Family sitosterolemia: report of two cases in Colombia

Sitosterolemia is an autosomal recessive and very rare disease. Its main characteristic is that there is a greater absorption and a decrease in the excretion of sterols, which leads to them being deposited in tissues. It is given by mutations in the ABCG5 or ABCG8 genes found on chromosome 2p21. In this clinical note, we describe the first two patients with familial sitosterolemia described in Colombia, brothers, one of them with xanthomas in extremities as the only symptom, and the other, completely asymptomatic. Genetic studies were performed as a diagnostic test in both patients, where a pathogenic homozygous variant could be identified in the ABCG8 gene in the first case (symptomatic), and a heterozygous variant in the ABCG8 gene in the second case (asymptomatic); the first patient has responded to treatment with ezetimibe. In conclusion, xanthomas should be studied in depth in pediatric age as they may be the only visible sign of such complex and hereditary diseases as familial sitosterolemia, which can be controlled and prevent cardiovascular complications of the disease.

A Clinical Case of Probable Sitosterolemia

Sitosterolemia is a rare genetic lipid disorder characterized by elevated plant sterols in the serum. A 24-year-old Japanese woman was referred to our hospital due to a high serum low-density lipoprotein cholesterol (LDL-C) level of 332 mg/dL. At first, she was suspected to suffer from familial hypercholesterolemia, and thus received lipid-lowering agents. Although her LDL-C level remained high (220 mg/dL) with diet therapy plus 10 mg/day rosuvastatin, it was drastically decreased to 46 mg/dL with the addition of 10 mg/day ezetimibe. Finally, her LDL-C level was well-controlled at about 70 mg/dL with 10 mg/day ezetimibe alone. Furthermore, while her serum sitosterol level was elevated at 10.5 μg/mL during the first visit to our hospital, it decreased to 3.6 μg/mL with the 10 mg/day ezetimibe treatment alone. These observations suggest that she might probably suffer from sitosterolemia. Therefore, targeted gene sequencing analysis was performed using custom panels focusing on the exome regions of 21 lipid-associated genes, including ABCG5, ABCG8, and familial hypercholesterolemia-causing genes (LDL receptor, LDLRAP1, PCSK9, and apolipoprotein B). We finally identified a heterozygous ABCG8 variant (NM_022437.2:c.1285A>G or NP_071882.1:p.Met429Val) in our patient. The same gene mutation was detected in her mother. We report here a rare case exhibiting probable sitosterolemia caused by a heterozygous Met429Val variant in the ABCG8 gene and additional unknown variants.

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.

Correlation of serum biochemical characteristics and ABCG8 genetic variant (rs 11887534) with gall stone compositions and risk of gallstone disease in Egyptian patients

BACKGROUND

The overexpression of the cholesterol transporter: ATP-binding cassette transporter (ABCG8) due to the effect of ABCG8 genetic variant (rs11887534) leads to the precipitation of cholesterol crystals and gallstone disease (GSD).

OBJECTIVE

To evaluate the chemical composition of gallstones and the role of ABCG8 (rs11887534) in the susceptibility to GSD.

METHODS

We enrolled 77 patients with GSD treated with standard laparoscopic or open cholecystectomy and 75 age and sex-matched healthy controls. Chemical analysis of the extracted gallstones was performed. Analysis of the rs11887534 was performed by real-time PCR TaqMan technique for both cases and controls.

RESULTS

Pure cholesterol stones were the main type of stones in GSD patients. The CC genotype carriers of rs11887534 were more prone to gallstone formation than other genotypes. The CC genotype carriers were 100 folds at increased risk to develop pure cholesterol stones.

CONCLUSION

The most prevalent type of gallbladder stones is pure cholesterol stone. ABCG8 (rs11887534) could be associated with increased risk for cholesterol gallstones formation, this risk was more pronounced in female patients.

Differential Effects of ABCG5/G8 Gene Region Variants on Lipid Profile, Blood Pressure Status, and Gallstone Disease History in Taiwan

ABCG5 and ABCG8 are two key adenosine triphosphate-binding cassette (ABC) proteins that regulate whole-body sterol trafficking. This study aimed to elucidate the association between ABCG5/G8 gene region variants and lipid profile, cardiometabolic traits, and gallstone disease history in Taiwan. A total of 1494 Taiwan Biobank participants with whole-genome sequencing data and 117,679 participants with Axiom Genome-Wide CHB Array data were enrolled for analysis. Using genotype-phenotype and stepwise linear regression analyses, we found independent associations of four Asian-specific ABCG5 variants, rs119480069, rs199984328, rs560839317, and rs748096191, with total, low-density lipoprotein (LDL), and non-high-density lipoprotein (HDL) cholesterol levels (all p ≤ 0.0002). Four other variants, which were in nearly complete linkage disequilibrium, exhibited genome-wide significant associations with gallstone disease history, and the ABCG8 rs11887534 variant showed a trend of superiority for gallstone disease history in a nested logistic regression model (p = 0.074). Through regional association analysis of various other cardiometabolic traits, two variants of the PLEKHH2, approximately 50 kb from the ABCG5/G8 region, exhibited significant associations with blood pressure status (p < 10^-6). In conclusion, differential effects of ABCG5/G8 region variants were noted for lipid profile, blood pressure status, and gallstone disease history in Taiwan. These results indicate the crucial role of individualized assessment of ABCG5/G8 variants for different cardiometabolic phenotypes.

