Familial hypercholesterolemia. The determination of phenotype - 2 Phenotypic variability in familial hypercholesterolemia: an update

Familial hypercholesterolemia. The determination of phenotype

Jansen, A.C.M.

Publication date

2003

Link to publication

Citation for published version (APA):

Jansen, A. C. M. (2003). Familial hypercholesterolemia. The determination of phenotype.

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hypercholesterolemia: :

ann update

Angeliquee CM Jansen, Sanne van Wissen, Joep C Defesche and

Johnn JP Kastelein

Departmentt of Vascular Medicine, Academic Medical Center,

Universityy of Amsterdam, the Netherlands.

Abstract t

Heterozygouss Familial Hypercholesterolemia is one of the most common inherited dominant disorders,, characterized by severely elevated LDL-cholesterol levels and premature cardiovascularr disease. Although the cause of FH is monogenic, there is a substantial variation inn the onset and severity of atherosclerotic disease symptoms. Additional atherogenic risk factors,, of environmental, metabolic and genetic origin, in conjunction with the LDL receptor defect,, are presumed to influence the clinical phenotype in FH, In this review, recent developmentss are discussed.

Introduction n

Familiall Hypercholesterolemia (FH) is a common inherited disorder of lipoprotein metabolism. Itt is caused by a plethora of mutations in the gene encoding the low-density lipoprotein receptorr (LDL-R). Consequently, a reduced number of functional LDL-cholesterol receptors iss present on the liver, resulting in insufficient uptake of plasma LDL, and a 2- to 3-fold elevatedd plasma LDL-cholesterol level.' Patients are clinically characterized by the presence off tendon xanthomas, xanthelasmata or arcus lipoides corneae. In most patients there also

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andd premature cardiovascular disease (CVD). Untreated, 75% of male FH patients suffer fromm coronary artery disease (CAD) before the age of 60 years.2-3-4

Characteristically,, the mean age of onset of CVD is between 40 and 45 years in male FH patientss and in female FH patients 10 years later. Once the age of 60 to 70 years is reached, thee risk seems to wane and to approach that of the general population ,3-5 This might be due too survival bias, since patients who are most susceptible to elevated LDL-cholesterol may have diedd at younger age.

Althoughh FH is monogenic, the phenotypic expression, in terms of onset and severity of atheroscleroticc disease, varies considerably. Even within subjects that share an identical genee defect, the clinical presentation differs substantially.2-6 Additional factors are presumed too influence the course of FH and lead to the enormous variation in clinical manifestations (figuree 1). In this review, we present the latest developments in this field. The influence of (a)) environmental, (b) metabolic, and (c) genetic factors and also (d) therapy will be discussed.

Figuree 1. The clinical phenotype of FH is influenced by environmental and metabolic factors, LDL-receptor

mutation,, other genetic factors and therapy.

Metauoii L iatAur:> LDL-RR mutation 1 1 '' <

' '

Otherr genetic factors

_ _

bnviroo unci u

Therapy y

A.. Environmental factors

Environmentall factors obviously play a crucial role in the clinical expression of FH and the influencess of smoking and dietary habits have been well-established. However, the specific contributionn of the environment is incompletely understood, as demonstrated by two recent studiess of Sijbrands et al.56

Inn one study, a large pedigree with the V408M mutation was investigated.6 Mortality of FH individualss varied significantly over time. During the 19lh and early 20th century, mortality wass not increased. It rose in the 20th century, with a maximum between 1935 and 1964, andd fell thereafter. During the decades with excess mortality of FH patients, survival ranged fromm severe excess mortality to normal life expectancy. Furthermore, mortality differed significantlyy between several branches of the pedigree. The large variability of mortality overr time could be explained by changes in lifestyle. A number of behavioral factors are knownn to influence cardiovascular risk.7 A growing awareness of the effects of smoking, diett and exercise may have led to an improvement of lifestyle during the last decades. Indeed,, cigarette smoking was recently shown to be one of the strongest predictors of CADD mortality in FH ,8

Anotherr study addressed all-cause mortality in 855 untreated first-degree relatives of 113 unrelatedd FH patients.5 As in the previous study, many carriers of a LDL-R mutation reached aa normal life span, but mortality varied with age, gender, family history of CVD and ,strikingly, againn calendar time, fn this study individuals with a family history of CVD had a significantly higherr mortality risk than individuals without. Probably, these families are characterized by aa combination of risk factors,

AA remarkable novel mutation was reported in FH patients in rural Tunesia.9 This severe mutation resultedd in mean LDL-cholesterol levels of 16 mmol/L (621 mg/dl) in 11 homozygous patients, andd a high prevalence of xanthomas. However, most homozygotes did not suffer from very earlyy CAD, and heterozygotes were almost free of clinical signs and had cholesterol levels in thee normal range. Additional environmental and genetic factors were suggested to be of influence.. A possible modulating factor could be the traditional Tunesian diet, which is very enrichedd in unsaturated fat (otive oil), while low in total fat and high in complex carbohydrates. Thiss diet may have reduced the incidence of cardiovascular events.

