RSH/SLO ("Smith-Lemli-Opitz") syndrome: historical, genetic, and developmental considerations

Inhibition of 7-dehydrocholesterol reductase by the teratogen AY9944: a rat model for Smith-Lemli-Opitz syndrome

Our aim is to verify the validity of a rat model proposed for Smith-Lemli-Opitz (SLO) syndrome, a developmental disorder characterized by a defect in 7-dehydrocholesterol-delta 7 (7DHC)-reductase and by facial dysmorphism close to the holoprosencephaly caused by the teratogen AY9944. We investigated the sterol profile in rats treated with AY9944 blocking 7DHC-reductase. AY9944 was given orally to rats on gestation day 3 (D3). The sera were sampled for kinetic data on D3, D6, D9, D12, and D21. Cholesterol was measured in parallel by the routine enzymatic method and by the gas chromatography/mass spectrometry (GC-MS) procedure used in SLO diagnosis. In addition to sterols, we dosed steroid hormones punctually on D4 and on D10, and examined D21 fetuses in other animals. The enzymatic method was not specific for cholesterol, and measured 70% pure 7DHC added to a normal serum. On D21, 77% live fetuses showed pituitary agenesis. Cholesterol was rapidly reduced by more than 50% on D6 involving an accumulation of 7DHC, 8DHC, and trienols, as identified in SLO-affected children. The most abundant 7DHC reached a maximum from D9 to D12, equaling cholesterol on D9 (11 mg/dl). On D10, the magnitudes of hypocholesterolemic and of 7DHC accumulation were found to be dose-dependent. Progesterone was reduced as early as 24 hr after treatment and dropped to 40% of the levels in the controls on D10, correlating to the decrease in cholesterolemia. This rat model reproduces the same biochemical perturbations as seen in SLO, strongly suggesting that aberrant sterols (7DHC, 8DHC, or nortrienol) may contribute to the developmental defects.

Abnormal cholesterol biosynthesis in sitosterolaemia and the Smith-Lemli-Opitz syndrome

We investigated the enzyme defects in two inherited disorders of cholesterol biosynthesis: sitosterolaemia and the Smith-Lemli-Opitz syndrome. In sitosterolaemic homozygotes, plasma plant sterols (sitosterol and campesterol) concentrations are elevated because of enhanced intestinal absorption and diminished removal. Underlying these changes is very low cholesterol biosynthesis to provide extra sterol for cell growth. Extremely reduced activities of HMG-CoA reductase, the rate-controlling enzyme for cholesterol biosynthesis, caused by deficient HMG-CoA reductase mRNA is responsible and is the suspected inherited abnormality. The Smith-Lemli-Opitz syndrome is caused by a block in the last reaction in the cholesterol biosynthetic pathway, the conversion of 7-dehydrocholesterol to cholesterol, which is catalysed by 7-dehydrocholesterol delta 7-reductase. As a result, low plasma and tissue cholesterol with high 7-dehydrocholesterol levels are found in homozygotes, who show characteristic phenotypes of mental retardation, facial dysmorphism, and organ and limb congenital anomalies. Similar biochemical findings are produced in rats fed BM 15,766, an inhibitor of 7-dehydrocholesterol delta 7-reductase. Interestingly, feeding cholesterol can suppress abnormal cholesterol biosynthesis and improve symptoms in homozygotes and rats fed BM 15,766.

Treatment of the cholesterol biosynthetic defect in Smith-Lemli-Opitz syndrome reproduced in rats by BM 15.766

BACKGROUND & AIMS

The Smith-Lemli-Opitz syndrome is a recessive inherited disorder characterized by neurological developmental defects and dysmorphic features with a defect in cholesterol synthesis at the conversion of 7-dehydrocholesterol to cholesterol. BM 15.766 inhibits 7-dehydrocholesterol-delta 7-reductase and reproduces the biochemical defect. The aim of this study was to investigate the effects of cholesterol, cholic acid, and lovastatin feeding on rats fed BM 15.766.

METHODS

Plasma cholesterol and 7-dehydrocholesterol concentrations were related to the hepatic 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase.

RESULTS

With the inhibitor treatment, plasma cholesterol concentrations decreased 67%; 7-dehydrocholesterol concentrations increased from trace to 17 mg/dL; and hepatic HMG-CoA reductase activity and messenger RNA levels were stimulated 74% and two times, respectively. In inhibitor-treated rats, feeding cholesterol increased plasma cholesterol concentrations 3.7 times, decreased 7-dehydrocholesterol concentrations 88%, and reduced elevated HMG-CoA reductase activity and messenger RNA levels 74% and 49%. Feeding cholic acid increased plasma cholesterol without reducing 7-dehydrocholesterol concentrations. The combination of cholic acid and cholesterol enhanced plasma cholesterol 9.5 times without decreasing 7-dehydrocholesterol levels. Feeding lovastatin depressed plasma cholesterol further without reducing 7-dehydrocholesterol levels.

