Accumulation of dietary cholesterol in sitosterolemia caused by mutations in adjacent ABC transporters

In healthy individuals, acute changes in cholesterol intake produce modest changes in plasma cholesterol levels. A striking exception occurs in sitosterolemia, an autosomal recessive disorder characterized by increased intestinal absorption and decreased biliary excretion of dietary sterols, hypercholesterolemia, and premature coronary atherosclerosis. We identified seven different mutations in two adjacent, oppositely oriented genes that encode new members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter family (six mutations in ABCG8 and one in ABCG5) in nine patients with sitosterolemia. The two genes are expressed at highest levels in liver and intestine and, in mice, cholesterol feeding up-regulates expressions of both genes. These data suggest that ABCG5 and ABCG8 normally cooperate to limit intestinal absorption and to promote biliary excretion of sterols, and that mutated forms of these transporters predispose to sterol accumulation and atherosclerosis.

Effect of mutations in the PCSK9 gene on the cell surface LDL receptors

The proprotein convertase subtilisin/kexin type 9 (PCSK9) gene is involved in the post-transcriptional regulation of the low-density lipoprotein (LDL) receptors (LDLR). Mutations in the PCSK9 gene have been associated with both hypocholesterolemia and hypercholesterolemia through 'loss-of-function' and 'gain-of-function' mechanisms, respectively. We have studied the effect of the four loss-of-function mutations R46L, G106R, N157K and R237W and the two gain-of-function mutations S127R and D374Y on the autocatalytic activity of PCSK9, as well as on the amount of the cell surface LDLR and internalization of LDL in transiently transfected HepG2 cells. The two groups of mutations did not differ with respect to autocatalytic activity of PCSK9, but they did differ with respect to the amount of cell surface LDLR and internalization of LDL. The four loss-of-function mutations had a 16% increased level of cell surface LDLR and a 35% increased level of internalization of LDL as compared with WT-PCSK9. The two gain-of-function mutations had a 23% decreased level of cell surface LDLR and a 38% decreased level of internalization of LDL as compared with WT-PCSK9. Our studies have also shown that transfer of media from transiently transfected HepG2 cells to untransfected HepG2 cells, reduces the amount of cell surface LDLR and internalization of LDL in the untransfected cells within 20 min of media transfer. Thus, PCSK9 or a factor acted upon by PCSK9, is secreted from the transfected cells and degrades LDLR both in transfected and untransfected cells.

Lamin A/C cardiomyopathy: young onset, high penetrance, and frequent need for heart transplantation

Aims

Lamin A/C (LMNA) mutations cause familial dilated cardiomyopathy (DCM) with frequent conduction blocks and arrhythmias. We explored the prevalence, cardiac penetrance, and expressivity of LMNA mutations among familial DCM in Norway. Furthermore, we explored the risk factors and the outcomes in LMNA patients.

Methods and results

During 2003–15, genetic testing was performed in patients referred for familial DCM. LMNA genotype-positive subjects were examined by electrocardiography, Holter monitoring, cardiac magnetic resonance imaging, and echocardiography. A positive cardiac phenotype was defined as the presence of atrioventricular (AV) block, atrial fibrillation/flutter (AF), ventricular tachycardia (VT), and/or echocardiographic DCM. Heart transplantation was recorded and compared with non-ischaemic DCM of other origin. Of 561 unrelated familial DCM probands, 35 (6.2%) had an LMNA mutation. Family screening diagnosed an additional 93 LMNA genotype-positive family members. We clinically followed up 79 LMNA genotype-positive [age 42 ± 16 years, ejection fraction (EF) 45 ± 13%], including 44 (56%) with VT. Asymptomatic LMNA genotype-positive family members (age 31 ± 15 years) had a 9% annual incidence of a newly documented cardiac phenotype and 61% (19/31) of cardiac penetrance during 4.4 ± 2.9 years of follow-up. Ten (32%) had AV block, 7 (23%) AF, and 12 (39%) non-sustained VT. Heart transplantation was performed in 15 of 79 (19%) LMNA patients during 7.8 ± 6.3 years of follow-up.

Conclusion

LMNA mutation prevalence was 6.2% of familial DCM in Norway. Cardiac penetrance was high in young asymptomatic LMNA genotype-positive family members with frequent AV block and VT, highlighting the importance of early family screening and cardiological follow-up. Nearly 20% of the LMNA patients required heart transplantation.

