice and enhances metabolism in the cecum, periphery and brain, as demonstrated by increases in the levels of SCFAs, tryptophan-derived metabolites, bile acids, glycolytic metabolites and scyllo-inositol. We show that inulin also reduces inflammatory gene expression in the hippocampus. This knowledge can be utilized to design early precision nutrition intervention strategies that use a prebiotic diet to enhance systemic metabolism and may be useful for reducing AD risk in asymptomatic APOE4 carriers.

Minimal detection of nuclear mutations in XP-V and normal cells treated with oxidative stress inducing agents.

Elevated levels of reactive oxygen species (ROS) can be induced by exposure to various chemicals and radiation. One type of damage in DNA produced by ROS is modification of guanine to 7,8-dihydro-8-oxoguanine (8-oxoG). This particular alteration to the chemistry of the base can inhibit the replication fork and has been linked to mutagenesis, cancer, and aging. In vitro studies have shown that the translesion synthesis polymerase, DNA polymerase η (pol η), is able to efficiently bypass 8-oxoG in DNA. In this study, we wanted to investigate the mutagenic effects of oxidative stress, and in particular 8-oxoG, in the presence and absence of pol η. We quantified levels of oxidative stress, 8-oxoG levels in DNA, and nuclear mutation rates. We found that most of the 8-oxoG detected were localized to the mitochondrial DNA, opposed to the nuclear DNA. We also saw a corresponding lack of mutations in a nuclear-encoded gene. This suggests that oxidative stress' primary mutagenic effects are not predominantly on genomic DNA.

Biochemical analysis of DNA polymerase η fidelity in the presence of replication protein A.

DNA polymerase η (pol η) synthesizes across from damaged DNA templates in order to prevent deleterious consequences like replication fork collapse and double-strand breaks. This process, termed translesion synthesis (TLS), is an overall positive for the cell, as cells deficient in pol η display higher mutation rates. This outcome occurs despite the fact that the in vitro fidelity of bypass by pol η alone is moderate to low, depending on the lesion being copied. One possible means of increasing the fidelity of pol η is interaction with replication accessory proteins present at the replication fork. We have previously utilized a bacteriophage based screening system to measure the fidelity of bypass using purified proteins. Here we report on the fidelity effects of a single stranded binding protein, replication protein A (RPA), when copying the oxidative lesion 7,8-dihydro-8-oxo-guanine(8-oxoG) and the UV-induced cis-syn thymine-thymine cyclobutane pyrimidine dimer (T-T CPD). We observed no change in fidelity dependent on RPA when copying these damaged templates. This result is consistent in multiple position contexts. We previously identified single amino acid substitution mutants of pol η that have specific effects on fidelity when copying both damaged and undamaged templates. In order to confirm our results, we examined the Q38A and Y52E mutants in the same full-length construct. We again observed no difference when RPA was added to the bypass reaction, with the mutant forms of pol η displaying similar fidelity regardless of RPA status. We do, however, observe some slight effects when copying undamaged DNA, similar to those we have described previously. Our results indicate that RPA by itself does not affect pol η dependent lesion bypass fidelity when copying either 8-oxoG or T-T CPD lesions.

Detrimental effects of UV-B radiation in a xeroderma pigmentosum-variant cell line.

DNA polymerase η (pol η), of the Y-family, is well known for its in vitro DNA lesion bypass ability. The most well-characterized lesion bypassed by this polymerase is the cyclobutane pyrimidine dimer (CPD) caused by ultraviolet (UV) light. Historically, cellular and whole-animal models for this area of research have been conducted using UV-C (λ=100-280 nm) owing to its ability to generate large quantities of CPDs and also the more structurally distorting 6-4 photoproduct. Although UV-C is useful as a laboratory tool, exposure to these wavelengths is generally very low owing to being filtered by stratospheric ozone. We are interested in the more environmentally relevant wavelength range of UV-B (λ=280-315 nm) for its role in causing cytotoxicity and mutagenesis. We evaluated these endpoints in both a normal human fibroblast control line and a Xeroderma pigmentosum variant cell line in which the POLH gene contains a truncating point mutation, leading to a nonfunctional polymerase. We demonstrate that UV-B has similar but less striking effects compared to UV-C in both its cytotoxic and its mutagenic effects. Analysis of the mutation spectra after a single dose of UV-B shows that a majority of mutations can be attributed to mutagenic bypass of dipyrimidine sequences. However, we do note additional types of mutations with UV-B that are not previously reported after UV-C exposure. We speculate that these differences are attributed to a change in the spectra of photoproduct lesions rather than other lesions caused by oxidative stress.

