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

Genetic identification of familial hypercholesterolemia within whole genome sequences in 6820 newborns

Familial hypercholesterolemia (FH) is defined as a monogenic disease, characterized by elevated low-density lipoprotein cholesterol (LDL-C) levels. FH remains underdiagnosed and undertreated in Chinese. We whole-genome sequenced 6820 newborns from Qingdao of China to investigate the FH-related gene (LDLR, APOB, PCSK9) mutation types, carrier ratio and genotype-phenotype correlation. In this study, the prevalence of FH in Qingdao of China was 0.47% (95% CI: 0.32%-0.66%). The plasma lipid levels of FH-related gene mutation carriers begin to increase as early as infant. T-CHO and LDL-C of FH infants was higher by 48.1% (p < 0.001) and 42.9% (p < 0.001) relative to non-FH infants. A total of 22 FH infants and their parent participate in further studies. The results indicated that FH infant parent noncarriers have the normal plasma lipid level, while T-CHO and LDL-C increased in FH infants and FH infant parent carriers, but no difference between the groups. This highlights the importance of genetic factors. In conclusion, the spectrum of FH-causing mutations in the newborns of Qingdao, China was described for the first time. These data can serve as a considerable dataset for next-generation sequencing analysis of the Chinese population with FH and potentially helping reform regional policies for early detection and prevention of FH.

Identification of a Variant in APOB Gene as a Major Cause of Hypobetalipoproteinemia in Lebanese Families

Familial hypobetalipoproteinemia (FHBL) is a codominant genetic disorder characterized by reduced plasma levels of low-density lipoprotein cholesterol and apolipoprotein B. To our knowledge, no study on FHBL in Lebanon and the Middle East region has been reported. Therefore, we conducted genetic studies in unrelated families and probands of Lebanese origin presenting with FHBL, in order to identify the causes of this disease. We found that 71% of the recruited probands and their affected relatives were heterozygous for the p.(Arg490Trp) variant in the APOB gene. Haplotype analysis showed that these patients presented the same mutant haplotype. Moreover, there was a decrease in plasma levels of PCSK9 in affected individuals compared to the non-affected and a significant positive correlation between circulating PCSK9 and ApoB levels in all studied probands and their family members. Some of the p.(Arg490Trp) carriers suffered from diabetes, hepatic steatosis or neurological problems. In conclusion, the p.(Arg490Trp) pathogenic variant seems a cause of FHBL in patients from Lebanese origin, accounting for approximately 70% of the probands with FHBL presumably as a result of a founder mutation in Lebanon. This study is crucial to guide the early diagnosis, management and prevention of the associated complications of this disease.

Gene network analysis to determine the effect of hypoxia-associated genes on brain damages and tumorigenesis using an avian model

BACKGROUND

Hypoxia refers to the condition of low oxygen pressure in the atmosphere and characterization of response to hypoxia as a biological complex puzzle, is challenging. Previously, we carried out a comparative genomic study by whole genome resequencing of highland and lowland Iranian native chickens to identify genomic variants associated with hypoxia conditions. Based on our previous findings, we used chicken as a model and the identified hypoxia-associated genes were converted to human's orthologs genes to construct the informative gene network. The main goal of this study was to visualize the features of diseases due to hypoxia-associated genes by gene network analysis.

RESULTS

It was found that hypoxia-associated genes contained several gene networks of disorders such as Parkinson, Alzheimer, cardiomyopathy, drug toxicity, and cancers. We found that biological pathways are involved in mitochondrion dysfunctions including peroxynitrous acid production denoted in brain injuries. Lewy body and neuromelanin were reported as key symptoms in Parkinson disease. Furthermore, calmodulin, and amyloid precursor protein were detected as leader proteins in Alzheimer's diseases. Dexamethasone was reported as the candidate toxic drug under the hypoxia condition that implicates diabetes, osteoporosis, and neurotoxicity. Our results suggested DNA damages caused by the high doses of UV radiation in high-altitude conditions, were associated with breast cancer, ovarian cancer, and colorectal cancer.

CONCLUSIONS

Our results showed that hypoxia-associated genes were enriched in several gene networks of disorders including Parkinson, Alzheimer, cardiomyopathy, drug toxicity, and different types of cancers. Furthermore, we suggested, UV radiation and low oxygen conditions in high-altitude regions may be responsible for the variety of human diseases.

