ANGPTL3 inhibition, dyslipidemia, and cardiovascular diseases

Optimal management of low-density lipoprotein cholesterol (LDL-C) is a central tenet in the primary and secondary prevention of atherosclerotic cardiovascular disease (ASCVD). However, significant residual cardiovascular risk remains despite achieving guideline-directed LDL-C levels, in part due to mixed hyperlipidemia with elevated fasting and non-fasting triglyceride-rich lipoprotein levels. Advances in human genetics have identified angiopoietin-like 3 (ANGPTL3) as a promising therapeutic target to lower cardiovascular risk. Evidence accrued from genetic epidemiological studies demonstrate that ANGPTL3 loss of function is strongly associated with lowering of circulating LDL-C, triglyceride-rich lipoproteins and concurrent risk reduction in development of coronary artery disease. Pharmacological inhibition of ANGPTL3 with monoclonal antibodies, antisense oligonucleotides and gene editing are in development with early studies showing their safety and efficacy in lowering in both, LDL-C and TGs, circumventing a key limitation of previous therapies. Monoclonal antibodies targeting ANGPTL3 are approved for clinical use in homozygous familial hypercholesteremia in USA and Europe. Although promising, future studies focusing on long-term beneficial effect in reducing cardiovascular events with inhibition of ANGPTL3 are warranted.

Ergothioneine boosts mitochondrial respiration and exercise performance via direct activation of MPST

Ergothioneine (EGT) is a diet-derived, atypical amino acid that accumulates to high levels in human tissues. Reduced EGT levels have been linked to age-related disorders, including neurodegenerative and cardiovascular diseases, while EGT supplementation is protective in a broad range of disease and aging models in mice. Despite these promising data, the direct and physiologically relevant molecular target of EGT has remained elusive. Here we use a systematic approach to identify how mitochondria remodel their metabolome in response to exercise training. From this data, we find that EGT accumulates in muscle mitochondria upon exercise training. Proteome-wide thermal stability studies identify 3-mercaptopyruvate sulfurtransferase (MPST) as a direct molecular target of EGT; EGT binds to and activates MPST, thereby boosting mitochondrial respiration and exercise training performance in mice. Together, these data identify the first physiologically relevant EGT target and establish the EGT-MPST axis as a molecular mechanism for regulating mitochondrial function and exercise performance.

Endothelial lipase mediates efficient lipolysis of triglyceride-rich lipoproteins

Triglyceride-rich lipoproteins (TRLs) are circulating reservoirs of fatty acids used as vital energy sources for peripheral tissues. Lipoprotein lipase (LPL) is a predominant enzyme mediating triglyceride (TG) lipolysis and TRL clearance to provide fatty acids to tissues in animals. Physiological and human genetic evidence support a primary role for LPL in hydrolyzing TRL TGs. We hypothesized that endothelial lipase (EL), another extracellular lipase that primarily hydrolyzes lipoprotein phospholipids may also contribute to TRL metabolism. To explore this, we studied the impact of genetic EL loss-of-function on TRL metabolism in humans and mice. Humans carrying a loss-of-function missense variant in LIPG, p.Asn396Ser (rs77960347), demonstrated elevated plasma TGs and elevated phospholipids in TRLs, among other lipoprotein classes. Mice with germline EL deficiency challenged with excess dietary TG through refeeding or a high-fat diet exhibited elevated TGs, delayed dietary TRL clearance, and impaired TRL TG lipolysis in vivo that was rescued by EL reconstitution in the liver. Lipidomic analyses of postprandial plasma from high-fat fed Lipg-/- mice demonstrated accumulation of phospholipids and TGs harboring long-chain polyunsaturated fatty acids (PUFAs), known substrates for EL lipolysis. In vitro and in vivo, EL and LPL together promoted greater TG lipolysis than either extracellular lipase alone. Our data positions EL as a key collaborator of LPL to mediate efficient lipolysis of TRLs in humans and mice.