CONCLUSIONS

Cholesterol is essential to correct abnormal cholesterol synthesis induced by BM 15.766 in rats by expanding the pool and inhibiting HMG-CoA reductase. Neither cholic acid nor lovastatin are effective separately, but cholic acid plus cholesterol may offer some additional benefit.

Increased first trimester nuchal translucency as a prenatal manifestation of Smith-Lemli-Opitz syndrome

Routine ultrasound examination at 11 weeks of gestation in a woman with no family history of genetic disease demonstrated increased accumulation of fluid in the fetal nuchal region. In view of the association of this defect with chromosomal abnormalities, fetal karyotyping was performed by chorion villus sampling and this demonstrated a normal 46,XY karyotype. Subsequent scans showed resolution of the nuchal fluid, and at the 20-week scan the fetal genitalia appeared to be female. Fetal blood sampling confirmed a normal male karyotype and fetoscopy confirmed the presence of female external genitalia. The parents elected to terminate the pregnancy, and postmortem findings were indicative of Smith-Lemli-Opitz syndrome. This was confirmed by the finding of increased levels of 7-dehydrocholesterol in cultured skin fibroblasts.

Correlation of severity and outcome with plasma sterol levels in variants of the Smith-Lemli-Opitz syndrome

OBJECTIVES

To determine whether type I and the more severe type II variant of Smith-Lemli-Opitz syndrome have the same metabolic defect and to learn which plasma sterol measurements best predict survival.

METHODS

Plasma sterols were measured in 33 individuals (24 type I, 9 type II) with a clinical diagnosis of the syndrome.

RESULTS

Cholesterol levels were abnormally low (61 +/- 34 mg/dl) in type I subjects, whereas concentrations of the cholesterol precursor 7-dehydrocholesterol and its isomer 8-dehydrocholesterol were elevated 40- to 10,000-fold. Plasma cholesterol levels were significantly lower and total dehydrocholesterol levels higher in type II than in type I. Six children with the type II variant died by 13 weeks with mean plasma cholesterol levels 6.2 +/- 3.1 mg/dl, versus 17 +/- 11 mg/dl in the three surviving children with type II (p < 0.05). No child with a cholesterol level 7 mg/dl or less lived longer than 13 weeks.

CONCLUSIONS

Patients with type I and type II variants of Smith-Lemli-Opitz syndrome have markedly reduced activity of the enzyme that converts 7-dehydrocholesterol to cholesterol, but the extent of the block is far more complete in type II. Survival correlates strongly with higher plasma cholesterol concentrations.

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.

Cholesterol metabolism in the RSH/Smith-Lemli-Opitz syndrome: summary of an NICHD conference

During the evolution of multicellularity and attendant processes of development, cholesterol played a key role in the formation of the plasma membrane and outer mitochondrial membrane of every cell in the organism. Later functions include pivotal involvement in steroid, bile acid, and vitamin D metabolism and myelination of the nervous system. In the CNS myelination does not begin until the third trimester, and subcortical myelination not until after birth. The cholesterol of the cell membrane of the ovum is maternally derived. It is not known when the zygote begins making its own cholesterol during morphogenesis and histogenesis, but it must occur early to keep up with the dramatic rate of cell division in the embryo. Thus, it is a startling surprise that human embryos and fetuses apparently able to synthesize little cholesterol (because of a presumed defect of the delta 5,7-sterol, delta 7-reductase that converts 7-dehydrocholesterol (7-DHC) into cholesterol) frequently live to term and, rarely, may be so mildly affected as to attend school with only mild MR. The discovery by G. Stephen Tint and his co-workers of the apparent 7-DHC reductase deficiency makes the RSH (Smith-Lemli-Opitz) syndrome the first true metabolic malformation syndrome. A teratological animal model which has been known for 30 years now appears applicable to the RSH/SLO syndrome. A multidisciplinary NICHD conference held on September 20-21, 1993 reviewed the numerous implications of this discovery and agreed unanimously that research in this field be given highest priority in order to better understand cholesterol synthesis in the mammalian brain, cholesterol transport from mother to embryo and fetus, pre- and postnatal metabolic compensation in structure and function for a profound block in cholesterol synthesis, the nature of the blood-brain barrier for cholesterol, treatment of affected infants, children, and adults, structure and genetic specification of a 7-DHC reductase enzyme (which has never been purified!) and its evolution, the variability of the syndrome and whether it is genetically homo- or heterogeneous, the population genetics of the RSH syndrome, possible selective advantages (or disadvantages) of heterozygotes, and means of newborn screening, carrier detection, and prenatal diagnosis.