Genotype-phenotype correlation in a large cohort of pediatric patients with heterozygous and homozygous familial hypercholesterolemia.

BACKGROUND: Familial hypercholesterolemia (FH) is a genetic disorder characterized by elevated low-density lipoprotein cholesterol (LDL-C) levels and premature cardiovascular disease (CVD). Both the heterozygous form and the very severe homozygous form can be diagnosed by genetic testing and by clinical criteria. Genetic testing can discern FH in a form caused by complete absence of the LDL-receptors, the negative variant and a form leading to reduced activity of the LDL receptors, the defective variant. The aim of this study is to provide more insight in the genotype-phenotype correlation in children and adolescents diagnosed with heterozygous FH (HeFH) and with homozygous FH (HoFH), specifically in relation to the clinical and therapeutic consequences of the negative and defective variant of FH. METHODS AND RESULTS: Data of 5904 children with a tentative diagnosis of FH referred to our center for genetic testing were collected. A lipid-profile was present in 3494 children, who became the study cohort. In this large cohort of children, which includes 2714 HeFH and 41 HoFH patients, it is shown that receptor negative variants are associated with significant higher LDL-C levels in HeFH patients than receptor defective variants (6.0 versus 4.9 mmol/L; p  < 0.001). A negative/negative variant is associated with a significant higher LDL-C level jn HoFH patients than a negative/defective variant, which in itself has a higher LDL-C level than a defective/defective variant. Significantly more premature CVD is present in close relatives of children with HeFH with negative variants compared to close relatives of HeFH children with defective variants (75% vs 59%; p  < 0.001). CONCLUSIONS: Performing genetic testing and identifying the type of underlying genetic variant is of added value in order to distinguish between pediatric patients with higher risks of premature CVD and to identify those that will benefit most from new types of lipid-lowering therapies. Since in children the phenotype of FH is less affected by environmental factors, the study substantiates the genotype-phenotype correlation in this large pediatric population.

APOE1 mutation in a patient with type III hyperlipoproteinaemia: detailed genetic analysis required.

We present the case of a patient with clinical features of familial dysbetalipoproteinaemia (FD) including high levels of total cholesterol, hypertriglyceridaemia and the presence of palmar xanthomas. Whereas genotype analysis identified the APOE3E3 isoform, sequence analysis revealed the presence of one APOE1 allele due to a mutation, p.Lys164Glu, which leads to loss of function of apolipoprotein E (ApoE), a rare cause of dominant FD.

Mutations in LPL, APOC2, APOA5, GPIHBP1 and LMF1 in patients with severe hypertriglyceridaemia.

OBJECTIVES: The severe forms of hypertriglyceridaemia (HTG) are caused by mutations in genes that lead to the loss of function of lipoprotein lipase (LPL). In most patients with severe HTG (TG > 10 mmol L(-1) ), it is a challenge to define the underlying cause. We investigated the molecular basis of severe HTG in patients referred to the Lipid Clinic at the Academic Medical Center Amsterdam. METHODS: The coding regions of LPL, APOC2, APOA5 and two novel genes, lipase maturation factor 1 (LMF1) and GPI-anchored high-density lipoprotein (HDL)-binding protein 1 (GPIHBP1), were sequenced in 86 patients with type 1 and type 5 HTG and 327 controls. RESULTS: In 46 patients (54%), rare DNA sequence variants were identified, comprising variants in LPL (n = 19), APOC2 (n = 1), APOA5 (n = 2), GPIHBP1 (n = 3) and LMF1 (n = 8). In 22 patients (26%), only common variants in LPL (p.Asp36Asn, p.Asn318Ser and p.Ser474Ter) and APOA5 (p.Ser19Trp) could be identified, whereas no mutations were found in 18 patients (21%). In vitro validation revealed that the mutations in LMF1 were not associated with compromised LPL function. Consistent with this, five of the eight LMF1 variants were also found in controls and therefore cannot account for the observed phenotype. CONCLUSIONS: The prevalence of mutations in LPL was 34% and mostly restricted to patients with type 1 HTG. Mutations in GPIHBP1 (n = 3), APOC2 (n = 1) and APOA5 (n = 2) were rare but the associated clinical phenotype was severe. Routine sequencing of candidate genes in severe HTG has improved our understanding of the molecular basis of this phenotype associated with acute pancreatitis and may help to guide future individualized therapeutic strategies.

