First case of sitosterolemia caused by double heterozygous mutations in ABCG5 and ABCG8 genes

Dried blood spot-based free sterol signatures in sitosterolemia diagnostics

BACKGROUND

Sitosterolemia is a rare inherited lipid metabolic disorder characterized by increased levels of plant sterols and accelerated atherosclerosis. Although early detection is beneficial for the prevention of disease progression, it is largely underdiagnosed by routine screening based on conventional lipid profiles.

MATERIALS AND METHODS

A gas chromatography-mass spectrometry (GC-MS)-based profiling has been developed and validated to measure the levels of biologically active free sterols, including five endogenous sterols and three plant sterols (sitosterol, campesterol, and stigmasterol) in dried blood spot (DBS).

RESULTS

Within- and between-run precisions were 1.4-11.1 % and 2.2-14.1 %, respectively, while the accuracies were all 86.3 ∼ 121.9 % with the correlation coefficients (r2) > 0.988 for all the sterols. In the patients (four girls and two boys, 6.5 ± 2.8 years), sitosterol levels were significantly increased, with an optimal cut-off value of 2.5 µg/mL distinguishing them from ninety-three age-matched healthy children. A cut-off value of 31.9 µg/mL differentiated the patients from six ABCG5/ABCG8 heterozygous carriers. In addition, the molecular ratios of sitosterol to cholesterol, desmosterol, and 7-dehydrocholesterol provided excellent cut-off values of 26.3, 67.6, and 21.6, respectively, to distinguish patients from both healthy controls and heterozygous carriers.

CONCLUSIONS

The novel DBS-based GC-MS profiling of free sterols accurately identified patients with sitosterolemia, with a performance comparable to that of a serum assay. The DBS profiling could be more feasible method in clinical practice as well as population screening programs, and it can provide diagnostic cut-off values for individual plant sterols.

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.

Sitosterolemia: A Case Report and a Concise Literature Review

Background

Sitosterolemia is a relatively rare metabolism lipid disorder, with about 110 cases worldwide and only a few known cases from the Middle East. Sitosterolemia is characterized by excessive uptake of phytosterols and their deposition in various tissues, leading to complications. Mutations in the ABCG5 and ABCG8 genes are associated with pathological changes in sitosterolemia. Case Presentation. An adult patient from Saudi Arabia with dyslipidemia who did not respond to statin therapy. Based on genetic testing, the patient was eventually diagnosed with sitosterolemia. Ezetimibe significantly improved his cholesterol levels.

Conclusion

The diagnosis of sitosterolemia is confirmed by the detection of high-phytosterol levels and pathological mutation in the ABCG5 and ABCG8 genes. Treatment of sitosterolemia is based on dietary changes and drugs to inhibit cholesterol absorption, such as ezetimibe.

Pediatric patients with familially inherited sitosterolemia: Two case reports

Background

Sitosterolemia is a rare recessive genetic abnormality of hyperlipidemia; it is characterized by increased levels and accumulation of sitosterol in the plasma and local tissues.

Case descriptions

The study subjects were two siblings (brother and sister) who had sitosterolemia with systemic multiple xanthomas as the main manifestation. The main clinical manifestations were hypercholesterolemia, premature atherosclerosis, arrhythmia, systemic multiple xanthomas, etc. After genetic testing, it was found that the patients had a compound heterozygous mutation of c.1324+1de1G in exon 7 and exon 9 of chromosome 2p21 of the adenosine triphosphate binding cassette transporter G family member 5(ABCG5) gene; the mutation at c.904+1G>A was of maternal origin, and the mutation at c. 1324+1de1G was of paternal origin. The compound heterozygous mutation of these two genes led to a metabolic disorder of plant sterols in vivo.

Conclusion

Sitosterolemia is an autosomal recessive disease that could be effectively controlled after dietary control and oral lipid-lowering therapy with Ezetimibe. Xanthomas, which affects function and appearance, could be surgically removed, and primary wound healing could be achieved.

