Malformation syndromes due to inborn errors of cholesterol synthesis

Effective Parameters Controlling Sterol Transfer: A Time-Resolved Small-Angle Neutron Scattering Study

Though cholesterol is the most prevalent and essential sterol in mammalian cellular membranes, its precursors, post-synthesis cholesterol products, as well as its oxidized derivatives play many other important physiological roles. Using a non-invasive in situ technique, time-resolved small angle neutron scattering, we report on the rate of membrane desorption and corresponding activation energy for this process for a series of sterol precursors and post-synthesis cholesterol products that vary from cholesterol by the number and position of double bonds in B ring of cholesterol's steroid core. In addition, we report on sterols that have oxidation modifications in ring A and ring B of the steroid core. We find that sterols that differ in position or the number of double bonds in ring B have similar time and energy characteristics, while oxysterols have faster transfer rates and lower activation energies than cholesterol in a manner generally consistent with known sterol characteristics, like Log P, the n-octanol/water partitioning coefficient. We find, however, that membrane/water partitioning which is dependent on lipid-sterol interactions is a better predictor, shown by the correlation of the sterols' tilt modulus with both the desorption rates and activation energy.

Sterol Biosynthesis Inhibition in Pregnant Women Taking Prescription Medications

Sterol biosynthesis is a critical homeostatic mechanism of the body. Sterol biosynthesis begins during early embryonic life and continues throughout life. Many commonly used medications, prescribed >200 million times in the United States annually, have a sterol biosynthesis inhibition side effect. Using our high-throughput LC-MS/MS method, we assessed the levels of post-lanosterol sterol intermediates (lanosterol, desmosterol, and 7-dehydrocholesterol (7-DHC)) and cholesterol in 1312 deidentified serum samples from pregnant women. 302 samples showing elevated 7-DHC were analyzed for the presence of 14 medications known to inhibit the 7-dehydrocholesterol reductase enzyme (DHCR7) and increase 7-DHC. Of the 302 samples showing 7-DHC elevation, 43 had detectable levels of prescription medications with a DHCR7-inhibiting side effect. Taking more than one 7-DHC-elevating medication in specific combinations (polypharmacy) might exacerbate the effect on 7-DHC levels in pregnant women, suggesting a potentially additive or synergistic effect. As 7-DHC and 7-DHC-derived oxysterols are toxic, and as DHCR7-inhibiting medications are considered teratogens, our findings raise potential concerns regarding the use of prescription medication with a DHCR7-inhibiting side effect during pregnancy. The use of prescription medications during pregnancy is sometimes unavoidable, but choosing a medication without a DHCR7-inhibiting side effect might lead to a heathier pregnancy and prevent putatively adverse outcomes for the developing offspring.

Activation of WNT signaling restores the facial deficits in a zebrafish with defects in cholesterol metabolism

Inborn errors of cholesterol metabolism occur as a result of mutations in the cholesterol synthesis pathway (CSP). Although mutations in the CSP cause a multiple congenital anomaly syndrome, craniofacial abnormalities are a hallmark phenotype associated with these disorders. Previous studies have established that mutation of the zebrafish hmgcs1 gene (Vu57 allele), which encodes the first enzyme in the CSP, causes defects in craniofacial development and abnormal neural crest cell (NCC) differentiation. However, the molecular mechanisms by which the products of the CSP disrupt NCC differentiation are not completely known. Cholesterol is known to regulate the activity of WNT signaling, an established regulator of NCC differentiation. We hypothesized that defects in cholesterol synthesis are associated with reduced WNT signaling, consequently resulting in abnormal craniofacial development. To test our hypothesis we performed a combination of pharmaceutical inhibition, gene expression assays, and targeted rescue experiments to understand the function of the CSP and WNT signaling during craniofacial development. We demonstrate reduced expression of four canonical WNT downstream target genes in homozygous carriers of the Vu57 allele and reduced axin2 expression, a known WNT target gene, in larvae treated with Ro-48-8071, an inhibitor of cholesterol synthesis. Moreover, activation of WNT signaling via treatment with WNT agonist I completely restored the craniofacial defects present in a subset of animals carrying the Vu57 allele. Collectively, these data suggest interplay between the CSP and WNT signaling during craniofacial development.

