Compound heterozygosity for mutant hepatic lipase in familial hepatic lipase deficiency

Whole-exome sequencing identifies novel protein-altering variants associated with serum apolipoprotein and lipid concentrations

BACKGROUND

Dyslipidemia is a major risk factor for cardiovascular disease, and diabetes impacts the lipid metabolism through multiple pathways. In addition to the standard lipid measurements, apolipoprotein concentrations provide added awareness of the burden of circulating lipoproteins. While common genetic variants modestly affect the serum lipid concentrations, rare genetic mutations can cause monogenic forms of hypercholesterolemia and other genetic disorders of lipid metabolism. We aimed to identify low-frequency protein-altering variants (PAVs) affecting lipoprotein and lipid traits.

METHODS

We analyzed whole-exome (WES) and whole-genome sequencing (WGS) data of 481 and 474 individuals with type 1 diabetes, respectively. The phenotypic data consisted of 79 serum lipid and apolipoprotein phenotypes obtained with clinical laboratory measurements and nuclear magnetic resonance spectroscopy.

RESULTS

The single-variant analysis identified an association between the LIPC p.Thr405Met (rs113298164) and serum apolipoprotein A1 concentrations (p=7.8×10-8). The burden of PAVs was significantly associated with lipid phenotypes in LIPC, RBM47, TRMT5, GTF3C5, MARCHF10, and RYR3 (p<2.9×10-6). The RBM47 gene is required for apolipoprotein B post-translational modifications, and in our data, the association between RBM47 and apolipoprotein C-III concentrations was due to a rare 21 base pair p.Ala496-Ala502 deletion; in replication, the burden of rare deleterious variants in RBM47 was associated with lower triglyceride concentrations in WES of >170,000 individuals from multiple ancestries (p=0.0013). Two PAVs in GTF3C5 were highly enriched in the Finnish population and associated with cardiovascular phenotypes in the general population. In the previously known APOB gene, we identified novel associations at two protein-truncating variants resulting in lower serum non-HDL cholesterol (p=4.8×10-4), apolipoprotein B (p=5.6×10-4), and LDL cholesterol (p=9.5×10-4) concentrations.

CONCLUSIONS

We identified lipid and apolipoprotein-associated variants in the previously known LIPC and APOB genes, as well as PAVs in GTF3C5 associated with LDLC, and in RBM47 associated with apolipoprotein C-III concentrations, implicated as an independent CVD risk factor. Identification of rare loss-of-function variants has previously revealed genes that can be targeted to prevent CVD, such as the LDL cholesterol-lowering loss-of-function variants in the PCSK9 gene. Thus, this study suggests novel putative therapeutic targets for the prevention of CVD.

Hyperalphalipoproteinemia and Beyond: The Role of HDL in Cardiovascular Diseases

Hyperalphalipoproteinemia (HALP) is a lipid disorder characterized by elevated plasma high-density lipoprotein cholesterol (HDL-C) levels above the 90th percentile of the distribution of HDL-C values in the general population. Secondary non-genetic factors such as drugs, pregnancy, alcohol intake, and liver diseases might induce HDL increases. Primary forms of HALP are caused by mutations in the genes coding for cholesteryl ester transfer protein (CETP), hepatic lipase (HL), apolipoprotein C-III (apo C-III), scavenger receptor class B type I (SR-BI) and endothelial lipase (EL). However, in the last decades, genome-wide association studies (GWAS) have also suggested a polygenic inheritance of hyperalphalipoproteinemia. Epidemiological studies have suggested that HDL-C is inversely correlated with cardiovascular (CV) risk, but recent Mendelian randomization data have shown a lack of atheroprotective causal effects of HDL-C. This review will focus on primary forms of HALP, the role of polygenic inheritance on HDL-C, associated risk for cardiovascular diseases and possible treatment options.

