Genetic Testing for Children with Familial Hypercholesterolemia

Authors: Keiko Nagahara, Hayato Tada, Kazushige Dobashi

Abstract

Familial hypercholesterolemia (FH) is a highly prevalent genetic disorder that occurs in approximately one in 300 people in the general population. In cases of heterozygous FH, which are encountered frequently, cardiovascular disease, the main complication, typically manifests after adulthood. However, if the diagnosis and treatment begin in childhood, the onset of such complications can be prevented. Therefore, it can be said that the diagnosis and treatment of this disease from childhood is extremely important; even more so in the case of homozygous FH. However, specific indicators for diagnosing FH physical findings such as Achilles tendon thickening and tendon xanthomas rarely manifest in childhood. It is also difficult to obtain detailed medical histories from relatives. Therefore, it is not always easy to make a clinical diagnosis. In this context, since 2022, genetic testing for FH has been covered by national health insurance in Japan, and it can be considered for children as needed. This paper presents the previous research concerning genetic testing for children, its importance and application, as well as the latest findings on universal screening that includes genetic testing. It is expected that the development of pediatric FH management in our country, which has not been particularly proactive until now, will contribute to the suppression of cardiovascular complications in this condition.

Introduction

Familial hypercholesterolemia (FH) is an autosomal dominant disorder characterized by hyper-low-density lipoprotein (LDL)-cholesterolemia, premature coronary artery disease (CAD), and tendon xanthomas^ 1) ^. FH is caused commonly by pathogenic variants in genes encoding the LDL receptor (LDLR)^ 1) ^, apolipoprotein B (APOB)^ 2) ^, and proprotein convertase subtilisin/kexin type 9 (PCSK9)^ 3) ^. The LDLR is the main causative gene for FH^ 1) ^. FH is one of the most common inherited disorders, and FH patients in Japan are observed in one in 200-500 of the general population^ 4) ^. Two meta-analyses including Japanese, reported a frequency of frequency of FH is approximately one in 300 individuals in the general population^ 5 , 6) ^. Nevertheless, there are national differences in the diagnosis rate of FH^ 7) ^. Recently, the prevalence of FH in Japan was reported to be 2.6% (314/11983)^ 8) ^. In many countries with very low diagnosis rates, including Japan, the problem is that it does not lead to early treatment.

Patients with FH have high serum LDL-cholesterol (LDL-C) levels from birth and are at risk of developing atherosclerosis at a younger age in comparison to individuals with high cholesterol, who have no genetic background^ 1 , 7) ^. The carotid intima-media thickness in children with a molecular diagnosis of heterozygous FH is significantly increased from 12 years of age^ 9) ^. Therefore, it is important to diagnose FH by no later than 10 years of age and provide appropriate treatment, such as proper diet, exercise programs, and pharmacotherapies as soon as possible after the diagnosis. However, diagnosing FH in childhood is not easy for the following they have no clinical findings, they have few opportunities to receive blood tests, confirming their detailed family history of FH or premature CAD is difficult, and the levels of serum LDL-C are physiologically variable during puberty^ 10) ^. Apart from the clinical diagnosis, FH is confirmed genetically when patients have pathogenic variants in genes causing FH. Despite the limitation that the positive rate of pathogenic variants is approximately 60-80% in FH patients^ 11) ^, FH patients with pathogenic variants have a higher risk of developing CAD than FH patients without pathogenic variants^ 12 , 13) ^. In addition, the identification of genetic variants in children with FH can lead to the diagnosis of their parents, playing an important role in so-called the reverse cascade screening (RCS). Therefore, genetic testing for FH has been shown to provide additional value in risk stratification for CAD, treatment decisions, and RCS.

This review presents the previous research concerning genetic testing for children, its importance and application, as well as the latest findings on universal screening that includes genetic testing.

Genetics

Pathogenic Variations of FH

FH is caused commonly by pathogenic variants in LDLR, APOB, and PCSK9. It is reported that, heterozygous LDLR, APOB, and PCSK9 variants are found in ＞90%, ＜10%, ＜1%, respectively of patients with FH with identified genetic variants, respectively^ 7 , 14 , 15) ^. Among patients with FH with identifiable genetic variants, the majority are LDLR variants. The LDL receptor (LDLR) plays a central role in LDL-C metabolism.

