Mass Spectrometry-Based Method to Measure Aflatoxin B1 DNA Adducts in Formalin-Fixed Paraffin-Embedded Tissues

Aflatoxin B1 (AFB1) is a potent human liver carcinogen produced by certain molds, particularly Aspergillus flavus and Aspergillus parasiticus, which contaminate peanuts, corn, rice, cottonseed, and ground and tree nuts, principally in warm and humid climates. AFB1 undergoes bioactivation in the liver to produce AFB1-exo-8,9-epoxide, which forms the covalently bound cationic AFB1-N7-guanine (AFB1-N7-Gua) DNA adduct. This adduct is unstable and undergoes base-catalyzed opening of the guanine imidazolium ring to form two ring-opened diastereomeric 8,9-dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)-9-hydroxy-aflatoxin B1 (AFB1-FapyGua) adducts. The AFB1 formamidopyrimidine (Fapy) adducts induce G → T transversion mutations and are likely responsible for the carcinogenic effects of AFB1. Quantitative liquid chromatography-mass spectrometry (LC-MS) methods have shown that AFB1-N7-Gua is eliminated in rodent and human urine, whereas ring-opened AFB1-FapyGua adducts persist in rodent liver. However, fresh frozen biopsy tissues are seldom available for biomonitoring AFB1 DNA adducts in humans, impeding research advances in this potent liver carcinogen. In contrast, formalin-fixed paraffin-embedded (FFPE) specimens used for histopathological analysis are often accessible for molecular studies. However, ensuring nucleic acid quality presents a challenge due to incomplete reversal of formalin-mediated DNA cross-links, which can preclude accurate quantitative measurements of DNA adducts. In this study, employing ion trap or high-resolution accurate Orbitrap mass spectrometry, we demonstrate that ring-opened AFB1-FapyGua adducts formed in AFB1-exposed newborn mice are stable to the formalin fixation and DNA de-cross-linking retrieval processes. The AFB1-FapyGua adducts can be detected at levels comparable to those in a match of fresh frozen liver. Orbitrap MS2 measurements can detect AFB1-FapyGua at a quantification limit of 4.0 adducts per 108 bases when only 0.8 μg of DNA is assayed on the column. Thus, our breakthrough DNA retrieval technology can be adapted to screen for AFB1 DNA adducts in FFPE human liver specimens from cohorts at risk of this potent liver carcinogen.

Tissue Organoid Cultures Metabolize Dietary Carcinogens Proficiently and Are Effective Models for DNA Adduct Formation

Human tissue three-dimensional (3D) organoid cultures have the potential to reproduce in vitro the physiological properties and cellular architecture of the organs from which they are derived. The ability of organoid cultures derived from human stomach, liver, kidney, and colon to metabolically activate three dietary carcinogens, aflatoxin B1 (AFB1), aristolochic acid I (AAI), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), was investigated. In each case, the response of a target tissue (liver for AFB1; kidney for AAI; colon for PhIP) was compared with that of a nontarget tissue (gastric). After treatment cell viabilities were measured, DNA damage response (DDR) was determined by Western blotting for p-p53, p21, p-CHK2, and γ-H2AX, and DNA adduct formation was quantified by mass spectrometry. Induction of the key xenobiotic-metabolizing enzymes (XMEs) CYP1A1, CYP1A2, CYP3A4, and NQO1 was assessed by qRT-PCR. We found that organoids from different tissues can activate AAI, AFB1, and PhIP. In some cases, this metabolic potential varied between tissues and between different cultures of the same tissue. Similarly, variations in the levels of expression of XMEs were observed. At comparable levels of cytotoxicity, organoids derived from tissues that are considered targets for these carcinogens had higher levels of adduct formation than a nontarget tissue.

Screening DNA Damage in the Rat Kidney and Liver by Untargeted DNA Adductomics

Air pollution, tobacco smoke, and red meat are associated with renal cell cancer (RCC) risk in the United States and Western Europe; however, the chemicals that form DNA adducts and initiate RCC are mainly unknown. Aristolochia herbaceous plants are used for medicinal purposes in Asia and worldwide. They are a significant risk factor for upper tract urothelial carcinoma (UTUC) and RCC to a lesser extent. The aristolochic acid (AA) 8-methoxy-6-nitrophenanthro-[3,4-d]-1,3-dioxolo-5-carboxylic acid (AA-I), a component of Aristolochia herbs, contributes to UTUC in Asian cohorts and in Croatia, where AA-I exposure occurs from ingesting contaminated wheat flour. The DNA adduct of AA-I, 7-(2'-deoxyadenosin-N6-yl)-aristolactam I, is often detected in patients with UTUC, and its characteristic A:T-to-T:A mutational signature occurs in oncogenes and tumor suppressor genes in AA-associated UTUC. Identifying DNA adducts in the renal parenchyma and pelvis caused by other chemicals is crucial to gaining insights into unknown RCC and UTUC etiologies. We employed untargeted screening with wide-selected ion monitoring tandem mass spectrometry (wide-SIM/MS2) with nanoflow liquid chromatography/Orbitrap mass spectrometry to detect DNA adducts formed in rat kidneys and liver from a mixture of 13 environmental, tobacco, and dietary carcinogens that may contribute to RCC. Twenty DNA adducts were detected. DNA adducts of 3-nitrobenzanthrone (3-NBA), an atmospheric pollutant, and AA-I were the most abundant. The nitrophenanthrene moieties of 3-NBA and AA-I undergo reduction to their N-hydroxy intermediates to form 2'-deoxyguanosine (dG) and 2'-deoxyadenosine (dA) adducts. We also discovered a 2'-deoxycytidine AA-I adduct and dA and dG adducts of 10-methoxy-6-nitro-phenanthro-[3,4-d]-1,3-dioxolo-5-carboxylic acid (AA-III), an AA-I isomer and minor component of the herbal extract assayed, signifying AA-III is a potent kidney DNA-damaging agent. The roles of AA-III, other nitrophenanthrenes, and nitroarenes in renal DNA damage and human RCC warrant further study. Wide-SIM/MS2 is a powerful scanning technology in DNA adduct discovery and cancer etiology characterization.

Mass Spectral Library for DNA Adductomics

Endogenous electrophiles, ionizing and non-ionizing radiation, and hazardous chemicals present in the environment and diet can damage DNA by forming covalent adducts. DNA adducts can form in critical cancer driver genes and, if not repaired, may induce mutations during cell division, potentially leading to the onset of cancer. The detection and quantification of specific DNA adducts are some of the first steps in studying their role in carcinogenesis, the physiological conditions that lead to their production, and the risk assessment of exposure to specific genotoxic chemicals. Hundreds of different DNA adducts have been reported in the literature, and there is a critical need to establish a DNA adduct mass spectral database to facilitate the detection of previously observed DNA adducts and characterize newly discovered DNA adducts. We have collected synthetic DNA adduct standards from the research community, acquired MSn (n = 2, 3) fragmentation spectra using Orbitrap and Quadrupole-Time-of-Flight (Q-TOF) MS instrumentation, processed the spectral data and incorporated it into the MassBank of North America (MoNA) database, and created a DNA adduct portal Web site (https://sites.google.com/umn.edu/dnaadductportal) to serve as a central location for the DNA adduct mass spectra and metadata, including the spectral database downloadable in different formats. This spectral library should prove to be a valuable resource for the DNA adductomics community, accelerating research and improving our understanding of the role of DNA adducts in disease.

High-Field Asymmetric Waveform Ion Mobility Spectrometry Analysis of Carcinogenic Aromatic Amines in Tobacco Smoke with an Orbitrap Tribrid Mass Spectrometer

Smoking is a risk factor for bladder cancer (BC), although the specific chemicals responsible for BC remain uncertain. Considerable research has focused on aromatic amines (AAs), including o-toluidine (o-tol), o-anisidine (o-anis), 2-naphthylamine (2-NA), and 4-aminobiphenyl (4-ABP), which are linked to human BC based on elevated BC incidence in occupationally exposed factory workers. These AAs arise at nanogram levels per combusted cigarette. The unambiguous identification of AAs, particularly low-molecular-weight monocyclic AAs in tobacco smoke extracts, by liquid chromatography-mass spectrometry (LC-MS) is challenging due to their poor performance on reversed-phase columns and co-elution with isobaric interferences from the complex tobacco smoke matrix. We employed a tandem liquid-liquid and solid-phase extraction method to isolate AAs from the basic fraction of tobacco smoke condensate (TSC) and utilized high-field asymmetric waveform ion mobility spectrometry (FAIMS) coupled to high-resolution accurate mass (HRAM) Orbitrap LC-MS2 to assay AAs in TSC. The employment of FAIMS greatly reduced sample complexity by removing precursor co-isolation interfering species at the MS1 scan stage, resulting in dramatically improved signal-to-noise of the precursor ions and cleaner, high-quality MS2 spectra for unambiguous identification and quantification of AAs in TSC. We demonstrate the power of LC/FAIMS/MS2 by characterizing and quantifying two low-molecular-weight carcinogenic AAs, o-tol and o-anis, in TSC, using stable isotopically labeled internal standards. These results demonstrate the power of FAIMS in trace-level analyses of AA carcinogens in the complex tobacco smoke matrix.

