Diacylglycerols and Lysophosphatidic Acid, Enriched on Lipoprotein(a), Contribute to Monocyte Inflammation.

BACKGROUND: Oxidized phospholipids play a key role in the atherogenic potential of lipoprotein(a) (Lp[a]); however, Lp(a) is a complex particle that warrants research into additional proinflammatory mediators. We hypothesized that additional Lp(a)-associated lipids contribute to the atherogenicity of Lp(a). METHODS: Untargeted lipidomics was performed on plasma and isolated lipoprotein fractions. The atherogenicity of the observed Lp(a)-associated lipids was tested ex vivo in primary human monocytes by RNA sequencing, ELISA, Western blot, and transendothelial migratory assays. Using immunofluorescence staining and single-cell RNA sequencing, the phenotype of macrophages was investigated in human atherosclerotic lesions. RESULTS: Compared with healthy individuals with low/normal Lp(a) levels (median, 7 mg/dL [18 nmol/L]; n=13), individuals with elevated Lp(a) levels (median, 87 mg/dL [218 nmol/L]; n=12) demonstrated an increase in lipid species, particularly diacylglycerols (DGs) and lysophosphatidic acid (LPA). DG and the LPA precursor lysophosphatidylcholine were enriched in the Lp(a) fraction. Ex vivo stimulation with DG(40:6) demonstrated a significant upregulation in proinflammatory pathways related to leukocyte migration, chemotaxis, NF-κB (nuclear factor kappa B) signaling, and cytokine production. Functional assessment showed a dose-dependent increase in the secretion of IL (interleukin)-6, IL-8, and IL-1ß after DG(40:6) and DG(38:4) stimulation, which was, in part, mediated via the NLRP3 (NOD [nucleotide-binding oligomerization domain]-like receptor family pyrin domain containing 3) inflammasome. Conversely, LPA-stimulated monocytes did not exhibit an inflammatory phenotype. Furthermore, activation of monocytes by DGs and LPA increased their transendothelial migratory capacity. Human atherosclerotic plaques from patients with high Lp(a) levels demonstrated colocalization of Lp(a) with M1 macrophages, and an enrichment of CD68+IL-18+TLR4+ (toll-like receptor) TREM2+ (triggering receptor expressed on myeloid cells) resident macrophages and CD68+CASP1+ (caspase) IL-1B+SELL+ (selectin L) inflammatory macrophages compared with patients with low Lp(a). Finally, potent Lp(a)-lowering treatment (pelacarsen) resulted in a reduction in specific circulating DG lipid subspecies in patients with cardiovascular disease with elevated Lp(a) levels (median, 82 mg/dL [205 nmol/L]). CONCLUSIONS: Lp(a)-associated DGs and LPA have a potential role in Lp(a)-induced monocyte inflammation by increasing cytokine secretion and monocyte transendothelial migration. This DG-induced inflammation is, in part, NLRP3 inflammasome dependent.

N-Glycosylation Defects in Humans Lower Low-Density Lipoprotein Cholesterol Through Increased Low-Density Lipoprotein Receptor Expression.

BACKGROUND: The importance of protein glycosylation in regulating lipid metabolism is becoming increasingly apparent. We set out to further investigate this by studying patients with type I congenital disorders of glycosylation (CDGs) with defective N-glycosylation. METHODS: We studied 29 patients with the 2 most prevalent types of type I CDG, ALG6 (asparagine-linked glycosylation protein 6)-deficiency CDG and PMM2 (phosphomannomutase 2)-deficiency CDG, and 23 first- and second-degree relatives with a heterozygous mutation and measured plasma cholesterol levels. Low-density lipoprotein (LDL) metabolism was studied in 3 cell models-gene silencing in HepG2 cells, patient fibroblasts, and patient hepatocyte-like cells derived from induced pluripotent stem cells-by measuring apolipoprotein B production and secretion, LDL receptor expression and membrane abundance, and LDL particle uptake. Furthermore, SREBP2 (sterol regulatory element-binding protein 2) protein expression and activation and endoplasmic reticulum stress markers were studied. RESULTS: We report hypobetalipoproteinemia (LDL cholesterol [LDL-C] and apolipoprotein B below the fifth percentile) in a large cohort of patients with type I CDG (mean age, 9 years), together with reduced LDL-C and apolipoprotein B in clinically unaffected heterozygous relatives (mean age, 46 years), compared with 2 separate sets of age- and sex-matched control subjects. ALG6 and PMM2 deficiency led to markedly increased LDL uptake as a result of increased cell surface LDL receptor abundance. Mechanistically, this outcome was driven by increased SREBP2 protein expression accompanied by amplified target gene expression, resulting in higher LDL receptor protein levels. Endoplasmic reticulum stress was not found to be a major mediator. CONCLUSIONS: Our study establishes N-glycosylation as an important regulator of LDL metabolism. Given that LDL-C was also reduced in a group of clinically unaffected heterozygotes, we propose that increasing LDL receptor-mediated cholesterol clearance by targeting N-glycosylation in the LDL pathway may represent a novel therapeutic strategy to reduce LDL-C and cardiovascular disease.