The LDLR gene is on the short arm of chromosome 19. It is an 893-ammino acid cell surface glycoprotein that binds and internalizes LDL particles, primarily in the liver^ 1) ^. Most cases of monogenic FH are caused by LDLR variants, with ＞2,000 variants reported^ 15) ^, and ~3,000 deposited in the Clin Var database. Variant types include large-scale DNA copy number variations (CNVs)^ 16) ^; nonsense variants; small insertions or deletions (insertion–deletion variants or ‘indels’) within or near the coding sequence, some of which might shift the reading frame; splicing variants, typically non-coding and occurring at intron–exon boundaries; and missense variants altering a single amino acid residue^ 7 , 14

^. In Japan, the genetic testing of 796 patients with FH from 472 families at two major FH-care centers from a different district of Japan identified 132 pathogenic varians^ 11) ^. In the report, patients with FH at Kanazawa University in the Hokuriku region, a single variant LDLR K811X accounted for 41% of FH patients. This was attributed to the founder effect and low population mobility. In contrast, many single variants were identified at the National Cerebral and Cardiovascular Center in Osaka, a second largest city in Japan, which may be due to population mobility and may reflect the general variant situation in Japan^ 11) ^.

The ApoB is the major protein constituent of LDL and acts as a ligand for LDLR. Heterozygous pathogenic variants in APOB block the binding of LDL containing apoB-100 to LDLR, resulting in severely elevated levels of LDL-C. In Caucasians, 5-10% of patients with FH were reported to have APOB variants^ 7 , 14 , 15) ^. In particular, the APOB R3527Q variant is relatively common in Caucasians in Europe and originated from the Mesolithic Celts. The LDL-C level is mildly elevated in the Amish population in which the APOB R3527Q variant is clustered^ 19) ^. In Japan, there have been few reports of APOB variants, and a common variant in the Amish population described above, the APOB R3527Q variant, was first identified in one family in 2020 ^ 20) ^.

The PCSK9 was reported in 2003 as the ninth gene of the proprotein convertase (PC) family, which cleaves precursors of hormones and growth factors at specific sites and regulates their activity^ 3) ^. The PCSK9 is a circulating protein that terminates the lifecycle of LDLR by binding to it and targeting it to lysosomal degradation. A heterozygous gain-of-function variant in PCSK9 leads to a phenotype of FH. In Europe, the prevalence of FH resulting from pathogenic variants in PCSK9 is ＜1%^ 7 , 15) ^. In Japan, it was reported that 5-10% of FH heterozygotes are due to gain-of-function variants of PCSK9 ^ 11 , 21) ^, and the high prevalence of PCSK9 variants is a characteristic of Japanese patients with FH. An international collaborative study reported that the gain-of-function variant in PCSK9 E32K is a high-frequency but mild variant^ 22) ^. On the other hand, several patients with deficiency of PCSK9 activity were reported to have low LDL-C levels but no health problems, and reduced the risk of coronary heart disease (CHD)^ 23) ^. These results suggest that PCSK9 inhibitors are expected to reduce cardiovascular risk and have a high safety profile. Currently, PCSK9 inhibitors such as PCSK9 monoclonal antibodies and a small interfering ribonucleic acid (siRNA) product are in clinical use. Details are described in the treatment section.

There are also limitations in current genetic testing. As mentioned previously, the detection rate of pathogenic variants in the analysis of clinically diagnosed FH cases is reported to be about 60-80% in adults^ 11) ^. Furthermore, approximately 10% of the causative genetic variants of FH are thought to be structural variants in LDLR, and cannot be identified by some analytical methods^ 11) ^.