Lipids Responsible for Intestinal or Hepatic Disorder: When to Suspect a Familial Intestinal Hypocholesterolemia?

ABSTRACT

Familial intestinal hypocholesterolemias, such as abetalipoproteinemia, hypobetalipoproteinemia, and chylomicron retention disease, are rare genetic diseases that result in a defect in the synthesis or secretion of lipoproteins containing apolipoprotein B.In children, these conditions present with diarrhoea and growth failure, whereas adults present with neuromuscular, ophthalmological, and hepatic symptoms. Simple laboratory investigations have shown that diagnosis can be made from findings of dramatically decreased cholesterol levels, deficiencies in fat-soluble vitamins (mostly vitamin E), endoscopic findings of the characteristic white intestinal mucosa, and fat-loaded enterocytes in biopsy samples. Genetic analysis is used to confirm the diagnosis. Treatment is based on a low-fat diet with essential fatty acid supplementation, high doses of fat-soluble vitamins, and regular and life-long follow-up.The present study examines cases and literature findings of these conditions, and emphasises the need to explore severe hypocholesterolemia and deficiencies in fat-soluble vitamins to not miss these rare, but easy to diagnose and treat, disorders.

Impaired Cytoskeletal and Membrane Biophysical Properties of Acanthocytes in Hypobetalipoproteinemia - A Case Study

Familial hypobetalipoproteinemia is a metabolic disorder mainly caused by mutations in the apolipoprotein B gene. In its homozygous form it can lead without treatment to severe ophthalmological and neurological manifestations. In contrast, the heterozygous form is generally asymptomatic but associated with a low risk of cardiovascular disease. Acanthocytes or thorny red blood cells (RBCs) are described for both forms of the disease. However, those morphological changes are poorly characterized and their potential consequences for RBC functionality are not understood. Thus, in the present study, we asked whether, to what extent and how acanthocytes from a patient with heterozygous familial hypobetalipoproteinemia could exhibit altered RBC functionality. Acanthocytes represented 50% of the total RBC population and contained mitoTracker-positive surface patches, indicating the presence of mitochondrial fragments. While RBC osmotic fragility, calcium content and ATP homeostasis were preserved, a slight decrease of RBC deformability combined with an increase of intracellular free reactive oxygen species were observed. The spectrin cytoskeleton was altered, showing a lower density and an enrichment in patches. At the membrane level, no obvious modification of the RBC membrane fatty acids nor of the cholesterol content were detected but the ceramide species were all increased. Membrane stiffness and curvature were also increased whereas transversal asymmetry was preserved. In contrast, lateral asymmetry was highly impaired showing: (i) increased abundance and decreased functionality of sphingomyelin-enriched domains; (ii) cholesterol enrichment in spicules; and (iii) ceramide enrichment in patches. We propose that oxidative stress induces cytoskeletal alterations, leading to increased membrane stiffness and curvature and impaired lipid lateral distribution in domains and spicules. In addition, ceramide- and spectrin-enriched patches could result from a RBC maturation defect. Altogether, the data indicate that acanthocytes are associated with cytoskeletal and membrane lipid lateral asymmetry alterations, while deformability is only mildly impaired. In addition, familial hypobetalipoproteinemia might also affect RBC precursors leading to disturbed RBC maturation. This study paves the way for the potential use of membrane biophysics and lipid vital imaging as new methods for diagnosis of RBC disorders.

Molecular analysis of APOB, SAR1B, ANGPTL3, and MTTP in patients with primary hypocholesterolemia in a clinical laboratory setting: Evidence supporting polygenicity in mutation-negative patients

BACKGROUND AND AIMS

Primary hypobetalipoproteinemia is generally considered a heterogenic group of monogenic, inherited lipoprotein disorders characterized by low concentrations of LDL cholesterol and apolipoprotein B in plasma. Lipoprotein disorders include abetalipoproteinemia, familial hypobetalipoproteinemia, chylomicron retention disease, and familial combined hypolipidemia. Our aim was to review and analyze the results of the molecular analysis of hypolipidemic patients studied in our laboratory over the last 15 years.

METHODS

The study included 44 patients with clinical and biochemical data. Genomic studies were performed and genetic variants were characterized by bioinformatics analysis. A weighted LDL cholesterol gene score was calculated to evaluate common variants associated with impaired lipid concentrations and their distribution among patients.