Inn conclusion, the above mentioned studies dearly illustrate a pivotal role of environmental factors.. However, many unknown factors remain to be clarified.

B.. Metabolic factors

Lipidd m e t a b o l i s m

Manyy studies have shown LDL-cholesterol level persé to be crucial for the clinical phenotype off FH. In a recent study, LDL-particle size, together with age and gender, were the most importantt risk factors for CAD in FH.S Besides LDL, other aspects of lipid metabolism are alsoo supposed to influence the phenotype in FH. Several epidemiological studies have demonstratedd the importance of high density lipoprotein (HDL) -cholesterol as independent coronaryy risk factor in the general population and also in FH.10 HDL-cholesterol, and probably reversee cholesterol transport in general, plays an important role. Additionally, variations in triglyceridee (TG) metabolism, especially remnants of TG-rich üoooroteins (TRL) or rpmnant likee particles (RLP), may also explain some of the heterogeneity of CVD among FH patients.'' Inn the Framingham Heart Study, RLP were an independent risk factor for CVD in women.12 Alsoo in a large FH cohort, RLP levels were clearly elevated and reacted favorably to simvastatin treatment.13 3

Lipoproteinn (a)

AA recent meta-analysis of prospective studies showed a moderate to strong influence of elevatedd Lp(a) on CAD risk.14 Lp(a) levels have been reported higher in FH patients''^, but thee association between Lp(a) levels and the risk for CAD in FH is unclear. However, recent reportss indicate that Lp(a) levels in FH patients may be associated with risk only in very early coronaryy cases.8

Highh sensitive C-reactive protein

Inflammationn plays a major role in atherogenesis. High sensitive C-reactive protein (hs-CRP) seemss the most promising marker related to this process as elevated levels are associated withh an elevated risk of CVD.'5 The role of hs-CRP in FH is still ambiguous. A recent study reportedd an association of hs-CRP with the presence of CVD in 337 FH patients.'7 However, inn another study no difference in hs-CRP levels were found in FH patients with and without CVD.BB Furthermore, in a cohort of 325 FH patients, treatment with HMG-CoA reductase

Figuree 2. Percentage change in hs-CRP

afterr one and t w o years of treatment w i t h eitherr atorvastatin 80 mg or simvastatin 40 m g .. *P<0.001 vs simvastatin; tP=0.02 vs simvastatin. . Atorvastatin, , Simvastatin 11 year 2 years

I I

1 1

inhibitorss reduced hs-CRP levels (figure 2). This was significantly associated with a reduction inn intima media thickness.18

Fibrinogen n

Neww insights in the pathogenesis of atherosclerosis have revealed some potential new risk factors.. A review of 18 prospective studies showed fibrinogen to be an independent risk factorr for atherosclerosis.'9 Recently, FH patients were described to have increased plasma fibrinogenn levels.20 Unfortunately, no data are yet available on the relationship between fibrinogenn and CVD in FH patients.

Inn conclusion, risk factors of metabolic origin seem to be important. Lipid abnormalities, in termss of low HDL and elevated TRL and RLP, play a key role.

C.. Genetic factors

Bothh the type of LDL-R mutation, as well as other genetic factors, may contribute to the phenotypicc variability of FH.

LDLL receptor mutations

Thee LDL-receptor is a 160-kD transmembrane glycoprotein, pivotal in the receptor-mediated endocytosiss of LDL particles, it is a multifunctional protein with several structural domains. Thee LDL-R protein is encoded by the LDL-R gene, localized on chromosome 19. In this gene, too date, over 700 different mutations have been described.31 Mutations have been categorizedd in 5 different classes. The so-called 'null-alleles' result in failure to produce any protein,, while other mutations lead to impairment of binding capacity, post-translational processingg or recycling.

Thee class of receptor mutation and resulting defective protein are suggested to influence thee clinical phenotype. The effects of mutation type on lipoprotein levels and on the risk of CVDD have been studied extensively. Several investigators have shown significant differences inn lipid levels among various types of mutations ,9'22'"-24 While some have found an increased CADD risk in patients with receptor-defective mutations compared to receptor-negative mutations",, others could not confirm this .5" However, these inconsistencies are probably duee to methodological differences and selection bias. Recently, in the first large and unselectedd FH cohort, CVD risk varied largely among different types of mutations, with carrierss of null-alleles having the most atherogenic lipid profile, combined with the most severee increased risk2B (table 1). This undoubtedly emphasizes an important role of mutation typee in the clinical expression of FH. Remarkably, LDL-cholesterol levels did not alone explain thee difference in CVD risk.

