Original ArticleABCG5 and ABCG8 genetic variants in familial hypercholesterolemia
Introduction
Familial hypercholesterolemia (FH) is an autosomal dominant disorder of lipoprotein metabolism characterized by high LDL cholesterol (LDL-C) plasma levels with an estimated prevalence of 1 in 250 individuals in Western Europe.1 Lifelong exposure to elevated plasma levels of LDL-C leads to atherosclerosis at an early age, and as such, FH patients are at increased risk for cardiovascular disease (CVD).2
FH is primarily caused by genetic variants that affect the uptake of LDL particles from the circulation by the LDL receptor (LDLR). Variants in LDLR, APOB, and PCSK9 result in clinical FH3: a mutation in one of these three genes can been found in 42.1% of patients with LDL-C levels between 5.00 and 5.99 mmol/L and in 88% of patients with an LDL-C >8 mmol/L.4 It is widely acknowledged that mutations in other genes may be present in those FH patients who do not have a pathogenic variant in LDLR, APOB, or PCSK9. Recently, mutations in the genes encoding for ATP-binding cassette subfamily G members 5 and 8 (ABCG5 and ABCG8) have been described to result in FH.5
ABCG5 and ABCG8 form a heterodimer, responsible for transmembrane transport of sterols, in particular plant sterols. In the intestine, this heterodimer is located in the apical membrane and transports sterols from the enterocyte into the intestinal lumen while the hepatic heterodimer transports sterols into bile.6 Homozygosity or compound heterozygosity for functional variants in ABCG5 or ABCG8 cause sitosterolemia, an autosomal recessive disorder characterized by severely elevated plasma plant sterols and cholesterol levels.7 Sitosterolemic patients also typically present with xanthomas, accelerated atherosclerosis, and premature coronary artery disease, characteristics that closely mimic the clinical FH phenotype.8 A recent study showed an association between hypercholesterolemia and sitosterolemia in the general population. Among the more than 200,000 subjects studied, plasma ß-sitosterol concentrations were above the 99th percentile in 4% of the subjects with plasma LDL-C levels greater than 4.9 mmol/L while such high plasma ß-sitosterol concentrations were presented in only 1% of those with plasma LDL-C levels less than 3.3 mmol/L.9
The relationship between increased plasma lipid concentrations, CVD risk, and genetic variations in ABCG5 or ABCG8 has been investigated in multiple studies in the general population.10, 11, 12, 13 However, the role of heterozygous (likely) pathogenic variants in the FH phenotype is less well understood, despite reports of the presence of variants in these two genes in hypercholesterolemic subjects and inclusion of these genes in FH sequencing panels.5,14,15 Because variants in ABCG5 and ABCG8 are frequently discovered with the expansion of next-generation sequencing panels in FH patients, we set out to investigate the prevalence of these variants in a cohort of patients who were considered to suffer from FH based on clinical symptoms and to investigate their association with lipid levels. Moreover, we determined the cosegregation of one ABCG5 and two ABCG8 variants with plasma plant sterol and cholesterol levels in three families.
Section snippets
Study population
The Amsterdam University Medical Center in Amsterdam is the national referral center for DNA diagnostics in dyslipidemia in the Netherlands. Patients are referred for genetic FH analysis based on the clinical judgment of the referring physician. An in-house next-generation sequencing capture covering, among others, LDLR, APOB, PCSK9, ABCG5, and ABCG8 (SeqCap easy choice, Roche NimbleGen Inc., Pleasanton), is used to establish the possible genetic origin FH. The full list of genes sequenced by
Frequency of ABCG5 and ABCG8 variants
In our clinical FH cohort of 3031 subjects, the mean LDL-C level was 5.8 ± 1.4 mmol/L. At least 14.2% of the FH patients were using LLT at moment of lipid profile measurement. 11.42%, 2.84%, and 0.69% was carrier of an FH-causing mutation in LDLR, APOB, or PCSK9, respectively (Table 1). 2.44% of patients were heterozygous carrier of (likely) pathogenic variants in either ABCG5 or ABCG8. Clinical details are described in Supplementary Table 3. We found 79 variants in ABCG5 and 112 variants in
Discussion
This study provides an in-depth investigation into the consequences of heterozygosity for deleterious variants (class 4 and class 5) in ABCG5 or ABCG8 in an unprecedented large cohort of 3031 FH patients. We identified 191 variants in ABCG5 or ABCG8 and performed frequency analysis and lipid profile comparison of variant carriers. In addition, a cosegregation analysis in three FH families carrying an ABCG5 or ABCG8 variant was performed. Three major findings stand out from our research. First,
Acknowledgments
The authors would like to thank J.F. Los for her invaluable work in including FH patients in their studies and D. Collard for his advice on advanced statistical methods. This study was funded by a research grant from The Netherlands Organisation for Health Research and Development (Vidi no. 016.156.445).