Mutation of the little finger domain in human DNA polymerase η alters fidelity when copying undamaged DNA.

DNA polymerase η (pol η) synthesizes past cyclobutane pyrimidine dimer and possibly 7,8-dihydro-8-oxoguanine (8-oxoG) lesions during DNA replication. Loss of pol η is associated with an increase in mutation rate, demonstrating its indispensable role in mutation suppression. It has been recently reported that ß-strand 12 (amino acids 316-324) of the little finger region correctly positions the template strand with the catalytic core of the enzyme. The authors hypothesized that modification of ß-strand 12 residues would disrupt correct enzyme-DNA alignment and alter pol η's activity and fidelity. To investigate this, the authors purified proteins containing the catalytic core of the polymerase, incorporated single amino acid changes to select ß-strand 12 residues, and evaluated DNA synthesis activity for each pol η. Lesion bypass efficiencies and replication fidelities when copying DNA-containing cis-syn cyclobutane thymine-thymine dimer and 8-oxoG lesions were determined and compared with the corresponding values for the wild-type polymerase. The results confirm the importance of the ß-strand in polymerase function and show that fidelity is most often altered when undamaged DNA is copied. Additionally, it is shown that DNA-protein contacts distal to the active site can significantly affect the fidelity of synthesis.

Biochemical analysis of active site mutations of human polymerase η.

DNA polymerase η (pol η) plays a critical role in suppressing mutations caused by the bypass of cis-syn cyclobutane pyrimidine dimers (CPD) that escape repair. There is evidence this is also the case for the oxidative lesion 7,8-dihydro-8-oxo-guanine (8-oxoG). Both of these lesions cause moderate to severe blockage of synthesis when encountered by replicative polymerases, while pol η displays little no to pausing during translesion synthesis. However, since lesion bypass does not remove damaged DNA from the genome and can possibly be accompanied by errors in synthesis during bypass, the process is often called 'damage tolerance' to delineate it from classical DNA repair pathways. The fidelity of lesion bypass is therefore of importance when determining how pol η suppresses mutations after DNA damage. As pol η has been implicated in numerous in vivo pathways other than lesion bypass, we wanted to better understand the molecular mechanisms involved in the relatively low-fidelity synthesis displayed by pol η. To that end, we have created a set of mutant pol η proteins each containing a single amino acid substitution in the active site and closely surrounding regions. We determined overall DNA synthesis ability as well as the efficiency and fidelity of bypass of thymine-thymine CPD (T-T CPD) and 8-oxoG containing DNA templates. Our results show that several amino acids are critical for normal polymerase function, with changes in overall activity and fidelity being observed. Of the mutants that retain polymerase activity, we demonstrate that amino acids Q38, Y52, and R61 play key roles in determining polymerase fidelity, with substation of alanine causing both increases and decreases in fidelity. Remarkably, the Q38A mutant displays increased fidelity during synthesis opposite 8-oxoG but decreased fidelity during synthesis opposite a T-T CPD.

The efficiency and fidelity of 8-oxo-guanine bypass by DNA polymerases delta and eta.