A novel mutation in the F5 gene (factor V Amsterdam) associated with bleeding independent of factor V procoagulant function.

We investigated a small Dutch family with a bleeding diathesis, prolonged prothrombin, and activated partial thromboplastin times, in whom no classifying diagnosis was made. The 2 affected relatives had severely decreased in vitro thrombin generation, and levels of tissue factor pathway inhibitor (TFPI) were strongly increased. To identify the genetic cause of the bleeding diathesis, we performed whole exome sequencing analysis of all living relatives. We found a novel gain-of-function mutation in the F5 gene (c.C2588G), which leads to an aberrant splicing of F5 and ultimately to a short factor V protein (missing 623 amino acids from the B domain), which we called factor V Amsterdam. Factor V Amsterdam binds to TFPI, prolonging its half-life and concentration. This is the second report of an association between a shorter form of factor V and increased TFPI levels, resulting in severely reduced thrombin generation and a bleeding tendency.

Proprotein Convertase Subtilisin Kexin Type 9 Inhibition for Autosomal Recessive Hypercholesterolemia-Brief Report.

OBJECTIVE: Proprotein convertase subtilisin kexin type 9 (PCSK9) inhibitors lower low-density lipoprotein (LDL) cholesterol in the vast majority of patients with autosomal dominant familial hypercholesterolemia. Will PCSK9 inhibition with monoclonal antibodies, in particular alirocumab, be of therapeutic value for patients with autosomal recessive hypercholesterolemia (ARH)? APPROACH AND RESULTS: Primary lymphocytes were obtained from 28 genetically characterized ARH patients and 11 controls. ARH lymphocytes treated with mevastatin were incubated with increasing doses of recombinant PCSK9 with or without saturating concentrations of alirocumab. Cell surface LDL receptor expression measured by flow cytometry and confocal microscopy was higher in ARH than in control lymphocytes. PCSK9 significantly reduced LDL receptor expression in ARH lymphocytes albeit to a lower extent than in control lymphocytes (25% versus 76%, respectively), an effect reversed by alirocumab. Fluorescent LDL cellular uptake, also measured by flow cytometry, was reduced in ARH lymphocytes compared with control lymphocytes. PCSK9 significantly lowered LDL cellular uptake in ARH lymphocytes, on average by 18%, compared with a 46% reduction observed in control lymphocytes, an effect also reversed by alirocumab. Overall, the effects of recombinant PCSK9, and hence of alirocumab, on LDL receptor expression and function were significantly less pronounced in ARH than in control cells. CONCLUSIONS: PCSK9 inhibition with alirocumab on top of statin treatment has the potential to lower LDL cholesterol in some autosomal recessive hypercholesterolemia patients.

Evidence of a polygenic origin of extreme high-density lipoprotein cholesterol levels.