Potential role of hepatic lipase in the accretion of docosahexaenoic acid (DHA) by the brain

DHA (docosahexaenoic acid) is an essential fatty acid that is required for the normal development and function of the brain. Because of its inability to synthesize adequate amounts of DHA from the precursors, the brain has to acquire DHA from plasma through the blood brain barrier (BBB). Recent studies demonstrated the presence of a transporter at the BBB that specifically transports DHA into the brain in the form of lysophosphatidylcholine (LPC-DHA). However, the mechanism by which LPC-DHA is generated in the plasma is not known. Our previous studies showed that there are at least three different enzymes - lecithin cholesterol acyltransferase (LCAT), endothelial lipase (EL), and hepatic lipase (HL), which can generate LPC-DHA from sn-2 DHA phosphatidylcholine. Here we determined the relative contributions of these enzymes in the delivery of DHA to the brain by measuring the brain DHA levels in the mice deficient in each of these enzymes. The results show that the brain DHA levels of LCAT-deficient mice or EL-deficient mice were not significantly lower than those of their littermates. However, brain DHA was significantly decreased in HL deficient mice (13.5% of total fatty acids) compared to their littermates (17.1%) (p < 0.002), and further decreased to 8.3% of total fatty acids in mice deficient in both HL and EL. These results suggest that HL activity may be the major source for the generation of LPC-DHA in the plasma necessary for transport into the brain, and EL might contribute to this process in the absence of HL.

Clonal Hematopoiesis of Indeterminate Potential Reshapes Age-Related CVD: JACC Review Topic of the Week

The incidence of cardiovascular diseases increases with age and is also correlated with increased inflammatory burden. Recently, human genetics provided a new paradigm linking aging, inflammation, and atherosclerotic cardiovascular disease (ASCVD). Next-generation genetic sequencing of whole blood-derived DNA in humans showed that clonal expansion of hematopoietic cells with somatic mutations in leukemogenic genes was associated with age and correlated with increased mortality. This phenomenon, termed clonal hematopoiesis of indeterminate potential (CHIP), was associated with hematologic malignancy as well as ASCVD independently of age and other traditional risk factors. Because the implication of CHIP with ASCVD, genetic loss-of-function studies of Tet2 and Dnmt3a in murine models have supported a mechanistic role for CHIP in promoting vascular disease. Despite the potential contribution of CHIP to myriad cardiovascular and aging-related diseases, the epidemiology and biology surrounding this phenomenon remains incompletely appreciated and understood, especially as applied to clinical practice and prognostication. Here, the authors review this emerging key risk factor, defining its discovery, relationship to cardiovascular diseases, preclinical evidence for causality, and implications for risk prediction and mitigation.

Novel congenital disorder of O-linked glycosylation caused by GALNT2 loss of function

Congenital disorders of glycosylation are a growing group of rare genetic disorders caused by deficient protein and lipid glycosylation. Here, we report the clinical, biochemical, and molecular features of seven patients from four families with GALNT2-congenital disorder of glycosylation (GALNT2-CDG), an O-linked glycosylation disorder. GALNT2 encodes the Golgi-localized polypeptide N-acetyl-d-galactosamine-transferase 2 isoenzyme. GALNT2 is widely expressed in most cell types and directs initiation of mucin-type protein O-glycosylation. All patients showed loss of O-glycosylation of apolipoprotein C-III, a non-redundant substrate for GALNT2. Patients with GALNT2-CDG generally exhibit a syndrome characterized by global developmental delay, intellectual disability with language deficit, autistic features, behavioural abnormalities, epilepsy, chronic insomnia, white matter changes on brain MRI, dysmorphic features, decreased stature, and decreased high density lipoprotein cholesterol levels. Rodent (mouse and rat) models of GALNT2-CDG recapitulated much of the human phenotype, including poor growth and neurodevelopmental abnormalities. In behavioural studies, GALNT2-CDG mice demonstrated cerebellar motor deficits, decreased sociability, and impaired sensory integration and processing. The multisystem nature of phenotypes in patients and rodent models of GALNT2-CDG suggest that there are multiple non-redundant protein substrates of GALNT2 in various tissues, including brain, which are critical to normal growth and development.

Natural human genetic variation determines basal and inducible expression of PM20D1, an obesity-associated gene

PM20D1 is a candidate thermogenic enzyme in mouse fat, with its expression cold-induced and enriched in brown versus white adipocytes. Thiazolidinedione (TZD) antidiabetic drugs, which activate the peroxisome proliferator-activated receptor-γ (PPARγ) nuclear receptor, are potent stimuli for adipocyte browning yet fail to induce Pm20d1 expression in mouse adipocytes. In contrast, PM20D1 is one of the most strongly TZD-induced transcripts in human adipocytes, although not in cells from all individuals. Two putative PPARγ binding sites exist near the gene's transcription start site (TSS) in human but not mouse adipocytes. The -4 kb upstream site falls in a segmental duplication of a nearly identical intronic region +2.5 kb downstream of the TSS, and this duplication occurred in the primate lineage and not in other mammals, like mice. PPARγ binding and gene activation occur via this upstream duplicated site, thus explaining the species difference. Furthermore, this functional upstream PPARγ site exhibits genetic variation among people, with 1 SNP allele disrupting a PPAR response element and giving less activation by PPARγ and TZDs. In addition to this upstream variant that determines PPARγ regulation of PM20D1 in adipocytes, distinct variants downstream of the TSS have strong effects on PM20D1 expression in human fat as well as other tissues. A haplotype of 7 tightly linked downstream SNP alleles is associated with very low PMD201 expression and correspondingly high DNA methylation at the TSS. These PM20D1 low-expression variants may account for human genetic associations in this region with obesity as well as neurodegenerative diseases.

