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Cholesterol Metabolism and Genetic Syndromes

Forbes D. Porter, MD, PhD, Head, Section on Molecular Dysmorphology
Sandra K. Conley, CPNP, Nurse Practitioner
Michelle Dail, BA, Postbaccalaureate Fellow
Rao Fu, BA, Doctoral Student
Helen Huang, PhD, Postdoctoral Fellow
Xiao-sheng Jiang, PhD, Postdoctoral Fellow
Grace Koo, BA, Postbaccalaureate Fellow
Andrea Silvis, RN, Postbaccalaureate Fellow
Elaine Tierney, MD, Special Volunteer
Chris A. Wassif, MSc, Technical Specialist
Nicole M. Yanjanin, CPNP, Nurse Practitioner

We study the molecular, biochemical, and cellular processes that underlie dysmorphic syndromes and birth defects attributable to inborn errors of cholesterol synthesis. Human malformation/mental retardation syndromes caused by inborn errors of cholesterol synthesis include the Smith-Lemli-Opitz syndrome (SLOS), lathosterolosis, desmosterolosis, X-linked dominant chondrodysplasia punctata type 2 (CDPX2), and the CHILD syndrome. We conduct both basic and clinical research. Our basic research uses mouse models of these disorders to understand the biochemical, molecular, cellular, and developmental processes that underlie the birth defects and clinical problems found in the various disorders. Our clinical research focuses on characterization and treatment of patients with SLOS. Our emphasis on both basic and clinical research allows us to integrate laboratory and clinical data in order to increase our understanding of the pathological mechanisms underlying SLOS and to improve the clinical care of SLOS patients. Understanding the pathophysiology of rare genetic disorders such as SLOS has yielded insights into potential contributing factors to more common disorders such as Autistic Spectrum Disorder. We also study clinical and translational aspects of Niemann-Pick disease, type C (NPC). NPC is a neurodegenerative disorder caused by impaired intracellular transport of cholesterol and lipids. Similar to our work with SLOS, our goals are to improve the clinical care of NPC patients.

Smith-Lemli-Opitz syndrome

SLOS is an autosomal recessive, multiple-malformation syndrome characterized by dysmorphic facial features, mental retardation, hypotonia, poor growth, and variable structural anomalies of the heart, lungs, brain, limbs, gastrointestinal tract, and genitalia. The SLOS phenotype is extremely variable. At the severe end of the phenotypic spectrum, infants often die as result of multiple major malformations. At the mild end of the phenotypic spectrum, SLOS combines minor physical malformations with behavioral and learning problems. The syndrome is attributable to an inborn error of cholesterol biosynthesis that blocks the conversion of 7-dehydrocholesterol (7-DHC) to cholesterol. Our laboratory initially cloned the human 3beta-hydroxysterol delta7-reductase gene (DHCR7) and demonstrated mutations of the gene in SLOS patients (Correa-Cerro et al., J Med Genet 2005;42:350). Together with others, we have identified over 100 mutations of DHCR7 to date. To further our understanding of the mechanisms underlying the broad phenotypic spectrum in SLOS, we used deuterium oxide labeling to measure residual DHCR7 activity in fibroblasts from patients with known genotypes and well-characterized phenotypes. In collaboration with other laboratories, we have been identifying and characterizing the biological activity of aberrant oxysterols, steroids, and neuroactive steroids.

In addition to our basic science work on the pathophysiological processes underlying SLOS, we are actively recruiting and following SLOS patients in three clinical protocols. Given that SLOS patients have a cholesterol deficiency, they may be treated with dietary cholesterol supplementation. To date, we have evaluated over 75 SLOS patients and continue to follow many of them in a longitudinal natural history study. Parents frequently report improved behavior in SLOS children on dietary cholesterol supplementation. However, we had yet to conduct a blinded protocol studying the efficacy of dietary cholesterol therapy to ameliorate behavioral problems associated with SLOS. We therefore designed and implemented a protocol to study the short-term efficacy of dietary cholesterol therapy to improve behavioral problems in 40 SLOS children. Basic laboratory experiments using patient cells and our SLOS mouse model suggested that simvastatin may be efficacious in improving the sterol abnormality in SLOS. Accordingly, we initiated a controlled, blinded cross-over protocol to study the safety and efficacy of simvastatin therapy in SLOS. Previously, we showed that SLOS fibroblasts have a secondary defect in intracellular cholesterol transport. As part of a Bench-to-Bedside proposal, we continue, in collaboration with Fran Platt's laboratory, to study impaired cholesterol and glycosphingolipid transport in SLOS and to investigate novel therapeutic interventions.

