Introduction

Hereditary lipid disorders can result in high levels of low-density lipoprotein cholesterol (LDL-C), triglycerides and lipoprotein(a) [Lp(a)], or, in some cases, very low levels of LDL-C and high-density lipoprotein cholesterol (HDL-C). Genetic testing can be important in screening, diagnosis, and potentially in treatment of lipid disorders, particularly for familial hypercholesterolemia (FH). This article briefly addresses the value of genetic testing in a lipid clinic with a focus on FH.

Familial Hypercholesterolemia

FH is the most common monogenic inherited lipid disorder resulting in very high LDL-C levels and causing preventable premature cardiovascular death, present in approximately 1 in 3-500 individuals.¹ The risk of premature coronary heart disease increases 20-fold, and myocardial infarction is often the first presenting sign in FH.^2,3 It is believed that <10% of FH patients are formally diagnosed in the U.S., and many are not adequately treated.⁴ If optimal LDL-C levels can be obtained, the risk for ischemic cardiovascular events can be reduced to that of the non-FH population.¹ Therefore, better screening and diagnostic tools can be helpful.

FH can result from one or more genetic mutations known to eventually increase LDL-C levels and are most commonly inherited as autosomal dominant traits. The most common genetic abnormality involves one of the possible 1,600 different mutations in low-density lipoprotein receptor (LDLR) gene.⁵ Other genetic defects include the following: familial defective apolipoprotein B-100;⁶ proprotein convertase subtilisin/kexin 9 (PCSK9) gain-of-function mutation,⁷ and rare autosomal recessive mutation in the adapter protein.⁸ These genetic mutations may affect only one allele (heterozygous FH [HeFH]), both alleles (homozygous FH [HoFH]), or two different mutations affecting two alleles (compound heterozygous FH). There is a gene dosage effect on phenotype, with HoFH having significantly higher LDL-C levels and earlier onset of cardiovascular disease (CVD) compared to HeFH.¹¹

When seeing a patient in a lipid clinic, it is important to take a detailed history; draw a pedigree including at least three generations' history of CVD and risk factors; perform a focused physical examination, such as assessment for tendon xanthomas, eruptive xanthomas, xanthelasma, and corneal arcus; and order a lipid profile (Figure 1). One should rule out secondary causes of dyslipidemia by screening for nephrotic syndrome and performing thyroid and liver function tests. Although FH is most frequently associated with elevated LDL-C and relatively normal triglycerides (Fredrickson type IIa lipid phenotype or ICD 9 code 272.0), the phenotype can be variable and some individuals can also manifest with elevated triglycerides because of lifestyle-related factors and other coexisting genetic defects. Therefore, although Fredrickson classification provides useful information about lipoprotein phenotype, this should not be confused with a genetic diagnosis.

Figure 1: Approach to Assess a Patient With a Lipid Disorder

Take full personal and family history of CVD and risk factors; three-generational family pedigree recommended. ↓ General and focused physical examination to assess for stigmata of lipid disorders.* ↓ Obtain lipid profile – consider what Fredrickson lipoprotein phenotypic class could the patient could have. ↓ Consider screening for lipoprotein (a),# particularly with strong personal or family history of ischemic CVD (particularly early onset), presence of ischemic CVD or recurrent CVD despite absence of other risk factors, or if the patient has very high LDL-C levels. ↓ Assess for secondary causes¶ of lipid disorder if lipid profile was abnormal. Tests to consider include urinalysis, TSH, and LFT. ↓ Does the patient meet clinical diagnosis of FH (using MedPed, Simon Broome, Dutch Lipid Clinic, NLA, 2013 ACC/AHA criteria)? ↓ If yes, consider screening of first- and second-degree relatives (in that order) using lipid profile, history, and physical examination (CASCADE screening), ↓ Consider identification of causative mutation in patients with clinical diagnosis of FH for possibility of targeted genetic testing for the causal mutation in first-degree family members,

