Recent insights into the Smith-Lemli-Opitz syndrome

Recent insights into the Smith-Lemli-Opitz syndrome. The Smith-Lemli-Opitz syndrome (SLOS) is an autosomal recessive multiple congenital anomaly/mental retardation disorder caused by an inborn error of post-squalene cholesterol biosynthesis. Deficient cholesterol synthesis in SLOS is caused by inherited mutations of 3beta-hydroxysterol-Delta7 reductase gene (DHCR7). DHCR7 deficiency impairs both cholesterol and desmosterol production, resulting in elevated 7DHC/8DHC levels, typically decreased cholesterol levels and, importantly, developmental dysmorphology. The discovery of SLOS has led to new questions regarding the role of the cholesterol biosynthesis pathway in human development. To date, a total of 121 different mutations have been identified in over 250 patients with SLOS who represent a continuum of clinical severity. Two genetic mouse models have been generated which recapitulate some of the developmental abnormalities of SLOS and have been useful in elucidating the pathogenesis. This mini review summarizes the recent insights into SLOS genetics, pathophysiology and potential therapeutic approaches for the treatment of SLOS.

Sitosterolaemia in Switzerland: molecular genetics links the US Amish-Mennonites to their European roots

Sitosterolaemia is a rare autosomal recessive disease characterized by increased intestinal absorption of plant sterols, decreased hepatic excretion into bile and elevated concentrations in plasma phytosterols. Homozygous or compound heterozygous loss of function mutations in either of the ATP-binding cassette (ABC) proteins ABCG5 and ABCG8 explain the increased absorption of plant sterols. Here we report a Swiss index patient with sitosterolaemia, who presented with the classical symptoms of xanthomas, but also had mitral and aortic valvular heart disease. Her management over the last 20 years included a novel therapeutic approach of high-dose cholesterol feeding that was semi-effective. Mutational and extended haplotype analyses showed that our patient shared this haplotype with that of the Amish-Mennonite sitosterolaemia patients, indicating they are related ancestrally.

Anthrax lethal factor inhibition

The primary virulence factor of Bacillus anthracis is a secreted zinc-dependent metalloprotease toxin known as lethal factor (LF) that is lethal to the host through disruption of signaling pathways, cell destruction, and circulatory shock. Inhibition of this proteolytic-based LF toxemia could be expected to provide therapeutic value in combination with an antibiotic during and immediately after an active anthrax infection. Herein is shown the crystal structure of an intimate complex between a hydroxamate, (2R)-2-[(4-fluoro-3-methylphenyl)sulfonylamino]-N-hydroxy-2-(tetrahydro-2H-pyran-4-yl)acetamide, and LF at the LF-active site. Most importantly, this molecular interaction between the hydroxamate and the LF active site resulted in (i) inhibited LF protease activity in an enzyme assay and protected macrophages against recombinant LF and protective antigen in a cell-based assay, (ii) 100% protection in a lethal mouse toxemia model against recombinant LF and protective antigen, (iii) approximately 50% survival advantage to mice given a lethal challenge of B. anthracis Sterne vegetative cells and to rabbits given a lethal challenge of B. anthracis Ames spores and doubled the mean time to death in those that died in both species, and (iv) 100% protection against B. anthracis spore challenge when used in combination therapy with ciprofloxacin in a rabbit "point of no return" model for which ciprofloxacin alone provided 50% protection. These results indicate that a small molecule, hydroxamate LF inhibitor, as revealed herein, can ameliorate the toxemia characteristic of an active B. anthracis infection and could be a vital adjunct to our ability to combat anthrax.

Ezetimibe effectively reduces plasma plant sterols in patients with sitosterolemia

BACKGROUND

Sitosterolemia is a recessively inherited disorder that results from mutations in either ABCG5 or G8 proteins, with hyperabsorption of dietary sterols and decreased hepatic excretion of plant sterols and cholesterol. As a consequence of markedly elevated plasma and tissue sitosterol and campesterol levels, premature atherosclerosis develops.