Desmosterolosis and desmosterol homeostasis in the developing mouse brain

Cholesterol serves as a building material for cellular membranes and plays an important role in cellular metabolism. The brain relies on its own cholesterol biosynthesis, which starts during embryonic development. Cholesterol is synthesized from two immediate precursors, desmosterol and 7-dehydrocholesterol (7-DHC). Mutations in the DHCR24 enzyme, which converts desmosterol into cholesterol, lead to desmosterolosis, an autosomal recessive developmental disorder. In this study, we assessed the brain content of desmosterol, 7-DHC, and cholesterol from development to adulthood, and analyzed the biochemical, molecular, and anatomical consequences of Dhcr24 mutations on the sterol profile in a mouse model of desmosterolosis and heterozygous Dhcr24^+/- carriers. Our HPLC-MS/MS studies revealed that by P0 desmosterol almost entirely replaced cholesterol in the Dhcr24-KO brain. The greatly elevated desmosterol levels were also present in the Dhcr24-Het brains irrespective of maternal genotype, persisting into adulthood. Furthermore, Dhcr24-KO mice brains showed complex changes in expression of lipid and sterol transcripts, nuclear receptors, and synaptic plasticity transcripts. Cultured Dhcr24-KO neurons showed increased arborization, which was also present in the Dhcr24-KO mouse brains. Finally, we observed a shared pathophysiological mechanism between the mouse models of desmosterolosis and Smith-Lemli-Opitz syndrome (a genetic disorder of conversion of 7-DHC to cholesterol).

From Cannabinoids and Neurosteroids to Statins and the Ketogenic Diet: New Therapeutic Avenues in Rett Syndrome?

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene, being one of the leading causes of mental disability in females. Mutations in the MECP2 gene are responsible for 95% of the diagnosed RTT cases and the mechanisms through which these mutations relate with symptomatology are still elusive. Children with RTT present a period of apparent normal development followed by a rapid regression in speech and behavior and a progressive deterioration of motor abilities. Epilepsy is one of the most common symptoms in RTT, occurring in 60 to 80% of RTT cases, being associated with worsening of other symptoms. At this point, no cure for RTT is available and there is a pressing need for the discovery of new drug candidates to treat its severe symptoms. However, despite being a rare disease, in the last decade research in RTT has grown exponentially. New and exciting evidence has been gathered and the etiopathogenesis of this complex, severe and untreatable disease is slowly being unfolded. Advances in gene editing techniques have prompted cure-oriented research in RTT. Nonetheless, at this point, finding a cure is a distant reality, highlighting the importance of further investigating the basic pathological mechanisms of this disease. In this review, we focus our attention in some of the newest evidence on RTT clinical and preclinical research, evaluating their impact in RTT symptomatology control, and pinpointing possible directions for future research.

Methods for Monitoring ABCA1-Dependent Sterol Release

Releasing sterols to the extracellular milieu is an important part of sterol homeostasis in cells and in the body. ATP-binding cassette transporter A1 (ABCA1) plays an essential role in cellular phospholipid and sterol release to lipid-free or lipid-poor apolipoprotein A-I (apoA-I), the major apolipoprotein in high-density lipoprotein (HDL), and constitutes the first step in the formation of nascent HDL. Loss-of-function mutations in the ABCA1 gene lead to a rare disease known as Tangier disease that causes severe deficiency in plasma HDL level. Mammalian cells receive exogenous cholesterol mainly from low-density lipoprotein. In addition, they synthesize cholesterol endogenously, as well as multiple precursor sterols that are sterol intermediates en route to be converted to cholesterol. HDL contains phospholipids, cholesterol, and precursor sterols, and ABCA1 has an ability to release phospholipids and various sterol molecules. Recent studies using model cell lines showed that ABCA1 prefers to use sterols newly synthesized endogenously as its preferred substrate, rather than cholesterol derived from LDL or cholesterol being recycled within the cells. Here, we describe several methods at the cell culture level to monitor ABCA1-dependent release of sterol molecules to apoA-I present at the cell exterior. Sterol release can be assessed by using a simple colorimetric enzymatic assay, and/or by monitoring the radioactivities of radiolabeled cholesterol incorporated into the cells, and/or of sterols biosynthesized from radioactive acetate, and/or by using gas chromatography-mass spectrometry analysis of various sterols present in medium and in cells. We also discuss the pros and cons of these methods. Together, these methods allow researchers to detect the release not only of cholesterol but also of other sterols present in minor quantities.

Sterol Metabolism and Transport in Atherosclerosis and Cancer

Cholesterol is a vital lipid molecule for mammalian cells, regulating fluidity of biological membranes, and serving as an essential constituent of lipid rafts. Mammalian cells acquire cholesterol from extracellular lipoproteins and from de novo synthesis. Cholesterol biosynthesis generates various precursor sterols. Cholesterol undergoes metabolic conversion into oxygenated sterols (oxysterols), bile acids, and steroid hormones. Cholesterol intermediates and metabolites have diverse and important cellular functions. A network of molecular machineries including transcription factors, protein modifiers, sterol transporters/carriers, and sterol sensors regulate sterol homeostasis in mammalian cells and tissues. Dysfunction in metabolism and transport of cholesterol, sterol intermediates, and oxysterols occurs in various pathophysiological settings such as atherosclerosis, cancers, and neurodegenerative diseases. Here we review the cholesterol, intermediate sterol, and oxysterol regulatory mechanisms and intracellular transport machineries, and discuss the roles of sterols and sterol metabolism in human diseases.

