Desmosterol in brain is elevated because DHCR24 needs REST for Robust Expression but REST is poorly expressed.

Cholesterol synthesis in the fetal brain is inhibited because activity of DHCR24 (24-dehydrocholesterol reductase) is insufficient, causing concentrations of the precursor desmosterol to increase temporarily to 15-25% of total sterols at birth. We demonstrate that failure of DHCR24 to be adequately upregulated during periods of elevated cholesterol synthesis in the brain results from the presence in its promoter of the repressor element 1 (RE1) nucleotide sequence that binds the RE1-silencing transcription factor (REST) and that REST, generally reduced in neural tissues, uncharacteristically but not without precedent, enhances DHCR24 transcription. DHCR24 and REST mRNA levels are reduced 3- to 4-fold in fetal mouse brain compared to liver (p < 0.001). Chromatin immunoprecipitation assays suggested that REST binds to the human DHCR24 promoter in the vicinity of the predicted human RE1 sequence. Luminescent emission from a human DHCR24 promoter construct with a mutated RE1 sequence was reduced 2-fold compared to output from a reporter with wild-type RE1 (p < 0.005). Silencing REST in HeLa cells resulted in significant reductions of DHCR24 mRNA (2-fold) and DHCR24 protein (4-fold). As expected, relative concentrations of Δ(24)-cholesterol precursor sterols increased 3- to 4-fold, reflecting the inhibition of DHCR24 enzyme activity. In contrast, mRNA levels of DHCR7 (sterol 7-dehydrocholesterol reductase), a gene essential for cholesterol synthesis lacking an RE1 sequence, and concentrations of HMGR (3-hydroxy-3-methyl-glutaryl-CoA reductase) enzyme protein were both unaffected. Surprisingly, a dominant negative fragment of REST consisting of just the DNA binding domain (about 20% of the protein) and full-length REST enhanced DHCR24 expression equally well. Furthermore, RE1 and the sterol response element (SRE), the respective binding sites for REST and the SRE binding protein (SREBP), are contiguous. These observations led us to hypothesize that REST acts because it is bound in close proximity to SREBP, thus amplifying its ability to upregulate DHCR24. It is likely that modulation of DHCR24 expression by REST persisted in the mammalian genome either because it does no harm or because suppressing metabolically active DHCR24 while providing abundant quantities of the multifunctional sterol desmosterol during neural development proved useful.

A biochemical abnormality in cerebrotendinous xanthomatosis. Impairment of bile acid biosynthesis associated with incomplete degradation of the cholesterol side chain.

Bile acid production in cerebrotendinous xanthomatosis (CTX) is subnormal, yet the activity of cholesterol 7alpha-hydroxylase, the rate-determining enzyme of bile acid synthesis, is elevated. To explain this discrepancy, bile acid precursors were sought in bile and feces of three CTX subjects. Over 10% of the total sterols excreted in bile and feces consisted of compounds more polar than cholesterol. Chromatographic analysis of the polar fractions in conjunction with gasliquid chromatography (GLC)-mass spectrometry indicated two major constituents, 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol and 5beta-cholestane-3alpha,7alpha,12alpha,24xi,25-pentol. After i.v. injection of [4-(14)C]cholesterol both bile alcohols were radioactive proving that they were derived from cholesterol. The accumulation of alcohols hydroxylated at C-25 and C-24,25 suggests that decreased bile acid synthesis in CTX results from impaired oxidation of the cholesterol side chain. This finding and the virtual absence of intermediates hydroxylated at C-26 indicate that current views of the major pathway of bile acid synthesis may require revision.

Increased formation of ursodeoxycholic acid in patients treated with chenodeoxycholic acid.