2019 George Lyman Duff Memorial Lecture

Dyslipidemias include both rare single gene disorders and common conditions that have a complex underlying basis. In London, ON, there is fortuitous close physical proximity between the Lipid Genetics Clinic and the London Regional Genomics Centre. For >30 years, we have applied DNA sequencing of clinical samples to help answer scientific questions. More than 2000 patients referred with dyslipidemias have participated in an ongoing translational research program. In 2013, we transitioned to next-generation sequencing; our targeted panel is designed to concurrently assess both monogenic and polygenic contributions to dyslipidemias. Patient DNA is screened for rare variants underlying 25 mendelian dyslipidemias, including familial hypercholesterolemia, hepatic lipase deficiency, abetalipoproteinemia, and familial chylomicronemia syndrome. Furthermore, polygenic scores for LDL (low-density lipoprotein) and HDL (high-density lipoprotein) cholesterol, and triglycerides are calculated for each patient. We thus simultaneously document both rare and common genetic variants, allowing for a broad view of genetic predisposition for both individual patients and cohorts. For instance, among patients referred with severe hypertriglyceridemia, defined as ≥10 mmol/L (≥885 mg/dL), <1% have a mendelian disorder (ie, autosomal recessive familial chylomicronemia syndrome), ≈15% have heterozygous rare variants (a >3-fold increase over normolipidemic individuals), and ≈35% have an extreme polygenic score (a >3-fold increase over normolipidemic individuals). Other dyslipidemias show a different mix of genetic determinants. Genetic results are discussed with patients and can support clinical decision-making. Integrating DNA testing into clinical care allows for a bidirectional flow of information, which facilitates scientific discoveries and clinical translation.

Genetic and Epigenetic Fine Mapping of Complex Trait Associated Loci in the Human Liver

Deciphering the impact of genetic variation on gene regulation is fundamental to understanding common, complex human diseases. Although histone modifications are important markers of gene regulatory elements of the genome, any specific histone modification has not been assayed in more than a few individuals in the human liver. As a result, the effects of genetic variation on histone modification states in the liver are poorly understood. Here, we generate the most comprehensive genome-wide dataset of two epigenetic marks, H3K4me3 and H3K27ac, and annotate thousands of putative regulatory elements in the human liver. We integrate these findings with genome-wide gene expression data collected from the same human liver tissues and high-resolution promoter-focused chromatin interaction maps collected from human liver-derived HepG2 cells. We demonstrate widespread functional consequences of natural genetic variation on putative regulatory element activity and gene expression levels. Leveraging these extensive datasets, we fine-map a total of 74 GWAS loci that have been associated with at least one complex phenotype. Our results reveal a repertoire of genes and regulatory mechanisms governing complex disease development and further the basic understanding of genetic and epigenetic regulation of gene expression in the human liver tissue.

Mining the genome for lipid genes

Mining of the genome for lipid genes has since the early 1970s helped to shape our understanding of how triglycerides are packaged (in chylomicrons), repackaged (in very low density lipoproteins; VLDL), and hydrolyzed, and also how remnant and low-density lipoproteins (LDL) are cleared from the circulation. Gene discoveries have also provided insights into high-density lipoprotein (HDL) biogenesis and remodeling. Interestingly, at least half of these key molecular genetic studies were initiated with the benefit of prior knowledge of relevant proteins. In addition, multiple important findings originated from studies in mouse, and from other types of non-genetic approaches. Although it appears by now that the main lipid pathways have been uncovered, and that only modulators or adaptor proteins such as those encoded by LDLRAP1, APOA5, ANGPLT3/4, and PCSK9 are currently being discovered, genome wide association studies (GWAS) in particular have implicated many new loci based on statistical analyses; these may prove to have equally large impacts on lipoprotein traits as gene products that are already known. On the other hand, since 2004 - and particularly since 2010 when massively parallel sequencing has become de rigeur - no major new insights into genes governing lipid metabolism have been reported. This is probably because the etiologies of true Mendelian lipid disorders with overt clinical complications have been largely resolved. In the meantime, it has become clear that proving the importance of new candidate genes is challenging. This could be due to very low frequencies of large impact variants in the population. It must further be emphasized that functional genetic studies, while necessary, are often difficult to accomplish, making it hazardous to upgrade a variant that is simply associated to being definitively causative. Also, it is clear that applying a monogenic approach to dissect complex lipid traits that are mostly of polygenic origin is the wrong way to proceed. The hope is that large-scale data acquisition combined with sophisticated computerized analyses will help to prioritize and select the most promising candidate genes for future research. We suggest that at this point in time, investment in sequence technology driven candidate gene discovery could be recalibrated by refocusing efforts on direct functional analysis of the genes that have already been discovered. This article is part of a Special Issue entitled: From Genome to Function.

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.