Nuclear DNA and Mitochondrial Damage of the Cooked Meat Carcinogen 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in Human Neuroblastoma Cells

Animal fat and iron-rich diets are risk factors for Parkinson's disease (PD). The heterocyclic aromatic amines (HAAs) harman and norharman are neurotoxicants formed in many foods and beverages, including cooked meats, suggesting a role for red meat in PD. The structurally related carcinogenic HAAs 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-9H-pyrido[2,3-b]indole (AαC) also form in cooked meats. We investigated the cytotoxicity, DNA-damaging potential, and mitochondrial damage of HAAs and their genotoxic HONH-HAA metabolites in galactose-dependent SH-SY5Y cells, a human neuroblastoma cell line relevant for PD-related neurotoxicity. All HAAs and HONH-HAAs induced weak toxicity except HONH-PhIP, which was 1000-fold more potent than the other chemicals. HONH-PhIP DNA adduct formation occurred at 300-fold higher levels than adducts formed with HONH-MeIQx and HONH-AαC, assuming similar cellular uptake rates. PhIP-DNA adduct levels occurred at concentrations as low as 1 nM and were threefold or higher and more persistent in mitochondrial DNA than nuclear DNA. N-Acetyltransferases (NATs), sulfotransferases, and kinases catalyzed PhIP-DNA binding and converted HONH-PhIP to highly reactive ester intermediates. DNA binding assays with cytosolic, mitochondrial, and nuclear fractions of SH-SY5Y fortified with cofactors revealed that cytosolic AcCoA-dependent enzymes, including NAT1, mainly carried out HONH-PhIP bioactivation to form N-acetoxy-PhIP, which binds to DNA. Furthermore, HONH-PHIP and N-acetoxy-PhIP inhibited mitochondrial complex-I, -II, and -III activities in isolated SH-SY5Y mitochondria. Mitochondrial respiratory chain complex dysfunction and DNA damage are major mechanisms in PD pathogenesis. Our data support the possible role of PhIP in PD etiology.

Anthracyclines React with Apurinic/Apyrimidinic Sites in DNA

The combination of doxorubicin (Adriamycin) and cyclophosphamide, referred to as AC chemotherapy, is commonly used for the clinical treatment of breast and other cancers. Both agents target DNA with cyclophosphamide causing alkylation damage and doxorubicin stabilizing the topoisomerase II-DNA complex. We hypothesize a new mechanism of action whereby both agents work in concert. DNA alkylating agents, such as nitrogen mustards, increase the number of apurinic/apyrimidinic (AP) sites through deglycosylation of labile alkylated bases. Herein, we demonstrate that anthracyclines with aldehyde-reactive primary and secondary amines form covalent Schiff base adducts with AP sites in a 12-mer DNA duplex, calf thymus DNA, and MDA-MB-231 human breast cancer cells treated with nor-nitrogen mustard and the anthracycline mitoxantrone. The anthracycline-AP site conjugates are characterized and quantified by mass spectrometry after NaB(CN)H3 or NaBH4 reduction of the Schiff base. If stable, the anthracycline-AP site conjugates represent bulky adducts that may block DNA replication and contribute to the cytotoxic mechanism of therapies involving combinations of anthracyclines and DNA alkylating agents.

Synthesis and Characterization of 15N5-Labeled Aflatoxin B1-Formamidopyrimidines and Aflatoxin B1-N7-Guanine from a Partial Double-Stranded Oligodeoxynucleotide as Internal Standards for Mass Spectrometric Measurements

Aflatoxin B₁ (AFB₁) exposure through contaminated food is a primary contributor to hepatocellular carcinogenesis worldwide. Hepatitis B viral infections in livers dramatically increase the carcinogenic potency of AFB₁ exposures. Liver cytochrome P450 oxidizes AFB₁ to the epoxide, which in turn reacts with N7-guanine in DNA, producing the cationic trans-8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B₁ adduct (AFB₁-N7-Gua). The opening of the imidazole ring of AFB₁-N7-Gua under physiological conditions causes the formation of the cis- and trans-diastereomers of 8,9-dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)-9-hydroxyaflatoxin B₁ (AFB₁-FapyGua). These adducts primarily lead to G → T mutations, with AFB₁-FapyGua being significantly more mutagenic than AFB₁-N7-Gua. The unequivocal identification and accurate quantification of these AFB₁-Gua adducts as biomarkers are essential for a fundamental understanding and prevention of AFB₁-induced hepatocellular carcinogenesis. Among a variety of analytical techniques used for this purpose, liquid chromatography-tandem mass spectrometry, with the use of the stable isotope-labeled analogues of AFB₁-FapyGua and AFB₁-N7-Gua as internal standards, provides the greatest accuracy and sensitivity. cis-AFB₁-FapyGua-¹⁵N₅, trans-AFB₁-FapyGua-¹⁵N₅, and AFB₁-N7-Gua-¹⁵N₅ have been synthesized and used successfully as internal standards. However, the availability of these standards from either academic institutions or commercial sources ceased to exist. Thus, quantitative genomic data regarding AFB₁-induced DNA damage in animal models and humans remain challenging to obtain. Previously, AFB₁-N7-Gua-¹⁵N₅ was prepared by reacting AFB₁-exo-8,9-epoxide with the uniformly ¹⁵N₅-labeled DNA isolated from algae grown in a pure ¹⁵N-environment, followed by alkali treatment, resulting in the conversion of AFB₁-N7-Gua-¹⁵N₅ to AFB₁-FapyGua-¹⁵N₅. In the present work, we used a different and simpler approach to synthesize cis-AFB₁-FapyGua-¹⁵N₅, trans-AFB₁-FapyGua-¹⁵N₅, and AFB₁-N7-Gua-¹⁵N₅ from a partial double-stranded 11-mer Gua-¹⁵N₅-labeled oligodeoxynucleotide, followed by isolation and purification. We also show the validation of these ¹⁵N₅-labeled standards for the measurement of cis-AFB₁-FapyGua, trans-AFB₁-FapyGua, and AFB₁-N7-Gua in DNA of livers of AFB₁-treated mice.

DNA Damage and Oxidative Stress of Tobacco Smoke Condensate in Human Bladder Epithelial Cells

Smoking is a major risk factor for bladder cancer (BC), with up to 50% of BC cases being attributed to smoking. There are 70 known carcinogens in tobacco smoke; however, the principal chemicals responsible for BC remain uncertain. The aromatic amines 4-aminobiphenyl (4-ABP) and 2-naphthylamine (2-NA) are implicated in BC pathogenesis of smokers on the basis of the elevated BC risk in factory workers exposed to these chemicals. However, 4-ABP and 2-NA only occur at several nanograms per cigarette and may be insufficient to induce BC. In contrast, other genotoxicants, including acrolein, occur at 1000-fold or higher levels in tobacco smoke. There is limited data on the toxicological effects of tobacco smoke in human bladder cells. We have assessed the cytotoxicity, oxidative stress, and DNA damage of tobacco smoke condensate (TSC) in human RT4 bladder cells. TSC was fractionated by liquid-liquid extraction into an acid-neutral fraction (NF), containing polycyclic aromatic hydrocarbons (PAHs), nitro-PAHs, phenols, and aldehydes, and a basic fraction (BF) containing aromatic amines, heterocyclic aromatic amines, and N-nitroso compounds. The TSC and NF induced a time- and concentration-dependent cytotoxicity associated with oxidative stress, lipid peroxide formation, glutathione (GSH) depletion, and apurinic/apyrimidinic (AP) site formation, while the BF showed weak effects. LC/MS-based metabolomic approaches showed that TSC and NF altered GSH biosynthesis pathways and induced more than 40 GSH conjugates. GSH conjugates of several hydroquinones were among the most abundant conjugates. RT4 cell treatment with synthetic hydroquinones and cresol mixtures at levels present in tobacco smoke accounted for most of the TSC-induced cytotoxicity and the AP sites formed. GSH conjugates of acrolein, methyl vinyl ketone, and crotonaldehyde levels also increased owing to TSC-induced oxidative stress. Thus, TSC is a potent toxicant and DNA-damaging agent, inducing deleterious effects in human bladder cells at concentrations of <1% of a cigarette in cell culture media.

The Cooked Meat Carcinogen 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine Hair Dosimeter, DNA Adductomics Discovery, and Associations with Prostate Cancer Pathology Biomarkers

Well-done cooked red meat consumption is linked to aggressive prostate cancer (PC) risk. Identifying mutation-inducing DNA adducts in the prostate genome can advance our understanding of chemicals in meat that may contribute to PC. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), a heterocyclic aromatic amine (HAA) formed in cooked meat, is a potential human prostate carcinogen. PhIP was measured in the hair of PC patients undergoing prostatectomy, bladder cancer patients under treatment for cystoprostatectomy, and patients treated for benign prostatic hyperplasia (BPH). PhIP hair levels were above the quantification limit in 123 of 205 subjects. When dichotomizing prostate pathology biomarkers, the geometric mean PhIP hair levels were higher in patients with intermediate and elevated-risk prostate-specific antigen values than lower-risk values <4 ng/mL (p = 0.03). PhIP hair levels were also higher in patients with intermediate and high-risk Gleason scores ≥7 compared to lower-risk Gleason score 6 and BPH patients (p = 0.02). PC patients undergoing prostatectomy had higher PhIP hair levels than cystoprostatectomy or BPH patients (p = 0.02). PhIP-DNA adducts were detected in 9.4% of the patients assayed; however, DNA adducts of other carcinogenic HAAs, and benzo[a]pyrene formed in cooked meat, were not detected. Prostate specimens were also screened for 10 oxidative stress-associated lipid peroxidation (LPO) DNA adducts. Acrolein 1,N²-propano-2'-deoxyguanosine adducts were detected in 54.5% of the patients; other LPO adducts were infrequently detected. Acrolein adducts were not associated with prostate pathology biomarkers, although DNA adductomic profiles differed between PC patients with low and high-grade Gleason scores. Many DNA adducts are of unknown origin; however, dG adducts of formaldehyde and a series of purported 4-hydroxy-2-alkenals were detected at higher abundance in a subset of patients with elevated Gleason scores. The PhIP hair biomarker and DNA adductomics data support the paradigm of well-done cooked meat and oxidative stress in aggressive PC risk.