Lipid metabolism fattens up hedgehog signaling

Signaling pathways direct organogenesis, often through concentration-dependent effects on cells. The hedgehog pathway enables cells to sense and respond to hedgehog ligands, of which the best studied is sonic hedgehog. Hedgehog signaling is essential for development, proliferation, and stem cell maintenance, and it is a driver of certain cancers. Lipid metabolism has a profound influence on both hedgehog signal transduction and the properties of the ligands themselves, leading to changes in the strength of hedgehog signaling and cellular functions. Here we review the evolving understanding of the relationship between lipids and hedgehog signaling.

Sequence analysis of four vitamin D family genes (VDR, CYP24A1, CYP27B1 and CYP2R1) in Vogt-Koyanagi-Harada (VKH) patients: identification of a potentially pathogenic variant in CYP2R1

BACKGROUND

VKH is a rare autoimmune disease. Decreased level of vitamin D has recently been found to be involved in the pathogenesis of Vogt-Koyanagi-Harada (VKH) disease. This study was designed to screen the vitamin D pathway genes for pathogenic mutations, if any, in VKH patients.

METHODS

Genomic DNA was extracted from blood samples collected from patients with VKH disease and healthy controls. Entire coding region, exon-intron junctions of four genes were sequenced in DNA from 39 Saudi VKH patients and 50 ethnically matched healthy individuals. All patients and controls were unrelated.

RESULTS

Vitamin D levels in VKH patients were found either insufficient (21-29 ng/mL) or deficient (<20 ng/mL). Sequencing analysis of the VDR, CYP24A1, CYP27B1 and CYP2R1 detected twelve nucleotide changes in these genes in our cohort of 39 patients; 4 of which were non-coding, 6 were synonymous coding and 2 were non-synonymous coding sequence changes. All synonymous coding variants were benign polymorphisms with no apparent clinical significance. A non-synonymous coding sequence variant (c.2 T > C; p.1Met?) found in VDR is an initiation coding change and was detected in control individuals as well, while another variant (c.852G > A; p.284 M > I) found in CYP2R1 is predicted to be disease causing by mutationtaster software. This potentially pathogenic variant was found in 17 out of 39 VKH patients.

CONCLUSIONS

Screening of four Vitamin D pathway genes in 39 VKH patients shows that a potentially pathogenic sequence variant in CYP2R1 may cause VKH in a subset of patients. These findings support the previous observation that low vitamin D levels might play a role in VKH pathogenesis and mutations in genes involved in vitamin D anabolism and catabolism might be of importance in VKH pathobiology.

ABCA1-dependent sterol release: sterol molecule specificity and potential membrane domain for HDL biogenesis

Mammalian cells synthesize various sterol molecules, including the C30 sterol, lanosterol, as cholesterol precursors in the endoplasmic reticulum. The build-up of precursor sterols, including lanosterol, displays cellular toxicity. Precursor sterols are found in plasma HDL. How these structurally different sterols are released from cells is poorly understood. Here, we show that newly synthesized precursor sterols arriving at the plasma membrane (PM) are removed by extracellular apoA-I in a manner dependent on ABCA1, a key macromolecule for HDL biogenesis. Analysis of sterol molecules by GC-MS and tracing the fate of radiolabeled acetate-derived sterols in normal and mutant Niemann-Pick type C cells reveal that ABCA1 prefers newly synthesized sterols, especially lanosterol, as the substrates before they are internalized from the PM. We also show that ABCA1 resides in a cholesterol-rich membrane domain resistant to the mild detergent, Brij 98. Blocking ACAT activity increases the cholesterol contents of this domain. Newly synthesized C29/C30 sterols are transiently enriched within this domain, but rapidly disappear from this domain with a half-life of less than 1 h. Our work shows that substantial amounts of precursor sterols are transported to a certain PM domain and are removed by the ABCA1-dependent pathway.

Novel activities of CYP11A1 and their potential physiological significance

CYP11A1, found only in vertebrates, catalyzes the first step of steroidogenesis where cholesterol is converted to pregnenolone. The purified enzyme, also converts desmosterol and plant sterols including campesterol and β-sitosterol, to pregnenolone. Studies, initially with purified enzyme, reveal that 7-dehydrocholesterol (7DHC), ergosterol, lumisterol 3, and vitamins D3 and D2 also serve as substrates for CYP11A1, with 7DHC being better and vitamins D3 and D2 being poorer substrates than cholesterol. Adrenal glands, placenta, and epidermal keratinocytes can also carry out these conversions and 7-dehydropregnenolone has been detected in the epidermis, adrenal glands, and serum, and 20-hydroxyvitamin D3 was detected in human serum and the epidermis. Thus, this metabolism does appear to occur in vivo, although its quantitative importance and physiological role remain to be established. CYP11A1 action on 7DHC in vivo is further supported by detection of Δ(7)steroids in Smith-Lemli-Opitz syndrome patients. The activity of CYP11A1 is affected by the structure of the substrate with sterols having steroidal or Δ(7)-steroidal structures undergoing side chain cleavage following hydroxylations at C22 and C20. In contrast, metabolism of vitamin D involves sequential hydroxylations that start at C20 but do not lead to cleavage. Molecular modeling using the crystal structure of CYP11A1 predicts that other intermediates of cholesterol synthesis could also serve as substrates for CYP11A1. Finally, CYP11A1-derived secosteroidal hydroxy-derivatives and Δ(7)steroids are biologically active when administered in vitro in a manner dependent on the structure of the compound and the lineage of the target cells, suggesting physiological roles for these metabolites. This article is part of a special issue entitled 'SI: Steroid/Sterol signaling'.

