A 60-y-old chylomicronemia patient homozygous for missense mutation (G188E) in the lipoprotein lipase gene showed no accelerated atherosclerosis

Clinical significance of small dense low-density lipoprotein cholesterol measurement in type 2 diabetes

Low-density lipoprotein cholesterol (LDL-C) is known to be a causal substance of atherosclerosis, but its usefulness as a predictive biomarker for atherosclerotic cardiovascular disease (ASCVD) is limited. In patients with type 2 diabetes (T2D), LDL-C concentrations do not markedly increase, while triglycerides (TG) concentrations are usually elevated. Although TG is associated with ASCVD risk, they do not play a direct role in the formation of atheromatous plaques. TG changes the risk of ASCVD in a way that is dependent on LDL-C, and TG is the primary factor in reducing LDL particle size. Small dense (sd)LDL, a potent atherogenic LDL subfraction, best explains the "Atherogenic Duo" of TG and LDL-C. Although hypertriglyceridemia is associated with small-sized LDL, patients with severe hypertriglyceridemia and low LDL-C rarely develop ASCVD. This suggests that quantifying sdLDL is more clinically relevant than measuring LDL size. We developed a full-automated direct sdLDL-C assay, and it was proven that sdLDL-C is a better predictor of ASCVD than LDL-C. The sdLDL-C level is specifically elevated in patients with metabolic syndrome and T2D who have insulin resistance. Due to its clear link to metabolic dysfunction, sdLDL-C could be named "metabolic LDL-C." Insulin resistance/hyperinsulinemia promotes TG production in the liver, causing steatosis and overproduction of VLDL1, a precursor of sdLDL. sdLDL-C is closely associated with steatotic liver disease and chronic kidney disease, which are common complications in T2D. This review focuses on T2D and discusses the clinical significance of sdLDL-C including its composition, pathophysiology, measurements, association with ASCVD, and treatments.

Anti-Lipoprotein Lipase Antibody as a Useful Marker for Plaque Vulnerability in Patients with Stable Angina

AIMS

Identifying patients with vulnerable plaque who have poor prognosis among those with coronary artery disease (CAD) is crucial to deciding future therapeutic interventions. We previously reported that male CAD patients with low anti-apolipoprotein B-100 autoantibody (anti-apoB-100 Ab) levels were at an increased risk of developing unstable plaque lesions. This study focused on the autoantibodies against lipoprotein lipase (LPL), a key enzyme in triglyceride metabolism, which is another risk factor for atherosclerosis, and investigated their association with plaque characteristics.

METHODS

We measured serum anti-LPL Ab levels using a homemade enzyme-linked immunosorbent assay in 80 male CAD patients. Coronary plaque properties were evaluated using iMAP®-intravascular ultrasound.

RESULTS

Serum anti-LPL Ab levels were not correlated with plaque burden but were significantly negatively and positively correlated with fibrotic and necrotic plaques, respectively. High-risk patients with low anti-apoB-100 Ab levels were divided into groups according to their anti-LPL Ab levels. The group with high anti-LPL Ab levels exhibited more necrotic plaques and fewer fibrotic plaques as well as higher remnant-like lipoprotein particle levels than the group with low anti-LPL Ab levels.

CONCLUSIONS

Serum anti-LPL Ab levels can serve as a marker of plaque instability in CAD patients and can help identify higher-risk cases when combined with anti-apoB-100 Ab levels.

Homozygous familial lipoprotein lipase deficiency without obvious coronary artery stenosis

