Doxorubicin induced cardiotoxicity in childhood cancer survivors is mediated by mitochondrial dysfunction. The aims of this research are to determine (i) the frequency of mitochondrial DNA (mtDNA) mutations in long-term survivors of childhood acute lymphoblastic leukemia (ALL), and (ii) to determine if co-administration of dexrazoxane reduces the occurrence of mutations. Patients previously treated on Dana-Farber Cancer Institute Childhood ALL protocols and at least 4 years from the date of ALL diagnosis were enrolled in this study. MtDNA was isolated from samples of peripheral blood lymphocytes. A vertical denaturing gradient gel electrophoresis method was used for detecting sequence variants in the 22 transfer RNA (tRNA) genes and flanking regions of the human mitochondrial genome. The patients were divided into two cohorts, those with mutations and those without, and compared. The patients and variants were also compared to healthy controls. Mutational status was compared with echocardiographic measurements. Of the patients who received chemotherapy, there were 51/167 (31

Mutations in T lymphocytes (T-cells) are informative quantitative markers for environmental mutagen exposures, but risk extrapolations from rodent models to humans also require an understanding of how T-cell development and proliferation kinetics impact mutagenic outcomes. Rodent studies have shown that patterns in chemical-induced mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene of T-cells differ between lymphoid organs. The current work was performed to obtain knowledge of the relationships between maturation events during T-cell development and changes in chemical-induced mutant frequencies over time in differing immune compartments of a mouse model. A novel reverse transcriptase-polymerase chain reaction based method was developed to determine the specific T-cell receptor beta (Tcrb) gene mRNA expressed in mouse T-cell isolates, enabling sequence analysis of the PCR product that then identifies the specific hypervariable CDR3 junctional region of the expressed Tcrb gene for individual isolates. Characterization of spontaneous Hprt mutant isolates from the thymus, spleen, and lymph nodes of control mice for their Tcrb gene expression found evidence of in vivo clonal amplifications of Hprt mutants and their trafficking between tissues in the same animal. Concurrent analyses of Hprt mutations and Tcrb gene rearrangements in different lymphoid tissues of control versus N-ethyl-N-nitrosourea-exposed mice permitted elucidation of the localization and timing of mutational events in T-cells, establishing that mutagenesis occurs primarily in the pre-rearrangement replicative period in pre-thymic/thymic populations. These findings demonstrate that chemical-induced mutagenic burden is determined by the combination of mutagenesis and T-cell clonal expansion, processes with roles in immune function and in the pathogenesis of autoimmune disease and cancer.

The lack of anticancer agents that overcome innate/acquired drug resistance is the single biggest barrier to achieving a durable complete response to cancer therapy. To address this issue, a new drug family was developed for intracellular delivery of the bioactive aminothiol WR1065 by conjugating it to discrete thiol-PEG polymers: 4-star-PEG-S-S-WR1065 (4SP65) delivers four WR1065s/molecule and m-PEG6-S-S-WR1065 (1LP65) delivers one. Infrequently, WR1065 has exhibited anticancer effects when delivered via the FDA-approved cytoprotectant amifostine, which provides one WR1065/molecule extracellularly. The relative anticancer effectiveness of 4SP65, 1LP65, and amifostine was evaluated in a panel of 15 human cancer cell lines derived from seven tissues. Additional experiments assessed the capacity of 4SP65 co-treatments to potentiate the anticancer effectiveness and overcome drug resistance to cisplatin, a chemotherapeutic, or gefitinib, a tyrosine kinase inhibitor (TKI) targeting oncogenic EGFR mutations. The CyQUANT®-NF proliferation assay was used to assess cell viability after 48-h drug treatments, with the National Cancer Institute COMPARE methodology employed to characterize dose-response metrics. In normal human epithelial cells, 4SP65 or 1LP65 enhanced or inhibited cell growth but was not cytotoxic. In cancer cell lines, 4SP65 and 1LP65 induced dose-dependent cytostasis and cytolysis achieving 99% cell death at drug concentrations of 11.2 ± 1.2 µM and 126 ± 15.8 µM, respectively. Amifostine had limited cytostatic effects in 11/14 cancer cell lines and no cytolytic effects. Binary pairs of 4SP65 plus cisplatin or gefitinib increased the efficacy of each partner drug and surmounted resistance to cytolysis by cisplatin and gefitinib in relevant cancer cell lines. 4SP65 and 1LP65 were significantly more effective against TP53-mutant than TP53-wild-type cell lines, consistent with WR1065-mediated reactivation of mutant p53. Thus, 4SP65 and 1LP65 represent a unique prodrug family for innovative applications as broad-spectrum anticancer agents that target p53 and synergize with a chemotherapeutic and an EGFR-TKI to prevent or overcome drug resistance.