Contribution statement: Reeskamp helped in data collection, data analysis, interpreting results, and writing the manuscript. Volta helped in data collection, data analysis, interpreting results,
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Cited by (27)
Unraveling the genetic background of individuals with a clinical familial hypercholesterolemia phenotype
2024, Journal of Lipid ResearchA Bibliometric Analysis of Familial Hypercholesterolemia From 2011 to 2021
2023, Current Problems in CardiologyCitation Excerpt :Mutations in genes encoding ATP-binding cassette subfamily G members 5 and 8 (ABCG5 and ABCG8) may cause FH.62–64 Laurens et al.65 reported that nearly 2.4% of FH patients had (possibly) pathogenic heterozygous variants at the ABCG5 and ABCG8 sites. Carriers of these mutations have lower levels of LDL-C, when compared to FH patients with LDLR mutations.
Sitosterolemia
2022, Advances in Clinical ChemistryCitation Excerpt :Accordingly, we propose a new stratification regarding monogenic lipid disorders associated with elevated risk for CAD (Fig. 10). Moreover, we, and other groups, have shown that at least a portion of individuals with a clinical diagnosis of FH are actually sitosterolemia or heterozygous mutation carriers of ABCG5 or ABCG8 [40,70,71]. It is of note that hypercholesterolemic patients with an ABCG5 or ABCG8 mutation exhibit greater responses to ezetimibe to lower LDL cholesterol [72].
Improvement of Definite Diagnosis of Familial Hypercholesterolemia Using an Expanding Genetic Analysis
2021, JACC: AsiaCitation Excerpt :Originally, FH was described as elevated LDL-C due to the loss-function of LDLR (15). With the advancement of DNA sequencing and a deeper understanding of the genetic basis of FH, its pathogenesis has been nearly redefined (16,17). In the present study, we implemented an NGS strategy for sequencing the promoter, coding, and exon-intron boundary regions of 9 FH-related genes as well as CNVs and harbored FH-associated variants in 68.5% of the index cases.
Mammalian ABCG-transporters, sterols and lipids: To bind perchance to transport?
2021, Biochimica et Biophysica Acta - Molecular and Cell Biology of LipidsCitation Excerpt :Since the discovery of the genes, a vast number of disease causing mutations have been found in both the ABCG5 and ABCG8 genes in patients [120,125–129]. More recently, missense mutations in the transporter genes were found that are associated with plasma lipid abnormalities in non-sitosterolaemia patients while others were shown to have minimal effects [130,131]. The ABCG5 and ABCG8 protein sequences are quite distinct from each other, with only 28% amino acid identity [120].
When a “normal” cholesterol level is not normal: Exposing an unusual presentation of familial hypercholesterolemia
2020, Journal of Clinical LipidologyCitation Excerpt :ABCG8 variants have been implicated as a contributing factor to elevated LDL-C levels in some patients with polygenic hypercholesterolemia.18 A study using next-generation sequence analysis of a Dutch cohort of 3031 hypercholesterolemic patients who had been referred for genetic testing to a national referral center for DNA diagnostics identified 9 patients with clinical FH who were double heterozygotes for mutations of LDLR and ABCG8.19 These double heterozygotes had similar LDL-C levels as compared with the remainder of the cohort (P = .259).
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Both authors contributed equally to this work.