Mutations in LMF1 cause combined lipase deficiency and severe hypertriglyceridemia. Nat Genet. 2007;39:1483-1487. [PMID: 17994020 DOI: 10.1038/ng.2007.24]

Identification of a Compound Heterozygous LMF1 Variants in a Patient with Severe Hypertriglyceridemia - Case Report and Literature Review

Familial chylomicronemia syndrome (FCS) and multifactorial chylomicronemia (MCM), characterized by highly variable triglyceride levels with acute episodes of severe hypertriglyceridemia (HTG), are caused by rare variants in genes associated with the catabolism of circulating lipoprotein triglycerides, mainly including LPL, APOC2, APOA5, GPIHBP1, and LMF1. Among them, the LMF1 gene only accounts for 1%. This study described a Chinese patient with severe HTG carrying compound heterozygous variants of a rare nonsense variant p.W168X in exon 3 and a missense variant p.R416Q in exon 9 in the LMF1 gene. These heterozygous variants account for his family's decreased lipase activity and mass, which caused the FCS phenotype.

Pancreatitis as a Main Consequence of APOC2-Related Hypertriglyceridemia: The Role of Nonsense and Frameshift Variants

APOC2-related hypertriglyceridemia occurs due to biallelic variants of this gene. Here, genotype-phenotype architecture of all pathogenic APOC2 variants is investigated among heterozygous and homozygous individuals. Clinical heterogeneity of various types of the variants is also described, and pancreatitis in more than half of homozygotes carrying chain-termination variants is highlighted as well. For this study, patients were selected who had a plasma triglyceride level above 250 mg/dL. The coding and intronic regions of the APOC2 gene were amplified using the Sanger sequencing to investigate the presence of variants. The genotypes, lipid profiles, and detailed clinical features were documented for all APOC2-related patients and heterozygous individuals. Pathogenicity of the variants was predicted and categorized using available bioinformatics tools such as MutationTaster and PolyPhen-2 and ACMG criteria. MetaDome and Phyre2 were applied for structural and functional in silico analyses. 40% (12 out of 30) of APOC2 variants were chain-termination (nonsense and frameshift) variants. These types of variants were determined in 60.53% of patients. 55% of these patients showed pancreatitis followed by lipemia retinalis (29%), abdominal pain (24%), hepatosplenomegaly (24%), and xanthomas (18%). The mean age of onset was about 22 years old. In at least 50% of 38 homozygous individuals, the TG level was more than 2000 mg/dL. More than 25% of heterozygous individuals showed at least one symptom. Pancreatitis and a severe form of HTG were found in 5 and 2% of heterozygous individuals, respectively. The main clinical features of APOC2-related hypertriglyceridemia include pancreatitis, lipemia retinalis, abdominal pain, hepatosplenomegaly, and xanthomas. Nonsense and frameshift homozygous variants usually lead to a severe form of hypertriglyceridemia. Pancreatitis is one of the main consequences of these types of mutations; thus, it is important to consider this point when evaluating asymptomatic individuals. Heterozygous individuals may become symptomatic due to the role of unknown modifying agent including environmental genetic factors.

Genetic and biochemical analysis of severe hypertriglyceridemia complicated with acute pancreatitis or with low post-heparin lipoprotein lipase mass

Severe hypertriglyceridemia is a pathological condition caused by genetic factors alone or in combination with environmental factors, sometimes leading to acute pancreatitis (AP). In this study, exome sequencing and biochemical analyses were performed in 4 patients with hypertriglyceridemia complicated by obesity or diabetes with a history of AP or decreased post-heparin LPL mass. In a patient with a history of AP, SNP rs199953320 resulting in LMF1 nonsense mutation and APOE rs7412 causing apolipoprotein E2 were both found in heterozygous form. Three patients were homozygous for APOA5 rs2075291, and one was heterozygous. ELISA and Western blot analysis of the serum revealed the existence of apolipoprotein A-V in the lipoprotein-free fraction regardless of the presence or absence of rs2075291; furthermore, the molecular weight of apolipoprotein A-V was different depending on the class of lipoprotein or lipoprotein-free fraction. Lipidomics analysis showed increased serum levels of sphingomyelin and many classes of glycerophospholipid; however, when individual patients were compared, the degree of increase in each class of phospholipid among cases did not coincide with the increases seen in total cholesterol and triglycerides. Moreover, phosphatidylcholine, lysophosphatidylinositol, and sphingomyelin levels tended to be higher in patients who experienced AP than those who did not, suggesting that these phospholipids may contribute to the onset of AP. In summary, this study revealed a new disease-causing gene mutation in LMF1, confirmed an association between overlapping of multiple gene mutations and severe hypertriglyceridemia, and suggested that some classes of phospholipid may be involved in the pathogenesis of AP.