RESULTS

Twenty-three patients were genetically confirmed as affected by primary hypobetalipoproteinemia. In this group of patients, the most prevalent mutated genes were APOB (in 17 patients, with eight novel mutations identified), SAR1B (in 3 patients, with one novel mutation identified), ANGPTL3 (in 2 patients), and MTTP (in 1 patient). The other 21 patients could not be genetically diagnosed with hypobetalipoproteinemia despite presenting suggestive clinical and biochemical features. In these patients, two APOB genetic variants associated with lower LDL cholesterol were more frequent than in controls. Moreover, the LDL cholesterol gene score, calculated with 11 SNPs, was significantly lower in mutation-negative patients.

CONCLUSIONS

Around half of the patients could be genetically diagnosed. The results suggest that, in at least some of the patients without an identified mutation, primary hypobetalipoproteinemia may have a polygenic origin.

Rare and common variants of APOB and PCSK9 in Korean patients with extremely low low-density lipoprotein-cholesterol levels

Background

Screening of variants, related to lipid metabolism in patients with extreme cholesterol levels, is a tool used to identify targets affecting cardiovascular outcomes. The aim of this study was to examine the prevalence and characteristics of rare and common variants of APOB and PCSK9 in Korean patients with extremely low low-density lipoprotein-cholesterol (LDL-C) levels.

Methods

Among 13,545 participants enrolled in a cardiovascular genome cohort, 22 subjects, whose LDL-C levels without lipid-lowering agents were ≤1 percentile (48 mg/dL) of Korean population, were analyzed. Two target genes, APOB and PCSK9, were sequenced by targeted next-generation sequencing. Prediction of functional effects was conducted using SIFT, PolyPhen-2, and Mutation Taster, and matched against a public database of variants.

Results

Eight rare variants of the two candidate genes (five in APOB and three in PCSK9) were found in nine subjects. Two subjects had more than two different rare variants of either gene (one subject in APOB and another subject in APOB/PCSK9). Conversely, 12 common variants (nine in APOB and three in PCSK9) were discovered in 21 subjects. Among all variants, six in APOB and three in PCSK9 were novel. Several variants previously reported functional, including c.C277T (p.R93C) and c.G2009A (p.G670E) of PCSK9, were found in our population.

Conclusions

Rare variants of APOB or PCSK9 were identified in nine of the 22 study patients with extremely low LDL-C levels, whereas most of them had common variants of the two genes. The common novelty of variants suggested polymorphism of the two genes among them. Our results provide rare genetic information associated with this lipid phenotype in East Asian people.

Unmet Needs in LDL-C Lowering: When Statins Won't Do!

The use of low-density lipoprotein cholesterol (LDL-C)-lowering medications has led to a significant reduction of cardiovascular risk in both primary and secondary prevention. Statin therapy, one of the cornerstones for the prevention and treatment of cardiovascular disease (CVD), has been demonstrated to be effective in lowering LDL-C levels and in reducing the risk for CVD and is generally well-tolerated. However, compliance with statins remains suboptimal. One of the main reasons is limitations by adverse events, notably myopathies, which can lead to non-compliance with the prescribed statin regimen. Reducing the burden of elevated LDL-C levels is critical in patients with CVD as well as in patients with very high baseline levels of LDL-C (e.g. patients with familial hypercholesterolaemia), as statin therapy is insufficient for optimally reducing LDL-C below target values. In this review, we discuss alternative treatment options after maximally tolerated doses of statin therapy, including ezetimibe, proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, and cholesteryl ester transfer protein (CETP) inhibitors. Difficult-to-treat patients may benefit from combination therapy with ezetimibe or a PCSK9 inhibitor (evolocumab or alirocumab, which are now available). Updates of treatment guidelines are needed to guide the management of patients who will best benefit from these new treatments.

Identification of novel APOB mutations by targeted next-generation sequencing for the molecular diagnosis of familial hypobetalipoproteinemia

BACKGROUND AND AIMS

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by decreased plasma levels of LDL-cholesterol and apolipoprotein B (ApoB). Currently, genetic diagnosis in FHBL relies largely on Sanger sequencing to identify APOB and PCSK9 gene mutations and on western blotting to detect truncated ApoB species.

METHODS

Here, we applied targeted enrichment and next-generation sequencing (NGS) on a panel of three FHBL genes and two abetalipoproteinemia genes (APOB, PCSK9, ANGPTL3, MTTP and SAR1B).