Extreme xanthomatosis in patients with both familial hypercholesterolemia and cerebrotendinous xanthomatosis.

Two unrelated individuals were referred to Lipid Clinics in The Netherlands and Chile with extreme xanthomatosis and hypercholesterolemia. Both were diagnosed with heterozygous familial hypercholesterolemia (heFH) after molecular genetic analysis of the low-density lipoprotein (LDL) receptor gene. Since heFH by itself could not account for the massive xanthomas, the presence of an additional hereditary lipid or lipoprotein disorder was suspected. Further genetic analysis revealed homozygozity for mutations in the sterol 27-hydroxylase gene, confirming the diagnosis of cerebrotendinous xanthomatosis (CTX). Markedly, the typical neurological manifestations of CTX were absent, suggestive of a protective role of LDL-receptor deficiency against the severe neurological consequences of CTX.

Founder mutations in the Netherlands: geographical distribution of the most prevalent mutations in the low-density lipoprotein receptor and apolipoprotein B genes.

BACKGROUND: In the Netherlands, a screening programme was set up in 1994 in order to identify all patients with familial hypercholesterolaemia (FH). After 15 years of screening, we evaluated the geographical distribution, possible founder effects and clinical phenotype of the 12 most prevalent FH gene mutations. METHODS: Patients who carried one of the 12 most prevalent mutations, index cases and those identified between 1994 and 2009 through the screening programme and whose postal code was known were included in the study. Low-density lipoprotein cholesterol (LDL-C) levels at the time of screening were retrieved. The prevalence of identified FH patients in each postal code area was calculated and visualised in different maps. RESULTS: A total of 10,889 patients were included in the study. Mean untreated LDL-C levels ranged from 4.4 to 6.4 mmol/l. For almost all mutations, a region of high prevalence could be observed. In total, 51 homozygous patients were identified in the Netherlands, of which 13 true homozygous for one of the 12 most prevalent mutations. The majority of them were living in high-prevalence areas for that specific mutation. CONCLUSIONS: Phenotypes with regard to LDL-C levels varied between the 12 most prevalent FH mutations. For most of these mutations, a founder effect was observed. Our observations can have implications with regard to the efficiency of molecular screening and physician's perception of FH and to the understanding of the prevalence and distribution of homozygous patients in the Netherlands.

Proteinuria in early childhood due to familial LCAT deficiency caused by loss of a disulfide bond in lecithin:cholesterol acyl transferase.

INTRODUCTION: Familial lecithin:cholesterol acyltransferase (LCAT) deficiency (FLD) is a rare recessive disorder of cholesterol metabolism characterized by the absence of high density lipoprotein (HDL) and the triad of corneal opacification, hemolytic anemia and glomerulopathy. PATIENTS: We here report on FLD in three siblings of a kindred of Moroccan descent with HDL deficiency. In all cases (17, 12 and 3 years of age) corneal opacification and proteinuria were observed. In the 17-year-old female proband, anemia with target cells was observed. RESULTS: Homozygosity for a mutation in LCAT resulted in the exchange of cysteine to tyrosine at position 337, disrupting the second disulfide bond in LCAT. LCAT protein and activity were undetectable in the patients' plasma and in media of COS7 cells transfected with an expression vector with mutant LCAT cDNA. Upon treatment with an ACE inhibitor and a thiazide diuretic, proteinuria in the proband decreased from 6g to 2g/24h. CONCLUSION: This is the first report that FLD can cause nephropathy at a very early age.

Molecular characterization of Polish patients with familial hypercholesterolemia: novel and recurrent LDLR mutations.

