NORMAL AND INHIBITED CHOLESTEROL-SYNTHESIS IN THE CULTURED RAT EMBRYO

The Smith-Lemli-Opitz syndrome-affected fetus presents a deficiency in Delta(7)-dehydrocholesterol reductase, the last enzymatic step in the cholesterol biosynthesis pathway. Development of the abnormal human f etus takes place in a normal environment as the heterozygous mother's cholesterolemia remains normal. An animal model for this disease has b een obtained from the offspring of pregnant rats treated with ''distal '' inhibitors of Delta(7)-dehydrocholesterol reductase, AY-9944 or BM1 5766. In the animal model, embryonic development occurs in a disturbed environment characterized by hypocholesterolemia and accumulation of Delta(7)-dehydrocholesterol and Delta(8)-dehydrocholesterol in the mat ernal serum. The purpose of the present study was to assess, in cultur ed rat embryos at early developmental stages, the relative contributio ns of exogenous and de novo synthesized cholesterol in the total embry onic cholesterol, according to the conditions of normal and altered de novo biosynthesis. Cultured rat embryos are able to synthesize choles terol as shown by C-13-incorporation into cholesterol from C-13-labele d precursors added to the culture medium. De novo cholesterol biosynth esis is altered by addition to the culture medium of AY-9944 which inh ibits the Delta(7)-dehydrocholesterol reductase and the Delta(8)-Delta (7)-sterol isomerase as suggested by the emergence of characteristic a berrant sterols in the embryonic tissues. Cholesterol-rich serum used for embryo culture alters the pattern in a way that confirms that the rat embryos are able to import exogenous cholesterol which down-regula tes de novo cholesterol biosynthesis. Exogenous cholesterol substitute s for the deficit in a manner efficient enough to prevent the embryoni c abnormalities induced by AY-9944.