Snapshots of ABCG1 and ABCG5/G8: A Sterol's Journey to Cross the Cellular Membranes

The subfamily-G ATP-binding cassette (ABCG) transporters play important roles in regulating cholesterol homeostasis. Recent progress in the structural data of ABCG1 and ABCG5/G8 disclose putative sterol binding sites that suggest the possible cholesterol translocation pathway. ABCG1 and ABCG5/G8 share high similarity in the overall molecular architecture, and both transporters appear to use several unique structural motifs to facilitate cholesterol transport along this pathway, including the phenylalanine highway and the hydrophobic valve. Interestingly, ABCG5/G8 is known to transport cholesterol and phytosterols, whereas ABCG1 seems to exclusively transport cholesterol. Ligand docking analysis indeed suggests a difference in recruiting sterol molecules to the known sterol-binding sites. Here, we further discuss how the different and shared structural features are relevant to their physiological functions, and finally provide our perspective on future studies in ABCG cholesterol transporters.

Structural analysis of cholesterol binding and sterol selectivity by ABCG5/G8

The ATP-binding cassette (ABC) sterol transporters are responsible for maintaining cholesterol homeostasis in mammals by participating in reverse cholesterol transport (RCT) or transintestinal cholesterol efflux (TICE). The heterodimeric ABCG5/G8 carries out selective sterol excretion, preventing the abnormal accumulation of plant sterols in human bodies, while homodimeric ABCG1 contributes to the biogenesis and metabolism of high-density lipoproteins. A sterol-binding site on ABCG5/G8 was proposed at the interface of the transmembrane domain and the core of lipid bilayers. In this study, we have determined the crystal structure of ABCG5/G8 in a cholesterol-bound state. The structure combined with amino acid sequence analysis shows that in the proximity of the sterol-binding site, a highly conserved phenylalanine array supports functional implications for ABCG cholesterol/sterol transporters. Lastly, in silico docking analysis of cholesterol and stigmasterol (a plant sterol) suggests sterol-binding selectivity on ABCG5/G8, but not ABCG1. Together, our results provide a structural basis for cholesterol binding on ABCG5/G8 and the sterol selectivity by ABCG transporters.

Identification and action mechanism of lipid regulating components from Rhei Radix et rhizoma

ETHNOPHARMACOLOGICAL RELEVANCE

Rhei Radix et Rhizoma is widely used in Traditional Chinese Medicine to attack stagnation, clear damp heat, relieve fire, cool blood, remove blood stasis and detoxify recorded in Chinese Pharmacopoeia. Modern pharmacological research has showed the extract of Rhei Radix et Rhizoma has the effect of lowering blood lipids, but the main active components and their mechanisms are still not clear.

AIM OF THE STUDY

To reveal the lipid regulating components from Rhei Radix et Rhizoma and preliminarily explore their related action mechanisms.

MATERIALS AND METHODS

A rat model of dyslipidemia was established by administration of a high-fat emulsion via gavage, and the intervention effect of different polar fractions of Rhei Radix et Rhizoma on rat blood lipids as well as their related action mechanisms were preliminarily investigated. The effective components were inferred based on the above tests and identified by high performance liquid chromatography in comparison with reference substances, their UV absorption and high resolution mass spectra characteristics.

RESULTS

The extract with dichloromethane fraction (DF) containing rhubarb free anthraquinones (aloe-emodin, rhein, emodin, chrysophanol and physcion) significantly regulated the disordered blood lipids, lowered TC and LDLC, reversed TG and increased HDLC level in dyslipidemic rats and also showed lipid-lowering effect on lipid abnormalities in HepG2 cells. DF could alter the signaling pathways such as PPARα and AMPK implicated in lipid metabolism, and it down-regulated the mRNA expression of liver APOA2, SCD-1, HMGCR, SREBP-2 and PCSK9, but up-regulated the expressions of liver APOE, LPL and intestinal ABCG8. Besides, it could change the composition of Firmicutes, Bacteroidetes and Proteobacteria in dyslipidemic rat feces samples.

CONCLUSIONS

Rhubarb free anthraquinones have a significant regulating effect on the levels of serum TC, LDLC and HDLC, and probably possess a bidirectional regulatory effect on TG level in dyslipidemic rats. These effects may be achieved by regulating the expressions of the liver PPARα and SREBP target genes, PCSK9 and the intestinal ABCG8 genes, which are involved in blood cholesterol transport, liver lipid metabolism and intestinal cholesterol excretion. Rhubarb free anthraquinones may also affect energy metabolism by changing the composition of gut microflora related to lipid metabolism in dyslipidemic rats.

Features of chinese patients with sitosterolemia

Background

Sitosterolemia is a lipid disorder characterized by the accumulation of phytosterols in plasma and organs, caused by mutations in the ABCG5 and/or ABCG8 genes. The disease is frequently misdiagnosed and mistreated as familial hypercholesterolemia (FH). To gain a better understanding of the disease, the current status of diagnosis and treatment of Chinese patients with sitosterolemia was reviewed and summarized.

Method

Literature search was performed. The clinical features and molecular characteristics of Chinese patients with sitosterolemia were analysed. Four children with sitosterolemia and the treatment experience were described.

Results

Fifty-five patients with sitosterolemia have been reported in China. These patients were aged from 3 months to 67 years at diagnosis, and the median was 8 years of age. Several complications, such as xanthomas in 47 patients (85%), thrombocytopenia in 17 patients (31%), anemia in 14 patients (25%), and cardiovascular damage in 12 patients (22%), were observed. Thirty-nine patients (71%) exhibited mutations in the ABCG5 gene, 15 patients (27%) showed mutations in ABCG8, and variations in both genes occurred in one patient (2%). A patient with two clinically rare diseases, namely, sitosterolemia and glycogen storage disease type VI (GSD VI)), is reported here for the first time. The four reported patients were treated with low cholesterol and phytosterol-limited diet alone or combined with cholestyramine. Even though decreases were observed for total plasma cholesterol (TC) and low-density-lipoprotein cholesterol (LDL-C), and these levels were as low as normal in some patients, the levels of plant sterols remained above the normal range. However, TC, LDL-C and plant sterol levels remained at high levels in patients treated with a control diet control only.