A DNA lesion created by oxidative stress is 7,8-dihydro-8-oxo-guanine (8-oxoG). Because 8-oxoG can mispair with adenine during DNA synthesis, it is of interest to understand the efficiency and fidelity of 8-oxoG bypass by DNA polymerases. We quantify bypass parameters for two DNA polymerases implicated in 8-oxoG bypass, Pols delta and eta. Yeast Pol delta and yeast Pol eta both bypass 8-oxoG and misincorporate adenine during bypass. However, yeast Pol eta is 10-fold more efficient than Pol delta, and following bypass Pol eta switches to less processive synthesis, similar to that observed during bypass of a cis-syn thymine-thymine dimer. Moreover, yeast Pol eta is at least 10-fold more accurate than yeast Pol delta during 8-oxoG bypass. These differences are maintained in the presence of the accessory proteins RFC, PCNA and RPA and are consistent with the established role of Pol eta in suppressing ogg1-dependent mutagenesis in yeast. Surprisingly different results are obtained with human and mouse Pol eta. Both mammalian enzymes bypass 8-oxoG efficiently, but they do so less processively, without a switch point and with much lower fidelity than yeast Pol eta. The fact that yeast and mammalian Pol eta have intrinsically different catalytic properties has potential biological implications.

The fidelity of DNA synthesis by eukaryotic replicative and translesion synthesis polymerases.

In their seminal publication describing the structure of the DNA double helix, Watson and Crick wrote what may be one of the greatest understatements in the scientific literature, namely that "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." Half a century later, we more fully appreciate what a huge challenge it is to replicate six billion nucleotides with the accuracy needed to stably maintain the human genome over many generations. This challenge is perhaps greater than was realized 50 years ago, because subsequent studies have revealed that the genome can be destabilized not only by environmental stresses that generate a large number and variety of potentially cytotoxic and mutagenic lesions in DNA but also by various sequence motifs of normal DNA that present challenges to replication. Towards a better understanding of the many determinants of genome stability, this chapter reviews the fidelity with which undamaged and damaged DNA is copied, with a focus on the eukaryotic B- and Y-family DNA polymerases, and considers how this fidelity is achieved.

Mutator alleles of yeast DNA polymerase zeta.

The yeast REV3 gene encodes the catalytic subunit of DNA polymerase zeta (pol zeta), a B family polymerase that performs mutagenic DNA synthesis in cells. To probe pol zeta mutagenic functions, we generated six mutator alleles of REV3 with amino acid replacements for Leu979, a highly conserved residue inferred to be at the pol zeta active site. Replacing Leu979 with Gly, Val, Asn, Lys, Met or Phe resulted in yeast strains with elevated UV-induced mutant frequencies. While four of these strains had reduced survival following UV irradiation, the rev3-L979F and rev3-L979M strains had normal survival, suggesting retention of pol zeta catalytic activity. UV mutagenesis in the rev3-L979F background was increased when photoproduct bypass by pol eta was eliminated by deletion of RAD30. The rev3-L979F mutation had little to no effect on mutagenesis in an ogg1Delta background, which cannot repair 8-oxo-guanine in DNA. UV-induced can1 mutants from rev3-L979F and rad30Deltarev3-L979F strains primarily contained base substitutions and complex mutations, suggesting error-prone bypass of UV photoproducts by L979F pol zeta. Spontaneous mutation rates in rev3-L979F and rev3-L979M strains are elevated by about two-fold overall and by two- to eight-fold for C to G transversions and complex mutations, both of which are known to be generated by wild-type pol zetain vitro. These results indicate that Rev3p-Leu979 replacements reduce the fidelity of DNA synthesis by yeast pol zetain vivo. In conjunction with earlier studies, the data establish that the conserved amino acid at the active site location occupied by Leu979 is critical for the fidelity of all four yeast B family polymerases. Reduced fidelity with retention of robust polymerase activity suggests that the homologous rev3-L979F allele may be useful for analyzing pol zeta functions in mammals, where REV3 deletion is lethal.

Effects of accessory proteins on the bypass of a cis-syn thymine-thymine dimer by Saccharomyces cerevisiae DNA polymerase eta.