Mutations in T lymphocytes (T-cells) are informative quantitative markers for environmental mutagen exposures, but risk extrapolations from rodent models to humans also require an understanding of how T-cell development and proliferation kinetics impact mutagenic outcomes. Rodent studies have shown that patterns in chemical-induced mutations in the hypoxanthine-guanine phosphoribosyltransferase ( Hprt ) gene of T-cells differ between lymphoid organs. The current work was performed to obtain knowledge of the relationships between maturation events during T-cell development and changes in chemical-induced mutant frequencies over time in differing immune compartments of a mouse model. A novel reverse transcriptase-polymerase chain reaction based method was developed to determine the specific T-cell receptor beta ( Tcrb ) gene mRNA expressed in mouse T-cell isolates, enabling sequence analysis of the PCR product that then identifies the specific hypervariable CDR3 junctional region of the expressed Tcrb gene for individual isolates. Characterization of spontaneous Hprt mutant isolates from the thymus, spleen, and lymph nodes of control mice for their Tcrb gene expression found evidence of in vivo clonal amplifications of Hprt mutants and their trafficking between tissues in the same animal. Concurrent analyses of Hprt mutations and Tcrb gene rearrangements in different lymphoid tissues of control versus N -ethyl- N -nitrosourea-exposed mice permitted elucidation of the localization and timing of mutational events in T-cells, establishing that mutagenesis occurs primarily in the pre-rearrangement replicative period in pre-thymic/thymic populations. These findings demonstrate that chemical-induced mutagenic burden is determined by the combination of mutagenesis and T-cell clonal expansion, processes with roles in immune function and in the pathogenesis of autoimmune disease and cancer.

Der wachsende Einfluss der künstlichen Intelligenz auf die Radiologie führt nicht nur zu einem fundamentalen Wandel bei der Gewinnung der Diagnosen, sondern stellt auch eine Fülle zusätzlicher Informationen bereit. Viele Programme korrelieren die Parameter der Bildauswertung mit den Resultaten histologischer, molekularbiologischer und genetischer Untersuchungen und leiten daraus therapeutische und prognostische Aussagen ab, die der individualisierten Präzisionsmedizin dienen sollen. Diese Informationen gehen in den Befundbericht ein und stehen in vollem Umfang auch den Patienten zur Verfügung. Sie nehmen jedoch keine Rücksicht auf Einflussgrößen wie den Zeitabstand zwischen Diagnose und Behandlungsbeginn, Begleiterkrankungen sowie die Verfügbarkeit und Verträglichkeit von Medikamenten. Es ist abzusehen, dass die ergänzenden Aussagen der Expertensysteme die Gesprächskontakte zwischen Arzt und Patient erheblich beeinflussen werden.

Acrylonitrile (ACN), which is a widely used industrial chemical, induces cancers in the mouse via unresolved mechanisms. For this report, complementary and previously described methods were used to assess in vivo genotoxicity and/or mutagenicity of ACN in several mouse models, including (i) female mice devoid of cytochrome P450 2E1 (CYP2E1), which yields the epoxide intermediate cyanoethylene oxide (CEO), (ii) male lacZ transgenic mice, and (iii) female (wild-type) B6C3F1 mice. Exposures of wild-type mice and CYP2E1-null mice to ACN at 0, 2.5 (wild-type mice only), 10, 20, or 60 (CYP2E1-null mice only) mg/kg body weight by gavage for 6 weeks (5 days/week) produced no elevations in the frequencies of micronucleated erythrocytes, but induced significant dose-dependent increases in DNA damage, detected by the alkaline (pH >13) Comet assay, in one target tissue (forestomach) and one nontarget tissue (liver) of wild-type mice only. ACN exposures by gavage also caused significant dose-related elevations in the frequencies of mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) reporter gene of T-lymphocytes from spleens of wild-type mice; however, Hprt mutant frequencies were significantly increased in CYP2E1-null mice only at a high dose of ACN (60 mg/kg) that is lethal to wild-type mice. Similarly, drinking water exposures of lacZ transgenic mice to 0, 100, 500, or 750 ppm ACN for 4 weeks caused significant dose-dependent elevations in Hprt mutant frequencies in splenic T-cells; however, these ACN exposures did not increase the frequency of lacZ transgene mutations above spontaneous background levels in several tissues from the same animals. Together, the Comet assay and Hprt mutant frequency data from these studies indicate that oxidative metabolism of ACN by CYP2E1 to CEO is central to the induction of the majority of DNA damage and mutations in ACN-exposed mice, but ACN itself also may contribute to the carcinogenic modes of action via mechanisms involving direct and/or indirect DNA reactivity.