Metabolism and biomarkers of heterocyclic aromatic amines in humans

Heterocyclic aromatic amines (HAAs) form during the high-temperature cooking of meats, poultry, and fish. Some HAAs also arise during the combustion of tobacco. HAAs are multisite carcinogens in rodents, inducing cancer of the liver, gastrointestinal tract, pancreas, mammary, and prostate glands. HAAs undergo metabolic activation by N-hydroxylation of the exocyclic amine groups to produce the proposed reactive intermediate, the heteroaryl nitrenium ion, which is the critical metabolite implicated in DNA damage and genotoxicity. Humans efficiently convert HAAs to these reactive intermediates, resulting in HAA protein and DNA adduct formation. Some epidemiologic studies have reported an association between frequent consumption of well-done cooked meats and elevated cancer risk of the colorectum, pancreas, and prostate. However, other studies have reported no associations between cooked meat and these cancer sites. A significant limitation in epidemiology studies assessing the role of HAAs and cooked meat in cancer risk is their reliance on food frequency questionnaires (FFQ) to gauge HAA exposure. FFQs are problematic because of limitations in self-reported dietary history accuracy, and estimating HAA intake formed in cooked meats at the parts-per-billion level is challenging. There is a critical need to establish long-lived biomarkers of HAAs for implementation in molecular epidemiology studies designed to assess the role of HAAs in health risk. This review article highlights the mechanisms of HAA formation, mutagenesis and carcinogenesis, the metabolism of several prominent HAAs, and the impact of critical xenobiotic-metabolizing enzymes on biological effects. The analytical approaches that have successfully biomonitored HAAs and their biomarkers for molecular epidemiology studies are presented.

Dietary Heterocyclic Amine Intake and Risk of Esophageal Squamous Cell Carcinoma in Rural Uganda

Dietary exposure to 2-amino-1-methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) in cooked meats maybe responsible for the high burden of Esophageal squamous cell carcinoma (ESCC) in southwestern Uganda. We conducted a pilot case-control study among 31 histologically confirmed ESCC cases and 54 age, gender, and residence matched healthy community controls sampled from the general population at the time of accrual of each case in southwestern Uganda. We collected data including smoking, alcohol consumption, diet, and scalp hair samples analyzed for normalized PhlP (adjusted per gram of melanin). We used logistic regression to determine the association of PhlP and ESCC. Overall, the mean normalized PhIP (ng/g melanin) was 44.79 (SD 148.08), higher among women compared to men (130.68 vs. 9.00, p = 0.03), lowest among healthy men [8.31 (SD 8.52) ng/g melanin] and highest among healthy women 158.39 (SD 288.75) ng/g melanin. In fully adjusted models, covariates associated with greater odds of ESCC included ever smoking 2 to 3 pack years of cigarettes (aOR 7.75 (95% CI 1.90, 31.50) and those 3 or more pack years (aOR5.82, 95%CI 1.25, 27.11), drinking 3 to 4 alcoholic drinks daily (aOR8.00, 95%CI 2.31, 27.74), and normalized PhIP above 75th percentile (8.65 ng/g of melanin) (aOR4.27, 95%CI 1.12, 16.24). In conclusion, high PhIP levels maybe associated with ESCC in a rural Uganda, a high ESCC burden setting. Further study with larger sample with a wider geographical representation is needed to validate scalp hair PhIP for assessment of ESCC risk.

Neuromelanin Modulates Heterocyclic Aromatic Amine-Induced Dopaminergic Neurotoxicity

Heterocyclic aromatic amines (HAAs) are mutagens and potential human carcinogens. Our group and others have demonstrated that HAAs may also produce selective dopaminergic neurotoxicity, potentially relevant to Parkinson's disease (PD). The goal of this study was to elucidate mechanisms of HAA-induced neurotoxicity through examining a translational biochemical weakness of common PD models. Neuromelanin is a pigmented byproduct of dopamine metabolism that has been debated as being both neurotoxic and neuroprotective in PD. Importantly, neuromelanin is known to bind and potentially release dopaminergic neurotoxicants, including HAAs (eg, β-carbolines such as harmane). Binding of other HAA subclasses (ie, aminoimidazoaazarenes) to neuromelanin has not been investigated, nor has a specific role for neuromelanin in mediating HAA-induced neurotoxicity been examined. Thus, we investigated the role of neuromelanin in modulating HAA-induced neurotoxicity. We characterized melanin from Sepia officinalis and synthetic dopamine melanin, proposed neuromelanin analogs with similar biophysical properties. Using a cell-free assay, we demonstrated strong binding of harmane and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) to neuromelanin analogs. To increase cellular neuromelanin, we transfected SH-SY5Y neuroblastoma cells with tyrosinase. Relative to controls, tyrosinase-expressing cells exhibited increased neuromelanin levels, cellular HAA uptake, cell toxicity, and oxidative damage. Given that typical cellular and rodent PD models form far lower neuromelanin levels than humans, there is a critical translational weakness in assessing HAA-neurotoxicity. The primary impacts of these results are identification of a potential mechanism by which HAAs accumulate in catecholaminergic neurons and support for the need to conduct neurotoxicity studies in systems forming neuromelanin.

Cytotoxicity and genotoxicity of the carcinogen aristolochic acid I (AA-I) in human bladder RT4 cells

Aristolochic acid (AA-I) induces upper urothelial tract cancer (UUTC) and bladder cancer (BC) in humans. AA-I forms the 7-(2'-deoxyadenosin-N⁶-yl)aristolactam I (dA-AL-I) adduct, which induces multiple A:T-to-T:A transversion mutations in TP53 of AA-I exposed UTUC patients. This mutation is rarely reported in TP53 of other transitional cell carcinomas and thus recognized as an AA-I mutational signature. A:T-to-T:A transversion mutations were recently detected in bladder tumors of patients in Asia with known AA-I-exposure, implying that AA-I contributes to BC. Mechanistic studies on AA-I genotoxicity have not been reported in human bladder. In this study, we examined AA-I DNA adduct formation and mechanisms of toxicity in the human RT4 bladder cell line. The biological potencies of AA-I were compared to 4-aminobiphenyl, a recognized human bladder carcinogen, and several structurally related carcinogenic heterocyclic aromatic amines (HAA), which are present in urine of smokers and omnivores. AA-I (0.05-10 µM) induced a concentration- and time-dependent cytotoxicity. AA-I (100 nM) DNA adduct formation occurred at over a thousand higher levels than the principal DNA adducts formed with 4-ABP or HAAs (1 µM). dA-AL-I adduct formation was detected down to a 1 nM concentration. Studies with selective chemical inhibitors provided evidence that NQO1 is the major enzyme involved in AA-I bio-activation in RT4 cells, whereas CYP1A1, another enzyme implicated in AA-I toxicity, had a lesser role in bio-activation or detoxification of AA-I. AA-I DNA damage also induced genotoxic stress leading to p53-dependent apoptosis. These biochemical data support the human mutation data and a role for AA-I in BC.

Comprehensive Analysis of DNA Adducts Using Data-Independent wSIM/MS2 Acquisition and wSIM-City

A novel software has been created to comprehensively characterize covalent modifications of DNA through mass spectral analysis of enzymatically hydrolyzed DNA using the neutral loss of 2'-deoxyribose, a nearly universal MS2 fragmentation process of protonated 2'-deoxyribonucleosides. These covalent modifications termed DNA adducts form through xenobiotic exposures or by reaction with endogenous electrophiles and can induce mutations during cell division and initiate carcinogenesis. DNA adducts are typically present at trace levels in the human genome, requiring a very sensitive and comprehensive data acquisition and analysis method. Our software, wSIM-City, was created to process mass spectral data acquired by a wide selected ion monitoring (wSIM) with gas-phase fractionation and coupled to wide MS2 fragmentation. This untargeted approach can detect DNA adducts at trace levels as low as 1.5 adducts per 109 nucleotides. This level of sensitivity is sufficient for comprehensive analysis and characterization of DNA modifications in human specimens.

Additive Effects of Arsenic and Aristolochic Acid in Chemical Carcinogenesis of Upper Urinary Tract Urothelium

BACKGROUND

Aristolochic acids (AA) and arsenic are chemical carcinogens associated with urothelial carcinogenesis. Here we investigate the combined effects of AA and arsenic toward the risk of developing upper tract urothelial carcinoma (UTUC).

METHODS

Hospital-based (n = 89) and population-based (2,921 cases and 11,684 controls) Taiwanese UTUC cohorts were used to investigate the association between exposure to AA and/or arsenic and the risk of developing UTUC. In the hospital cohort, AA exposure was evaluated by measuring aristolactam-DNA adducts in the renal cortex and by identifying A>T TP53 mutations in tumors. In the population cohort, AA exposure was determined from prescription health insurance records. Arsenic levels were graded from 0 to 3 based on concentrations in well water and the presence of arseniasis-related diseases.

RESULTS

In the hospital cohort, 43, 26, and 20 patients resided in grade 0, 1+2, and 3 arseniasis-endemic areas, respectively. Aristolactam-DNA adducts were present in >90% of these patients, indicating widespread AA exposure. A>T mutations in TP53 were detected in 28%, 44%, and 22% of patients residing in grade 0, 1+2, and 3 arseniasis-endemic areas, respectively. Population studies revealed that individuals who consumed more AA-containing herbs had a higher risk of developing UTUC in both arseniasis-endemic and nonendemic areas. Logistic regression showed an additive effect of AA and arsenic exposure on the risk of developing UTUC.

CONCLUSIONS

Exposure to both AA and arsenic acts additively to increase the UTUC risk in Taiwan.

IMPACT

This is the first study to investigate the combined effect of AA and arsenic exposure on UTUC.