METHODS AND RESULTS

In this multicenter, double-blind, randomized, placebo-controlled study, we examined whether treatment with ezetimibe, an inhibitor of cholesterol absorption, reduces plant sterol levels in patients with sitosterolemia. After a 3-week placebo run-in, 37 patients were randomized to receive placebo (n=7) or ezetimibe 10 mg/d (n=30) for 8 weeks. Sitosterol concentrations decreased by 21% (P<0.001) in patients treated with ezetimibe compared with a nonsignificant 4% rise in those on placebo (between-group P<0.001). The reduction in sitosterol from baseline was progressive, with further decline observed at each subsequent biweekly visit. Campesterol also progressively declined, with a mean decrease after 8 weeks of 24% with ezetimibe and a mean increase of 3% with placebo treatment (between-group P<0.001). Reductions in plant sterol concentrations were similar irrespective of whether patients were undergoing concomitant treatment with resin or statin. Reductions in total sterols and apolipoprotein B were also observed. Ezetimibe was well tolerated, with no serious treatment-related adverse events or discontinuations due to adverse events being reported.

CONCLUSIONS

Ezetimibe produced significant and progressive reductions in plasma plant sterol concentrations in patients with sitosterolemia, consistent with the hypothesis that ezetimibe inhibits the intestinal absorption of plant sterols as well as cholesterol, leading to reductions in plasma concentrations.

Oesophageal atresia: a simplified approach to early management

Management following the repair of oesophageal atresia (OA) with tracheooesophageal fistula (TOF) in the past included the routine use of an intercostal chest drain, a gastrostomy, or a transanastomotic tube (TAT) for enteral nutrition and a routine contrast swallow (CS) before oral feeds. There has been a trend towards simplification of the management, but this is not universal. The aim of this study was to evaluate the safety of a simplified management regime in infants undergoing primary repair of OA in a retrospective case note review of infants undergoing surgery for OA with TOF under the care of one consultant over a 12-year period. Intercostal chest drains, TATs, and CSs were not routinely used. Early enteral feeding was initiated and oral feeding was allowed in babies of adequate birth weight (BW) and gestation. A CS was only performed when there were specific anastomotic concerns. Parameters recorded included demographic details, time to first enteral feed by tube or mouth, time to full oral feeds, and complications. Forty patients were studied; 17 were managed without (group 1) and 23 with (group 2) a TAT. Sex distribution, gestational age, and BW were comparable in the two groups. In group 1, the time to the establishment of full oral feeds was 2-8 days (average 3.9). Four infants developed strictures; 2 were managed with dilatation alone and 2 required surgery. In group 2, the time to the establishment of full enteral feeds was 2-12 days (average 5.9). Four patients developed strictures; 2 underwent an anti-reflux procedure and a 3rd resection of a cartilaginous remnant. There was 1 death in a patient with intractable cardiac failure. The majority of infants with OA and TOF can thus be safely managed without routine chest drainage or CS. A sizeable minority do not require a TAT. Early introduction of oral feeds in the non-TAT group is not associated with an increased complication rate.

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.

Dietary cholesterol absorption; more than just bile

Absorption of dietary cholesterol from the intestine is an important part of cholesterol homeostasis and represents the first step that allows dietary cholesterol to exert its metabolic effects. Although the role of bile salts in the initial absorption of dietary cholesterol, by the formation of emulsions, is readily appreciated, the recognition that other molecular mechanisms might govern this process is only recently gaining momentum. Not only does the intestine regulate the amount of dietary cholesterol that enters the body; it is very selective with regard to the sterols that are allowed in. The human intestine is responsible for absorbing a significant amount of cholesterol each day. In addition to approximately 0.5 g d(-1) of dietary cholesterol, many other sterols are also present in almost equal abundance in the normal diet. Approximately 0.4 g of plant sterols, such as sitosterol, brassicasterol and avanesterol, are also present. However, the human body seems to allow only cholesterol to enter and remain in the body, with almost negligible amounts of plant sterols being retained. That specific molecular mechanisms are responsible for this behavior is supported by the identification of the genetic defect(s) in a rare disorder, beta-sitosterolemia (MIM 210250), where this process is disrupted. Such studies are now beginning to throw light on sterol absorption and excretion and elucidate the molecular mechanisms that govern these processes.