Missense mutations in the PCSK9 gene are associated with hypocholesterolemia and possibly increased response to statin therapy

OBJECTIVE

The proprotein convertase subtilisin/kexin type 9 (PCSK9) gene encodes a proprotein convertase that causes degradation of cell surface low-density lipoprotein receptors (LDLRs). Mutations in the PCSK9 gene that disrupt the normal function of PCSK9 could therefore result in increased number of LDLRs and hypocholesterolemia. Also, the cholesterol-lowering effect of statins could be increased in subjects carrying mutations in the PCSK9 gene.

METHODS AND RESULTS

We have screened 38 unrelated hypocholesterolemic subjects as well as 25 unrelated familial hypercholesterolemia (FH) heterozygotes who responded particularly well to statin therapy for mutations in the 12 exons of the PCSK9 gene by DNA sequencing. Six of the 38 (15.8%) hypocholesterolemic subjects were heterozygous for 1 of the 3 mutations R46L, G106R, or R237W in the PCSK9 gene. In the group of 25 FH heterozygotes who responded particularly well to statin therapy, 3 (8.8%) were heterozygous for mutations R46L or N157K in the PCSK9 gene. None of 441 hypercholesterolemic subjects without mutations in the LDLR gene or in the apolipoprotein B-100 gene possessed any of the 4 mutations.

CONCLUSIONS

The 4 missense mutations R46L, G106R, N157K, and R237W are associated with hypocholesterolemia and possibly increased response to statin therapy.

Berberine decreases PCSK9 expression in HepG2 cells

BACKGROUND

Proprotein convertase subtilisin/kexin type 9 (PCSK9) post-transcriptionally downregulates the low-density lipoprotein receptor (LDLR) by binding to the receptor's epidermal growth factor repeat A on the cell surface and shuttling the LDLR to the lysosomes for degradation. Mutations in the PCSK9 gene have been shown to cause either hypo- or hypercholesterolemia. Here we investigated the effect of berberine, a natural plant extract, on PCSK9 expression in HepG2 cells.

RESULTS

Berberine decreases PCSK9 mRNA and protein levels in a time- and dose-dependent manner. This was not due to increased degradation of PCSK9 mRNA but most likely due to a decreased transcription of the PCSK9 gene. We also show that a combination of berberine and mevastatin increases LDLR mRNA and protein levels, while suppressing the increase in PCSK9 mRNA levels caused by mevastatin alone.

CONCLUSION

Berberine may be a useful supplement to statin treatment due to its effects on PCSK9 mRNA and protein levels.

Mutations in ATP-cassette binding proteins G5 (ABCG5) and G8 (ABCG8) causing sitosterolemia

Sitosterolemia is an autosomal recessive disorder caused by mutations in two adjacent genes encoding coordinately regulated ATP binding cassette (ABC) half transporters (ABCG5 and ABCG8). In this paper we describe three novel mutations causing sitosterolemia: 1) a frameshift mutation (c.336-337insA) in ABCG5 that results in premature termination of the protein at amino acid 197; 2) a missense mutation that changes a conserved residue c.1311C>G; N437K) in ABCG5 and 3) a splice site mutation in ABCG8 (IVS1-2A>G). This study expands the spectrum of the ABCG5 and ABCG8 mutations that cause sitosterolemia. Nine nonsynonymous polymorphisms are also reported: I523V, C600Y, Q604E, and M622V in ABCG5; and D19H, Y54C, T400K, A632V, and Y641F in ABCG8.

Transmural differences in myocardial contraction in long-QT syndrome: mechanical consequences of ion channel dysfunction

BACKGROUND

Long-QT syndrome (LQTS) is characterized by prolonged myocardial action potential duration. The longest action potential duration is reported in the endomyocardium and midmyocardium. Prolonged action potential duration in LQTS may cause prolonged cardiac contraction, which can be assessed by strain echocardiography. We hypothesized that myocardial contraction is most prolonged in subendocardial myofibers in LQTS patients and that inhomogeneous transmural contraction is related to the risk of spontaneous arrhythmia.