Autosomal dominant hypercholesterolemia (ADH) is caused by mutations in the genes coding for the low-density lipoprotein receptor (LDLR), apolipoprotein B-100 (APOB), or proprotein convertase subtilisin/kexin type 9 (PCSK9). In this study, a molecular analysis of LDLR and APOB was performed in a group of 378 unrelated ADH patients, to explore the mutation spectrum that causes hypercholesterolemia in Poland. All patients were clinically diagnosed with ADH according to a uniform protocol and internationally accepted WHO criteria. Mutational analysis included all exons, exon-intron boundaries and the promoter sequence of the LDLR, and a fragment of exon 26 of APOB. Additionally, the MLPA technique was applied to detect rearrangements within LDLR. In total, 100 sequence variations were identified in 234 (62%) patients. Within LDLR, 40 novel and 59 previously described sequence variations were detected. Of the 99 LDLR sequence variations, 71 may be pathogenic mutations. The most frequent LDLR alteration was a point mutation p.G592E detected in 38 (10%) patients, followed by duplication of exons 4-8 found in 16 individuals (4.2%). Twenty-five cases (6.6%) demonstrated the p.R3527Q mutation of APOB. Our findings imply that major rearrangements of the LDLR gene as well as 2 point mutations (p.G592E in LDLR and p.R3527Q in APOB) are frequent causes of ADH in Poland. However, the heterogeneity of LDLR mutations detected in the studied group confirms the requirement for complex molecular studies of Polish ADH patients.

Silent exonic mutations in the low-density lipoprotein receptor gene that cause familial hypercholesterolemia by affecting mRNA splicing.

In a large group of patients with the clinical phenotype of familial hypercholesterolemia, such as elevated low-density lipoprotein (LDL) cholesterol and premature atherosclerosis, but without functional mutations in the genes coding for the LDL receptor and apolipoprotein B, we examined the effect of 128 seemingly neutral exonic and intronic DNA variants, discovered by routine sequencing of these genes. Two variants, G186G and R385R, were found to be associated with altered splicing. The nucleotide change leading to G186G resulted in the generation of new 3'-splice donor site in exon 4 and R385R was associated with a new 5'-splice acceptor site in exon 9 of the LDL receptor gene. Splicing of these alternate splice sites leads to an in-frame 75-base pair deletion in a stable mRNA of exon 4 in case of G186G and R385R resulted in a 31-base pair frame-shift deletion in exon 9 and non-sense-mediated mRNA decay.

The discrete and combined effect of SREBP-2 and SCAP isoforms in the control of plasma lipids among familial hypercholesterolaemia patients.

BACKGROUND AND AIM: Hypercholesterolaemia is a major risk factor for atherosclerosis. Cholesterol is modulated by genetic and environmental factors. An important regulatory pathway is controlled by the sterol-regulatory element-binding proteins (SREBPs) and the SREBP cleavage-activating protein (SCAP). Both SREBP-2 and SCAP are candidates to contribute to the development of atherosclerosis. We investigated the possible effects of the variability of proteins involved in this regulatory pathway on plasma lipids among familial hypercholesterolaemia patients. METHODS AND RESULTS: Single nucleotide polymorphisms (SNPs) in the genes encoding SREBP-2 and SCAP causing amino acid changes at positions 595 (595G/A) and 796 (796I/V), respectively, were genotyped in 801 FH individuals originating from Israel, The Netherlands, and Switzerland. A linear regression model to examine the associations between SREBP-2 and SCAP isoforms and lipid and lipoprotein levels was used. In females, homozygosity either for the SREBP-2-595A or for the SCAP-796I isoform was associated with higher LDL-cholesterol plasma concentrations (14.7 mg/dl and 20.3 mg/dl, respectively). Surprisingly, heterozygosity for the combination SREBP-2-595A/SCAP-796I was associated with a decrease of 30.28 mg/dl in LDL-C (p-value for gene-gene interaction=0.09). No such effect was observed among FH males. Subgroup analysis considering the most frequent (N>/=24) LDL receptor mutations (del191-2, ins313+1-2, C660X, E207K, S285L) revealed further gene-dosage- and gender-dependent effects of the SCAP mutations on LDL-cholesterol concentrations (p=0.0345). These effects were, however, not present when less frequent LDL receptor mutations were investigated. CONCLUSIONS: These results suggest a possible gene-gene interaction between the genes encoding SREBP-2 and SCAP that modulate plasma lipids in a strictly gender-specific fashion. Further investigation is needed to confirm this effect. A study in a larger FH group or in non-FH hypercholesterolaemic subjects may further define the role of this regulatory mechanism in determining plasma lipid concentration.

