Molecular genetic testing for familial hypercholesterolemia in the Netherlands: a stepwise screening strategy enhances the mutation detection rate

Genetic analysis in a compound heterozygote family with familial hypercholesterolemia

Homozygous familial hypercholesterolemia (FH) is rare, with an incidence of ~one in a million and commonly presents with a genetic mutation. The genetic variations of families with FH were clinically analyzed to investigate the association between the phenotype and genotype of patients. Direct sequencing was conducted for the proband and her parents to detect mutations in the fragment of 18 exons of the low‑density lipoprotein receptor (LDLR) and apolipoprotein B100 Q3500R in the peripheral blood genomic DNA. The gene sequences were compared with normal ones to find mutations using GenBank. The QX200 Droplet Digital PCR system was used to detect target DNA copy number variations of the proband and her parents. The functional alterations resulting from the novel mutations were verified by quantitative polymerase chain reaction, western blotting and flow cytometric analyses. The lipid levels of the proband and her parents were all elevated. Genetic testing results indicated that the proband and her mother had a novel heterozygous missense mutation (C377G, 28893T>G) in exon 8 of the LDLR gene, whereas the proband and her father had LDLR gene DNA fragment deletions in exon 18. Clinically, the proband was of a compound heterozygous genotype and her parents were of the simple heterozygous genotype. Furthermore, both mutations led to impaired expression and LDL binding and internalization function of LDLR in vitro. The proband's genotype was confirmed to be compound heterozygous FH, leading to clinical manifestations in line with the homozygous FH phenotype. The phenotype is highly associated with the genotype in this type of compound heterozygous FH.

Diagnostic Yield and Clinical Utility of Sequencing Familial Hypercholesterolemia Genes in Patients With Severe Hypercholesterolemia

BACKGROUND

Approximately 7% of American adults have severe hypercholesterolemia (untreated low-density lipoprotein [LDL] cholesterol ≥190 mg/dl), which may be due to familial hypercholesterolemia (FH). Lifelong LDL cholesterol elevations in FH mutation carriers may confer coronary artery disease (CAD) risk beyond that captured by a single LDL cholesterol measurement.

OBJECTIVES

This study assessed the prevalence of an FH mutation among those with severe hypercholesterolemia and determined whether CAD risk varies according to mutation status beyond the observed LDL cholesterol level.

METHODS

Three genes causative for FH (LDLR, APOB, and PCSK9) were sequenced in 26,025 participants from 7 case-control studies (5,540 CAD case subjects, 8,577 CAD-free control subjects) and 5 prospective cohort studies (11,908 participants). FH mutations included loss-of-function variants in LDLR, missense mutations in LDLR predicted to be damaging, and variants linked to FH in ClinVar, a clinical genetics database.

RESULTS

Among 20,485 CAD-free control and prospective cohort participants, 1,386 (6.7%) had LDL cholesterol ≥190 mg/dl; of these, only 24 (1.7%) carried an FH mutation. Within any stratum of observed LDL cholesterol, risk of CAD was higher among FH mutation carriers than noncarriers. Compared with a reference group with LDL cholesterol <130 mg/dl and no mutation, participants with LDL cholesterol ≥190 mg/dl and no FH mutation had a 6-fold higher risk for CAD (odds ratio: 6.0; 95% confidence interval: 5.2 to 6.9), whereas those with both LDL cholesterol ≥190 mg/dl and an FH mutation demonstrated a 22-fold increased risk (odds ratio: 22.3; 95% confidence interval: 10.7 to 53.2). In an analysis of participants with serial lipid measurements over many years, FH mutation carriers had higher cumulative exposure to LDL cholesterol than noncarriers.

CONCLUSIONS

Among participants with LDL cholesterol ≥190 mg/dl, gene sequencing identified an FH mutation in <2%. However, for any observed LDL cholesterol, FH mutation carriers had substantially increased risk for CAD.

Premature coronary heart disease and autosomal dominant hypercholesterolemia: Increased risk in women with LDLR mutations

BACKGROUND

For patients with autosomal dominant hypercholesterolemia (ADH), it remains unclear whether differences exist in the risk of premature coronary heart disease (CHD) between patients with confirmed mutations in low-density lipoprotein receptor (LDLR) vs those without detectable mutations.

OBJECTIVE

This study sought to assess the risk of premature CHD in ADH patients with mutations in LDLR (referred to as familial hypercholesterolemia [FH]) vs those without detectable mutations (unexplained ADH), stratified by sex.

METHODS

Comparative study of premature CHD in a multiethnic cohort of 111 men and 165 women meeting adult Simon-Broome criteria for ADH.