METHODS AND RESULTS

We included 101 genotyped LQTS mutation carriers and 35 healthy individuals. A history of cardiac arrhythmias was present in 48 mutations carriers, and 53 were asymptomatic. Myocardial contraction duration was assessed by strain echocardiography as time from the ECG Q wave to peak strain in 16 LV segments. Strain was assessed along the longitudinal axis, predominantly representing subendocardial fibers, and along the circumferential axis, representing midmyocardial fibers. Mean contraction duration was longer in LQTS mutation carriers compared with healthy individuals (445 ± 45 versus 390 ± 40 milliseconds; P<0.001) and longer in symptomatic compared with asymptomatic LQTS mutation carriers (460 ± 40 versus 425 ± 45 milliseconds; P<0.001). Contraction duration by longitudinal strain was longer than by circumferential strain in symptomatic LQTS patients (460 ± 45 versus 445±45 milliseconds; P=0.008) but not in asymptomatic patients and healthy individuals, indicating transmural mechanical dispersion. This time difference was present in a majority of LV segments and was most evident in patients with LQT2 and the Jervell and Lange-Nielsen syndrome.

CONCLUSION

Contraction duration in symptomatic LQTS mutation carriers was longer in the subendocardium than in the midmyocardium, indicating transmural mechanical dispersion, which was not present in asymptomatic and healthy individuals.

Eicosapentaenoic and docosahexaenoic acid affect mitochondrial and peroxisomal fatty acid oxidation in relation to substrate preference

Decreased triacylglycerol synthesis within hepatocytes due to decreased diacylglycerol acyltransferase (DGAT) activity has been suggested to be an important mechanism by which diets rich in fish oil lower plasma triacylglycerol levels. New findings suggest that eicosapentaenoic acid (EPA), and not docosahexaenoic acid (DHA), lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation and decreased availability of fatty acids for triacylglycerol synthesis. To contribute to the understanding of the triacylglycerol-lowering mechanism of fish oil, the different metabolic properties of EPA and DHA were studied in rat liver parenchymal cells and isolated rat liver organelles. EPA-CoA was a poorer substrate than DHA-CoA for DGAT in isolated rat liver microsomes, and in the presence of EPA, a markedly lower value for the triacyl[3H]glycerol/diacyl[3H]glycerol ratio was observed. The distribution of [1-14C]palmitic acid was shifted from incorporation into secreted glycerolipids toward oxidation in the presence of EPA (but not DHA) in rat liver parenchymal cells. [1-14C]EPA was oxidized to a much greater extent than [1-14C]DHA in rat liver parenchymal cells, isolated peroxisomes, and especially in purified mitochondria. As the oxidation of EPA was more effective and sensitive to the CPT-I inhibitor, etomoxir, when measured in a combination of both mitochondria and peroxisomes, we hypothesized that both are involved in EPA oxidation, whereas DHA mainly is oxidized in peroxisomes. In rats, EPA treatment lowered plasma triacylglycerol and increased hepatic mitochondrial fatty acid oxidation and carnitine palmitoyltransferase (CPT)-I activity in both the presence and absence of malonyl-CoA. Whereas only EPA treatment increased the mRNA levels of CPT-I, DHA treatment increased the mRNA levels of peroxisomal fatty acyl-CoA oxidase and fatty acid binding protein more effectively than EPA treatment. In conclusion, EPA and DHA affect cellular organelles in relation to their substrate preference. The present study strongly supports the hypothesis that EPA, and not DHA, lowers plasma triacylglycerol by increased mitochondrial fatty acid oxidation.

Description of a large family with autosomal dominant hypercholesterolemia associated with the APOE p.Leu167del mutation

Apolipoprotein (apo) E mutants are associated with type III hyperlipoproteinemia characterized by high cholesterol and triglycerides levels. Autosomal dominant hypercholesterolemia (ADH), due to the mutations in the LDLR, APOB, or PCSK9 genes, is characterized by an isolated elevation of cholesterol due to the high levels of low-density lipoproteins (LDLs). We now report an exceptionally large family including 14 members with ADH. Through genome-wide mapping, analysis of regional/functional candidate genes, and whole exome sequencing, we identified a mutation in the APOE gene, c.500_502delTCC/p.Leu167del, previously reported associated with sea-blue histiocytosis and familial combined hyperlipidemia. We confirmed the involvement of the APOE p.Leu167del in ADH, with (1) a predicted destabilization of an alpha-helix in the binding domain, (2) a decreased apo E level in LDLs, and (3) a decreased catabolism of LDLs. Our results show that mutations in the APOE gene can be associated with bona fide ADH.