OBJECTIVE: There are several known monogenic causes of high and low high-density lipoprotein cholesterol (HDL-C) levels, but traditional sequencing studies have had limited success in identifying mutations in the majority of individuals with extreme HDL-C levels. The aim of this study was to assess the power of a targeted high-throughput sequencing strategy to elucidate the genetic basis of extreme HDL-C phenotypes. APPROACH AND RESULTS: We sequenced 195 genes with either established or implicated roles in lipid and lipoprotein metabolism plus 78 lipid-unrelated genes in patients with HDL-C <1st (n=40) or >99th (n=40) percentile values, and the results were compared with those of 498 individuals representative of the Dutch general population and 95 subjects with normal HDL-C (between 40th and 60th percentile values). The extreme HDL cohort carried more rare nonsynonymous variants in the lipid geneset than both the general population (odds ratio, 1.39; P=0.019) and normal HDL-C (odds ratio, 1.43; P=0.040) cohorts. The prevalence of such variants in the lipid-related and lipid-unrelated genesets was similar in the control groups, indicative of equal mutation rates. In the extreme HDL cohort, however, there was enrichment of rare nonsynonymous variants in the lipid versus the control geneset (odds ratio, 2.23; P<0.0001), and 70% of the lipid-related variants altered conserved nucleotides. The lipid geneset comprised 4 nonsense, 10 splice-site, and 8 coding indel variants, whereas the control geneset contained only 1 such variant. In the lipid geneset, 87% and 28% of the patients carried ≥ 2 and ≥ 5 rare variants. CONCLUSIONS: This study suggests that most extreme HDL-C phenotypes have a polygenic origin.

Advances in genetics show the need for extending screening strategies for autosomal dominant hypercholesterolaemia.

Aims Autosomal dominant hypercholesterolaemia (ADH) is a major risk factor for coronary artery disease. This disorder is caused by mutations in the genes coding for the low-density lipoprotein receptor (LDLR), apolipoprotein B (APOB), and proprotein convertase subtilisin/kexin 9 (PCSK9). However, in 41% of the cases, we cannot find mutations in these genes. In this study, new genetic approaches were used for the identification and validation of new variants that cause ADH. Methods and results Using exome sequencing, we unexpectedly identified a novel APOB mutation, p.R3059C, in a small-sized ADH family. Since this mutation was located outside the regularly screened APOB region, we extended our routine sequencing strategy and identified another novel APOB mutation (p.K3394N) in a second family. In vitro analyses show that both mutations attenuate binding to the LDLR significantly. Despite this, both mutations were not always associated with ADH in both families, which prompted us to validate causality through using a novel genetic approach. Conclusion This study shows that advances in genetics help increasing our understanding of the causes of ADH. We identified two novel functional APOB mutations located outside the routinely analysed APOB region, suggesting that screening for mutations causing ADH should encompass the entire APOB coding sequence involved in LDL binding to help identifying and treating patients at increased cardiovascular risk.

Use of Lipoprotein(a) to improve diagnosis and management in clinical familial hypercholesterolemia.

BACKGROUND AND AIMS: Lipoprotein(a) (Lp(a)) is an LDL-like particle whose plasma levels are largely genetically determined. The impact of measuring Lp(a) in patients with clinical familial hypercholesterolemia (FH) referred for genetic testing is largely unknown. We set out to evaluate the contribution of (genetically estimated) Lp(a) in a large nation-wide referral population of clinical FH. METHODS: In 1504 patients referred for FH genotyping, we used an LPA genetic instrument (rs10455872 and rs3798220) as a proxy for plasma Lp(a) levels. The genetic Lp(a) proxy was used to correct LDL-cholesterol and reclassify patients with clinical FH based on Dutch Lipid Criteria Network (DLCN) scoring. Finally, we used estimated Lp(a) levels to reclassify ASCVD risk using the SCORE and SMART risk scores. RESULTS: LPA SNPs were more prevalent among mutation-negative compared with mutation-positive patients (296/1280 (23.1%) vs 35/224 (15.6%), p = 0.016). Among patients with genetically defined high Lp(a) levels, 9% were reclassified to the DLCN category 'unlikely FH' using Lp(a)-corrected LDL-cholesterol (LDL-Ccor) and all but one of these patients indeed carried no FH variant. Furthermore, elevated Lp(a) reclassified predicted ASCVD risk into a higher category in up to 18% of patients. CONCLUSIONS: In patients referred for FH molecular testing, we show that taking into account (genetically estimated) Lp(a) levels not only results in reclassification of probability of genetic FH, but also has an impact on individual cardiovascular risk evaluation. However, to avoid missing the diagnosis of an FH variant, clear thresholds for the use of Lp(a)-cholesterol adjusted LDL-cholesterol levels in patients referred for genetic testing of FH must be established.

Low-Cost High-Throughput Genotyping for Diagnosing Familial Hypercholesterolemia.