High frequency of APOB gene mutations causing familial hypobetalipoproteinaemia in patients of Dutch and Spanish descent.

BACKGROUND: Familial hypobetalipoproteinaemia (FHBL) is an autosomal co-dominant hereditary disorder of lipoprotein metabolism characterised by decreased low density lipoprotein (LDL) cholesterol and apolipoprotein B (APOB) plasma levels. High levels of plasma APOB and LDL cholesterol are strong predictors for risk of cardiovascular disease (CVD), while individuals with low APOB and LDL cholesterol levels are thought to have lower than average risk for CVD, and in fact, heterozygous FHBL patients appear to be asymptomatic. METHODS: Rather than identifying truncated APOB proteins in plasma fractions separated by gel electrophoresis, which will miss any mutations in proteins smaller than 30 kb, we analysed the APOB gene directly, using PCR. RESULTS: We identified nine different mutations, six of which are novel. Each mutation showed complete co-segregation with the FHBL phenotype in the families, and statistically significant differences between carriers and non-carriers were found for plasma total, LDL, and HDL cholesterol, triglycerides, and APOB levels, but not for APOA1 levels. All carriers of an APOB mutation were completely free from CVD. CONCLUSIONS: Prolonged low levels of LDL cholesterol and elevated levels of HDL cholesterol may reduce the progression of atherosclerotic disease, but this has not been unequivocally shown that this is indeed the case in individuals with FHBL, and is the subject of a current study.

Allelic variation in the promoter region of the LDL receptor gene: analysis of an African-specific variant in the FP2 cis-acting regulatory element.

DNA samples of 2303 individuals from nine different population groups were screened for variant -175g-->t in the promoter region of the low-density lipoprotein receptor (LDLR) gene. The -175g-->t variant detected at carrier frequencies of 3-10% in different African population groups was absent in the Caucasian and Asian (Chinese) individuals studied. In contrast to previous findings in Black South Africans where this polymorphism predominated in patients with familial hypercholesterolaemia (FH), it occurred at a significantly lower frequency in hypercholesterolaemics from the recently admixed Coloured population of South Africa compared with population-matched controls (P<0.0001). Haplotype and mutation analysis excluded the likelihood that this finding is due to association with a specific disease-related mutation in FH patients, although reversal of the positive association with FH observed in the Black population may, at least in part, be due to admixture linkage disequilibrium. Transient transfection studies in HepG2 cells demonstrated that the -175t allele is associated with a non-significant decrease ( approximately 7%) of LDLR transcription in the absence of sterols. The data presented in this study raise the possibility that the -175g-->t polymorphism may have subtle effects that become clinically important within certain genetic and/or environmental contexts.

Prevalence and significance of cardiovascular risk factors in a large cohort of patients with familial hypercholesterolaemia.

OBJECTIVE: Patients with familial hypercholesterolaemia (FH) vary widely in terms of onset of cardiovascular disease (CVD). DESIGN: The association between cardiovascular risk factors and prevalent CVD was examined in a cross-sectional study in order to elucidate their contribution to atherogenesis. SETTING AND SUBJECTS: Patients were recruited from 37 Dutch Lipid Clinics. The diagnosis of FH was based on a uniform diagnostic protocol, confirmed by DNA analysis in 62% of the cases. All patients were investigated free from any lipid-lowering drug for at least 6 weeks. MAIN OUTCOME MEASURES: Differences in lipids, lipoproteins and other risk factors for CVD were analysed in FH patients with and without CVD. RESULTS: A total of 526 patients were assessed and more than 37% had a history of CVD with a mean age of onset of 46.8 years. Mean LDL cholesterol (LDL-C) levels were severely elevated (8.38 +/- 2.13 mmol L-1). In univariate analysis, age, presence of hypertension or diabetes, body mass index, triglycerides (TG) and low HDL cholesterol (HDL-C) were all significantly associated with CVD. Also in multivariate analysis, all these risk factors, except TG and diabetes, were significantly linked to CVD. CONCLUSION: A high CVD risk in this large well-documented characterized sample of FH patients is not only conferred by elevated LDL-C but also by low HDL-C.