Polygenic FH

Recently, genome-wide association studies have shown that common genetic variations in the apolipoprotein E gene (APOE), LDL receptor adaptor protein 1 gene (LDLRAP1), and ATP-binding cassette sub-family G member 5/8 genes (ABCG5/8) are associated with plasma LDL-C levels^ 24) ^. The APOE is associated with hyper-LDL-cholesterolemia in a recessive manner^ 24 , 25) ^. Autosomal recessive hypercholesterolemia (ARH) is an autosomal recessive genetic disorder caused by an abnormality in LDLRAP1 involved in LDL receptor endocytosis^ 26) ^. The double variants in LDLR and LDLRAP1 associated with autosomal dominant and recessive form of hypercholesterolemia^ 27) ^. ABCG5/8 are the causative genes of sitosterolemia^ 28) ^, and associated with hyper-LDL-cholesterolemia^ 24 , 29 , 30) ^. APOE, LDLRAP1, and ABCG5/8 were defined as “LDL-altering genes”, and FH due to the accumulation of variants in “LDL-altering genes” is called “polygenic FH”^ 24 , 31) ^. It was reported that individuals from 15% of families carried a significant burden of common lipid-related alleles, suggesting that complex inheritance can masquerade as monogenic disease^ 31) ^. However, there is no evidence that a superposition of high-frequency genetic polymorphisms causes FH. Those polymorphisms are thought to influence the phenotype of FH or other forms of hyper-LDL-cholesterolemia.

Homozygous FH

As a rule, children with homozygous FH (HoFH) inherited FH-causing pathogenic variants from their parents and their untreated LDL-C levels are usually higher than approximately 600 mg/dL (15.6 mmol/L)^ 1) ^. HoFH patients are exposed to very high levels of LDL-C from the fetal period, and cutaneous xanthomas that occur at skin flexures during infancy are the trigger for their examination. Sitosterolemia and cerebrotendinous xanthomatosis (CTX) are differential conditions for HoFH presenting with xanthomas in infancy. Sitosterolemia is characterized by high LDL-C levels in infancy (during breastfeeding), which are comparable to HoFH and CTX, but even without treatment, LDL-C levels improve after infancy. On the other hand, HoFH has a very poor prognosis because atherosclerotic diseases such as coronary atherosclerosis and aortic valve disease progress rapidly from childhood and death may occur at a young age, even in children. Therefore, an early diagnosis and initiation of treatment are particularly important in HoFH, and genetic testing plays an important role in its early diagnosis.

In genetic testing, the detection of two pathogenic variants in any of the FH causative genes, LDLR, APOB, or PCSK9 confirms the diagnosis of HoFH. A ‘true homozygote’ carries two identical variants in the same gene, a ‘compound heterozygote’ carries two different variants in the same gene, and a ‘double heterozygote’ carries variants in two different gene. All of the above are treated as HoFH. The ARH is clinically a phenotype of HoFH and is included in HoFH. Double heterozygotes carrying an LDLR variant together with the PCSK9 gain‑of‑function variant E32K have been reported in Japan^ 21 , 32 , 33) ^. These cases were milder than either true homozygotes or compound heterozygotes in LDLR and responded better to drug therapy, including statins^ 33) ^. In compound heterozygous with two LDLR variants, the receptor-defective type (2-30% of normal LDL receptor activity) is considered less severe than the receptor-negative type (＜2% of normal LDL receptor activity)^ 1) ^. Since the response to treatment varies depending on the type of genetic variants, it is advisable to confirm the genetic variant as much as possible in cases of suspected HoFH.

Recently, the clinical characteristics of 201 patients with definite or probable HoFH in Japan were reported^ 34) ^. Although many patients (n = 30) were diagnosed at 0 years old, substantial proportions of the patients were diagnosed in their adulthood. Genetic testing was conducted on 65 patients (32.3%), of which 52% (n = 34) had variants in LDLR and 26% (n = 17) had variants in LDLR and PCSK9. Other types of double heterozygous FH with LDLR and PCSK9 variants were observed. These included LDLR＋LDLRAP1＋PCSK9 (1.5%, n = 1), LDLR＋PCSK9＋Others (1.5%, n = 1), LDLR＋PCSK9＋LDLRAP1＋Others (6.2%, n = 4). Moreover, a patient had variants in LDLRAP1＋PCSK9 (1.5%, n = 1). Previous studies from Europe reported that ＞90% of patients with HoFH had pathogenic variants in both LDLR alleles^ 35 , 36) ^. In contrast, as much as a quarter of the HoFH cases were caused by double heterozygous variants in LDLR and PCSK9 in Japanese patients^ 34) ^. Therefore, it is important to consider these differences when we compare phenotypic differences of HoFH across difference ethnicities.

