DNA testing for familial hypercholesterolemia: improving disease recognition and patient care

Pharmacogenomics of statins and familial hypercholesterolemia

PURPOSE OF REVIEW

To collect evidence on statin pharmacogenomics, and review what is known in this field for familial hypercholesterolemia (FH) patients.

RECENT FINDINGS

There are well-known associations between specific single nucleotide polymorphisms involved in statin transport and metabolism and either adverse effects or altered lipid-lowering efficacy. However, the applicability of this knowledge is uncertain, especially in high-risk populations. There are alternative approaches to study plasma concentrations of statins and new insights on why some association studies fail to be replicated.

SUMMARY

Statin therapy recommendations are not always followed in primary and secondary prevention and, even when followed, patients often fail to reach therapeutic target values. Considering the stringent 2019 European Atherosclerosis Society and European Society of Cardiology recommended target lipid levels, as well as the persistently high cost for alternative lipid-lowering therapies such as PCSK9 inhibitors, the variability in low-density lipoprotein cholesterol reductions on statin therapy is still an important factor that needs to be addressed to ensure better cardiovascular disease risk management, especially for FH patients, who have not been well studied historically in this context.

Homozygous familial hypercholesterolaemia in two boys aged 5 and 10 years

Familial hypercholesterolaemia (FH) is an autosomal dominant lipid disorder. Homozygous FH (HFH), though rare, presents in early childhood. Two different presentations of HFH are reported. The first child presented at 5 years of age with xanthomas on the knees, elbows and buttocks and failure to thrive since the second year of life. He was found to be hypertensive with moderate aortic regurgitation. He is now stable on statins and antihypertensives. The second child presented at 10 years of age with multiple xanthomas and severe aortic stenosis. He died of refractory cardiac failure despite emergency aortic balloon valvoplasty due to diffuse coronary artery disease. Strong clinical suspicion can aid early diagnosis and delay cardiovascular complications.

Single step PCR for the identification of Low Density Lipoprotein Receptor (LDL-R) gene mutations

Background and Objective: This study was conducted to determine the common mutation of low density lipoprotein receptor in patients with familial hypercholesterolemia (FH) in our population and identify the different point mutation in the LDL-receptor gene. The main aim of this study was to reduce the cost of PCR without extracting DNA and do the diagnosis at single step.

Methods: This study was carried out in the period of one year, from 2009- 2011. All the patients selected for this study were from Dr. Ziauddin Hospital, National Institute of Cardiovascular Diseases, and Dr. Rubina Ghani’s Pathological & Molecular Laboratories. While collecting the blood sample, the patients were in overnight fasting condition. The clinical and biochemical analysis was performed on hyperlipidemic patients (n=120) to determine the frequency of familial hypercholesterolemia in our population. After lipid profile the patients were selected and direct multiplex PCR (Polymerase chain reaction) was performed from whole blood collected in a single tube using forward and reverse primers of exons 3, 4, 9 and 14 of without extracting DNA.

Results: Genomic DNA was extracted from blood samples as well as direct whole ETDA blood of healthy control group and hypercholesterolemia patients to detect mutations in exons 3, 4, 9, and 14 of the LDLR gene, with modification in the technique by using type-specific primers. These results for exon 4 mutation were confirmed by DNA sequencing.

Conclusion: Screening method based on PCR by using Kappa direct PCR could be a faster and cheaper method with least contamination for screening a large number of FH patients for mutation of LDLR gene.

Review of the cost effectiveness of pharmacogenetic-guided treatment of hypercholesterolaemia

Hypercholesterolaemia is a highly prevalent condition that has major health and cost implications for society. Pharmacotherapy is an important and effective treatment modality for hypercholesterolaemia, with 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors ('statins') the most commonly used class of drugs. Over the past decade, there has been intensive research to identify pharmacogenetic markers to guide treatment of hypercholesterolaemia. This study aimed to review the evidence of incremental cost, effect and cost effectiveness of pharmacogenetic-guided treatment of hypercholesterolaemia. Three cost-effectiveness analyses (CEAs) were identified that studied the value of screening for genotypes of angiotensin I converting enzyme (ACE), cholesteryl ester transfer protein (CETP), and kinesin family member 6 (KIF6) prior to initiating statin therapy. For all three CEAs, a major limitation identified was the reproducibility of the evidence supporting the clinical effect of screening for the pharmacogenetic marker. Associated issues included the uncertain value of pharmacogenetic markers over or in addition to existing approaches for monitoring lipid levels, and the lack of evidence to assess the effectiveness of alternative therapeutic options for individuals identified as poor responders to statin therapy. Finally, the economic context of the market for diagnostic tests (is it competitive or is there market power?) and the practicality of large-scale screening programmes to inform prescribing in a complex and varied market may limit the generalizability of the results of the specific CEAs to policy outcomes. The genotype of solute carrier organic anion transporter family member 1B1 (SLCO1B1) has recently been associated with increased risk of muscle toxicity with statin therapy and the review identified that exploration of cost effectiveness of this pharmacogenetic marker is likely warranted.