Conclusions

The analysis reveals that different from Caucasians carrying mainly variations in ABCG8, most Chinese patients have mutations in the ABCG5 gene, and Arg446Ter, Gln251Ter, anArg389His might be hot-spot mutations in Chinese patients. The current survey provides clinical data to enable the development of a standardized protocol for the diagnosis and treatment of sitosterolemia in China.

Risk of gallstones based on ABCG8 rs11887534 single nucleotide polymorphism among Taiwanese men and women

BACKGROUND

Gallstones are abnormal masses caused by impaired metabolism of cholesterol, bilirubin, or bile salts in the gallbladder or biliary tract. ATP-binding cassette subfamily G member 8 (ABCG8) is a protein that regulates cholesterol efflux from the liver. Genome-wide association studies (GWAS) and meta-analyses of GWAS revealed the ABCG8 rs11887534 variant as the most common genetic determinant of gallstones in humans. These findings have not been extensively replicated in Taiwanese. Therefore, we appraised the relationship between gallstones and rs11887534 in a relatively large Taiwanese sample.

METHODS

We retrieved data collected through questionnaires, physical and biochemical tests from the Taiwan Biobank Bank (TWB). The study participants comprised 7388 men and 13,880 women who voluntarily enrolled in the Taiwan Biobank project between 2008 and 2019. Gallstones were self-reported.

RESULTS

The overall sample size was 21,268 comprising 938 gallstone patients and 20,330 non-gallstone individuals. Among the participants, 20,640 had the GG and 628 had the GC + CC genotype. At p-value < 0.05, the baseline genotypes and gallstone status between men and women were not significantly different. The risk of gallstones was higher in participants having the GC + CC compared to the GG genotype: odds ratio (OR); 95% confidence interval (CI) = 1.698; 1.240-2.325), but was lower in men compared to women (OR = 0.763; 95% CI = 0.638-0.913). Compared to men with the rs11887534 GG genotype, women with the GG and GC + CC genotypes had a higher risk of gallstone (OR; 95% CI = 1.304; 1.087-1.565 for GG and 2.291; 1.514-3.467 for GC + CC). The positive association between GC + CC and gallstones was retained after we restricted the analysis to the female participants (OR; 95% CI = 1.789 = 1.208-2.648). Hormone use was associated with an elevated risk of gallstones (OR; 95% CI = 1.359; 1.107-1.668). Relative to GG and no hormone use, we found a significantly high risk among hormone users with the GC + CC genotype (OR; 95% CI = 3.596; 1.495-8.650).

CONCLUSIONS

The rs11887534 GC + CC genotype was independently associated with a higher risk of gallstones. This risk was much higher among women, especially those who used hormones for various gynecological purposes.

Clinical, genetic profile and therapy evaluation of 55 children and 5 adults with sitosterolemia

BACKGROUND

Sitosterolemia is a rare autosomal recessive disease characterized by phytosterol accumulation in the blood and tissues. However, the detailed clinical and genetic spectra are lacking.

OBJECTIVE

To describe and compare the clinical, biochemical, genetic, therapeutic, and follow-up characteristics of 55 pediatric and five adult sitosterolemia patients.

METHODS

Clinical, genetic and therapeutic data from 60 patients at Xinhua Hospital from January 2016 to June 2021 were retrospectively collected.

RESULTS

Pediatric patients' manifestations included xanthomas(93%), hematological disorders(30%), arthralgia(24%), splenomegaly(11%), atherosclerosis(10%). Adult patients had symptoms such as atherosclerosis(5/5), xanthomas(4/5), hematological disorders(3/5), arthralgia(3/5), splenomegaly(3/5). Elevated total cholesterol(TC) and low-density lipoprotein cholesterol(LDL-C) were observed in 96% patients (pediatric 98%, adult 3/4), and phytosterol levels in 100% patients. The age of onset was also negatively correlated with blood TC (P < 0.0001, r = -0.5548) and LDL-C (P = 0.0001, r = -0.4859) levels. Targeted treatments resulted in symptomatic remission(pediatric 96%, adult 4/5), and significantly decreased lipid and phytosterol levels(all P<0.05). In the dietary-therapy cohort(n=34), blood lipid levels decreased(all P<0.05). In the 13 pediatric patients from the dietary-therapy cohort who switched from dietary to combination therapy with ezetimibe, dietary therapy decreased TC and LDL-C levels by 54% and 52%, and ezetimibe further decreased them by 18% and 20%, respectively. Further, we identified 15 novel ABCG5/ABCG8 variants.

CONCLUSIONS

This study expands the clinical and genetic spectra of sitosterolemia. The low-phytosterol diet is the cornerstone of sitosterolemia treatment. Ezetimibe can further decrease blood lipid levels and increase daily dietary phytosterol tolerance.

The association of cytochrome 7A1 and ATP-binding cassette G8 genotypes with type 2 diabetes among Jordanian patients

OBJECTIVES

Increased cholesterol levels were found to be associated with diabetes mellitus type II (DM2). The cholesterol is metabolized by cytochrome 7A1 (CYP7A1) and transported in the intestine by ATP-binding cassette G8 (ABCG8). Genetic variants in CYP7A1 and ABCG8 genes can affect the cholesterol levels. The aim of this study is to compare the frequency of CYP7A1 rs3808607 and ABCG8 rs11887534 and rs4148217 genotypes between healthy and DM2 subjects from Jordanian population.

