The Molecular Genetic Background of Familial Hypercholesterolemia: Data From the Slovak Nation-Wide Survey

Familial hypercholesterolemia (FH) is most frequently caused by LDLR or APOB mutations. Therefore, the aim of our study was to examine the genetic background of Slovak patients suspected of FH. Patients with clinical suspicion of FH (235 unrelated probands and 124 family relatives) were recruited throughout Slovakia during the years 2011-2015. The order of DNA analyses in probands was as follows: 1. APOB mutation p.Arg3527Gln by real-time PCR method, 2. direct sequencing of the LDLR gene 3. MLPA analysis of the LDLR gene. We have identified 14 probands and 2 relatives with an APOB mutation p.Arg3527Gln, and 89 probands and 75 relatives with 54 different LDLR mutations. Nine of LDLR mutations were novel (i.e. p.Asp90Glu, c.314-2A>G, p.Asp136Tyr, p.Ser177Pro, p.Lys225_Glu228delinsCysLys, p.Gly478Glu, p.Gly675Trpfs*42, p.Leu680Pro, p.Thr832Argfs*3). This is the first study on molecular genetics of FH in Slovakia encompassing the analysis of whole LDLR gene. Genetic etiology of FH was confirmed in 103 probands (43.8 %). Out of them, 86.4 % of probands carried the LDLR gene mutation and remaining 13.6 % probands carried the p.Arg3527Gln APOB mutation.

Genetics of familial hypercholesterolemia

Familial hypercholesterolemia (FH) is a genetic disorder characterized by elevated low-density lipoprotein (LDL) cholesterol and premature cardiovascular disease, with a prevalence of approximately 1 in 200-500 for heterozygotes in North America and Europe. Monogenic FH is largely attributed to mutations in the LDLR, APOB, and PCSK9 genes. Differential diagnosis is critical to distinguish FH from conditions with phenotypically similar presentations to ensure appropriate therapeutic management and genetic counseling. Accurate diagnosis requires careful phenotyping based on clinical and biochemical presentation, validated by genetic testing. Recent investigations to discover additional genetic loci associated with extreme hypercholesterolemia using known FH families and population studies have met with limited success. Here, we provide a brief overview of the genetic determinants, differential diagnosis, genetic testing, and counseling of FH genetics.

The roles of genetic polymorphisms and human immunodeficiency virus infection in lipid metabolism

Dyslipidemia has been frequently observed among individuals infected with human immunodeficiency virus type 1 (HIV-1), and factors related to HIV-1, the host, and antiretroviral therapy (ART) are involved in this phenomenon. This study reviews the roles of genetic polymorphisms, HIV-1 infection, and highly active antiretroviral therapy (HAART) in lipid metabolism. Lipid abnormalities can vary according to the HAART regimen, such as those with protease inhibitors (PIs). However, genetic factors may also be involved in dyslipidemia because not all patients receiving the same HAART regimen and with comparable demographic, virological, and immunological characteristics develop variations in the lipid profile. Polymorphisms in a large number of genes are involved in the synthesis of structural proteins, and enzymes related to lipid metabolism account for variations in the lipid profile of each individual. As some genetic polymorphisms may cause dyslipidemia, these allele variants should be investigated in HIV-1-infected patients to identify individuals with an increased risk of developing dyslipidemia during treatment with HAART, particularly during therapy with PIs. This knowledge may guide individualized treatment decisions and lead to the development of new therapeutic targets for the treatment of dyslipidemia in these patients.