Mutagenicity of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP) in human TP53 knock-in (Hupki) mouse embryo fibroblasts

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a possible human carcinogen formed in cooked fish and meat. PhIP is bioactivated by cytochrome P450 enzymes to form 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (N-OH-PhIP), a genotoxic metabolite that reacts with DNA leading to the mutation-prone DNA adduct N-(deoxyguanosin-8-yl)-PhIP (dG-C8-PhIP). Here, we studied N-OH-PhIP-induced whole genome mutagenesis in human TP53 knock-in (Hupki) mouse embryo fibroblasts (HUFs) immortalised and subjected to whole genome sequencing (WGS). In addition, mutagenicity of N-OH-PhIP in TP53 and the lacZ reporter gene were assessed. TP53 mutant frequency in HUF cultures treated with N-OH-PhIP (2.5 μM for 24 h, n = 90) was 10% while no TP53 mutations were found in untreated controls (DMSO for 24 h, n = 6). All N-OH-PhIP-induced TP53 mutations occurred at G:C base pairs with G > T/C > A transversions accounting for 58% of them. TP53 mutations characteristic of those induced by N-OH-PhIP have been found in human tumours including breast and colorectal, which are cancer types that have been associated with PhIP exposure. LacZ mutant frequency increased 25-fold at 5 μM N-OH-PHIP and up to ~350 dG-C8-PhIP adducts/10⁸ nucleosides were detected by ultra-performance liquid chromatography-electrospray ionisation multistage scan mass spectrometry (UPLC-ESI-MS³) at this concentration. In addition, a WGS mutational signature defined by G > T/C > A transversions was present in N-OH-PhIP-treated immortalised clones, which showed similarity to COSMIC SBS4, 18 and 29 signatures found in human tumours.

Applying Tobacco, Environmental, and Dietary-Related Biomarkers to Understand Cancer Etiology and Evaluate Prevention Strategies

Many human cancers are caused by environmental and lifestyle factors. Biomarkers of exposure and risk developed by our team have provided critical data on internal exposure to toxic and genotoxic chemicals and their connection to cancer in humans. This review highlights our research using biomarkers to identify key factors influencing cancer risk as well as their application to assess the effectiveness of exposure intervention and chemoprevention protocols. The use of these biomarkers to understand individual susceptibility to the harmful effects of tobacco products is a powerful example of the value of this type of research and has provided key data confirming the link between tobacco smoke exposure and cancer risk. Furthermore, this information has led to policy changes that have reduced tobacco use and consequently, the tobacco-related cancer burden. Recent technological advances in mass spectrometry led to the ability to detect DNA damage in human tissues as well as the development of adductomic approaches. These new methods allowed for the detection of DNA adducts in tissues from patients with cancer, providing key evidence that exposure to carcinogens leads to DNA damage in the target tissue. These advances will provide valuable insights into the etiologic causes of cancer that are not tobacco-related.See all articles in this CEBP Focus section, "Environmental Carcinogenesis: Pathways to Prevention."

Mass Spectrometric Quantitation of Apurinic/Apyrimidinic Sites in Tissue DNA of Rats Exposed to Tobacco-Specific Nitrosamines and in Lung and Leukocyte DNA of Cigarette Smokers and Nonsmokers

Metabolic activation of the carcinogenic tobacco-specific nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN) results in formation of reactive electrophiles that modify DNA to produce a variety of products including methyl, 4-(3-pyridyl)-4-oxobutyl (POB)-, and 4-(3-pyridyl)-4-hydroxybutyl adducts. Among these are adducts such as 7-POB-deoxyguanosine (N7POBdG) which can lead to apurinic/apyrimidinic (AP) sites by facile hydrolysis of the base-deoxyribonucleoside bond. In this study, we used a recently developed highly sensitive mass spectrometric method to quantitate AP sites by derivatization with O-(pyridin-3-yl-methyl)hydroxylamine (PMOA) (detection limit, 2 AP sites per 108 nucleotides). AP sites were quantified in DNA isolated from tissues of rats treated with NNN and NNK and from human lung tissue and leukocytes of cigarette smokers and nonsmokers. Rats treated with 5 or 21 mg/kg bw NNK for 4 days by s.c. injection had 2-6 and 2-17 times more AP sites than controls in liver and lung DNA (p < 0.05). Increases in AP sites were also found in liver DNA of rats exposed for 10 and 30 weeks (p < 0.05) but not for 50 and 70 weeks to 5 ppm of NNK in their drinking water. Levels of N7POBG were significantly correlated with AP sites in rats treated with NNK. In rats treated with 14 ppm (S)-NNN in their drinking water for 10 weeks, increased AP site formation compared to controls was observed in oral and nasal respiratory mucosa DNA (p < 0.05). No significant increase in AP sites was found in human lung and leukocyte DNA of cigarette smokers compared to nonsmokers, although AP sites in leukocyte DNA were significantly correlated with urinary levels of the NNK metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL). This is the first study to use mass spectrometry based methods to examine AP site formation by carcinogenic tobacco-specific nitrosamines in laboratory animals and to evaluate AP sites in DNA of smokers and nonsmokers.

Abstract A33: Untapped biospecimens and novel mass spectrometry scanning techniques for DNA adductomics

A major impediment in the biomonitoring of DNA adducts is the lack of fresh-frozen biopsy samples available for biomarker research. However, archived formalin-fixed, paraffin-embedded (FFPE) tissues with a clinical diagnosis of disease are often accessible. We have established a method to fully unravel DNA crosslinks in FFPE specimens under mild conditions that preserve the structural integrity of DNA adducts. Our targeted, quantitative mass spectrometry measurements employing ion trap or high-resolution Orbitrap mass spectrometry require less than ten micrograms of DNA with limits of quantification at three adducts per 109 nucleotides. We have successfully screened FFPE tissues of rodents exposed to tobacco and dietary carcinogens and reported adduct levels comparable to those of matching fresh-frozen tissues. Our technology has been employed to identify a DNA adduct of aristolochic acid, a potent urothelial carcinogen present in Chinese herbal medicines, in human FFPE kidney blocks stored at ambient temperature for up to nine years. The method also detected DNA adducts of the bladder carcinogen 4-aminobiphenyl in human FFPE bladder, and a DNA adduct of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, a heterocyclic aromatic amine formed in cooked meats and a potential prostate carcinogen, in FFPE prostate specimens of prostate cancer patients. Thus, the technology is versatile and can be employed to screen for DNA adducts formed with a wide range of environmental and dietary carcinogens. The ability to retrospectively analyze FFPE tissues for DNA adducts for which there is a clinical diagnosis of disease opens a previously untapped source of biospecimens for molecular epidemiology studies seeking the causal role of environmental chemicals in cancer etiology. With the recent improvements in the sensitivity and scanning rates of high-resolution MS instruments, such as quadrupole time-of-flight and Orbitrap MS detectors, it is now possible to screen for a wide array of DNA damage in the human genome using DNA adductomics approaches. We have adapted data-dependent and data-independent scanning techniques originated from proteomics and metabolomics to screen for DNA adducts of the genome. DNA adductomics is a new and developing technology for human exposure assessment. As the analytic technology matures and bioinformatics tools become available for analysis of the mass spectral data, DNA adductomics can advance our understanding of the role chemical exposures play in DNA damage and disease risk.

Citation Format: Byeong Hwa Yun, Jingshu Guo, Scott Walmsley, Paari Murugan, Christopher J. Weight, Peter W. Villalta, Robert J. Turesky. Untapped biospecimens and novel mass spectrometry scanning techniques for DNA adductomics [abstract]. In: Proceedings of the AACR Special Conference on Environmental Carcinogenesis: Potential Pathway to Cancer Prevention; 2019 Jun 22-24; Charlotte, NC. Philadelphia (PA): AACR; Can Prev Res 2020;13(7 Suppl): Abstract nr A33.

Kinetics of DNA Adducts and Abasic Site Formation in Tissues of Mice Treated with a Nitrogen Mustard

Nitrogen mustards (NM) are an important class of chemotherapeutic drugs used in the treatment of malignant tumors. The accepted mechanism of action of NM is through the alkylation of DNA bases. NM-adducts block DNA replication in cancer cells by forming cytotoxic DNA interstrand cross-links. We previously characterized several adducts formed by reaction of bis(2-chloroethyl)ethylamine (NM) with calf thymus (CT) DNA and the MDA-MB-231 mammary tumor cell line. The monoalkylated N7-guanine (NM-G) adduct and its cross-link (G-NM-G) were major lesions. The cationic NM-G undergoes a secondary reaction through depurination to form an apurinic (AP) site or reacts with hydroxide to yield the stable ring-opened N⁵-substituted formamidopyrimidine (NM-Fapy-G) adduct. Both of these lesions are mutagenic and may contribute to secondary tumor development, a major clinical limitation of NM chemotherapy. We established a kinetic model with NM-treated female mice and measured the rates of formation and removal of NM-DNA adducts and AP sites. We employed liquid chromatography-mass spectrometry (LC-MS) to measure NM-G, G-NM-G, and NM-Fapy-G adducts in liver, lung, and spleen over 168 h. NM-G reached a maximum level within 6 h in all organs and then rapidly declined. The G-NM-G cross-link and NM-FapyG were more persistent with half-lives over three-times longer than NM-G. We quantified AP site lesions in the liver and showed that NM treatment increased AP site levels by 3.7-fold over the basal levels at 6 h. The kinetics of AP site repair closely followed the rate of removal of NM-G; however, AP sites remained 1.3-fold above basal levels 168 h post-treatment with NM. Our data provide new insights into NM-induced DNA damage and biological processing in vivo. The quantitative measurement of the spectrum of NM adducts and AP sites can serve as biomarkers in the design and assessment of the efficacy of novel chemotherapeutic regimens.

Development of a DNA Adductome Mass Spectral Database

Mass spectrometry-based DNA adductomics is an emerging approach for the human biomonitoring of hazardous chemicals. A mass spectral database of DNA adducts will be created for the scientific community to investigate the associations between chemical exposures, DNA damage, and disease risk.

DNA adducts: Formation, biological effects, and new biospecimens for mass spectrometric measurements in humans

Hazardous chemicals in the environment and diet or their electrophilic metabolites can form adducts with genomic DNA, which can lead to mutations and the initiation of cancer. In addition, reactive intermediates can be generated in the body through oxidative stress and damage the genome. The identification and measurement of DNA adducts are required for understanding exposure and the causal role of a genotoxic chemical in cancer risk. Over the past three decades, 32 P-postlabeling, immunoassays, gas chromatography/mass spectrometry, and liquid chromatography/mass spectrometry (LC/MS) methods have been established to assess exposures to chemicals through measurements of DNA adducts. It is now possible to measure some DNA adducts in human biopsy samples, by LC/MS, with as little as several milligrams of tissue. In this review article, we highlight the formation and biological effects of DNA adducts, and highlight our advances in human biomonitoring by mass spectrometric analysis of formalin-fixed paraffin-embedded tissues, untapped biospecimens for carcinogen DNA adduct biomarker research.