Acrylonitrile (ACN), which is a widely used industrial chemical, induces cancers in the mouse via unresolved mechanisms. For this report, complementary and previously described methods were used to assess in vivo genotoxicity and/or mutagenicity of ACN in several mouse models, including (i) female mice devoid of cytochrome P450 2E1 (CYP2E1), which yields the epoxide intermediate cyanoethylene oxide (CEO), (ii) male lacZ transgenic mice, and (iii) female (wild-type) B6C3F1 mice. Exposures of wild-type mice and CYP2E1-null mice to ACN at 0, 2.5 (wild-type mice only), 10, 20, or 60 (CYP2E1-null mice only) mg/kg body weight by gavage for 6 weeks (5 days/week) produced no elevations in the frequencies of micronucleated erythrocytes, but induced significant dose-dependent increases in DNA damage, detected by the alkaline (pH >13) Comet assay, in one target tissue (forestomach) and one nontarget tissue (liver) of wild-type mice only. ACN exposures by gavage also caused significant dose-related elevations in the frequencies of mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) reporter gene of T-lymphocytes from spleens of wild-type mice; however, Hprt mutant frequencies were significantly increased in CYP2E1-null mice only at a high dose of ACN (60 mg/kg) that is lethal to wild-type mice. Similarly, drinking water exposures of lacZ transgenic mice to 0, 100, 500, or 750 ppm ACN for 4 weeks caused significant dose-dependent elevations in Hprt mutant frequencies in splenic T-cells; however, these ACN exposures did not increase the frequency of lacZ transgene mutations above spontaneous background levels in several tissues from the same animals. Together, the Comet assay and Hprt mutant frequency data from these studies indicate that oxidative metabolism of ACN by CYP2E1 to CEO is central to the induction of the majority of DNA damage and mutations in ACN-exposed mice, but ACN itself also may contribute to the carcinogenic modes of action via mechanisms involving direct and/or indirect DNA reactivity.

Acrylonitrile (ACN), which is a widely used industrial chemical, induces cancers in multiple organs/tissues of rats by unresolved mechanisms. For this report, evidence for ACN-induced direct/indirect DNA damage and mutagenesis was investigated by assessing the ability of ACN, or its reactive metabolite, 2-cyanoethylene oxide (CEO), to bind to DNA in vitro, to form select DNA adducts [N7-(2'-oxoethyl)guanine, N-2,3-ethenoguanine, 1,N-6-ethenodeoxyadenosine, and 3,N-4-ethenodeoxycytidine] in vitro and/or in vivo, and to perturb the frequency and spectra of mutations in the hypoxanthine-guanine phosphoribosyltransferase (Hprt) gene in rats exposed to ACN in drinking water. Adducts and frequencies and spectra of Hprt mutations were analyzed using published methods. Treatment of DNA from human TK6 lymphoblastoid cells with [2,3-C-14]-CEO produced dose-dependent binding of C-14-CEO equivalents, and treatment of DNA from control rat brain/liver with CEO induced dose-related formation of N7-(2'-oxoethyl)guanine. No etheno-DNA adducts were detected in target tissues (brain and forestomach) or nontarget tissues (liver and spleen) in rats exposed to 0, 3, 10, 33, 100, or 300 ppm ACN for up to 105 days or to 0 or 500 ppm ACN for similar to 15 months; whereas N7-(2'-oxoethyl)guanine was consistently measured at nonsignificant concentrations near the assay detection limit only in liver of animals exposed to 300 or 500 ppm ACN for >= 2 weeks. Significant dose-related increases in Hprt mutant frequencies occurred in T-lymphocytes from spleens of rats exposed to 33-500 ppm ACN for 4 weeks. Comparisons of "mutagenic potency estimates" for control rats versus rats exposed to 500 ppm ACN for 4 weeks to analogous data from rats/mice treated at a similar age with N-ethyl-N-nitrosourea or 1,3-butadiene suggest that ACN has relatively limited mutagenic effects in rats. Considerable overlap between the sites and types of mutations in ACN-exposed rats and butadiene-exposed rats/mice, but not controls, provides evidence that the carcinogenicity of these epoxide-forming chemicals involves corresponding mutagenic mechanisms.