Fine-mapping, mutation analyses, and structural mapping of cerebrotendinous xanthomatosis in U.S. pedigrees

Cerebrotendinous xanthomatosis (CTX) is a rare autosomal recessive disorder of bile acid biosynthesis. Clinically, CTX patients present with tendon xanthomas, juvenile cataracts, and progressive neurological dysfunction and can be diagnosed by the detection of elevated plasma cholestanol levels. CTX is caused by mutations affecting the sterol 27-hydroxylase gene (CYP27 ). CTX has been identified in a number of populations, but seems to have a higher prevalence in the Japanese, Sephardic Jewish, and Italian populations. We have assembled 12 previously unreported pedigrees from the United States. The CYP27 locus had been previously mapped to chromosome 2q33-qter. We performed linkage analyses and found no evidence of genetic heterogeneity. All CTX patients showed segregation with the CYP27 locus, and haplotype analysis and recombinant events allowed us to precisely map CYP27 to chromosome 2q35, between markers D2S1371 and D2S424. Twenty-three mutations were identified from 13 probands analyzed thus far; 11 were compound heterozygotes and 2 had homozygous mutations. Of these, five are novel mutations [Trp100Stop, Pro408Ser, Gln428Stop, a 10-base pair (bp) deletion in exon 1, and a 2-bp deletion in exon 6 of the CYP27 gene]. Three-dimensional structural modeling of sterol 27-hydroxylase showed that, while the majority of the missense mutations disrupt the heme-binding and adrenodoxin-binding domains critical for enzyme activity, two missense mutations (Arg94Trp/Gln and Lys226Arg) are clearly located outside these sites and may identify a potential substrate-binding or other protein contact site.

Spectrum of Delta(7)-dehydrocholesterol reductase mutations in patients with the Smith-Lemli-Opitz (RSH) syndrome

The Smith-Lemli-Opitz syndrome (SLOS; also known as the RSH syndrome) is an autosomal recessive genetic disorder, leading to characteristic multi-organ developmental abnormalities, dysmorphic facies, limb malformations and mental retardation. Mutations in the gene for Delta(7)-dehydrocholesterol reductase (Delta(7)-reductase), which catalyzes the last step in cholesterol biosynthesis, cause the disease. We screened 32 patients with SLOS, 28 from the USA and four from Sweden. Twenty-two different nucleotide changes, predicted to be disease-causing mutations, were identified; 20 missense mutations, one nonsense mutation and one splice-site mutation involving the exon 9 acceptor site (IVS8 -1G-->C) were detected. All probands were heterozygous for mutations. Twelve of these mutations have not been reported previously, including missense mutations L148R, F168I, D175H, P179L, P243R, F284L, N287K, F302L, R404S, Y462H, R469P and one nonsense mutation W37X [corrected]. Coupled with previously reported mutations, these findings bring the total of different Delta(7)-reductase mutations to 36. These are distributed throughout the coding sequence of the Delta(7)-reductase gene except exons 3 and 5, with a clustering in exon 9. Three mutations account for 54% of those observed in our cohort, the splice acceptor site mutation IVS8 -1G-->C (22/64 alleles, 34%), T93M (8/64, 12.5%) and V326L (5/64, 7.8%). Severity of SLOS was negatively correlated with both plasma cholesterol and relative plasma cholesterol, but not with 7-dehydrocholesterol, the immediate precursor, confirming previous observations. However, no correlation was observed between mutations and phenotype, suggesting that the degree of severity may be affected by other factors. We estimate that between 33 and 42% of the variation in the SLOS severity score is accounted for by variation in plasma cholesterol. Thus, factors other than plasma cholesterol are additionally involved in determining severity.