Regulation and deregulation of cholesterol homeostasis: The liver as a metabolic "power station"

Cholesterol plays several structural and metabolic roles that are vital for human biology. It spreads along the entire plasma membrane of the cell, modulating fluidity and concentrating in specialized sphingolipid-rich domains called rafts and caveolae. Cholesterol is also a substrate for steroid hormones. However, too much cholesterol can lead to pathological pictures such as atherosclerosis, which is a consequence of the accumulation of cholesterol into the cells of the artery wall. The liver is considered to be the metabolic power station of mammalians, where cholesterol homeostasis relies on an intricate network of cellular processes whose deregulations can lead to several life-threatening pathologies, such as familial and age-related hypercholesterolemia. Cholesterol homeostasis maintenance is carried out by: biosynthesis, via 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) activity; uptake, through low density lipoprotein receptors (LDLr); lipoprotein release in the blood; storage by esterification; and degradation and conversion into bile acids. Both HMGR and LDLr are transcribed as a function of cellular sterol amount by a family of transcription factors called sterol regulatory element binding proteins that are responsible for the maintenance of cholesterol homeostasis through an intricate mechanism of regulation. Cholesterol obtained by hepatic de novo synthesis can be esterified and incorporated into apolipoprotein B-100-containing very low density lipoproteins, which are then secreted into the bloodstream for transport to peripheral tissues. Moreover, dietary cholesterol is transferred from the intestine to the liver by high density lipoproteins (HDLs); all HDL particles are internalized in the liver, interacting with the hepatic scavenger receptor (SR-B1). Here we provide an updated overview of liver cholesterol metabolism regulation and deregulation and the causes of cholesterol metabolism-related diseases. Moreover, current pharmacological treatment and novel hypocholesterolemic strategies will also be introduced.

Mechanisms of disease: genetic causes of familial hypercholesterolemia

Familial hypercholesterolemia (FH) is characterized by raised serum LDL cholesterol levels, which result in excess deposition of cholesterol in tissues, leading to accelerated atherosclerosis and increased risk of premature coronary heart disease. FH results from defects in the hepatic uptake and degradation of LDL via the LDL-receptor pathway, commonly caused by a loss-of-function mutation in the LDL-receptor gene (LDLR) or by a mutation in the gene encoding apolipoprotein B (APOB). FH is primarily an autosomal dominant disorder with a gene-dosage effect. An autosomal recessive form of FH caused by loss-of-function mutations in LDLRAP1, which encodes a protein required for clathrin-mediated internalization of the LDL receptor by liver cells, has also been documented. The most recent addition to the database of genes in which defects cause FH is one encoding a member of the proprotein convertase family, PCSK9. Rare dominant gain-of-function mutations in PCSK9 cosegregate with hypercholesterolemia, and one mutation is associated with a particularly severe FH phenotype. Expression of PCSK9 normally downregulates the LDL-receptor pathway by indirectly causing degradation of LDL-receptor protein, and loss-of-function mutations in PCSK9 result in low plasma LDL levels. Thus, PCSK9 is an attractive target for new drugs aimed at lowering serum LDL cholesterol, which should have additive lipid-lowering effects to the statins currently used.

Familial defective apolipoprotein B-100 in Slovakia: are differences in prevalence of familial defective apolipoprotein B-100 explained by ethnicity?

The objective of this study was to examine frequency of familial defective apo-B-100 (FDB, R3500Q mutation) in probands with the phenotype of familial hypercholesterolemia (FH) and in the general population of 40-year-old subjects in Slovakia and to characterize their lipid and clinical criteria and to compare the frequency of FDB with other populations. We identified 35 patients with FDB among 362 probands with clinical diagnosis of FH and two cases of FDB in the 40-year-old cohort of 2323 subjects from general Slovak population. Probands with FDB differed from those with FH only in plasma triglyceride concentrations (1.84+/-1.4 mmol/l versus 1.45+/-0.98 mmol/l, respectively, p<0.01). Evaluation of personal history of premature atherosclerosis did not show any differences (11.4% in FDB versus 20% in FH, p<0.16). The FDB patients had similar manifestation of xanthomatosis as the FH patients (17.1% versus 8.25%, p<0.25). The frequency of FDB of 9.7% found in the FH patients is among the highest of those reported to date. The frequency of R3500Q mutation of 0.09% found in Slovak 40-year-old subjects did not differ significantly from published population molecular data. Our comparison of estimated FDB frequencies with those which were found by DNA analysis demonstrated that estimated frequencies were not only wider in range, but also significantly higher than those which were assessed by the analysis. The definitive answer to the prevalence of FDB and its biochemical and clinical characteristics requires screening of unbiased samples of the general population from different ethnic groups based on molecular genetic methods.