The unique role of proprotein convertase subtilisin/kexin 9 in cholesterol homeostasis

The LDL receptor (LDLR) plays an essential role in the regulation of plasma (LDL) cholesterol concentrations by virtue of its ability to clear plasma LDL. Down-regulation of the LDLR by proprotein convertase subtilisin/kexin 9 (PCSK9) has recently emerged as a regulatory mechanism that controls plasma LDL cholesterol concentrations. Studies in which PCSK9 is over-expressed in mice, have demonstrated that PCSK9, by enhancing hepatic LDLR degradation, decreases the availability of the LDLR for LDL uptake, resulting in increased plasma LDL cholesterol levels. However, PCSK9 has also recently been shown to mediate down-regulation of surface receptors other than the LDLR, suggesting that it may have much broader roles than initially thought.

Insertion/deletion (I/D) polymorphism at the locus for angiotensin I-converting enzyme and myocardial infarction

Male (n = 185) and female (n = 49) survivors of myocardial infarction (MI) below 56 and 61 years of age, respectively, were compared to 366 controls with respect to distribution of genotypes in an insertion/deletion (ID) polymorphism at the angiotensin I-converting enzyme (ACE) locus. The frequency of the DD genotype (homozygosity for the deletion allele) was significantly lower among male patients than controls (22.7% versus 34.9%, p = 0.011). In a "low-risk" group, defined as having less than the sex-specific, age-adjusted median values of body mass index (BMI) and apolipoprotein B (apoB), respectively, and absence of treatment with lipid-lowering drugs, the prevalence of the DD genotype was not statistically different between male patients and controls. In a male "high-risk" group (those individuals who had not been defined as "low-risk" subjects), the prevalence of the DD genotype was 20.9% in patients and 38.3% in controls (p = 0.002). In women, no significant differences in genotype frequencies between patients and controls were found in the whole sample or in any subgroup. These results appear to be at variance with data reported recently by Cambien et al. (1992). The difference may be due to chance, undetected selection biases, different gene-environment interactions between Norway and France or Ireland, or to preferential loss of DD individuals in our male "high-risk" group.

The prevalence of mutations in KCNQ1, KCNH2, and SCN5A in an unselected national cohort of young sudden unexplained death cases

INTRODUCTION

Sudden unexplained death account for one-third of all sudden natural deaths in the young (1-35 years). Hitherto, the prevalence of genopositive cases has primarily been based on deceased persons referred for postmortem genetic testing. These deaths potentially may represent the worst of cases, thus possibly overestimating the prevalence of potentially disease causing mutations in the 3 major long-QT syndrome (LQTS) genes in the general population. We therefore wanted to investigate the prevalence of mutations in an unselected population of sudden unexplained deaths in a nationwide setting.

METHODS

DNA for genetic testing was available for 44 cases of sudden unexplained death in Denmark in the period 2000-2006 (equaling 33% of all cases of sudden unexplained death in the age group). KCNQ1, KCNH2, and SCN5A were sequenced and in vitro electrophysiological studies were performed on novel mutations.

RESULTS

In total, 5 of 44 cases (11%) carried a mutation in 1 of the 3 genes corresponding to 11% of all investigated cases (R190W KCNQ1, F29L KCNH2 (2 cases), P297S KCNH2 and P1177L SCN5A). P1177L SCN5A has not been reported before. In vitro electrophysiological studies of P1177L SCN5A revealed an increased sustained current suggesting a LQTS phenotype.

CONCLUSION

In a nationwide setting, the genetic investigation of an unselected population of sudden unexplained death cases aged 1-35 years finds a lower than expected number of mutations compared to referred populations previously reported. We therefore conclude that the prevalence of mutations in the 3 major LQTS associated genes may not be as abundant as previously estimated.