The prevalence of familial lipoprotein lipase deficiency (LPLD) is approximately one in 1,000,000 in the general population. There are conflicting reports on whether or not LPLD is atherogenic. We conducted coronary computed tomographic (CT) angiography on two patients in their 70 s who had genetically confirmed LPLD. Patient 1 was a 73 year old woman with a body mass index (BMI) of 27.5 kg/m², no history of diabetes mellitus and no history of drinking alcohol or smoking. At the time of her first visit, her serum total cholesterol, triglycerides and high-density lipoprotein cholesterol levels were 4.8 mmol/L, 17.3 mmol/L, and 0.5 mmol/L, respectively. She was treated with a lipid-restricted diet and fibrate but her serum TG levels remained extremely high. Next-generation sequencing analysis revealed a missense mutation (homo) in the LPL gene, c.662T>C (p. Ile221Thr), leading to the diagnosis of homozygous familial LPL deficiency (LPLD). Patient 2 was another 73- year- old woman. She also had marked hypertriglyceridemia with no history of diabetes mellitus, drinking alcohol, or smoking. Previous genetic studies showed she had a nonsense mutation (homozygous) in the LPL gene, c.1277G>A (p.Trp409Ter). To clarify the degree of coronary artery stenosis in these two cases, we conducted coronary CT angiography and found that no coronary artery stenosis in either the right or left coronary arteries. Based on the findings in these two elderly women along with previous reports on patients in their 60 s with LPLD and hypertriglyceridemia, we suggest that LPLD may not be associated with the development or progression of coronary artery disease.

Serum HDL-C values: An extremely useful marker for differentiating homozygous lipoprotein lipase deficiency from severe hypertriglyceridemia with other causes in Japan: A meta-analysis based on literatures on Japanese homozygous lipoprotein lipase deficiency

BACKGROUNDS AND AIM

Lipoprotein lipase (LPL) deficiency is a genetic disorder with a defective gene for lipoprotein lipase, leading to very high triglycerides. In the daily practice it is much more common to come across severely hypertriglyceridemia without homozygous or compound heterozygous LPL deficiency (SHTG).

METHODS

We investigated on how to screen homozygous or compound heterozygous LPL deficiency using lipid parameters by meta-analyzing past 20 subjects on this genetic disease reported by Japanese investigators. As a comparison with LPL deficiency, 21 subjects with SHTG from recent two studies were included in this study.

RESULTS

Serum HDL-C levels were significantly lower in LPL deficiency than in SHTG (0.38 ± 0.13 vs 0.94 ± 0.28 mmol/L (mean ± SD), p < 0.001), whereas other serum lipids did not differ between the two groups. The ROC curve ± standard error for serum HDL-C for discriminating the two groups was 0.97 ± 0.019. Sensitivity and specificity for distinguishing the two groups were 90% and 95%, respectively when serum HDL-C 0.62 mmol/L was adopted as cut point.

CONCLUSION

We found for the first time that serum HDL-C is an extremely useful marker for discriminating LPL deficiency from SHTG in Japanese population.

Hypertriglyceridemia and Atherosclerosis: Using Human Research to Guide Mechanistic Studies in Animal Models

Human studies support a strong association between hypertriglyceridemia and atherosclerotic cardiovascular disease (CVD). However, whether a causal relationship exists between hypertriglyceridemia and increased CVD risk is still unclear. One plausible explanation for the difficulty establishing a clear causal role for hypertriglyceridemia in CVD risk is that lipolysis products of triglyceride-rich lipoproteins (TRLs), rather than the TRLs themselves, are the likely mediators of increased CVD risk. This hypothesis is supported by studies of rare mutations in humans resulting in impaired clearance of such lipolysis products (remnant lipoprotein particles; RLPs). Several animal models of hypertriglyceridemia support this hypothesis and have provided additional mechanistic understanding. Mice deficient in lipoprotein lipase (LPL), the major vascular enzyme responsible for TRL lipolysis and generation of RLPs, or its endothelial anchor GPIHBP1, are severely hypertriglyceridemic but develop only minimal atherosclerosis as compared with animal models deficient in apolipoprotein (APO) E, which is required to clear TRLs and RLPs. Likewise, animal models convincingly show that increased clearance of TRLs and RLPs by LPL activation (achieved by inhibition of APOC3, ANGPTL3, or ANGPTL4 action, or increased APOA5) results in protection from atherosclerosis. Mechanistic studies suggest that RLPs are more atherogenic than large TRLs because they more readily enter the artery wall, and because they are enriched in cholesterol relative to triglycerides, which promotes pro-atherogenic effects in lesional cells. Other mechanistic studies show that hepatic receptors (LDLR and LRP1) and APOE are critical for RLP clearance. Thus, studies in animal models have provided additional mechanistic insight and generally agree with the hypothesis that RLPs derived from TRLs are highly atherogenic whereas hypertriglyceridemia due to accumulation of very large TRLs in plasma is not markedly atherogenic in the absence of TRL lipolysis products.

