Adrenal insufficiency in phytosterolaemia

Association of ABCG5 and ABCG8 Transporters with Sitosterolemia

Sitosterolemia is a rare genetic lipid disorder, mainly characterized by the accumulation of dietary xenosterols in plasma and tissues. It is caused by inactivating mutations in either ABCG5 or ABCG8 subunits, a subfamily-G ATP-binding cassette (ABCG) transporters. ABCG5/G8 encodes a pair of ABC half transporters that form a heterodimer (G5G8). This heterodimeric ATP-binding cassette (ABC) sterol transporter, ABCG5/G8, is responsible for the hepatobiliary and transintestinal secretion of cholesterol and dietary plant sterols to the surface of hepatocytes and enterocytes, promoting the secretion of cholesterol and xenosterols into the bile and the intestinal lumen. In this way, ABCG5/G8 function in the reverse cholesterol transport pathway and mediate the efflux of cholesterol and xenosterols to high-density lipoprotein and bile salt micelles, respectively. Here, we review the biological characteristics and function of ABCG5/G8, and how the mutations of ABCG5/G8 can cause sitosterolemia, a loss-of-function disorder characterized by plant sterol accumulation and premature atherosclerosis, among other features.

Pediatric Patients with Sitosterolemia: Next-Generation Sequencing and Biochemical Examination in Clinical Practice

Here, we report the pediatric cases of sitosterolemia, a rare autosomal-recessive genetic disorder, characterized by high concentrations of plant sterols in blood and heterogeneity manifestations. All three patients (two girls aged 2 and 6 years old, and one boy aged 14 years old) were initially diagnosed with hypercholesterinemia. Next-generation sequencing (NGS) revealed homozygous (p.Leu572Pro/p.Leu572Pro) and compound (p.Leu572Pro/p.Gly512Arg and p.Leu572Pro/p.Trp361*) variants in the ABCG8 gene that allowed for the diagnosis of sitosterolemia. Two patients whose blood phytosterol levels were estimated before the diet demonstrated high levels of sitosterol/campesterol (69.6/29.2 and 28.3/12.4 μmol/L, respectively). Here, we demonstrate that NGS-testing led to the proper diagnosis that is essential for patients' management. The variant p.Leu572Pro might be prevalent among patients with sitosterolemia in Russia.

Case Report: Next Generation Sequencing in Clinical Practice–A Real Tool for Ending the Protracted Diagnostic Odyssey

We reported a case of sitosterolemia, which is a rare genetic disease, characterized by increased plant sterol absorption and great heterogeneity of clinical manifestations. Our patient was initially referred to the lipid clinic due to high cholesterol levels and premature cardiovascular disease. Diagnosis of familial hypercholesterolemia was established in accordance with the Dutch Lipid Clinic Network criteria. Next-generation sequencing was later performed, which revealed a nonsense mutation in the ABCG8 gene, which led to the diagnosis of sitosterolemia. The aim of our report is to demonstrate, how genetic testing helped to make the correct diagnosis and to explain many of the patient's health problems, which etiology remained unclear for many years.

Sitosterolemia: Twenty Years of Discovery of the Function of ABCG5ABCG8

Sitosterolemia is a lipid disorder characterized by the accumulation of dietary xenosterols in plasma and tissues caused by mutations in either ABCG5 or ABCG8. ABCG5 ABCG8 encodes a pair of ABC half transporters that form a heterodimer (G5G8), which then traffics to the surface of hepatocytes and enterocytes and promotes the secretion of cholesterol and xenosterols into the bile and the intestinal lumen. We review the literature from the initial description of the disease, the discovery of its genetic basis, current therapy, and what has been learned from animal, cellular, and molecular investigations of the transporter in the twenty years since its discovery. The genomic era has revealed that there are far more carriers of loss of function mutations and likely pathogenic variants of ABCG5 ABCG8 than previously thought. The impact of these variants on G5G8 structure and activity are largely unknown. We propose a classification system for ABCG5 ABCG8 mutants based on previously published systems for diseases caused by defects in ABC transporters. This system establishes a framework for the comprehensive analysis of disease-associated variants and their impact on G5G8 structure-function.