The formation of ursodeoxycholic acid, the 7 beta-hydroxy epimer of chenodeoxycholic acid, was investigated in three subjects with cerebrotendinous xanthomatosis and in four subjects with gallstones. Total biliary bile acid composition was analyzed by gas-liquid chromatography before and after 4 months of treatment with 0.75 g/day of chenodeoxycholic acid. Individual bile acids were identified by mass spectrometry. Before treatment, bile from cerebrotendinous xanthomatosis (CTX) subjects contained cholic acid, 85%; chenodeoxycholic acid, 7%; deoxycholic acid, 3%; allocholic acid, 3%; and unidentified steroids, 2%; while bile from gallstone subjects contained cholic acid, 45%; chenodeoxycholic acid, 43%; deoxycholic acid, 11%, and lithocholic acid, 1%. In all subjects, 4 months of chenodeoxycholic acid therapy increased the proportion of this bile acid to approximately 80% and decreased cholic acid to 3% of the total biliary bile acids, the remaining 17% of bile acids were identified as ursodeoxycholic acid. After the intravenous injection of [3H]chenodeoxycholic acid, the specific activity of biliary ursodeoxycholic acid exceeded the specific activity of chenodeoxycholic acid, and the resulting specific activity decay curves suggested precursor-product relationships. When [3H]7-ketolithocholic acid was administrated to another patient treated with chenodeoxycholic acid, radioactivity was detected in both the ursodeoxycholic acid and chenodeoxycholic acid fractions. These results indicate that substantial amounts of ursodeoxycholic acid are formed in patients treated with chenodeoxycholic acid. The ursodeoxycholic acid was synthesized from chenodeoxycholic acid presumably via 7-ketolithocholic acid.

Cholic acid biosynthesis: the enzymatic defect in cerebrotendinous xanthomatosis.

Cholic acid biosynthesis is defective in individuals with cerebrotendinous xanthomatosis (CTX) and is associated with the excretion of 5beta-cholestane-3alpha,7alpha, 12alpha,25-tetrol, an intermediate in the 25-hydroxylation pathway of cholic acid in CTX. To define the enzymatic defect in CTX, two suspected precursors of cholic acid, namely 5beta-[7beta-(3)H]cholestane-3alpha,7alpha, 12alpha-triol and 5beta-[24-(14)C]cholestane-3alpha,7alpha, 12alpha,24S,25-pentol were examined by both in vivo and in vitro experiments. A third precursor, 5beta-[7beta-(3)H]-cholestane-3alpha,7alpha, 12alpha,25-tetrol, was compared with them in vitro. In the in vivo experiments, each one of the labeled precursors was administered intravenously to two CTX and two control subjects. In the controls, 5beta-[7beta-(3)H]cholestane-3alpha,7alpha, 12alpha-triol as well as 5beta-[24-(14)C]-cholestane-3alpha,7alpha, 12alpha,24S,25-pentol were rapidly converted to labeled cholic acid. Maximum specific activity values were reached within 1 d after pulse labeling, followed by exponential decay of the cholic acid specific activity curves. In contrast, these two precursors differed widely when administered to two CTX patients. While 5beta-[24-(14)C]cholestane-3alpha,7alpha, 12alpha,24S,25-pentol was rapidly converted to [24-(14)C]cholic acid and yielded identical decay curves with those obtained in the control subjects, maximum specific activity values in [7beta-(3)H]cholic acid were much lower and peaked only on the second day after the injection of 5beta-[7beta-(3)H]cholestane-3alpha,7alpha, 12alpha-triol. Furthermore, an appreciable amount of (3)H label was present in the 5beta-cholestane-3alpha,7alpha, 12alpha,25-tetrol isolated from the bile of the subjects with CTX. In the in vitro experiments, three enzymes on the 25-hydroxylation pathway of cholic acid were examined in both control and CTX subjects. The rate of the 25-hydroxylation of 5beta-cholestane-3alpha,7alpha, 12alpha-triol in CTX patients was comparable to that in the controls. Similarly, the transformation of 5beta-cholestane-3alpha,7alpha, 12alpha,24S,25-pentol to cholic acid, catalyzed by soluble enzymes, proceeded at approximately equal rates in CTX and in control individuals. On the other hand, the rate of 5beta-cholestane-3alpha,7alpha, 12alpha,24S,25-pentol formation was about four times greater in the control subjects than in the CTX patients.The results of the in vivo as well as the in vitro experiments suggest that the site of the enzymatic defect in CTX is at the 24S-hydroxylation of 5beta-cholestane-3alpha,7alpha, 12alpha,25-tetrol. The relative deficiency of this hydroxylase in CTX patients, accompanied by the accumulation of its substrate in bile and feces, probably accounts for the subnormal production of bile acids in CTX patients.