Lipoprotein Lipase Deficiency Arising in Type V Dyslipidemia

A 40-year-old Japanese man presented with child-onset hypertriglyceridemia recently complicated by diabetes mellitus. The patient's diabetes mellitus was maintained, but he had persistent insulin resistance. The patient also had persistent severe hypertriglyceridemia (1,224-4,104 mg/dL), despite the administration of bezafibrate and ezetimibe. Type V dyslipidemia was revealed by agarose gel electrophoresis and the refrigerator test, and a significantly reduced post-heparin lipoprotein lipase mass of 26 ng/mL was confirmed. Genetic testing confirmed two heterozygous LPL variants, p.Tyr88X and p.Gly215Glu in trans; thus, the patient was diagnosed with lipoprotein lipase deficiency. Lipoprotein lipase deficiency typically arises in type I dyslipidemia, but is latent in type V dyslipidemia.

Molecular and functional characterization of familial chylomicronemia syndrome

BACKGROUND AND AIMS

Familial chylomicronemia syndrome is a rare autosomal recessive disorder leading to severe hypertriglyceridemia (HTG) due to mutations in lipoprotein lipase (LPL)-associated genes. Few data exist on the clinical features of the disorder or on comprehensive genetic approaches to uncover the causative genes and mutations.

METHODS

Eight patients diagnosed with familial hyperchylomicronemia with recessive inheritance were included in this study (two males and six females; median age of onset 23.0 years; mean triglyceride level 3446 mg/dl). We evaluated their clinical features, including coronary artery disease using coronary computed tomography, and performed targeted next-generation sequencing on a panel comprising 4813 genes associated with known clinical phenotypes. After standard filtering for allele frequency <1% and in silico annotation prediction, we used three types of variant filtering to identify causative mutations: homozygous mutations in known familial hyperchylomicronemia-associated genes, homozygous mutations with high damaging scores in novel genes, and deleterious mutations within 37 genes known to be associated with HTG.

RESULTS

A total of 1810 variants out of the 73,389 identified with 94.3% mean coverage (×20) were rare and nonsynonymous. Among these, our schema detected four pathogenic or likely pathogenic mutations in the LPL gene (p.Ala248LeufsTer4, p.Arg270Cys, p.Ala361Thr, and p.Val227Gly), including one novel mutation and a variant of uncertain significance. Patients harboring LPL gene mutations showed no severe atherosclerotic changes in the coronary arteries, but recurrent pancreatitis with long-term exposure to HTG was observed.

CONCLUSIONS

These results demonstrate that LPL gene plays a major role in extreme HTG associated with hyperchylomicronemia, although the condition may not cause severe atherosclerosis.

Severe hypertriglyceridemia in Japan: Differences in causes and therapeutic responses

BACKGROUND

Severe hypertriglyceridemia (>1000 mg/dL) has a variety of causes and frequently leads to life-threating acute pancreatitis. However, the origins of this disorder are unclear for many patients.

OBJECTIVE

We aimed to characterize the causes of and responses to therapy in rare cases of severe hypertriglyceridemia in a group of Japanese patients.

METHODS

We enrolled 121 patients from a series of case studies that spanned 30 years. Subjects were divided into 3 groups: (1) primary (genetic causes); (2) secondary (acquired); and (3) disorders of uncertain causes. In the last group, we focused on 3 possible risks factors for hypertriglyceridemia: obesity, diabetes mellitus, and heavy alcohol intake.

RESULTS

Group A (n = 20) included 13 patients with familial lipoprotein lipase deficiency, 3 patients with apolipoprotein CII deficiency, and other genetic disorders in the rest of the group. Group B patients (n = 15) had various metabolic and endocrine diseases. In Group C (uncertain causes; n = 86), there was conspicuous gender imbalance (79 males, 3 females) and most male subjects were heavy alcohol drinkers. In addition, 18 of 105 adult patients (17%) had histories of acute pancreatitis.