BACKGROUND: Familial hypercholesterolemia (FH) is a common but underdiagnosed genetic disorder characterized by high low-density lipoprotein cholesterol levels and premature cardiovascular disease. Current sequencing methods to diagnose FH are expensive and time-consuming. In this study, we evaluated the accuracy of a low-cost, high-throughput genotyping array for diagnosing FH. METHODS: An Illumina Global Screening Array was customized to include probes for 636 variants, previously classified as FH-causing variants. First, its theoretical coverage was assessed in all FH variant carriers diagnosed through next-generation sequencing between 2016 and 2022 in the Netherlands (n=1772). Next, the performance of the array was validated in another sample of FH variant carriers previously identified in the Dutch FH cascade screening program (n=1268). RESULTS: The theoretical coverage of the array for FH-causing variants was 91.3%. Validation of the array was assessed in a sample of 1268 carriers of whom 1015 carried a variant in LDLR, 250 in APOB, and 3 in PCSK9. The overall sensitivity was 94.7% and increased to 98.2% after excluding participants with variants not included in the array design. Copy number variation analysis yielded a 89.4% sensitivity. In 18 carriers, the array identified a total of 19 additional FH-causing variants. Subsequent DNA analysis confirmed 5 of the additionally identified variants, yielding a false-positive result in 16 subjects (1.3%). CONCLUSIONS: The FH genotyping array is a promising tool for genetically diagnosing FH at low costs and has the potential to greatly increase accessibility to genetic testing for FH. Continuous customization of the array will further improve its performance.

High prevalence of mutations in LCAT in patients with low HDL cholesterol levels in The Netherlands: identification and characterization of eight novel mutations.

Lecithin:cholesterol acyltransferase (LCAT) is crucial to the maturation of high-density lipoprotein (HDL). Homozygosity for LCAT mutations underlies rare disorders characterized by HDL-cholesterol (HDL-c) deficiency while heterozygotes have half normal HDL-c levels. We studied the prevalence of LCAT mutations in referred patients with low HDL-c to better understand the molecular basis of low HDL-c in our patients. LCAT was sequenced in 98 patients referred for HDL-c <5th percentile and in four patients referred for low HDL-c and corneal opacities. LCAT mutations were highly prevalent: in 28 of the 98 participants (29%), heterozygosity for nonsynonymous mutations was identified while 18 patients carried the same mutation (p.T147I). The four patients with corneal opacity were compound heterozygotes. All previously identified mutations are documented to cause loss of catalytic activity. Nine novel mutations-c.402G>T (p.E134D), c.403T>A (p.Y135N), c.964C>T (p.R322C), c.296G>C (p.W99S), c.736G>T (p.V246F), c.802C>T (p.R268C), c.945G>A (p.W315X), c.1012C>T (p.L338F), and c.1039C>T (p.R347C)--were shown to be functional through in vitro characterization. The effect of several mutations on the core protein structure was studied by a three-dimensional (3D) model. Unlike previous reports, functional mutations in LCAT were found in 29% of patients with low HDL-c, thus constituting a common cause of low HDL-c in referred patients in The Netherlands.

Intronic variant screening with targeted next-generation sequencing reveals first pseudoexon in LDLR in familial hypercholesterolemia.

BACKGROUND AND AIMS: Familial hypercholesterolemia (FH) is caused by pathogenic variants in LDLR, APOB, or PCSK9 genes (designated FH+). However, a significant number of clinical FH patients do not carry these variants (designated FH-). Here, we investigated whether variants in intronic regions of LDLR attribute to FH by affecting pre-mRNA splicing. METHODS: LDLR introns are partly covered in routine sequencing of clinical FH patients using next-generation sequencing. Deep intronic variants, >20 bp from intron-exon boundary, were considered of interest once (a) present in FH- patients (n = 909) with LDL-C >7 mmol/L (severe FH-) or after in silico analysis in patients with LDL-C >5 mmol/L (moderate FH-) and b) absent in FH + patients (control group). cDNA analysis and co-segregation analysis were performed to assess pathogenicity of the identified variants. RESULTS: Three unique variants were present in the severe FH- group. One of these was the previously described likely pathogenic variant c.2140+103G>T. Three additional variants were selected based on in silico analyses in the moderate FH- group. One of these variants, c.2141-218G>A, was found to result in a pseudo-exon inclusion, producing a premature stop codon. This variant co-segregated with the hypercholesterolemic phenotype. CONCLUSIONS: Through a screening approach, we identified a deep intronic variant causal for FH. This finding indicates that filtering intronic variants in FH- patients for the absence in FH + patients might enrich for true FH-causing variants and suggests that intronic regions of LDLR need to be considered for sequencing in FH- patients.