Update of the molecular basis of familial hypercholesterolemia in The Netherlands

Autosomal-dominant hypercholesterolemia (ADH) has been identified as a major risk factor for coronary vascular disease (CVD) and is associated with mutations in the low-density lipoprotein receptor (LDLR) and the apolipoprotein B (APOB) gene. Since 1991 DNA samples from clinically diagnosed ADH patients have been routinely analyzed for the presence of LDLR and APOB gene mutations. As of 2001, 1,641 index patients (164 index patients per year) had been identified, while from 2001 onward a more sensitive, high-throughput system was used, resulting in the identification of 1,177 new index patients (average=294 index patients per year). Of these 1,177 index cases, 131 different causative genetic variants in the LDLR gene and six different causative mutations in the APOB gene were new for the Dutch population. Of these 131 mutations, 83 LDLR and four APOB gene mutations had not been reported before. The inclusion of all 2,818 index cases into the national screening program for familial hypercholesterolemia (FH) resulted in the identification of 7,079 relatives who carried a mutation that causes ADH. Screening of the LDLR and APOB genes in clinically diagnosed FH patients resulted in approximately 77% of the patients being identified as carriers of a causative mutation. The population of patients with ADH was divided into three genetically distinct groups: carriers of an LDLR mutation (FH), carriers of an APOB mutation (FDB), and non-LDLR/non-APOB patients (FH3). No differences were found with regard to untreated cholesterol levels, response to therapy, and onset of CVD. However, all groups were at an increased risk for CVD. Therefore, to ultimately identify all individuals with ADH, the identification of new genes and mutations in the genes that cause ADH is of crucial importance for the ongoing national program to identify patients with ADH by genetic cascade screening.

Primary hypercholesterolemia: genetic causes and treatment of five monogenic disorders

Coronary heart disease is a major cause of death in Europe and the USA. Insudation of atherogenic lipoproteins, including low-density lipoprotein (LDL), into the artery wall is integral to atherosclerosis. It is clear that numerous genetic loci contribute to increased plasma levels of LDL. However, five specific monogenic disorders, three of which have been reported recently, are known to increase LDL. These are familial hypercholesterolemia (LDL receptor gene: LDLR); familial ligand-defective apoB- 100 (apoB gene: APOB); autosomal recessive hypercholesterolemia (ARH gene); sitosterolemia (ABCG5 or ABCG8 genes) and cholesterol 7alpha-hydroxylase deficiency (CYP7A1 gene). This review relates the mechanisms underlying these five disorders with specific therapeutic interventions.

A new but frequent mutation of apoB-100—apoB His3543Tyr

ApolipoproteinB 100 (apoB-100) is an important component of atherogenic lipoproteins such as LDL and serves as a ligand for the LDL-receptor. Familial defective apolipoproteinB 100 (FDB) is caused by a R3500Q mutation of the apoB gene and results in decreased binding of LDL to the LDL-receptor. So far FDB is the most frequent and best studied alteration of apoB-100. Apart from this, three other apoB mutations, R3500W, R3531C and R3480W, affecting binding to the LDL-receptor are known to date. We screened the apoB gene segment of codons 3448-3561 by denaturing gradient gel electrophoresis (DGGE) analysis in a total of 853 consecutively sampled German patients undergoing diagnostic coronary angiography for suspected CAD. By this, a new single base mutation was detected and confirmed by DNA sequencing. The mutation, CAC(3543)TAC results in a His3543Tyr substitution in apoB-100 (H3543Y). The prevalence of heterozygotes for H3543Y in the study population was 0.47% compared to 0.12% for the known Arg 3500 Gln (R3500Q) mutation. In conclusion, the new mutation is four times more frequent than "classical" FDB and thus appears to be the most common apoB mutation in Germany.