Multiplexed Targeted Resequencing Identifies Coding and Regulatory Variation Underlying Phenotypic Extremes of High-Density Lipoprotein Cholesterol in Humans

BACKGROUND

Genome-wide association studies have uncovered common variants at many loci influencing human complex traits, such as high-density lipoprotein cholesterol (HDL-C). However, the contribution of the identified genes is difficult to ascertain from current efforts interrogating common variants with small effects. Thus, there is a pressing need for scalable, cost-effective strategies for uncovering causal variants, many of which may be rare and noncoding.

METHODS

Here, we used a molecular inversion probe target capture approach to resequence both coding and regulatory regions at 7 HDL-C-associated loci in 797 individuals with extremely high HDL-C versus 735 low-to-normal HDL-C controls. Our targets included protein-coding regions of GALNT2, APOA5, APOC3, SCARB1, CCDC92, ZNF664, CETP, and LIPG (>9 kb) and proximate noncoding regulatory features (>42 kb).

RESULTS

Exome-wide genotyping in 1114 of the 1532 participants yielded a >90% genotyping concordance rate with molecular inversion probe-identified variants in ≈90% of participants. This approach rediscovered nearly all established genome-wide association studies associations in GALNT2, CETP, and LIPG loci with significant and concordant associations with HDL-C from our phenotypic extremes design at 0.1% of the sample size of lipid genome-wide association studies. In addition, we identified a novel, rare, CETP noncoding variant enriched in the extreme high HDL-C group (P<0.01, score test).

CONCLUSIONS

Our targeted resequencing of individuals at the HDL-C phenotypic extremes offers a novel, efficient, and cost-effective approach for identifying rare coding and noncoding variation differences in extreme phenotypes and supports the rationale for applying this methodology to uncover rare variation-particularly noncoding variation-underlying myriad complex traits.

HDL Cholesterol Metabolism and the Risk of CHD: New Insights from Human Genetics

PURPOSE OF REVIEW

Elevated high-density lipoprotein cholesterol levels in the blood (HDL-C) represent one of the strongest epidemiological surrogates for protection against coronary heart disease (CHD), but recent human genetic and pharmacological intervention studies have raised controversy about the causality of this relationship. Here, we review recent discoveries from human genome studies using new analytic tools as well as relevant animal studies that have both addressed, and in some cases, fueled this controversy.

RECENT FINDINGS

Methodologic developments in genotyping and sequencing, such as genome-wide association studies (GWAS), exome sequencing, and exome array genotyping, have been applied to the study of HDL-C and risk of CHD in large, multi-ethnic populations. Some of these efforts focused on population-wide variation in common variants have uncovered new polymorphisms at novel loci associated with HDL-C and, in some cases, CHD risk. Other efforts have discovered loss-of-function variants for the first time in genes previously implicated in HDL metabolism through common variant studies or animal models. These studies have allowed the genetic relationship between these pathways, HDL-C and CHD to be explored in humans for the first time through analysis tools such as Mendelian randomization. We explore these discoveries for selected key HDL-C genes CETP, LCAT, LIPG, SCARB1, and novel loci implicated from GWAS including GALNT2, KLF14, and TTC39B. Recent human genetics findings have identified new nodes regulating HDL metabolism while reshaping our current understanding of known candidate genes to HDL and CHD risk through the study of critical variants across model systems. Despite their effect on HDL-C, variants in many of the reviewed genes were found to lack any association with CHD. These data collectively indicate that HDL-C concentration, which represents a static picture of a very dynamic and heterogeneous metabolic milieu, is unlikely to be itself causally protective against CHD. In this context, human genetics represent an extremely valuable tool to further explore the biological mechanisms regulating HDL metabolism and investigate what role, if any, HDL plays in the pathogenesis of CHD.