Chylomicronemia with a mutant GPIHBP1 (Q115P) that cannot bind lipoprotein lipase.

OBJECTIVE: GPIHBP1 is an endothelial cell protein that binds lipoprotein lipase (LPL) and chylomicrons. Because GPIHBP1 deficiency causes chylomicronemia in mice, we sought to determine whether some cases of chylomicronemia in humans could be attributable to defective GPIHBP1 proteins. METHODS AND RESULTS: Patients with severe hypertriglyceridemia (n=60, with plasma triglycerides above the 95th percentile for age and gender) were screened for mutations in GPIHBP1. A homozygous GPIHBP1 mutation (c.344A>C) that changed a highly conserved glutamine at residue 115 to a proline (p.Q115P) was identified in a 33-year-old male with lifelong chylomicronemia. The patient had failure-to-thrive as a child but had no history of pancreatitis. He had no mutations in LPL, APOA5, or APOC2. The Q115P substitution did not affect the ability of GPIHBP1 to reach the cell surface. However, unlike wild-type GPIHBP1, GPIHBP1-Q115P lacked the ability to bind LPL or chylomicrons (d < 1.006 g/mL lipoproteins from Gpihbp1(-/-) mice). Mouse GPIHBP1 with the corresponding mutation (Q114P) also could not bind LPL. CONCLUSIONS: A homozygous missense mutation in GPIHBP1 (Q115P) was identified in a patient with chylomicronemia. The mutation eliminated the ability of GPIHBP1 to bind LPL and chylomicrons, strongly suggesting that it caused the patient's chylomicronemia.

The identification and function of a Netrin-1 mutation in a pedigree with premature atherosclerosis.

BACKGROUND AND AIMS: Neuroimmune guidance cues have been shown to play a role in atherosclerosis, but their exact role in human pathophysiology is largely unknown. In the current study, we investigated the role of a c.1769G > T variant in Netrin-1 in (premature) atherosclerosis. METHODS: To determine the effect of the genetic variation, purified Netrin-1, either wild type (wtNetrin-1) or the patient observed variation (mutNetrin-1), was used for migration, adhesion, endothelial barrier function and bindings assays. Expression of adhesion molecules and transcription proteins was analyzed by RT-PCR, Western blot or ELISA. To further delineate how mutNetrin-1 mediates its effect on cell migration, lenti-viral knockdown of UNC5B or DCC was used. RESULTS: Bindings assays revealed a decreased binding capacity of mutNetrin-1 to the receptors UNC5B, DCC and ß3-integrin and an increased binding capacity to neogenin, heparin and heparan sulfate compared to wtNetrin-1. Exposure of endothelial cells to mutNetrin-1 resulted in enhanced monocyte adhesion and expression of IL-6, CCL2 and ICAM-1 compared to wtNetrin-1. In addition, mutNetrin-1 lacks the inhibitory effect on the NF-κB pathway that is observed for wtNetrin-1. Moreover, the presence of mutNetrin-1 diminished migration of macrophages and smooth muscle cells. Importantly, UNC5B or DCC specific knockdown showed that mutNetrin-1 is unable to act through DCC resulting in enhanced inhibition of migration. CONCLUSIONS: Our data demonstrates that mutNetrin-1 fails to exert anti-inflammatory effects on endothelial cells and more strongly blocks macrophage migration compared to wtNetrin-1, suggesting that the carriers of this genetic molecular variant may well be at risk for premature atherosclerosis.

Genetic variants in SUSD2 are associated with the risk of ischemic heart disease.