Cryptogenic Cirrhosis and Sitosterolemia: A Treatable Disease If Identified but Fatal If Missed

Sitosterolemia is an autosomal recessive metabolic disease caused by mutations in ABCG5 or ABCG8 genes which encode for the (ATP)-binding cassette (ABC) transporters that are responsible for the trafficking of xenosterols. Liver involvement is not a recognized manifestation of this disease, and cirrhosis has been reported only once in the medical literature. We describe a fatal case of a 21-year old South Asian male who presented with decompensated cirrhosis, and biochemical abnormalities consistent with sitosterolemia. Genetic testing showed a homozygous pathogenic mutation in ABCG5, confirming the diagnosis. Sitosterolemia is a rare, but likely under-recognized condition, and a high degree of suspicion is imperative to make the diagnosis. We propose that sitosterolemia should be included in the differential diagnosis for patients with cryptogenic cirrhosis, especially as there are effective oral therapies to treat this condition. Newly diagnosed sitosterolemia patients should undergo a thorough hepatology evaluation and follow-up to evaluate for the presence, development, and progression of any hepatic involvement.

Genetic Control of Serum Marinobufagenin in the Spontaneously Hypertensive Rat and the Relationship to Blood Pressure

BACKGROUND

We have investigated serum levels of immunoreactive marinobufagenin (MBG) in 16- to 20-week-old spontaneously hypertensive rats (SHRs)-A3 and in the normotensive Wistar-Kyoto (WKY) rat strain in the absence of salt loading, and we have investigated the genetic control of serum MBG.

METHODS AND RESULTS

We genotyped the F2 progeny of an SHR-A3×WKY intercross using a genome-wide panel of 253 single-nucleotide polymorphism markers that were dimorphic between SHR-A3 and WKY and measured serum MBG by ELISA. Serum MBG levels were lower in SHR-A3 than WKY rats (0.39±0.07 and 1.27±0.40 nmol/L, respectively), suggesting that MBG may not play a role in the markedly divergent blood pressure measured by telemetry in rats of these 2 strains (SHR-A3 and WKY, 198.3±4.43 and 116.8±1.51 mm Hg, respectively). The strain difference in serum MBG was investigated to determine whether genomic regions influencing MBG might be identified by genetic mapping. Quantitative trait locus mapping indicated a single locus influencing serum MBG in the region of chromosome 6q12. Homozygosity of WKY alleles at this locus was associated with increased serum MBG levels. We surveyed whole genome sequences from our SHR-A3 and WKY lines, seeking coding sequence variation between SHR-A3 and WKY within the mapped locus that might explain the inherited strain difference in serum MBG.

CONCLUSIONS

We identified amino acid substitution in the sterol transport protein Abcg5, present in SHR-A3, but absent in WKY, that is a potential mechanism influencing MBG levels.

Effects of Phytosterols as Food Additives on Adrenal and Reproductive Endocrine Function during Sexual Maturation in Male Japanese Quail (Coturnix coturnix japonica)