RESULTS

In this study, we identified five likely pathogenic heterozygous rare variants. These include four novel nonsense mutations in APOB (p.Gln845*, p.Gln2571*, p.Cys2933* and p.Ser3718*) and a rare variant in PCSK9 (Minor Allele Frequency <0.1%). The affected family members tested were shown to be carriers, suggesting co-segregation with low LDL-C.

CONCLUSIONS

Our study further demonstrates that NGS is a reliable and practical approach for the molecular screening of FHBL-causative genes that may provide a mean for deciphering the genetic basis in FHBL.

Homozygous familial hypobetalipoproteinemia: two novel mutations in the splicing sites of apolipoprotein B gene and review of the literature

OBJECTIVE

Familial hypobetalipoproteinemia (FHBL) is autosomal codominant disorder of lipoprotein metabolism characterized by low plasma levels of total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C) and apolipoprotein B (apoB) below the 5(th) percentile of the distribution in the population. Patients with the clinical diagnosis of homozygous FHBL (Ho-FHBL) are extremely rare and few patients have been characterized at the molecular level. Here we report the medical history and the molecular characterization of one paediatric patient with clinical features of Ho-FHBL.

METHODS

A one month old infant with failure to thrive, severe hypocholesterolemia and acanthocytosis was clinically and genetically characterized. Molecular characterization of the proband and her parents was performed by direct sequencing of the APOB gene and functional role of the identified mutations was assessed by the minigene methodology.

RESULTS

The proband was found carrying two novel splicing mutations of the APOB gene (c.3696+1G > C and c.3697-1G > A). CHOK1H8 cells expressing minigenes harbouring the mutations showed that these two mutations were associated with the retention of intron 23 and skipping of exon 24, resulting in two truncated apoB fragments of approximate size of 26-28 % of ApoB-100 and the total absence of apoB.

CONCLUSION

We describe the first case of Ho-FHBL due to two splicing mutations affecting both the donor and the acceptor splice sites of the same intron of the APOB gene occurring in the same patient. The clinical management of the proband is discussed and a review of the clinical and genetic features of the published Ho-FHBL cases is reported.

Homozygous MTTP and APOB mutations may lead to hepatic steatosis and fibrosis despite metabolic differences in congenital hypocholesterolemia

BACKGROUND & AIMS

Non-alcoholic steatohepatitis leading to fibrosis occurs in patients with abetalipoproteinemia (ABL) and homozygous or compound heterozygous familial hypobetalipoproteinemia (Ho-FHBL). We wanted to establish if liver alterations were more frequent in one of both diseases and were influenced by comorbidities.

METHODS

We report genetic, clinical, histological and biological characteristics of new cases of ABL (n =7) and Ho-FHBL (n = 7), and compare them with all published ABL (51) and Ho-FHBL (22) probands.

RESULTS

ABL patients, diagnosed during infancy, presented mainly with diarrhea, neurological and ophthalmological impairments and remained lean, whereas Ho-FHBL were diagnosed later, with milder symptoms often becoming overweight in adulthood. Despite subtle differences in lipid phenotype, liver steatosis was observed in both groups with a high prevalence of severe fibrosis (5/27 for Ho-FHBL vs. 4/58 for ABL (n.s.)). Serum triglycerides concentration was higher in Ho-FHBL whereas total and HDL-cholesterol were similar in both groups. In Ho-FHBL liver alterations were found to be independent from the apoB truncation size and apoB concentrations.

CONCLUSIONS

Our findings provide evidence for major liver abnormalities in both diseases. While ABL and Ho-FHBL patients have subtle differences in lipid phenotype, carriers of APOB mutations are more frequently obese. These results raise the question of a complex causal link between apoB metabolism and obesity. They suggest that the genetic defect in VLDL assembly is critical for the occurrence of liver steatosis leading to fibrosis and shows that obesity and insulin resistance might contribute by increasing lipogenesis.

