Genetic testing for familial hypercholesterolemia

Abstract

Purpose of review:

Despite familial hypercholesterolemia (FH) being the most common genetic cause of cardiovascular disease (CVD), genetic testing is rarely utilized in the US. This review summarizes what is known about the clinical utility of genetic testing and its role in the diagnosis and screening of FH.

Recent findings:

The presence of an FH-causative variant is associated with a substantially higher risk of CVD, even when low-density lipoprotein cholesterol (LDL-C) levels are only modestly elevated. Genetic testing can facilitate the identification of FH cases who may be missed by clinical diagnostic criteria, improve risk stratification beyond LDL-C and family history, guide treatment decisions, and improve treatment initiation and adherence. Genetic testing can be incorporated into FH screening and diagnosis algorithms, including cascade, targeted, and universal screening. Integrating genetic testing into cascade screening can enhance the effectiveness of the process. Several models of universal FH screening with coordinated genetic and lipid testing are feasible and effective.

Summary:

More systematic integration of genetic testing into FH diagnosis and screening can significantly reduce the burden of this condition through early detection and treatment. Further pragmatic implementation studies are needed to determine how to more effectively and affordably integrate genetic testing into clinical lipid screening programs.

Introduction

Familial hypercholesterolemia (FH) is a genetic disorder that results in premature atherosclerotic cardiovascular disease (CVD) due to lifelong exposure to markedly elevated low-density lipoprotein cholesterol (LDL-C) levels (1). FH is the most common genetic cause of CVD, affecting 1 in 250 individuals in the US (2). Early FH detection is important as this condition is treatable and early lipid-lowering therapy can effectively reduce the risk of premature CVD (3). FH can be diagnosed with established clinical criteria, with or without genetic testing (1, 4, 5). Despite national guideline recommendations of universal cholesterol screening beginning in childhood, it is estimated that 90% of FH cases remain undiagnosed in the US (6–8). Genetic testing has the potential to improve FH diagnosis, however, it is rarely utilized in the US (1, 3). This review will summarize the clinical utility of genetic testing and its role in the diagnosis and screening of FH.

Genetics of FH

FH was traditionally considered an autosomal dominant inherited disorder most frequently associated with variants in three genes: the LDL receptor (LDLR) gene, apolipoprotein B (APOB) gene, and proprotein convertase subtilisin/kexin 9 (PCSK9) gene (3, 9). Variants in the LDLR gene lead to a decrease in LDLR activity and consequently impaired clearance of LDL-C by the receptors (Figure 1) (3, 9). FH-causative variants in the APOB gene are mostly in regions encoding for the LDLR binding domains and can cause impaired binding of LDL-C to LDLR (3, 9). Gain-of-function variants in the PCSK9 gene increase the ability of the PCSK9 protein to promote degradation of LDLRs, leading to decreased LDLR availability for LDL-C uptake (3, 9). Variants in LDLR account for > 90% of genetically confirmed FH cases, while APOB variants account for 5–10% and PCSK9 variants <1% (3, 10).

Figure 1. The molecular pathway of LDL-C degradation.

The hepatocytes remove LDL-C from the bloodstream via LDL receptors (LDLR), which specifically binds APOB in the LDL particles. The LDL-LDLR complex is then internalized by endocytosis via clathrin-coated pits through interactions involving the LDLR adaptor protein (LDLRAP1). The complex is transported via early endosomes to the late endosomal compartment, where the acidic environment causes dissociation of the LDL-LDLR complex. The LDLR is recycled to the cell surface while the LDL particle is degraded in lysosome. When PCSK9 is present, it binds to LDLR and promotes its degradation in lysosome, subsequently lowering LDLR levels and increasing LDL-C levels. Mutations in LDLR, APOB, PCSK9, and LDLRAP1 are known to cause FH.

APOB: apolipoprotein B; FH: familial hypercholesterolemia; LDL: low-density lipoprotein; PCSK9 = proprotein convertase subtilisin/kexin type 9.