RESULTS

Women with FH (n = 51) had an increased risk of premature CHD compared with unexplained ADH women (n = 111; hazard ratio [HR], 2.74; 95% confidence interval, 1.40-5.34; P = .003) even after adjustment for lipid levels and traditional CHD risk factors (HR, 2.53 [1.10-5.83]; P = .005). Men with FH (n = 42), in contrast, had a similar risk of premature CHD when compared with unexplained ADH men (n = 66; unadjusted: HR, 1.48 [0.84-2.63]; P = .18; adjusted: HR, 1.04 [0.46-2.37]; P = .72). To address whether mutation status provides additional information beyond LDL-cholesterol level, we analyzed premature CHD risk for FH vs unexplained ADH at various percentiles of LDL-cholesterol: the risk ratios were significant for women at 25th percentile (HR, 4.90 [1.69-14.19]) and 50th percentile (HR, 3.44 [1.42-8.32]) but not at 75th percentile (HR, 1.99 [0.95-4.17]), and were not significant for men at any percentile.

CONCLUSIONS

Our findings suggest that genetic confirmation of ADH may be important to identify patient's risk of CHD, especially for female LDLR mutation carriers.

Importance of early recognition of heterozygous familial hypercholesterolaemia

PURPOSE OF REVIEW

To outline recent updates in the diagnosis and management of heterozygous familial hypercholesterolaemia.

RECENT FINDINGS

Recent guidelines have suggested that familial hypercholesterolaemia is vastly underdiagnosed in most countries worldwide. Improvements in next-generation sequencing have led to the detection of novel mutations and the cheaper cost of this technology makes the early identification of asymptomatic individuals a feasible option. With more widespread use of high doses of more potent statins in affected adults, cardiovascular mortality has decreased in adults with hypercholesterolaemia.

SUMMARY

Barriers to cascade testing of relatives of index cases remain worldwide despite improvements in gene technology and the marked recent decrease in costs of genetic testing. Recent guidelines recommending screening of young children, for example, 8-10 years with measurement of LDL cholesterol concentrations will increase the diagnosis of familial hypercholesterolaemia among children but long-term safety data of the use of statins in this young age group are not available. To date, the benefit of statin-induced decreases in LDL cholesterol concentration in children is based on effects of treatment on proxy measures of cardiovascular disease and not a reduction in cardiovascular events.

Cascade Screening in Familial Hypercholesterolemia: Advancing Forward

Familial hypercholesterolemia is a genetic disorder associated with elevated LDL-cholesterol and high lifetime cardiovascular risk. Both clinical and molecular cascade screening programs have been implemented to increase early definition and treatment. In this systematic review, we discuss the main issues found in 65 different articles related to cascade screening and familial hypercholesterolemia, covering a range of topics including different types/strategies, considerations both positive and negative regarding cascade screening in general and associated with the different strategies, cost and coverage consideration, direct and indirect contact with patients, public policy around life insurance and doctor-patient confidentiality, the "right to know," and public health concerns regarding familial hypercholesterolemia.

JCL Roundtable: diagnosis of severe familial hypercholesterolemia

The diagnosis of familial hypercholesterolemia is usually straightforward. The severely elevated low-density lipoprotein cholesterol and the occurrence of high concentrations of low-density lipoprotein cholesterol in the parents provide the diagnosis. The presence of tendon xanthomata is confirmation but not necessary. However, this relatively simple picture becomes much more complicated when one attempts to define the genetic variants that actually produced this clinical syndrome. In this Roundtable discussion, I am joined by two experts in the identification of genetic abnormalities discovered in those with phenotypic familial hypercholesterolemia. Dr. John Kane from the University of California, San Francisco, and Dr. Daniel Rader from the University of Pennsylvania share their knowledge in and experience with this topic.

Low prevalence of mutations in known loci for autosomal dominant hypercholesterolemia in a multiethnic patient cohort

BACKGROUND- Autosomal dominant hypercholesterolemia (ADH), characterized by elevated plasma levels of low-density lipoprotein (LDL)-cholesterol, is caused by variants in at least 3 different genes: LDL receptor (LDLR), apolipoprotein B-100, and proprotein convertase subtilisin-like kexin type 9. There is paucity of data about the molecular basis of ADH among ethnic groups other than those of European or Japanese descent. Here, we examined the molecular basis of ADH in a multiethnic patient cohort from lipid clinics in a large, urban US city. METHODS AND RESULTS- A total of 38 men and 53 women, aged 22 to 76 years, met modified Simon-Broome criteria for ADH and were screened for mutations in the exons and consensus splice sites of LDLR, and in selected exons of apolipoprotein B-100 and proprotein convertase subtilisin-like kexin type 9. Deletions and duplications of LDLR exons were detected with multiplex ligation-dependent probe amplification. Heterozygous variants in LDLR were identified in 30 patients and in apolipoprotein B-100 in 1 patient. The remaining 60 patients (65%) had unexplained ADH. A higher proportion of blacks (77%) than either non-Hispanic whites (57%) or Hispanics (53%) had unexplained ADH. Compared with patients with LDLR variants, those with unexplained ADH had lower levels of LDL-cholesterol (292 ± 47 mg/dL versus 239 ± 42 mg/dL, respectively; P<0.0001) and higher levels of high-density lipoprotein cholesterol (45 ± 12 mg/dL versus 54 ± 13 mg/dL, respectively; P=0.003).