Feeding impairments associated with plasma sterols in Smith-Lemli-Opitz syndrome

OBJECTIVE

To quantitatively evaluate feeding impairment in children with Smith-Lemli-Opitz syndrome (SLOS) and to correlate feeding impairment with clinical and biochemical indices of disease severity.

STUDY DESIGN

The study subjects were 26 children with SLOS ranging in age from 0.4 to 19 years. Clinical severity was measured using an existing scoring system. We created a tool to quantitatively evaluate feeding. Plasma sterol concentrations were measured, and statistical associations (correlations) with feeding scores were calculated.

RESULTS

Oral hyposensitivity or hypersensitivity, adverse behaviors, and risk for dysphagia were seen in ∼65% of the children with SLOS. Thirteen of the 26 children experienced failure to thrive, and 10 children required gastrostomy. Plasma concentration of 7-dehydrocholesterol, as a measure of severity, was correlated with total feeding score and oral function subcategory score (P < .001) and less so with oral structure score, adverse behaviors, or dysphagia. Correlations with cholesterol concentrations were less statistically significant. A plasma 7-dehydrocholesterol concentration >0.24 mmol/L or cholesterol concentration <1.95 mmol/L was predictive of gastrostomy tube use. Feeding impairments may improve with age.

CONCLUSION

Feeding impairment is common and complex in patients with SLOS. Our findings confirm that oral sensitivities, adverse feeding behaviors, and risk of oral phase dysphagia are amenable to quantitative evaluation and analysis. Feeding difficulties in children with SLOS are correlated with plasma sterol concentrations, suggesting a link between the biochemical severity of SLOS and feeding function. These findings expand the behavioral phenotype of SLOS and begin to provide insight into the biological causes of feeding difficulties.

Steroidal triterpenes of cholesterol synthesis

Cholesterol synthesis is a ubiquitous and housekeeping metabolic pathway that leads to cholesterol, an essential structural component of mammalian cell membranes, required for proper membrane permeability and fluidity. The last part of the pathway involves steroidal triterpenes with cholestane ring structures. It starts by conversion of acyclic squalene into lanosterol, the first sterol intermediate of the pathway, followed by production of 20 structurally very similar steroidal triterpene molecules in over 11 complex enzyme reactions. Due to the structural similarities of sterol intermediates and the broad substrate specificity of the enzymes involved (especially sterol-Δ²⁴-reductase; DHCR24) the exact sequence of the reactions between lanosterol and cholesterol remains undefined. This article reviews all hitherto known structures of post-squalene steroidal triterpenes of cholesterol synthesis, their biological roles and the enzymes responsible for their synthesis. Furthermore, it summarises kinetic parameters of enzymes (V_max and K_m) and sterol intermediate concentrations from various tissues. Due to the complexity of the post-squalene cholesterol synthesis pathway, future studies will require a comprehensive meta-analysis of the pathway to elucidate the exact reaction sequence in different tissues, physiological or disease conditions. A major reason for the standstill of detailed late cholesterol synthesis research was the lack of several steroidal triterpene standards. We aid to this efforts by summarizing commercial and laboratory standards, referring also to chemical syntheses of meiosis-activating sterols.

Effects of diet and development on the Drosophila lipidome

Cells produce tens of thousands of different lipid species, but the importance of this complexity in vivo is unclear. Analysis of individual tissues and cell types has revealed differences in abundance of individual lipid species, but there has been no comprehensive study comparing tissue lipidomes within a single developing organism. Here, we used quantitative shotgun profiling by high-resolution mass spectrometry to determine the absolute (molar) content of 250 species of 14 major lipid classes in 6 tissues of animals at 27 developmental stages raised on 4 different diets. Comparing these lipidomes revealed unexpected insights into lipid metabolism. Surprisingly, the fatty acids present in dietary lipids directly influence tissue phospholipid composition throughout the animal. Furthermore, Drosophila differentially regulates uptake, mobilization and tissue accumulation of specific sterols, and undergoes unsuspected shifts in fat metabolism during larval and pupal development. Finally, we observed striking differences between tissue lipidomes that are conserved between phyla. This study provides a comprehensive, quantitative and expandable resource for further pharmacological and genetic studies of metabolic disorders and molecular mechanisms underlying dietary response.