Familial hypobetalipoproteinemia: analysis of three Spanish cases with two new mutations in the APOB gene

Extremely low LDL-cholesterol concentrations are very unusual and generally related with comorbidities accompanying malnutrition. Less frequently low LDL-cholesterol levels result from mutations in the APOB, PCSK9, ANGPTL3, SAR1B and MTTP genes (primary hypobetalipoproteinemia). We investigated three patients with plasma LDL-cholesterol levels below the fifth percentile of the Spanish population. We recorded data on demographic and anthropometric characteristics, life style habits, physical examination, liver ultrasound and lipid and lipoprotein levels, in the probands and their first-degree relatives. Secondary causes of hypocholesterolemia were ruled out by clinical study, complementary tests and follow-up. The APOB, MTTP and SAR1B genes were sequenced. Patients were found to be heterozygotes for point mutations located in the exon 26 of the APOB gene. One patient, with fatty liver, carried a previously described mutation (c.7600C>T) (Arg2507X), causing the formation of truncated Apo B-55.25. The other two mutations producing truncations are new. One asymptomatic patient carried the Arg3672X (Apo B-80.93) and the other with fatty liver and steatorrhea carried the Ser2184fsVal2193X (Apo B-48.32). Our study reinforces the concept that in the heterozygous carriers of truncated Apo Bs, the clinical manifestations of FHBL are dependent on the size of the truncations.

Genetic variation in APOB, PCSK9, and ANGPTL3 in carriers of pathogenic autosomal dominant hypercholesterolemic mutations with unexpected low LDL-Cl Levels

Autosomal Dominant Hypercholesterolemia (ADH) is caused by LDLR and APOB mutations. However, genetically diagnosed ADH patients do not always exhibit the expected hypercholesterolemic phenotype. Of 4,669 genetically diagnosed ADH patients, identified through the national identification screening program for ADH, 75 patients (1.6%) had LDL-cholesterol (LDL-C) levels below the 50th percentile for age and gender prior to lipid-lowering therapy. The genes encoding APOB, PCSK9, and ANGPTL3 were sequenced in these subjects to address whether monogenic dominant loss-of-function mutations underlie this paradoxical phenotype. APOB mutations, resulting in truncated APOB, were found in five (6.7%) probands, reducing LDL-C by 56%. Rare variants in PCSK9, and ANGPTL3 completely correcting the hypercholesterolemic phenotype were not found. The common variants p.N902N, c.3842+82T>A, p.D2312D, and p.E4181K in APOB, and c.1863+94A>G in PCSK9 were significantly more prevalent in our cohort compared to the general European population. Interestingly, 40% of our probands carried at least one minor allele for all four common APOB variants compared to 1.5% in the general European population. While we found a low prevalence of rare variants in our cohort, our data suggest that regions in proximity of the analyzed loci, and linked to specific common haplotypes, might harbor additional variants that correct an ADH phenotype.

Hepatic steatosis does not cause insulin resistance in people with familial hypobetalipoproteinaemia

AIMS/HYPOTHESIS

Hepatic steatosis is strongly associated with hepatic and whole-body insulin resistance. It has proved difficult to determine whether hepatic steatosis itself is a direct cause of insulin resistance. In patients with familial hypobetalipoproteinaemia (FHBL), hepatic steatosis is a direct consequence of impaired hepatic VLDL excretion, independently of metabolic derangements. Thus, patients with FHBL provide a unique opportunity to investigate the relation between increased liver fat and insulin sensitivity.

METHODS

We included seven male participants with FHBL and seven healthy matched controls. Intrahepatic triacylglycerol content and intramyocellular lipid content were measured using localised proton magnetic resonance spectroscopy (¹H-MRS). A two-step hyperinsulinaemic-euglycaemic clamp, using stable isotopes, was assessed to determine hepatic and peripheral insulin sensitivity.

RESULTS

¹H-MRS showed moderate to severe hepatic steatosis in patients with FHBL. Basal endogenous glucose production (EGP) and glucose levels did not differ between the two groups, whereas insulin levels tended to be higher in patients compared with controls. Insulin-mediated suppression of EGP during lower dose insulin infusion and insulin-mediated peripheral glucose uptake during higher dose insulin infusion were comparable between FHBL participants and controls. Baseline fatty acids and lipolysis (glycerol turnover) at baseline and during the clamp did not differ between groups.

CONCLUSIONS/INTERPRETATION

In spite of moderate to severe hepatic steatosis, people with FHBL do not display a reduction in hepatic or peripheral insulin sensitivity compared with healthy matched controls. These results indicate that hepatic steatosis per se is not a causal factor leading to insulin resistance.

TRIAL REGISTRATION

ISRCTN35161775.