Microsomal triglyceride transfer protein expression during mouse development

Feto-maternal transfer of lipophilic nutrients is an important factor in the normal development of the fetus and may be mediated by lipoproteins as carriers of these nutrients. Two proteins that may be important in this process are apolipoprotein B (apoB, the major structural protein of secreted lipoproteins) and microsomal triglyceride transfer protein (MTP) whose normal activity is required for the secretion of apoB-containing lipoproteins. Although no abnormalities of conception and embryonic lethality are known in humans who inherit genetic deficiencies of either of these proteins, homozygous mice bearing knockouts of either apoB or MTP show early embryonic lethality. To characterize the ontogeny of MTP expression during embryonic mouse development, we have used in situ hybridization to characterize the pattern of expression. By using microwave heating of tissue sections to optimize hybridization, we show that there is robust MTP expression in the yolk sac tissues followed by expression in the primordial liver cell nests as early as day 9 post-coitum (E9.5). Intestinal expression is detected around E12.5 and attains full adult expression patterns by E14.5. No expression in any other tissues was observed, including developing heart, kidney, placenta, and maternal decidua. Thus the pattern of MTP expression is compatible with a role in the transfer of lipophilic nutrients from the yolk sac, prior to hepatic development and to the liver, once the circulatory system has been established.

Removal of Foscarnet by hemodialysis using dialysate-side values

Foscarnet is an antiviral agent widely used in the treatment of cytomegalovirus (CMV) infection. We describe a cardiac transplant patient, who while being maintained with hemodialysis because of tobramycin-induced acute renal failure, was given Foscarnet for disseminated CMV infection. Using dialysate-side clearance methodology, we found the dialyzer clearance of Foscarnet to be in the order of 89 ml/min.

Detection of a common mutation in the RSH or Smith-Lemli-Opitz syndrome by a PCR-RFLP assay: IVS8-G-->C is found in over sixty percent of US propositi

The RSH or Smith-Lemli-Opitz syndrome (SLOS) is a relatively common autosomal recessive disorder of cholesterol biosynthesis resulting from a deficiency of the enzyme 7-dehydrocholesterol delta7-reductase (7-DHCR). Mutations in 7-DHCR gene cause SLOS. Among these, a G-->C transversion in the splice acceptor site of exon 9 (IVS8-1G-->C) was suspected to be a frequent mutation, having been detected in about 18% of SLOS patients so far. This mutation results in the elimination of a AlwN1 restriction endonuclease site. We report a simple PCR-RFLP assay to detect the IVS8-1G-->C mutation. Using this method, we identified the IVS8-1G-->C mutation in 21 of 33 SLOS propositi. This mutation was detected in one of 90 normal adult Caucasian Americans; but not among 121 Africans from Sierra Leone, 120 Caucasians from Finland, 95 Chinese or 103 Japanese adults. The results of this study provide further evidence that IVS8-1G-->C transversion is a very common mutation in SLOS patients from the US and that the carrier rate in US caucasians may be high. The simple PCR-RFLP assay developed makes identification of this mutation convenient for diagnosis and for carrier detection.

Cyclosporin-induced dyslipoproteinemia is associated with selective activation of SREBP-2

The use of cyclosporin A has contributed greatly to the success of organ transplantation. However, cyclosporin-associated side effects of hypertension, nephrotoxicity, and dyslipoproteinemia have tempered these benefits. Cyclosporin-induced dyslipoproteinemia may be an important risk factor for the accelerated atherosclerosis observed posttransplantation. Using a mouse model, we treated Swiss-Webster mice for 6 days with a daily dose of 20 microg/g body wt of cyclosporin and observed significant elevations of plasma cholesterol, triglyceride, and apolipoprotein B (apoB) levels relative to vehicle-alone treated control animals. Measurement of the rate of secretion of very low-density lipoprotein (VLDL) by the liver in vivo showed that cyclosporin treatment led to a significant increase in the rate of hepatic VLDL triglyceride secretion. Total apoB secretion was unaffected. Northern analysis showed that cyclosporin A treatment increased the abundance of hepatic mRNA levels for a number of key genes involved in cholesterol biosynthesis relative to vehicle-alone treated animals. Two key transcriptional factors, sterol regulatory element-binding protein (SREBP)-1 and SREBP-2, also showed differential expression; SREBP-2 expression was increased at the mRNA level, and there was an increase in the active nuclear form, whereas the mRNA and the nuclear form of SREBP-1 were reduced. These results show that the molecular mechanisms by which cyclosporin causes dyslipoproteinemia may, in part, be mediated by selective activation of SREBP-2, leading to enhanced expression of lipid metabolism genes and hepatic secretion of VLDL triglyceride.

