3Rs‐friendly approach to exogenous metabolic activation that supports high‐throughput genetic toxicology testing

MultiFlow® DNA Damage—p53, γH2AX, Phospho‐Histone H3 is a miniaturized, flow cytometry‐based assay that provides genotoxic mode of action information by distinguishing clastogens, aneugens, and nongenotoxicants. Work to date has focused on the p53‐competent human cell line TK6. While mammalian cell genotoxicity assays typically supply exogenous metabolic activation in the form of concentrated rat liver S9, this is a less‐than‐ideal approach for several reasons, including 3Rs considerations. Here, we describe our experiences with low concentration S9 and saturating co‐factors which were allowed to remain in contact with cells and test chemicals for 24 continuous hours. We exposed TK6 cells in 96‐well plates to each of 15 reference chemicals over a range of concentrations, both in the presence and absence of 0.25% v/v phenobarbital/β‐naphthoflavone‐induced rat liver S9. After 4 and 24 hr of treatment cell aliquots were added to wells of a microtiter plate containing the working detergent/stain/antibody cocktail. After a brief incubation robotic sampling was employed for walk‐away flow cytometric data acquisition. PROAST benchmark dose (BMD) modeling was used to characterize the resulting dose–response curves. For each of the 8 reference pro‐genotoxicants studied, relative nuclei count, γH2AX, and/or p53 biomarker BMD values were order(s) of magnitude lower for 0.25% S9 conditions compared to 0% S9. Conversely, several of the direct‐acting reference chemicals exhibited appreciably lower cytotoxicity and/or genotoxicity BMD values in the presence of S9 (eg, resorcinol). These results prove the efficacy of the low concentration S9 system, and indicate that an efficient and highly scalable multiplexed assay can effectively identify chemicals that require bioactivation to exert their genotoxic effects.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154945/1/em22361_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154945/2/em22361.pd

Simultaneous Measurement of Benzo[a]pyrene-induced Pig-a and lacZ Mutations, Micronuclei and DNA Adducts in Muta™ Mouse

In this study we compared the response of the Pig-a gene mutation assay to that of the lacZ transgenic rodent mutation assay, and demonstrated that multiple endpoints can be measured in a 28-day repeat dose study. Muta™Mouse were dosed daily for 28 days with benzo[a]pyrene (BaP; 0, 25, 50 and 75 mg/kg body weight/day) by oral gavage. Micronucleus (MN) frequency was determined in reticulocytes (RETs) 48 hr following the last dose. 72 h following the last dose, mice were euthanized, and tissues (glandular stomach, small intestine, bone marrow and liver) were collected for lacZ mutation and DNA adduct analysis, and blood was evaluated for Pig-a mutants. BaP-derived DNA adducts were detected in all tissues examined and significant dose-dependent increases in mutant Pig-a phenotypes (i.e., RETCD24- and RBC CD24-) and lacZ mutants were observed. We estimate that mutagenic efficiency (i.e., rate of conversion of adducts into mutations) was much lower for Pig-a compared to lacZ, and speculate that this difference is likely explained by differences in repair capacity between the gene targets, and differences in the cell populations sampled for Pig-a versus lacZ. The BaP doubling doses for both gene targets, however, were comparable, suggesting that similar mechanisms are involved in the accumulation of gene mutations. Significant dose-related increases in % MN were also observed; however, the doubling dose was considerably higher for this endpoint. The similarity in dose response kinetics of Pig-a and lacZ provides further evidence for the mutational origin of glycosylphosphatidylinositol (GPI)-anchor deficiencies detected in the Pig-a assay. Environ. Mol. Mutagen. 2011. © 2011 Wiley-Liss, Inc

Benchmark dose analyses of multiple genetic toxicity endpoints permit robust, cross-tissue comparisons of MutaMouse responses to orally delivered benzo[a]pyrene.

Genetic damage is a key event in tumorigenesis, and chemically induced genotoxic effects are a human health concern. Although genetic toxicity data have historically been interpreted using a qualitative screen-and-bin approach, there is increasing interest in quantitative analysis of genetic toxicity dose-response data. We demonstrate an emerging use of the benchmark dose (BMD)-approach for empirically ranking cross-tissue sensitivity. Using a model environmental carcinogen, we quantitatively examined responses for four genetic damage endpoints over an extended dose range, and conducted cross-tissue sensitivity rankings using BMD100 values and their 90% confidence intervals (CIs). MutaMouse specimens were orally exposed to 11 doses of benzo[a]pyrene. DNA adduct frequency and lacZ mutant frequency (MF) were measured in up to 8 tissues, and Pig-a MF and micronuclei (MN) were assessed in immature (RETs) and mature red blood cells (RBCs). The cross-tissue BMD pattern for lacZ MF is similar to that observed for DNA adducts, and is consistent with an oral route-of-exposure and differences in tissue-specific metabolism and proliferation. The lacZ MF BMDs were significantly correlated with the tissue-matched adduct BMDs, demonstrating a consistent adduct conversion rate across tissues. The BMD CIs, for both the Pig-a and the MN endpoints, overlapped for RETs and RBCs, suggesting comparable utility of both cell populations for protracted exposures. Examination of endpoint-specific response maxima illustrates the difficulty of comparing BMD values for a fixed benchmark response across endpoints. Overall, the BMD-approach permitted robust comparisons of responses across tissues/endpoints, which is valuable to our mechanistic understanding of how benzo[a]pyrene induces genetic damage