One of the most interesting facets of SLOS is its distinct behavioral phenotype. Most patients with SLOS exhibit autistic characteristics. We are currently working in collaboration with groups from both NHGRI and the Kennedy Krieger Institute in Baltimore to analyze this association further. We received funding from Autism Speaks to initiate a multicenter double-masked placebo-controlled trial of cholesterol in hypocholesterolemic autistic spectrum disorder (ASD). We also recently received funding from the NIH Directors Office to support studies aimed at elucidating the genetic mechanism underlying hypocholesterolemia in ASD patients.

Mouse models of SLOS

Using gene targeting in murine embryonic stem cells, we produced three SLOS mouse models: a null mutation, a hypomorphic point mutation, and a conditional mutation. Mouse pups homozygous for the null mutation (Dhcr7^Δ3-5/Δ3-5) exhibit variable craniofacial anomalies, are growth-retarded, feed poorly, and appear weak. Dhcr7^Δ3-5/Δ3-5 pups die during the first day of life because they fail to feed. Biochemical characterization showed that the mutant pups had markedly elevated serum and tissue 7-DHC levels as well as reduced serum and tissue cholesterol levels. Cleft palate was present in 9% of the Dhcr7^Δ3-5/Δ3-5 pups and is found in approximately one-third of all SLOS patients. Further investigation of the neurological abnormalities in these mice showed that cortical neurons had an impaired response to glutamate. A decreased glutamate response is consistent with the phenotypic observation of poor feeding by the mutant animals. Neurological dysfunctions, including poor feeding, hypotonia, mental retardation, and behavioral dysfunction, are major clinical problems in SLOS. The impaired glutamate response observed in our mouse model may yield insights into the etiology of some of the neurological dysfunction in SLOS.

Given that the Dhcr7^Δ3-5/Δ3-5 pups die during the first day of life, we were not able to study postnatal brain development, myelination, or behavior or to test therapeutic interventions. For this reason, we developed a mis-sense allele (Dhcr7T93M) and a conditional Dhcr7 mutant allele (Dhcr7^{loxPΔ3-5loxP}). The T93M mutation is the second most common mutation found in human patients. Dhcr7^T93M/T93M and Dhcr7^T93M/Δ3-5 mice are viable and demonstrate SLOS with a gradient of biochemical severity (Dhcr7^Δ3-5/Δ3-5 > Dhcr7^T93M/Δ3-5 > Dhcr7^T93M/T93M). We used Dhcr7^T93M/Δ3-5 mice to test the efficacy of therapeutic interventions on tissue sterol profiles. As expected, dietary cholesterol therapy improved the sterol composition in peripheral tissues but not in the central nervous system. Treatment of mice with simvastatin improved the biochemical defect in both peripheral and central nervous system tissue, suggesting that simvastatin therapy may be used to treat some of the behavioral and learning problems in children with SLOS. In collaboration with Cedric Shackleton and Gordon Watson, we are using our hypomorphic mouse model to investigate adeno-associated viral gene therapy for SLOS.

As part of our clinical studies on SLOS, we previously identified a novel oxysterol, 27-hydroxy-7-dehydrocholesterol (27-7DHC), derived from 7-DHC in SLOS patients. We therefore investigated whether 27-7DHC contributes to the pathology of SLOS and found a strong negative correlation between plasma 27-7DHC and cholesterol levels in SLOS patients. In addition, previous work showed that low cholesterol levels impair hedgehog signaling. Therefore, we hypothesized that increased 27-7DHC levels would have detrimental effects during development in response to suppression of cholesterol levels. To test our hypothesis, we produced SLOS mice (Dhcr7^Δ3-5/Δ3-5) expressing a CYP27 transgene. CYP27Tg mice have elevated CYP27 expression and 27-hydroxycholesterol levels but normal cholesterol levels. Dhcr7^Δ3-5/Δ3-5 mice are growth-retarded, exhibit a low incidence of cleft palate (9%), and die during the first day of life. Dhcr7^Δ3-5/Δ3-5:CYP27Tg embryos are stillborn and have multiple malformations, including growth retardation, micrognathia, cleft palate (77%), lingual and dental hypoplasia, ankyloglossia, umbilical hernia, cardiac defects, cloacae, curled tails, and limb defects. We observed autopod defects (polydactyly, syndactyly, and oligodactyly) in 77% of the mice. Consistent with our hypothesis, sterol levels were halved in the liver and were 20-fold lower in the brain tissue of Dhcr7^Δ3-5/Δ3-5:CYP27Tg than of Dhcr7^Δ3-5/Δ3-5 embryos. Recognition of the role of 27-7DHC in SLOS may explain some of the phenotypic variability and may lead to the development of a therapeutic intervention.

Recognizing its potential impact on prenatal therapy, we have been characterizing maternal-fetal cholesterol transport in Abca1, Abcg1, and Sr-b1 mutant mice. We found that disruption of Abca1 reduced maternal-fetal cholesterol transport; conversely the elevated expression of Abca1 caused an increase in maternal-fetal cholesterol transport. In our SLOS mouse model, we were able to show, as a proof of principle, that increased expression of Abca1 raised cholesterol content in SLOS mutant embryos.