Probiotic yogurt regulates gut microbiota homeostasis and alleviates hepatic steatosis and liver injury induced by high-fat diet in golden hamsters

This study aimed to investigate the beneficial effects of probiotic yogurt on lipid metabolism and gut microbiota in metabolic-related fatty liver disease (MAFLD) golden hamsters fed on a high-fat diet (HFD). The results demonstrated that probiotic yogurt significantly reversed the adverse effects caused by HFD, such as body and liver weight gain, liver steatosis and damage, sterol deposition, and oxidative stress after 8 weeks of intervention. qRT-PCR analysis showed that golden hamsters fed HFD had upregulated genes related to adipogenesis, increased free fatty acid infiltration, and downregulated genes related to lipolysis and very low-density lipoprotein secretion. Probiotic yogurt supplements significantly inhibited HFD-induced changes in the expression of lipid metabolism-related genes. Furthermore, 16S rRNA gene sequencing of the intestinal content microbiota suggested that probiotic yogurt changed the diversity and composition of the gut microbiota in HFD-fed hamsters. Probiotic yogurt decreased the ratio of the phyla Firmicutes/Bacteroidetes, the relative abundance of the LPS-producing genus Desulfovibrio, and bacteria involved in lipid metabolism, whereas it increased the relative abundance of short-chain fatty acids producing bacteria in HFD-fed hamsters. Predictive functional analysis of the microbial community showed that probiotic yogurt-modified genes involved in LPS biosynthesis and lipid metabolism. In summary, these findings support the possibility that probiotic yogurt significantly improves HFD-induced metabolic disorders through modulating intestinal microflora and lipid metabolism and effectively regulating the occurrence and development of MAFLD. Therefore, probiotic yogurt supplementation may serve as an effective nutrition strategy for the treatment of patients with MAFLD clinically.

Skeletal growth is enhanced by a shared role for SOX8 and SOX9 in promoting reserve chondrocyte commitment to columnar proliferation

SOX8 was linked in a genome-wide association study to human height heritability, but roles in chondrocytes for this close relative of the master chondrogenic transcription factor SOX9 remain unknown. We undertook here to fill this knowledge gap. High-throughput assays demonstrate expression of human SOX8 and mouse Sox8 in growth plate cartilage. In situ assays show that Sox8 is expressed at a similar level as Sox9 in reserve and early columnar chondrocytes and turned off when Sox9 expression peaks in late columnar and prehypertrophic chondrocytes. Sox8-/- mice and Sox8fl/flPrx1Cre and Sox9fl/+Prx1Cre mice (inactivation in limb skeletal cells) have a normal or near normal skeletal size. In contrast, juvenile and adult Sox8fl/flSox9fl/+Prx1Cre compound mutants exhibit a 15 to 20% shortening of long bones. Their growth plate reserve chondrocytes progress slowly toward the columnar stage, as witnessed by a delay in down-regulating Pthlh expression, in packing in columns and in elevating their proliferation rate. SOX8 or SOX9 overexpression in chondrocytes reveals not only that SOX8 can promote growth plate cell proliferation and differentiation, even upon inactivation of endogenous Sox9, but also that it is more efficient than SOX9, possibly due to greater protein stability. Altogether, these findings uncover a major role for SOX8 and SOX9 in promoting skeletal growth by stimulating commitment of growth plate reserve chondrocytes to actively proliferating columnar cells. Further, by showing that SOX8 is more chondrogenic than SOX9, they suggest that SOX8 could be preferred over SOX9 in therapies to promote cartilage formation or regeneration in developmental and degenerative cartilage diseases.