CONCLUSION

The cause of severe hypertriglyceridemia is uncertain in many patients. In primary genetic forms of severe hypertriglyceridemia, genetic diversity between populations is unknown. In the acquired forms, we found fewer cases of estrogen-induced hypertriglyceridemia than in Western countries. In our clinical experience, the cause of most hypertriglyceridemia is uncertain. Our work suggests that genetic factors for plasma triglyceride sensitivity to alcohol should be explored.

Loss of Transcription Factor CREBH Accelerates Diet-Induced Atherosclerosis in Ldlr-/- Mice

OBJECTIVE

Liver-enriched transcription factor cAMP-responsive element-binding protein H (CREBH) regulates plasma triglyceride clearance by inducing lipoprotein lipase cofactors, such as apolipoprotein A-IV (apoA-IV), apoA-V, and apoC-II. CREBH also regulates apoA-I transcription. This study aims to determine whether CREBH has a role in lipoprotein metabolism and development of atherosclerosis.

APPROACH AND RESULTS

CREBH-deficient Creb3l3(-/-) mice were bred with Ldlr(-/-) mice creating Ldlr(-/-) Creb3l3(-/-) double knockout mice. Mice were fed on a high-fat and high-sucrose Western diet for 20 weeks. We showed that CREBH deletion in Ldlr(-/-) mice increased very low-density lipoprotein-associated triglyceride and cholesterol levels, consistent with the impairment of lipoprotein lipase-mediated triglyceride clearance in these mice. In contrast, high-density lipoprotein cholesterol levels were decreased in CREBH-deficient mice, which was associated with decreased production of apoA-I from the liver. The results indicate that CREBH directly activated Apoa1 gene transcription. Accompanied by the worsened atherogenic lipid profile, Ldlr(-/-) Creb3l3(-/-) mice developed significantly more atherosclerotic lesions in the aortas than Ldlr(-/-) mice.

CONCLUSIONS

We identified CREBH as an important regulator of lipoprotein metabolism and suggest that increasing hepatic CREBH activity may be a novel strategy for prevention and treatment of atherosclerosis.

Genetics of Lipid and Lipoprotein Disorders and Traits

PURPOSE OF REVIEW

Plasma lipids, namely cholesterol and triglyceride, and lipoproteins, such as low-density lipoprotein (LDL) and high-density lipoprotein, serve numerous physiological roles. Perturbed levels of these traits underlie monogenic dyslipidemias, a diverse group of multisystem disorders. We are on the verge of having a relatively complete picture of the human dyslipidemias and their components.

RECENT FINDINGS

Recent advances in genetics of plasma lipids and lipoproteins include the following: (1) expanding the range of genes causing monogenic dyslipidemias, particularly elevated LDL cholesterol; (2) appreciating the role of polygenic effects in such traits as familial hypercholesterolemia and combined hyperlipidemia; (3) accumulating a list of common variants that determine plasma lipids and lipoproteins; (4) applying exome sequencing to identify collections of rare variants determining plasma lipids and lipoproteins that via Mendelian randomization have also implicated gene products such as NPC1L1, APOC3, LDLR, APOA5, and ANGPTL4 as causal for atherosclerotic cardiovascular disease; and (5) using naturally occurring genetic variation to identify new drug targets, including inhibitors of apolipoprotein (apo) C-III, apo(a), ANGPTL3, and ANGPTL4.

SUMMARY

Here, we compile this disparate range of data linking human genetic variation to plasma lipids and lipoproteins, providing a "one stop shop" for the interested reader.

Lipoprotein lipase and atherosclerosis

Lipoprotein lipase has long been known to hydrolyse triglycerides from triglycerides-rich lipoproteins. More recently, it has been shown to promote the binding of lipoproteins to various lipoprotein receptors. Evidence is also presented regarding the possible atherogenic role of lipoprotein lipase. In theory, lipoprotein lipase deficiency should help to clarify this question. However, the rarity of this condition means that it has not been possible to conduct epidemiological studies. An alternative approach is to investigate the correlation of lipoprotein lipase with onset of cardiovascular disease in prospective studies in large population-based cohorts. Complementary with this approach, animal models have been used to explore the atherogenicity of lipoprotein lipase expressed by macrophages.