Among several hypotheses to explain how translesion synthesis (TLS) by DNA polymerase eta (pol eta) suppresses ultraviolet light-induced mutagenesis in vivo despite the fact that pol eta copies DNA with low fidelity, here we test whether replication accessory proteins enhance the fidelity of TLS by pol eta. We first show that the single-stranded DNA binding protein RPA, the sliding clamp PCNA, and the clamp loader RFC slightly increase the processivity of yeast pol eta and its ability to recycle to new template primers. However, these increases are small, and they are similar when copying an undamaged template and a template containing a cis-syn TT dimer. Consequently, the accessory proteins do not strongly stimulate the already robust TT dimer bypass efficiency of pol eta. We then perform a comprehensive analysis of yeast pol eta fidelity. We show that it is much less accurate than other yeast DNA polymerases and that the accessory proteins have little effect on fidelity when copying undamaged templates or when bypassing a TT dimer. Thus, although accessory proteins clearly participate in pol eta functions in vivo, they do not appear to help suppress UV mutagenesis by improving pol eta bypass fidelity per se.

Specialized mismatch repair function of Glu339 in the Phe-X-Glu motif of yeast Msh6.

The major eukaryotic mismatch repair (MMR) pathway requires Msh2-Msh6, which, like Escherichia coli MutS, binds to and participates in repair of the two most common replication errors, single base-base and single base insertion-deletion mismatches. For both types of mismatches, the side chain of E. coli Glu38 in a conserved Phe-X-Glu motif interacts with a mismatched base. The Ovarepsilon of Glu38 forms a hydrogen bond with either the N7 of purines or the N3 of pyrimidines. We show here that changing E. coli Glu38 to alanine results in nearly complete loss of repair of both single base-base and single base deletion mismatches. In contrast, a yeast strain with alanine replacing homologous Glu339 in Msh6 has nearly normal repair for insertion-deletion and most base-base mismatches, but is defective in repairing base-base mismatches characteristic of oxidative stress, e.g. 8-oxo-G.A mismatches. The results suggest that bacterial MutS and yeast Msh2-Msh6 differ in how they recognize and/or process replication errors involving undamaged bases, and that Glu339 in Msh6 may have a specialized role in repairing mismatches containing oxidized bases.

Replication of 2-hydroxyadenine-containing DNA and recognition by human MutSalpha.

2-Hydroxyadenine (2-OH-A), a product of DNA oxidation, is a potential source of mutations. We investigated how representative DNA polymerases from the A, B and Y families dealt with 2-OH-A in primer extension experiments. A template 2-OH-A reduced the rate of incorporation by DNA polymerase alpha (Pol alpha) and Klenow fragment (Kf(exo-)). Two Y family DNA polymerases, human polymerase eta (Pol eta) and the archeal Dpo4 polymerase were affected differently. Bypass by Pol eta was very inefficient whereas Dpo4 efficiently replicated 2-OH-A. Replication of a template 2-OH-A by both enzymes was mutagenic and caused base substitutions. Dpo4 additionally introduced single base deletions. Thermodynamic analysis showed that 2-OH-A forms stable base pairs with T, C and G, and to a lesser extent with A. Oligonucleotides containing 2-OH-A base pairs, including the preferred 2-OH-A:T, were recognized by the human MutSalpha mismatch repair (MMR). MutSalpha also recognized 2-OH-A located in a repeat sequence that mimics a frameshift intermediate.

Multiple solutions to inefficient lesion bypass by T7 DNA polymerase.

We hypothesize that enzymatic switching during translesion synthesis (TLS) to relieve stalled replication forks occurs during transitions from preferential to disfavored use of damaged primer-templates, and that the polymerase or 3'-exonuclease used for each successive nucleotide incorporated is the one whose properties result in the highest efficiency and the highest fidelity of bypass. Testing this hypothesis requires quantitative determination of the relative lesion bypass ability of both TLS polymerases and major replicative polymerases. As a model of the latter, here we measure the efficiency and fidelity of cis-syn TT dimer and abasic site bypass using the structurally well-characterized T7 DNA polymerase. No bypass of either lesion occurred during a single round of synthesis, and the exonuclease activity of wild-type T7 DNA polymerase was critical in preventing TLS. When repetitive cycling of the exonuclease-deficient enzyme was allowed, limited bypass did occur but hundreds to thousands of cycles were required to achieve even a single bypass event. Analysis of TLS fidelity indicated that these rare bypass events involved rearrangements of the template and primer strands, insertions opposite the lesion, and combinations of these events, with the choice among these strongly depending on the sequence context of the lesion. Moreover, the presence of a lesion affected the fidelity of copying adjacent undamaged template bases, even when lesion bypass itself was correct. The results also indicate that a TT dimer presents a different type of block to the polymerase than an abasic site, even though both lesions are extremely potent blocks to processive synthesis. The approaches used here to quantify the efficiency and fidelity of TLS can be applied to other polymerase-lesion combinations, to provide guidance as to which of many possible polymerases is most likely to bypass various lesions in biological contexts.