Epidemiological studies of 1,3-butadiene (BD) exposures have reported a possible association with chronic myelogenous leukemia (CML), which is defined by the presence of the t(9;22) translocation (Philadelphia chromosome) creating an oncogenic BCR-ABL fusion gene. Butadiene diepoxide (DEB), the most mutagenic of three epoxides resulting from BD, forms DNA-DNA crosslink adducts that can lead to DNA double-strand breaks (DSBs). Thus, a study was designed to determine if ( +/- )-DEB exposure of HL60 cells, a promyelocytic leukemia cell line lacking the Philadelphia chromosome, can produce t(9;22) translocations. In HL60 cells exposed for 3 h to 0-10 mu M DEB, overlapping dose-response curves suggested a direct relationship between 1,4-bis-(guan-7-yl)-2,3-butanediol crosslink adduct formation (R = 0.977, P = 0.03) and cytotoxicity (R = 0.961, P = 0.002). Experiments to define the relationships between cytotoxicity and the induction of micronuclei (MN), a dosimeter of DNA DSBs, showed that 24 h exposures of HL60 cells to 0-5.0 mu M DEB caused significant positive correlations between the concentration and (i) the degree of cytotoxicity (R = 0.998, p = 0.002) and (ii) the frequency of MN (R = 0.984, p = 0.016) at 48 h post exposure. To determine the relative induction of MN and 49;22) translocations following exposures to DEB, or x-rays as a positive control for formation of t(9;22) translocations, HL60 cells were exposed for 24 h to 0, 1, 2.5, or 5 mu M DEB or to 0, 2.0, 3.5, or 5.0 Gy x-rays, or treatments demonstrated to yield 0, 20%, 50%, or 80% cytotoxicity. Treatments between 0 and 3.5 Gy x-rays caused significant dose-related increases in both MN (p < 0.001) and t(9;22) translocations (p = 0.01), whereas DEB exposures causing similar cytotoxicity levels did not increase translocations over background. These data indicate that, while DEB induces DNA DSBs required for formation of MN and translocations, acute DEB exposures of HL60 cells did not produce the Philadelphia chromosome obligatory for CML.

The use of computed tomography (CT scans) has increased dramatically in recent decades, raising questions about the long-term safety of CT-emitted x-rays especially in infants who are more sensitive to radiation-induced effects. Cancer risk estimates for CT scans typically are extrapolated from models; therefore, new approaches measuring actual DNA damage are needed for improved estimations. Hence, changes in a dosimeter of DNA double-strand breaks, micronucleated reticulocytes (MN-RETs) measured by flow cytometry, were investigated in mice and infants exposed to CT scans. In male C57BL/6N mice (6-8 weeks-of-age), there was a dose-related increase in MN-RETs in blood samples collected 48 h after CT scans delivering targeted exposures of 1-130 cGy x-rays (n = 5-10/group, r = 0.994, p = 0.01), with significant increases occurring at exposure levels as low as 0.83 cGy x-rays compared to control mice (p = 0.002). In paired blood specimens from infants with no history of a prior CT scan, there was no difference in MN-RET frequencies found 2 h before (mean, 0.10 +/- 0.07%) versus 48 h after (mean, 0.11 +/- 0.05%) a scheduled CT scan/cardiac catheterization. However, in infants having prior CT scan(s), MN-RET frequencies measured at 48 h after a scheduled CT scan (mean = 0.22 +/- 0.12%) were significantly higher than paired baseline values (mean, 0.17 +/- 0.07%; p = 0.032). Increases in baseline (r = 0.722, p < 0.001) and 48-h post exposure (r = 0.682, p < 0.001) levels of MN-RETs in infants with a history of prior CT scans were significantly correlated with the number of previous CT scans. These preliminary findings suggest that prior CT scans increase the cellular responses to subsequent CT exposures. Thus, further investigation is needed to characterize the potential cancer risk from single versus repeated CT scans or cardiac catheterization in infants.