Polygenic determinants in extremes of high-density lipoprotein cholesterol

HDL cholesterol (HDL-C) remains a superior biochemical predictor of CVD risk, but its genetic basis is incompletely defined. In patients with extreme HDL-C concentrations, we concurrently evaluated the contributions of multiple large- and small-effect genetic variants. In a discovery cohort of 255 unrelated lipid clinic patients with extreme HDL-C levels, we used a targeted next-generation sequencing panel to evaluate rare variants in known HDL metabolism genes, simultaneously with common variants bundled into a polygenic trait score. Two additional cohorts were used for validation and included 1,746 individuals from the Montréal Heart Institute Biobank and 1,048 individuals from the University of Pennsylvania. Findings were consistent between cohorts: we found rare heterozygous large-effect variants in 18.7% and 10.9% of low- and high-HDL-C patients, respectively. We also found common variant accumulation, indicated by extreme polygenic trait scores, in an additional 12.8% and 19.3% of overall cases of low- and high-HDL-C extremes, respectively. Thus, the genetic basis of extreme HDL-C concentrations encountered clinically is frequently polygenic, with contributions from both rare large-effect and common small-effect variants. Multiple types of genetic variants should be considered as contributing factors in patients with extreme dyslipidemia.

A human APOC3 missense variant and monoclonal antibody accelerate apoC-III clearance and lower triglyceride-rich lipoprotein levels

Recent large-scale genetic sequencing efforts have identified rare coding variants in genes in the triglyceride-rich lipoprotein (TRL) clearance pathway that are protective against coronary heart disease (CHD), independently of LDL cholesterol (LDL-C) levels. Insight into the mechanisms of protection of these variants may facilitate the development of new therapies for lowering TRL levels. The gene APOC3 encodes apoC-III, a critical inhibitor of triglyceride (TG) lipolysis and remnant TRL clearance. Here we report a detailed interrogation of the mechanism of TRL lowering by the APOC3 Ala43Thr (A43T) variant, the only missense (rather than protein-truncating) variant in APOC3 reported to be TG lowering and protective against CHD. We found that both human APOC3 A43T heterozygotes and mice expressing human APOC3 A43T display markedly reduced circulating apoC-III levels. In mice, this reduction is due to impaired binding of A43T apoC-III to lipoproteins and accelerated renal catabolism of free apoC-III. Moreover, the reduced content of apoC-III in TRLs resulted in accelerated clearance of circulating TRLs. On the basis of this protective mechanism, we developed a monoclonal antibody targeting lipoprotein-bound human apoC-III that promotes circulating apoC-III clearance in mice expressing human APOC3 and enhances TRL catabolism in vivo. These data reveal the molecular mechanism by which a missense variant in APOC3 causes reduced circulating TG levels and, hence, protects from CHD. This protective mechanism has the potential to be exploited as a new therapeutic approach to reduce apoC-III levels and circulating TRL burden.

Targeting ApoC-III to Reduce Coronary Disease Risk

Triglyceride-rich lipoproteins (TRLs) are causal contributors to the risk of developing coronary artery disease (CAD). Apolipoprotein C-III (apoC-III) is a component of TRLs that elevates plasma triglycerides (TGs) through delaying the lipolysis of TGs and the catabolism of TRL remnants. Recent human genetics approaches have shown that heterozygous loss-of-function mutations in APOC3, the gene encoding apoC-III, lower plasma TGs and protect from CAD. This observation has spawned new interest in therapeutic efforts to target apoC-III. Here, we briefly review both currently available as well as developing therapies for reducing apoC-III levels and function to lower TGs and cardiovascular risk. These therapies include existing options including statins, fibrates, thiazolidinediones, omega-3-fatty acids, and niacin, as well as an antisense oligonucleotide targeting APOC3 currently in clinical development. We review the mechanisms of action by which these drugs reduce apoC-III and the current understanding of how reduction in apoC-III may impact CAD risk.

Abstract 367: Extreme High-Density Lipoprotein Cholesterol Genetics: An Assortment of Large and Small Polygenic Effects

Rationale: Although HDL-C levels are known to have a complex genetic basis, most studies have focused solely on identifying rare variants with large phenotypic effects to explain extreme HDL-C phenotypes.

Objective: Here we concurrently evaluate the contribution of both rare and common genetic variants, as well as large-scale copy number variations (CNVs), towards extreme HDL-C concentrations.

Methods: In clinically ascertained patients with low ( N =136) and high ( N =119) HDL-C profiles, we applied our targeted next-generation sequencing panel (LipidSeq TM ) to sequence genes involved in HDL metabolism, which were subsequently screened for rare variants and CNVs. We also developed a novel polygenic trait score (PTS) to assess patients’ genetic accumulations of common variants that have been shown by genome-wide association studies to associate primarily with HDL-C levels. Two additional cohorts of patients with extremely low and high HDL-C (total N =1,746 and N =1,139, respectively) were used for PTS validation.