Mutational spectrum of smith-lemli-opitz syndrome patients in hungary

Smith-Lemli-Opitz (SLO) syndrome is an autosomal recessive disorder characterized by multiple congenital abnormalities and mental retardation. The condition is caused by the deficiency of 7-dehydrocholesterol reductase (DHCR7) which catalyzes the final step in cholesterol biosynthesis. Biochemical diagnosis is based on increased concentration of 7-dehydrocholesterol (7-DHC) in the patient serum. Both life expectancy and quality of life are severely affected by the disease. The estimated prevalence of SLO syndrome ranges between 1:20,000 and 1:40,000 among Caucasians. Although the mutational spectrum of the disease is wide, approximately 10 mutations are responsible for more than 80% of the cases. These mutations show a large interethnic variability. There are no mutation distribution data from Hungary to date. Thirteen patients were diagnosed with SLO syndrome in our laboratory. As first-line tests, serum 7-DHC and total cholesterol were measured and, in positive cases, molecular genetic analysis of the DHCR7 gene was performed. Complete genetic background of the disease could be identified in 12 cases. In 1 case only 1 mutation was detected in a heterozygote form. One patient was homozygous for the common splice site mutation c.964-1G>C, while all other patients were compound heterozygotes. One novel missense mutation, c.374A>G (p.Tyr125Cys) was identified.

Influence of fatty acids in maternal diet on atherogenesis in offspring of LDL receptor-deficient mice

AIMS

The present study investigated the effect of a maternal diet rich in omega-6 (E6D) or omega-9 (E9D) on atherogenesis in the offspring of mice.

MAIN METHODS

LDL receptor-deficient mice were fed a diet rich in either omega-6 (E6D) or omega-9 (E9D) for 45 days prior to mating and until the birth of the offspring, evaluating the effect on the offspring aorta in comparison to a standard diet (STD), by immunohistochemical analysis, morphometric analysis and electron microscopy.

KEY FINDINGS

Hypercholesterolemic female mice fed E6D generated offspring with high levels of total cholesterol, triglycerides (TG) and CC-chemokine ligand 2/monocyte chemoattractant protein 1 (CCL2/ MCP-1) as well as a reduction in high-density lipoprotein. The ascending aorta of these animals exhibited an increase in arterial wall thickness as well as increased expression of CCL2/MCP-1 and vascular cell adhesion molecule 1. The ultrastructural analysis revealed severe alterations in endothelial cells. The offspring from mothers fed E9D exhibited a reduction in TG and an increase in low-density lipoprotein. The ultrastructural analysis revealed a well-preserved aortic endothelium in these animals.

SIGNIFICANCE

The results suggest that hypercholesterolemic mothers feed a diet rich in omega-6 predispose their offspring to endothelial dysfunction.

Hypothesis: the role of sterols in autism spectrum disorder

A possible role for sterols in the development of autism spectrum disorder (ASD) has not been proven, but studies in disorders of sterol biosynthesis, chiefly Smith-Lemli-Opitz syndrome (SLOS), enable hypotheses on a causal relationship to be discussed. Advances in genetic technology coupled with discoveries in membrane physiology have led to renewed interest for lipids in the nervous system. This paper hypothesizes on the role of sterol dysfunction in ASD through the framework of SLOS. Impaired sonic hedgehog patterning, alterations in membrane lipid rafts leading to abnormal synaptic plasticity, and impaired neurosteroid synthesis are discussed. Potential therapeutic agents include the development of neuroactive steroid-based agents and enzyme-specific drugs. Future investigations should reveal the specific mechanisms underlying sterol dysfunction in neurodevelopmental disorders by utilizing advanced imaging and molecular techniques.

Malformation syndromes caused by disorders of cholesterol synthesis

Cholesterol homeostasis is critical for normal growth and development. In addition to being a major membrane lipid, cholesterol has multiple biological functions. These roles include being a precursor molecule for the synthesis of steroid hormones, neuroactive steroids, oxysterols, and bile acids. Cholesterol is also essential for the proper maturation and signaling of hedgehog proteins, and thus cholesterol is critical for embryonic development. After birth, most tissues can obtain cholesterol from either endogenous synthesis or exogenous dietary sources, but prior to birth, the human fetal tissues are dependent on endogenous synthesis. Due to the blood-brain barrier, brain tissue cannot utilize dietary or peripherally produced cholesterol. Generally, inborn errors of cholesterol synthesis lead to both a deficiency of cholesterol and increased levels of potentially bioactive or toxic precursor sterols. Over the past couple of decades, a number of human malformation syndromes have been shown to be due to inborn errors of cholesterol synthesis. Herein, we will review clinical and basic science aspects of Smith-Lemli-Opitz syndrome, desmosterolosis, lathosterolosis, HEM dysplasia, X-linked dominant chondrodysplasia punctata, Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects Syndrome, sterol-C-4 methyloxidase-like deficiency, and Antley-Bixler syndrome.