Role of Genetic Testing in the Diagnosis of FH in Children

The Diagnosis of FH in Children

Except for HoFH, diagnosing FH in children on the basis of clinical findings such as tendon xanthomas or corneal arcus is difficult, as these manifestations are rarely evident in childhood and typically appear in adulthood^ 1) ^. In fact, tendon xanthomas are usually absent in childhood and start to develop with aging. The frequency of xanthomas is reported to be 2.6% at 1-9 years of age, 12.5% at 10-19 years of age, 69.2% at 20-29 years of age, and 90% at 30-39 years of age^ 1) ^. Another report states that even in adulthood, tendon xanthomas are present in approximately 60-70% and absent in 20-30%^ 12) ^. Therefore, genetic testing is even more useful in diagnosing FH in childhood. Furthermore, due to parental divorce and the shift toward nuclear families, obtaining detailed family histories has become difficult, making it harder to diagnose FH based on family history. Under these circumstances, genetic testing plays an important role in the diagnosis of pediatric FH.

Diagnostic Criteria

Currently, three formal diagnostic criteria for FH are widely used in Western the MEDPED Criteria^ 37) ^ ** ( Table 1 ) **, the Simon Broome Criteria^ 14 , 38 , 39) ^ ** ( Table 2 ) **, and the FH Dutch Lipid Clinic Network (DLCN) Criteria^ 40 , 41) ^ ** ( Table 3 ) **. The Simon Broome and Dutch criteria include the results of the genetic testing as a part of the FH diagnostic criteria. In the Simon Broome criteria, TC ＞260 mg/dL or LDL-C ＞155mg/dL and DNA mutation are diagnosed with “Definite FH” in children (＜16 years). In the Dutch criteria, children with only functional mutations in LDLR, APOB, or PCSK9 are diagnosed with “Probable FH” regardless of serum LDL-C levels. In these cases, if other criteria are present, the diagnosis is “Definite FH.” In Japan, the Japan Atherosclerosis Society (JAS) diagnostic criteria were published in 2022 ^ 42) ^ ** ( Fig.1 ) **. In the JAS criteria, it is stated that genetic testing is considered when the diagnosis of FH is difficult, and FH is diagnosed when a child has a pathogenic variant of FH ** ( Fig.1 ) **. Furthermore, not only “Definite FH” but also “Probable FH” and “Possible FH” groups, recommending careful follow-up for children in whom FH cannot be completely ruled out ** ( Fig.1 ) **. In any case, the genetic diagnosis is confirmed when a pathogenic variant in the FH causative gene is identified. Therefore, genetic testing has come to play an important role in the diagnosis of FH in children.

Detection Rate of Pathogenic Variants

It should be noted genetic variants are not identified in all patients. In adults, the detection rate of pathogenic variants in clinically diagnosed FH is reported to be approximately 60-80%^ 11) ^. In pediatric FH, the reported detection rates of of pathogenic variants in LDLR, APOB, or PCSK9 are as follows, 95% (255/269) in the Netherlands^ 43) ^, 88% (2531/2866) in eight European countries^ 44) ^, 57% (155/272) in Slovenia^ 45) ^ , 56% (9/16) in Iran^ 46) ^, 50% (39/78) in Italy^ 47) ^, and 49% (16/33)^ 48) ^ and 48% (22/46)^ 49) ^ in Japan ** ( Table 4 ) **. Note that the absence of pathogenic variants in the gene responsible for FH does not rule out FH.

Genetic Testing in Risk Stratification for CAD in Children with FH: Phenotype Correlations by gene

Clinical Characteristics of Children with FH, with or without of Pathogenic Variants

Patients with FH are at extremely high risk for CAD. In adults, serum LDL-C levels are reported to be higher in FH patients with pathogenic variants than in those without variants^ 12 , 13 , 50) ^. Similar results of LDL-C levels have been reported in children with FH^ 43

^ ** ( Table 4 ) **. Patients with hypercholesterolemia develop CAD when their cumulative LDL-C levels reach approximately 6,200 mg/dL . year (160 mmol/L . year)^ 7) ^. Untreated pathogenic variant-positive children with FH are expected to reach levels of cumulative LDL-C at which they will develop cardiovascular events sooner in comparison to variant-negative children with FH. Moreover, in children with FH, the rate of family history of CAD is higher in the variant-positive group than in the variant-negative group^ 43 , 45

^ ** ( Table 4 ) **. In adults with FH, those with FH causative variants without clinical signs are reported to be 3.4 times more likely to develop CAD than FH patients with no clinical signs who are negative for pathogenic variants^ 12) ^. Therefore, the pathogenic variant-positive group is considered to be at high risk for the development of CAD, and genetic testing is important in risk stratification for CAD.