The metabolic syndrome and the hepatic fatty acid drainage hypothesis

Much data indicates that lowering of plasma triglyceride levels by hypolipidemic agents is caused by a shift in the liver metabolism towards activation of peroxisome proliferator activated receptor (PPAR)alpha-regulated fatty acid catabolism in mitochondria. Feeding rats with lipid lowering agents leads to hypolipidemia, possibly by increased channeling of fatty acids to mitochondrial fatty acid oxidation at the expense of triglyceride synthesis. Our hypothesis is that increased hepatic fatty acid oxidation and ketogenesis drain fatty acids from blood and extrahepatic tissues and that this contributes significantly to the beneficial effects on fat mass accumulation and improved peripheral insulin sensitivity. To investigate this theory we employ modified fatty acids that change the plasma profile from atherogenic to cardioprotective. One of these novel agents, tetradecylthioacetic acid (TTA), is of particular interest due to its beneficial effects on lipid transport and utilization. These hypolipidemic effects are associated with increased fatty acid oxidation and altered energy state parameters of the liver. Experiments in PPAR alpha-null mice have demonstrated that the effects hypolipidemic of TTA cannot be explained by altered PPAR alpha regulation alone. TTA also activates the other PPARs (e.g., PPAR delta) and this might compensate for deficiency of PPAR alpha. Altogether, TTA-mediated clearance of blood triglycerides may result from a lowered level of apo C-III, with a subsequently induction of hepatic lipoprotein lipase activity and (re)uptake of fatty acids from very low density lipoprotein (VLDL). This is associated with an increased hepatic capacity for fatty acid oxidation, causing drainage of fatty acids from the blood stream. This can ultimately be linked to hypolipidemia, anti-adiposity, and improved insulin sensitivity.

Role of the C-terminal domain of PCSK9 in degradation of the LDL receptors

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the low density lipoprotein receptor (LDLR) at the cell surface and disrupts the normal recycling of the LDLR. In this study, we investigated the role of the C-terminal domain for the activity of PCSK9. Experiments in which conserved residues and histidines on the surface of the C-terminal domain were mutated indicated that no specific residues of the C-terminal domain, apart from those responsible for maintaining the overall structure, are required for the activity of PCSK9. Rather, the net charge of the C-terminal domain is important. The more positively charged the C-terminal domain, the higher the activity toward the LDLR. Moreover, replacement of the C-terminal domain with an unrelated protein of comparable size led to significant activity of the chimeric protein. We conclude that the role of the evolutionary, poorly conserved C-terminal domain for the activity of PCSK9 reflects its overall positive charge and size and not the presence of specific residues involved in protein-protein interactions.

High prevalence of exercise-induced arrhythmias in catecholaminergic polymorphic ventricular tachycardia mutation-positive family members diagnosed by cascade genetic screening

AIM

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited cardiac disease predisposing to life-threatening arrhythmias. We aimed to determine the prevalence of arrhythmias and efficacy of beta-blocker treatment in mutation-positive family members diagnosed by cascade genetic screening.

METHODS AND RESULTS

Relatives of six unrelated CPVT patients were tested for the relevant mutation in the ryanodine receptor-2 gene. Mutation carriers underwent an exercise test at inclusion time and 3 months after the initiation of beta-blocker therapy in the highest tolerable dose. The occurrence of ventricular premature beats, couplets, and non-sustained ventricular arrhythmias (nsVT) were recorded in addition to the heart rate at which they occurred. Thirty family members were mutation carriers and were followed for 22 (13-288) months. Previous undiagnosed CPVT-related symptoms were reported by eight subjects. Exercise test induced ventricular arrhythmias in 23 of the 30 mutation carriers. On beta-blocker treatment, exercise-induced arrhythmias occurred at a lower heart rate (117 +/- 17 vs. 135 +/- 34 beats/min, P = 0.02) but at similar workload (P = 0.78). Beta-blocker treatment suppressed the occurrence of exercise-induced nsVT in three of the four patients, while less severe arrhythmias were unchanged. One patient died during follow-up.

CONCLUSION

Exercise test revealed a high prevalence of arrhythmias in CPVT mutation carriers diagnosed by cascade genetic screening. beta-Blocker therapy appeared to suppress the most severe exercise-induced arrhythmias, while less severe arrhythmias occurred at a lower heart rate.

Tetradecylthioacetic acid inhibits growth of rat glioma cells ex vivo and in vivo via PPAR-dependent and PPAR-independent pathways

The peroxisome proliferator-activated receptors (PPARs) are transcription factors involved in fatty acid metabolism and energy homeostasis. The PPARs also play crucial roles in the control of cellular growth and differentiation. Especially, the recently emerged concept of ligand-dependent PPARgamma-mediated inhibition of cancer cell proliferation through induction of G(1)-phase arrest and differentiation is of clinical interest to cancer therapy. Tetradecylthioacetic acid (TTA) is a sulphur-substituted saturated fatty acid analog with unique biochemical properties. In this study, we investigated the effects of TTA-administration on cell proliferation in glioma cancer models. The rat glioma cell line BT4Cn, whether grown in culture or implanted in rats, expressed significant levels of PPARgamma and PPARdelta, with PPARgamma being the predominant PPAR subtype. In BT4Cn cells, TTA activated all PPAR subtypes in a dose-dependent manner. In cell culture experiments, the PPARgamma-selective ligand BRL49653 moderately inhibited growth of BT4Cn cells, whereas administration of TTA resulted in a marked growth inhibition. Administration of the PPARgamma-selective antagonist GW9662 abolished BRL49653-induced growth inhibition, but only marginally reduced the effect of TTA. TTA reduced tumor growth and increased the survival time of rats with implanted BT4Cn tumor. TTA-induced apoptosis in BT4Cn cells, and the administration of TTA led to cytochrome c release from mitochondria and increased the glutathione content in glioma cells. In conclusion, our results indicate that TTA inhibits proliferation of glioma cancer cells through both PPARgamma-dependent and PPARgamma-independent pathways, of which the latter appears to predominate.