Molecular genetics and gene expression in atherosclerosis.

Although molecular cardiology is a relative young discipline, the impact of the new techniques on diagnosis and therapy in cardiovascular disease are extensive. Our insight into pathophysiological mechanisms is rapidly expanding and is changing our understanding of cardiovascular disease radically and irrevocably. Molecular cardiology has many different aspects. In this paper the importance of molecular cardiology and genetics for every day clinical practice are briefly outlined. It is expected that in the genetic predisposition for atherosclerotic disease multiple genes are involved (genetics). The role of only a minority of genes involved in the atherosclerotic process is known. Far less is known about particular gene-gene and gene-environment interactions. In some families disease can be explained mostly by a single, major gene (monogenic), of which the lipid disorder Familial Hypercholesterolemia is an example. In other cases, one or several variations in minor genes (multigenic) contribute to an atherosclerotic predisposition, for instance the lipoprotein lipase gene. Although mutations in this gene influence lipoprotein levels, disease development is predominantly depending on environmental influences. Recently several additional genetic risk factors were identified including elevated levels of lipoprotein (a) [Lp(a)], the DD genotype of angiotensin converting enzyme (ACE), and elevated levels of homocysteine. This illustrates the complexity of genetics in relation to atherosclerosis and the difficulty to assign predictive values to separate genetic risk factors. Furthermore, little attention has been given to protective genes thus far, explaining why some high risk patients are protected from vascular disease. Genetics based treatment or elimination of the genetic risk factor requires complete understanding of the pathogenic molecular basis. Once this requirement is fulfilled, disease management can be strived for, provided that adequate medical management is available. Recent studies suggest that such treatment should be genotype specific, as the genetic makeup can determine the outcome of a pharmacological intervention (pharmacogenetics). Once the trigger for atherosclerosis has initiated disease development, various genes are activated or silenced and contribute to lesion progression. Every stage of lesion development depends on a different gene expression programme (genomics). In this review paper an introduction is provided into genetics, pharmacogenetics and gene expression with respect to atherosclerotic disease.

Recent origin and spread of a common Lithuanian mutation, G197del LDLR, causing familial hypercholesterolemia: positive selection is not always necessary to account for disease incidence among Ashkenazi Jews.

G197del is the most prevalent LDL receptor (LDLR) mutation causing familial hypercholesterolemia (FH) in Ashkenazi Jew (AJ) individuals. The purpose of this study was to determine the origin, age, and population distribution of G197del, as well as to explore environmental and genetic effects on disease expression. Index cases from Israel (n=46), South Africa (n=24), Russia (n=7), The Netherlands (n=1), and the United States (n=1) were enlisted. All trace their ancestry to Lithuania. A highly conserved haplotype (D19S221:104-D19S865:208-D19S413:74) was identified in G197del chromosomes, suggesting the occurrence of a common founder. When two methods were used for analysis of linkage disequilibrium (LD) between flanking polymorphic markers and the disease locus and for the study of the decay of LD over time, the estimated age of the deletion was found to be 20 +/- 7 generations (the 95% confidence interval is 15-26 generations), so that the most recent common ancestor of the mutation-bearing chromosomes would date to the 14th century. This corresponds with the founding of the Jewish community of Lithuania (1338 a.d.), as well as with the great demographic expansion of AJ individuals in eastern Europe, which followed this settlement. The penetrance of mutation-linked severe hypercholesterolemia is high (94% of heterozygotes have a baseline concentration of LDL cholesterol (LDL-C) that is >160 mg/dl), and no significant differences in the mean baseline lipid level of G197del carriers from different countries were found. Polymorphisms of apolipoprotein E and of scavenger-receptor class B type I were observed to have minor effects on the plasma lipid profile. With respect to determinative genetic influences on the biochemical phenotype, there is no evidence that could support the possibility of a selective evolutionary metabolic advantage. Therefore, the founder effect in a rapidly expanding population from a limited number of families remains a simple, parsimonious hypothesis explaining the spread of G197del-LDLR-linked FH in AJ individuals.