Functionality of sequence variants in the genes coding for the low-density lipoprotein receptor and apolipoprotein B in individuals with inherited hypercholesterolemia

Patients with familial hypercholesterolemia (FH) have elevated LDL-C levels, usually above the 90th percentile (P90) for age and gender. However, large-scale genetic cascade screening for FH showed that 15% of the LDL-receptor (LDLR) or Apolipoprotein B (APOB) mutation carriers have LDL-C levels below P75. Nonpathogenicity of sequence changes may explain this phenomenon. To assess pathogenicity of a mutation we proposed three criteria: (1) mean LDL-C 4P75 in untreated mutation carriers; (2) higher mean LDL-C level in untreated carriers than in untreated noncarriers; and (3) higher percentage of medication users in carriers than in noncarriers at screening. We considered a mutation nonpathogenic when none of the three criteria were met. We applied these criteria to mutations that had been determined in more than 50 untreated adults. Segregation analysis was performed to confirm nonpathogenicity. Forty-six mutations had been tested in more than 50 untreated subjects, and three were nonpathogenic according to our criteria: one in LDLR (c.108C4A, exon 2) and two in APOB (c.13154T4C and c.13181T4C, both in exon 29). Segregation analysis also indicated nonpathogenicity. According to our criteria, three sequence variants were nonpathogenic. The criteria may help to identify nonpathogenic sequence changes in genetic cascade screening programs.

Recent progress in understanding protein and lipid factors affecting hepatic VLDL assembly and secretion

Excess lipid induced metabolic disorders are one of the major existing challenges for the society. Among many different causes of lipid disorders, overproduction and compromised catabolism of triacylglycerol-rich very low density lipoproteins (VLDL) have become increasingly prevalent leading to hyperlipidemia worldwide. This review provides the latest understanding in different aspects of VLDL assembly process, including structure-function relationships within apoB, mutations in APOB causing hypobetalipoproteinemia, significance of modulating microsomal triglyceride-transfer protein activity in VLDL assembly, alterations of VLDL assembly by different fatty acid species, and hepatic proteins involved in vesicular trafficking, and cytosolic lipid droplet metabolism that contribute to VLDL assembly. The role of lipoprotein receptors and exchangeable apolipoproteins that promote or diminish VLDL assembly and secretion is discussed. New understanding on dysregulated insulin signaling as a consequence of excessive triacylglycerol-rich VLDL in the plasma is also presented. It is hoped that a comprehensive view of protein and lipid factors that contribute to molecular and cellular events associated with VLDL assembly and secretion will assist in the identification of pharmaceutical targets to reduce disease complications related to hyperlipidemia.

PCSK9 dominant negative mutant results in increased LDL catabolic rate and familial hypobetalipoproteinemia

OBJECTIVE

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a central player in the regulation of cholesterol homeostasis, increasing the low-density lipoprotein (LDL) receptor degradation. Our study aimed at exploring the pathogenic consequences in vivo and in vitro of a PCSK9 prodomain mutation found in a family with hypobetalipoproteinemia (FHBL).

METHODS AND RESULTS

A white 49-year-old diabetic man had profound FBHL (LDLC: 16 mg/dL) whereas his daughter and sister displayed a milder phenotype (LDLC 44 mg/dL and 57 mg/dL, respectively), all otherwise healthy with a normal liver function. A monoallelic PCSK9 double-mutant R104C/V114A cosegregated with FBHL, with no mutation found at other FHBL-causing loci. A dose-effect was also found in FBHL relatives for plasma APOB and PCSK9 (very-low to undetectable in proband, approximately 50% decreased in sister and daughter) and LDL catabolic rate (256% and 88% increased in proband and daughter). Transient transfection in hepatocytes showed severely impaired processing and secretion of the double mutant which acted as a dominant negative over secretion of wild-type PCSK9.

CONCLUSIONS

These results show that heterozygous PCSK9 missense mutations may associate with profound hypobetalipoproteinemia and constitute the first direct evidence in human that decrease of plasma LDLC concentrations associated to PCSK9 LOF mutations are attributable to an increased clearance rate of LDL.

[Familial hypobetalipoproteinemia: clinical characterization of a new mutation in the APOB gene]

BACKGROUND AND OBJECTIVE

Familial hypobetalipoproteinemia (FHB) is usually due to mutations in the APOB gene. Almost 60 different mutations have been reported. We report a Spanish family with FHB phenotype and a new mutation.

PATIENT AND METHODS

We performed an analytical, localizing and molecular study of the APOB gene in the proband and in two relatives phenotypically affected.

