genetics to elucidate further the mechanisms by which chemicals cause developmental toxicity.
An example of a chemical interaction with a membrane receptor that leads to adverse developmental outcome is the case of class III antiarrhythmic drugs, such as dofetilide and almokalant. In rat embryos, these drugs block specific potassium channels in myocardial cells, presumably by high-affinity interactions with adrenergic or muscarinic receptors. This action on potassium flux leads to a profound bradycardia, which in turn progresses to malformations (probably through hypoxia in the affected structures) or death in utero (Webster et al. 1996). This example also illustrates the process of pathogenesis, as the molecular interaction produces an effect on embryonal organ function, which in turn affects the further development and viability of the embryo.
Binns et al. (1963) reported that an epidemic of cyclopia with associated holoprosencephaly in sheep was caused by teratogenic compounds present in the subalpine lily, Veratrum californicum. Subsequent work by Keeler and Binns (1968) showed that the active teratogenic agents in this plant were cyclopamine, its glycoside alkaloid X, jervine, and veratosine. Of these teratogenic agents, cyclopamine and jervine are the most active. In addition, it was noted that these two compounds closely resemble cholesterol in structure. In an early study, Roux and Aubry (1966) showed that alterations in cholesterol metabolism induced by AY-9944, an inhibitor of the final step in cholesterol synthesis, induced holoprosencephaly in rats. Almost 30 years then passed before additional insights were gained into the teratogenic mechanism of cyclopamine-induced holoprosencephaly.
In the 1990s, new data from several unrelated fields converged to suggest that cyclopamine-induced holoprosencephaly was caused by interference with cholesterol metabolism and SHH signaling. First, SHH is synthesized as a precursor that must be cleaved and covalently linked with cholesterol to be active (Roelink et al. 1995; Porter et al. 1996). Second, SHH is necessary and sufficient for patterning the ventral neural tube (Tanabe and Jessell 1996). Third, mutations in SHH were shown to cause holoprosencephaly in mice (Chiang et al. 1996). Fourth, holoprosencephaly associated with Smith-Lemli-Opitz syndrome results from a genetic decrease in Δ7-DHC (Δ7-dehydrocholesterol) reductase activity (Kelley et al. 1996; Tint et al. 1994). These discoveries, coupled with earlier studies linking cylopamine to holoprosencephaly, led to the hypothesis that cyclopamine causes holoprosencephaly by interfering with SHH signaling. Direct confirmation of this hypothesis came in 1998 from two independent labora-