Interaction between the ligand-binding domain of the LDL receptor and the C-terminal domain of PCSK9 is required for PCSK9 to remain bound to the LDL receptor during endosomal acidification

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the epidermal growth factor homology domain repeat A of the low-density lipoprotein receptor (LDLR) at the cell surface and disrupts recycling of the internalized LDLR. As a consequence, the LDLR is rerouted to the lysosomes for degradation. Although PCSK9 may bind to an LDLR lacking the ligand-binding domain, at least three ligand-binding repeats of the ligand-binding domain are required for PCSK9 to reroute the LDLR to the lysosomes. In this study, we have studied the binding of PCSK9 to an LDLR with or without the ligand-binding domain at increasingly acidic conditions in order to mimic the milieu of the LDLR:PCSK9 complex as it translocates from the cell membrane to the sorting endosomes. These studies have shown that PCSK9 is rapidly released from an LDLR lacking the ligand-binding domain at pH in the range of 6.9-6.1. A similar pattern of release at acidic pH was also observed for the binding to the normal LDLR of mutant PCSK9 lacking the C-terminal domain. Together these data indicate that an interaction between the negatively charged ligand-binding domain of the LDLR and the positively charged C-terminal domain of PCSK9 is required for PCSK9 to remain bound to the LDLR during the early phase of endosomal acidification as the LDLR translocates from the cell membrane to the sorting endosome.

Characterization of novel mutations in the catalytic domain of the PCSK9 gene

OBJECTIVES

To expand our understanding of the structure and function of proprotein convertase subtilisin/kexin type 9 (PCSK9) by studying how naturally occurring mutations in PCSK9 disrupt the function of PCSK9.

DESIGN

Mutations in PCSK9 were identified by sequencing of DNA from subjects with hypo- or hypercholesterolemia. The effect of the identified mutations on the autocatalytic cleavage and secretion of PCSK9, as well as the effect on PCSK9-mediated degradation of the low density lipoprotein receptors, were determined in HepG2 or HEK293 cells transiently transfected with mutant PCSK9-containing plasmids. The findings were collated to the clinical characteristics of the subjects possessing these mutations, and the phenotypic effects were analysed in terms of available structural data for PCSK9.

RESULTS

Five novel mutations in PCSK9 were identified. Mutation R215H was a gain-of-function mutation which causes hypercholesterolemia. Mutation G236S and N354I were loss-of-function mutations due to failure to exit the endoplasmic reticulum or failure to undergo autocatalytic cleavage, respectively. Mutations A245T and R272Q were most likely normal genetic variants. By comparing the number of patients with gain-of-function mutations in PCSK9 with the number of familial hypercholesterolemia heterozygotes among subjects with hypercholesterolemia, the prevalence of subjects with gain-of-function mutations in PCSK9 in Norway can be estimated to one in 15,000.

CONCLUSION

This study has provided novel information about the structural requirements for the normal function of PCSK9. However, more studies are needed to determine the mechanisms by which gain-of-function mutations in PCSK9 cause hypercholesterolemia.

Diagnosis of familial hypercholesterolemia in general practice using clinical diagnostic criteria or genetic testing as part of cascade genetic screening

BACKGROUND

Too few familial hypercholesterolemia (FH) patients are diagnosed. The most cost-effective strategy to diagnose FH is to examine first-degree relatives of already diagnosed patients. This is referred to as cascade genetic screening.

