A Practical Approach, Second Edition=Ronald D. Ho.pdf

40 DEVELOPMENTAL REPRODUCTIVE TOXICOLOGY: A PRACTICAL APPROACH, SECOND EDITIONwhereas control cells remained as relatively undifferentiated, pluripotent cells. 222 The phenotypeof differentiated cells can also be transformed by transfection with proto-oncogenes. For example,adult human pigment epithelial cells acquire characteristics of neuronal cells when transfected withthe H-ras proto-oncogene. 223 Results from studies such as these suggest that mutations in developmentallyimportant genes could result in developmental abnormalities.Another useful method for studying effects of specific mutations is generation of transgenicmice carrying activated proto-oncogenes. Transgenic mice expressing high levels of the mos protooncogenetransgene in brain tissue exhibit degeneration of neurons, axons, and spiral ganglia, aswell as gliosis. These physiological abnormalities are accompanied by neurobehavioral anomalies,such as circling, head tilting, and head bobbing. 224 Expression of a mutated WT-1 tumor suppressorgene in transgenic mice caused abnormal development of kidney, mesothelium, heart, and lungs. 225Transgenic mice have been developed that lack functional HRas, KRas2, and N-Ras genes, resultingin tumors in all three mutants, with variable to midgestational deaths reported. In particular, KRastransgenic animals display CNS tissue effects. 1Although studies using transfected cells and transgenic mice are useful for demonstrating thatmutations in specific genes can cause developmental abnormalities, it would also be interesting todetermine if mutated proto-oncogenes are observed in animals or cells exposed to mutagens. Insummary, the role of mutation in the mechanistic process of developmental toxicants still remainsto be clarified. Questions regarding whether a significant level of mutation would occur in viableembryos and lead to teratogenic events are still at issue. Obviously, additional studies will berequired to separate correlation and phenomenology versus true mechanistic paths.F. Alterations in Gene Expression1. Retinoic AcidDevelopment occurs according to very specific and well-orchestrated patterns of gene expression.Hence, alterations in these expression patterns can result in serious adverse developmental consequences.Retinoic acids (RAs), the biologically active metabolites of Vitamin A, play an importantrole in controlling these synchronized expression patterns. Exogenous retinoids can be equallyeffective in disrupting these processes.Retinoids are well characterized developmental toxicants, with their potency being determinedby both kinetic and dynamic factors, including the timing of exposure, dose, and structural formof the retinoid under evaluation. 1 Structure-activity relationships established for retinoids havedemonstrated that an acidic polar terminus is essential for developmental toxicity 15,226 and thatgreater than a 1000-fold difference in potency between different retinoids can be observed. Manyof these differences appear to be due to kinetic differences resulting from changed elimination ratesand reduced affinity for cellular retinoid acid binding proteins. 227 Kinetic studies have shown thatthe area under the curve correlates better with RA teratogenicity than does the peak high dose. 228RA can produce many dysmorphogenic effects, such as truncation of the forebrain and posteriorizationof the hindbrain. Cross-species evaluations have revealed that these same effects are observedin mammals, birds, amphibia, and fishes.There are minimal strain and species differences when specific metabolites of RA (e.g., alltrans-RAor 13-cis-RA) are evaluated. However, species differences in metabolism of RA can affectthe form and type of retinoid at target sites and underscores the significance of kinetic studies indiscussing mechanistic differences in cross-species RA responses. 229,230Our knowledge of retinoic acid receptors indicates that they belong to the nuclear hormoneligand–dependent transcription-factor superfamily. This information has greatly facilitated ourmechanistic understanding of RA developmental toxicity 1 . Two classes of receptors have beenidentified: RARs and RXRs. 231 Each class of receptors has three receptor types: α, β, and γ; eachis encoded by a separate gene, and multiple isomers can be formed by differential promoter usage© 2006 by Taylor & Francis Group, LLC