LDL-C Levels according to Pathogenic Variant in LDLR

FH is a complex clinical genetic disease with wide genetic heterogeneity and large phenotypic variations^ 51) ^. Among adult FH patients with LDLR variants, the LDL-C levels in patients with loss-of-function variants, such as nonsense, splice-site, and frameshift variants, were reported to be higher than in those with missense variants^ 13 , 17) ^. Although the number of patients is very small, even in children, the mean serum LDL-C levels in individuals with nonsense, splice-site, and frameshift variants in LDLR were higher than those in children with missense variants in LDLR^ 48) ^.

LDL-C Levels in LDLR vs PCSK9 or APOB Variants

The LDL receptor is at the center of LDL-C metabolism. and the serum LDL-C levels of patients with FH with LDLR variants are often higher than those of FH patients with APOB or PCSK9 variants, both in adults^ 50) ^ and children^ 44 , 45) ^. In the Myocardial Infarction Genetics Consortium case-control studies, the effects of an increased LDL-C level and the increase in odds of CAD were most pronounced in those with loss-of-function in LDLR^ 13) ^.

The PCSK9 E32K variant has been frequently reported in Japanese individuals, it is associated with a milder and more variable degree of hyper-LDL-cholesterolemia in comparison to LDLR variants^ 21 , 32 , 33) ^. FH patients carrying both LDLR variants and PCSK9 V4I variants had higher LDL‑C levels and a greater incidence of concomitant CAD^ 21) ^. In contrast, the loss-of-function of PCSK9 variants reduce LDL-C levels. A cohort study found that 3.2% (301/9524) Caucasians had a PCSK9 R46L loss-of-function variant. This was associated with a significant reduction in the plasma level of LDL-C (15%)^ 23) ^. Moreover, Individuals who were heterozygous or homozygous for the PCSK9 R46L variant had a 47% reduction in the incidence of coronary events^ 23) ^. In Black individuals, nonsense variants in PCSK9 (Y142X or C679X) were identified in 2.6% (85/3363), and were associated with a 28% reduction in LDL‑C levels and an 88% reduction in the risk of CHD^ 23) ^. Therefore, LDL-C levels vary greatly depending on genetic variants, and genetic testing of individuals with FH is also beneficial for stratifying the risk of CAD.

Role of Genetic Testing in the Treatment of Children with FH

Heterozygous FH

Therapeutic interventions to improve lipid abnormalities in children with FH include lifestyle modification and pharmacotherapy. In Japan, lifestyle modifications include regular physical activity, limiting screen time, healthy diet (high in fiber, low in saturated fat ≤ 7% of calories, avoidance of trans fat, ≤ 200 mg/dL of dietary cholesterol) and maintenance of healthy diet^ 42 , 52) ^. In pharmacotherapy, the first-line drugs are statins, starting at the lowest dose. Seven statins are approved by the Food and Drug Administration (FDA) as the lipid-lowering drug for children with FH^ 53 , 54) ^. Rosuvastatin was recently approved for use in children from 7 years of age (initially it was authorized for use in children from 10 years of age); pitavastatin and pravastatin are indicated for children as young as 8 years of age; atorvastatin, fluvastatin, lovastatin and simvastatin may be used in children ≥ 10 years of age^ 53) ^. In children and adolescents (≥ 10 years of age) with an LDL‑C level persistently ≥ 190 mg/dL, or ≥ 160 mg/dL in the presence of a clinical presentation consistent with FH, who do not respond adequately after 3–6 months of lifestyle therapy, initiation of statin therapy is considered reasonable^ 55) ^. In Japan, pitavastatin has been indicated for children ≥ 10 years of age since 2015. In the JAS guidelines, if the level of LDL-C remains ≥ 180 mg/dL, pitavastatin should be considered at 10 years of age^ 42) ^. Regarding treatment goals, the European Society for Arteriosclerosis recommends an LDL-C target of ＜135 mg/dL in individuals of ＞10 years of age^ 41) ^, and the JAS recommends an LDL-C target of ＜140 mg/dL^ 42) ^. However, the decision to initiate treatment is difficult during puberty because of the physiologic decline in serum LDL-C levels^ 10) ^. In such cases, the identification of genetic variants provides a rationale for the initiation of treatment.