METHODS AND RESULTS

One thousand eight hundred and five first-degree relatives of index patients with molecularly defined FH consented to cascade genetic screening by the use of molecular genetic testing. Of these, 44.8% were mutation carriers and 55.2% were noncarriers. Only 44.2% of the mutation carriers were on lipid-lowering drugs at the time of genetic testing. Of these, only 9.4% had a value for total serum cholesterol below 5 mM. Among adult mutation carriers who were not on lipid-lowering treatment at the time of genetic testing, reductions in total serum cholesterol and low-density lipoprotein cholesterol of 18.4% (p < 0.0001) and 25.3% (p < 0.0001), respectively, were observed 6 months after genetic testing. It is assumed that this improvement in the lipid profile is due to a definite diagnosis obtained by molecular genetic testing. By using the results of genetic testing as the gold standard for diagnosis of FH, data from a questionnaire filled out by the relatives showed that the use of clinical criteria to diagnose FH in general practice had a sensitivity of 46.2% and a specificity of 88.0%.

CONCLUSIONS

The use of clinical diagnostic criteria to diagnose FH in general practice identifies only approximately 50% of FH patients. Molecular genetic testing as part of cascade genetic screening is an efficient tool to diagnose patients, leading to significant improvement in the lipid profile. It should therefore be implemented in clinical medicine.

A DNA polymorphism at the angiotensin II type 1 receptor (AT1R) locus and myocardial infarction

Two hundred and thirty-five survivors of myocardial infarction (MI) were compared to 384 controls with respect to distribution of genotypes and gene frequencies in the A1166C polymorphism at the angiotensin II type 1 receptor (AT1R) locus. No differences in allele frequencies or genotype distribution were observed when all patients were compared with all controls. When comparing CC homozygotes with the combined group of CA heterozygotes and AA homozygotes (CA/AA), a difference in borderline significance between the MI group and controls was observed (p=0.05). In males alone, this difference was much more pronounced because of the larger proportion of males with the CC genotype in MI cases than in male controls (p=0.01). No significant differences were observed between female cases and controls. No interaction between the insertion/deletion (I/D) polymorphism at the angiotensin I-converting enzyme (ACE) locus and the polymorphism at the AT1R locus was detected. When subdividing the subjects into a "low-risk" and a "high-risk" group, based on levels of apolipoprotein B (apoB) and body mass index (BMI), and whether or not the person used lipid-lowering drugs, the frequency of CC homozygotes in male cases of the "low-risk" group differed significantly compared to the frequency in male controls of the "low-risk" group (p<0.001). No differences were observed in females, but the number of "low-risk" group female cases was low (n=3). Thus, CC homozygosity appears to be associated with MI in Norwegian males, especially among those with a "low-risk" phenotype.

Insertion/deletion (I/D) polymorphism at the locus for angiotensin I-converting enzyme and parental history of myocardial infarction

One hundred and eighty-one male and 48 female myocardial infarction (MI) survivors and 172 male and 194 female controls were studied with respect to a possible association between premature parental MI (before age 61 years in mothers and/or before age 56 years in fathers) and an insertion/deletion (I/D) polymorphism in the gene encoding angiotensin I-converting enzyme (ACE). In the total series, the frequency of premature parental MI was 14% in the DD (homozygotes for the deletion (D) allele) genotypic group, 10.6% in the ID (heterozygotes) genotypic group and 6.1% in the II (homozygotes for the insertion (I) allele) genotypic group. In all males (male MI survivors and male controls combined), and in the total series, there was a significant excess of DD individuals as compared to II individuals among those with a parental history of premature MI (odds ratio 3.1 (p = 0.03) and 3.1 (p = 0.009), respectively). The ACE polymorphism may be an important genetic marker of MI risk and contribute to clustering of premature MI in families.

Effects of intronic mutations in the LDLR gene on pre-mRNA splicing: Comparison of wet-lab and bioinformatics analyses

Screening for mutations in the low density lipoprotein receptor (LDLR) gene has identified more than 1000 mutations as the cause of familial hypercholesterolemia (FH). In addition, numerous intronic mutations with uncertain effects on pre-mRNA splicing have also been identified. In this study, we have selected 18 intronic mutations in the LDLR gene for comprehensive studies of their effects on pre-mRNA splicing. Epstein-Barr virus (EBV) transformed lymphocytes from subjects heterozygous for these mutations were established and mRNA was studied by Northern blot analyses and reverse transcription polymerase chain reactions. Furthermore, functional studies of the LDLRs were performed by flow cytometry. The results of the wet-lab analyses were compared to the predictions obtained from bioinformatics analyses using the programs MaxEntScan, NetGene2 and NNSplice 0.9, which are commonly used software packages for prediction of abnormal splice sites. Thirteen of the 18 intronic mutations were found to affect pre-mRNA splicing in a biologically relevant way as determined by wet-lab analyses. Skipping of one or two exons was observed for eight of the mutations, intron inclusion was observed for four of the mutations and activation of a cryptic splice site was observed for two of the mutations. Transcripts from eight of the mutant alleles were subjected to degradation. The computational analyses of the normal and mutant splice sites, predicted abnormal splicing with a sensitivity of 100% and a specificity of 60%. Thus, bioinformatics analyses are valuable tools as a first screening of the effects of intronic mutations in the LDLR gene on pre-mRNA splicing.