Heterozygous FH is caused by a single pathogenic variant in one of the three FH-causative genes, while homozygous FH is caused by two pathogenic variants (3). The rare homozygous FH causes markedly elevated LDL-C >400 mg/dL and very early onset of CVD (11). In patients with heterozygous FH, LDL-C is usually 190–400 mg/dL but may be higher and overlaps significantly with LDL-C ranges seen in homozygous FH (3).

Variants in other genes can also cause FH. There is an autosomal recessive form of hypercholesterolemia caused by pathogenic variants in the LDLR adapter protein 1 (LDLRAP1) gene (12). Additionally, a rare variant in the apolipoprotein E (APOE) gene (p.Leu167del) was reported to cause an autosomal dominant FH phenotype (13, 14).

Given that many individuals with FH phenotype do not carry any of these variants, it is likely that additional monogenetic and polygenetic causes of FH will be identified (9). Of note, although more than 2,000 unique variants have been reported in clinical FH patients, less than 8% have been validated as disease-causing by complete in vitro functional assays, and another 52% were considered pathogenic or likely pathogenic when applying American College of Medical Genetics and Genomics guidelines for variant classification (15). This suggests that about 40% of all variants identified in FH patients need further functional evidence to be considered disease-causing, highlighting the urgent need for more functional assays to validate the pathogenicity of FH variants to improve FH diagnosis (15).

Prevalence of genetically confirmed FH

FH can be diagnosed with established clinical criteria such as the Dutch Lipid Clinic Network (DLCN), Simon Broome, and American Heart Association criteria, with or without including FH genetic variant criteria that require genetic testing (1, 4, 5). In the US, the majority of epidemiology studies used clinical criteria alone to define FH, with estimates ranging from 1:200 to 1:300 depending on the diagnostic definitions (2, 16, 17). The prevalence of genetically confirmed FH in the general US population is not well characterized, as genetic testing for FH is uncommon in clinical practice and has not been included in most population-based health surveys (3). An analysis of 20,485 participants from seven case-control studies and five cohort studies found an FH variant in 97 (1:211) individuals (Table) (18). In 50,726 individuals in the Geisinger Health System, 229 (1:222) had an FH variant (19). Additionally, in a study of 21,426 US adults without a history of coronary heart disease (CHD) from six population-based cohort studies, 63 (1:340) had an FH variant (20).

Table.

Selected studies examining the interplay between FH genotype and phenotype in association with cardiovascular outcomes

Author (year)	Study population	Prevalence of FH variants	Association with cardiovascular outcomes
Khera et al. (2016) (18)	14,117 participants from 7 case-control studies (5,540 coronary artery disease [CAD] cases, 8,577 CAD-free controls), and 11,908 participants from 5 prospective cohort studies	Of the 20,485 CAD-free control and prospective cohort participants, 1,386 (6.7%) had LDL-C ≥190 mg/dL; of these, 24 (1.7%) carried an FH variant.	Among participants from the case-control studies, the presence of an FH variant was associated with a 3.8-fold (95% CI: 2.6 to 5.4) increase in odds of CAD. Within any stratum of observed LDL-C (<130, 130–160, 160–190, 190–220, ≥220 mg/dL), CAD risk was about two-fold higher in those with an FH variant than in those without.
Abul-Husn et al. (2016) (19)	50,726 participants from the MyCode Community Health Initiative at Geisinger Health System	(1) Of the 50,926 participants, 229 (0.45%) had an FH variant. (2) Of the 4435 participants with LDL-C ≥190 mg/dL, 112 (2.5%) had an FH variant. (3) An LDL-C ≥190mg/dL was absent in 45% of FH variant carriers.	FH variant carriers had an increased risk of general (odds ratio 2.6; 95% CI: 2.0 to 3.5) and premature (odds ratio 3.7; 95% CI: 2.6 to 5.2) coronary artery disease (CAD) compared with noncarriers.
Zhang et al. (2023) (20)	21,426 participants without existing coronary heart disease (CHD) at baseline from 6 US prospective cohort studies	Of the 21,426 participants, 1,334 (6.2%) had an LDL-C ≥190 mg/dL, 63 (0.3%) had an FH variant, and 30 (0.1%) had both an LDL-C ≥190 mg/dL and an FH variant.	Compared with the reference group of LDL-C <190 mg/dL and no FH variant, the hazard ratios for CHD were 1.4 (95% CI: 1.2 to 1.6) for those with LDL-C ≥190 mg/dL alone, 2.8 (95% CI: 1.3 to 5.9) for those with an FH variant alone, and 4.3 (95% CI: 2.3 to 7.8) for those with both LDL-C ≥190 mg/dL and an FH variant.