RESULTS

The proband was a 32-year-old woman with moderate to severe mental retardation, morbid obesity, hypocholesterolemia, hypertransaminasemia, and hepatic steatosis. The familial phenotypic study was positive in other 6 relatives. The genetic study confirmed the presence of a novel mutation (apoB-69.27) in the APOB gene. The proband, her mother and one maternal great aunt were heterozygote for that mutation.

CONCLUSIONS

FHB has a variable phenotypic expression that can range from oligosymptomatic disease to severe neurological damage.

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

Identification of mutations in the apolipoprotein B-100 gene and in the PCSK9 gene as the cause of hypocholesterolemia

BACKGROUND

Characterization of the normally occurring mutations as the cause of hypocholesterolemia may increase our understanding of the normal lipid metabolism.

METHODS

DNA from 93 unrelated hypocholesterolemic subjects with a mean (+/-SD) value for total serum cholesterol of 3.3 (+/-0.5) mmol/l) were subjected to DNA sequencing of the individual exons of the apolipoprotein B-100 (apoB-100) gene and of the proprotein convertase subtilisin/kexin 9 (PCSK9) gene. The same analyses were also performed in 23 unrelated subjects with autosomal dominant hypercholesterolemia who had unusually low levels of total serum cholesterol.

RESULTS

Of the 93 hypocholesterolemic subjects, 9 subjects (9.7%) were heterozygous for a truncating mutation in the apoB-100 gene and six subjects (6.5%) were heterozygous for a loss-of-function mutation in the PCSK9 gene. Of the 23 subjects with autosomal dominant hypercholesterolemia, four subjects (17.4%) were heterozygous for mutations in the apoB-100 gene.

CONCLUSION

Truncating mutations in the apoB-100 gene are slightly more common as the cause of hypocholesterolemia compared to loss-of-function mutations in the PCSK9 gene. It appears that mutations in the apoB-100 gene may completely normalize the lipid profile in subjects with autosomal dominant hypercholesterolemia, whereas loss-of-function mutations in the PCSK9 gene do not have a sufficient cholesterol-lowering capacity.

High-resolution melting analysis for detection of familial ligand-defective apolipoprotein B-100 mutations

BACKGROUND

Familial ligand-defective apolipoprotein B-100 (FDB) is characterized by elevated plasma concentrations of LDL-cholesterol and apolipoprotein (apo) B, normal triglyceride and HDL-cholesterol levels, the presence of tendon xanthomas, and premature coronary artery disease. FDB cannot be clinically distinguished from heterozygous LDL-receptor-defective familial hypercholesterolaemia (FH) without genetic testing.

METHODS

Amplicons in exon 26 and exon 29 of the APOB gene were screened for established genetic variants including mutations and polymorphisms using high-resolution melting analysis. Six novel variants associated with FDB in hypercholesterolaemic Dutch patients (S3476L, S3488G, Y3533C, T3540M, I4350T, G4368D) were also studied.

RESULTS

All positive controls, a total of 10 mutations in exon 26 and four mutations in exon 29, were readily detectable by melting curve analysis. In addition, a patient previously not known to be heterozygous for the H3543Y mutation was identified in a screen of hypercholesterolaemic subjects. The method was validated by comparison of high-resolution melting analysis with DNA sequence data in a 'blinded' manner in 35 consecutive patients attending a lipid disorders clinic. These patients were classified as 'definite FH' by the Dutch Lipid Clinic Network criteria. Five patients were found to be heterozygous for the R3500Q and one for H3543Y.

CONCLUSIONS

We have established a novel, robust method of FDB mutation detection using high-resolution melting analysis in conjunction with DNA sequencing. Compared with existing methods it is not only more cost-effective, but is also capable of detecting new sequence changes and will have importance in cascade screening of affected subjects.

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.