Biochemistry and pathophysiology of intravascular and intracellular lipolysis

All organisms use fatty acids (FAs) for energy substrates and as precursors for membrane and signaling lipids. The most efficient way to transport and store FAs is in the form of triglycerides (TGs); however, TGs are not capable of traversing biological membranes and therefore need to be cleaved by TG hydrolases ("lipases") before moving in or out of cells. This biochemical process is generally called "lipolysis." Intravascular lipolysis degrades lipoprotein-associated TGs to FAs for their subsequent uptake by parenchymal cells, whereas intracellular lipolysis generates FAs and glycerol for their release (in the case of white adipose tissue) or use by cells (in the case of other tissues). Although the importance of lipolysis has been recognized for decades, many of the key proteins involved in lipolysis have been uncovered only recently. Important new developments include the discovery of glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), the molecule that moves lipoprotein lipase from the interstitial spaces to the capillary lumen, and the discovery of adipose triglyceride lipase (ATGL) and comparative gene identification-58 (CGI-58) as crucial molecules in the hydrolysis of TGs within cells. This review summarizes current views of lipolysis and highlights the relevance of this process to human disease.

Reciprocal Hemizygosity Analysis of Mouse Hepatic Lipase Reveals Influence on Obesity

OBJECTIVES

We previously demonstrated coincident quantitative trait loci (QTLs) for percentage body fat, plasma hepatic lipase (HL) activity, and plasma cholesterol on mouse chromosome 7. In the present study, we investigated whether hepatic lipase (Lipc) is an obesity gene, whether Lipc interacts with an unknown gene on chromosome 7, and how HL activity is linked to the chromosome 7 locus.

RESEARCH METHODS AND PROCEDURES

BSB mice are a model of complex obesity due to interactions among genes from C57BL/6J and Mus spretus (SPRET) in (C57BL/6J x SPRET) x C57BL/6J backcross mice. Five crosses tested the impact on obesity of combinations of inactive (knockout) and wild-type Lipc alleles from C57BL/6J or SPRET in a reciprocal hemizygosity analysis.

RESULTS

The combined data from this allelic series suggest that Lipc alleles, and not alleles from a gene linked to Lipc, influence obesity. No interaction between Lipc and chromosome 7 was demonstrated. We confirmed the chromosome 7 QTLs for obesity, HL activity, and cholesterol. Because obesity and HL activity are not consistently associated in the BSB model, linkage of HL activity to chromosome 7 is not secondary to obesity per se. We also report, for the first time to our knowledge, a QTL in mammals for food intake.

DISCUSSION

This use of reciprocal hemizygosity analysis in mammals, which, to our knowledge, is the first reported, reveals its power to detect previously unknown effects of Lipc on obesity.

VLDL hydrolysis by hepatic lipase regulates PPARδ transcriptional responses

BACKGROUND

PPARs (α,γ,δ) are a family of ligand-activated transcription factors that regulate energy balance, including lipid metabolism. Despite these critical functions, the integration between specific pathways of lipid metabolism and distinct PPAR responses remains obscure. Previous work has revealed that lipolytic pathways can activate PPARs. Whether hepatic lipase (HL), an enzyme that regulates VLDL and HDL catabolism, participates in PPAR responses is unknown.

METHODS/PRINCIPAL FINDINGS

Using PPAR ligand binding domain transactivation assays, we found that HL interacted with triglyceride-rich VLDL (>HDL≫LDL, IDL) to activate PPARδ preferentially over PPARα or PPARγ, an effect dependent on HL catalytic activity. In cell free ligand displacement assays, VLDL hydrolysis by HL activated PPARδ in a VLDL-concentration dependent manner. Extended further, VLDL stimulation of HL-expressing HUVECs and FAO hepatoma cells increased mRNA expression of canonical PPARδ target genes, including adipocyte differentiation related protein (ADRP), angiopoietin like protein 4 and pyruvate dehydrogenase kinase-4. HL/VLDL regulated ADRP through a PPRE in the promoter region of this gene. In vivo, adenoviral-mediated hepatic HL expression in C57BL/6 mice increased hepatic ADRP mRNA levels by 30%. In ob/ob mice, a model with higher triglycerides than C57BL/6 mice, HL overexpression increased ADRP expression by 70%, demonstrating the importance of triglyceride substrate for HL-mediated PPARδ activation. Global metabolite profiling identified HL/VLDL released fatty acids including oleic acid and palmitoleic acid that were capable of recapitulating PPARδ activation and ADRP gene regulation in vitro.

CONCLUSIONS

These data define a novel pathway involving HL hydrolysis of VLDL that activates PPARδ through generation of specific monounsaturated fatty acids. These data also demonstrate how integrating cell biology with metabolomic approaches provides insight into specific lipid mediators and pathways of lipid metabolism that regulate transcription.