CVD risk associated with FH-causative variants

Recent studies have shown that for any given observed LDL-C level, the risk for CVD is substantially higher in those with an FH-causative variant than in those without, even when LDL-C is only moderately elevated (Table) (18, 20). In a case-control study of 14,117 individuals from the Myocardial Infarction Genetics Consortium, the presence of an FH-causative variant was associated with 3.8-fold increase in odds of CHD (18). Within any stratum of observed LDL-C (<130, 130–160, 160–190, 190–220, ≥220 mg/dL), CHD risk was about two-fold higher in those with an FH variant than in those without (18). A recent cohort study of 21,426 US adults further showed that genotype effects compounded phenotype effects in a dose-response manner, with progressively higher CHD risk going from those with FH phenotype only (LDL-C ≥190 mg/dL; hazard ratio [HR]=1.4; 95% CI 1.2 to 1.6), to genotype only (HR=2.8; 95% CI 1.3 to 5.9), and to phenotype and genotype combined (HR=4.3; 95% CI 2.3 to 7.8) (20). The higher CHD risk is likely explained by the substantially higher cumulative LDL-C burden in those with an FH variant since these individuals will have had genetically determined lifelong high LDL-C levels (18). Taken together, these findings highlight the potential utility of genetic testing to refine risk stratification beyond LDL-C alone.

Clinical utility of genetic testing for FH

Genetic testing may facilitate FH diagnosis by identifying those with FH-causative variants who do not meet diagnostic criteria based on lipid levels, clinical features, or available family history (3). Studies have shown that FH screening based on LDL-C levels alone may miss a significant number of FH cases (18–20). Two large studies reported that >50% of FH variant carriers had LDL-C levels <190 mg/dL (Figure 2) (18, 20). Similarly, in an analysis of 50,726 individuals from the Geisinger Health System, when retrospectively applying the DLCN criteria to electronic health record data, a probable or definite FH clinical diagnosis could be made by available health record data in only 24% of those who had an FH-causative variant, and 45% of the FH variant carriers had LDL-C <190 mg/dL (19). These findings suggest that FH screening based on severely elevated LDL-C alone may miss a significant number of FH cases, especially at younger ages. Because these individuals have a high risk of premature CHD, genetic testing may aid FH diagnosis by identifying those at-risk individuals who may not have otherwise been diagnosed and could benefit from treatment (20).

Figure 2. Prevalence of FH-causative variants by LDL-C levels.

Data were based on 21,426 US adults without a history of CHD from six population-based prospective cohort studies (20). (A) Prevalence of FH-causative variants by LDL-C levels among 21,426 US adults without a history of CHD. (B) Distribution of LDL-C among the 63 individuals with an FH-causative variant.

Genetic testing can also refine risk stratification by providing prognostic information beyond clinical data alone. In a study of 626 individuals with a clinical diagnosis of FH based on DLCN criteria, CVD risk was higher in those with an FH-causative variant compared with those with an elevated LDL-C level due to a polygenic etiology (21). As discussed earlier, CHD risk was two-fold higher in those with an FH variant than in those without, even when their LDL-C levels were similar (18, 20). The presence of an FH variant could be particularly informative for individuals with LDL-C levels that would not otherwise meet criteria for lipid lowering therapy.