Methods and Challenges for Computational Data Analysis for DNA Adductomics

Frequent exposure to chemicals in the environment, diet, and endogenous electrophiles leads to chemical modification of DNA and the formation of DNA adducts. Some DNA adducts can induce mutations during cell division and, when occurring in critical regions of the genome, can lead to the onset of disease, including cancer. The targeted analysis of DNA adducts over the past 30 years has revealed that the human genome contains many types of DNA damages. However, a long-standing limitation in conducting DNA adduct measurements has been the inability to screen for the total complement of DNA adducts derived from a wide range of chemicals in a single assay. With the advancement of high-resolution mass spectrometry (MS) instrumentation and new scanning technologies, nontargeted "omics" approaches employing data-dependent acquisition and data-independent acquisition methods have been established to simultaneously screen for multiple DNA adducts, a technique known as DNA adductomics. However, notable challenges in data processing must be overcome for DNA adductomics to become a mature technology. DNA adducts occur at low abundance in humans, and current softwares do not reliably detect them when using common MS data acquisition methods. In this perspective, we discuss contemporary computational tools developed for feature finding of MS data widely utilized in the disciplines of proteomics and metabolomics and highlight their limitations for conducting nontargeted DNA-adduct biomarker discovery. Improvements to existing MS data processing software and new algorithms for adduct detection are needed to develop DNA adductomics into a powerful tool for the nontargeted identification of potential cancer-causing agents.

Quantitation of Lipid Peroxidation Product DNA Adducts in Human Prostate by Tandem Mass Spectrometry: A Method That Mitigates Artifacts

Reactive oxygen species (ROS) and chronic inflammation contribute to DNA damage of many organs, including the prostate. ROS cause oxidative damage to biomolecules, such as lipids, proteins, and nucleic acids, resulting in the formation of toxic and mutagenic intermediates. Lipid peroxidation (LPO) products covalently adduct to DNA and can lead to mutations. The levels of LPO DNA adducts reported in humans range widely. However, a large proportion of the DNA adducts may be attributed to artifact formation during the steps of isolation and nuclease digestion of DNA. We established a method that mitigates artifacts for most LPO adducts during the processing of DNA. We have applied this methodology to measure LPO DNA adducts in the genome of prostate cancer patients, employing ultrahigh-performance liquid chromatography electrospray ionization ion trap multistage mass spectrometry. Our preliminary data show that DNA adducts of acrolein, 6-hydroxy-1,N²-propano-2'-deoxyguanosine (6-OH-PdG) and 8-hydroxy-1,N²-propano-2'-deoxyguanosine (8-OH-PdG) (4-20 adducts per 10⁷ nucleotides) are more prominent than etheno (ε) adducts (<0.5 adducts per 10⁸ nucleotides). This analytical methodology will be used to examine the correlation between oxidative stress, inflammation, and LPO adduct levels in patients with benign prostatic hyperplasia and prostate cancer.

Bioactivation of the tobacco carcinogens 4-aminobiphenyl (4-ABP) and 2-amino-9H-pyrido[2,3-b]indole (AαC) in human bladder RT4 cells

Occupational and tobacco exposure to aromatic amines (AAs) including 4-aminobiphenyl (4-ABP) and 2-naphthylamine (2-NA) are associated with bladder cancer (BC) risk. Several epidemiological studies have also reported a possible role for structurally related heterocyclic aromatic amines (HAAs) formed in tobacco smoke or cooked meats with BC risk. We had screened for DNA adducts of 4-ABP, 2-NA, and several prominent HAAs formed in tobacco smoke or grilled meats including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-9H-pyrido[2,3-b]indole (AαC) in the bladder DNA of BC patients, using liquid chromatography/mass spectrometry. We detected DNA adducts of 4-ABP, but not adducts of the other carcinogens. In this study, we have examined the capacity of RT4 cells, an epithelial human bladder cell line, to bioactivate AAs and HAAs to DNA damaging agents, which may contribute to BC. 4-ABP and AαC formed DNA adducts, but DNA adducts of 2-NA, PhIP, and MeIQx were not detected. 4-ABP DNA adducts were formed at tenfold higher levels than AαC adducts. Pretreatment of RT4 cells with α-naphthoflavone (1-10 µM), a specific cytochrome P450 1 (CYP1) inhibitor, decreased AαC adduct formation by 50% but did not affect the level of 4-ABP adducts. However, cell pretreatment with 8-methoxypsoralen (0.1-1 µM), a potent inhibitor of CYP2A, resulted in a 90% decrease of 4-ABP DNA adducts levels. These data signify that CYP2A and CYP1A isoforms expressed in the target urothelium bioactivate 4-ABP and AαC, respectively, and may be a critical feature of aromatic amine-induced urinary bladder carcinogenesis. The bioactivation of other tobacco and environmental AAs by bladder CYPs and their ensuing bladder DNA damage warrants further study.

Biomonitoring an albumin adduct of the cooked meat carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in humans

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is formed in cooked meats and may be linked to dietary-associated colorectal, prostate and mammary cancers. Genotoxic N-oxidized metabolites of PhIP react with the Cys34 of albumin (Alb) to form a sulfinamide adduct, a biomarker of the biologically effective dose. We examined the kinetics of PhIP-Alb adduct formation in plasma of volunteers on a 4-week semicontrolled diet of cooked meat containing known quantities of PhIP. The adduct was below the limit of detection (LOD) (10 femtograms PhIP/mg Alb) in most subjects before the meat feeding but increased by up to 560-fold at week 4 in subjects who ate meat containing 8.0 to 11.7 μg of PhIP per 150-200 g serving. In contrast, the adduct remained below the LOD in subjects who ingested 1.2 or 3.0 μg PhIP per serving. Correlations were not seen between PhIP-Alb adduct levels and PhIP intake levels (P = 0.76), the amount of PhIP accrued in hair (P = 0.13), the amounts of N-oxidized urinary metabolites of PhIP (P = 0.66) or caffeine CYP1A2 activity (P = 0.55), a key enzyme involved in the bioactivation of PhIP. The half-life of the PhIP-Alb adduct was <2 weeks, signifying that the adduct was not stable. PhIP-Alb adduct formation is direct evidence of bioactivation of PhIP in vivo. However, the PhIP hair biomarker is a longer lived and more sensitive biomarker to assess exposure to this potential human carcinogen.

Quantitation of Apurinic/Apyrimidinic Sites in Isolated DNA and in Mammalian Tissue with a Reduced Level of Artifacts

The apurinic/apyrimidinic (AP) site is a common lesion of DNA damage. The levels of AP sites reported in the literature cover a wide range, which is primarily due to the artifactual generation or loss of AP sites during processing of the DNA. Herein, we have developed a method for quantitating AP sites with a largely reduced level of artifacts by derivatizing AP sites before DNA isolation. A rapid digestion of nuclear protein was performed to minimize enzymatic DNA repair, followed by direct derivatization of AP sites in the nuclear lysate with O-(pyridin-3-yl-methyl)hydroxylamine, yielding an oxime derivative that is stable through the subsequent DNA processing steps. Quantitation was done using highly selective and sensitive liquid chromatography-tandem mass spectrometry, with a limit of quantitation at 2.2 lesions per 10⁸ nucleotides (nts, 0.9 fmol on column). The method was applied in vivo to measure AP sites in rats undergoing oxidative stress [liver, 3.31 ± 0.47/10⁷ nts (dosed) vs 0.91 ± 0.06/10⁷ nts (control); kidney, 1.60 ± 0.07/10⁷ nts (dosed) vs 1.13 ± 0.12/10⁷ nts (control)]. The basal AP level was significantly lower than literature values. The method was also used to measure AP sites induced by the chemotherapeutic nitrogen mustard in vitro.

Metabolic Activation of the Cooked Meat Carcinogen 2-Amino-1-Methyl-6-Phenylimidazo[4,5-b]Pyridine in Human Prostate

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), an heterocyclic aromatic amine (HAA) formed in cooked meat, is a rodent and possible human prostate carcinogen. Recently, we identified DNA adducts of PhIP in the genome of prostate cancer patients, but adducts of 2-amino-3, 8-dimethylmidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-9 H-pyrido[2,3-b]indole (AαC), other prominent HAAs formed in cooked meats, were not detected. We have investigated the bioactivation of HAAs by Phase I and II enzymes in the human prostate (LNCaP) cell line using cytotoxicity and DNA adducts as endpoints. PhIP, MeIQx, and 2-amino-3-methylimidazo[4,5-f]quinoline, another HAA found in cooked meats, were poorly bioactivated and not toxic. The synthetic genotoxic N-hydroxylated-HAAs were also assayed in LNCaP cells with Phase II enzyme inhibitors. Notably, 2-hydroxy-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), but not other HONH-HAAs, induced cytotoxicity. Moreover, PhIP-DNA adduct formation was 20-fold greater than adducts formed with other HONH-HAAs. Pretreatment of LNCaP cells with mefenamic acid, a specific inhibitor of sulfotransferase (SULT1A1), decreased PhIP-DNA adducts by 25%, whereas (Z)-5-(2'-hydroxybenzylidene)-2-thioxothiazolidin-4-one and pentachlorophenol, inhibitors of SULTs and N-acetyltransferases (NATs), decreased the PhIP-DNA adduct levels by 75%. NATs in cytosolic fractions of LNCaP cells and human prostate catalyzed DNA binding of HONH-PhIP by up to 100-fold greater levels than for SULT and kinase activities. Recombinant NAT2 is catalytically superior to recombinant NAT1 in the bioactivation of HONH-PhIP; however, the extremely low levels of NAT2 activity in prostate suggest that NAT1 may be the major isoform involved in PhIP-DNA damage. Thus, the high susceptibility of LNCaP cells recapitulates the DNA-damaging effect of HONH-PhIP in rodent and human prostate.