Association of an intronic haplotype of the LIPC gene with hyperalphalipoproteinemia in two independent populations

Hepatic lipase (HL) plays a major role in the regulation of plasma lipids. Several groups seeking to find association between the gene encoding HL (LIPC) and plasma concentrations of high-density lipoprotein cholesterol (HDLc) using various methods and populations have reported conflicting results. We have approached the problem of demonstrating a relationship between the LIPC locus and HDLc by means of haplotype association using four single nucleotide polymorphisms (SNPs) (rs12594375G/A, rs8023503C/T, rs4775047C/T, and rs11634134T/A) located in intron 1 of the LIPC gene in two independent Japanese populations consisting of 2,970 and 1,638 individuals, respectively. Significant association between hyperalphalipoproteinemia and a specific haplotype in this intron was detected in both populations. When HDLc levels among the three haplotypic categories were analyzed [haplotype rs8023503C/rs12594375G (haplotype-1; H1) homozygotes (H1H1), haplotype rs8023503T/rs12594375A (haplotype-2; H2) homozygotes (H2H2), and heterozygotes (H1H2)], HDLc levels were lowest among H1H1 [mean +/- standard error (SE) = 58.4 +/- 0.4 mg/dl], highest among H2H2 (62.5 +/- 0.8 mg/dl), and intermediate among H1H2 (59.2 +/- 0.4 mg/dl) (P = 0.00011), indicating that H2 haplotype elevates plasma HDLc levels. This association was validated in the second population (n = 1,638) (P = 0.00070). The results provide convincing evidence that the LIPC locus influences HDL metabolism.

Characterization of a novel mutation causing hepatic lipase deficiency among French Canadians

Individuals with hepatic lipase (HL) deficiency are often characterized by elevated levels of triglycerides (TGs) and cholesterol. The aim of the present study was to characterize the molecular defect leading to severe HL deficiency in a Québec-based kindred. In the proband and two of her brothers, the very low to undetectable HL activity resulted from compound heterozygosity for two rare HL gene mutations, a previously unknown missense mutation in exon 5 designated A174T and the previously reported T383M mutation in exon 8 of the HL gene. The mutation at codon 174 resulted in the substitution of alanine for threonine, a polar amino acid, in a highly conserved nonpolar region of the protein involved in the catalytic activity of the enzyme. The severe HL deficiency among the three related compound heterozygotes was associated with a marked TG enrichment of LDL and HDL particles. The two men with severe HL deficiency also presented with abdominal obesity, which appeared to amplify the impact of HL deficiency on plasma TG-rich lipoprotein levels. Our results demonstrated that HL deficiency in this Québec kindred is associated with an abnormal lipoprotein-lipid profile, which may vary considerably in the presence of secondary factors such as abdominal obesity.