Regulating intestinal function to reduce atherogenic lipoproteins

Significant knowledge regarding different molecules involved in the transport of dietary fat into the circulation has been garnered. Studies point to the possibility that accumulation of intestine-derived lipoproteins in the plasma could contribute to atherosclerosis. This article provides a brief overview of dietary lipid metabolism and studies in mice supporting the hypothesis that intestinal lipoproteins contribute to atherosclerosis. Deficiencies in lipoprotein lipase and Gpihbp1, and overexpression of heparanse in mice, are associated with increases in atherosclerosis, suggesting that defects in catabolism of larger lipoproteins in the plasma contribute to atherosclerosis. Furthermore, inositol-requiring enzyme 1β-deficient mice that produce more intestinal lipoproteins also develop more atherosclerosis. Thus, increases in plasma intestinal lipoproteins due to either overproduction or reduced catabolism result in augmented atherosclerosis. Intestinal lipoproteins tend to adhere strongly to subendothelial proteoglycans, elicit an inflammatory response by endothelial cells and activate macrophages, contributing to the initiation and progression of the disease. Thus, molecules that reduce intestinal lipid absorption can be useful in lowering atherosclerosis.

Novel combined GPIHBP1 mutations in a patient with hypertriglyceridemia associated with CAD

AIM

Lipoprotein lipase (LPL) deficiency is a rare autosomal recessive disorder characterized by severe hypertriglyceridemia. Similar clinical phenotypes have been reported with respect to defects in several LPL-associated proteins. However, it remains controversial whether severe hypertriglyceridemia itself is atherogenic. We herein present a case of LPL deficiency due to novel combined mutations of glycosylphosphatidylinositol (GPI)-anchored high-density lipoprotein (HDL)-binding protein 1 (GPIHBP1) in a patient with coronary artery disease (CAD).

PATIENT

We evaluated a 54-year-old woman with severe hypertriglyceridemia and double vessel CAD. Although the LPL mass and activity in the postheparin plasma were extremely low, no mutations were detected in the LPL gene itself.

RESULTS

Genetic analyses revealed that the patient had double homozygous mutations at 41 bp (c.41 G > T) and 202 bp (c.202 T > C) in the GPIHBP1 gene, resulting in C14F and C68R, respectively. Although the C14F/C68R GPIHBP1 exhibited a normal LPL-binding activity, the levels of mutant proteins were extremely reduced compared to those of the wild-type proteins in vitro.

CONCLUSION

We found novel combined mutations of GPIHBP1 in a patient with hypertriglyceridemia and severe CAD. The present case provides important insight into the pathogenesis of severe hypertriglyceridemia associated with atherosclerosis.

Identification of mutations in the lipoprotein lipase (LPL) and apolipoprotein C-II (APOC2) genes using denaturing high performance liquid chromatography (DHPLC)

BACKGROUND

Endothelial lipoprotein lipase (LPL) hydrolyzes triglycerides of chylomicrons and very low density lipoproteins, releasing free fatty acids for local and systemic use. Mutations in the LPL gene or its cofactor APOC2 may result in a decrease or complete loss of enzyme function and subsequently to type I hyperlipoproteinemia.

METHODS

We used PCR to amplify all exons and the promoter region of LPL and APOC2. Nine blinded DNA samples with known LPL mutations were used as positive controls. In addition, nine patients from our lipid clinic and twelve healthy subjects were analyzed. DNA was screened for sequence variants by denaturing HPLC (DHPLC) followed by direct sequencing of PCR fragments showing distinct elution profiles.

RESULTS

All LPL sequence variants in the positive controls (D9N, V69L, delAACTG386, I225T, N291S, and S447X) were correctly identified. In the remaining patients, additional variants were detected in LPL and APOC2. These variants were also present in healthy subjects, indicating that they constituted silent variation with no relevant effect on plasma triglycerides, at least in the heterozygous state.

CONCLUSIONS

A semi-automated DHPLC screening method was developed for the detection of sequence variants in the LPL and APOC2 genes. Our results demonstrate that the method was robust and sensitive.