Results: In the discovery cohort, 32.4% of low HDL-C patients carried rare variants or CNVs in primary ( ABCA1 , APOA1 , LCAT ) and secondary ( LPL , LMF1 , GPD1 , APOE ) HDL-C–altering genes. Additionally, 13.4% of high HDL-C patients carried rare variants or CNVs in primary ( SCARB1 , CETP , LIPC , LIPG ) and secondary ( APOC3 , ANGPTL4 ) HDL-C–altering genes. For polygenic effects, patients with abnormal HDL-C profiles but without rare variants or CNVs were ~2-fold more likely to have an extreme PTS compared to normolipidemic individuals, indicating an increased frequency of common HDL-C–associated variants in these patients. Similar results in the two validation cohorts demonstrate that this novel PTS successfully quantifies common variant accumulation, further characterizing the polygenic basis for extreme HDL-C phenotypes.

Conclusions: Patients with extreme HDL-C levels have various combinations of rare variants, common variants, or CNVs driving their phenotypes. Fully characterizing the genetic basis of HDL-C levels must extend to encompass multiple types of genetic determinants—not just rare variants—to further our understanding of this complex, controversial quantitative trait.

Loss of Function of GALNT2 Lowers High-Density Lipoproteins in Humans, Nonhuman Primates, and Rodents

Human genetics studies have implicated GALNT2, encoding GalNAc-T2, as a regulator of high-density lipoprotein cholesterol (HDL-C) metabolism, but the mechanisms relating GALNT2 to HDL-C remain unclear. We investigated the impact of homozygous GALNT2 deficiency on HDL-C in humans and mammalian models. We identified two humans homozygous for loss-of-function mutations in GALNT2 who demonstrated low HDL-C. We also found that GALNT2 loss of function in mice, rats, and nonhuman primates decreased HDL-C. O-glycoproteomics studies of a human GALNT2-deficient subject validated ANGPTL3 and ApoC-III as GalNAc-T2 targets. Additional glycoproteomics in rodents identified targets influencing HDL-C, including phospholipid transfer protein (PLTP). GALNT2 deficiency reduced plasma PLTP activity in humans and rodents, and in mice this was rescued by reconstitution of hepatic Galnt2. We also found that GALNT2 GWAS SNPs associated with reduced HDL-C also correlate with lower hepatic GALNT2 expression. These results posit GALNT2 as a direct modulator of HDL metabolism across mammals.

Abstract 17: APOC3 A43T Variant Promotes ApoC-III Catabolism and Accelerates TG-rich Lipoprotein Clearance in Mice and Humans

Humans with loss-of-function (LoF) variants in APOC3 , the gene encoding apolipoprotein C-III (apoC-III), have significantly reduced plasma triglycerides (TG) and protection from coronary disease. These findings suggest that apoC-III may be a viable therapeutic target for decreasing vascular risk through TG reduction, and that elucidation of the protective mechanism of APOC3 LoF variants would inform such strategies. We report here the protective mechanism of the APOC3 A43T missense variant, one of four recently identified CAD-protective variants. By genotyping >8,000 human participants with low TG, we identified 17 APOC3 A43T carriers and phenotyped 6 carriers and 54 matched controls. A43T heterozygotes demonstrate a significant reduction in apoC-III levels relative to non-carriers (50% reduction, P<0.05), resulting in decreased plasma TG (50% reduction, P<0.05). We generated viral vectors expressing WT or A43T apoC-III and expressed these in humanized mouse models to further explore the mechanism of reduced apoC-III levels due to the A43T variant. Mice expressing human CETP and the apoC-III A43T variant exhibit reduced plasma apoC-III (50% reduction, P<0.0001) despite equal hepatic expression and secretion relative to controls expressing WT human apoC-III. These mice also exhibit reduced plasma TG and VLDL-C, and increased HDL-C relative to WT-expressing mice, fully recapitulating the protective lipoprotein profile of the human A43T carriers. Radioisotope-labeled apoC-III turnover studies showed that the A43T mutation causes a >3-fold higher apoC-III clearance rate in vivo (P<0.0001) due to defective integration into lipoprotein particles and accelerated renal catabolism (40% increase, P<0.01). This results in increased lipoprotein lipase (LPL) activity (27% increase, P<0.01) and faster chylomicron-TG clearance (97% increase, P<0.01) in vivo . We are currently performing analogous studies of WT vs. A43T apoC-III turnover and VLDL clearance in human APOC3 A43T carriers. Collectively, our results support the rationale for therapeutic efforts to target circulating apoC-III through disruption of its binding to lipoproteins, mirroring the genetics-driven approaches for targeting PCSK9 that have recently yielded novel therapies.