Understanding Hypertriglyceridemia: Integrating Genetic Insights

Hypertriglyceridemia is an exceptionally complex metabolic disorder characterized by elevated plasma triglycerides associated with an increased risk of acute pancreatitis and cardiovascular diseases such as coronary artery disease. Its phenotype expression is widely heterogeneous and heavily influenced by conditions as obesity, alcohol consumption, or metabolic syndromes. Looking into the genetic underpinnings of hypertriglyceridemia, this review focuses on the genetic variants in LPL, APOA5, APOC2, GPIHBP1 and LMF1 triglyceride-regulating genes reportedly associated with abnormal genetic transcription and the translation of proteins participating in triglyceride-rich lipoprotein metabolism. Hypertriglyceridemia resulting from such genetic abnormalities can be categorized as monogenic or polygenic. Monogenic hypertriglyceridemia, also known as familial chylomicronemia syndrome, is caused by homozygous or compound heterozygous pathogenic variants in the five canonical genes. Polygenic hypertriglyceridemia, also known as multifactorial chylomicronemia syndrome in extreme cases of hypertriglyceridemia, is caused by heterozygous pathogenic genetic variants with variable penetrance affecting the canonical genes, and a set of common non-pathogenic genetic variants (polymorphisms, using the former nomenclature) with well-established association with elevated triglyceride levels. We further address recent progress in triglyceride-lowering treatments. Understanding the genetic basis of hypertriglyceridemia opens new translational opportunities in the scope of genetic screening and the development of novel therapies.

Macromolecular Interactions of Lipoprotein Lipase (LPL)

Lipoprotein lipase (LPL) is a critical enzyme in humans that provides fuel to peripheral tissues. LPL hydrolyzes triglycerides from the cores of lipoproteins that are circulating in plasma and interacts with receptors to mediate lipoprotein uptake, thus directing lipid distribution via catalytic and non-catalytic functions. Functional losses in LPL or any of its myriad of regulators alter lipid homeostasis and potentially affect the risk of developing cardiovascular disease-either increasing or decreasing the risk depending on the mutated protein. The extensive LPL regulatory network tunes LPL activity to allocate fatty acids according to the energetic needs of the organism and thus is nutritionally responsive and tissue dependent. Multiple pharmaceuticals in development manipulate or mimic these regulators, demonstrating their translational importance. Another facet of LPL biology is that the oligomeric state of the enzyme is also central to its regulation. Recent structural studies have solidified the idea that LPL is regulated not only by interactions with other binding partners but also by self-associations. Here, we review the complexities of the protein-protein and protein-lipid interactions that govern LPL structure and function.

The Role of Triglyceride-rich Lipoproteins and Their Remnants in Atherosclerotic Cardiovascular Disease

Atherosclerotic cardiovascular disease (ASCVD) is the world's leading cause of death. ASCVD has multiple mediators that therapeutic interventions target, such as dyslipidaemia, hypertension, diabetes and heightened systemic inflammatory tone, among others. LDL cholesterol is one of the most well-studied and established mediators targeted for primary and secondary prevention of ASCVD. However, despite the strength of evidence supporting LDL cholesterol reduction by multiple management strategies, ASCVD events can still recur, even in patients whose LDL cholesterol has been very aggressively reduced. Hypertriglyceridaemia and elevated levels of triglyceride-rich lipoproteins (TRLs) may be key contributors to ASCVD residual risk. Several observational and genetic epidemiological studies have highlighted the causal role of triglycerides within the TRLs and/or their remnant cholesterol in the development and progression of ASCVD. TRLs consist of intestinally derived chylomicrons and hepatically synthesised very LDL. Lifestyle modification has been considered the first line intervention for managing hypertriglyceridaemia. Multiple novel targeted therapies are in development, and have shown efficacy in the preclinical and clinical phases of study in managing hypertriglyceridaemia and elevated TRLs. This comprehensive review provides an overview of the biology, pathogenicity, epidemiology, and genetics of triglycerides and TRLs, and how they impact the risk for ASCVD. In addition, we provide a summary of currently available and novel emerging triglyceride-lowering therapies in development.

A novel homozygous nonsense variant of LMF1 in pregnancy-induced hypertriglyceridemia with acute pancreatitis