Varying amounts of phytosterols (PS) occur naturally in several foods of plant origin. PS, which are structurally and functionally similar to cholesterol, have been shown to reduce plasma total cholesterol (TC) and low-density lipoprotein-cholesterol (LDL-C) levels. Moreover, PS disrupts endocrine function in certain animals. In the present study, we investigated the effects of high doses of PS on adrenal and reproductive endocrine function during sexual maturation in Japanese male quails. Two experiments were conducted; in the first experiment, quail chicks were subjected to long-term chronic feeding of PS (8, 80, and 800 mg/kg body weight [BW]) and the chemicals were gavaged into the crop sac from 7-50 days post-hatching. From the forty-fourth day, half of the animals in each group were subjected to a 6-day adrenocorticotropic hormone (ACTH) challenge for artificial stimulation of the adrenal gland and evaluation of long-term PS effects; in the second experiment, single doses of PS were subcutaneously injected (SC) into adult males (10-weeks-old) to assess the acute direct effect. Results indicated that chronically PS-fed animals showed a better adrenal response to ACTH challenge, and the corticosterone levels were higher (P<0.05) than those of the controls. Moreover, corticosterone levels were also high (P<0.05) 3 h after SC injection of PS. In contrast, testosterone levels and the testes weights were significantly lower (P<0.05) in the groups chronically administered with PS. No differences were observed in the testosterone levels in the acute experiment or luteinizing hormone (LH) levels in either experiment. In conclusion, the differential effects of PS on the adrenal gland and testis might be due to preferential use of different lipoprotein-cholesterol forms for steroid production. In addition, PS might locally perturb testosterone production by its accumulation or delay in testicular maturation.

Thyroid Hormone Status in Sitosterolemia Is Modified by Ezetimibe

OBJECTIVES

To assess the association between biomarkers of thyroid status and 5α-stanols in patients with sitosterolemia treated with ezetimibe (EZE).

STUDY DESIGN

Eight patients with sitosterolemia (16-56 years of age) were studied during 14 weeks off EZE therapy and 14 weeks on EZE (10 mg/day). Serum thyroid biomarkers (free triiodothyronine [FT3], free thyroxine [FT4], FT3/FT4 ratio, thyroid-stimulating hormone), 5α-stanols (sitostanol and cholestanol), and cholestanol precursors (total cholesterol and its synthesis marker lathosterol, and 7α-hydroxy-4-cholesten-3-one cholestenol) were measured at baseline and during the 14 weeks off EZE and on EZE.

RESULTS

EZE increased FT3/FT4 (10% ± 4%; P = .02). EZE reduced plasma and red blood cells sitostanol (-38% ± 6% and -20% ± 4%; all P < .05) and cholestanol (-18% ± 6% and -13% ± 3%; all P < .05). The change in plasma cholestanol level on EZE inversely correlated with the change in FT3/FT4 (r = -0.86; P = .01). EZE lowered total cholesterol (P < .0001) and did not affect 7α-hydroxy-4-cholesten-3-one cholestanol. EZE increased (P < .0001) lathosterol initially, but the level was not sustained, resulting in similar levels at week 14 off EZE and on EZE.

CONCLUSION

In patients with STSL, 5α-stanols levels might be associated with thyroid function. EZE reduces circulating 5α-stanols while increasing FT3/FT4, implying increased conversion of T4 to T3, thus possibly improving thyroid hormone status.

TRIAL REGISTRATION

ClinicalTrials.govNCT01584206.

Investigating Sitosterolemia to Understand Lipid Physiology

The cholesterol molecule is at the center of the pathophysiology of many vascular diseases. Whole-body cholesterol pools are maintained by a balance of endogenous synthesis, dietary absorption and elimination from our bodies. While the cellular aspects of cholesterol metabolism received significant impetus from the seminal work of Goldstein and Brown investigating LDL receptor trafficking, how dietary cholesterol was absorbed and eliminated was relatively neglected. The identification of the molecular defect a rare human disorder, Sitosterolemia, led to elucidation of a key mechanism of how we regulate the excretory pathway in the liver and in the intestine. Two proteins, ABCG5 and ABCG8, constitute a heterodimeric transporter that facilitates the extrusion of sterols from the cell into the biliary lumen, with a preference for xenosterols. This mechanism explained how dietary xenosterols are prevented from accumulating in our bodies. In addition, this disease has also highlighted the potential harm of xenosterols; macrothrombocytopenia, liver disease and endocrine disruption are seen when xenosterols accumulate. Mouse models of this disease suggest that there are more dramatic alterations of physiology, suggesting that these highly conserved mechanisms have evolved to prevent these xenosterols from accumulating in our bodies.