Common genetic determinants of vitamin D insufficiency: a genome-wide association study

BACKGROUND

Vitamin D is crucial for maintenance of musculoskeletal health, and might also have a role in extraskeletal tissues. Determinants of circulating 25-hydroxyvitamin D concentrations include sun exposure and diet, but high heritability suggests that genetic factors could also play a part. We aimed to identify common genetic variants affecting vitamin D concentrations and risk of insufficiency.

METHODS

We undertook a genome-wide association study of 25-hydroxyvitamin D concentrations in 33 996 individuals of European descent from 15 cohorts. Five epidemiological cohorts were designated as discovery cohorts (n=16 125), five as in-silico replication cohorts (n=9367), and five as de-novo replication cohorts (n=8504). 25-hydroxyvitamin D concentrations were measured by radioimmunoassay, chemiluminescent assay, ELISA, or mass spectrometry. Vitamin D insufficiency was defined as concentrations lower than 75 nmol/L or 50 nmol/L. We combined results of genome-wide analyses across cohorts using Z-score-weighted meta-analysis. Genotype scores were constructed for confirmed variants.

FINDINGS

Variants at three loci reached genome-wide significance in discovery cohorts for association with 25-hydroxyvitamin D concentrations, and were confirmed in replication cohorts: 4p12 (overall p=1.9x10(-109) for rs2282679, in GC); 11q12 (p=2.1x10(-27) for rs12785878, near DHCR7); and 11p15 (p=3.3x10(-20) for rs10741657, near CYP2R1). Variants at an additional locus (20q13, CYP24A1) were genome-wide significant in the pooled sample (p=6.0x10(-10) for rs6013897). Participants with a genotype score (combining the three confirmed variants) in the highest quartile were at increased risk of having 25-hydroxyvitamin D concentrations lower than 75 nmol/L (OR 2.47, 95% CI 2.20-2.78, p=2.3x10(-48)) or lower than 50 nmol/L (1.92, 1.70-2.16, p=1.0x10(-26)) compared with those in the lowest quartile.

INTERPRETATION

Variants near genes involved in cholesterol synthesis, hydroxylation, and vitamin D transport affect vitamin D status. Genetic variation at these loci identifies individuals who have substantially raised risk of vitamin D insufficiency.

FUNDING

Full funding sources listed at end of paper (see Acknowledgments).

Age-dependent increase in desmosterol restores DRM formation and membrane-related functions in cholesterol-free DHCR24-/- mice

Cholesterol is a prominent modulator of the integrity and functional activity of physiological membranes and the most abundant sterol in the mammalian brain. DHCR24-knock-out mice lack cholesterol and accumulate desmosterol with age. Here we demonstrate that brain cholesterol deficiency in 3-week-old DHCR24^−/− mice was associated with altered membrane composition including disrupted detergent-resistant membrane domain (DRM) structure. Furthermore, membrane-related functions differed extensively in the brains of these mice, resulting in lower plasmin activity, decreased β-secretase activity and diminished Aβ generation. Age-dependent accumulation and integration of desmosterol in brain membranes of 16-week-old DHCR24^−/− mice led to the formation of desmosterol-containing DRMs and rescued the observed membrane-related functional deficits. Our data provide evidence that an alternate sterol, desmosterol, can facilitate processes that are normally cholesterol-dependent including formation of DRMs from mouse brain extracts, membrane receptor ligand binding and activation, and regulation of membrane protein proteolytic activity. These data indicate that desmosterol can replace cholesterol in membrane-related functions in the DHCR24^−/− mouse.

Prolactin receptor-associated protein/17beta-hydroxysteroid dehydrogenase type 7 gene (Hsd17b7) plays a crucial role in embryonic development and fetal survival

Our laboratory has previously cloned and purified a protein named PRAP (prolactin receptor-associated protein) that was shown to be a novel 17beta-hydroxysteroid dehydrogenase (HSD) enzyme with dual activity. This enzyme, renamed HSD17B7 or PRAP/17beta-HSD7, converts estrone to estradiol and is also involved in cholesterol biosynthesis. The major site of its expression is the corpus luteum of a great number of species including rodents and humans. To examine the functional significance of HSD17B7 in pregnancy, we generated a knockout mouse model with targeted deletions of exons 1-4 of this gene. We anticipated a mouse with a severe fertility defect due to its inability to regulate estrogen levels during pregnancy. The heterozygous mutant mice are normal in their development and gross anatomy. The females cycle normally, and both male and female are fertile with normal litter size. To our surprise, the breeding of heterozygous mice yielded no viable HSD17B7 null mice. However, we found HSD17B7 null embryo alive in utero on d 8.5 and d 9.5. By d 10.5, the fetuses grow and suffer from severe brain malformation and heart defect. Because the brain depends on in situ cholesterol biosynthesis for its development beginning at d 10, the major cause of fetal death appears to be due to the cholesterol synthetic activity of this enzyme. By ablating HSD17B7 function, we have uncovered, in vivo, an important requirement for this enzyme during fetal development.