Identification and characterization of LDL receptor gene mutations in hyperlipidemic Chinese

DNA screening for LDL receptor mutations was performed in 170 unrelated hyperlipidemic Chinese patients and two clinically diagnosed familial hypercholesterolemia patients. Two deletions (Del e3-5 and Del e6-8), eight point mutations (W-18X, D69N, R94H, E207K, C308Y, I402T, A410T, and A696G), and two polymorphisms (A370T and I602V) were identified. Of these mutations, C308Y and Del e6-8 were found in homozygosity, and D69N and C308Y were seen in unrelated patients. The effects of mutations on LDL receptor function were characterized in COS-7 cells. The LDL receptor level and activity were close to those of wild type in A696G transfected cells. A novel intermediate protein and reduction of LDL receptor activity were seen in D69N transfected cells. For R94H, E207K, C308Y, I402T, and A410T mutations, only approximately 20-64% of normal receptor activities were seen. Conversely, Del e3-5 and Del e6-8 lead to defective proteins with approximately 0-13% activity. Most of the mutant receptors were localized intracellularly, with a staining pattern resembling that of the endoplasmic reticulum and Golgi apparatus (D69N, R94H, E207K, C308Y, and I402T) or endosome/lysosome (A410T and Del e6-8). Molecular analysis of the LDL receptor gene will clearly identify the cause of the patient's hyperlipidemia and allow appropriate early treatment as well as antenatal and family studies.

Familial hypercholesterolemia due to ligand-defective apolipoprotein B100: first case report in a Mexican family

BACKGROUND

Familial defective apolipoprotein B100 (FDB) is one of the known causes of familial hypercholesterolemia (FH). Its frequency among subjects with FH varies among ethnic groups; information on FH is insufficient for populations from Latin America. We proposed to describe prevalence of FDB in a cohort of Mexican FH probands (n = 30).

METHODS

We searched for the known FDB mutations using polymerase chain reaction assays. In this set of patients, mean lipid values were representative of FH (cholesterol 351 mg/dL, LDL cholesterol 274 mg/dL, HDL cholesterol 51 mg/dL, and triglycerides 132 mg/dL).

RESULTS

One subject with Arg3500Gln mutation was found: a 44-year-old male with a history of coronary heart disease (CHD) among paternal relatives. His lipid profile was cholesterol 370 mg/dL, LDL-cholesterol 300 mg/dL, HDL-cholesterol 32 mg/dL, and triglycerides 189 mg/dL. Tendinous xanthomata were detected. Three of four siblings, one of three sons, and one of nine nieces and nephews carried the mutation. The mutation was confirmed by automated sequencing. Tendinous xanthomata were absent in affected subjects younger than age 20 years; additionally, the subjects had borderline cholesterol levels.

CONCLUSIONS

Our data suggest that FDB explains the small number of FH cases in Mexico. Inclusion of molecular biology assays to the clinical laboratory makes it possible to diagnose affected individuals with borderline cholesterol levels or without tendinous xanthomata.

Rapid detection of 3500Q and 3531 mutations and MspI polymorphism in exon 26 at the apolipoprotein B gene