There are several other reasons why using clinical criteria for FH diagnosis alone may not be very reliable.¹² Patients with FH can have a varying range of LDL-C levels. Some genetic FH patients may not have very high LDL-C levels, which can be missed using clinical criteria alone.^13-16 Such individuals may possess other favorable genes or epigenetic factors that can reduce the impact of the mutation, but there continues to be 50% chance of passing the mutation to his/her offspring, who may have an adverse net cardiovascular risk. Furthermore, the index patient will still have higher lifetime LDL-C exposure, making the argument that earlier identification of the mutation could offer long-term benefit. On the contrary, depending upon the referral criteria, 10-40% of those with clinical FH diagnosis may not have detectable causal mutation. In addition, family history may not always be very reliable. An example includes a lack of family history of premature onset of CVD because of the trend for early initiation of preventive therapies, which usually means later manifestation of ischemic event, particularly in women with FH. Therefore, a male proband may present with premature onset of coronary heart disease, while his mother may have high levels of LDL-C but no history of an ischemic cardiac event. Genetic testing in the mother would have been very helpful so that a directed screening for the genetic variant can be considered for her offspring.¹²

There are three commonly used criteria to diagnose FH: Make Early Diagnosis to Prevent Early Deaths (MedPed) in the U.S., Simon Broome diagnostic criteria, and Dutch Lipid Clinic Network Criteria.^17-19 The MedPed criteria use various total cholesterol cut-off points based on family history of FH. Simon-Broome and Dutch criteria utilize lipid profiles, history of premature CVD, and physical examination findings such as tendon xanthomas from both the patient and family. Simon Broome and Dutch criteria consider FH to be definite, probable, or possible with presence of specific genetic mutation(s) carrying the most points. In addition, the National Lipid Association Expert Panel published clinical guidelines to address screening, diagnosis, and management of FH and recommended cascade screening with clinical criteria and using genetic testing in cases of diagnostic uncertainty.²⁰ The 2013 American College of Cardiology/American Heart Association guidelines on the treatment of blood cholesterol suggest to consider FH in all patients with LDL-C >190 mg/dL.^21,22

One should remember that the cholesterol levels used in all of these criteria refer to off-treatment levels. However, it is difficult to obtain off-treatment cholesterol levels for most patients in contemporary practice because of the emphasis on early initiation of statins in FH patients. For the same reason, physical examination findings such as tendon xanthomas, which usually require life-long accumulation of very high cholesterol levels, may not be present.

Identification of the mutated gene(s) provides a definite diagnosis of FH, which is not always the case with a clinical diagnosis of FH. In both the Simon Broome criteria and the Dutch Lipid Clinic Network Criteria, positive DNA mutation(s) alone is sufficient to make the diagnosis of FH, whereas many individuals who may clinically have an autosomal dominant inheritance of high LDL-C and CVD may no longer meet these criteria.

Genetic Testing in the Screening of FH

Since most causative mutations for FH are inherited in autosomal dominant fashion, approximately half of first-degree relatives of the proband also inherit the mutations. Using this principle cascade FH screening employs screening first- and second-degree relatives (in that order) of affected FH patients with lipid profile, which is guided by family history of premature coronary heart disease. This cost-effective approach to identify new FH cases has been used in Europe but has just begun in the U.S.⁴ If genetic testing is available and the causative mutation is known, this is the most effective screening strategy, which has been employed very successfully in the Netherlands for almost two decades, and has resulted in identification of 71% of estimated FH cases, the highest rate seen among 22 countries.¹ Without genetic screening, only 39% of these individuals were on cholesterol-lowering medications, and after two years of genetic screening, 85% were on these medications.²³ Mutation detection ranged from 36% to 59%, and the rates differed based on the original likelihood for having an FH diagnosis and on the methods used for genetic screening.^24-26 Such a targeted genetic testing in affected family members has been shown to be cost-effective.²⁷ The cost per life year gained for genetic cascade testing and intensive statin therapy in FH is €3-4000, which is similar to that for mammography screening for breast cancer.²⁸

It should be remembered that in the U.S., cascade screening using lipids and medical history has just begun and should be widely implemented, while the potential benefit and risks of using the genetic information, particularly the targeted genetic tests for screening the family members once the causative mutation is identified in the proband, should also be considered.