Tablee 1: Risk of coronary

genderr and family ties

Alll mutations N543H/2393del9bp p V408M M 1359-1 1 313+1/2 2 Nulll alleles Otherr alleles

arteryy disease in FH patients relatives to

NN RR 608/10877 8.54 241/4011 7.83 77/1811 7.01 102/1766 15.95 83/1511 11.14 124/2366 10.43 484/8511 7.97

unaffectedd relatives, adjusted for age.

CADD risk

CAD,, coronary artery disease; N, number of carriers/number of nonorriers interval. . 95%% CI 5.29-13.80 0 3.11-19.67 7 2.18-22.59 9 5.52-46.14 4 4.67-26.57 7 4.08-26.69 9 4.60-13.81 1

RR,, relative risk; CI, confidence

Otherr genetic factors

Inn the last decade, the insight into the role of genetics in the pathogenesis of atherosclerosis hass increased enormously. Different genes and gene-environment interactions were found too influence the risk of CVD. Some genetic variations in genes might induce minor effects onn phenotype in the normal population, but these effects might be remarkably more pronouncedd in FH patients due to their a priori increased CAD risk.

LipoproteinLipoprotein lipase gene

LPLL is a crucial enzyme in the metabolism of TRL. Reduced activity results in elevated levels off triglycerides, a risk factor for CVD.27 Two common mutations in the LPL gene, D9N and N291S,, were shown to significantly modulate HDL-cholesterol levels and triglycerides in bothh men and women in the Framingham Offspring Study.23 However, no increased risk for coronaryy heart disease (CHD) could be attributed to these mutations. In contrast, when thesee same two mutations were studied in FH cohorts, not only significant changes in the biochemicall phenotypes were found, the mutations also conferred significantly greater risk forr CVD.29'30

ApolipoproteinApolipoprotein E gene

Apolipoproteinn E is the ligand protein via which TRL such as chylomicron- and VLDL-remnants aree cleared by the LDL-R on the liver cell. Three variants of the apolipoprotein F_ gene give risee to different isoforms: E2, E3 and E4. Patients with the E2/E2 genotype are characterized byy higher TG-rich lipoprotein concentrations and lower LDL-cholesterol levels, whilst in individualss with E4/E4-genotype opposite effects on lipid levels are found.

FHH patients with coexistent dysbetalipoproteinemia have lower LDL-cholesterol- and higher TG-levels.. In these patients a similar frequency of cardiovascular complications is seen as in

FH.311 In earlier studies, apo E phenotype did not influence the phenotype of FH.32 As recently studied,, in 450 children with FH, the apo E4 allele did not cause a rise of LDL-cholesterol levelss but a decrease of HDL-cholesterol levels compared to the carriers of the other alleles, whichh might confer additional risk in these children (Wiegman A, unpublished data).

CholesterylCholesteryl ester transfer protein gene

Thee cholesteryl ester transfer protein (CETP) is a key enzyme in human reverse cholesterol transport.. It mediates the transfer of cholesteryl esters from HDL-cholesterol to TG-rich particless in exchange for triglycerides. Variations in the CETP gene are determinants of CETPP activity and HDL-cholesterol and appear to translate into a tower CVD risk among menn with the Taq1 B2-allele.33 In line with these findings, in a study of 101 FH subjects, the CETPP Taq1 B2-allele was associated with a less atherogenic lipid profile, consisting of lower LDL-- and higher HDL-cholesterol levels. Moreover, the B2 allele was associated with al lowerr prevalence of arcus cornealis, xanthomas and CVD.34

ParaoxonaseParaoxonase gene

Oxidationn of LDL is hypothesized to play a central role in the initiation on atherosclerosis. Paraoxonasee (PON) is a calcium dependent ester hydrolase that protects LDL in vitro from oxidativee stress by hydrolyzing lipid peroxides. This could implicate the paraoxonase gene inn the development of atherosclerosis in FH subjects. The first study on the role of the paraoxonasee gene in FH showed an association between the PON, LL/QQ genotype and carotidd wall thickness.35 This association could not be confirmed by a second study on the associationn between this mutation and CVD. In the latter study, however, a common polymorphismm in the PON2 gene (Cys —*Ser at codon 311) was associated with CVD.

36

The rolee of the PON gene in FH is certainly intriguing and warrants further investigation.