Inability to fully suppress sterol synthesis rates with exogenous sterol in embryonic and extraembyronic fetal tissues

The requirement for cholesterol is greater in developing tissues (fetus, placenta, and yolk sac) as compared to adult tissues. Here, we compared cholesterol-induced suppression of sterol synthesis rates in the adult liver to the fetal liver, fetal body, placenta, and yolk sac of the Golden Syrian hamster. Sterol synthesis rates were suppressed maximally in non-pregnant adult livers when cholesterol concentrations were increased. In contrast, sterol synthesis rates were suppressed only marginally in fetal livers, fetal bodies, placentas, and yolk sacs when cholesterol concentrations were increased. To begin to elucidate the mechanism responsible for the blunted response of sterol synthesis rates in fetal tissues to exogenous cholesterol, the ratio of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) to Insig-1 was measured in these same tissues since the ratio of SCAP to the Insigs can impact SREBP processing. The fetal tissues had anywhere from a 2- to 6-fold greater ratio of SCAP to Insig-1 than did the adult liver, suggesting constitutive processing of the SREBPs. As expected, the level of mature, nuclear SREBP-2 was not different in the fetal tissues with different levels of cholesterol whereas it was different in adult livers. These findings indicate that the suppression of sterol synthesis to exogenous sterol is blunted in developing tissues and the lack of response appears to be mediated at least partly through relative levels of Insigs and SCAP.

Biomarkers, metabonomics, and drug development: can inborn errors of metabolism help in understanding drug toxicity?

Application of "omics" technology during drug discovery and development is rapidly evolving. This review evaluates the current status and future role of "metabonomics" as a tool in the drug development process to reduce the safety-related attrition rates and bridge the gaps between preclinical and clinical, and clinical and market. Particularly, the review looks at the knowledge gap between the pharmaceutical industry and pediatric hospitals, where metabonomics has been successfully applied to screen and treat newborn babies with inborn errors of metabolism. An attempt has been made to relate the clinical pathology associated with inborn errors of metabolism with those of drug-induced pathology. It is proposed that extending the metabonomic biomarkers used in pediatric hospitals, as "advanced clinical chemistry" for preclinical and clinical drug development, is immediately warranted for better safety assessment of drug candidates. The latest advances in mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy should help replace the traditional approaches of laboratory clinical chemistry and move the safety evaluation of drug candidates into the new millennium.

Partial rescue of retinal function and sterol steady-state in a rat model of Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is the first-described in a growing family of hereditary defects in cholesterol biosynthesis, and presents with a spectrum of serious abnormalities, including multiple dysmorphologies, failure to thrive, cognitive and behavioral impairments, and retinopathy. Using a pharmacologically induced rat model of SLOS that exhibits key hallmarks of the disease, including progressive retinal degeneration and dysfunction, we show that a high-cholesterol diet can substantially correct abnormalities in retinal sterol composition, with concomitant improvement of visual function, particularly within the cone pathway. Although histologic degeneration still occurred, a high-cholesterol diet reduced the number of pyknotic photoreceptor nuclei, relative to animals on a cholesterol-free diet. These findings demonstrate that cholesterol readily crosses the blood-retina barrier (unlike the blood-brain barrier) and suggest that cholesterol supplementation may be efficacious in treating SLOS-associated retinopathy.

Cholesterol precursors and facial clefting

Inborn errors of cholesterol synthesis cause human malformation syndromes, including Smith-Lemli-Opitz syndrome, lathosterolosis, desmosterolosis, X-linked dominant chondrodysplasia punctata type 2, and congenital hemidysplasia with ichthyosiform erythroderma and limb defects. Because adequate cholesterol is not transported across the placenta, low cholesterol and elevated sterol precursor levels are present during embryogenesis. It has been debated whether the malformations result from low cholesterol or the buildup of sterol precursors. In this issue of the JCI, Engelking et al. provide evidence that sterol precursor accumulation plays a pivotal role in the genesis of facial malformations (see the related article beginning on page 2356).

Lanosterol synthase mutations cause cholesterol deficiency-associated cataracts in the Shumiya cataract rat

The Shumiya cataract rat (SCR) is a hereditary cataractous strain. It is thought that the continuous occurrence of poorly differentiated epithelial cells at the bow area of the lens forms the pathophysiological basis for cataract formation in SCRs. In this study, we attempted to identify the genes associated with cataract formation in SCRs by positional cloning. Genetic linkage analysis revealed the presence of a major cataract locus on chromosome 20 as well as a locus on chromosome 15 that partially suppressed cataract onset. Hypomorphic mutations were identified in genes for lanosterol synthase (Lss) on chromosome 20 and farnesyl diphosphate farnesyl transferase 1 (Fdft1) on chromosome 15, both of which function in the cholesterol biosynthesis pathway. A null mutation for Lss was also identified. Cataract onset was associated with the specific combination of Lss and Fdft1 mutant alleles that decreased cholesterol levels in cataractous lenses to about 57% of normal. Thus, cholesterol insufficiency may underlie the deficient proliferation of lens epithelial cells in SCRs, which results in the loss of homeostatic epithelial cell control of the underlying fiber cells and eventually leads to cataractogenesis. These findings may have some relevance to other types of cataracts, inborn defects of cholesterol synthesis, and the effects of cholesterol-lowering medication.