Elevated frequencies of micronucleated erythrocytes in infants exposed to zidovudine in utero and postpartum to prevent mother-to-child transmission of HIV

Zidovudine-based antiretroviral therapies (ART) for treatment of HIV-infected pregnant women have markedly reduced mother-to-child transmission of the human immunodeficiency virus (HIV-1) from ~25% to <1%. However, zidovudine (ZDV; AZT), a nucleoside analogue, induces chromosomal damage, gene mutations, and cancer in animals following direct or transplacental exposures. To determine if chromosomal damage is induced by ZDV in infants exposed transplacentally, we evaluated micronucleated reticulocyte frequencies (%MN-RET) in 16 HIV-infected ART-treated mother-infant pairs. Thirteen women received prenatal ART containing ZDV; 3 received ART without ZDV. All infants received ZDV for 6 weeks postpartum. Venous blood was obtained from women at delivery, and from infants at 1–3 days, 4–6 weeks, and 4–6 months of life; cord blood was collected immediately after delivery. Ten cord blood samples (controls) were obtained from infants of HIV-uninfected women who did not receive ART. %MN-RET was measured using a single laser 3-color flow cytometric system. Ten-fold increases in %MN-RET were seen in women and infants who received ZDV-containing ART prenatally; no increases were detected in 3 women and infants who received prenatal ART without ZDV. Specifically, mean %MN-RET in cord blood of ZDV-exposed infants was 1.67±0.34 compared with 0.16±0.06 in non-ZDV ART-exposed infants (P=0.006) and 0.12±0.02 in control cord bloods (p<0.0001). %MN-RET in ZDV-exposed newborns decreased over the first 6 months of life to levels comparable to cord blood controls. These results demonstrate that transplacental ZDV exposure is genotoxic in humans. Long-term monitoring of HIV-uninfected ZDV-exposed infants is recommended to ensure their continued health

Building on the past, shaping the future: The environmental mutagenesis and genomics society

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97167/1/em21765.pd

Assessment of Systemic Genetic Damage in Pediatric Inflammatory Bowel Disease.

The etiology of distal site cancers in inflammatory bowel disease (IBD) is not well understood and requires further study. We investigated whether pediatric IBD patients' blood cells exhibit elevated levels of genomic damage by measuring the frequency of mutant phenotype (CD59-/CD55-) reticulocytes (MUT RET) as a reporter of PIG-A mutation, and the frequency of micronucleated reticulocytes (MN-RET) as an indicator of chromosomal damage. IBD patients (n = 18 new onset disease, 46 established disease) were compared to age-matched controls (constipation or irritable bowel syndrome patients from the same clinic, n = 30) and young healthy adults age 19 - 24 (n = 25). IBD patients showed no indication of elevated MUT RET relative to controls (mean ± std. dev. = 3.1 ± 2.3 x 10-6 versus 3.6 ± 5.6 x 10-6 , respectively). In contrast, of 59 IBD patients where %MN-RET measurements were obtained, 10 exceeded the upper bound 90% tolerance interval derived from control subjects (i.e., 0.42%). Furthermore, each of the 10 IBD patients with elevated MN-RET had established disease (10/42), none were new onset (0/17) (p = 0.049). Interestingly, each of the subjects with increased chromosomal damage was receiving anti-TNF based monotherapy at the time blood was collected (10/10, 100%), whereas this therapy was less common (20/32, 63%) among patients that exhibited ≤ 0.42% MN-RET (p = 0.040). The results clearly indicate the need for further work to understand whether the results presented herein are reproducible, and if so, to elucidate the causative factor(s) responsible for elevated MN-RET frequencies in some IBD patients

Enhanced Detection of β-Galactosidase Reporter Activation Is Achieved by a Reduction of Hemoglobin Content in Tissue Lysates

β-galactosidase (β-gal), the product of the E. coli LacZ gene, has been used extensively as a reporter in numerous systems. Until recently, the most commonly used method of detecting β-gal reporter enzymatic activity was a colormetric assay based on the cleavage of the β-gal substrate 5-bromo-4-chloro-3-indolyl β-d-galactopyranoside (X-gal) to form a blue precipitate. However, when increased sensitivity is needed, many investigators now turn to alternate substrates that produce fluorescent or luminescent products upon cleavage by β-gal. These products are much more easily quantified than X-gal. The luminescent and fluorometric assays work very well in cultured cells but are often less sensitive in whole tissue lysates. In this study, we have evaluated the sensitivity of a fluorescent and a luminescent substrate in whole tissue lysates cleared of red blood cells or washed with PBS only. We have found that both assays show increased low-end sensitivity in tissues with reduced levels of hemoglobin (Hb). Hb is apparently able to quench luminescent and, to a lesser degree, fluorescent reporter light emission. Therefore, steps should be taken to reduce Hb levels either by lysis, perfusion, or both to enhance the sensitivity of these assays