Molecular diagnosis of hypobetalipoproteinemia: an ENID review

Primary hypobetalipoproteinemia (HBL) includes a group of genetic disorders: abetalipoproteinemia (ABL) and chylomicron retention disease (CRD), with a recessive transmission, and familial hypobetalipoproteinemia (FHBL) with a co-dominant transmission. ABL and CRD are rare disorders due to mutations in the MTP and SARA2 genes, respectively. Heterozygous FHBL is much more frequent. FHBL subjects often have fatty liver and, less frequently, intestinal fat malabsorption. FHBL may be linked or not to the APOB gene. Most mutations in APOB gene cause the formation of truncated forms of apoB which may or may be not secreted into the plasma. Truncated apoBs with a size below that of apoB-30 are not detectable in plasma; they are more frequent in patients with the most severe phenotype. Only a single amino acid substitution (R463W) has been reported as the cause of FHBL. Approximately 50% of FHBL subjects are carriers of pathogenic mutations in APOB gene; therefore, a large proportion of FHBL subjects have no apoB gene mutations or are carriers of rare amino acid substitutions in apoB with unknown effect. In some kindred FHBL is linked to a locus on chromosome 3 (3p21) but the candidate gene is unknown. Recently a FHBL plasma lipid phenotype was observed in carriers of mutations of the PCSK9 gene causing loss of function of the encoded protein, a proprotein convertase which regulates LDL-receptor number in the liver. Inactivation of this enzyme is associated with an increased LDL uptake and hypobetalipoproteinemia. HBL carriers of PCSK9 mutations do not develop fatty liver disease.

Postprandial lipoprotein metabolism in familial hypobetalipoproteinemia

OBJECTIVE

Familial hypobetalipoproteinemia (FHBL) is an autosomal codominantly inherited disorder of lipoprotein metabolism characterized by decreased plasma concentrations of low-density lipoprotein-cholesterol and apolipoprotein (apo) B. We examined the effect of truncated apoB variants (<apoB-48) causing FHBL on postprandial triglyceride-rich lipoprotein (TRL) metabolism.

METHODS AND RESULTS

A standardized oral fat load was given after a 12-h fast to six heterozygous [apoB-6.9 (n=3), apoB-25.8 (n=1), apoB-40.3 (n=2)] FHBL subjects and 10 normolipidemic controls. Plasma was obtained every 2 h for 10 h. Large TRLs [containing chylomicrons (CM)] and small TRLs (containing CM remnants) were isolated by ultracentrifugation. Compared with controls, FHBL subjects had significantly decreased fasting plasma cholesterol (2.3+/-0.5 vs. 4.8+/-0.5 mmol/liter), triglyceride (0.4+/-0.3 vs. 1.5+/-0.5 mmol/liter), low-density lipoprotein-cholesterol (0.6+/-0.4 vs. 3.0+/-0.5 mmol/liter), and apoB (0.22+/-0.05 vs. 0.95+/-0.14 g/liter) concentrations (all P<0.001). The postprandial incremental area under the curve in FHBL subjects was decreased for large TRL-triglyceride (-61%; P<0.005), small TRL-cholesterol (-86%; P<0.001), and small TRL-triglyceride (-86%; P<0.001) relative to controls. Multicompartmental modeling analysis showed that the delay time of apoB-48 was shorter and that apoB-48 production was decreased in FHBL subjects compared with controls.

CONCLUSIONS

We have demonstrated that heterozygous FHBL subjects with apoB truncations shorter than apoB-48, and therefore only a single fully-functional apoB-48 allele, have decreased TRL production but normal postprandial TRL particle clearance.

Hepatic and cardiovascular consequences of familial hypobetalipoproteinemia

OBJECTIVE

Individuals with familial hypobetalipoproteinemia (FHBL) have been reported to be prone to fatty liver disease (FLD). Conversely, the profound reduction of low-density lipoprotein (LDL) cholesterol in this disorder might decrease cardiovascular risk. In the present study, we assessed hepatic steatosis as well as noninvasive surrogate markers for cardiovascular disease (CVD) in subjects with FHBL and in matched controls.

METHODS AND RESULTS

Hepatic steatosis was assessed by abdominal ultrasonography. Carotid intima-media thickness (IMT) and distal common carotid arterial wall stiffness as surrogate markers for CVD risk were measured using high-resolution B-mode ultrasonography. Whereas transaminase levels were only modestly elevated, both prevalence (54% versus 29%; P=0.01) and severity of steatosis were significantly higher in FHBL individuals compared with controls. Despite similar IMT measurements, arterial stiffness was significantly lower in FHBL (P=0.04) compared with controls. Additionally, the increase in arterial stiffness as seen in the presence of traditional risk factors was attenuated, suggesting that very low levels of apoB-containing lipoproteins can negate the adverse effects of other risk factors on the vasculature.

CONCLUSIONS

FHBL is characterized by an increased prevalence and severity of fatty liver disease. The observed decreased level of arterial wall stiffness, most pronounced in the presence of nonlipid risk factors, is indicative of cardiovascular protection in these subjects.