Identifying the specific type of FH variant may also influence clinical management, as genetic background can affect the therapeutic response in FH patients (3). Null variants in LDLR, which lead to no functional LDLR being produced, are more severe and often associated with higher LDL-C compared with defective LDLR variants (where some functional LDLRs may still be formed) and variants in APOB and PSCK9 (3, 22). A study of 156 patients with heterozygous FH found that those with an LDLR null variant had poorer response to atorvastatin than those with defective or with no LDLR variant, and those with LDLR variants were 9 times less likely to achieve targeted LDL-C than those without (23). In homozygous FH patients, PCSK9 inhibitors had no effect on LDL-C level among those with 2 LDLR null alleles (i.e., no functional LDLR), but reduced LDL-C levels by 20–40% in those with at least one allele with residual LDLR activity (24, 25).

Additionally, FH genetic testing has been shown to have a positive impact on the initiation of lipid-lowering therapy, adherence to therapy, and LDL-C reduction (3, 26–28). In a study 747 individuals with FH in the Netherlands, the percentage of treated patients increased from 37.6% to 85.9% two years after genetic testing (27). Even among FH variant carriers who were already receiving lipid-lowering therapy at the time of genetic testing, significant reduction in LDL-C was observed after confirmation by genetic testing (27). Individuals diagnosed with FH through genetic testing also had higher perceived CVD risk, more likely to believe lipid-lowering therapy was highly important, and to recommend genetic screening to family members (3, 29, 30). These findings suggest that individuals who receive a genetic diagnosis are more engaged with their care, which could hold important implications for the development of genetic testing programs for FH.

Genetic testing in the diagnosis and screening of FH

While the utility of FH genetic testing has been clearly established, there is no consensus on how genetic testing should be carried out in the diagnosis and screening of FH (31). In the US, genetic testing for FH is rarely utilized and is typically reserved for individuals already demonstrating severely elevated LDL-C (1, 3). FH genetic testing has been performed more extensively in the Netherlands, Norway, United Kingdom, Australia, New Zealand, Spain, among others (3, 7). In general, genetic testing can be incorporated into three models of FH diagnosis and screening programs: cascade, targeted, and universal screening.

Cascade screening

Cascade screening is a systematic approach to test the at-risk relatives of people with FH (proband or index case) (3). It is the most studied FH screening strategy and has been shown to be highly effective in identifying additional FH cases, can diagnose FH at younger ages, and is cost-effective (3, 32–34). Integrating genetic testing into cascade screening can enhance the overall effectiveness of the process (35). Study showed that new case detection rate after cascade screening was significantly greater when an FH-causative variant was identified in the proband than in those without, suggesting that coordinated genetic and lipid testing can improve the uptake and yield of cascade screening (35). However, cascade genetic testing for FH is not currently systematically performed in the US (3, 36). Cascade screening in a fragmented US health care system requires motivated index cases; privacy rules also prevent direct contact of at-risk relatives by medical systems, making cascade screening dependent on patients and families. A recent study showed that even with knowledge of a known FH variant, only 23% of index cases contacted relatives to alert them about possible FH (37).

Of note, cascade screening may not be a suitable method of population-level screening for FH due to its limited ability to identify new FH index cases (38). A computer simulation study of the UK population showed that for cascade screening of first-degree relatives to achieve a detection rate of 80%, 25% of unrelated FH index cases need to be identified, which would require 45% of all FH cases being identified independently of cascade screening (38).

Targeted screening

Studies of targeted FH genetic testing in individuals presenting to cardiovascular specialty clinics have shown mixed results (31, 39–41). In a study of 103 patients with acute coronary syndrome (ASC), age ≤65 years, and LDL-C ≥160 mg/dL in Spain, the prevalence of genetically confirmed FH was 8.7%, and DLCN criteria failed to diagnose 44% patients with an FH-causative variant (39). In a study of 66 individuals with suspected premature ACS in Sweden, the prevalence of genetically confirmed FH was 4.5% (40). After excluding individuals with traditional CVD risk factors other than hypercholesterolemia, the prevalence increased to 15.8% (40). In contrast, in a study of 231 UK adults presenting with premature myocardial infarction, the prevalence of genetically confirmed FH was only 1.3% (41). Some of the differences in diagnostic yield may be due to differences in the patient population, LDL-C levels, and genetic testing platforms (31, 39–41).