Hepatic lipase deficiency

Hepatic lipase (HL) is an enzyme that is made primarily by hepatocytes (and also found in adrenal gland and ovary) and hydrolyzes phospholipids and triglycerides of plasma lipoproteins. It is secreted and bound to the hepatocyte surface and readily released by heparin. It is a member of the lipase superfamily and is homologous to lipoprotein lipase and pancreatic lipase. The enzyme can be divided into an NH2-terminal domain containing the catalytic site joined by a short spanning region to a smaller COOH-terminal domain. The NH2-terminal portion contains an active site serine in a pentapeptide consensus sequence, Gly-Xaa-Ser-Xaa-Gly, as part of a classic Ser-Asp-His catalytic triad, and a putative hinged loop structure covering the active site. The COOH-terminal domain contains a putative lipoprotein-binding site. The heparin-binding sites may be distributed throughout the molecule, with the characteristic elution pattern from heparin-sepharose determined by the COOH-terminal domain. Of the three N-linked glycosylation sites, Asn-56 is required for efficient secretion and enzymatic activity. HL is hypothesized to directly couple HDL lipid metabolism to tissue/cellular lipid metabolism. The potential significance of the HL pathway is that it provides the hepatocyte with a mechanism for the uptake of a subset of phospholipids enriched in unsaturated fatty acids and may allow the uptake of cholesteryl ester, free cholesterol, and phospholipid without catabolism of HDL apolipoproteins. HL can hydrolyze triglyceride and phospholipid in all lipoproteins, but is predominant in the conversion of intermediate density lipoproteins to LDL and the conversion of post-prandial triglyceride-rich HDL into the postabsorptive triglyceride-poor HDL. HL plays a secondary role in the clearance of chylomicron remnants by the liver. Human post-heparin HL activity is inversely correlated with intermediate density lipoprotein cholesterol concentration only in subjects with a hyperlipidemia involving VLDL. This is consistent with intermediate-density lipoproteins being a substrate for HL. HDL cholesterol has been reported to be inversely correlated to HL activity, and on this basis it has been suggested that lowering HL would increase HDL cholesterol. However, the correlation could also be due to a common hormonal factor such as estrogen, which has been shown to up-regulate apoAI and HDL cholesterol and lower HL. A striking feature of severe deficiency of HL is the increase in HDL cholesterol and apolipoprotein AI and an approximately 10-fold increase in HDL triglyceride. Hyper-alpha-triglyceridemia is not a feature of antiatherogenic HDL. HL binds not only to heparan, but also to the LDL receptor-related protein. It has been suggested that enzymatically inactive HL can play a role in hepatic lipoprotein uptake, forming a "bridge" by binding to the lipoprotein and to the cell surface. This raises the interesting possibility that production and secretion of mutant inactive HL could promote clearance of VLDL remnants. We have described a rare family with HL deficiency. Affected patients are compound heterozygotes for a mutation of Ser267 to Phe that results in an inactive enzyme and a mutation of Thr383 to Met that results in impaired secretion and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease. Recently, it has been reported that mutations affecting the structure of HL (e.g., T383M) are relatively frequent in the Finnish population. A C-to-T polymorphism in the promotor region of the HL gene is associated with lowered HL activity and less strongly with increased HDL cholesterol. In summary, there is a good understanding of what HL does in lipoprotein metabolism; however, there is little understanding of its physiological importance, that is, why HL does what it does. (ABSTRACT TRUNCATED)

Association of variation in hepatic lipase activity with promoter variation in the hepatic lipase gene. The LOCAT Study Invsestigators

The associations between six genetic polymorphisms in the hepatic lipase (HL) gene (LIPC) and variation in postheparin HL activity and fasting serum lipoproteins were evaluated in 395 male Finnish coronary heart disease patients with HDL cholesterol concentrations </= 1.1 mmol/liter. The LIPC promoter polymorphism at position -514 was highly significantly associated with variation in HL activity (P = 0.0000008), with mean activities of 20.4, 17.5, and 13.2 mumol free fatty acid/ml per hour in subjects having C/C, C/T, and T/T genotypes, respectively. Furthermore, the triglyceride content of low density lipoprotein, intermediate density lipoprotein and HDL, and the cholesterol content of intermediate density lipoprotein were found to be associated with variation at LIPC position -514. However, there was no association of this polymorphism with coronary heart disease. These data suggest that the LIPC promoter variation is likely to be the basis for variation in HL activity, which underlies the variation in serum lipoprotein phenotypes in this sample.

Overexpression of hepatic lipase in transgenic mice decreases apolipoprotein B-containing and high density lipoproteins. Evidence that hepatic lipase acts as a ligand for lipoprotein uptake

To determine the mechanisms by which human hepatic lipase (HL) contributes to the metabolism of apolipoprotein (apo) B-containing lipoproteins and high density lipoproteins (HDL) in vivo, we developed and characterized HL transgenic mice. HL was localized by immunohistochemistry to the liver and to the adrenal cortex. In hemizygous (hHLTg+/0) and homozygous (hHLTg+/+) mice, postheparin plasma HL activity increased by 25- and 50-fold and plasma cholesterol levels decreased by 80% and 85%, respectively. In mice fed a high fat, high cholesterol diet to increase endogenous apoB-containing lipoproteins, plasma cholesterol decreased 33% (hHLTg+/0) and 75% (hHLTg+/+). Both apoB-containing remnant lipoproteins and HDL were reduced. To extend this observation, the HL transgene was expressed in human apoB transgenic (huBTg) and apoE-deficient (apoE-/-) mice, both of which have high plasma levels of apoB-containing lipoproteins. (Note that the huBTg mice that were used in these studies were all hemizygous for the human apoB gene.) In both the huBTg,hHLTg+/0 mice and the apoE-/-,hHLTg+/0 mice, plasma cholesterol decreased by 50%. This decrease was reflected in both the apoB-containing and the HDL fractions. To determine if HL catalytic activity is required for these decreases, we expressed catalytically inactive HL (HL-CAT) in apoE-/- mice. The postheparin plasma HL activities were similar in the apoE-/- and the apoE-/-,HL-CAT+/0 mice, reflecting the activity of the endogenous mouse HL and confirming that the HL-CAT was catalytically inactive. However, the postheparin plasma HL activity was 20-fold higher in the apoE-/-,hHLTg+/0 mice, indicating expression of the active human HL. Immunoblotting demonstrated high levels of human HL in postheparin plasma of both apoE-/-,hHLTg+/0 and apoE-/-,HL-CAT+/0 mice. Plasma cholesterol and apoB-containing lipoprotein levels were approximately 60% lower in apoE-/-,HL-CAT+/0 mice than in apoE-/- mice. However, the HDL were only minimally reduced. Thus, the catalytic activity of HL is critical for its effects on HDL but not for its effects on apoB-containing lipoproteins. These results provide evidence that HL can act as a ligand to remove apoB-containing lipoproteins from plasma.