METHODS

A total of 117 DM2 patients and 100 healthy controls, of Jordanian Arabic origin, were genotyped for CYP7A1 rs3808607 and ABCG8 rs11887534 and rs4148217 genetic variants using polymerase chain reaction (PCR) followed by restriction fragment length polymorphism technique.

RESULTS

The study showed that homozygosity of rs3808607 (A-204C) genotype in CYP7A1 was significantly higher in DM2 patients (ANOVA, p<0.05) with an odd ratio of 2.66, but rs11887534 (G55C) and rs4148217 (C1199A) genetic polymorphisms in ABCG8 were found in comparable frequencies in both healthy and DM2 subjects.

CONCLUSIONS

The results of this study indicate that CYP7A1 rs3808607 genetic polymorphism is associated with DM2. Further clinical studies are required to confirm this finding among DM2 patients of Jordanian origin.

Effects of SNVs in ABCA1, ABCG1, ABCG5, ABCG8, and SCARB1 Genes on Plasma Lipids, Lipoproteins, and Adiposity Markers in a Brazilian Population

Several proteins are involved in cholesterol homeostasis, as scavenger receptor class B type I and ATP-binding cassette (ABC) transporters including ABCA1, ABCG1, ABCG5, and ABCG8. This study aimed to determine the effects of single nucleotide variants (SNVs) rs2275543 (ABCA1), rs1893590 (ABCG1), rs6720173 (ABCG5), rs6544718 (ABCG8), and rs5888 (SCARB1) on plasma lipids, lipoproteins, and adiposity markers in an asymptomatic population and its sex-specific effects. Volunteers (n = 590) were selected and plasma lipids, lipoproteins, and adiposity markers (waist-to-hip and waist-to-height ratios, lipid accumulation product and body adiposity index) were measured. Genomic DNA was isolated from peripheral blood cells according to the method adapted from Gross-Bellard. SNVs were detected in the TaqMan® OpenArray® Real-Time polymerase chain reaction platform and data analyses were performed using the TaqMan® Genotyper Software. The rs2275543*C point to an increase of high-density lipoprotein size in females while in males very-low-density lipoprotein, cholesterol, and triglycerides were statistically lower (P value < 0.05). The rs1893590*C was statistically associated with lower apolipoprotein A-I levels and higher activities of paraoxonase-1 and cholesteryl ester transfer protein (P value < 0.05). The rs6720173 was statistically associated with an increase in cholesterol and low-density lipoprotein cholesterol in males; moreover, rs6544718*T reduced adiposity markers in females (P value < 0.05). Regarding the rs5888, a decreased adiposity marker in the total population and in females occurred (P value < 0.05). Multivariate analysis of variance showed that SNVs could influence components of high-density lipoprotein metabolism, mainly through ABCG1 (P value < 0.05). The ABCA1 and ABCG5 variants showed sex-specific effects on lipids and lipoproteins, while SCARB1 and ABCG8 variants might influence adiposity markers in females. Our data indicate a possible role of ABCG1 on HDL metabolism.

Molecular basis of cholesterol efflux via ABCG subfamily transporters

The ABCG1 homodimer (G1) and ABCG5-ABCG8 heterodimer (G5G8), two members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter G family, are required for maintenance of cellular cholesterol levels. G5G8 mediates secretion of neutral sterols into bile and the gut lumen, whereas G1 transports cholesterol from macrophages to high-density lipoproteins (HDLs). The mechanisms used by G5G8 and G1 to recognize and export sterols remain unclear. Here, we report cryoelectron microscopy (cryo-EM) structures of human G5G8 in sterol-bound and human G1 in cholesterol- and ATP-bound states. Both transporters have a sterol-binding site that is accessible from the cytosolic leaflet. A second site is present midway through the transmembrane domains of G5G8. The Walker A motif of G8 adopts a unique conformation that accounts for the marked asymmetry in ATPase activities between the two nucleotide-binding sites of G5G8. These structures, along with functional validation studies, provide a mechanistic framework for understanding cholesterol efflux via ABC transporters.

An association of ABCG8: rs11887534 polymorphism and HDL-cholesterol response to statin treatment in the Polish population

Background

Variation in lipid changes in response to statin treatment is associated with genetic polymorphism. Sterolin-1, encoded by ABCG5, and sterolin-2, encoded by ABCG8, together form a sterol transporter. There are some reports indicating association of rs11887534 (ABCG8:c.55G > C) polymorphism with lipid concentrations, both prior to and after statin treatment. The aim of this study was to analyze both baseline plasma lipids and their concentrations in response to statin treatment with regard to ABCG8: rs11887534 polymorphism in Caucasian patients of Polish origin.

Methods

The study group consisted of 170 consecutive adult out-patients treated with atorvastatin or simvastatin for a minimum of 2 months. Concentrations of triglycerides (TG), total cholesterol (TC), LDL-cholesterol (LDL-C) and HDL-cholesterol (HDL-C) were measured before and after statin treatment. The ABCG8 polymorphism was identified by mini-sequencing genomic DNA extracted from peripheral blood leukocytes.

Results

There were no significant differences in regard to ABCG8 variants for baseline TG, TC, LDL-C and HDL-C as well as for TG, TC or LDL-C concentrations after statin treatment. However, patients carrying at least one C allele showed a decrease in post-statin HDL-C concentrations and the absolute and relative changes between post- and pre-statin HDL-C concentrations were negative in contrast to positive values in wild-type homozygotes.