Universal screening

Several universal screening programs have been tested, including universal lipid and/or genetic testing in childhood, and child-parent screening during routine childhood immunization (42–45). Slovenia is currently the only country with a universally implemented FH screening program, developed for the pediatric population and consisted of two steps: (1) universal hypercholesterolemia screening of preschool children in primary care; (2) children with elevated LDL-C are then referred to tertiary care for genetic testing, with additional cascade screening of family members (42, 43). It has been shown to be effective, with 30–50% of the children referred through the program had genetically confirmed FH (42, 43).

Another model of universal screening that has been explored in research studies in the UK and Australia is child-parent screening, initiated at the time of routine immunization during age 1–2 years (44–46). This method screens two generations simultaneously when parents and children are already visiting their primary care for preventive health reasons: the child provides the screening entry point; if a child with FH is identified, the parent with FH may then be identified (44–46). This approach has been shown to be feasible and highly cost-effective to detect FH, with 8 new cases identified every 1,000 children screened (45). When further coupled with cascade screening, one new case was identified every 56 to 70 children screened (44, 46).

Universal screening in children may have unique advantages. Currently, diagnosis of FH occurs at a mean age of 50 years, by which time more than one third with FH have already experienced a CVD event (47). Early FH detection leading to guideline-based therapy beginning in childhood has the potential to reserve the disease course and reduce the incidence of CVD (3, 43). Further, diagnosing index cases in childhood may help identify affected parents and relatives with unrecognized FH at a much younger age than a strategy based on the occurrence of CVD events (i.e., reverse cascade screening).

Implementation considerations

Currently genetic testing for FH in the US is predominantly conducted within specialized clinics accompanied by traditional genetic counseling, which typically includes the construction of family pedigrees, risk assessments, and counseling about the benefits, limitations, and family implications (31). The limited availability of trained professionals and resources available in these settings presents significant barriers if genetic testing were to be more broadly implemented (31). Several alternatives to the traditional genetic testing and counseling approach are being explored, including expanding screening in primary care settings, training nurses as specialized patient advocates, utilizing technology-based tools such as chatbots and web-based applications to streamline genetic counseling and cascade screening, and leveraging more active approaches such as direct contact to increase uptake of cascade screening (31). Also, the current out-of-pocket cost of FH genetic testing is about $500 or lower (3). With costs of next-generation sequencing continuing to fall, genetic testing for FH on a population-level may become more accessible (3).

Conclusions

The clinical utility of genetic testing has been clearly established, including identification of those who may be missed by existing clinical diagnostic criteria, improving risk stratification beyond LDL-C, informing treatment decisions, and improving treatment initiation and adherence. As genetic testing is becoming more affordable and scalable, it can be incorporated into several models of FH diagnosis and screening programs, including cascade, targeted, and universal screening, and has the potential to substantially reduce the burden of FH by early detection and treatment.

Key Points.

• Despite being the most common genetic cause of CVD, an estimated 90% of FH cases are undiagnosed and genetic testing is rarely utilized in the US.

• The presence of an FH-causative variant is associated with a substantially higher risk of CVD, even among those with only moderately elevated LDL-C.

• Genetic testing can facilitate the identification of FH cases who may be missed by existing clinical diagnostic criteria, improve risk stratification beyond LDL-C and family history, guide treatment decisions, and improve treatment initiation and adherence.

• Genetic testing can be incorporated into several FH diagnosis and screening models, including cascade, targeted, and universal screening. However, not all screening programs are equally effective and the most effective approach for identifying and diagnosing FH in the US remains to be determined; a combination of approaches will likely be necessary.