Statins exert their LDL-C-lowering effects by suppressing cholesterol synthesis and enhancing the expression of the LDL receptor via a transcription factor (sterol regulatory element-binding protein-2 [SREBP-2]). On the other hand, statins simultaneously increase the expression of PCSK9 from SREBP-2 ^ 56) ^. This is one reason the LDL‑C–lowering effect of higher statin doses reaches a plateau.

PCSK9 inhibitors increase the LDL receptor expression in hepatocytes, and thereby significantly reduce blood LDL-C levels, even when used in combination with statins, and suppress the onset of cardiovascular disease. Therefore, the combination of statins with PCSK9 inhibitors is a reasonable treatment. In Japan, PSCK9 inhibitors are approved for adult FH patients whose LDL-C levels fail to reach treatment goals despite maximum statin doses and concomitant use of other medications. As PCSK9 inhibitors, a monoclonal PCSK9 antibody (evolocumab) and an siRNA formulation that targets mRNA encoding the PCSK9 protein (inclisiran sodium) are approved in Japan.

In a randomized, double-blind, placebo‑controlled trial in children and adlescents, evolocumab reduced LDL‑C levels and other lipid parameters in heterozygous FH patients of 10–17 years of age^ 57) ^. The results of a comparison of 104 patients in the evolocumab group and 53 patients in the placebo group showed that LDL-C levels in the evolocumab group (–44.5%, –77.5 mg/dL) were significantly lower than those in the placebo group (–6.2%, –9.0 mg/dL) after 24 weeks^ 57) ^. Moreover, regarding inclisiran sodium, randomized controlled trials are currently underway to evaluate the efficacy and safety of inclisiran sodium in children of 6 to ＜12 years of age (ORION-20)^ 58) ^ and 12 to ＜18 years of age (ORION-16)^ 59) ^ with heterozygous FH.

As a general rule, treatment for children with heterozygous FH consists of a combination of lifestyle modifications and statin therapy, regardless of the results of genetic testing^ 42) ^.

Homozygous FH

In patients with HoFH, regardless of whether the patient is receptor-negative or receptor-defective type, statin therapy and lifestyle modifications should be initiated as soon as possible upon the diagnosis of HoFH. Recently, the JAS guidelines were updated with additional revisions in relation to the treatment of pediatric HoFH^ 60) ^. The primary mechanism of action for statins, ezetimibe, and resins is to increase LDL receptor activity; however, patients with HoFH often show poor responsiveness to these agents^ 60) ^. In receptor‑negative HoFH, the LDL‑C–lowering efficacy of these drugs is low; nevertheless, statin therapy has been reported to improve the prognosis^ 61 , 62) ^. In contrast, for HoFH of the receptor‑defective type, several reports have described marked efficacy of combination therapy with statins, resins, nicotinic acid, and other agents^ 63) ^.

Since responsiveness to drugs that enhance LDL receptor activity (e.g., statins and PCSK9 inhibitors) varies, it is desirable to confirm the presence of genetic variants when HoFH is suspected^ 42 , 60) ^. In the receptor-negative type HoFH, statins and PCSK9 inhibitors that act via the LDL receptor are less effective. In contrast, PCSK9 inhibitors can be expected to provide some efficacy in FH patients who are double heterozygotes, carrying an LDLR loss‑of‑function variant together with a PCSK9 gain‑of‑function variant^ 60 , 64) ^. In special cases, double heterozygotes carrying an LDLR variant together with the PCSK9 E32K variant present clinical features of HoFH, such as cutaneous xanthomas from childhood, yet retain a preserved response to lipid-lowering therapy including statins^ 32) ^. When statins or PCSK9 inhibitors show limited therapeutic efficacy, consider drugs that act independently of LDL receptor activity. Evinacumab, an antibody against angiopoietin-like protein 3 (ANGPTL3)^ 65 , 66) ^ and lomitapide, a microsomal triglyceride transfer protein (MTP) inhibitor^ 67) ^, have been reported to be effective as drugs that do not depend on LDL receptor activity. In Japan, evinacumab was approved in January 2024 as a treatment for HoFH. Equivalent efficacy and safety to adults have been reported in pediatric patients^ 65) ^, and its administration to pediatric HoFH patients is approved without age restrictions^ 60) ^. Lomitapide has been associated with numerous adverse events, including gastrointestinal symptoms, liver dysfunction, and fatty liver disease. Consequently, it is not approved for pediatric FH in any country. If the target level for lowering LDL-C is not achieved, lipoprotein-apheresis should be considered as needed. In view of the above, the results of genetic testing is also important for treatment selection.