Competitive inhibition of bile acid synthesis by endogenous cholestanol and sitosterol in sitosterolemia with xanthomatosis. Effect on cholesterol 7 alpha-hydroxylase.

The 7 alpha-hydroxylation of two cholesterol analogues, sitosterol and cholestanol, and their effect on the 7 alpha-hydroxylation of cholesterol were measured in rat and human hepatic microsomes. In untreated rat liver microsomes, the 7 alpha-hydroxylation of cholesterol was higher than that of cholestanol (1.4-fold) and sitosterol (30-fold). After removal of endogenous sterols from the microsomes by acetone treatment, the 7 alpha-hydroxylation of cholesterol was similar to that of cholestanol and only fourfold higher than that of sitosterol. Cholestanol and sitosterol competitively inhibited cholesterol 7 alpha-hydroxylase in both rat and human liver microsomes, with cholestanol the more potent inhibitor. Patients with sitosterolemia with xanthomatosis, who have elevated microsomal cholestanol and sitosterol, showed reduced cholesterol 7 alpha-hydroxylase activity relative to the activity in control subjects (13.9 and 14.7 vs. 20.3 +/- 0.9 pmol/nmol P-450 per min, P less than 0.01). Enzyme activity in these patients was 40% higher when measured in microsomes from which competing sterols had been removed. Ileal bypass surgery in one sitosterolemic patient decreased plasma cholestanol and sitosterol concentrations and resulted in a 30% increase in hepatic microsomal cholesterol 7 alpha-hydroxylase activity. Cholesterol 7 alpha-hydroxylase appears to have a specific apolar binding site for the side chain of cholesterol and is affected by the presence of cholestanol and sitosterol in the microsomal substrate pool. Reduced bile acid synthesis in sitosterolemia with xanthomatosis may be related to the inhibition of cholesterol 7 alpha-hydroxylase activity by endogenous cholesterol analogues.

Biosynthesis of bile acids in cerebrotendinous xanthomatosis. Relationship of bile acid pool sizes and synthesis rates to hydroxylations at C-12, C-25, and C-26.

To examine the defect in side-chain oxidation during the formation of bile acids in cerebrotendinous xanthomatosis, we measured in vitro hepatic microsomal hydroxylations at C-12 and C-25 and mitochondrial hydroxylation at C-26 and related them to the pool size and synthesis rates of cholic acid and chenodeoxycholic acid as determined by the isotope dilution technique. Hepatic microsomes and mitochondria were prepared from seven subjects with cerebrotendinous xanthomatosis and five controls. Primary bile acid synthesis was markedly reduced in cerebrotendinous xanthomatosis as follows: cholic acid, 133 +/- 30 vs. 260 +/- 60 mg/d in controls; and chenodeoxycholic acid, 22 +/- 10 vs. 150 +/- 30 mg/d in controls. As postulated for chenodeoxycholic acid synthesis, mitochondrial 26-hydroxylation of 5 beta-cholestane-3 alpha, 7 alpha-diol was present in all specimens and was 30-fold more active than the corresponding microsomal 25-hydroxylation. However, mean mitochondrial 26-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha-diol was less active in cerebrotendinous xanthomatosis than in controls: 59 +/- 17 compared with 126 +/- 21 pmol/mg protein per min. As for cholic acid synthesis, microsomal 25-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol was substantially higher in cerebrotendinous xanthomatosis and control preparations (620 +/- 103 and 515 +/- 64 pmol/mg protein per min, respectively) than the corresponding control mitochondrial 26-hydroxylation of the same substrate (165 +/- 25 pmol/mg protein per min). Moreover in cerebrotendinous xanthomatosis, mitochondrial 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol-26-hydroxylase activity was one-seventh as great as in controls. Hepatic microsomal 12 alpha-hydroxylation, which may be rate-controlling for the cholic acid pathway, was three times more active in cerebrotendinous xanthomatosis than in controls: 1,600 vs. 500 pmol/mg protein per min. These results demonstrate severely depressed primary bile acid synthesis in cerebrotendinous xanthomatosis with a reduction in chenodeoxycholic acid formation and pool size disproportionately greater than that for cholic acid. The deficiency of chenodeoxycholic acid can be accounted for by hyperactive microsomal 12 alpha-hydroxylation that diverts precursors into the cholic acid pathway combined with decreased side-chain oxidation (mitochondrial 26-hydroxylation). However, side-chain oxidation in cholic acid biosynthesis may be initiated via microsomal 25-hydroxylation of 5beta-cholestane-3alpha,7alpha,12alpha-triol was substantially lower in control and cerebrotendinous xanthomatosis liver. Thus, separate mechanisms may exist for the cleavage of the cholesterol side chain in cholic acid and chenodeoxycholic acid biosynthesis.

