Reproducing abnormal cholesterol biosynthesis as seen in the Smith-Lemli-Opitz syndrome by inhibiting the conversion of 7-dehydrocholesterol to cholesterol in rats.

Abstract

The Smith-Lemli-Opitz syndrome is a recessive inherited disorder characterized by neurologic developmental defects and dysmorphic features in many organs. Recently, abnormal cholesterol biosynthesis with impaired conversion of 7-dehydrocholesterol to cholesterol has been discovered in homozygotes. To reproduce the biochemical abnormality, BM 15.766, a competitive inhibitor of 7-dehydrocholesterol-A7-reductase, the enzyme that catalyzes the conversion of 7-dehydrocholesterol into cholesterol was fed by gavage to rats. After 14 d, plasma cholesterol concentrations declined from 48 mg/dl to 16 mg/dl and 7-dehydro-cholesterol levels rose from trace to 17 mg/dl. Hepatocytes surrounding the central vein developed balloon necrosis. Stimulating cholesterol synthesis with cholestyramine followed by BM 15.766 produced an additional 40% decline (P < 0.05) in plasma cholesterol and 34% increase in 7-dehydrocholesterol levels compared to the inhibitor alone. Adding 2% cholesterol to the diet during the second week of BM 15.766 treatment increased plasma cholesterol threefold and decreased 7-dehydrocholesterol concentrations 55 %. Hepatic 3-hydroxy-3-methylglutaryl co-enzyme A (HMG-CoA) reductase activity increased 73% with a 3.9-fold rise in mRNA levels but cholesterol 7a-hydroxylase activity decreased slightly though mRNA levels increased 1.4 times with BM 15.766 treatment. These results demonstrate that BM 15.766 is a potent inhibitor of 7-dehydrocholesterol-A7-reductase. The model reproduces abnormal cholesterol biosynthesis as seen in the Smith-Lemli-Opitz syndrome and is useful to test different treatment strategies. Stimulating early steps of cholesterol synthesis worsens the biochemical abnormalities while feeding cholesterol inhibits abnormal synthesis, improves the biochemical abnormalities and prevents liver damage. (J. Clin. Invest. 1995. 95:76-81.)

Markedly inhibited 7-dehydrocholesterol-delta 7-reductase activity in liver microsomes from Smith-Lemli-Opitz homozygotes.

“…1) which is the penultimate reaction in the biosynthetic pathway (7,8,(11)(12)(13). In support of this reasoning, rats treated with BM 15.766, an inhibitor of 7-dehydrocholesterol-L\7-reductase became deficient in cholesterol and accumulated 7-dehydrocholesterol in all tissues reproducing the biochemical abnormalities as seen in SmithLemli-Opitz homozygotes (14,15).…”

A membrane defect in the pathogenesis of the Smith-Lemli-Opitz syndrome

“…Additional evidence supporting a defect in DHCR7 enzyme activity as the biochemical basis for clinical features of SLOS is found in the early (19) and more recent work (20) from Roux's group demonstrating that inhibitors of DHCR7 (AY 9944 and BM 15.766) produce teratogenic malformations and sterol abnormalities in rats similar to those seen in humans. Xu et al (21) have confirmed these findings with BM 15.766, and Honda et al (22) have extended these observations to human skin fibroblasts, in which BM 15.766 inhibited DHCR7 enzyme activity and resulted in the accumulation of 7-DHC and the depletion of cholesterol, thereby inducing SLOS at the cellular level. Because cholesterol is absolutely required for the biosynthesis of cell membranes, steroids, and sex hormones, the altered sterol synthesis in SLOS patients may underlie the widespread tissue and organ malformations.…”

“…However, treatment with cholesterolrich diets may have a greater benefit when given to infants than when initiated later in life, although improvement has been documented in adults with this dietary approach (50). In adult rats treated with BM 15.766, Xu et al (21) have shown that cholesterol feeding fully restores plasma cholesterol levels and reduces plasma 7-DHC levels by z50%. Further improvements were seen when the feeding protocol included cholic acid to improve hepatic bile formation or lovastatin to induce the upregulation of cell surface LDL levels.…”

“…Preliminary experiments demonstrated that this wash protocol removed extracellular 45 Ca 21 while retaining intracellular 45 Ca 21 and was equally effective in normal and SLOS cells. The cells were then lysed with SDS (1 mg/ml), and aliquots of the lysate were analyzed for protein content and radioactivity.…”

How and when the regional competence of chick epidermis is established: feathers vs. scutate and reticulate scales, a problem en route to a solution.

“…4F). One other method of blocking Shh signalling is by using the molecule BM 15 766, which inhibits the last step of cholesterol synthesis (Xu et al, 1995). By treating 7-day chick embryo by intra-amniotic injection (Prin, unpublished data), a few embryos survived until 11 days and were characterized by a generally small size.…”