Reverse cholesterol transport--a review of the process and its clinical implications

OBJECTIVES

This review article will summarize the current knowledge surrounding the reverse cholesterol transport system; the process, the effect of mutations in genes coding for proteins which function in the system, and the possible clinical implications of these alterations.

RESULTS

High-density lipoprotein-cholesterol (HDL-C) concentration is a marker for the reverse cholesterol transport (RCT) system, whereby cholesterol is returned from peripheral cells to the liver for reuse or excretion in the bile. Increased HDL-C concentrations are generally accepted to be protective against the future development of atherosclerosis and coronary artery disease (CAD), but recent evidence has indicated that the underlying cause of the increased HDL-C may affect whether it is protective or detrimental. The major steps in the RCT pathway are the efflux of free cholesterol from cells and binding by pre-beta HDL, esterification of HDL-bound cholesterol by lecithin cholesterol acyl transferase (LCAT), cholesteryl ester transfer protein (CETP) mediated exchange of cholesteryl ester and triglycerides between HDL and apo B-containing particles, and hepatic lipase (HL) mediated uptake of cholesterol and triglycerides by the liver. Mutations in proteins active in the RCT pathway can shed light on the functions and control of the various steps in the system. LCAT deficiency, leading to greatly reduced HDL and fish eye disease, is not usually associated with increased risk of CAD. Several new mutations in LCAT have recently been reported, however, which do result in CAD. Mutations leading to reduced CETP activity result in less CE being directed into apo-B containing particles and more remaining in the HDL. This has been associated with increased HDL-C concentrations. The generally accepted hypothesis that reduced CETP activity leads to reduced CAD risk has been challenged by a number of recent publications, and has become an area of active investigation. Mutations leading to reduced HL activity are rare occurrences. To date, all have been associated with increased HDL-C concentrations and CAD.

CONCLUSION

The development of techniques to identify and characterize the functional significance of mutations in proteins involved in RCT will aid in the understanding of the mechanisms and control of this pathway.

Hepatic lipase gene therapy in hepatic lipase-deficient mice. Adenovirus-mediated replacement of a lipolytic enzyme to the vascular endothelium

Hepatic lipase (HL) is an endothelial-bound lipolytic enzyme which functions as a phospholipase as well as a triacylglycerol hydrolase and is necessary for the metabolism of IDL and HDL. To evaluate the feasibility of replacing an enzyme whose in vivo physiologic function depends on its localization on the vascular endothelium, we have infused recombinant replication-deficient adenovirus vectors expressing either human HL (HL-rAdV; n = 7) or luciferase cDNA (Lucif-rAdV; n = 4) into HL-deficient mice with pretreatment plasma cholesterol, phospholipid, and HDL cholesterol values of 176 +/- 9, 314 +/- 12, and 129 +/- 9, respectively. After infusion of HL-rAdV, HL could be detected in the postheparin plasma of HL-deficient mice by immunoblotting and postheparin plasma HL activities were 25,700 +/- 4,810 and 1,510 +/- 688 nmol/min/ml on days 5 and 15, respectively. Unlike the mouse HL, 97% of the newly synthesized human HL was heparin releasable, indicating that the human enzyme was virtually totally bound to the mouse vascular endothelium. Infusion of HL-rAdV in HL-deficient mice was associated with a 50-80% decrease in total cholesterol, triglyceride, phospholipids, cholesteryl ester, and HDL cholesterol (P < 0.001) as well as normalization of the plasma fast protein liquid chromatography lipoprotein profile by day 8. These studies demonstrate successful expression and delivery of a lipolytic enzyme to the vascular endothelium for ultimate correction of the HL gene defect in HL-deficient mice and indicate that recombinant adenovirus vectors may be useful in the replacement of endothelial-bound lipolytic enzymes in human lipolytic deficiency states.