Measuring the fidelity of translesion DNA synthesis.

A method is described to measure the fidelity of copying past a DNA lesion in a defined sequence on a synthetic oligonucleotide primer-template. The DNA product is the result of a complete lesion bypass reaction, i.e., containing all four deoxynucleotide triphosphates and requiring both insertion opposite the lesion and multiple extensions from the resulting primer termini containing the lesion. The nascent strand is recovered and hybridized to a gapped region of the lacZalpha complementation gene of the M13mp2 genome. When this DNA is introduced into Escherichia coli, errors made during translesion DNA synthesis are detected by M13 plaque colors. Sequencing of DNA from mutant plaques defines the types of errors and permits calculation of error rates for base substitutions, insertions, and deletions. The method is illustrated here for bypass of a cis-syn thymine-thymine dimer by human DNA polymerase eta. The assay can be used with other lesions in various sequence contexts and with other polymerases with or without accessory proteins.

Increased susceptibility to UV-induced skin carcinogenesis in polymerase eta-deficient mice.

Xeroderma pigmentosum variant (XPV) patients with mutations in the DNA polymerase eta (pol eta) gene are hypersensitive to sunlight and have greatly increased susceptibility to sunlight-induced skin cancer. Consistent with the ability of Pol eta to efficiently bypass UV light-induced cyclobutane pyrimidine dimers, XPV cells lacking Pol eta have diminished capacity to replicate UV-damaged DNA and are sensitive to UV light-induced killing and mutagenesis. To better understand these and other Pol eta functions, we generated Pol eta-deficient mice. Mice homozygous for a null mutation in pol eta are viable, fertile, and do not show any obvious spontaneous defects during the first year of life. However, fibroblasts derived from these mutant mice are sensitive to killing by exposure to UV light, and all Pol eta-deficient mice develop skin tumors after UV irradiation, in contrast to the wild-type littermate controls that did not develop such tumors. These results and biochemical studies of translesion synthesis by mouse Pol eta indicate that Pol eta-dependent bypass of cyclobutane pyrimidine dimers suppresses UV light-induced skin cancer in mice. Moreover, 37.5% of pol eta heterozygous mice also developed skin cancer during 5 months after a 5-month exposure to UV light, suggesting that humans who are heterozygous for mutations in pol eta may also have an increased risk of skin cancer.

Enzymatic switching for efficient and accurate translesion DNA replication.

When cyclobutane pyrimidine dimers stall DNA replication by DNA polymerase (Pol) delta or epsilon, a switch occurs to allow translesion synthesis by DNA polymerase eta, followed by another switch that allows normal replication to resume. In the present study, we investigate these switches using Saccharomyces cerevisiae Pol delta, Pol epsilon and Pol eta and a series of matched and mismatched primer templates that mimic each incorporation needed to completely bypass a cis-syn thymine-thymine (TT) dimer. We report a complementary pattern of substrate use indicating that enzymatic switching involving localized translesion synthesis by Pol eta and mismatch excision and polymerization by a major replicative polymerase can account for the efficient and accurate dimer bypass known to suppress sunlight-induced mutagenesis and skin cancer.

Efficiency, fidelity and enzymatic switching during translesion DNA synthesis.

More than half of the 16 human DNA polymerases may have some role in DNA replication and potentially modulate the biological effects of DNA template lesions that impede replication fork progression. As one approach to understand how multiple polymerases are coordinated at the fork, we recently quantified the efficiency and fidelity with which one particular translesion synthesis enzyme, human DNA polymerase eta, copies templates containing cis-syn thymine dimers. Several observations from that study were unanticipated. Here we discuss the structural and biological implications of those results in light of earlier studies of translesion synthesis.