Dietary Carcinogens and DNA Adducts in Prostate Cancer

Prostate cancer (PC) is the most commonly diagnosed non-cutaneous cancer and the second leading cause of cancer-related to death in men. The major risk factors for PC are age, family history, and African American ethnicity. Epidemiological studies have reported large geographical variations in PC incidence and mortality, and thus lifestyle and dietary factors influence PC risk. High fat diet, dairy products, alcohol and red meats, are considered as risk factors for PC. This book chapter provides a comprehensive, literature-based review on dietary factors and their molecular mechanisms of prostate carcinogenesis. A large portion of our knowledge is based on epidemiological studies where dietary factors such as cancer promoting agents, including high-fat, dairy products, alcohol, and cancer-initiating genotoxicants formed in cooked meats have been evaluated for PC risk. However, the precise mechanisms in the etiology of PC development remain uncertain. Additional animal and human cell-based studies are required to further our understandings of risk factors involved in PC etiology. Specific biomarkers of chemical exposures and DNA damage in the prostate can provide evidence of cancer-causing agents in the prostate. Collectively, these studies can improve public health research, nutritional education and chemoprevention strategies.

Targeted and Untargeted Detection of DNA Adducts of Aromatic Amine Carcinogens in Human Bladder by Ultra-Performance Liquid Chromatography-High-Resolution Mass Spectrometry

Epidemiological studies have linked aromatic amines (AAs) from tobacco smoke and some occupational exposures with bladder cancer risk. Several epidemiological studies have also reported a plausible role for structurally related heterocyclic aromatic amines present in tobacco smoke or formed in cooked meats with bladder cancer risk. DNA adduct formation is an initial biochemical event in bladder carcinogenesis. We examined paired fresh-frozen (FR) and formalin-fixed paraffin-embedded (FFPE) nontumor bladder tissues from 41 bladder cancer patients for DNA adducts of 4-aminobiphenyl (4-ABP), a bladder carcinogen present in tobacco smoke, and 2-amino-9 H-pyrido[2,3- b]indole, 2-amino-1-methyl-6-phenylimidazo[4,5- b]pyridine and 2-amino-3,8-dimethylimidazo[4,5- f]quinoxaline, possible human carcinogens, which occur in tobacco smoke and cooked meats. These chemicals are present in urine of tobacco smokers or omnivores. Targeted DNA adduct measurements were done by ultra-performance liquid chromatography-electrospray ionization multistage hybrid Orbitrap MS. N-(2'-Deoxyguanosin-8-yl)-4-ABP ( N-(dG-C8)-4-ABP) was the sole adduct detected in FR and FFPE bladder tissues. Twelve subjects (29%) had N-(dG-C8)-4-ABP levels above the limit of quantification, ranging from 1.4 to 33.8 adducts per 109 nucleotides (nt). DNA adducts of other human AA bladder carcinogens, including 2-naphthylamine (2-NA), 2-methylaniline (2-MA), 2,6-dimethylaniline (2,6-DMA), and lipid peroxidation (LPO) adducts, were screened for in bladder tissue, by our untargeted data-independent adductomics method, termed wide-selected ion monitoring (wide-SIM)/MS2. Wide-SIM/MS2 successfully detected N-(dG-C8)-4-ABP, N-(2'-deoxyadenosin-8-yl)-4-ABP and the presumed hydrazo linked adduct, N-(2'-deoxyguanosin- N2-yl)-4-ABP, and several LPO adducts in bladder DNA. Wide-SIM/MS2 detected multiple DNA adducts of 2-NA, 2-MA, and, 2,6-DMA, when calf thymus DNA was modified with reactive intermediates of these carcinogens. However, these AA-adducts were below the limit of detection in unspiked human bladder DNA (<1 adduct per 108 nt). Wide-SIM/MS2 can screen for many types of DNA adducts formed with exogenous and endogenous electrophiles and will be employed to identify DNA adducts of other chemicals that may contribute to the etiology of bladder cancer.

Mechanistic Evidence for Red Meat and Processed Meat Intake and Cancer Risk: A Follow-up on the International Agency for Research on Cancer Evaluation of 2015

The Working Group of the International Agency for Research on Cancer classified the consumption of processed meat as carcinogenic to humans (Group 1), and classified red meat as probably carcinogenic to humans (Group 2A); consumption of both meat types is associated with an increased risk of colorectal cancer. These classifications are based on a compilation of epidemiology data and mechanistic evidence from animal and human studies. The curing of meats with nitrite can produce carcinogenic N-nitroso compounds (NOCs), and the smoking of meat produces polycyclic aromatic hydrocarbons (PAHs). The high-temperature cooking of meat also produces carcinogenic heterocyclic aromatic amines (HAAs). The ingestion of heme from meat can catalyze the formation of NOCs and lipid peroxidation products (LPOs) in the digestive tract. Many of these chemicals form DNA adducts, some of which can induce mutations and initiate carcinogenesis. Another recent hypothesis is that N-glycolylneuraminic acid, a non-human sialic acid sugar present in red meat, becomes incorporated in the cell membrane, triggering the immune response with associated inflammation and reactive oxygen species, which can contribute to DNA damage, tumor promotion, and cancer. The mechanisms by which these chemicals in meat induce DNA damage, and the impact of dietary and host factors that influence the biological potency of these chemicals are highlighted in this updated report.

Method for Biomonitoring DNA Adducts in Exfoliated Urinary Cells by Mass Spectrometry

Tobacco smoking contributes to about 50% of the bladder-cancer (BC) cases in the United States. Some aromatic amines in tobacco smoke are bladder carcinogens; however, other causal agents of BC are uncertain. Exfoliated urinary cells (EUCs) are a promising noninvasive biospecimen to screen for DNA adducts of chemicals that damage the bladder genome, although the analysis of DNA adducts in EUCs is technically challenging because of the low number of EUCs and limiting quantity of cellular DNA. Moreover, EUCs and their DNA adducts must remain viable during the time of collection and storage of urine to develop robust screening methods. We employed RT4 cells, a well-differentiated transitional epithelial bladder cell line, as a cell-model system in urine to investigate cell viability and the chemical stability of DNA adducts of two prototypical bladder carcinogens: 4-aminobiphenyl (4-ABP), an aromatic amine found in tobacco smoke, and aristolochic acid I (AA-I), a nitrophenanthrene found in Aristolochia herbaceous plants used for medicinal purposes worldwide. The cell viability of RT4 cells pretreated with 4-ABP or AA-I in urine exceeded 80%, and the major DNA adducts of 4-ABP and AA-I, quantified by liquid chromatography-mass spectrometry, were stable for 24 h. Thereafter, we successfully screened EUCs of mice treated with AA-I to measure DNA adducts of AA-I, which were still detected 25 days following treatment with the carcinogen. EUCs are promising biospecimens that can be employed for the screening of DNA adducts of environmental and dietary genotoxicants that may contribute to the development of BC.

Formalin-Fixed Paraffin-Embedded Tissues-An Untapped Biospecimen for Biomonitoring DNA Adducts by Mass Spectrometry

The measurement of DNA adducts provides important information about human exposure to genotoxic chemicals and can be employed to elucidate mechanisms of DNA damage and repair. DNA adducts can serve as biomarkers for interspecies comparisons of the biologically effective dose of procarcinogens and permit extrapolation of genotoxicity data from animal studies for human risk assessment. One major challenge in DNA adduct biomarker research is the paucity of fresh frozen biopsy samples available for study. However, archived formalin-fixed paraffin-embedded (FFPE) tissues with clinical diagnosis of disease are often available. We have established robust methods to recover DNA free of crosslinks from FFPE tissues under mild conditions which permit quantitative measurements of DNA adducts by liquid chromatography-mass spectrometry. The technology is versatile and can be employed to screen for DNA adducts formed with a wide range of environmental and dietary carcinogens, some of which were retrieved from section-cuts of FFPE blocks stored at ambient temperature for up to nine years. The ability to retrospectively analyze FFPE tissues for DNA adducts for which there is clinical diagnosis of disease opens a previously untapped source of biospecimens for molecular epidemiology studies that seek to assess the causal role of environmental chemicals in cancer etiology.

Non-invasive detection of urothelial cancer through the analysis of driver gene mutations and aneuploidy

Current non-invasive approaches for detection of urothelial cancers are suboptimal. We developed a test to detect urothelial neoplasms using DNA recovered from cells shed into urine. UroSEEK incorporates massive parallel sequencing assays for mutations in 11 genes and copy number changes on 39 chromosome arms. In 570 patients at risk for bladder cancer (BC), UroSEEK was positive in 83% of those who developed BC. Combined with cytology, UroSEEK detected 95% of patients who developed BC. Of 56 patients with upper tract urothelial cancer, 75% tested positive by UroSEEK, including 79% of those with non-invasive tumors. UroSEEK detected genetic abnormalities in 68% of urines obtained from BC patients under surveillance who demonstrated clinical evidence of recurrence. The advantages of UroSEEK over cytology were evident in low-grade BCs; UroSEEK detected 67% of cases whereas cytology detected none. These results establish the foundation for a new non-invasive approach for detection of urothelial cancer.