Several environmental and genetic factors are associated with high levels of cholesterol. Hypercholesterolemia is the main phenotype of Familial Defective Apolipoprotein B and Familial Hypercholesterolemia that are caused by mutations at the apolipoprotein (apo) B and LDL receptor genes, respectively. Identification of the specific genetic alteration associated with hypercholesterolemia is an important issue in clinical diagnosis of high risk for CAD. Apo B gene mutations and polymorphisms are usually screened by SSCP, DGGE, and heteroduplex, which must be confirmed by DNA sequencing or by direct detection using PCR techniques. In this study, we have optimized a PCR-RFLP procedure for identification of 3500Q and 3531 mutations and MspI polymorphism at the apo B gene. The technique can be performed in a single reaction, using the restriction endonuclease MspI for simultaneous detection of 3500Q mutation and MspI polymorphism, and NsiI for detection of 3531 mutation. The procedure was validated by analysis of control DNA samples from individuals carrying these mutations. Screening of 186 Brazilian hypercholesterolemic individuals showed that the frequency of the M-allele (7.8%) of MspI polymorphism was similar to that found in other individuals with CAD. However, neither 3500Q nor 3531 mutations were detected in this group. In conclusion, this procedure is simple and rapid, being easily introduced in clinical laboratories for direct detection of the more frequent mutations at the apo B gene associated with hypercholesterolemia.

Apolipoprotein B gene polymorphisms: prevalence and impact on serum lipid concentrations in hypercholesterolemic individuals from Brazil

Genetic polymorphisms at the apolipoprotein B (apo B) have been associated with elevated plasma concentrations of low-density lipoprotein (LDL) cholesterol, atherosclerosis and increased risk for coronary artery disease (CAD). In the present study, four apo B gene polymorphisms (MspI, XbaI, Ins/Del and 3'HVR) have been investigated to determine their frequencies and influence on the lipid profile of 177 hypercholesterolemic white Brazilian subjects (HG) and 100 control individuals (CG). The genotype distribution and allele frequency of MspI, XbaI and Ins/Del polymorphisms of apo B gene were similar between HG and CG groups. The frequency of the alleles smaller than 43 repeats (< or =43) of 3'HVR polymorphism in the HG group was higher when compared to controls (16.4 vs. 8.5%, P<0.05). Moreover, these alleles were associated with higher total cholesterol concentrations in serum of hypercholesterolemic individuals (P<0.05). In addition, an association between Ins/Del and 3'HVR polymorphism was observed. The alleles < or =43 and Del were more frequent in the HG when compared to the CG individuals (P<0.05). We concluded that 3'HVR polymorphism at the apo B gene may be an important genetic marker to evaluate atherosclerotic disease risk.

Familial defective apolipoprotein B-100: detection and haplotype analysis of the Arg(3500)-->Gln mutation in hyperlipidemic Chinese

Familial defective apolipoprotein (apo) B-100 (FDB) is caused by R3500Q mutation of the apo B gene resulting in decreased binding of LDL to the LDL receptor. Two other apo B mutations, R3500W and R3531C, affecting binding are known to date. We screened the apo B gene segment around codon 3500 by heteroduplex analysis and single strand conformation polymorphism (SSCP) analysis in a total of 373 hyperlipidemic individuals. Two single-base mutations were detected and confirmed by DNA sequencing. One mutation, ACA(3528)-->ACG change, resulted in degenerate codon with no amino acid substitution. The other mutation, CGG(3500)-->CAG mutation, resulted in an Arg(3500)-->Gln substitution (R3500Q). The prevalence of heterozygote in this selected population was 0.3% (95% confidence interval, 0.01-1.5%) for the R3500Q mutation, and 2.4% (95% confidence interval, 1.1-4.5%) for the previously described R3500W mutation. The results suggest that the R3500Q mutation is not a significant factor contributing to moderate hypercholesterolemia in Chinese (P=0.027). Family studies of the R3500Q carrier revealed a further two individuals heterozygous for the mutation, both of whom were hypercholesterolemic. Analysis of the R3500Q allele using six diallelic markers and the 3'HVR marker revealed a haplotype which was the same as that reported in a Chinese American but differed from that reported in a Chinese Canadian. Our data support limited multiple recurrent origins for R3500Q in Chinese population.