Plant Sterols, Stanols, and Sitosterolemia

Phytosterolemia (sitosterolemia) is a rare autosomal recessive sterol storage disease caused by mutations in either of the adenosine triphosphate (ATP) binding cassette transporter genes; (ABC) G5 or ABCG8, leading to impaired elimination of plant sterols and stanols, with their increased accumulation in the blood and tissues. Thus the disease is characterized by substantially elevated serum plant sterols and stanols, with moderate to high plasma cholesterol levels, and increased risk of premature atherosclerosis. Hematologic abnormalities including macrothrombocytopenia, stomatocytosis and hemolysis are frequently observed in sitosterolemia patients. Currently, ezetimibe, a sterol absorption inhibitor, is used as the routine treatment for sitosterolemia, with reported improvement in plant sterol levels and hemolytic parameters. This review summarizes the research related to the health impact of plant sterols and stanols on sitosterolemia.

Non-cholesterol sterols and cholesterol metabolism in sitosterolemia

Sitosterolemia (STSL) is a rare autosomal recessive disease, manifested by extremely elevated plant sterols (PS) in plasma and tissue, leading to xanthoma and premature atherosclerotic disease. Therapeutic approaches include limiting PS intake, interrupting enterohepatic circulation of bile acid using bile acid binding resins such as cholestyramine, and/or ileal bypass, and inhibiting intestinal sterol absorption by ezetimibe (EZE). The objective of this review is to evaluate sterol metabolism in STSL and the impact of the currently available treatments on sterol trafficking in this disease. The role of PS in initiation of xanthomas and premature atherosclerosis is also discussed. Blocking sterols absorption with EZE has revolutionized STSL patient treatment as it reduces circulating levels of non-cholesterol sterols in STSL. However, none of the available treatments including EZE have normalized plasma PS concentrations. Future studies are needed to: (i) explore where cholesterol and non-cholesterol sterols accumulate, (ii) assess to what extent these sterols in tissues can be mobilized after blocking their absorption, and (iii) define the factors governing sterol flux.

Dietary xenosterols lead to infertility and loss of abdominal adipose tissue in sterolin-deficient mice

The investigation of the human disease sitosterolemia (MIM 210250) has shed light not only on the pathways by which dietary sterols may traffic but also on how the mammalian body rids itself of cholesterol and defends against xenosterols. Two genes, ABCG5 and ABCG8, located at the sitosterolemia locus, each encodes a membrane-bound ABC half-transporter and constitutes a functional unit whose activity has now been shown to account for biliary and intestinal sterol excretion. Knockout mice deficient in Abcg5 or Abcg8 recapitulate many of the phenotypic features of sitosterolemia. During the course of our studies to characterize these knockout mice, we noted that these mice, raised on normal rodent chow, exhibited infertility as well as loss of abdominal fat. We show that, although sitosterolemia does not lead to any structural defects or to any overt endocrine defects, fertility could be restored if xenosterols are specifically blocked from entry and that the loss of fat is also reversed by a variety of maneuvers that limit xenosterol accumulation. These studies show that xenosterols may have a significant biological impact on normal mammalian physiology and that the Abcg5 or Abcg8 knockout mouse model may prove useful in investigating the role of xenosterols on mammalian physiology.

Plant sterols and stanols: their role in health and disease

Mammalian physiological processes, and likely any organism with a biliary tree, can distinguish between dietary cholesterol and non-cholesterols, retaining very little of the non-cholesterol in their bodies. Historically, the distinction between plant sterols and cholesterol has been known about for a century or more. That plants sterols were not 'absorbed' has been investigated for almost half a century. Indeed, the oral of plant sterols in gram quantities was shown to interfere with cholesterol absorption and is one of the oldest pharmacological therapies for hypercholesterolemia. Although the basis for the latter was shown to be caused by exclusion of cholesterol from intestinal micelles by plant sterols, it was not until the identification of the a rare genetic disease, sitosterolemia, first described in 1974, that led to the hypothesis that specific molecular mechanism(s) governed both the entry and excretion of sterols by the body. This talk will cover the physiology of dietary sterol metabolism, genetics and pathophysiology of sitosterolemia. Additionally, the role of plant sterols in normal and abnormal metabolism in humans as well as selected animal models will be discussed.