Saline-induced dilutional acidosis in a maintenance hemodialysis patient

A patient with end-stage renal disease developed severe hyperchloremic acidosis (venous serum total CO2 level of 10 mmol/L) after treatment with 16 L of isotonic saline. Analysis of this case and published literature indicates that dilutional acidosis may result when very large volumes of isotonic saline are administered intravenously, especially in patients with impaired or absent renal function.

A gene for congenital generalized lipodystrophy maps to human chromosome 9q34

Congenital generalized lipodystrophy (CGL, Berardinelli-Seip Syndrome, OMIM # 269700) is a rare autosomal recessive disorder characterized by near complete absence of adipose tissue from birth. Affected individuals have marked insulin resistance, hypertriglyceridemia and acanthosis nigricans, and develop diabetes mellitus during teenage years. The genetic defect for CGL is unknown. A semi-automated genome-wide scan with a set of highly polymorphic short tandem repeats (STR) was carried out in 17 well-characterized pedigrees and identified a locus for CGL to chromosome 9q34. The maximum two-point lod score obtained was 3.6 at D9S1818 (theta(max) = 0.05). There was evidence for genetic heterogeneity (alpha = 0.73) and 2 of the pedigrees were unlinked. Multipoint linkage analysis excluding the 2 unlinked families yielded a peak lod score of 5.4 between loci D9S1818 and D9S1826. The CGL1 critical region harbors a plausible candidate gene encoding the retinoid X receptor alpha (RXRA) that plays a central role in adipocyte differentiation. Identification of the CGL gene(s) will contribute to our understanding of the adipocyte differentiation and elucidation of the mechanisms of insulin resistance in disorders of adipose tissue.

Mapping a gene involved in regulating dietary cholesterol absorption. The sitosterolemia locus is found at chromosome 2p21

The molecular mechanisms regulating the amount of dietary cholesterol retained in the body as well as the body's ability to selectively exclude other dietary sterols are poorly understood. Studies of the rare autosomal recessively inherited disease sitosterolemia (OMIM 210250) may shed some light on these processes. Patients suffering from this disease appear to hyperabsorb both cholesterol and plant sterols from the intestine. Additionally, there is failure of the liver's ability to preferentially and rapidly excrete these non-cholesterol sterols into bile. Consequently, people who suffer from this disease have very elevated plasma plant sterol levels and develop tendon and tuberous xanthomas, accelerated atherosclerosis, and premature coronary artery disease. Identification of this gene defect may therefore throw light on regulation of net dietary cholesterol absorption and lead to an advancement in the management of this important cardiovascular risk factor. By studying 10 well-characterized families with this disorder, we have localized the genetic defect to chromosome 2p21, between microsatellite markers D2S1788 and D2S1352 (maximum lodscore 4.49, theta = 0.0).