Acute coronary syndrome in an 8-year-old child with familial hypercholesterolemia: a case report

Background

Familial hypercholesterolemia (FH) is the most commonly inherited metabolic disease and has an autosomal dominant mode of inheritance. Patients with FH usually present with high levels of low-density lipoprotein-cholesterol, xanthomas and early coronary artery disease. Children with FH are subject to developing early and enhanced atherosclerosis, leading to an increased risk of coronary events. We report here an 8-year-old patient with acute coronary syndrome (ACS) who was diagnosed with homozygous FH (HoFH) due to mutations in the low-density lipoprotein receptor (LDLR) gene and subsequently treated with percutaneous transluminal coronary angioplasty (PTCA) after filtration plasma. To the best of our knowledge, this patient is the youngest case of HoFH treated with filtration plasma followed by PTCA reported to date.

Case presentation

An 8-year-old Asian patient was admitted to Gansu Provincial Hospital presenting symptoms of ACS. Laboratory tests showed that the patient’s cholesterol and low-density lipid levels were extremely high. An electrocardiogram (ECG) revealed sinus arrhythmia and electric axis deviation to the right, but the ECG was roughly normal. Multiple cardiac function abnormalities were diagnosed on the ECG. Multiple sites of coronary artery stenosis were determined by computed tomography angiography. DNA sequencing of exons showed a C-to-A substitution at nucleotide 126 in exon 2, resulting in a LDLR mutation in the patient and seven other family members. Following combination treatment with lipid-lowering drugs, anti-thrombosis drugs, filtration plasma and PTCA, the patient’s symptoms were significantly improved and the patient discharged.

Conclusion

We report the rare case of a patient with ACS attributable to HoFH who was treated with PTCA following filtration plasma. The patient was shown to have clinically diagnosed and molecularly confirmed HoFH that resulted from a mutation in the LDLR gene. Children with HoFH have higher risk of early coronary events and death from myocardial infarction due to premature atherosclerosis than adults. Earlier intervention and treatment will bring great benefits to the long-term survival of pediatric patients.

Acute Coronary Syndrome Developed in a 17-year-old Boy with Sitosterolemia Comorbid with Takayasu Arteritis: A Rare Case Report and Review of the Literature

A 17-year-old boy with acute coronary syndrome was admitted to our hospital. He had xanthomas over his elbow and Achilles tendon and a high level of low-density lipoprotein cholesterol; therefore, his initial diagnosis was familial hypercholesterolemia. However, a genetic analysis revealed a compound heterozygous mutation in the ABCG5 gene with a high serum level of sitosterol, leading to the diagnosis of sitosterolemia. After lipid-lowering treatment, percutaneous coronary intervention was performed. Furthermore, a persistently high C-reactive protein level and images of large arteries led to a diagnosis of Takayasu arteritis. To our knowledge, this is the first case of sitosterolemia complicated by Takayasu arteritis.

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.

Recent advances in ABCG5 and ABCG8 variants

PURPOSE OF REVIEW

In this review, we summarize the genetics and mechanisms of sitosterolemia and sterol trafficking, and provide an update on the understanding of the prevalence of ABCG5 and ABCG8 variants and their role in human disease.

RECENT FINDINGS

Defects in ABCG5/G8 result in the accumulation of xenosterols. It had been previously thought that near total LoF of one of the proteins was required to cause pathology. However, recently there was the first report of a patient with Sitosterolemia who was heterozygous for mutations in both genes. Moreover, large population studies have demonstrated the even simple heterozygous carriers are associated with altered lipid profiles and cardiovascular risk. Broader screening has added to the rapidly growing list of gene variants indicating that the prevalence of ABCG5/G8 variants is higher than previous thought, especially in patients with hypercholesterolemia.