Phytosterol additive boosts adrenal response to ACTH in male Japanese quail (Coturnix coturnix japonica)

To investigate the adrenal effect of a phytosterol (PS) additive, 80 male Japanese quail were divided into four sub-groups and fed 0, 40, 400, and 4,000 ppm of PS, respectively, for 21 days. Subsequently, 50% of the birds from each dosage group were subjected to a 6-day adrenal function test, whereby they were injected with long-lasting adrenocorticotropin (ACTH). The remaining quail in each PS dosage group were raised under normal conditions. The groups receiving 400 and 4000 ppm PS exhibited decreased serum levels of LDL-cholesterol with and without ACTH stimulation (P < 0.01). No amount of dose of PS changed serum corticosterone (CORT) under normal conditions (P > 0.05). Enhancement of CORT was observed on the 2nd and the 6th days of the ACTH challenge in birds receiving 400 ppm (P < 0.05). Average ACTH-induced CORT levels in the 400 ppm group were higher than in the 0 ppm group (P < 0.01). Our results demonstrated that PS can boost ACTH-induced CORT levels in male Japanese quail.

Mechanisms and genetic determinants regulating sterol absorption, circulating LDL levels, and sterol elimination: implications for classification and disease risk

This review integrates historical biochemical and modern genetic findings that underpin our understanding of the low-density lipoprotein (LDL) dyslipidemias that bear on human disease. These range from life-threatening conditions of infancy through severe coronary heart disease of young adulthood, to indolent disorders of middle- and old-age. We particularly focus on the biological aspects of those gene mutations and variants that impact on sterol absorption and hepatobiliary excretion via specific membrane transporter systems (NPC1L1, ABCG5/8); the incorporation of dietary sterols (MTP) and of de novo synthesized lipids (HMGCR, TRIB1) into apoB-containing lipoproteins (APOB) and their release into the circulation (ANGPTL3, SARA2, SORT1); and receptor-mediated uptake of LDL and of intestinal and hepatic-derived lipoprotein remnants (LDLR, APOB, APOE, LDLRAP1, PCSK9, IDOL). The insights gained from integrating the wealth of genetic data with biological processes have important implications for the classification of clinical and presymptomatic diagnoses of traditional LDL dyslipidemias, sitosterolemia, and newly emerging phenotypes, as well as their management through both nutritional and pharmaceutical means.

The ABCG5 ABCG8 sterol transporter and phytosterols: implications for cardiometabolic disease

PURPOSE OF REVIEW

This review summarizes recent developments in the activity, regulation, and physiology of the ABCG5 ABCG8 (G5G8) transporter and the use of its xenobiotic substrates, phytosterols, as cholesterol lowering agents in the treatment of cardiovascular disease. Recent progress has significant implications for the role of G5G8 and its substrates in complications associated with features of the metabolic syndrome.

RECENT FINDINGS

Recent reports expand the clinical presentation of sitosterolemia to include platelet and adrenal dysfunction. The G5G8 sterol transporter is critical to hepatobiliary excretion of cholesterol under nonpathological conditions and has been linked to the cholesterol gallstone susceptibility. Finally, the cardiovascular benefits of cholesterol lowering through the use of phytosterol supplements were offset by vascular dysfunction, suggesting that alternative strategies to reduced cholesterol absorption offer greater benefit.

SUMMARY

Insulin resistance elevates G5G8 and increases susceptibility to cholesterol gallstones. However, this transporter is critical for the exclusion of phytosterols from the absorptive pathways in the intestine. Challenging the limits of this protective mechanism through phytosterol supplementation diminishes the cardioprotective benefits of cholesterol lowering in mouse models of cardiovascular disease.