Abstract 400: Determining the Contribution of Endothelial Lipase-mediated Lipolysis to Brain Phospholipid Metabolism

Polyunsaturated fatty acids (PUFA) are essential constituents of the cell membrane. Brain PUFA content is critical for synaptic function and neuroinflammation and PUFA deficiency may underlie several neurological disorders. Prior work in murine models has showed that the predominant source of central nervous system (CNS) PUFAs is 2-acyl-lysophophatidylcholine (2-lysoPC), a class of phospholipids (PLs) circulating on blood lipoproteins that is formed from the enzymatic activity of phospholipases. Endothelial lipase (EL) is a phospholipase that is a critical determinant of HDL metabolism in humans and the predominant generator of 2-lysoPC on HDL particles. While the contribution of EL to circulating HDL metabolism is well-established, its contribution to CNS fatty acid availability and brain function has not been explored. In order to investigate the physiological contribution of EL to brain FA availability in vivo, we studied PL-derived PUFA uptake in mice with EL deficiency. We adopted a method for measuring EL activity using a fluorescent synthetic HDL particle containing sn2-TopFluor-PC, a fluorescent precursor of 2-lysoPC and demonstrated its utility in vitro. Next, we tested the clearance of this synthetic HDL-derived 2-lysoPC in C57BL/6 (WT) vs EL KO mice (n=12/group). Circulating plasma 2-lysoPC clearance was significantly impaired in in EL KO mice (-32%, p=0.024) and was accompanied by a slower remodeling of the HDL particles. Measurement of labeled PL content in tissues showed a significant reduction in hepatic uptake in KO mice (22% decrease, p=0.002). No significant differences were observed in total brain PL uptake, but the distribution of PL uptake differed between WT and KOs. Ongoing studies are evaluating the physiological consequences of reduced circulating 2-lysoPC clearance, reduced hepatic uptake, and altered distribution of brain PL uptake in EL KO mice. Collectively, our data demonstrate that EL is a key physiological regulator of systemic PL catabolism.

Rare variant in scavenger receptor BI raises HDL cholesterol and increases risk of coronary heart disease

Scavenger receptor BI (SR-BI) is the major receptor for high-density lipoprotein (HDL) cholesterol (HDL-C). In humans, high amounts of HDL-C in plasma are associated with a lower risk of coronary heart disease (CHD). Mice that have depleted Scarb1 (SR-BI knockout mice) have markedly elevated HDL-C levels but, paradoxically, increased atherosclerosis. The impact of SR-BI on HDL metabolism and CHD risk in humans remains unclear. Through targeted sequencing of coding regions of lipid-modifying genes in 328 individuals with extremely high plasma HDL-C levels, we identified a homozygote for a loss-of-function variant, in which leucine replaces proline 376 (P376L), in SCARB1, the gene encoding SR-BI. The P376L variant impairs posttranslational processing of SR-BI and abrogates selective HDL cholesterol uptake in transfected cells, in hepatocyte-like cells derived from induced pluripotent stem cells from the homozygous subject, and in mice. Large population-based studies revealed that subjects who are heterozygous carriers of the P376L variant have significantly increased levels of plasma HDL-C. P376L carriers have a profound HDL-related phenotype and an increased risk of CHD (odds ratio = 1.79, which is statistically significant).

A systematic study of modulation of ADAM-mediated ectodomain shedding by site-specific O-glycosylation

Regulated shedding of the ectodomain of cell membrane proteins by proteases is a common process that releases the extracellular domain from the cell and activates cell signaling. Ectodomain shedding occurs in the immediate extracellular juxtamembrane region, which is also where O-glycosylation is often found and examples of crosstalk between shedding and O-glycosylation have been reported. Here, we systematically investigated the potential of site-specific O-glycosylation mediated by distinct polypeptide GalNAc-transferase (GalNAc-T) isoforms to coregulate ectodomain shedding mediated by the A Disintegrin And Metalloproteinase (ADAM) subfamily of proteases and in particular ADAM17. We analyzed 25 membrane proteins that are known to undergo ADAM17 shedding and where the processing sites included Ser/Thr residues within ± 4 residues that could represent O-glycosites. We used in vitro GalNAc-T enzyme and ADAM cleavage assays to demonstrate that shedding of at least 12 of these proteins are potentially coregulated by O-glycosylation. Using TNF-α as an example, we confirmed that shedding mediated by ADAM17 is coregulated by O-glycosylation controlled by the GalNAc-T2 isoform both ex vivo in isogenic cell models and in vivo in mouse Galnt2 knockouts. The study provides compelling evidence for a wider role of site-specific O-glycosylation in ectodomain shedding.