Role of Genetic Testing in Reverse Cascade Screening (RCS) in Children with FH

Cascade screening is intended to screen close relatives of patients with identified pathogenic variants to inform disease prevention strategies. FH is an autosomal dominant genetic disorder, which means that if a child is diagnosed with FH, one of the parents is FH patient. Therefore, RCS allows parents diagnosed with FH to begin treatment before the onset of CAD. If siblings have high LDL-C levels, they are likely to have the same variants.

In FH, RCS is beneficial in terms of healthcare economics. It was concluded that a strategy involving cholesterol screening (at age 1-2 years) followed by diagnostic genetic testing and RCS was the most cost-effective modelled in the UK (incremental cost-effectiveness ratio [ICER] vs. no universal screening [US] £12,480/quality adjusted life year [QALY]; probability of cost-effectiveness 96.8% at £20,000/QALY threshold)^ 68) ^. In Japan, the cost-effectiveness of Kagawa model FH screening (i.e., the combination of US in the universal health examination of children of 9–10 years of age conducted in Kagawa Prefecture and RCS of the probands’ relatives) was evaluated. An FH screening series (US in children + RCS in adult relatives) was cost-effective in comparison to no screening, with an ICER of approximately JPY 150,000 (USD 1,042) / QALY, which was below the willingness-to-pay threshold of JPY 5,000,000 (USD 34,722) / QALY for medical technology in Japan (USD 1 = JPY 144)^ 69) ^. Therefore, RCS is important in health economics, and genetic testing plays an important role in RCS.

Application of Genetic Testing in Children

As mentioned above, genetic testing for FH in children is useful for diagnosis, risk stratification, treatment, RCS, and medical economics. However, its application requires careful consideration. In Japan, The Japanese Association of Medical Science has issued the “Guidelines for Genetic Testing and Diagnosis in Medical Care”, which provide standards for conducting genetic testing^ 70) ^. According to the guidelines, genetic testing should be performed with consent as necessary in cases where a diagnosis is difficult to make or in severe cases. FH is a treatable disease with a definitive diagnosis, and genetic testing could be considered in children as needed. On the other hand, for non-affected individuals, it is recommended that the procedure should be postponed until the individual is an adult and able to make an autonomous decision to undergo genetic testing for FH^ 42 , 70) ^. In Japan, genetic testing for the clinical diagnosis of FH has been covered under the national health insurance system since April 2022. Accordingly, it is expected that the results of genetic testing will be used for the definitive diagnosis and selection of treatment for FH patients.

Ethics of Genetic Testing in Children

Genetic testing for FH in both adults and children requires ethical considerations. According to the above guidelines, in the case of genetic testing for a disease that has developed in a minor, it is necessary to obtain the consent of an individual standing as a surrogate representative^ 70) ^. In such cases, the surrogate should make the decision after careful consideration of the examinee’s best interests in health care. It is desirable to obtain informed assent from the examinee after giving the explanation of the test at a level that the patient can comprehend^ 70) ^.

In addition, genetic counseling should be provided at the appropriate time, if necessary, when conducting genetic testing. Genetic counseling requires not only the provision of information, but also psychological and social consideration to enable the child and their guardians to make autonomous decisions regarding genetic testing. It is desirable that genetic counseling be conducted in collaboration with physicians experienced in FH treatment and individuals who are skilled in genetic counseling (e.g., clinical genetic specialists and certified genetic counselors)^ 42 , 71) ^.

Conflicts of Interest

None.