HMG-CoA reductase is not the site of the primary defect in phytosterolemia

Phytosterolemia is an autosomal recessive disorder characterized by the excessive absorption, reduced excretion, and consequent high tissue and plasma levels of plant sterols, by the presence of tendon xanthomas, and by premature atherosclerosis. Low HMG-CoA reductase (HRase) activity and mass have been reported in liver and mononuclear leucocytes and low mRNA levels in liver from phytosterolemic subjects. These results led to the proposal that the primary defect in this condition involves the HRase gene locus. We examined this hypothesis in phytosterolemic subjects and heterozygous parents from four unrelated families. A variable number tandem repeat (VNTR) polymorphism of the HRase gene in the three informative families and a ScrFI restriction fragment length polymorphism (RFLP) within intron 2 of the gene in one of these families, segregated independently of the disease phenotype. Biological parentage was confirmed in the family in whom both polymorphisms failed to segregate with the disorder. These results conclusively exclude the HRase gene locus as the site of the primary defect in phytosterolemia. The study was extended by examining plasma levels of mevalonic acid and lathosterol, both markers of cholesterol biosynthesis, in response to cholestyramine, a bile acid sequestrant that is known to up-regulate HRase. Oral administration of cholestyramine resulted in a substantial (7.7-fold) increase in mevaIonic acid levels in two phytosterolemic subjects, compared with a 2.2-fold rise in their obligate heterozygote parents and a 2.3-fold increase in three healthy control subjects. The lathosterol/cholesterol (L/C) ratio showed a quantitatively similar response. Baseline levels of mevalonate and the L/C ratio were low in the phytosterolemic patients in conformity with reports of reduced cholesterol biosynthesis and HRase activity in this disorder. These functional data provide support for the concept that the primary defect in phytosterolemia does not affect a trans gene locus responsible for the constitutive expression or regulation of HMG-CoA reductase.

An adaptive noise reduction stethoscope for auscultation in high noise environments

Auscultation of lung sounds in patient transport vehicles such as an ambulance or aircraft is unachievable because of high ambient noise levels. Aircraft noise levels of 90-100 dB SPL are common, while lung sounds have been measured in the 22-30 dB SPL range in free space and 65-70 dB SPL within a stethoscope coupler. Also, the bandwidth of lung sounds and vehicle noise typically has significant overlap, limiting the utility of traditional band-pass filtering. In this study, a passively shielded stethoscope coupler that contains one microphone to measure the (noise-corrupted) lung sound and another to measure the ambient noise was constructed. Lung sound measurements were made on a healthy subject in a simulated USAF C-130 aircraft environment within an acoustic chamber at noise levels ranging from 80 to 100 dB SPL. Adaptive filtering schemes using a least-mean-squares (LMS) and a normalized least-mean-squares (NLMS) approach were employed to extract the lung sounds from the noise-corrupted signal. Approximately 15 dB of noise reduction over the 100-600 Hz frequency range was achieved with the LMS algorithm, with the more complex NLMS algorithm providing faster convergence and up to 5 dB of additional noise reduction. These findings indicate that a combination of active and passive noise reduction can be used to measure lung sounds in high noise environments.

Sitosterolemia: exclusion of genes involved in reduced cholesterol biosynthesis

Sitosterolemia (phytosterolemia) is a rare autosomal recessively inherited disorder that is characterized by premature coronary artery disease, xanthomas, and increased plasma plant sterols and 5alpha-stanols. Affected individuals show an increased absorption of both cholesterol and sitosterol from the diet, decreased bile clearance of these sterols and their metabolites resulting in markedly expanded whole body cholesterol and sitosterol pools. Biochemical studies have shown that the regulation of the cholesterol biosynthetic pathway may be abnormal in this condition. In particular, the activities and mRNA for the biosynthetic enzymes, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and HMG-CoA synthase are low in liver biopsy specimens isolated from affected individuals, suggesting replete intracellular cholesterol pools. However, the membrane expression of hepatocyte low density lipoprotein receptors was increased, suggesting discordant regulation. Segregation analyses in three families for the genes for HMG-CoA reductase, HMG-CoA synthase, and LDL-receptor excluded these as sites of mutation. In view of the previously described discordant regulation of the above genes in sitosterolemia, the two major regulatory genes for this pathway, sterol regulatory element binding proteins (SREBP-1 and -2), were also examined. These genes did not segregate with the disease and were thus excluded. Two other genes involved in cholesterol absorption and chylomicron secretion, namely acyl coenzyme A:cholesterol acyltransferase (ACAT) and microsomal triglyceride transfer protein (MTP) were also examined for segregation and similarly excluded. Although the gene defect in sitosterolemia therefore remains to be elucidated, important candidate genes have been excluded.