Feedback regulation of bile-acid synthesis in the rat. Differing effects of taurocholate and tauroursocholate.

We studied the effect of the orientation of the 7-hydroxyl group in taurocholate (7 alpha) and tauroursocholate (7 beta) on the feedback regulation of bile-acid synthesis and its rate-controlling enzyme, cholesterol 7 alpha-hydroxylase, in bile-fistula rats. To ensure a constant supply of cholesterol and to label newly synthesized bile acids, RS[2-14C]mevalonolactone was infused intraduodenally at 154 mumol/h before and during bile-acid infusion. Mevalonolactone inhibited hydroxymethyl-glutaryl CoA reductase activity 90% but did not increase bile-acid synthesis and cholesterol 7 alpha-hydroxylase activity. When sodium taurocholate was infused at the rate of 27 mumol/100 g rat per h (equivalent to the hourly hepatic bile-acid flux), bile-acid synthesis decreased 82% and cholesterol 7 alpha-hydroxylase activity declined 78%. This inhibitory effect was observed in the absence of hepatic damage. In contrast, sodium tauroursocholate infused at the same rate did not decrease bile-acid synthesis nor cholesterol 7 alpha-hydroxylase activity. Hepatic cholesterol content rose 36% with sodium taurocholate but did not change during sodium tauroursocholate administration. These results demonstrate that the feedback inhibition of bile-acid synthesis is mediated through the regulation of cholesterol 7 alpha-hydroxylase. In these experiments, taurocholate was a far more potent inhibitor than its 7 beta-hydroxy epimer, tauroursocholate.

A molecular defect in hepatic cholesterol biosynthesis in sitosterolemia with xanthomatosis.

We examined the relationship between cholesterol biosynthesis and total and high affinity LDL binding in liver specimens from two sitosterolemic and 12 healthy control subjects who died unexpectedly and whose livers became available when no suitable recipient for transplantation was identified. Accelerated atherosclerosis, unrestricted intestinal sterol absorption, increased plasma and tissue plant sterol concentrations, and low cholesterol synthesis characterize this disease. Mean total microsomal HMG-CoA reductase (rate-control controlling enzyme for cholesterol biosynthesis) activity was sevenfold higher (98.1 +/- 28.8 vs. 15.0 +/- 2.0 pmol/mg protein per min) and microsomal enzyme protein mass was eightfold larger (1.43 +/- 0.41 vs. 0.18 +/- 0.04 relative densitometric U/mg protein) in 11 controls than the average for two sitosterolemic liver specimens. HMG-CoA reductase mRNA probed with pRED 227 and pHRED 102 was decreased to barely detectable levels in the sitosterolemic livers. In addition, there was a 50% decrease in the rate [2-14C]mevalonic acid was converted to cholesterol by sitosterolemic liver slices compared with controls (112 vs. 224 +/- 32 pmol/g liver per h). In contrast, average total LDL binding was 60% greater (326 vs. 204 +/- 10 ng/mg), and high affinity (receptor-mediated) binding 165% more active (253 vs. 95.1 +/- 8.2 ng/mg) in two sitosterolemic liver membrane specimens than the mean for 12 controls. Liver morphology was intact although sitosterolemic hepatocytes and microsomes contained 24 and 14% less cholesterol, respectively, and 10-100 times more plant sterols and 5 alpha-stanols than control specimens. We postulate that inadequate cholesterol biosynthesis is an inherited abnormality in sitosterolemia and may be offset by augmented receptor-mediated LDL catabolism to supply cellular sterols that cannot be formed.