Post-prandial lipaemia

Post-prandial lipaemia represents the state of absorption during which TG metabolic capacity is under challenge. Low TG metabolic capacity imparts the risk of development of atherosclerosis. TG-intolerance has been shown to be an independent risk factor for CAD and impaired TG metabolic capacity could underlie a common high risk lipoprotein constellation of low HDL cholesterol and small sized HDL and LDL. Magnitude and duration of post-prandial lipaemia determine how much cholesterol is diverted from LDL and HDL into TG-rich lipoproteins through which it causes atherosclerosis. Potential means of intervention are improvement of TG metabolic capacity by reducing obesity, prescription of aerobic exercise, reduction of oxidizability of post-prandial lipoproteins by antioxidants and TG-lowering drugs.

Molecular characterization of human hepatic lipase deficiency. In vitro expression of two naturally occurring mutations

Individuals with hepatic lipase (HL) deficiency are often characterized by elevated levels of triglycerides and cholesterol and may be subject to premature atherosclerosis. Missense mutations in the HL gene have been identified in two affected families: substitutions of serine for phenylalanine at amino acid 267 and threonine for methionine at amino acid 383 (S267F and T383M, respectively). To confirm the role of S267F and T383M, respectively). To confirm the role of mutations separately into human HL cDNA by site-directed mutagenesis, and the resulting constructs were independently expressed in COS cells. HL activity and mass were measured and compared with wild-type HL transfectants to determine the effect of these mutations on lipase activity and secretion. Although similar amounts of HL protein were detected intracellularly after transfection with the wild-type and mutant constructs, S267F and T383M HL activity levels were markedly decreased: in S267F, no HL activity was detected, and activity levels in T383M were 38% of wild-type HL. Heparin-induced secretion of the two HL mutants was also severely affected: no detectable activity could be measured in the media of S267F, although some inactive mass (12% of wild-type HL) was secreted; mutant T383M secreted 4% and 20% of wild-type activity and mass, respectively. These results indicate that the single amino acid substitution present in HL S267F is sufficient to render the enzyme completely nonfunctional; in contrast, the T383M mutant retains partial activity but is poorly secreted. Thus, these defects appear capable of accounting for the HL-deficient phenotypes exhibited by individuals carrying the T383M and S267F mutations.

Beta-VLDL in hepatic lipase deficiency induces apoE-mediated cholesterol ester accumulation in macrophages

Hepatic lipase-deficient subjects in the Ontario kindred are compound heterozygotes for hepatic lipase mutations (Ser267-->Phe and Thr383-->Met). Cholesteryl ester-rich beta-very-low-density lipoprotein (beta-VLDL) accumulates in plasma and such subjects have premature atherosclerosis. To determine a possible mechanism, we hypothesized that hepatic lipase-deficient beta-VLDL, homozygous for apolipoprotein (apo) E3, would cause cholesteryl ester accumulation and foam cell formation in macrophages. beta-VLDL and pre-beta-VLDL were isolated by Pevikon electrophoresis and incubated with J774 macrophages, cells that do not secrete apoE. beta-VLDL increased cellular cholesteryl ester content 13-fold, whereas pre-beta-VLDL increased cholesteryl ester sevenfold. beta-VLDL increased acyl CoA:cholesterol acyltransferase activity fourfold (measured as [14C]oleate incorporation into cholesteryl ester). Preincubation of hepatic lipase-deficient beta-VLDL with the anti-apoE monoclonal antibody 1D7, which inhibits binding of apoE to low-density lipoprotein receptors, inhibited cellular cholesteryl ester accumulation by 75%, whereas the anti-apoB blocking monoclonal antibody 5E11 failed to inhibit cellular cholesteryl ester accumulation. In contrast to hepatic lipase deficiency, beta-VLDL from type III subjects (E2/E2) failed to increase cellular cholesteryl ester or acyl CoA:cholesterol acyltransferase more than 1.5-fold. Thus, hepatic lipase-deficient beta-VLDL readily induces cholesteryl ester accumulation in J774 macrophages, a process mediated by functional apoE3. This may explain the premature atherosclerosis observed in this kindred.