A Rapid Throughput Method To Extract DNA from Formalin-Fixed Paraffin-Embedded Tissues for Biomonitoring Carcinogenic DNA Adducts

Formalin-fixed paraffin-embedded (FFPE) tissues are rarely used for screening DNA adducts of carcinogens because the harsh conditions required to reverse the formaldehyde-mediated DNA cross-links can destroy DNA adducts. We recently adapted a commercial silica-based column kit used in genomics to manually isolate DNA under mild conditions from FFPE tissues of rodents and humans and successfully measured DNA adducts of several carcinogens including aristolochic acid I (AA-I), 4-aminobiphenyl (4-ABP), and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) (Yun et al. (2013) Anal. Chem. 85, 4251-8, and Guo et al. (2016) Anal. Chem. 88, 4780-7). The DNA retrieval methodology is robust; however, the procedure is time-consuming and labor intensive, and not amenable to rapid throughput processing. In this study, we have employed the Promega Maxwell 16 MDx system, which is commonly used in large scale genomics studies, for the rapid throughput extraction of DNA. This system streamlines the DNA isolation procedure and increases the sample processing rate by about 8-fold over the manual method (32 samples versus 4 samples processed per hour). High purity DNA is obtained in satisfactory yield for the measurements of DNA adducts by ultra performance liquid chromatography-electrospray-ionization-ion trap-multistage scan mass spectrometry. The measurements show that the levels of DNA adducts of AA-I, 4-ABP, and PhIP in FFPE rodent and human tissues are comparable to those levels measured in DNA from matching tissues isolated by the commercial silica-based column kits and in DNA from fresh frozen tissues isolated by the conventional phenol-chloroform extraction method. The isolation of DNA from tissues is one major bottleneck in the analysis of DNA adducts. This rapid throughput methodology greatly decreases the time required to process DNA and can be employed in large-scale epidemiology studies designed to assess the role of chemical exposures and DNA adducts in cancer risk.

Data-Independent Mass Spectrometry Approach for Screening and Identification of DNA Adducts

Long-term exposures to environmental toxicants and endogenous electrophiles are causative factors for human diseases including cancer. DNA adducts reflect the internal exposure to genotoxicants and can serve as biomarkers for risk assessment. Liquid chromatography-multistage mass spectrometry (LC-MSn) is the most common method for biomonitoring DNA adducts, generally targeting single exposures and measuring up to several adducts. However, the data often provide limited evidence for a role of a chemical in the etiology of cancer. An "untargeted" method is required that captures global exposures to chemicals, by simultaneously detecting their DNA adducts in the genome; some of which may induce cancer-causing mutations. We established a wide selected ion monitoring tandem mass spectrometry (wide-SIM/MS2) screening method utilizing ultraperformance-LC nanoelectrospray ionization Orbitrap MSn with online trapping to enrich bulky, nonpolar adducts. Wide-SIM scan events are followed by MS2 scans to screen for modified nucleosides by coeluting peaks containing precursor and fragment ions differing by -116.0473 Da, attributed to the neutral loss of deoxyribose. Wide-SIM/MS2 was shown to be superior in sensitivity, specificity, and breadth of adduct coverage to other tested adductomic methods with detection possible at adduct levels as low as 4 per 109 nucleotides. Wide-SIM/MS2 data can be analyzed in a "targeted" fashion by generation of extracted ion chromatograms or in an "untargeted" fashion where a chromatographic peak-picking algorithm can be used to detect putative DNA adducts. Wide-SIM/MS2 successfully detected DNA adducts, derived from chemicals in the diet and traditional medicines and from lipid peroxidation products, in human prostate and renal specimens.

Quantification of Hemoglobin and White Blood Cell DNA Adducts of the Tobacco Carcinogens 2-Amino-9H-pyrido[2,3-b]indole and 4-Aminobiphenyl Formed in Humans by Nanoflow Liquid Chromatography/Ion Trap Multistage Mass Spectrometry

Aromatic amines covalently bound to hemoglobin (Hb) as sulfinamide adducts at the cysteine 93 residue of the Hb β chain have served as biomarkers to assess exposure to this class of human carcinogens for the past 30 years. In this study, we report that 2-amino-9H-pyrido[2,3-b]indole (AαC), an abundant carcinogenic heterocyclic aromatic amine formed in tobacco smoke and charred cooked meats, also reacts with Hb to form a sulfinamide adduct. A novel nanoflow liquid chromatography/ion trap multistage mass spectrometry (nanoLC-IT/MS3) method was established to assess exposure to AαC and the tobacco-associated bladder carcinogen 4-aminobiphenyl (4-ABP) through their Hb sulfinamide adducts. Following mild acid hydrolysis of Hb in vitro, the liberated AαC and 4-ABP were derivatized with acetic anhydride to form the N-acetylated amines, which were measured by nanoLC-IT/MS3. The limits of quantification (LOQ) for AαC- and 4-ABP-Hb sulfinamide adducts were ≤7.1 pg/g Hb. In a pilot study, the mean level of Hb sulfinamide adducts of AαC and 4-ABP were, respectively, 3.4-fold and 4.8-fold higher in smokers (>20 cigarettes/day) than nonsmokers. In contrast, the major DNA adducts of 4-ABP, N-(2'-deoxyguanosin-8-yl)-4-aminobiphenyl, and AαC, N-(2'-deoxyguanosin-8-yl)-2-amino-9H-pyrido[2,3-b]indole, were below the LOQ (3 adducts per 109 bases) in white blood cell (WBC) DNA of smokers and nonsmokers. These findings reaffirm that tobacco smoke is a major source of exposure to AαC. Hb sulfinamide adducts are suitable biomarkers to biomonitor 4-ABP and AαC; however, neither carcinogen binds to DNA in WBC, even in heavy smokers, at levels sufficient for biomonitoring.

Metabolism of the Tobacco Carcinogen 2-Amino-9H-pyrido[2,3-b]indole (AαC) in Primary Human Hepatocytes

2-Amino-9H-pyrido[2,3-b]indole (AαC) is the most abundant carcinogenic heterocyclic aromatic amine (HAA) formed in mainstream tobacco smoke. AαC is a liver carcinogen in rodents, but its carcinogenic potential in humans is not known. To obtain a better understanding of the genotoxicity of AαC in humans, we have investigated its metabolism and its ability to form DNA adducts in human hepatocytes. Primary human hepatocytes were treated with AαC at doses ranging from 0.1-50 μM, and the metabolites were characterized by ultra-performance LC/ion trap multistage mass spectrometry (UPLC/MSn). Six major metabolites were identified: a ring-oxidized doubly conjugated metabolite, N2-acetyl-2-amino-9H-pyrido[2,3-b]indole-6-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-6-O-Gluc); two ring-oxidized glucuronide (Gluc) conjugates: 2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (AαC-3-O-Gluc) and 2-amino-9H-pyrido[2,3-b]indol-6-yl-oxo-(β-d-glucuronic acid) (AαC-6-O-Gluc); two sulfate conjugates, 2-amino-9H-pyrido[2,3-b]indol-3-yl sulfate (AαC-3-O-SO3H) and 2-amino-9H-pyrido[2,3-b]indol-6-yl sulfate (AαC-6-O-SO3H); and the Gluc conjugate, N2-(β-d-glucosidurony1)-2-amino-9H-pyrido[2,3-b]indole (AαC-N2-Gluc). In addition, four minor metabolites were identified: N2-acetyl-9H-pyrido[2,3-b]indol-3-yl sulfate (N2-acetyl-AαC-3-O-SO3H), N2-acetyl-9H-pyrido[2,3-b]indol-6-yl sulfate (N2-acetyl-AαC-6-O-SO3H), N2-acetyl-2-amino-9H-pyrido[2,3-b]indol-3-yl-oxo-(β-d-glucuronic acid) (N2-acetyl-AαC-3-O-Gluc), and O-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HN2-O-Gluc). The latter metabolite, AαC-HN2-O-Gluc is a reactive intermediate that binds to DNA to form the covalent adduct N-(2'-deoxyguanosin-8-yl)-2-amino-9H-pyrido[2,3-b]indole (dG-C8-AαC). Preincubation of hepatocytes with furafylline, a selective mechanism-based inhibitor of P450 1A2, resulted in a strong decrease in the formation of AαC-HN2-O-Gluc and a concomitant decrease in DNA adduct formation. Our findings describe the major pathways of metabolism of AαC in primary human hepatocytes and reveal the importance of N-acetylation and glucuronidation in metabolism of AαC. P450 1A2 is a major isoform involved in the bioactivation of AαC to form the reactive AαC-HN2-O-Gluc conjugate and AαC-DNA adducts.

Mass Spectrometric Characterization of an Acid-Labile Adduct Formed with 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and Albumin in Humans

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a carcinogenic heterocyclic aromatic amine formed during the high-temperature cooking of meats. The cytochrome P450-mediated N-hydroxylation of the exocyclic amine group of PhIP produces 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine, an electrophilic metabolite that forms adducts with DNA and proteins. Previous studies conducted by our laboratory showed that the reaction of N-oxidized PhIP metabolites with human albumin in vitro primarily occurs at the Cys³⁴ residue, to produce an acid-labile linked sulfinamide adduct. On the basis of these findings, we developed a sensitive ultraperformance liquid chromatography-mass spectrometry method to measure acid-labile albumin-PhIP adducts in human volunteers administered a dietary-relevant dose of ¹⁴C-labeled PhIP [Dingley, K. H., et al. (1999) Cancer Epidemiol., Biomarkers Prev. 8, 507-512]. Mild acid treatment of albumin (0.1 N HCl, 37 °C for 1 h) or proteolytic digestion with Pronase [50 mM ammonium bicarbonate buffer (pH 8.5) at 37 °C for 18 h] released similar amounts of covalently bound PhIP, which was characterized by multistage scanning and quantified by Orbitrap mass spectrometry. The amount of [¹⁴C]PhIP recovered by acid treatment of albumin 24 h following dosing accounted for 7.2-21.3% of the [¹⁴C]PhIP bound to albumin based on accelerator mass spectrometry measurements. 2-Amino-1-methyl-6-(5-hydroxy)phenylimidazo[4,5-b]pyridine, a hydrolysis product of the Cys³⁴ S-N linked sulfenamide adduct of PhIP, was not detected in either acid-treated or protease-treated samples. These findings suggest that a portion of the PhIP bound to albumin in vivo probably occurs as an acid-labile sulfinamide adduct formed at the Cys³⁴ residue.