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

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-delta 7-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 7 alpha-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-delta 7-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.

A 25-hydroxylation pathway of cholic acid biosynthesis in man and rat.

This paper describes a pathway of cholic acid synthesis, in man and in the rat, which involves 25-hydroxylated intermediates and is catalyzed by microsomal and soluble enzymes. The subcellular localization, stereospecificity, and other properties of the enzymes involved were studied with liver fractions of normolipidemic subjects, cerebrotendinous xanthomatosis patients, and rats. 5beta-Cholestane-3alpha,7alpha,12alpha,25-tetrol was converted to 5beta-cholestane-3alpha,7alpha,12alpha,24beta,25-pentol by the microsomal fraction in the presence of NADPH and O2. 5beta-Cholestane-3alpha,7alpha,12alpha,24alpha,25-pentol, 5beta-cholestane-3alpha,7alpha,12alpha,-23xi,25-pentol, and 5beta-cholestane-3alpha,7alpha,12alpha,25,26-pentol were also formed. In the presence of NAD, 5beta-cholestane-3alpha,7alpha,12alpha,24beta,25-pentol, but not the other 5beta-cholestanepentols formed, was converted to cholic acid by soluble enzymes in good yield. These experiments demonstrate the existence of a pathway for side-chain degradation in cholic acid synthesis which does not involve hydroxylation at C-26 or the participation of mitochondria.

Biosynthesis of chenodeoxycholic acid in man: stereospecific side-chain hydroxylations of 5beta-cholestane-3alpha,7alpha-diol.

Stereospecific side-chain hydroxylations of 5beta-cholestane-3alpha, 7alpha-diol were studied in mitochondrial and microsomal fractions of human liver. Incubation of 5beta-cholestane-3alpha, 7alpha-diol resulted in hydroxylations at C-12, C-24, C-25, and C-26. Hydroxylations at C-24 and C-26 were accompanied by the introduction of additional asymmetric carbon atoms at C-24 and C-25 respectively, that led to the formation of two distinct pairs of diastereoisomers, namely 5beta-cholestane-3alpha, 7alpha,24-triols (24R and 24S) and 5beta-cholestane-3alpha, 7alpha,26-triols (25R and 25S). A sensitive and reproducible radioactive assay to measure the formation of the different biosynthetic 5beta-cholestanetriols was developed. Optimal assay conditions for human mitochondrial and microsomal systems were tentatively established.The mitochondrial fraction was found to predominantly catalyze the 26-hydroxylation of 5beta-cholestane-3alpha, 7alpha-diol with the formation of the 25R-diastereoisomer of 5beta-cholestane-3alpha, 7alpha,26-triol as the major product. In the microsomal fraction, on the other hand, 25-hydroxylation was more efficient than 26-hydroxylation and accounted for 6.4% of the total hydroxylations. The microsomes catalyzed the formation of both diastereoisomers of 5beta-cholestane-3alpha, 7alpha,26-triol (25R and 25S, 4.2 and 1.6% respectively). These experiments suggest that the initial step in the degradation of the steroid side chain during the biosynthesis of chenodeoxycholic acid in man is mediated by the mitochondria, and involves the formation of the 25R-diastereoisomer of 5beta-cholestane-3alpha, 7alpha,26-triol. The role of the microsomal 25- and 26-hydroxylated intermediates requires further exploration.