MethylenetetrahydrofoiateMethylenetetrahydrofoiate reductase gene

Hyperhomocysteinemiaa is a potential risk factor as well. However, it is still unclear whether itt plays a causal role in atherosclerosis. Few studies have been reported on the association betweenn homocysteine and atherosclerotic disease in FH, and these show conflicting results.8-37'388 These contradicting data are likely due to limited numbers of patients and differentt definitions of outcome variables. Mutations in the methylenetetrahydrofoiate reductasee (MTHFR) form a well-known cause of life-long elevated plasma homocysteine levels.. Kawashiri et al. studied the MTHFR genotype in 199 FH patients. The C677T mutation wass more frequent in patients with CAD. Furthermore, homozygosity was an independent predictorr of early onset of CAD.39 We also recently studied the MTHFR TT-genotype in 11100 FH patients. Patients homozygous for the TT-genotype had increased homocysteine

Figuree 3 Univariate Pearson correlation

betweenn intima media thickness and log homocysteinee in 609 FH patients.

1,55 2.0 2,5 3,0 3,5 4,0 4,5 5,0 Logg Homocysteine (micromol/L)

inn a subgroup of 609 patients (figure 3). Therefore, we have to conclude that the MTHFR genotypee does not confer an additional risk for our FH patients/0

Renin-AngiotensinRenin-Angiotensin System

Thee renin-angiotensin system plays a critical role in the control of blood pressure, a significant riskk factor for CVD. Polymorphisms in three genes (genes encoding the angiotensin-l-converting enzyme,, the angiotensinogen enzyme and the angiotensin II type I receptor) have been postulatedd as risk factors for CVD, however, their roles are still controversial. In FH patients, a mutationn in the gene encoding the angiotensin II type I receptor was associated with a significantt increased risk for CVD.4'

MicrosomalMicrosomal Triglyceride Transfer Protein gene

Thee microsomal triglyceride transfer protein (MTP) has an important function in intracellular apolipoproteinn (apo) B lipidation and secretion of very low density lipoprotein. A functional polymorphismm in the promoter of the MTP gene affects plasma concentrations of LDL and VLDLL in healthy men.--7 This polymorphism was studied in a large cohort of FH patients. Whilee an effect on LDL-cholesterol levels could not be observed, a significant TG lowering effectt was demonstrated.43

Inn conclusion, with respect to genetic influences, the LDL-R mutation type constitutes a determinantt of the FH phenotype. The potential contribution to risk of other genes is certainlyy interesting. However, one should bear in mind that predominantly case-control studiess with rather small numbers of patients have been carried out so far. Further studies withh better designs on larger cohorts are warranted to confirm current results.

D.. Therapy

Treatmentt sharply influences the clinical phenotype of FH. The introduction of hydroxymethylglutaryll co-enzyme A (HMG CoA) reductase inhibitors has revolutionized the treatmentt of FH. The bile acids sequestrants reduced LDL-choiesterol by only 10% to 15%, whereass statins reduce these levels by 35% to 60%. The clinical relevance was highlighted inn a large cohort of treated FH patients in 19993. The relative risk for coronary mortality declinedd from 1992 onwards, and was especially pronounced in the younger age group (agee 20-59). This emphasizes the importance of treatment with HMG CoA reductase inhibitors off all FH adults. Furthermore, beneficial effects of HMG CoA reductase inhibitors in FH weree demonstrated in two recent studies. Lipid lowering with atorvastatin 80 mg for a periodd of two years reduced LDL-cholesterol 5 1 % and showed a decrease of intima media thicknesss (IMT), as measured by ultrasound. In contrast, lipid lowering with simvastatin 40 mgg reduced LDL-cholesterol 41 %, but showed an increase of IMT.44 Another study reported onn a beneficial effect of long-term treatment with simvastatin on endothelial function in FH patients.. In patients, treated with 40 or 80 mg simvastatin for one year, LDL-cholesterol reducedd 43%.Ab Different LDL-R mutations may lead to variable responses to therapy.46 Sincee statins cause a LDL-cholesterol reduction via an upregulation of the LDL-R gene, this couldd explain their decreased efficacy in carriers of null-alleles.

Conclusion n

Patientss with Familial Hypercholesterolemia are at increased risk for cardiovascular mortality andd disease. Age, gender, LDL-cholesterol levels and a positive family history for premature atherosclerosiss are the most important determinants for early and severe events. The clinical phenotypee is highly modifiable by the presence of environmental and metabolic factors, thee type of LDL-receptor mutation and co-inheritance of other genetic factors.

Thee variability in the clinical phenotype of FH demonstrates that even in a monogenic disorder,, environmental, metabolic and other genetic factors play an equally important role.. Therefore, FH provides an excellent study model for future studies on complex gene-gene,, and gene-environment interactions.

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