Conclusions

Our results suggest that the c.55C allele of the ABCG8: rs11887534 polymorphism might be associated with decrease in HDL-cholesterol in response to statin treatment in Polish patients.

Diagnosis and Management of Sitosterolemia 2021

Sitosterolemia is an inherited metabolic disorder characterized by increased levels of plant sterols, such as sitosterol. This disease is caused by loss-of-function genetic mutations in ATP-binding cassette (ABC) subfamily G member 5 or member 8 ( ABCG5 or ABCG8 , respectively), both of which play important roles in selective excretion of plant sterols from the liver and intestine, leading to failure to prevent absorption of food plant sterols. This disorder has been considered to be extremely rare. However, accumulated clinical data as well as genetics suggest the possibility of a much higher prevalence. Its clinical manifestations resemble those observed in patients with familial hypercholesterolemia (FH), including tendon xanthomas, hyper LDL-cholesterolemia, and premature coronary atherosclerosis. We provide an overview of this recessive genetic disease, diagnostic as well as therapeutic tips, and the latest diagnostic criteria in Japan.

Transmembrane Polar Relay Drives the Allosteric Regulation for ABCG5/G8 Sterol Transporter

The heterodimeric ATP-binding cassette (ABC) sterol transporter, ABCG5/G8, is responsible for the biliary and transintestinal secretion of cholesterol and dietary plant sterols. Missense mutations of ABCG5/G8 can cause sitosterolemia, a loss-of-function disorder characterized by plant sterol accumulation and premature atherosclerosis. A new molecular framework was recently established by a crystal structure of human ABCG5/G8 and reveals a network of polar and charged amino acids in the core of the transmembrane domains, namely, a polar relay. In this study, we utilize genetic variants to dissect the mechanistic role of this transmembrane polar relay in controlling ABCG5/G8 function. We demonstrated a sterol-coupled ATPase activity of ABCG5/G8 by cholesteryl hemisuccinate (CHS), a relatively water-soluble cholesterol memetic, and characterized CHS-coupled ATPase activity of three loss-of-function missense variants, R543S, E146Q, and A540F, which are respectively within, in contact with, and distant from the polar relay. The results established an in vitro phenotype of the loss-of-function and missense mutations of ABCG5/G8, showing significantly impaired ATPase activity and loss of energy sufficient to weaken the signal transmission from the transmembrane domains. Our data provide a biochemical evidence underlying the importance of the polar relay and its network in regulating the catalytic activity of ABCG5/G8 sterol transporter.

The ABCs of Sterol Transport

Cholesterol homeostasis and trafficking are critical to the maintenance of the asymmetric plasma membrane of eukaryotic cells. Disruption or dysfunction of cholesterol trafficking leads to numerous human diseases. ATP-binding cassette (ABC) transporters play several critical roles in this process, and mutations in these sterol transporters lead to disorders such as Tangier disease and sitosterolemia. Biochemical and structural information on ABC sterol transporters is beginning to emerge, with published structures of ABCA1 and ABCG5/G8; these two proteins function in the reverse cholesterol transport pathway and mediate the efflux of cholesterol and xenosterols to high-density lipoprotein and bile salt micelles, respectively. Although both of these transporters belong to the ABC family and mediate the efflux of a sterol substrate, they have many distinct differences. Here, we summarize the current understanding of sterol transport driven by ABC transporters, with an emphasis on these two extensively characterized transporters.

Picky ABCG5/G8 and promiscuous ABCG2 - a tale of fatty diets and drug toxicity

Structural data on ABCG5/G8 and ABCG2 reveal a unique molecular architecture for subfamily G ATP‐binding cassette (ABCG) transporters and disclose putative substrate‐binding sites. ABCG5/G8 and ABCG2 appear to use several unique structural motifs to execute transport, including the triple helical bundles, the membrane‐embedded polar relay, the re‐entry helices, and a hydrophobic valve. Interestingly, ABCG2 shows extreme substrate promiscuity, whereas ABCG5/G8 transports only sterol molecules. ABCG2 structures suggest a large internal cavity, serving as a binding region for substrates and inhibitors, while mutational and pharmacological analyses support the notion of multiple binding sites. By contrast, ABCG5/G8 shows a collapsed cavity of insufficient size to hold substrates. Indeed, mutational analyses indicate a sterol‐binding site at the hydrophobic interface between the transporter and the lipid bilayer. In this review, we highlight key differences and similarities between ABCG2 and ABCG5/G8 structures. We further discuss the relevance of distinct and shared structural features in the context of their physiological functions. Finally, we elaborate on how ABCG2 and ABCG5/G8 could pave the way for studies on other ABCG transporters.

The effect of ursodeoxycholic acid on the relative expression of the lipid metabolism genes in mouse cholesterol gallstone models

BACKGROUND

Many studies indicate that gallstone formation has genetic components. The abnormal expression of lipid-related genes could be the basis for particular forms of cholesterol gallstone disease. The aim of this study was to obtain insight into lipid metabolism disorder during cholesterol gallstone formation and to evaluate the effect of ursodeoxycholic acid (UDCA) on the improvement of bile lithogenicity and its potential influence on the transcription of lipid-related genes.