SUMMARY

These findings support a strategy of measuring xenosterol levels in patients with hypercholesterolemia to screen for ABCG5/G8 variants, and then tailoring treatment with a sterol absorption inhibitor, like ezetimibe, where indicated. Xenosterol trafficking affects remnant clearance and maybe pathogenically linked to the increased risk of atherosclerosis.

Clinical and structural insights into potential dominant negative triggers of proximal urea cycle disorders

Despite biochemical and genetic testing being the golden standards for identification of proximal urea cycle disorders (UCDs), genotype-phenotype correlations are often unclear. Co-occurring partial defects affecting more than one gene have not been demonstrated so far in proximal UCDs. Here, we analyzed the mutational spectrum of 557 suspected proximal UCD individuals. We probed oligomerizing forms of NAGS, CPS1 and OTC, and evaluated the surface exposure of residues mutated in heterozygously affected individuals. BN-PAGE and gel-filtration chromatography were employed to discover protein-protein interactions within recombinant enzymes. From a total of 281 confirmed patients, only 15 were identified as "heterozygous-only" candidates (i.e. single defective allele). Within these cases, the only missense variants to potentially qualify as dominant negative triggers were CPS1 p.Gly401Arg and NAGS p.Thr181Ala and p.Tyr512Cys, as assessed by residue oligomerization capacity and surface exposure. However, all three candidates seem to participate in critical intramolecular functions, thus, unlikely to facilitate protein-protein interactions. This interpretation is further supported by BN-PAGE and gel-filtration analyses revealing no multiprotein proximal urea cycle complex formation. Collectively, genetic analysis, structural considerations and in vitro experiments point against a prominent role of dominant negative effects in human proximal UCDs.

Sitosterolemia Exhibiting Severe Hypercholesterolemia with Tendon Xanthomas Due to Compound Heterozygous ABCG5 Gene Mutations Treated with Ezetimibe and Alirocumab

We herein report a rare case presenting with severe hypercholesterolemia, massive Achilles tendon xanthomas, and multi-vessel coronary artery disease. Initially, the patient was misdiagnosed with familial hypercholesterolemia. However, a genetic analysis using our custom sequencing panel covering genes associated with Mendelian lipid disorders revealed him to have a genetic basis of sitosterolemia with compound heterozygous mutations in the adenosine triphosphate binding cassette subfamily G5 (ABCG5) gene. A comprehensive genetic analysis can be particularly useful for diagnosing cases with severe phenotypes, leading to appropriate and medical therapies. Our patient was refractory to statins, whereas ezetimibe and PCSK9 inhibitor with a low-plant-sterol diet successfully reduced his serum levels of low-density lipoprotein cholesterol.

Whole exome sequencing for non-selective pediatric patients with hyperlipidemia

BACKGROUND

Hyperlipidemia is a group of conditions with abnormally elevated levels of any or all lipids or lipoproteins in the blood. It is highly heterogeneous both genetically and clinically, which contributes to diagnostic challenges and results in many patients to be underdiagnosed and undertreated in China. Precise diagnosis and early management are critical to reduce the incidence of potential coronary artery disease and cardiovascular disease.

RESULTS

We performed a single center study to demonstrate the clinical utility of the genome-first approach by whole exome sequencing (WES) for 12 pediatric patients with abnormal lipids or lipoproteins levels. In vitro experiments were performed in COS-7 cells to further evaluate the biological function of the novel variants. We identified ten pathogenic and likely pathogenic variants and three of them were novel. Molecular cause was uncovered in five (41.7%) patients including three lipoprotein lipase deficiency patients, one hypercholesterolemia patient and one sitosterolemia patient. We also found three patients with rare variants of uncertain significance. Copy number variant (CNV) analysis with WES data did not reveal any potential hyperlipidemia related CNVs in all patients.

CONCLUSION

We expanded the mutation and phenotype spectra of familial hyperlipidemia. Our study demonstrated the effectiveness of genome-first approach for evaluation pediatric hyperlipidemia patients and showed that WES can be used as the first-tier test for patients with suspected Mendelian hyperlipidemia disorder.