Abnormal high density lipoproteins in cerebrotendinous xanthomatosis.

The plasma lipoprotein profiles and high density lipoproteins (HDL) were characterized in patients with the genetic disease cerebrotendinous xanthomatosis (CTX). Abnormalities in the HDL may contribute to their increased atherogenesis and excessive deposits of tissue sterols in the presence of low or low-normal concentrations of plasma cholesterol (165 +/- 25 mg/dl) and low density lipoproteins (LDL). The mean HDL-cholesterol concentration in the CTX plasmas was 14.5 +/- 3.2 mg/dl, about one-third the normal value. The low HDL-cholesterol reflects a low concentration and an abnormal lipid composition of the plasma HDL. Relative to normal HDL, the cholesteryl esters are low, free cholesterol and phospholipids essentially normal, and triglycerides increased. The ratio of apoprotein (apo) to total cholesterol in the HDL of CTX was two to three times greater than normal. In the CTX HDL, the ratio of apoAI to apoAII was high, the proportion of apoC low, and a normally minor form of apoAI increased relative to other forms. The HDL in electron micrographs appeared normal morphologically and in particle size. The abnormalities in lipoprotein distribution profile and composition of the plasma HDL result from metabolic defects that are not understood but may be linked to the genetic defect in bile acid synthesis in CTX. As a consequence, it is probable that the normal functions of the HDL, possibly including modulation of LDL-cholesterol uptake and the removal of excess cholesterol from peripheral tissues, are perturbed significantly in this disease.

Increasing dietary cholesterol induces different regulation of classic and alternative bile acid synthesis.

We investigated the effect of increasing dietary cholesterol on bile acid pool sizes and the regulation of the two bile acid synthetic pathways (classic, via cholesterol 7alpha-hydroxylase, and alternative, via sterol 27-hydroxylase) in New Zealand white rabbits fed 3 g cholesterol/per day for up to 15 days. Feeding cholesterol for one day increased hepatic cholesterol 75% and cholesterol 7alpha-hydroxylase activity 1.6 times without significant change of bile acid pool size or sterol 27-hydroxylase activity. After three days of cholesterol feeding, the bile acid pool size increased 83% (P < 0.01), and further feeding produced 10%-20% increments, whereas cholesterol 7alpha-hydroxylase activity declined progressively to 60% below baseline. In contrast, sterol 27-hydroxylase activity rose 58% after three days of cholesterol feeding and remained elevated with continued intake. Bile drainage depleted the bile acid pool and stimulated downregulated cholesterol 7alpha-hydroxylase activity but did not affect sterol 27-hydroxylase activity. Thus, increasing hepatic cholesterol does not directly inhibit cholesterol 7alpha-hydroxylase and initially favors enzyme induction, whereas increased bile acid pool is the most powerful inhibitor of cholesterol 7alpha-hydroxylase. Sterol 27-hydroxylase is insensitive to the bile acid flux but is upregulated by increasing hepatic cholesterol.

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

We investigated the enzyme defect in late cholesterol biosynthesis in the Smith-Lemli-Opitz syndrome, a recessively inherited developmental disorder characterized by facial dysmorphism, mental retardation, and multiple organ congenital anomalies. Reduced plasma and tissue cholesterol with increased 7-dehydrocholesterol concentrations are biochemical features diagnostic of the inherited enzyme defect. Using isotope incorporation assays, we measured the transformation of the precursors, [3 alpha- 3H]lathosterol and [1,2-3H]7-dehydrocholesterol into cholesterol by liver microsomes from seven controls and four Smith-Lemli-Opitz homozygous subjects. The introduction of the double bond in lathosterol at C-5[6] to form 7-dehydrocholesterol that is catalyzed by lathosterol-5-dehydrogenase was equally rapid in controls and homozygotes liver microsomes (120 +/- 8 vs 100 +/- 7 pmol/mg protein per min, P = NS). In distinction, the reduction of the double bond at C-7 [8] in 7-dehydrocholesterol to yield cholesterol catalyzed by 7-dehydrocholesterol-delta 7-reductase was nine times greater in controls than homozygotes microsomes (365 +/- 23 vs 40 +/- 4 pmol/mg protein per min, P < 0.0001). These results demonstrate that the pathway of lathosterol to cholesterol in human liver includes 7-dehydrocholesterol as a key intermediate. In Smith-Lemli-Opitz homozygotes, the transformation of 7-dehydrocholesterol to cholesterol by hepatic microsomes was blocked although 7-dehydrocholesterol was produced abundantly from lathosterol. Thus, lathosterol 5-dehydrogenase is equally active which indicates that homozygotes liver microsomes are viable. Accordingly, microsomal 7-dehydrocholesterol-delta 7-reductase is inherited abnormally in Smith-Lemli-Opitz homozygotes.