METHODS

Gallstone-susceptible mouse models were induced by feeding with a lithogenic diet (LD) for 8 weeks. Bile and liver tissues were obtained from these mouse models after 0, 4 and 8 weeks. Bile lipids were measured enzymatically, and the cholesterol saturation index (CSI) was calculated to evaluate the bile lithogenicity by using Carey's critical tables. Real-time polymerase chain reaction (RT-PCR) was used to detect the mRNA expression levels of farnesoid X receptor (FXR), liver X receptor (LXR), adenosine triphosphate-binding cassette subfamily G member 5/8 (ABCG5/8), cholesterol 7-α hydroxylase (CYP7A1), oxysterol 7-α hydroxylase (CYP7B1), sterol 27-α hydroxylase (CYP27A1), peroxisome proliferator-activated receptor alpha (PPAR-α) and adenosine triphosphate-binding cassette subfamily B member 11 (ABCB11).

RESULTS

The rate of gallstone formation was 100% in the 4-week group but only 30% in the UDCA-treated group. The UDCA-treated group had a significantly lower CSI compared with other groups. Of special note, the data on the effects of UDCA showed higher expression levels of ABCG8, ABCB11 and CYP27A1, as well as lower expression levels of LXR and PPAR-α, compared to the model control group.

CONCLUSIONS

UDCA exhibits tremendously potent activity in restraining lipid accumulation, thus reversing the lithogenic effect and protecting hepatocytes from serious pathological damage. The abnormal expression of ABCG8, CYP7A1, CYP27A1, LXR and PPAR-α might lead to high lithogenicity of bile. These results are helpful in exploring new lipid metabolism pathways and potential targets for the treatment of cholesterol stones and for providing some basis for the study of the pathogenesis and genetic characteristics of cholelithiasis. Research on the mechanism of UDCA in improving lipid metabolism and bile lithogenicity may be helpful for clinical treatment and for reducing the incidence of gallstones.

Beneficial effect of ezetimibe-atorvastatin combination therapy in patients with a mutation in ABCG5 or ABCG8 gene

Background

Use of ezetimibe on top of statin therapy has been shown to be effective to reduce LDL cholesterol level in hypercholesterolemic patients. However, little is known regarding the individual variety of the effectiveness of ezetimibe. We hypothesized that hypercholesterolemic patients with a mutation in ABCG5 or ABCG8 gene exhibit better response to ezetimibe than those without, based on the fact that ezetimibe is hyper-effective for in patients with sitosterolemia caused by ABCG5 or ABCG8 genetic mutations.

Methods

Electronical medical record were reviewed in a total of 321 hypercholesterolemic patients (baseline LDL cholesterol = 192 ± 46 mg/dl) prescribed ezetimibe 10 mg daily on top of atorvastatin 10 mg daily who had undergone genetic analysis of ABCG5 or ABCG8 gene in our institute since 2006 to 2017. Pathogenicity of the variants were determined using standard variant filtering schema, including minor allele frequency, in silico annotation tools. Patients were divided into 2 groups based on the presence of ABCG5 or ABCG8 mutation. We compared the percent reduction of LDL cholesterol as well as the achieved LDL cholesterol levels between these 2 groups.

Results

We found 26 (8%) individuals who exhibit deleterious mutations in ABCG5 or ABCG8 gene. Baseline characteristics under the atorvastatin 10 mg therapy were comparable in age, gender, and LDL cholesterol level between 2 groups. Under these conditions, percent reduction of LDL cholesterol in mutation positive group was significantly larger than that of mutation negative group (28 ± 16% vs. 39 ± 21%, p < 0.05). As a result, the achieved LDL cholesterol level in mutation positive group was significantly lower than that of mutation negative group (87 ± 29 mg/dl vs. 72 ± 26% mg/dl, p < 0.05).

Conclusion

These results suggest that ezetimibe-atorvastatin combination therapy might be more beneficial in hypercholesterolemic patients with a mutation in ABCG5 or ABCG8 gene.

Genes Potentially Associated with Familial Hypercholesterolemia

This review addresses the contribution of some genes to the phenotype of familial hypercholesterolemia. At present, it is known that the pathogenesis of this disease involves not only a pathological variant of low-density lipoprotein receptor and its ligands (apolipoprotein B, proprotein convertase subtilisin/kexin type 9 or low-density lipoprotein receptor adaptor protein 1), but also lipids, including sphingolipids, fatty acids, and sterols. The genetic cause of familial hypercholesterolemia is unknown in 20%–40% of the cases. The genes STAP1 (signal transducing adaptor family member 1), CYP7A1 (cytochrome P450 family 7 subfamily A member 1), LIPA (lipase A, lysosomal acid type), ABCG5 (ATP binding cassette subfamily G member 5), ABCG8 (ATP binding cassette subfamily G member 8), and PNPLA5 (patatin like phospholipase domain containing 5), which can cause aberrations of lipid metabolism, are being evaluated as new targets for the diagnosis and personalized management of familial hypercholesterolemia.

Rare and Deleterious Mutations in ABCG5/ABCG8 Genes Contribute to Mimicking and Worsening of Familial Hypercholesterolemia Phenotype

BACKGROUND

A substantial proportion of patients clinically diagnosed as having familial hypercholesterolemia (FH) do not manifest causative mutation(s) in the FH genes such asLDLR,APOB, andPCSK9. We aimed to evaluate the effect of rare and deleterious mutation(s) inABCG5/ABCG8on hyper-low-density lipoprotein (LDL) cholesterolemia in individuals who meet the clinical criteria for FH.Methods and Results:We compared the LDL cholesterol (LDL-C) values among 487 subjects with FH; the subjects were grouped according to the presence of mutation(s) in FH andABCG5/ABCG8genes. We identified 276 individuals with a deleterious mutation in 1 FH gene (57%, monogenic FH), but found no causative mutations in 156 individuals (32%, mutation-negative). A total of 37 individuals had deleterious mutations inABCG5orABCG8, but not in FH genes (8%,ABCG5/ABCG8mutation carriers). Among these, 3 individuals had sitosterolemia (0.6%) with double mutations. We also identified 18 individuals with deleterious mutations in an FH gene andABCG5orABCG8(4%,ABCG5/ABCG8-oligogenic FH). Subjects without mutations had significantly higher polygenic scores than those in any other groups. LDL-C levels in oligogenic FH subjects were significantly higher than in the monogenic FH subjects. Moreover, sitosterol/lathosterol levels were significantly affected by those mutations.