A rational free energy-based approach to understanding and targeting disease-causing missense mutations

BACKGROUND AND SIGNIFICANCE

Intellectual disability is a condition characterized by significant limitations in cognitive abilities and social/behavioral adaptive skills and is an important reason for pediatric, neurologic, and genetic referrals. Approximately 10% of protein-encoding genes on the X chromosome are implicated in intellectual disability, and the corresponding intellectual disability is termed X-linked ID (XLID). Although few mutations and a small number of families have been identified and XLID is rare, collectively the impact of XLID is significant because patients usually are unable to fully participate in society.

OBJECTIVE

To reveal the molecular mechanisms of various intellectual disabilities and to suggest small molecules which by binding to the malfunctioning protein can reduce unwanted effects.

METHODS

Using various in silico methods we reveal the molecular mechanism of XLID in cases involving proteins with known 3D structure. The 3D structures were used to predict the effect of disease-causing missense mutations on the folding free energy, conformational dynamics, hydrogen bond network and, if appropriate, protein-protein binding free energy.

RESULTS

It is shown that the vast majority of XLID mutation sites are outside the active pocket and are accessible from the water phase, thus providing the opportunity to alter their effect by binding appropriate small molecules in the vicinity of the mutation site.

CONCLUSIONS

This observation is used to demonstrate, computationally and experimentally, that a particular condition, Snyder-Robinson syndrome caused by the G56S spermine synthase mutation, might be ameliorated by small molecule binding.

Pharmacogenetic aspects in familial hypercholesterolemia with the special focus on FHMarburg (FH p.W556R)

Objective

Familial hypercholesterolemia (FH) is an autosomal dominant inherited disorder caused by mutations in the low density lipoprotein receptor (LDLR) gene. FH is characterized by elevated plasma LDL cholesterol, premature atherosclerosis, and a high risk of premature myocardial infarction. In general, mutations within LDLR gene can cause five different classes of defects, namely: class I defect: no LDLR synthesis; class II defect: no LDLR transport; class III defect: no low density lipoprotein (LDL) to LDLR binding; class IV defect: no LDLR/LDL internalization; and class V defect: no LDLR recycling. One might expect that both the class of LDLR defect as well as the precise mutation influences the severity of hypercholesterolemia on one hand and the response on drug treatment on the other. To clarify this question we studied the effect of the LDLR mutation p.W556R in two heterozygote subjects.

Results

We found that two heterozygote FH patients with the LDLR mutation p.W556R causing a class II LDLR defect (transport defective LDLR) respond exceedingly well to the treatment with simvastatin 40 mg/ezetimibe 10 mg. There was a LDL cholesterol decrease of 55 and 64%, respectively. In contrast, two affected homozygote p.W556R FH patients, in the mean time undergoing LDL apheresis, had no response to statin but a 15% LDL cholesterol decrease on ezetimibe monotherapy.

Conclusions

The LDLR mutation p.W556R is a frequent and severe class II defect for FH. The affected homozygote FH patients have a total loss of the functional LDLR and—as expected—do not respond on statin therapy and require LDL apheresis. In contrast, heterozygote FH patients with the same LDLR defect respond exceedingly well to standard lipid-lowering therapy, illustrating that the knowledge of the primary LDLR defect enables us to foresee the expected drug effects.

Detection of familial hypercholesterolemia in a cohort of children with hypercholesterolemia: Results of a family and DNA-based screening

The diagnosis of familial hypercholesterolemia (FH) in unselected children is difficult due to the frequent overlap of cholesterol values in affected and non-affected and the paucity of physical signs. Nevertheless, detection and treatment of FH in childhood has been advocated to prevent atherosclerosis in these patients. Here, we report the results of a screening program in a cohort of 157 unrelated, hypercholesterolemic (HC) children (age range 2-15 years; mean 8.3+/-3.4 years) carried out by a combination of family study and molecular analysis of the LDLR gene. On the basis of the familial phenotype, 27 (17.2%) were classified as probable FH and 49 (31.2%) as affected by FCHL. Among probable FH children, 14 (51.8%) carried mutant LDLR alleles, giving an overall 8.9% prevalence of FH. Most of LDLR variants were already reported, but three new mutations G266C, T368M, and D451Y were identified. Beside increased TC and LDL-C (p<0.001), FH children showed decreased HDL-C (p<0.05) and higher prevalence of family history of CAD when compared to non-FH children. None presented tendon xanthomas. We estimated that LDL-C >3.9 mmol/L was the best cut off value for diagnosing FH in these children, showing 79% sensitivity and 71.0% specificity. We propose the use of a LDL-C cut off level associated with a family study to identify FH among HC children.