Biomonitoring Human Albumin Adducts: The Past, the Present, and the Future

Serum albumin (Alb) is the most abundant protein in blood plasma. Alb reacts with many carcinogens and/or their electrophilic metabolites. Studies conducted over 20 years ago showed that Alb forms adducts with the human carcinogens aflatoxin B1 and benzene, which were successfully used as biomarkers in molecular epidemiology studies designed to address the role of these chemicals in cancer risk. Alb forms adducts with many therapeutic drugs or their reactive metabolites such as β-lactam antibiotics, acetylsalicylic acid, acetaminophen, nonsteroidal anti-inflammatory drugs, chemotherapeutic agents, and antiretroviral therapy drugs. The identification and characterization of the adduct structures formed with Alb have served to understand the generation of reactive metabolites and to predict idiosyncratic drug reactions and toxicities. The reaction of candidate drugs with Alb is now exploited as part of the battery of screening tools to assess the potential toxicities of drugs. The use of gas chromatography-mass spectrometry, liquid chromatography, or liquid chromatography-mass spectrometry (LC-MS) enabled the identification and quantification of multiple types of Alb xenobiotic adducts in animals and humans during the past three decades. In this perspective, we highlight the history of Alb as a target protein for adduction to environmental and dietary genotoxicants, pesticides, and herbicides, common classes of medicinal drugs, and endogenous electrophiles, and the emerging analytical mass spectrometry technologies to identify Alb-toxicant adducts in humans.

Biomonitoring DNA Adducts of Cooked Meat Carcinogens in Human Prostate by Nano Liquid Chromatography-High Resolution Tandem Mass Spectrometry: Identification of 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine DNA Adduct

Epidemiologic studies have reported an association between frequent consumption of well-done cooked meats and prostate cancer risk. However, unambiguous physiochemical markers of DNA damage from carcinogens derived from cooked meats, such as DNA adducts, have not been identified in human samples to support this paradigm. We have developed a highly sensitive nano-LC-Orbitrap MS n method to measure DNA adducts of several carcinogens originating from well-done cooked meats, tobacco smoke, and environmental pollution, including 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-9H-pyrido[2,3-b]indole (AαC), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), benzo[a]pyrene (B[a]P), and 4-aminobiphenyl (4-ABP). The limit of quantification (LOQ) of the major deoxyguanosine (dG) adducts of these carcinogens ranged between 1.3 and 2.2 adducts per 10 9 nucleotides per 2.5 μg of DNA assayed. The DNA adduct of PhIP, N-(deoxyguanosin-8-yl)-PhIP (dG-C8-PhIP) was identified in 11 out of 35 patients, at levels ranging from 2 to 120 adducts per 10 9 nucleotides. The dG-C8 adducts of AαC and MeIQx, and the B[a]P adduct, 10-(deoxyguanosin-N 2 -yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (dG-N 2 -B[a]PDE) were not detected in any specimen, whereas N-(deoxyguanosin-8-yl)-4-ABP (dG-C8-4-ABP) was identified in one subject (30 adducts per 10 9 nucleotides). PhIP-DNA adducts also were recovered quantitatively from formalin fixed paraffin embedded (FFPE) tissues, signifying FFPE tissues can serve as biospecimens for carcinogen DNA adduct biomarker research. Our biomarker data provide support to the epidemiological observations implicating PhIP, one of the most mass-abundant heterocyclic aromatic amines formed in well-done cooked meats, as a DNA-damaging agent that may contribute to the etiology of prostate cancer.

Human T lymphocytes bioactivate heterocyclic aromatic amines by forming DNA adducts

Heterocyclic aromatic amines (HAA) are formed in cooked meat, poultry and fish but also arise in tobacco smoke and exhaust gases. HAA are potential human carcinogens, which require metabolic activation to exert their genotoxicity. Human tissues can bioactivate HAA to produce reactive intermediates that bind to DNA. HAA DNA adduct formation occurs in human hepatocytes; however, the potential of HAA to form DNA adducts has not been investigated in human T lymphocytes. In this study, we investigated the ability of human T lymphocytes activated with PMA/Ionomycin or CD3/CD28 to express functional CYP1 activity and bioactivate three major HAA: 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-9H-pyrido[2,3-b]indole (AαC) to form DNA adducts. Adducts were measured by ultraperformance liquid chromatography-electrospray ionization/multistage scan mass spectrometry. The highest level of DNA adducts occurred for AαC (16 adducts per 10⁹ nucleotides), followed by PhIP (9 adducts per 10⁹ nucleotides). In contrast, DNA adducts formed from MeIQx and the structurally related aromatic amine 4-aminobiphenyl, a known human carcinogen, were below the limit of detection (< 3 adducts per 10⁹ nucleotides). Moreover, we demonstrate that AαC is a potent inducer of CYP1A1 and CYP1B1 activity through a transcriptional mechanism involving the AhR pathway. Overall, our results highlight the capacity of activated human T lymphocytes to more efficiently bioactivate AαC to form DNA adducts than other prominent HAA or 4-ABP. Environ. Mol. Mutagen. 57:656-667, 2016. © 2016 Wiley Periodicals, Inc.

Human Biomonitoring of DNA Adducts by Ion Trap Multistage Mass Spectrometry

Humans are continuously exposed to hazardous chemicals in the environment. These chemicals or their electrophilic metabolites can form adducts with genomic DNA, which can lead to mutations and the initiation of cancer. The identification of DNA adducts is required for understanding exposure and the etiological role of a genotoxic chemical in cancer risk. The analytical chemist is confronted with a great challenge because the levels of DNA adducts generally occur at <1 adduct per 10(7) nucleotides, and the amount of tissue available for measurement is limited. Ion trap mass spectrometry has emerged as an important technique to screen for DNA adducts because of the high level sensitivity and selectivity, particularly when employing multi-stage scanning (MS(n) ). The product ion spectra provide rich structural information and corroborate the adduct identities even at trace levels in human tissues. Ion trap technology represents a significant advance in measuring DNA adducts in humans. © 2016 by John Wiley & Sons, Inc.

Aristolochic Acid in the Etiology of Renal Cell Carcinoma

BACKGROUND

Aristolochia species used in the practice of traditional herbal medicine contains aristolochic acid (AA), an established human carcinogen contributing to urothelial carcinomas of the upper urinary tract. AA binds covalently to genomic DNA, forming aristolactam (AL)-DNA adducts. Here we investigated whether AA is also an etiologic factor in clear cell renal cell carcinoma (ccRCC).

METHODS

We conducted a population-based case-control study to investigate the linkage between Aristolochia prescription history, cumulative AA consumption, and ccRCC incidence in Taiwan (5,709 cases and 22,836 matched controls). The presence and level of mutagenic dA-AL-I adducts were determined in the kidney DNA of 51 Taiwanese ccRCC patients. The whole-exome sequences of ccRCC tumors from 10 Taiwanese ccRCC patients with prior exposure to AA were determined.

RESULTS

Cumulative ingestion of more than 250 mg of AA increased risk of ccRCC (OR, 1.25), and we detected dA-AL-I adducts in 76% of Taiwanese ccRCC patients. Furthermore, the distinctive AA mutational signature was evident in six of 10 sequenced ccRCC exomes from Taiwanese patients.

CONCLUSIONS

This study strongly suggests that AA contributes to the etiology of certain RCCs.

IMPACT

The current study offers compelling evidence implicating AA in a significant fraction of the RCC arising in Taiwan and illustrates the power of integrating epidemiologic, molecular, and genetic data in the investigation of cancer etiology. Cancer Epidemiol Biomarkers Prev; 25(12); 1600-8. ©2016 AACR.

Bioactivation of Heterocyclic Aromatic Amines by UDP Glucuronosyltransferases

2-Amino-9H-pyrido[2,3-b]indole (AαC) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are carcinogenic heterocyclic aromatic amines (HAA) that arise during the burning of tobacco and cooking of meats. UDP-glucuronosyltransferases (UGT) detoxicate many procarcinogens and their metabolites. The genotoxic N-hydroxylated metabolite of AαC, 2-hydroxyamino-9H-pyrido[2,3-b]indole (HONH-AαC), undergoes glucuronidation to form the isomeric glucuronide (Gluc) conjugates N(2)-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HON(2)-Gluc) and O-(β-d-glucosidurony1)-2-hydroxyamino-9H-pyrido[2,3-b]indole (AαC-HN(2)-O-Gluc). AαC-HON(2)-Gluc is a stable metabolite but AαC-HN(2)-O-Gluc is a biologically reactive intermediate, which covalently adducts to DNA at levels that are 20-fold higher than HONH-AαC. We measured the rates of formation of AαC-HON(2)-Gluc and AαC-HN(2)-O-Gluc in human organs: highest activity occurred with liver and kidney microsomes, and lesser activity was found with colon and rectum microsomes. AαC-HN(2)-O-Gluc formation was largely diminished in liver and kidney microsomes, by niflumic acid, a selective inhibitor UGT1A9. In contrast, AαC-HON(2)-Gluc formation was less affected and other UGT contribute to N(2)-glucuronidation of HONH-AαC. UGT were reported to catalyze the formation of isomeric Gluc conjugates at the N(2) and N3 atoms of 2-hydroxyamino-1-methyl-6-phenylimidazo[4,5-b]pyridine (HONH-PhIP), the genotoxic metabolite of PhIP. However, we found that the N3-Gluc of HONH-PhIP also covalently bound to DNA at higher levels than HONH-PhIP. The product ion spectra of this Gluc conjugate acquired by ion trap mass spectrometry revealed that the Gluc moiety was linked to the oxygen atom of HONH-PhIP and not the N3 imidazole atom of the oxime tautomer of HONH-PhIP as was originally proposed. UGT1A9, an abundant UGT isoform expressed in human liver and kidney, preferentially forms the O-linked Gluc conjugates of HONH-AαC and HONH-PhIP as opposed to their detoxicated N(2)-Gluc isomers. The regioselective O-glucuronidation of HONH-AαC and HONH-PhIP, by UGT1A9, is a mechanism of bioactivation of these ubiquitous HAAs.