Unexpected inhibition of cholesterol 7 alpha-hydroxylase by cholesterol in New Zealand white and Watanabe heritable hyperlipidemic rabbits.

We investigated the effect of cholesterol feeding on plasma cholesterol concentrations, hepatic activities and mRNA levels of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase and hepatic LDL receptor function and mRNA levels in 23 New Zealand White (NZW) and 17 Watanabe heritable hyperlipidemic (WHHL) rabbits. Plasma cholesterol concentrations were 9.9 times greater in WHHL than NZW rabbits and rose significantly in both groups when cholesterol was fed. Baseline liver cholesterol levels were 50% higher but rose only 26% in WHHL as compared with 3.6-fold increase with the cholesterol diet in NZW rabbits. In both rabbit groups, hepatic total HMG-CoA reductase activity was similar and declined > 60% without changing enzyme mRNA levels after cholesterol was fed. In NZW rabbits, cholesterol feeding inhibited LDL receptor function but not mRNA levels. As expected, receptor-mediated LDL binding was reduced in WHHL rabbits. Hepatic cholesterol 7 alpha-hydroxylase activity and mRNA levels were 2.8 and 10.4 times greater in NZW than WHHL rabbits. Unexpectedly, cholesterol 7 alpha-hydroxylase activity was reduced 53% and mRNA levels were reduced 79% in NZW rabbits with 2% cholesterol feeding. These results demonstrate that WHHL as compared with NZW rabbits have markedly elevated plasma and higher liver cholesterol concentrations, less hepatic LDL receptor function, and very low hepatic cholesterol 7 alpha-hydroxylase activity and mRNA levels. Feeding cholesterol to NZW rabbits increased plasma and hepatic concentrations greatly, inhibited LDL receptor-mediated binding, and unexpectedly suppressed cholesterol 7 alpha-hydroxylase activity and mRNA to minimum levels similar to WHHL rabbits. Dietary cholesterol accumulates in the plasma of NZW rabbits, and WHHL rabbits are hypercholesterolemic because reduced LDL receptor function is combined with decreased catabolism of cholesterol to bile acids.

7-Dehydrocholesterol-dependent proteolysis of HMG-CoA reductase suppresses sterol biosynthesis in a mouse model of Smith-Lemli-Opitz/RSH syndrome.

Smith-Lemli-Opitz/RSH syndrome (SLOS), a relatively common birth-defect mental-retardation syndrome, is caused by mutations in DHCR7, whose product catalyzes an obligate step in cholesterol biosynthesis, the conversion of 7-dehydrocholesterol to cholesterol. A null mutation in the murine Dhcr7 causes an identical biochemical defect to that seen in SLOS, including markedly reduced tissue cholesterol and total sterol levels, and 30- to 40-fold elevated concentrations of 7-dehydrocholesterol. Prenatal lethality was not noted, but newborn homozygotes breathed with difficulty, did not suckle, and died soon after birth with immature lungs, enlarged bladders, and, frequently, cleft palates. Despite reduced sterol concentrations in Dhcr7(-/-) mice, mRNA levels for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-controlling enzyme for sterol biosynthesis, the LDL receptor, and SREBP-2 appeared neither elevated nor repressed. In contrast to mRNA, protein levels and activities of HMG-CoA reductase were markedly reduced. Consistent with this finding, 7-dehydrocholesterol accelerates proteolysis of HMG-CoA reductase while sparing other key proteins. These results demonstrate that in mice without Dhcr7 activity, accumulated 7-dehydrocholesterol suppresses sterol biosynthesis posttranslationally. This effect might exacerbate abnormal development in SLOS by increasing the fetal cholesterol deficiency.