CONCLUSIONS

Our findings suggest that additional loci may contribute to ADH, especially in understudied populations such as blacks.

Management of familial hypercholesterolaemia in children and adolescents

Familial hypercholesterolaemia is a disorder of low-density lipoprotein (LDL) cholesterol metabolism, which is associated with the onset of vascular changes associated with coronary heart disease in childhood. This disorder has co-dominant transmission with a prevalence of one in 500 in the general population. Cascade screening is the most effective method of identifying children. Children in the at-risk group should have their cholesterol levels checked between the age of 2 and 10 years. Children with LDL cholesterol levels ≥ 3.4 mmol/L are likely to suffer from this disorder, although at this level there is a significant false positive rate. Molecular genetic testing is available for the LDL receptor gene, APOB gene and the PCSK9 gene. This is the most specific test for familial hypercholesterolaemia but has a false negative rate of 20-50%. Once diagnosed, treatment should be considered in children with an LDL cholesterol level ≥ 4.9 mmol/L. If the child has two other risk factors or a positive family history, this threshold should be lowered to ≥4.1 mmol/L. Guidelines recommend that treatment should be commenced by the age of 10 years, although some advise waiting until menarche in females. Statin therapy is currently recommended as first line treatment. Randomised placebo trials have shown that statin therapy reduces LDL cholesterol levels by 25% and is not associated with increased risk of adverse events. These are short-term studies, and longer follow-up will be required to definitively prove efficacy and safety.

Familial hypercholesterolemia: the lipids or the genes?

Familial Hypercholesterolemia (FH) is a common cause of premature cardiovascular disease and is often undiagnosed in young people. Although the disease is diagnosed clinically by high LDL cholesterol levels and family history, to date there are no single internationally accepted criteria for the diagnosis of FH. Several genes have been shown to be involved in FH; yet determining the implications of the different mutations on the phenotype remains a hard task. The polygenetic nature of FH is being enhanced by the discovery of new genes that serve as modifiers. Nevertheless, the picture is still unclear and many unknown genes contributing to the phenotype are most likely involved. Because of this evolving polygenetic nature, the diagnosis of FH by genetic testing is hampered by its cost and effectiveness.

In this review, we reconsider the clinical versus genetic nomenclature of FH in the literature. After we describe each of the genetic causes of FH, we summarize the known correlation with phenotypic measures so far for each genetic defect. We then discuss studies from different populations on the genetic and clinical diagnoses of FH to draw helpful conclusions on cost-effectiveness and suggestions for diagnosis.

High-resolution melting analysis for detection of familial ligand-defective apolipoprotein B-100 mutations

BACKGROUND

Familial ligand-defective apolipoprotein B-100 (FDB) is characterized by elevated plasma concentrations of LDL-cholesterol and apolipoprotein (apo) B, normal triglyceride and HDL-cholesterol levels, the presence of tendon xanthomas, and premature coronary artery disease. FDB cannot be clinically distinguished from heterozygous LDL-receptor-defective familial hypercholesterolaemia (FH) without genetic testing.

METHODS

Amplicons in exon 26 and exon 29 of the APOB gene were screened for established genetic variants including mutations and polymorphisms using high-resolution melting analysis. Six novel variants associated with FDB in hypercholesterolaemic Dutch patients (S3476L, S3488G, Y3533C, T3540M, I4350T, G4368D) were also studied.

RESULTS

All positive controls, a total of 10 mutations in exon 26 and four mutations in exon 29, were readily detectable by melting curve analysis. In addition, a patient previously not known to be heterozygous for the H3543Y mutation was identified in a screen of hypercholesterolaemic subjects. The method was validated by comparison of high-resolution melting analysis with DNA sequence data in a 'blinded' manner in 35 consecutive patients attending a lipid disorders clinic. These patients were classified as 'definite FH' by the Dutch Lipid Clinic Network criteria. Five patients were found to be heterozygous for the R3500Q and one for H3543Y.

CONCLUSIONS

We have established a novel, robust method of FDB mutation detection using high-resolution melting analysis in conjunction with DNA sequencing. Compared with existing methods it is not only more cost-effective, but is also capable of detecting new sequence changes and will have importance in cascade screening of affected subjects.