Hypertriglyceridemia (HTG)-induced pancreatitis during pregnancy could lead to maternal and fetal death. However, its genetic bases are not fully understood, and its treatment strategies are yet to be established. Here we report a case with a novel homozygous nonsense variant of LMF1 in pregnancy-associated HTG with acute pancreatitis. Our patient had childhood-onset severe HTG that had been well-controlled by dietary management in the non-pregnant period with plasma triglyceride (TG) levels at around 200 mg/dL. Milky plasma was noted at the first-trimester pregnancy checkup, followed by a severe increase in plasma TG (10,500 mg/dL) that resulted in pancreatitis in the last trimester. The implementation of strict dietary fat restriction (less than 4 grams per day) reduced plasma TG levels and led to successful delivery. Exome sequencing revealed a novel homozygous nonsense variant in LMF1 (c.697C>T, p.Arg233Ter). The activities of lipoprotein lipase (LPL) and hepatic lipase in post-heparin plasma were not abolished but reduced. The use of pemafibrate decreased plasma TG levels with a concomitant increase in LPL activity. HTG in childhood or early pregnancy is commonly assumed to be polygenic in origin but should be regarded as a feature suggestive of monogenic hyperchylomicronemia. Adequate TG monitoring and dietary fat restriction should be implemented to prevent potentially lethal events of pancreatitis.

Integrated Control of Fatty Acid Metabolism in Heart Failure

Disrupted fatty acid metabolism is one of the most important metabolic features in heart failure. The heart obtains energy from fatty acids via oxidation. However, heart failure results in markedly decreased fatty acid oxidation and is accompanied by the accumulation of excess lipid moieties that lead to cardiac lipotoxicity. Herein, we summarized and discussed the current understanding of the integrated regulation of fatty acid metabolism (including fatty acid uptake, lipogenesis, lipolysis, and fatty acid oxidation) in the pathogenesis of heart failure. The functions of many enzymes and regulatory factors in fatty acid homeostasis were characterized. We reviewed their contributions to the development of heart failure and highlighted potential targets that may serve as promising new therapeutic strategies.

Comprehensive in-silico analysis of deleterious SNPs in APOC2 and APOA5 and their differential expression in cancer and cardiovascular diseases conditions

Genetic variations in APOC2 and APOA5 genes involve activating lipoprotein lipase (LPL), responsible for the hydrolysis of triglycerides (TG) in blood and whose impaired functions affect the TG metabolism and are associated with metabolic diseases. In this study, we investigate the biological significance of genetic variations at the DNA sequence and structural level using various computational tools. Subsequently, 8 (APOC2) and 17 (APOA5) non-synonymous SNPs (nsSNPs) were identified as high-confidence deleterious SNPs based on the effects of the mutations on protein conservation, stability, and solvent accessibility. Furthermore, based on our docking results, the interaction of native and mutant forms of the corresponding proteins with LPL depicts differences in root mean square deviation (RMSD), and binding affinities suggest that these mutations may affect their function. Furthermore, in vivo, and in vitro studies have shown that differential expression of these genes in disease conditions due to the influence of nsSNPs abundance may be associated with promoting the development of cancer and cardiovascular diseases. Preliminary screening using computational methods can be a helpful start in understanding the effects of mutations in APOC2 and APOA5 on lipid metabolism; however, further wet-lab experiments would further strengthen the conclusions drawn from the computational study.

The breast cancer microenvironment and lipoprotein lipase: Another negative notch for a beneficial enzyme?

The energy demand of breast cancers is in part met through the β-oxidation of exogenous fatty acids. Fatty acids may also be used to aid in cell signaling and toward the construction of new membranes for rapidly proliferating tumor cells. A significant quantity of fatty acids comes from the hydrolysis of lipoprotein triacylglycerols and phospholipids by lipoprotein lipase (LPL). The lipid obtained via LPL in the breast tumor microenvironment may thus promote breast tumor growth and development. In this hypothesis article, we introduce LPL, provide a meta-analysis of RNAseq data showing that LPL is associated with poor prognosis, and explain how LPL might play a role in breast cancer prognosis over time.

Exome-wide association analysis of CT imaging-derived hepatic fat in a medical biobank

Nonalcoholic fatty liver disease is common and highly heritable. Genetic studies of hepatic fat have not sufficiently addressed non-European and rare variants. In a medical biobank, we quantitate hepatic fat from clinical computed tomography (CT) scans via deep learning in 10,283 participants with whole-exome sequences available. We conduct exome-wide associations of single variants and rare predicted loss-of-function (pLOF) variants with CT-based hepatic fat and perform cross-modality replication in the UK Biobank (UKB) by linking whole-exome sequences to MRI-based hepatic fat. We confirm single variants previously associated with hepatic fat and identify several additional variants, including two (FGD5 H600Y and CITED2 S198_G199del) that replicated in UKB. A burden of rare pLOF variants in LMF2 is associated with increased hepatic fat and replicates in UKB. Quantitative phenotypes generated from clinical imaging studies and intersected with genomic data in medical biobanks have the potential to identify molecular pathways associated with human traits and disease.

