Complex gene-chemical interactions: hepatic uroporphyria as a paradigm.

Many toxicological disorders, in common with numerous human diseases, are probably the consequence of multigene interactions with a variety of chemical and physiological factors. The importance of genetic factors may not be obvious initially from association studies because of their complexity and variable penetrance. The human disease, porphyria cutanea tarda (PCT), is a skin disease caused by the photosensitizing action of porphyrins arising secondary to the decreased activity of an enzyme of heme biosynthesis, uroporphyrinogen decarboxylase (UROD), in the liver. It is triggered by idiosyncratic hepatic interaction between genetic factors and chemicals such as alcohol, estrogenic drugs, and polyhalogenated aromatics. PCT and its animal models are known collectively as the hepatic uroporphyrias. There is strong evidence for the participation of iron in the pathogenesis of these conditions. Mouse models have been used to explore the relative importance of a variety of agents such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), alcohol, and iron in the development of uroporphyria and to elucidate the mechanism of the depression of hepatic UROD activity. Mutations of the UROD and hemochromatosis (HFE) genes are genetic factors in some PCT patients which can be mimicked in mice heterozygous for the Hfe and Urod null genes. Association studies of uroporphyria induced by TCDD or hexachlorobenzene with DNA markers in mouse intercrosses have shown the participation of other, unknown, genetic factors in addition to the strong influence of the Ahr gene. The pathogenesis of hepatic uroporphyrias exemplifies the complexity of the interactions between chemical and genetic factors that can contribute to the hepatotoxicity of chemicals.