PCSK9-mediated degradation of the LDL receptor generates a 17 kDa C-terminal LDL receptor fragment

Proprotein convertase subtilisin/kexin type 9 (PCSK9) binds to the LDL receptor (LDLR) at the cell surface and reroutes the internalized LDLR to intracellular degradation. In this study, we have shown that PCSK9-mediated degradation of the full-length 160 kDa LDLR generates a 17 kDa C-terminal LDLR fragment. This fragment was not generated from mutant LDLRs resistant to PCSK9-mediated degradation or when degradation was prevented by chemicals such as ammonium chloride or the cysteine cathepsin inhibitor E64d. The observation that the 17 kDa fragment was only detected when the cells were cultured in the presence of the γ-secretase inhibitor DAPT indicates that this 17 kDa fragment undergoes γ-secretase cleavage within the transmembrane domain. The failure to detect the complementary 143 kDa ectodomain fragment is likely to be due to its rapid degradation in the endosomal lumen. The 17 kDa C-terminal LDLR fragment was also generated from a Class 5 mutant LDLR undergoing intracellular degradation. Thus, one may speculate that an LDLR with bound PCSK9 and a Class 5 LDLR with bound LDL are degraded by a similar mechanism that could involve ectodomain cleavage in the endosome.

Genome-wide expression analysis of cells expressing gain of function mutant D374Y-PCSK9

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a key regulator of serum cholesterol. The possibility that PCSK9 also functions in other pathways needs to be addressed. We have transfected HepG2 cells with mutant D374Y-PCSK9 or control vector. Gene expression signatures were determined using the Affymetrix GeneChip technology, and the expression pattern of selected genes was confirmed by quantitative real-time polymerase chain reaction (qRT-PCR). Data was normalized and analyzed using a model-based background adjustment for oligonucleotide expression arrays, then filtered based upon expression within treatments group, and subjected to moderated t-statistics. Five hundred twenty transcripts had altered expression levels between D374Y-PCSK9 and control vector. Among the 520 probes on our top list, 312 were found to have an assigned Gene Ontology (GO) term, and 96 were found in the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways. Genome-wide expression profiling revealed that "steroid biosynthesis," "sterol metabolism," and "cholesterol biosynthsis" were affected by D374Y-PCSK9. Also, the GO biological process terms "response to stresss," "response to virus," "response to unfolded protein," and "immune response" were influenced by D374Y-PCSK9. Our results suggest that D374Y-PCSK9 results in up-regulation of genes involved in sterol biosynthesis and down-regulation of stress-response genes and specific inflammation pathways.

Identification of deletions and duplications in the low density lipoprotein receptor gene by MLPA

BACKGROUND

Familial hypercholesterolemia (FH) is caused by mutations in the low density lipoprotein (LDL) receptor gene. In this study we have compared multiplex ligation-dependent probe amplification (MLPA) and long-range PCR to detect large deletions/duplications in the LDL receptor gene.

METHOD

DNA from 431 unrelated FH patients without mutations in the LDL receptor gene detectable by DNA sequencing and who had total serum cholesterol levels above 10.0 mmol/l, was subjected to analyses by MLPA and by five long-range PCRs.

RESULT

Eleven deletions and two duplications were detected by MLPA. Six of the deletions and one of the duplications were also detected by long-range PCR. A total of 44 of the 431 (10.2%) FH patients possessed a deletion or a duplication.

CONCLUSION

MLPA has a higher sensitivity than five long-range PCRs to detect large deletions/duplications in the LDL receptor gene. Even though the direct cost of MLPA is twice that of five long-range PCRs, it has replaced long-range PCR for routine diagnostics in our laboratory because of the higher sensitivity and the 30-50% reduction in hands-on time.