The events or factors that lead from normal cell function to conditions and diseases such as aging or cancer reflect complex interactions between cells and their environment. Cellular stress responses, a group of processes involved in homeostasis and adaptation to environmental change, contribute to cell survival under stress and can be resolved with damage avoidance or damage tolerance outcomes. To investigate the impact of environmental agents/conditions upon cellular stress response outcomes in epithelium, a novel quantitative assay, the “stress response resolution” (SRR) assay, was developed. The SRR assay consists of pretreatment with a test agent or vehicle followed later by a calibrated stress conditions exposure step (here, using 6‐thioguanine). Pilot studies conducted with a spontaneously‐immortalized murine mammary epithelial cell line pretreated with vehicle or 20 µg N‐ethyl‐N‐nitrososurea/ml medium for 1 hr, or two hTERT‐immortalized human bronchial epithelial cell lines pretreated with vehicle or 100 µM zidovudine/lamivudine for 12 days, found minimal alterations in cell morphology, survival, or cell function through 2 weeks post‐exposure. However, when these pretreatments were followed 2 weeks later by exposure to calibrated stress conditions of limited duration (for 4 days), significant alterations in stress resolution were observed in pretreated cells compared with vehicle‐treated control cells, with decreased damage avoidance survival outcomes in all cell lines and increased damage tolerance outcomes in two of three cell lines. These pilot study results suggest that sub‐cytotoxic pretreatments with chemical mutagens have long‐term adverse impact upon the ability of cells to resolve subsequent exposure to environmental stressors. Environ. Mol. Mutagen. 54:268–280, 2013. © 2013 Wiley Periodicals, Inc.

Cellular stress responses consist of a complex network of pathways and linked processes that, when perturbed, are postulated to have roles in the pathogenesis of various human diseases. To assess the impact of environmental insults upon this network, we developed a novel stress response resolution (SRR) assay for investigation of cellular stress resolution outcomes and the effects of environmental agents and conditions thereupon. SRR assay-based criteria identified three distinct groups of surviving cell clones, including those resembling parental cells, those showing Hprt/HPRT mutations, and a third type, “Phenotype-altered” clones, that occurred predominantly in cells pretreated with a chemical mutagen, was heterogeneous in nature, and expressed significant alterations in cell morphology and/or function compared with parental cells. Further evaluation of Phenotype-altered clones found evidence of various alterations that resembled epithelial-to-mesenchymal transition, phenotype switching, checkpoint dysfunction, senescence barrier bypass, and/or epigenetic reprogramming. Phenotype-altered clones were found to occur spontaneously in a cell line with a mutator phenotype, to represent the major surviving clone type in a variation of the SRR assay, and to be tumorigenic in nude mice. Assessment of SRR assay final results showed that pretreatment with a chemical mutagen induced significant changes in cellular stress response prosurvival capacity, in damage avoidance versus damage tolerance stress resolution outcomes, and in the damage burden in the final surviving cell populations. Taken together, these results support the conclusion that use of the SRR assay can provide novel insights into the role of environmental insults in the pathogenesis of cancer and other human diseases. Environ. Mol. Mutagen. 54:281–293, 2013. © 2013 Wiley Periodicals, Inc.

To delineate temporal changes in the integrity and function of mitochondria/cardiomyocytes in hearts from mice exposed in utero to commonly used nucleoside analogs (NRTIs), CD-1 mice were exposed in utero to 80 mg AZT/kg, 40 mg 3TC/kg, 80 mg AZT/kg plus 40 mg 3TC/kg, or vehicle alone during days 12–18 of gestation and hearts from female mouse offspring were examined at 13 and 26 weeks postpartum. Alterations in cardiac mitochondrial DNA (mtDNA) content, oxidative phosphorylation (OXPHOS) enzyme activities, mtDNA mutations, and echocardiography of NRTI-exposed mice were assessed and compared with findings in vehicle-exposed control mice. A hybrid capture-chemiluminescence assay showed significant twofold increases in mtDNA levels in hearts from AZT- and AZT/3TC-exposed mice at 13 and 26 weeks postpartum, consistent with near doubling in mitochondrial numbers over time compared with vehicle-exposed mice. Echocardiographic measurements at 13 and 26 weeks postpartum indicated progressive thinning of the left ventricular posterior wall in NRTI-exposed mice, relative to controls, with differences becoming statistically significant by 26 weeks. Overall, progressive functional changes occurred in mouse mitochondria and cardiac tissue several months after in utero NRTI exposures; AZT and 3TC acted in concert to cause additive cardiotoxic effects of AZT/3TC compared with either drug alone.