BACKGROUND: Genetic factors partly determine the risk for premature myocardial infarction (MI). OBJECTIVES: We report the identification of a novel rare genetic variant in a kindred with an autosomal dominant trait for premature MI and atherosclerosis and explored the association of a common nonsynonymous variant in the same gene with the risk of ischemic heart disease (IHD) in a population-based study. METHODS: Next-generation sequencing was performed in a small pedigree with premature MI or subclinical atherosclerosis. A common variant, rs8141797 A>G (p.Asn466Ser), in sushi domain-containing protein 2 (SUSD2) was studied in the prospective Copenhagen General Population Studies (N = 105,408) for association with IHD. RESULTS: A novel heterozygous nonsense mutation in SUSD2 (c.G583T; p.Glu195Ter) was associated with the disease phenotype in the pedigree. SUSD2 protein was expressed in aortic specimens in the subendothelial cell layer and around the vasa vasorum. Furthermore, the minor G-allele of rs8141797 was associated with per allele higher levels of SUSD2 mRNA expression in the heart and vasculature. In the Copenhagen General Population Study, hazard ratios for IHD were 0.92 (95% CI: 0.87-0.97) in AG heterozygotes and 0.86 (0.62-1.19) in GG homozygotes vs noncarrriers (P-trend = .002). Finally, in meta-analysis including 73,983 IHD cases and 215,730 controls, the odds ratio for IHD per G-allele vs A-allele was 0.93 (0.90-0.96) (P = 4.6 × 10-7). CONCLUSIONS: The identification of a truncating mutation in SUSD2, which was associated with premature MI and subclinical atherosclerosis, combined with the finding that a common missense variant in SUSD2 was strongly associated with a lower risk of IHD, suggest that SUSD2 may alter the risk of atherosclerosis.

A Deep Intronic Variant in LDLR in Familial Hypercholesterolemia.

BACKGROUND: Familial hypercholesterolemia (FH) is an inherited disorder characterized by high plasma LDL-C (low-density lipoprotein-cholesterol) levels. The vast majority of FH patients carry a mutation in the coding region of LDLR, APOB, or PCSK9. We set out to identify the culprit genetic defect in a large family with clinical FH, in whom no mutations were identified in the coding regions of these FH genes. METHODS: Whole genome sequencing was performed in 5 affected and 4 unaffected individuals from a family with an unexplained autosomal dominant FH trait. The effect on splicing of the identified novel intronic LDLR mutation was ascertained by cDNA sequencing. The prevalence of the novel variant was assessed in 1 245 FH patients without an FH causing mutation identified by Sanger sequencing and in 2 154 patients referred for FH analysis by next-generation sequencing (covering the intronic region). RESULTS: A novel deep intronic variant in LDLR (c.2140+103G>T) was found to cosegregate with high LDL-C in 5 patients, but was not present in 4 unaffected family members. The variant was shown to result in a 97 nucleotides insertion leading to a frameshift and premature stop codon in exon 15 of LDLR. The prevalence of the intronic variant was 0.24% (3/1245) in a cohort of FH patients without a known FH causing mutation and 0.23% (5/2154) in a population of FH patients referred for analysis by next-generation sequencing. Cosegregation analysis of a second family showed full penetrance of the novel variant with the FH phenotype over 3 generations. CONCLUSIONS: The c.2140+103G>T mutation in LDLR is a novel intronic variant identified in FH that cosegregates with the FH phenotype. Our findings underline the need to analyze the intronic regions of LDLR in patients with FH, especially those in whom no mutation is found in the coding regions of LDLR, APOB, or PCSK9.

Lipoprotein lipase gene analyses in one Turkish family and three different Chinese families with severe hypertriglyceridaemia: one novel and several established mutations.

Lipoprotein lipase (LPL, triacylglycerol acylhydrolase; EC 3.1.1.3) deficiency (OMIM 238600) is an autosomal recessive inherited condition caused by mutations in the LPL gene, either in a homozygous or in a compound heterozygous state, leading to loss of lipolytic activity and resulting in severe hypertriglyceridaemia and subsequent risk for developing pancreatitis. Numerous LPL gene mutations leading to loss of catalytic function have been described. In this present study, we describe full clinical, biochemical and molecular analyses of severe hypertriglyceridaemic individuals in one Turkish and three Chinese families. We established one novel mutation (delCT1312-1313), a new combination of mutations (S193R and I194T) and four previously reported mutations (L252R, L252V, S193R and I194T) of the LPL gene and report phenotypes for these and four previously described mutations. Finally, we show that two patients homozygous for the LPL gene delCT1312-1313 mutations are characterized by absence of LPL activity that coincides with absence of LPL protein.