The Toxicologist - Society of Toxicology

yet is induced in most bladder and breast tumors with correlation of expression to
the grade of the tumor in bladder cancer and a correlation to unfavorable clinical
outcome in breast cancer. In non-tumorigenic breast and bladder cell lines, MT-3
expression is also repressed and refractory to metal induction; however, during tumorigenesis,
the gene transitions from a dormant state, to an actively transcribed
gene. Initial investigations into the mechanism of oncogenic activation of MT-3
was fraught with difficulty due to the inability of MT-3 promoter-reporter constructs
to model expression in normal and transformed cell types. Using the epigenetic
modifying agents,5-azacytidine, and the histone deacetylating agent, MS-275,
the MT-3 gene can be activated in non-tumorigenic epithelial cells. The resulting
activated state allows for metal induction of MT-3 through the metal-regulatory elements
(MREs), previously thought to be inactive in MT-3 gene regulation in the
brain. This talk will detail studies on the epigenetic regulation of MT-3 expression
in normal, tumorigenic and metal-transformed cell types.
819 HUMAN VARIABILITY IN SUSCEPTIBILITY TO
ENVIRONMENTAL TOXICANTS.
H. Mortensen. NCCT, U.S. EPA, Research Triangle Park, NC.
Defining the differing levels of susceptibility across human populations in response
to environmental chemicals can provide information to define population risk factors
and in turn, allow for risk levels to be based on the most susceptible populations.
Data from high-throughput/high content (HT/HC) technologies, including
‘omics, have been integral in the identification and characterization of drug-target
or disease loci and have the potential to be informative for characterizing the effects
and dose-response assessment of chemical exposure and outcomes within genetically
heterogeneous populations. Many of the same ‘omics technologies have been
successfully utilized to provide data that informs the mechanism of action for environmental
chemicals, including the identification of perturbed toxicity pathways.
In addition, large scale population genotyping studies, such as the HapMap, can
help to establish levels of variability at chemical-associated, target loci across human
populations, and in comparison to genome-wide patterns. Individual genotypephenotype
combinations can then be verified using in vitro methods, or extrapolation
from animal models. This session includes talks on some of the latest approaches
to informing population variability and the identification of susceptible
populations through the use of HT/HC data, particularly from genomics technologies.
We will discuss how ‘omics data in combination with data from enhanced animal
models, publically available datasets and related computational tools can be
used to identify biomarkers, and subsequently define risk for genetically heterogeneous
populations, and how this variability translates to human risk and progression
of disease.
820 POPULATION-BASED DISCOVERY OF
TOXICOGENOMICS BIOMARKERS FOR
HEPATOTOXICITY.
I. Rusyn. Environmental Sciences and Engineering, University of North Carolina,
Chapel Hill, Chapel Hill, NC.
Current approaches to biomarker discovery through gene expression profiling usually
utilize a single or few strains of rodents, limiting the ability to detect biomarkers
that may represent the wide range of toxicity responses typically observed in genetically
heterogeneous human populations. To enhance the utility of animal
models to detect response biomarkers for genetically diverse populations, a genetically-defined
panel of mouse strains can be used. By taking into the account strainspecific
gene expression patterns it is possible to establish genetic polymorphismdependent
and -independent pathways perturbed by the toxicants. Since genetic
regulation of gene expression is a key contributor to population diversity, better understanding
of the relationship between toxicity responses and the individual’s genetic
background is needed.
821 EXPERIMENTALLY DEFINING TOXICITY PATHWAYS
USING IN VITRO HIGH-CONTENT SCREENING OF
EMBRYONIC FIBROBLASTS FROM THE MOUSE
DIVERSITY PANEL.
R. Thomas. Genomic Biology and Bioinformatics, The Hamner Institutes for Health
Sciences, Research Triangle Park, NC.