The current study was designed to delineate temporal changes in cardiomyocytes and mitochondria at the light and electron microscopic levels in hearts of mice exposed transplacentally to commonly used nucleoside analogs (NRTIs). Pregnant CD-1 mice were given 80 mg AZT/kg, 40 mg 3TC/kg, 80 mg AZT/kg plus 40 mg 3TC/kg, or vehicle alone during the last 7 days of gestation, and hearts from female mouse pups were examined at 13 and 26 weeks postpartum for histopathological or ultrastructural changes in cross-sections of both the ventricles and the interventricular septum. Using light microscopy and special staining techniques, transplacental exposure to AZT, 3TC, or AZT/3TC was shown to induce significant histopathological changes in myofibrils; these changes were more widespread at 13 weeks than at 26 weeks postpartum. While most light microscopic lesions resolved, some became more severe between 13 and 26 weeks postpartum. Transplacental NRTI exposure also resulted in progressive drug-specific changes in the number and ultrastructural integrity of cardiac mitochondria. These light and electron microscopic findings show that a subset of changes in cardiac mitochondria and myofibrils persisted and progressed months after transplacental exposure of an animal model to NRTIs, with combined AZT/3TC exposure yielding additive effects compared with either drug alone.

A sensitive vertical denaturing gradient gel electrophoresis (DGGE) method, using 13 unipolar psoralen-clamped PCR primer pairs, was developed for detecting sequence variants in the 22 tRNA genes and flanking regions (together spanning ∼21%) of the human mitochondrial genome. A study was conducted to determine (i) if mitochondrial DNA (mtDNA) polymorphisms and/or mutations were detectable in healthy newborns and (ii) if prepartum 3′-azido-2′,3′-dideoxythymidine (AZT) based HIV-1 prophylaxis was associated with significant increases in mtDNA mutations and changes in the degree of heteroplasmy of sequence variants in uninfected infants born to HIV-1-infected mothers. DGGE analysis of umbilical cord tissue (where vascular endothelium and smooth muscle cells are the major source of mtDNA) showed that mtDNA sequence variants were significantly elevated by threefold in AZT-treated infants compared with unexposed controls ( P < 0.001), with 24 changes observed in 19/52 (37%) treated newborns (averaging 0.46 changes/subject) versus only eight changes found in 7/55 (13%) unexposed newborns (averaging 0.15 changes/subject). Six distinct sequence variants occurring in unexposed controls were predominately synonymous and homoplasmic, representing previously reported polymorphisms. Uninfected infants exposed to a combination of AZT and 2′,3′-dideoxy-3′-thiacytidine and “maternal HIV-1” had a significant shift in the spectrum of mutations ( P = 0.04) driven by increases in nonsynonymous heteroplasmic sequence variants at polymorphic sites (10 distinct variants) and novel sites (four distinct variants). While the weight of evidence suggests that prepartum AZT-based prophylaxis produces mtDNA mutations, additional research is needed to determine the degree to which fetal responses to maternal HIV-1 infection, in the absence of antiretroviral treatment, contribute to prenatal mtDNA mutagenesis. Environ. Mol. Mutagen., 2009. © 2008 Wiley-Liss, Inc.