CONCLUSIONS

The results suggested that rare and deleterious mutations inABCG5/ABCG8contribute substantially to mimicking and exacerbation of the FH phenotype.

Heterozygous familial hypercholesterolaemia in a pair of identical twins: a case report and updated review

BACKGROUND

Familial hypercholesterolaemia (FH) is the most common inherited metabolic disease with an autosomal dominant mode of inheritance. It is characterised by raised serum levels of total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-c), leading to premature coronary artery disease. Children with FH are subjected to early and enhanced atherosclerosis, leading to greater risk of coronary events, including premature coronary artery disease. To the best of our knowledge, this is the first report of a pair of monochorionic diamniotic identical twins with a diagnosis of heterozygous FH, resulting from mutations in both LDLR and ABCG8 genes.

CASE PRESENTATION

This is a rare case of a pair of 8-year-old monochorionic diamniotic identical twin, who on family cascade screening were diagnosed as definite FH, according to the Dutch Lipid Clinic Criteria (DLCC) with a score of 10. There were no lipid stigmata noted. Baseline lipid profiles revealed severe hypercholesterolaemia, (TC = 10.5 mmol/L, 10.6 mmol/L; LDL-c = 8.8 mmol/L, 8.6 mmol/L respectively). Their father is the index case who initially presented with premature CAD, and subsequently diagnosed as FH. Family cascade screening identified clinical FH in other family members including their paternal grandfather who also had premature CAD, and another elder brother, aged 10 years. Genetic analysis by targeted next-generation sequencing using MiSeq platform (Illumina) was performed to detect mutations in LDLR, APOB100, PCSK9, ABCG5, ABCG8, APOE and LDLRAP1 genes. Results revealed that the twin, their elder brother, father and grandfather are heterozygous for a missense mutation (c.530C > T) in LDLR that was previously reported as a pathogenic mutation. In addition, the twin has heterozygous ABCG8 gene mutation (c.55G > C). Their eldest brother aged 12 years and their mother both had normal lipid profiles with absence of LDLR gene mutation.

CONCLUSION

A rare case of Asian monochorionic diamniotic identical twin, with clinically diagnosed and molecularly confirmed heterozygous FH, due to LDLR and ABCG8 gene mutations have been reported. Childhood FH may not present with the classical physical manifestations including the pathognomonic lipid stigmata as in adults. Therefore, childhood FH can be diagnosed early using a combination of clinical criteria and molecular analyses.

Sitosterolemia, Hypercholesterolemia, and Coronary Artery Disease

Sitosterolemia is a rare inherited disease characterized by increased levels of plant sterols, such as sitosterol. The cause of this disease is ATP-binding cassette (ABC) subfamily G member 5 or member 8 (ABCG5 or ABCG8, respectively) gene mutations. Recent advances in genetics have revealed that the prevalence of subjects with deleterious mutations in ABCG5 and/or ABCG8 genes could be more than 1 in ~200,000 individuals among the general population. Furthermore, accumulated evidence, including infantile cases exhibiting progression/regression of systemic xanthomas associated with LDL cholesterol levels, have shown that the elevation of LDL cholesterol seems to be the major cause of development of atherosclerosis and not the elevation of sitosterol. Regarding therapies, LDL apheresis, as well as proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, could be useful for sitosterolemia, in addition to ezetimibe and/or colestimide. In this study, we provide the current understanding and future perspectives of sitosterolemia, which is currently considered an extremely rare disorder but is expected to be much more prevalent in clinical settings.

Mutations in the ABCG8 gene are associated with sitosterolaemia in the homozygous form and xanthelasmas in the heterozygous form

Sitosterol is the most abundant plant sterol found in our diet. Sitosterolemia (OMIM 210250), also known as phytosterolaemia, is a rare autosomal recessive disease caused by the inability to efficiently excrete plant sterol, and is characterized by cutaneous xanthomas and accelerated atherosclerosis. Sitosterolaemia is caused by homozygous or compound heterozygous mutations in either ABCG5 or ABCG8 (both on chromosome 2p21), which encode the sterol efflux transporter ABCG5 (sterolin-1) and ABCG8 (sterolin-2), respectively. To investigate a Tunisian family with several members who manifested with generalized cutaneous xanthomas, whereas others had only isolated xanthelasmas. Genetic analysis was performed based on exome sequencing of DNA obtained from five affected individuals and one unaffected individual from a Tunisian family.

RESULTS

A novel mutation in the ABCG8 gene, designated c.965-1G>C, was identified by exome sequencing in the members of this family. The homozygous form was associated with generalized cutaneous xanthomatosis while the heterozygous form was linked to isolated xanthelasmas. Our results indicate a gene dosage effect of ABCG8 and suggest that individuals at risk should be followed closely.

ABCG5 and ABCG8 gene polymorphisms in type 2 diabetes mellitus in the Turkish population

OBJECTIVE

The aim of the present study was to investigate the relationship between ABCG5 and ABCG8 gene polymorphisms and plasma lipid concentrations in Turkish patients with type 2 diabetes mellitus.