The serotonin1A receptor: a representative member of the serotonin receptor family

1. Serotonin is an intrinsically fluorescent biogenic amine that acts as a neurotransmitter and is found in a wide variety of sites in the central and peripheral nervous system. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions. 2. Serotonin exerts its diverse actions by binding to distinct cell surface receptors which have been classified into many groups. The serotonin1A (5-HT1A) receptor is the most extensively studied of the serotonin receptors and belongs to the large family of seven transmembrane domain G-protein coupled receptors. 3. The tissue and sub-cellular distribution, structural characteristics, signaling of the serotonin1A receptor and its interaction with G-proteins are discussed. 4. The pharmacology of serotonin1A receptors is reviewed in terms of binding of agonists and antagonists and sensitivity of their binding to guanine nucleotides. 5. Membrane biology of 5-HT1A receptors is presented using the bovine hippocampal serotonin1A receptor as a model system. The ligand binding activity and G-protein coupling of the receptor is modulated by membrane cholesterol thereby indicating the requirement of cholesterol in maintaining the receptor organization and function. This, along with the reported detergent resistance characteristics of the receptor, raises important questions on the role of membrane lipids and domains in the function of this receptor.

Formation of 7-dehydrocholesterol-containing membrane rafts in vitro and in vivo, with relevance to the Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a recessive disease typified by 7-dehydrocholesterol (7DHC) accumulation and depletion of cholesterol. Because cholesterol is a primary component of detergent-resistant membrane domains ("rafts"), we examined the compatibility of 7DHC with raft formation. Liposomes containing bovine brain phosphatidylcholine, sphingomyelin, cerebrosides, and either cholesterol, 7DHC, or coprostanol (the latter being incompatible with raft formation) were prepared. 7DHC was indistinguishable from cholesterol in its ability to become incorporated into membrane rafts, as judged by physical and chemical criteria, whereas coprostanol did not form rafts. The in vivo compatibility of 7DHC with raft formation was evaluated in brains of rats treated with trans-1,4-bis(2-dichlorobenzylamino-ethyl)cyclohexane dihydrochloride (AY9944), which mimics the SLOS biochemical defect. 7DHC/cholesterol ratios in rafts and whole brains from AY9944-treated rats were similar, indicating comparable efficiency of 7DHC and cholesterol incorporation into brain rafts. In contrast, dolichol (a nonsterol isoprenoid incompatible with raft formation) was greatly depleted in brain rafts relative to whole brain. Although brain raft fractions prepared from AY9944-treated and control rats yielded similar sterol-protein ratios, their gel electrophoresis profiles exhibited multiple differences, suggesting that altered raft sterol composition perturbs raft protein content. These results are discussed in the context of the SLOS phenotype, particularly with regard to the associated central nervous system defects.

Autosomal recessive HEM/Greenberg skeletal dysplasia is caused by 3 beta-hydroxysterol delta 14-reductase deficiency due to mutations in the lamin B receptor gene

Hydrops-ectopic calcification-"moth-eaten" (HEM) or Greenberg skeletal dysplasia is an autosomal recessive chondrodystrophy with a lethal course, characterized by fetal hydrops, short limbs, and abnormal chondro-osseous calcification. We found elevated levels of cholesta-8,14-dien-3beta-ol in cultured skin fibroblasts of an 18-wk-old fetus with HEM, compatible with a deficiency of the cholesterol biosynthetic enzyme 3beta-hydroxysterol delta(14)-reductase. Sequence analysis of two candidate genes encoding putative human sterol delta(14)-reductases (TM7SF2 and LBR) identified a homozygous 1599-1605TCTTCTA-->CTAGAAG substitution in exon 13 of the LBR gene encoding the lamin B receptor, which results in a truncated protein. Functional complementation of the HEM cells by transfection with control LBR cDNA confirmed that LBR encoded the defective sterol delta(14)-reductase. Mutations in LBR recently have been reported also to cause Pelger-Huët anomaly, an autosomal dominant trait characterized by hypolobulated nuclei and abnormal chromatin structure in granulocytes. The fact that the healthy mother of the fetus showed hypolobulated nuclei in 60% of her granulocytes confirms that classic Pelger-Huët anomaly represents the heterozygous state of 3beta-hydroxysterol delta(14)-reductase deficiency.