Hepatic lipase deficiency. Clinical, biochemical, and molecular genetic characteristics

Hepatic lipase (HL) is an important enzyme in the metabolism of triglyceride-rich lipoproteins and high density lipoproteins. The clinical syndrome of HL deficiency is rare and difficult to identify. We studied carriers of mutant HL to ascertain whether there are distinctive clinical and/or biochemical characteristics of the heterozygous state. In an Ontario kindred, compound heterozygosity for two HL mutations, S267F and T383M, underlies the clinical syndrome of complete HL deficiency. We report that simple heterozygotes for either HL mutant do not have a discrete lipoprotein abnormality, except for relative triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL. Postheparin HL activity is depressed to a greater degree in carriers of S267F compared with carriers of T383M. Retinyl palmitate loading studies in a compound heterozygote revealed impaired clearance of chylomicron remnants. The dyslipoproteinemia in a compound heterozygote was ameliorated by lovastatin. There was no difference in the quantity and distribution of HL mRNA in the liver of a compound heterozygote when compared with that of a normal subject. Thus, HL deficiency associated with structural variation of the HL gene is characterized by premature atherosclerosis, triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL, the presence of circulating beta-very low density lipoproteins, and abnormal catabolism of postprandial triglyceride-rich lipoproteins.

Gene-environment interactions in atherosclerosis

The importance of environment and genetics working together to shape an individual's risk for atherosclerosis seems intuitively obvious. However, it is only recently that research strategies have begun to evolve that attempt to answer questions related to apportionment of risk that is due to specific environmental and genetic factors. These factors may impact upon risk either singly or, more likely, through a complex interaction that affects the metabolic history of the whole organism. Because the genetic bases of lipid and lipoprotein metabolism have been well-studied, and because of the epidemiologic and pathobiochemical associations between genetic disorders of lipid metabolism and atherosclerosis, researchers have searched for gene-environment interactions within animal and human systems in which the phenotype is defined by some index of lipoprotein metabolism. From work done in the lipoprotein area to this point a clear case can be made for: 1) the genetic influence over the phenotypic response to an environmental stimulus; 2) the environmental modulation of the phenotypic expression of severe genetic defects. In the realm of gene-environment interactions that affect lipoprotein phenotype, diet is the best-studied environmental factor.

Metabolic and genetic control of HDL cholesterol levels

Variation of HDL cholesterol levels in man shows a strong inverse relationship to the incidence of atherosclerotic vascular disease. Thus the regulation of HDL cholesterol levels has been the subject of intense investigation. Human genetic differences in cholesteryl ester transfer protein and hepatic lipase illustrate the importance of these factors in the normal catabolism of HDL, while metabolic and population studies show that lipoprotein lipase activity plays a central role in the transfer of lipids and apoproteins into HDL. Metabolic turnover studies in humans suggest that variations in the fractional catabolism of the HDL structural proteins, apoA-I and apoA-II, account for much of the variation of HDL levels in human populations. Although the catabolism of these apolipoproteins is poorly understood, changes in the core lipid composition of HDL may lead to changes in catabolism of the HDL proteins. The core lipid composition of HDL appears to be determined by lipid transfer processes, and the activities of lipoprotein and hepatic lipase. Thus many genetic and environmental factors that influence HDL levels appear to operate by changing the activities of the lipases or the lipid transfer process.

Human hepatic lipase mutations and polymorphisms

Human hepatic lipase (HL) is a 477 residue glycoprotein that hydrolyzes triglycerides from plasma lipoproteins. Familial HL deficiency is a rare recessive disorder that is characterized by premature atherosclerosis and abnormal circulating lipoproteins. While studying the HL gene from the world's index family with HL deficiency, we identified four coding sequence variants of HL, one in each of exons 4, 5, 6, and 8. In this report we present the genetic basis for two new HL gene variants, one in each of exons 3 and 5. All six HL DNA variants are single base pair changes. Two variants (at codons 133 and 202) are diallelic DNA polymorphisms that are silent at the amino acid level. One variant (V73M) is an allele that defines an uncommon HL isoprotein. One variant (N193S) has two alleles of approximately equal frequency in the population that specify two common HL isoproteins. Two variants (S267F and T383M) are rare mutations found to date only in HL deficient subjects and their relatives. Of the six HL variants described to date, only S267F and T383M are associated with hyperlipidemia.