A key component of the vision laid out in the NRC report ‘Toxicity Testing in the
21st Century’ is the development of assays based on toxicity pathways. These toxicity
pathways were defined as cellular response pathways that, when sufficiently
perturbed, are expected to result in adverse health effects. Despite the importance
of these pathways, no strategy was put forth on how to identify them. In this presentation,
we describe the use of primary embryo fibroblasts isolated from 32 genetically
characterized inbred mouse strains to experimentally identify potential toxicity
pathways. The mouse embryo fibroblasts were exposed to one of a set of diverse
environmental and pharmaceutical agents at multiple doses and for two different
exposure intervals. The effects of exposure were assessed using high-content screening
assays for integrated cellular responses. Cross-strain differences in sensitivity to
the different treatments allowed genetic mapping to be performed on each endpoint,
resulting in a collection of candidate loci that underlie these cellular responses.
Enrichment analysis on these candidate loci has identified a number of
pathways that can serve as the basis for developing toxicity pathway assays.
822 HERITABILITY IN DOSE RESPONSE - PUTTING THE
HORSE BEFORE THE CART.
A. Motsinger-Reif. Department of Statistics, North Carolina State University,
Raleigh, NC. Sponsor: H. Mortensen.
The investigation of genetic and genomic factors that determine or predict differential
response to environmental exposures is a crucial goal of toxicogenomics, and
relies on the unstated and often untested assumption that this differential response
is a heritable trait. While limitations in traditional genetic study designs often prohibit
the estimation of the relative contribution of genetics to response traits in such
data, new approaches may allow for such estimates. For example, we demonstrate
an ex vivo model system to determine the heritability of drug-induced cell killing
and performed genome-wide analysis for quantitative trait loci for prioritizing drug
response outcomes. Such an approach could be useful in toxicogenomics for prioritizing
response outcomes for genetic investigation.
823 ASSOCIATION OF GENETIC POLYMORPHISMS, MRNA
EXPRESSION OF P53 AND P21 WITH CHRONIC
BENZENE POISONING IN A CHINESE
OCCUPATIONAL POPULATION.
N. Rothman. Division of Cancer Epidemiology and Genetics, National Cancer
Institute, Bethesda, MD. Sponsor: H. Mortensen.
Dr. Nat Rothman of the NCI will describe his work using genomic approaches to
study precursor and disease outcomes associated with benzene exposure in humans.
Specifically, he will discuss (1) using genetic polymorphisms of white blood
cell count (WBC) in a Chinese worker population to study the relationship between
hematologic outcomes associated with benzene exposure and the disease
pathway; (2) the toxicity pathway in periphera blood mononuclear cells for benzene
using toxicogenomic approaches. He will discuss the relationship between
these biomarkers and disease risk in the general population and the implications
for risk assessment.
824 PERSONALIZED MEDICINE, INDIVIDUALIZED DRUG
THERAPY: CAN THESE GOALS BE REALISTICALLY
ACHIEVED?
D. W. Nebert. Departments of Environmental Health & Pediatrics, Division of
Human Genetics, University Cincinnati Medical Center, Cincinnati, OH.
For each environmental toxicant, an identical exposure is well known to elicit a different
response between virtually any two individuals. From this standpoint,
human risk assessment is no different from “personalized medicine”, or its subset,
“individualized drug therapy”. Can any number of DNA variant sites (genotype)
ever be associated—-100% of the time—-with diagnosis of a complex disease such
as benzene-induced leukemia (phenotype)? Can any genotype ever predict, 100%
of the time, each patient’s response to a metal, chemical or drug (phenotype)?
Ideally, a 100% level of success in genotype-phenotype association studies is what
physicians’ desire for risk assessment, personalized medicine, or individualized drug
therapy to be successful. Several researchers have insisted, however, that the genome
is far too complex for us to understand at the present time.
Contributions by ENCODE, the HapMap Consortium, genome-wide association
(GWA) studies, and “third-generation DNA deep sequencing” have given us
valuable breakthroughs in understanding a few complex diseases, whereas many
others have yielded puzzling results. Comparing “monogenic”, high-penetrance
predominantly monogenic (hPpM) toxicogenetic disorders, and “complex” diseases—-the
percentage that one gene might contribute to the trait varies: >90%,
15%-25%, and 0.01%-1.0%, respectively. I advocate that adverse chemical reactions
are not different from complex diseases.
Recent studies show that 14, 20 or 95 loci significantly associated (P