METHODS

Included in this study were 80 patients with type 2 diabetes and 73 healthy controls. Two selected single nucleotide polymorphisms in ABC transporter genes, ABCG5 (rs6720173) and ABCG8 (rs4148211), were genotyped by using the polymerase chain reaction-restriction fragment length polymorphism technique.

RESULTS

The rate of having the ABCG8 AA genotype (p=0.001) was significantly higher in the patients than in the control subjects. Correspondingly, the rates of having the AG genotype (p=0.001) and the G allele (p=0.001) were significantly lower in the patients than in controls. Upon comparing the groups regarding ABCG5, the frequencies of occurrence of the GG genotype (p=0.031) and G allele (p=0.003) were considerably higher in patients than in control subjects. In the patients, the rates of having the CC genotype (p=0.003) and the C allele (p=0.031) were also significantly lower than those in control subjects. There was no significant difference between G5 and G8 polymorphism and lipid levels in the study groups. The ABCG8 AA genotype carriers had higher triglyceride (p=0.045) and very low-density-cholesterol (p=0.045) levels than the ABCG8 GG genotype carriers in all study populations.

CONCLUSIONS

These results indicate that the AA genotype for ABCG8 and the GG genotype and G allele for ABCG5 are risk factors for diabetes. This study reveals the first data concerning the ABCG5 and ABCG8 gene polymorphisms in Turkish patients with diabetes.

The ABCG5/8 cholesterol transporter and myocardial infarction versus gallstone disease

OBJECTIVES

The study sought to test the hypothesis that genetic variation in ABCG5/8, the transporter responsible for intestinal and hepatobiliary cholesterol efflux, may simultaneously influence plasma and biliary cholesterol levels, and hence risk of myocardial infarction (MI) and gallstone disease in opposite directions.

BACKGROUND

High plasma levels of low-density lipoprotein (LDL) cholesterol are a causal risk factor for MI, whereas high levels of biliary cholesterol promote gallstone formation.

METHODS

A total of 60,239 subjects from Copenhagen were included, including 5,647 with MI and 3,174 with symptomatic gallstone disease. Subjects were genotyped for 6 common, nonsynonymous and functional variants in ABCG5/8, and a combined weighted genotype score was calculated.

RESULTS

Combined, weighted genotype scores were associated with stepwise decreases in LDL cholesterol of up to 5.9% (0.20 mmol/l) for individuals with a score ≥8.0 (prevalence = 11%) versus <2.0 (prevalence = 9%; p for trend across 5 groups = 2 × 10E-35). The cumulative incidences of MI and gallstone disease as a function of age and increasing genotype score were decreased and increased (log-rank ps for trend: 6 × 10E-4 and 9 × 10E-45), respectively. The multifactorially adjusted odds ratios were 0.83 (95% confidence interval: 0.73 to 0.94) for MI and 2.85 (95% confidence interval: 2.39 to 3.39) for symptomatic gallstone disease for individuals with a genotype score ≥8.0 versus <2.0.

CONCLUSIONS

Genetic variation in ABCG5/8, which associates with decreased levels of plasma LDL cholesterol protects against MI, but increases the risk of symptomatic gallstone disease. These results suggest that MI and gallstones, 2 seemingly unrelated diseases, are intrinsically linked via the function of the ABCG5/8 cholesterol transporter.

ABCG8 Gene Responses to 8 Weeks Treadmill Running With or Without Pistachia atlantica (Baneh) Extraction in Female Rats

BACKGROUND

It is well established that the excess cellular cholesterol concentration, as well as high density lipoprotein (HDL) and total cholesterol levels are strongly correlated with the incidence of coronary artery disease (CAD). Reverse cholesterol transport (RCT) is a term used to describe the efflux of excess cellular cholesterol. ABCG8 is a member of ABCG family that play a critical role in this process.

OBJECTIVES

The current study was conducted to investigate the effect of endurance exercise with or without Pistachia atlantica (Baneh) extraction on small intestine and kidney ABCG8 gene, also plasma high density lipoprotein (HDL-c), triglyceride (TG), total cholesterol (TC), glucose, and estrogen levels in female rats.

MATERIALS AND METHODS

In this study twenty Wistar female rats (six to eight weeks old, 125-135 g weight) were used. Animals were randomly assigned into training (n = 10) and control (n = 10) groups and further divided into saline-control (SC), saline-training (ST), Baneh-control (BC), and Baneh-training (BT) groups. Training groups was given exercise on a motor-driven treadmill at 25 m/min (0% grade) for 60 min/day, 5 days/week for eight weeks. Animals were fed orally with Baneh extraction and saline for four week. After the last training session, rats were sacrificed, small intestine and kidney were excised, and ABCG8 expression was detected by Real-time PCR method. Plasma also was collected for plasma variable measurements. Statistical analysis was performed using a one way analysis of variance, and significance was accepted at P < 0.05. Correlation was calculated using the Pearson Product Moment correlation.

RESULTS

Exercise increased (P < 0.01) and Baneh reduced intestinal ABCG8 mRNA (P < 0.05). In kidney tissue, there wasn't significant change between the groups (P < 0.40). Plasma HDL-C level was increased by exercise (P < 0.05) and decreased by Baneh (P < 0.02) that was correlated by intestine ABCG8 (r = 0.81, P < 0.001). Plasma TG and TC were unchanged, but glucose and estradiol were increased and decreased in Baneh groups (P < 0.02), respectively.

CONCLUSIONS

Our study shows that exercise increases intestinal ABCG8 mRNA, and Baneh can increase plasma glucose concentration and reduce ABCG8 expression, HDL-C, and estrogen levels probably due to high fatty acid components.