Genetic Testing in the Treatment of FH

Response to medications that eventually increase LDL-R depends on the effect of the causative mutations on LDL-R function. Therefore, identification of specific mutations can sometimes be helpful in treatment, for example treatment with statins.²⁹ The Trial Evaluating PCSK9 Antibody in Subjects with LDL Receptor Abnormalities (TESLA) evaluated the efficacy of a monoclonal antibody against PCSK9 for reducing LDL-C levels in clinically-defined HoFH patients. The LDL-C lowering effect was proportionate to the degree of functioning LDLR, suggesting that genetic data can sometimes provide incremental information to assess response to treatment.³⁰ Genetic information can also be helpful in other areas of pharmacogenomics, such as in estimating an adverse effect of a medication. Genetic sequencing may also identify new and unknown mutations, which may lead to novel therapeutic targets, such as the genetic studies that led to the development of monoclonal antibodies to PCSK9 molecules.

Disadvantages of Genetic Testing

Genetic tests may not be widely available or reimbursed by the U.S. health care plans. Presence of specific mutation depends on the screening criteria used and the population studied. Interpretation can be difficult with false-positive cases with nonpathogenic sequences. Genetic test can be expensive, but the cost has substantially decreased over time. However, focused screening of causative mutations in family members once identified in the proband can be cost effective. Psychological effects of "labeling" individuals with genetic defects should be considered, and ideally genetic testing should be offered in conjunction with a trained genetic counselor. One must be sensitive in communication of identified mutations to family members and should be considered in concert with the index patient. Presence of causative mutations can affect an individual's eligibility for long-term care, life, and disability insurance as well as employment. Finally, it should be remembered that for wider implementation of genetic testing in clinical practice, randomized trials and cost-effectiveness studies are needed to assess the incremental benefit of genetic testing over clinical criteria for screening and diagnosis of FH.¹²

L(a)

Currently there is no clear role of genetic testing in routine practice in the identification or management of individuals with elevated Lp(a) levels.

Hypertriglyceridemia

Familial chylomicronemia syndrome is a monogenic autosomal recessive disorder.³¹ Because of clustering of susceptibility alleles and lifestyle-related factors in families, screening for hypertriglyceridemia with a lipid profile for family members is important, but routine genetic testing is not warranted.³¹

References

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Keywords: Alleles, Antibodies, Monoclonal, Anticholesteremic Agents, Apolipoprotein B-100, Arcus Senilis, Breast Neoplasms, Cardiovascular Diseases, Cholesterol, Cholesterol, LDL, Cholesterol, HDL, Coronary Disease, Counseling, DNA, Early Diagnosis, Epigenesis, Genetic, Gene Dosage, Genetic Testing, Hyperlipoproteinemia Type I, Hyperlipoproteinemia Type II, Hyperthyroidism, Hypertriglyceridemia, Hypothyroidism, Insurance, Disability, International Classification of Diseases, Lipids, Lipoprotein(a), Lipoproteins, HDL, Lipoproteins, LDL, Liver Diseases, Liver Function Tests, Long-Term Care, Mammography, Mutation, Myocardial Infarction, Nephrotic Syndrome, Pedigree, Pharmacogenetics, Phenotype, Physical Examination, Proprotein Convertases, Receptors, LDL, Referral and Consultation, Risk Factors, Subtilisin, Tendons, Thyroid Function Tests, Triglycerides, Urinalysis, Xanthomatosis, Secondary Prevention

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