R3531C mutation in the apolipoprotein B gene is not sufficient to cause hypercholesterolemia

Familial hypercholesterolemia and familial ligand-defective apolipoprotein B-100 (FDB) are dominantly inherited disorders leading to impaired low-density lipoprotein receptor (LDLR) and apolipoprotein B-100 (APOB) interaction, plasma LDL elevation, and hypercholesterolemia. We previously identified the first French FDB-R3531C proband, a woman with very high total cholesterol, in a group of type IIa hypercholesterolemic families. We report here the investigation of her family at large that revealed the total absence of cosegregation with hypercholesterolemia. Six of the 10 subjects heterozygous for the R3531C mutation had plasma cholesterol lower than the 97.5th percentile for their age and gender, and mean cholesterol levels were not significantly different between affected and unaffected persons. Furthermore, 2 family members with similar high LDL-cholesterol levels were not carriers of the R3531C substitution, suggesting the implication of another mutation. Segregation analysis of the LDLR gene revealed statistically significant genetic linkage with hypercholesterolemia, and analysis of the proband LDLR gene led to the identification of the 664 proline to leucine defective mutation and its detection in all 6 hypercholesterolemic-related members of this family. Therefore, our results show that the family presents with familial hypercholesterolemia and give evidence that the R3531C substitution in the APOB gene is not an allelic variant leading to FDB. Furthermore, thorough analysis of our data suggests that the APOB-R3531C mutation enhances the hypercholesterolemic effect of the LDLR-P664L defect, suggesting that it is a susceptibility mutation.

Evaluation of the Roche Diagnostics LightCycler-Apo B 3500 Mutation Detection Kit

Familial defective apolipoprotein (apo) B-100 is an autosomal codominant disorder associated with hypercholesterolemia and an increased risk of coronary artery disease. Two independent mutations affecting the codon 3500 (Arg3500-->Gln and Arg3500-->Trp) have been shown to cause ligand-defective apo B-100. Identification of carriers of these mutations is an important step in the risk stratification of individuals and families with hypercholesterolemia. We evaluated a homogeneous assay for detection of mutations at codon 3500 that combines rapid-cycle PCR with allele-specific fluorescent probe melting profiles for product genotyping. This single-tube analysis is performed on the LightCycler, a microvolume fluorimeter integrated with a thermal cycler. Continuous acquisition of fluorescence data during a melting curve analysis at completion of PCR allows the detection of mutations, as loss of fluorescence occurs in an allele-specific manner. By plotting melting peaks, the three apo B-100 alleles were readily distinguishable. Using this method, genotyping of 32 samples is completed within 40 min without the need for any post-PCR sample manipulation, thereby eliminating the risks of end-product contamination and sample tracking errors. The specific detection of mutations at codon 3500 of the apo B gene on the LightCycler is a rapid and reliable method that is ideally suitable for typing both small and large numbers of samples.

Mucopolysaccharidosis type I: characterization of novel mutations affecting alpha-L-iduronidase activity

alpha-L-Iduronidase (IDUA) deficiency (mucopolysaccharidosis type I, MPS I) involves a broad spectrum of clinical severity ranging from a severe Hurler syndrome through an intermediate Hurler Scheie syndrome to a mild Scheie syndrome. To date, a number of mutations of the IDUA gene are known in Hurler syndrome, but only a few in Hurler Scheie or Scheie syndrome. The characterization of novel mutations in two patients with the Hurler-Scheie syndrome is reported on. The novel R619G mutation (C-G transversion in codon 619) was apparently homozygous. In transfected COS-7 cells, R619G caused significant reduction in enzyme activity (1.5% of normal activity), although it did not cause significant reduction in IDUA mRNA or protein level. Conversely, the previously described homozygous T364M mutation (C-T transition in codon 364) caused a decrease in the level of IDUA mRNA. Studies inhibiting RNA synthesis with actinomycin D or inhibiting protein synthesis with cycloheximide demonstrate that the decrease in the latter mutation is attributable to an increased rate of mRNA decay. By examining the stability of IDUA mRNA and protein, studies provide better insight into the effect of mutation on IDUA activity.