To gain insight into pathophysiological processes contributing to SLOS, we completed a series of proteomic experiments to identify proteins that are differentially expressed in the cortex of Dhcr7^Δ3-5/Δ3-5 embryos. Functional analysis is in progress; however, we have already identified alterations in dendrite and axon formation that may contribute to the cognitive deficits found in SLOS.

We continue to use the SLOS mouse models to understand the pathophysiological processes that underlie the birth defects and clinical problems in SLOS and are using various biochemical, molecular, and proteomic approaches to investigate these issues. We are also using the mouse models to test various prenatal and postnatal therapeutic interventions.

Lathosterolosis, desmosterolosis, and HEM dysplasia

Lathosterol 5-desaturase catalyzes the conversion of lathosterol to 7-dehydrocholesterol, representing the enzymatic step immediately preceding the defect in SLOS. Thus, to understand further the roles of decreased cholesterol versus increased 7-dehydrocholesterol in SLOS, we disrupted the mouse lathosterol 5-desaturase gene (Sc5d) by using targeted homologous recombination in embryonic stem cells. Sc5d^−/− pups are stillborn, present with micrognathia and cleft palate, and exhibit limb-patterning defects. Many of the malformations in these mutant mice resemble malformations in SLOS and are consistent with impaired hedgehog signaling during development. Biochemically, the mice have, in serum and tissue, markedly elevated lathosterol levels and reduced cholesterol levels.

One goal of producing a lathosterolosis mouse model was to gain phenotypic insight for the purpose of identifying a corresponding human malformation syndrome. We have now identified a human infant patient with lathosterolosis, a human malformation syndrome that has not yet been described. Biochemically, fibroblasts from the patient show decreased cholesterol and increased lathosterol levels. Mutation analysis showed that the patient is homozygous for a single A→C nucleotide change at position 137 in SC5D, resulting in a mutant enzyme in which the amino acid serine is substituted for tyrosine at position 46. Both parents were heterozygous for the mutation. The infant's phenotype resembled severe SLOS. Malformations found in both the human patient and mouse model include growth failure, abnormal nasal structure, abnormal palate, micrognathia, and postaxial polydactyly. A unique feature of lathosterolosis is the clinical finding of mucolipidosis in the affected infant. This clinical presentation is not reported in SLOS and may help distinguish SLOS clinically from lathosterolosis. This lysosomal storage disorder can be replicated in embryonic fibroblasts from the Sc5d mutant mouse model. To distinguish pathological changes that are attributable to decreased cholesterol from those that are a consequence of increased 7-DHC in SLOS, we are comparing proteomic changes in the Sc5d mutant mouse model with those found in the SLOS mouse model.

Desmosterolosis is another inborn error of cholesterol synthesis that resembles SLOS. It results from a mutation in the 3beta-hydroxysterol delta24-reductase gene (DHCR24). DHCR24 catalyzes the reduction of desmosterol to cholesterol. We disrupted the mouse Dhcr24 gene by using targeted homologous recombination in embryonic stem cells. Surprisingly, although most Dhcr24 mutant mice die at birth, the pups are phenotypically normal.

Others have shown that mutations of the lamin B receptor (LBR) cause HEM (hydrops, ectopic calcification, moth-eaten skeletal ) dysplasia in humans and ichthyosis in mice. LBR has both lamin B–binding and sterol Δ14-reductase domains. Although only a minor sterol abnormality has been reported, it has been proposed that LBR is the primary sterolΔ14-reductase and that impaired sterol Δ14-reduction underlies HEM dysplasia. However, DHCR14 also encodes a sterol Δ14-reductase. To test the hypothesis that LBR and DHCR14 are redundant sterol Δ14-reductases, we obtained ichthyosis mice (Lbr^−/−) and disrupted Dhcr14. Dhcr14^−/− mice are phenotypically normal. We found no sterol abnormalities in either Lbr^−/− or Dhcr14^−/− tissues at 1 and 21 days of age. We then bred the mice to obtain compound mutant mice. Lbr^−/−:Dhcr14^−/− and Lbr^−/−:Dhcr14^+/− died in utero. Lbr^+/−:Dhcr14^−/− mice appeared normal at birth but, by 10 days of age, were growth-retarded and neurologically abnormal (with ataxia and tremors) and, consistent with a demyelinating process, evidenced vacuolation and swelling of the myelin sheaths in the spinal cord upon pathological evaluation. We observed neither vacuolation nor swelling of the myelin sheaths in either Lbr^−/− or Dhcr14^−/− mice. In contrast to Lbr^−/− mice, Lbr^+/−:Dhcr14^−/− mice had normal skin and did not display the Pelger-Huët anomaly. Peripheral tissue sterols were normal in all three mutant mice, although we found significantly elevated levels (50% of total sterols) of cholesta-8,14-dien-3β-ol and cholesta-8,14,24-trien-3β-ol in brain tissue from 10-day-old Lbr^+/−:Dhcr14^−/− mice. In contrast, we observed relatively small transient elevations of Δ¹⁴-sterols in Lbr^−/− and Dhcr14Δ4-7/Δ4-7 brain tissue. Our data support the idea that HEM dysplasia and ichthyosis result from impaired lamin B receptor function rather than from impaired sterol Δ14-reduction. Impaired sterol Δ14-reduction gives rise to a novel murine phenotype for which a corresponding human disorder has yet to be identified.

