Elsevier

Journal of Clinical Lipidology

Volume 14, Issue 2, March–April 2020, Pages 207-217.e7
Journal of Clinical Lipidology

Original Article
ABCG5 and ABCG8 genetic variants in familial hypercholesterolemia

https://doi.org/10.1016/j.jacl.2020.01.007Get rights and content

Highlights

  • 2.4% of clinical familial hypercholesterolemia patients carry variants in ABCG5 and ABCG8.

  • ABCG5 or ABCG8 carriers have lower low-density lipoprotein cholesterol compared to LDLR mutation carriers.

  • ABCG5 and ABCG8 variants did not cosegregate with high low-density lipoprotein cholesterol in familial hypercholesterolemia families.

Background

Familial hypercholesterolemia (FH) is a common inherited disease characterized by elevated low-density lipoprotein cholesterol (LDL-C) plasma levels and increased cardiovascular disease risk. Most patients carry a mutation in the low-density lipoprotein receptor gene (LDLR). Common and rare variants in the genes encoding adenosine triphosphate–binding cassette transporters G5 and G8 (ABCG5 and ABCG8) have been shown to affect LDL-C levels.

Objective

The objective of this study was to investigate whether and to which extent heterozygous variants in ABCG5 and ABCG8 are associated with the hypercholesterolemic phenotype.

Methods

We sequenced ABCG5 and ABCG8 in a cohort of 3031 clinical FH patients and compared the prevalence of variants with a European reference population (gnomAD). Clinical characteristics of carriers of putative pathogenic variants in ABCG5 and/or ABCG8 were compared with heterozygous carriers of mutations in LDLR. Furthermore, we assessed the segregation of one ABCG5 and two ABCG8 variants with plasma lipid and sterol levels in three kindreds.

Results

The frequencies of (likely) pathogenic LDLR, APOB, PCSK9, ABCG5, and ABCG8 variants in our FH cohort were 11.42%, 2.84%, 0.69%, 1.48%, and 0.96%, respectively. We identified 191 ABCG5 and ABCG8 variants of which 53 were classified as pathogenic or likely pathogenic. Of these 53 variants, 51 were either absent from a reference population or more prevalent in our FH cohort than in the reference population. LDL-C levels were significantly lower in heterozygous carriers of a (likely) pathogenic ABCG5 or ABCG8 variant compared to LDLR mutation carriers (6.2 ± 1.7 vs 7.2 ± 1.7 mmol/L, P < .001). The combination of both an ABCG5 or ABCG8 variant and a LDLR variant was found not to be associated with significant higher LDL-C levels (7.8 ± 2.3 vs 7.2 ± 1.7 mmol/L, P = .259). Segregation analysis in three families (nine carriers, in addition to the index cases, and 16 noncarriers) did not show complete segregation of the ABCG5/G8 variants with high LDL-C levels, and LDL-C levels were not different (3.9 ± 1.3 vs 3.5 ± 0.6 mmol/L in carriers and noncarriers, respectively, P = .295), while plasma plant sterol levels were higher in carriers compared to noncarriers (cholestanol: 10.2 ± 1.7 vs 8.4 ± 1.6 μmol/L, P = .007; campesterol: 22.5 ± 10.1 vs 13.4 ± 3.5 μmol/L, P = .008; sitosterol: 17.0 ± 11.6 vs 8.2 ± 2.6 μmol/L, P = .024).

Conclusions

2.4% of subjects in our FH cohort carried putative pathogenic ABCG5 and ABCG8 variants but had lower LDL-C levels compared to FH patients who were heterozygous carriers of an LDLR variant. These results suggest a role for these genes in hypercholesterolemia in FH patients with less severely elevated LDL-C levels. We did not find evidence that these variants cause autosomal dominant FH.

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,

References (30)

  • H. Tada et al.

    Oligogenic familial hypercholesterolemia, LDL cholesterol, and coronary artery disease

    J Clin Lipidol

    (2018)
  • K. Lu et al.

    Two genes that map to the STSL locus cause sitosterolemia: genomic structure and spectrum of mutations involving sterolin-1 and sterolin-2, encoded by ABCG5 and ABCG8, respectively

    Am J Hum Genet

    (2001)
  • P.S. Buonuomo et al.

    Timely diagnosis of sitosterolemia by next generation sequencing in two children with severe hypercholesterolemia

    Atherosclerosis

    (2017)
  • B.G. Nordestgaard et al.

    Familial hypercholesterolaemia is underdiagnosed and undertreated in the general population: Guidance for clinicians to prevent coronary heart disease

    Eur Heart J

    (2013)
  • C.S. Bruikman et al.

    Molecular basis of familial hypercholesterolemia

    Curr Opin Cardiol

    (2017)
  • Cited by (27)

    • A Bibliometric Analysis of Familial Hypercholesterolemia From 2011 to 2021

      2023, Current Problems in Cardiology
      Citation 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 Chemistry
      Citation 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: Asia
      Citation 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 Lipids
      Citation 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 Lipidology
      Citation 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).

    View all citing articles on Scopus
    1

    Both authors contributed equally to this work.

    View full text