SONIC HEDGEHOG AND CYCLOPAMINE INDUCED HOLOPROSENCEPHALY

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DESCRIPTION: (Adapted from the Applicant's Abstract) Cyclopamine is a teratogenic steroidal alkaloid which was isolated from the desert corn lily. Vertebrate embryos exposed to cyclopamine around the period of gastrulation develop brain and craniofacial malformations analogous to humans with holoprosencephaly. Holoprosencephaly has also been associated with loss of function mutations in the Sonic Hedgehog gene and abnormalities in cholesterol metabolism. Sonic Hedgehog is expressed in the notochord and prechordal plate and encodes a protein (SHH) that directs pattern formation in the ventral portion of the neural tube. SHH synthesis is followed by an autoproteolytic event whereby the amino-terminal cleavage product is attached to cholesterol, thereby affixed to the outer surface of the cell. Because cyclopamine is structurally similar to cholesterol, it is hypothesized that cyclopamine interferes with SHH-mediated signal transduction. Preliminary data support a model in which cyclopamine blocks SHH autoproteolytic processing. The effect of cyclopamine might be direct disruption of SHH association with cholesterol or indirect suppression of cholesterol biosynthesis. Hypocholesterolemic drugs (BM-15.766, AY-9944) which interfere with cholesterol biosynthesis produce similar teratogenic effects. The hypothesis will be tested by addressing the several aims. First, the effects of cyclopamine and hypocholesterolemic agents on SHH-mediated patterning events in the neural tubes of avian and hamster embryos will be determined. It is expected that each of these agents will induce abnormal morphological and biochemical patterns in the neural tube, comparable to those observed in SHH-deficient embryos. The teratogenic potential of various natural and synthetic structural analogues of cyclopamine will also be examined and the teratogenic potential of various natural and synthetic structural analogues of cyclopamine will be examined and the teratogenicity of all these agents will be correlated with their effects on SHH autoproteolysis and cholesterol biosynthesis, as measured in vitro. The goals are to elucidate the molecular events which underly the teratogenic effects of cyclopamine and related steroidal alkaloids, and to characterize the structural properties of teratogens which mediate these effects. The proposed research may have important environmental health implications since steroidal alkaloids are found in many agricultural products.