Relation of atherogenic lipoproteins with estimated glomerular filtration rate decline: a longitudinal study

BACKGROUND

Chronic kidney disease (CKD) is associated with dyslipidemia, but the role of atherogenic lipid fractions in CKD progression remains unclear. Here we assess whether baseline plasma levels of lipoprotein(a) [Lp(a)] and apolipoprotein C-III (apoC-III), causal cardiovascular (CV) risk factors being studied as therapeutic targets, are associated with decreasing estimated glomerular filtration rate (eGFR) over time.

METHODS

In the Penn Diabetes Heart Study (PDHS), a single-center observational cohort of type 2 diabetes patients without clinical CV disease or pre-existing CKD, we performed linear mixed effects modeling with incremental multivariable analysis to evaluate the effects of baseline plasma Lp(a) and apoC-III on the slope of eGFR over time for subjects with longitudinal data (N = 400).

RESULTS

Each two-fold higher plasma Lp(a) level was associated with an additional decline in eGFR by 0.50 mL/min/year in the fully adjusted model (p < 0.001). Baseline Lp(a) levels greater than the atherogenic cut-point of 30 mg/dL were associated with a decline in eGFR by 2.75 mL/min/year compared to 1.01 mL/min/year in subjects with baseline Lp(a) less than 30 mg/dL (p < 0.001). Although each two-fold higher apoC-III level was also associated with statistically significant decline in eGFR over time, as expected the association was attenuated after adjusting for baseline triglycerides, the key lipid intermediary regulated by apoC-III in circulation.

CONCLUSIONS

Elevated baseline plasma Lp(a) levels are associated with a decrease in eGFR over time independent of race, lipid medication use, and albuminuria, whereas elevated baseline apoC-III levels are associated with eGFR decline in a triglyceride-dependent fashion.

Plasma apolipoprotein C-III levels, triglycerides, and coronary artery calcification in type 2 diabetics

OBJECTIVE

Triglyceride-rich lipoproteins have emerged as causal risk factors for developing coronary heart disease independent of low-density lipoprotein cholesterol levels. Apolipoprotein C-III (ApoC-III) modulates triglyceride-rich lipoprotein metabolism through inhibition of lipoprotein lipase and hepatic uptake of triglyceride-rich lipoproteins. Mutations causing loss-of-function of ApoC-III lower triglycerides and reduce coronary heart disease risk, suggestive of a causal role for ApoC-III. Little data exist about the relationship of ApoC-III, triglycerides, and atherosclerosis in patients with type 2 diabetes mellitus (T2DM). Here, we examined the relationships between plasma ApoC-III, triglycerides, and coronary artery calcification in patients with T2DM.

APPROACH AND RESULTS

Plasma ApoC-III levels were measured in a cross-sectional study of 1422 subjects with T2DM but without clinically manifest coronary heart disease. ApoC-III levels were positively associated with total cholesterol (Spearman r=0.36), triglycerides (r=0.59), low-density lipoprotein cholesterol (r=0.16), fasting glucose (r=0.16), and glycosylated hemoglobin (r=0.12; P<0.0001 for all). In age, sex, and race-adjusted analysis, ApoC-III levels were positively associated with coronary artery calcification (Tobit regression ratio, 1.78; 95% confidence interval, 1.27-2.50 per SD increase in ApoC-III; P<0.001). As expected for an intermediate mediator, these findings were attenuated when adjusted for both triglycerides (Tobit regression ratio, 1.43; 95% confidence interval, 0.94-2.18; P=0.086) and separately for very low-density lipoprotein cholesterol (Tobit regression ratio, 1.14; 95% confidence interval, 0.75-1.71; P=0.53).

CONCLUSIONS

In persons with T2DM, increased plasma ApoC-III is associated with higher triglycerides, less favorable cardiometabolic phenotypes, and higher coronary artery calcification, a measure of subclinical atherosclerosis. Therapeutic inhibition of ApoC-III may thus be a novel strategy for reducing plasma triglyceride-rich lipoproteins and cardiovascular risk in T2DM.