Dig deeper when it does not make sense: Juvenile xanthomas due to sitosterolemia

Sitosterolemia is an extremely rare autosomal recessive disease caused by mutations in either ABCG5 or ABCG8, which encode for a sterol efflux transporter (sterolin) that pumps sterols out into the intestinal lumen or into bile. This leads to progressive accumulation of plant sterols in blood and tissues. Clinical presentation is variable and may include xanthoma, arthritis, thyroid dysfunction, premature atherosclerotic disease, splenomegaly, and hematologic manifestations. We report a child presented with multiple xanthomas at age 5.5 years, located on the elbow, knee, and toe. Juvenile xanthogranuloma was considered based on histopathologic findings. At 8 years of age, a lipid profile showed markedly elevated total cholesterol (9.4 mmol/L) and low-density lipoprotein cholesterol (LDL-C, 7.4 mmol/L). Simvastatin therapy was initiated, however, the lipid profile was persistently abnormal. At age 8.5 years, genetic testing identified two novel variants: (NM_022437.3[ABCG8]:c.1444del;p.Leu482Trpfs*40) and (NM_022437.3[ABCG8]:c.1640T>C;p.Leu547Pro) in the ABCG8 gene. Plasma sitosterol was subsequently found to be very high, confirming the diagnosis. She was started on a low plant sterol and cholesterol diet for 6 weeks with insignificant response and therefore ezetimibe (10 mg daily) was added. This resulted in significant reduction of cholesterol, LDL, sitosterol levels, and no further increase in the size of the xanthomas. This case emphasizes the diagnostic odyssey, the benefits of genomic testing and importance of a correct diagnosis in order to initiate appropriate therapy. It also illustrates the importance of considering rare conditions, such as sitosterolemia, as a differential diagnosis in patients with hypercholesterolemia and increased LDL-C.

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.

Sitosterolemia-10 years observation in two sisters

Familial hypercholesterolemia due to heterozygous low-density lipoprotein-receptor mutations is a common inborn errors of metabolism. Secondary hypercholesterolemia due to a defect in phytosterol metabolism is far less common and may escape diagnosis during the work-up of patients with dyslipidemias. Here we report on two sisters with the rare, autosomal recessive condition, sitosterolemia. This disease is caused by mutations in a defective adenosine triphosphate-binding cassette sterol excretion transporter, leading to highly elevated plant sterol concentrations in tissues and to a wide range of symptoms. After a delayed diagnosis, treatment with a diet low in plant lipids plus ezetimibe to block the absorption of sterols corrected most of the clinical and biochemical signs of the disease. We followed the two patients for over 10 years and report their initial presentation and long-term response to treatment.

Familial Hypercholesterolemia: The Most Frequent Cholesterol Metabolism Disorder Caused Disease

Cholesterol is an essential component of cell barrier formation and signaling transduction involved in many essential physiologic processes. For this reason, cholesterol metabolism must be tightly controlled. Cell cholesterol is mainly acquired from two sources: Dietary cholesterol, which is absorbed in the intestine and, intracellularly synthesized cholesterol that is mainly synthesized in the liver. Once acquired, both are delivered to peripheral tissues in a lipoprotein dependent mechanism. Malfunctioning of cholesterol metabolism is caused by multiple hereditary diseases, including Familial Hypercholesterolemia, Sitosterolemia Type C and Niemann-Pick Type C1. Of these, familial hypercholesterolemia (FH) is a common inherited autosomal co-dominant disorder characterized by high plasma cholesterol levels. Its frequency is estimated to be 1:200 and, if untreated, increases the risk of premature cardiovascular disease. This review aims to summarize the current knowledge on cholesterol metabolism and the relation of FH to cholesterol homeostasis with special focus on the genetics, diagnosis and treatment.