Niemann-Pick type C

Niemann-Pick type C (NPC) is a neurodegenerative disorder caused by a defect in intracellular lipid and cholesterol transport. As part of a Bench-to-Bedside initiative, we initiated a clinical protocol to identify and characterize biomarkers that could be used in a subsequent therapeutic trial. The project continues to receive support from the Ara Parseghian Medical Research Foundation and Dana's Angels Research Trust. To date, we have enrolled 53 NPC patients and have identified a group of disease-specific and nonspecific biomarkers indicative of increased oxidative stress in NPC patients. In collaboration with Dan Ory, we have identified increased levels of oxysterols in the serum from NPC patients. This serum-based test holds the promise of being a screening or diagnostic test for NPC that would reduce the diagnostic delay experienced by many families. Based on this work, we initiated and completed a therapeutic trial to test the safety and efficacy of N-acetyl cysteine (NAC) in NPC. To complement the clinical work, we have begun to apply proteomic approaches to both mouse and human biomaterials to identify biological pathways disrupted in NPC.

Additional Funding

Ara Parseghian Medical Research Foundation
Dana's Angels Research Trust
Autism Speaks
NIH Office of the Director
Therapeutics for Rare and Neglected Diseases

Publications

Yanjanin NM, Vélez JI, Gropman A, King K, Bianconi SE, Conley SK, Brewer CC, Solomon B, Pavan WJ, Arcos-Burgos M, Patterson MC, Porter FD. Linear clinical progression, independent of age of onset, in Niemann-Pick disease, type C. Am J Med Genet B Neuropsychiatr Genet 2009;[Epub ahead of print].
Tierney E, Conley SK, Goodwin H, Porter FD. Analysis of short-term behavioral effects of dietary cholesterol supplementation in Smith-Lemli-Opitz Syndrome. Am J Med Genet A 2010;152A:91-95.
Jiang, X-S, Wassif, CA, Backlund, PS, Song, L, Holtzclaw, LA, Li, Z, Yergey, AL, Porter, FD. Activation of Rho GTPases in Smith-Lemli-Opitz Syndrome: Pathophysiological and Clinical Implications. Hum. Mol. Genet. 2010;19:1347-1357.
Jiang X-S, Backlund PS, Wassif CA, Yergey AL, Porter FD. Quantitative Proteomic Analysis of Inborn Errors of Cholesterol Synthesis: Identifcation of Altered Metabolic Pathways in DHCR7 and SC5D Deficiency. Molecular and Cellular Proteomics 2010;9:1461-1475.
Fu R, Yanjanin NM, Bianconi S, Pavan WJ, Porter FD. Oxidative Stress in Niemann-Pick Disease, type C. Molecular Genetics and Metabolism 2010;101:214-218.

Collaborators

Joan Bailey-Wilson, PhD, Inherited Diseases Research Branch, NHGRI, Bethesda, MD
Steven Fliesler, PhD, St. Louis University, St. Louis, MO
Yiannis Ioannou, PhD, Mount Sinai School of Medicine, New York, NY
Norman Javitt, MD, PhD, New York University Medical School, New York, NY
Zheng Li, PhD, Clinical Neuroscience Branch, NIMH, Bethesda, MD
Daniel Ory, MD, Washington University, St. Louis, MO
Marc Patterson, MD, Columbia University, New York, NY
William Pavan, PhD, Genetic Disease Research Branch, NHGRI, Bethesda, MD
Fran Platt, PhD, Oxford University, Oxford, UK
Alan Remaley, MD, PhD, Molecular Diseases Branch, NHLBI, Bethesda, MD
Cedric Shackleton, PhD, Children's Hospital Oakland Research Institute, Oakland, CA
Gordon Watson, PhD, Children's Hospital Oakland Research Institute, Oakland, CA
Alfred Yergey, PhD, Mass Spectrometry Core Facility, NICHD, Bethesda, MD

Contact

For more information, email fdporter@helix.nih.gov.

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