Mortality over two centuries in large pedigree with familial hypercholesterolaemia: family tree mortality study.

OBJECTIVE: To estimate all cause mortality from untreated familial hypercholesterolaemia free from selection for coronary artery disease. DESIGN: Family tree mortality study. SETTING: Large pedigree in Netherlands traced back to a single pair of ancestors in the 19th century. SUBJECTS: All members of pedigree aged over 20 years with 0.5 probability of carrying a mutation for familial hypercholesterolaemia. MAIN OUTCOME MEASURE: All cause mortality. RESULTS: A total of 70 deaths took place among 250 people analysed for 6950 person years. Mortality was not increased in carriers of the mutation during the 19th and early 20th century; it rose after 1915, reached its maximum between 1935 and 1964 (standardised mortality ratio 1.78, 95% confidence interval 1.13 to 2.76; P=0.003), and fell thereafter. Mortality differed significantly between two branches of the pedigree (relative risk 3.26, 95% confidence interval 1.74 to 6.11; P=0.001). CONCLUSIONS: Risk of death varies significantly among patients with familial hypercholesterolaemia. This large variability over time and between branches of the pedigree points to a strong interaction with environmental factors. Future research is required to identify patients with familial hypercholesterolaemia who are at extreme risk and need early and vigorous preventive measures.

Review of first 5 years of screening for familial hypercholesterolaemia in the Netherlands.

BACKGROUND: Familial hypercholesterolaemia is a common lipid disorder that predisposes for premature cardiovascular disease (CVD). We set up a screening programme in the Netherlands in 1994 to: establish the feasibility of active family screening supported by DNA diagnostics; assess whether or not active identification of these patients with familial hypercholesterolaemia would lead to more cholesterol-lowering treatment; and compare diagnosis by DNA analysis with that by cholesterol measurement. METHODS: Both DNA analysis and measurement of cholesterol concentrations were used to screen families in which a functional mutation in the LDL-receptor gene had been detected. FINDINGS: In the first 5 years, 5442 relatives of 237 people with familial hypercholesterolaemia were screened; 2039 individuals were identified as heterozygous by LDL-receptor gene mutation analysis. At the time of examination, 667 of these adults with familial hypercholesterolaemia (39%) received some form of lipid-lowering treatment; 1 year later, this percentage had increased to 93%. In addition, laboratory analysis showed that for carriers as well as non-carriers 18% would have been misdiagnosed by cholesterol measurement alone, with sex-specific and age-specific 90th percentiles of the general Dutch population as diagnostic criteria. INTERPRETATION: Targeted family screening with DNA analysis proved to be highly effective in identifying patients with hypercholesterolaemia. Most of the identified patients sought treatment and were successfully started on cholesterol-lowering treatment to lower the risk of premature CVD. Our findings could have wider relevance for the screening of other prevalent genetic disorders in the population at large.

The molecular basis of familial hypercholesterolemia in The Netherlands.

Mutations in the low-density lipoprotein (LDL) receptor gene are responsible for familial hypercholesterolemia (FH). At present, more than 600 mutations in this gene are known to underlie FH. However, the array of mutations varies considerably in different populations. Therefore, the delineation of essentially all LDL-receptor gene mutations in a population is a prerequisite for the implementation of nation-wide genetic testing for FH. In the Netherlands, mutation analysis by denaturing gradient gel electrophoresis and sequencing in 1641 clinically diagnosed FH patients resulted in the characterization of 159 different LDL-receptor gene defects. The nine most common mutations were responsible for 66.5% of our FH index cases. Of these, four mutations occurred with relatively high frequencies in specific parts of the Netherlands. The remaining mutations were only encountered in single FH patients, comprising 22.2% of the patient cohort analyzed. Subsequent genetic testing of relatives of the index cases within the national FH screening program resulted in the identification of 5,531 FH patients in total. The analysis for LDL-receptor mutations is a continuing effort to update the LDL-receptor mutation catalogue. Subsequently, with the newly generated index cases, the screening program can be extended and continued to identify and treat FH patients as early as possible and reduce cardiovascular morbidity and mortality in these patients at high risk.