Abstract 332: Coronary Artery Disease-Protective Variant A43T in APOC3 Alters Circulating ApoC-III Levels In vivo

Elevated plasma triglycerides (TG) raise risk of coronary artery disease (CAD) independently of low-density lipoprotein cholesterol (LDL-C). Recent human genetics studies have shown that genetically lower TG through loss-of-function (LoF) of APOC3 lowers risk of developing CAD. APOC3 encodes apolipoprotein C-III (ApoC-III), an apolipoprotein on VLDLs and HDLs that inhibits the lipoprotein lipase (LPL) pathway of postprandial TG clearance. The specific molecular mechanisms underlying the reduction of TG and CAD risk by APOC3 LoF mutations are not known. Here, we study the mechanism of LoF for one of the 4 disease-protective APOC3 coding variants, A43T, in humans and rodent models. We recruited human subjects with this variant for deep phenotyping of TG metabolism and show that carriers of this variant have lower plasma ApoC-III in vivo, and possibly increased LPL activity. Using an adeno-associated virus (AAV) vector to express WT vs. mutant human APOC3 in mice, we show that the A43T variant also lowers circulating ApoC-III levels. Additional studies are ongoing to determine the mechanism of lower stability of ApoC-III A43T on TG-rich lipoprotein particles.

Genetics of lipid traits: Genome-wide approaches yield new biology and clues to causality in coronary artery disease

A wealth of novel lipid loci have been identified through a variety of approaches focused on common and low-frequency variation and collaborative metaanalyses in multiethnic populations. Despite progress in identification of loci, the task of determining causal variants remains challenging. This work will undoubtedly be enhanced by improved understanding of regulatory DNA at a genomewide level as well as new methodologies for interrogating the relationships between noncoding SNPs and regulatory regions. Equally challenging is the identification of causal genes at novel loci. Some progress has been made for a handful of genes and comprehensive testing of candidate genes using multiple model systems is underway. Additional insights will be gleaned from focusing on low frequency and rare coding variation at candidate loci in large populations. This article is part of a Special Issue entitled: From Genome to Function.

Mining the LIPG allelic spectrum reveals the contribution of rare and common regulatory variants to HDL cholesterol

Genome-wide association studies (GWAS) have successfully identified loci associated with quantitative traits, such as blood lipids. Deep resequencing studies are being utilized to catalogue the allelic spectrum at GWAS loci. The goal of these studies is to identify causative variants and missing heritability, including heritability due to low frequency and rare alleles with large phenotypic impact. Whereas rare variant efforts have primarily focused on nonsynonymous coding variants, we hypothesized that noncoding variants in these loci are also functionally important. Using the HDL-C gene LIPG as an example, we explored the effect of regulatory variants identified through resequencing of subjects at HDL-C extremes on gene expression, protein levels, and phenotype. Resequencing a portion of the LIPG promoter and 5' UTR in human subjects with extreme HDL-C, we identified several rare variants in individuals from both extremes. Luciferase reporter assays were used to measure the effect of these rare variants on LIPG expression. Variants conferring opposing effects on gene expression were enriched in opposite extremes of the phenotypic distribution. Minor alleles of a common regulatory haplotype and noncoding GWAS SNPs were associated with reduced plasma levels of the LIPG gene product endothelial lipase (EL), consistent with its role in HDL-C catabolism. Additionally, we found that a common nonfunctional coding variant associated with HDL-C (rs2000813) is in linkage disequilibrium with a 5' UTR variant (rs34474737) that decreases LIPG promoter activity. We attribute the gene regulatory role of rs34474737 to the observed association of the coding variant with plasma EL levels and HDL-C. Taken together, the findings show that both rare and common noncoding regulatory variants are important contributors to the allelic spectrum in complex trait loci.

Loss-of-function variants in endothelial lipase are a cause of elevated HDL cholesterol in humans

Elevated plasma concentrations of HDL cholesterol (HDL-C) are associated with protection from atherosclerotic cardiovascular disease. Animal models indicate that decreased expression of endothelial lipase (LIPG) is inversely associated with HDL-C levels, and genome-wide association studies have identified LIPG variants as being associated with HDL-C levels in humans. We hypothesized that loss-of-function mutations in LIPG may result in elevated HDL-C and therefore performed deep resequencing of LIPG exons in cases with elevated HDL-C levels and controls with decreased HDL-C levels. We identified a significant excess of nonsynonymous LIPG variants unique to cases with elevated HDL-C. In vitro lipase activity assays demonstrated that these variants significantly decreased endothelial lipase activity. In addition, a meta-analysis across 5 cohorts demonstrated that the low-frequency Asn396Ser variant is significantly associated with increased HDL-C, while the common Thr111Ile variant is not. Functional analysis confirmed that the Asn396Ser variant has significantly decreased lipase activity both in vitro and in vivo, while the Thr111Ile variant has normal lipase activity. Our results establish that loss-of-function mutations in LIPG lead to increased HDL-C levels and support the idea that inhibition of endothelial lipase may be an effective mechanism to raise HDL-C.