Current risk assessments of 1,3-butadiene (BD*) are complicated by limited evidence of its carcinogenicity in humans. Hence, there is a critical need to identify early events and factors that account for the heightened sensitivity of mice to BD-induced carcinogenesis and to deter-mine which animal model, mouse or rat, is the more useful surrogate of potency for predicting health effects in BD-exposed humans. HEI sponsored an earlier investigation of mutagenic responses in mice and rats exposed to BD, or to the racemic mixture of 1,2-epoxy-3-butene (BDO) or of 1,2,3,4-diepoxybutane (BDO2; Walker and Meng 2000). In that study, our research team demonstrated (1) that the frequency of mutations in the hypoxanthine-guanine phosphoribosyl transferase (Hprt) gene of splenic T cells from BD-exposed mice and rats could be correlated with the species-related differences in cancer susceptibility; (2) that mutagenic-potency and mutagenic-specificity data from mice and rats exposed to BD or its individual epoxy intermediates could provide useful information about the BD metabolites responsible for mutations in each species; and (3) that our novel approach to measuring the mutagenic potency of a given chemical exposure as the change in Hprt mutant frequencies (Mfs) over time was valuable for estimating species-specific differences in mutagenic responses to BD exposure and for predicting the effect of BD metabolites in each species. To gain additional mode-of-action information that can be used to inform studies of human responses to BD exposure, experiments in the current investigation tested a new set of five hypotheses about species-specific patterns in the mutagenic effects in rodents of exposure to BD and BD metabolites: 1. Repeated BD exposures at low levels that approach the occupational exposure limit for BD workers (set by the U.S. Occupational Safety and Health Administration) are mutagenic in female mice. 2. The differences in mutagenic responses of the Hprt gene to BD in similarly exposed rodents of a given species (reported in various earlier studies) are primarily associated with age-related thymus activity and trafficking of T cells and with sex-related differences in BD metabolism. 3. The mutagenic potency of the stereochemical forms of BD's epoxy intermediates plays a significant role in the species-related mutagenicity of BD. 4. The hydrolysis-detoxification pathway of BD through 1,2-dihydroxy-3-butene (BD-diol) is a major contributor to mutagenicity at high-level BD exposures in mice and rats. 5. Significant and informative species-specific differences in mutation spectra can be identified by examining both large- and small-scale genetic alterations in the Hprt gene of BD-exposed mice and rats. The first four hypotheses were tested by exposing mice and rats to BD, meso-BDO2, or BD-diol and measuring Hprt Mfs as the primary biomarker. For this, we used the T-cell-cloning assay of lymphocytes isolated from the spleens of exposed and control (sham-exposed) mice and rats. The first hypothesis was tested by exposing female B6C3F1 mice (4 to 5 weeks of age) by inhalation for 2 weeks (6 hours/day, 5 days/week) to 0 or 3 ppm BD. Hprt Mfs were measured at the time of peak mutagenic response after exposure for this age of mice. We then compared the resulting data to those from mutagenicity studies with mice of the same age that had been exposed in a similar protocol to higher levels of BD (Walker and Meng 2000). In mice exposed to 3 ppm BD (n = 27), there was a significant 1.6-fold increase over the mean background Hprt Mf in control animals (n = 24, P = 0.004). Calculating the efficiency of Hprt mutant induction, by dividing induced Hprt Mfs by the respective BD exposure levels, demonstrated that the mutagenic potency of 3 ppm BD was twice that of 20 ppm BD and almost 20 times that of 625 or 1250 ppm BD in exposed female mice. Sample-size calculations based on the Hprt Mf data from this experiment demonstrated the feasibility of conducting a future experiment to find out whether induced Mfs at even lower exposure levels (between 0.1 and 1.0 ppm BD) fit the supralinear exposure-response curve found with exposures between 3.0 and 62.5 ppm BD, or whether they deviate from the curve as Mf values approach the background levels found in control animals. The second hypothesis was tested by estimating mutagenic potency for female mice exposed by inhalation for 2 weeks to 0 or 1250 ppm BD at 8 weeks of age and comparing this estimate to that reported for female mice exposed to BD in a similar protocol at 4 to 5 weeks of age (Walker and Meng 2000). For these two age groups, the shapes of the mutant splenic T-cell manifestation curves were different, but the mutagenic burden was statistically the same. These results support our contention that the disparity in responses reported in earlier Hprt-mutation studies of BD-exposed rodents is related more to age-related T-cell kinetics than to age-specific