Impact/Purpose:

Building Predictive Models for Hazard ID - Prioritization Method Development and Application

Description:

The EPA was recently mandated to evaluate the potential effects of chemicals on endocrine function and has identified Xenopus as a model organism to use as the basis for a thyroid disruption screening assay. The main objective of this work is to develop a hypothalamic-pituitary-thyroid (HPT) model for this bioassay. This model will provide a rational framework to organize and interpret toxicological data from the molecular to the organismal levels and will serve as a basis for development of predictive tools related to thyroid toxicity. Recent developments in understanding the molecular events involved in TH homeostasis and action suggest that thyroid toxicity might be identifiable using appropriate molecular endpoints in an abbreviated test. This potential improvement, which will lower costs and reduce animal use, is currently more likely to succeed due to expanding genomic information. This project will focus on building linkages between early molecular events associated with exposure and organismal-level effects. Understanding these linkages and their relative importance will be assessed using the HPT model. One of the major goals of this work is to populate the HPT model with data specifically at the molecular and biochemical levels which will provide a basis to interpret interspecies homology and comparative toxicity.

Record Details:

Record Type:PROJECT

Projected Completion Date:09/30/2008

OMB Category:Other

Record ID: 149126

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Project Information:

Progress :This is a New Start in the Computational Toxicology Research Program that began in January 2005. Past efforts have been focused on the development of a thyroid screening assay for OSCP. This assay, which is based primarily on whole organism responses, is proposed to last 14 - 21 days beginning in late pre-metamorphosis or early pro-metamorphosis using the amphibian, X. laevis. We have worked with three T4 synthesis inhibitors (6-propylthiouracil (PTU), perchlorate (PERC), and methimazole (METH)), two receptor agonists (T4 and T3), and a deiodinase inhibitor (iopanoic acid (IOP)) in an effort to test the responsiveness of this model system to chemicals with different modes of action. We have developed dose-response relationships for each of these chemicals. Interestingly, we have found that in the case of the T4 synthesis inhibitors, clear signs of thyroid system disruption, as evidenced by thyroid follicular cell hypertrophy and hyperplasia, occurs at concentrations below those which effect developmental rate. This observation suggests that the HPT executed a successful compensatory response that permitted normal development (Tietge et al, 2004; Degitz et al, 2004). More extreme glandular changes were observed at higher chemical concentrations accompanied by delayed development.

We have initiated studies to examine the changes in molecular and biochemical endpoints specific to components of the HPT system. As a prerequisite to understanding the changes associated with exposure to TH synthesis inhibitors, we have been determining the expression patterns of several genes throughout normal metamorphosis. TSH and type II deiodinase gene expression patterns in the pituitary, for example, have been developed and demonstrate patterns of up-regulation coincident with initiation of metamorphic climax These patterns of gene expression are critically important as they establish the baseline conditions against which changes induced by chemical exposure will be compared and interpreted.

Approach :Development of this assay is progressing, but its widespread use on Agency chemical inventories will be limited due to limited resources. As a consequence, a strategy to objectively rank and prioritize the order of chemical testing needs to be developed. One of the most likely uses for a HPT systems model is to aid in the understanding and discrimination of different toxic modes of action. As such, these models further enable the development of quantitative structure activity relationships (QSARs) by providing a basis for sorting chemicals by mode of action, a necessary step prior to quantifying features of chemical structure associated with a particular type of toxicity. If these relationships can ultimately be established, then predictive models can be developed to rank chemicals for future in vivo testing. In vivo testing for HPT effects will be improved through this research by providing a basis to link early molecular events to organismal outcomes.

There are four specific aims for this research. The first is to develop an HPT systems model which is capable of integrating data from different levels of biological organization, molecular to organismal, into a coherent system. The second specific aim is to develop an understanding of the compensatory mechanisms at the genomic level involved in TH homeostasis and how they respond to chemical perturbation. The third specific aim is to use in vitro models to help define the functions of component systems of the HPT system. Both pituitary and thyroid gland cultures will be used as experimental approaches to define tissue-specific outputs in response to endogenous and xenobiotic chemical inputs. The fourth specific aim is to use the emerging knowledge of the HPT system to develop usable, predictive toxicological tools. Once the input-output relationships are developed for the thyroid glands in culture, this work will be extended to specifically address chemicals known to inhibit T4 synthesis via disruption of iodine uptake by the sodium/iodide symporter (NIS).

Relevance :The primary benefit of this work is to develop a sufficient understanding of the HPT so that predictive models can be developed, testing protocols can be abbreviated, and efforts in inter-species extrapolation can be improved.

Project IDs:

ID Code :IA-4

Project type :Partner Specific