Normolipemic subcutaneous xanthomatosis.

A patient with diabetes mellitus is described in whom an unusual xanthomatosis developed involving large areas of the subcutaneous tissue and vocal cords. Few lesions were present on the skin. Plasma lipid, lipoprotein, apolipoprotein, and cholestanol levels revealed normal patterns. Electron microscopy showed macrophages with vacuolar and crystal lipid inclusions. Results of lipid and enzyme analysis of the subcutaneous xanthoma were similar to those of xanthomas derived from a patient with diabetes mellitus and type V hyperlipidemia. The mechanism of this xanthomatosis remains unknown.

Response to diet and cholestyramine in a patient with sitosterolemia.

In this report, an 11-year-old boy with diffuse tendinous and tuberous xanthomatosis and a plasma sterol concentration of 555 mg/dL, consisting primarily of cholesterol, is described. Three months after changing from an unrestricted diet to a cholesterol-lowering diet, his plasma sterol concentration decreased to 221 mg/dL. Because of the degree and rapidity of his response to diet, sitosterolemia was suspected. According to results of capillary gas-liquid chromatography of his plasma sterols, there was a sitosterol concentration of 31.3 mg/dL (normal less than 1.0 mg/dL), establishing the diagnosis of sitosterolemia. Addition of cholestyramine therapy (8 g/d) to a low sterol diet further lowered his plasma sterol concentration to 173 mg/dL and led to complete regression of all tuberous xanthomata. Tendinous xanthomata regressed at a slower rate. These findings show that the diagnosis of sitosterolemia should be suspected in severely hypercholesterolemic children (total cholesterol greater than 400 mg/dL) whose plasma cholesterol level is highly responsive to dietary manipulation. The rapid and sustained lowering of plasma cholesterol and regression of xanthomata after treatment with diet and cholestyramine suggest that sitosterolemia is a treatable cause of premature atherosclerosis.

Unusual case of Smith-Lemli-Opitz syndrome "type II".

We describe a fetus with abnormalities suggestive, but not typical, of severe Smith-Lemli-Opitz syndrome (SLO). Biochemical studies demonstrated that there was a defect of cholesterol biosynthesis similar to that recently discovered in children with SLO. The findings in this fetus extend even further the wide spectrum of abnormalities of the SLO phenotype, and emphasize that a genetic pathological examination and biochemical studies should always be undertaken on atypical cases, especially fetuses.

Pathogenesis of malformations in a rodent model for Smith-Lemli-Opitz syndrome.

The fact that Smith-Lemli-Opitz syndrome (SLOS), a syndrome comprising major malformations involving a number of organ systems, results from an abnormality in cholesterol biosynthesis, was discovered only recently. Utilizing a drug (BM 15.766) to inhibit the same step in cholesterol biosynthesis as is abnormal in those affected with SLOS, we have developed a rat model that presents with abnormalities observed as early as gestational day 12 that appear to be consistent with some of those subsequent malformations that comprise the human syndrome. Abnormalities of the brain and face include deficiency in the midline region of the upper face, narrowing of the forebrain hemispheres and of the cerebral aqueduct, and deficiency in the developing lower jaw. Associated pathogenesis, as observed on gestational day 11 in histological sections and with scanning electron microscopy, involves abnormal cell populations at the rim of the developing forebrain and in the alar plate of the lower midbrain and hind-brain. The affected cells appear abnormally rounded up, having apparently lost their normal cell contacts. The potential basis for the selective vulnerability of this cell population and the impact of its vulnerability relative to subsequent dysmorphogenesis is discussed.