differences in the metabolism of BD. The third hypothesis was tested by estimating mutagenic potency for female mice and rats (4 to 5 weeks of age) exposed by inhalation to 2 or 4 ppm meso-BDO2 and comparing these estimates to those previously obtained for female mice and rats of the same age and exposed in a similar protocol to (+/-)-BDO2 (Meng et al. 1999b; Walker and Meng 2000). These exposures to stereospecific forms of BDO2 caused equivalent mutagenic effects in each species. This suggests that the small differences in the mutagenic potency of the individual stereoisomers of BDO2 appear to be of less consequence in characterizing the sources of BD-induced mutagenicity than the much larger differences between the mutagenic potencies of BDO2 and the other two BD epoxides (BDO and 1,2-dihydroxy-3,4-epoxybutane [BDO-diol]). The fourth hypothesis was tested in several experiments. First, female and male mice and rats (4 to 5 weeks of age) were exposed by nose only for 6 hours to 0, 62.5, 200, 625, or 1250 ppm BD or to 0, 6, 18, 24, or 36 ppm BD-diol primarily to establish BD and BD-diol exposure levels that would yield similar plasma concentrations of BD-diol. Second, animals were exposed in inhalation chambers for 4 weeks to 0, 6, 18, or 36 ppm BD-diol to determine the mutagenic potency estimates for these exposure levels and to compare these estimates with those reported for BD-exposed female mice and rats (Walker and Meng 2000) in which similar blood levels of BD-diol had been achieved. Measurements of plasma concentrations of BD-diol (via a gas chromatography and mass spectrometry [GC/MS] method developed for this purpose) showed these results: First, BD-diol accumulated in a sublinear manner during a single 6-hour exposure to more than 200 ppm BD. Second, BD-diol accumulated in a linear manner during single (6-hour) or repeated (4-week) exposure to 6 or 18 ppm BD and in a sublinear manner with increasing levels of BD-diol exposure. Third, exposure of female mice and rats to 18 ppm BD-diol produced plasma concentrations equivalent to those produced by exposure to 200 ppm BD (exposure to 36 ppm BD-diol produced plasma concentrations of about 25% of those produced by exposure to 625 ppm BD). In general, 4-week exposure to 18 or 36 ppm BD-diol was significantly mutagenic in female and male mice and rats. The differences in mutagenic responses between the species and sexes were not remarkable, except that the mutagenic effects were greatest in female mice. The substantial differences in the exposure-related accumulation of BD-diol in plasma after rodents were exposed to more than 200 ppm BD compared with the relatively small differences in the mutagenic responses to direct exposures to 6, 18, or 36 ppm BD-diol in female mice provided evidence that the contribution of BD-diol-derived metabolites to the overall mutagenicity of BD has a narrow range of effect that is confined to relatively high-level BD exposures in mice and rats. This conclusion was supported by the results of parallel analyses of adducts in mice and rats concurrently exposed to BD-diol (Powley et al. 2005b), which showed that the exposure-response curves for the formation of N-(2,3,4-trihydroxybutyl)valine (THB-Val) in hemoglobin, formation of N7-(2,3,4-trihydroxybutyl)guanine (THB-Gua) in DNA, and induction of Hprt mutations in exposed rodents were remarkably similar in shape (i.e., supralinear). Combined, these data suggest that trihydroxybutyl (THB) adducts are good quantitative indicators of BD-induced mutagenicity and that BD-diol-derived BDO-diol (the major source of the adducts) might be largely responsible for mutagenicity in rodents exposed to BD-diol or to hight levels of BD. The mutagenic-potency studies of meso-BDO2 and BD-diol reported here, combined with our earlier studies of BD, (+/-) BDO, and(+/-)-BDO2 (Walker and Meng 2000), revealed important trends in species-specific mutagenic responses that distinguish the relative degree to which the epoxy intermediates contribute to mutation induction in rodents at selected levels of BD exposures. These data as a whole suggest that , in mice, BDO2 largely causes mutations at exposures less than 62.5 ppm BD and that BD-diol-derived metabolites add to these mutagenic effects at higher BD exposures. In rats, it appears that the BD-diol pathway might account for nearly all the mutagenicity at the hight-level BD exposures where significant increases in Hprt Mfs are found and cancers are induced. Additional exposure-response studies of hemoglobin and DNA adducts specifics to BDO2, BDO-diol, and other reactive intermediates are needed to determine more definitively the relative contribution of each metabolite to the DNA alkylation and mutation patterns induced by BD exposure in mice and rats. For the fifth hypothesis, a multiplex polymerase chain reaction (PCR) procedure for the analysis of genomic DNA mutations in the Hprt gene of mice was developed. (ABSTRACT TRUNCATED)