Clinical heterogeneity in monogenic chylomicronaemia

Chylomicronaemia accompanies hypertriglyceridaemia, usually due to a polygenic predisposition in combination with secondary risk factors. Monogenic chylomicronaemia represents a small subgroup of patients with hypertriglyceridaemia. This article describes three patients and illustrates the heterogeneity in the presentation of monogenic chylomicronaemia. The first case is a man with mild hypertriglyceridaemia who is a compound heterozygote for two variants in the LMF1 gene, without relevant medical history. The second case is a woman who is a double heterozygote of variants in the LPL and APOA5 genes. She experienced pancreatitis. The third case is a man, with recurrent pancreatitis attributed to severe hypertriglyceridaemia and homozygous for a variant in the APOC2 gene. This article highlights that in patients with hypertriglyceridaemia, the absence of pancreatitis or the presence of mild hypertriglyceridaemia does not exclude monogenic chylomicronaemia. Genetic screening should be considered in patients with unexplained or severe hypertriglyceridaemia, to determine appropriate treatment and follow-up.

The Regulation of Triacylglycerol Metabolism and Lipoprotein Lipase Activity

Triacylglycerol (TG) metabolism is tightly regulated to maintain a pool of TG within circulating lipoproteins that can be hydrolyzed in a tissue-specific manner by lipoprotein lipase (LPL) to enable the delivery of fatty acids to adipose or oxidative tissues as needed. Elevated serum TG concentrations, which result from a deficiency of LPL activity or, more commonly, an imbalance in the regulation of tissue-specific LPL activities, have been associated with an increased risk of atherosclerotic cardiovascular disease through multiple studies. Among the most critical LPL regulators are the angiopoietin-like (ANGPTL) proteins ANGPTL3, ANGPTL4, and ANGPTL8, and a number of different apolipoproteins including apolipoprotein A5 (ApoA5), apolipoprotein C2 (ApoC2), and apolipoprotein C3 (ApoC3). These ANGPTLs and apolipoproteins work together to orchestrate LPL activity and therefore play pivotal roles in TG partitioning, hydrolysis, and utilization. This review summarizes the mechanisms of action, epidemiological findings, and genetic data most relevant to these ANGPTLs and apolipoproteins. The interplay between these important regulators of TG metabolism in both fasted and fed states is highlighted with a holistic view toward understanding key concepts and interactions. Strategies for developing safe and effective therapeutics to reduce circulating TG by selectively targeting these ANGPTLs and apolipoproteins are also discussed.

Low circulating PCSK9 levels in LPL homozygous children with chylomicronemia syndrome in a syrian refugee family in Lebanon

Familial chylomicronemia syndrome is a rare autosomal recessive disorder of lipoprotein metabolism characterized by the presence of chylomicrons in fasting plasma and an important increase in plasma triglycerides (TG) levels that can exceed 22.58 mmol/l. The disease is associated with recurrent episodes of abdominal pain and pancreatitis, eruptive cutaneous xanthomatosis, lipemia retinalis, and hepatosplenomegaly. A consanguineous Syrian family who migrated to Lebanon was referred to our laboratory after perceiving familial chylomicronemia syndrome in two children. The LPL and PCSK9 genes were sequenced and plasma PCSK9 levels were measured. Sanger sequencing of the LPL gene revealed the presence of the p.(Val227Phe) pathogenic variant in exon 5 at the homozygous state in the two affected children, and at the heterozygous state in the other recruited family members. Interestingly, PCSK9 levels in homozygous carriers of the p.(Val227Phe) were ≈50% lower than those in heterozygous carriers of the variant (p-value = 0.13) and ranged between the 5th and the 7.5th percentile of PCSK9 levels in a sample of Lebanese children of approximately the same age group. Moreover, this is the first reported case of individuals carrying simultaneously an LPL pathogenic variant and PCSK9 variants, the L10 and L11 leucine insertion, which can lower and raise low-density lipoprotein cholesterol (LDL-C) levels respectively. TG levels fluctuated concomitantly between the two children, were especially high following the migration from a country to another, and were reduced under a low-fat diet. This case is crucial to raise public awareness on the risks of consanguineous marriages to decrease the emergence of inherited autosomal recessive diseases. It also highlights the importance of the early diagnosis and management of these diseases to prevent serious complications, such as recurrent pancreatitis in the case of familial hyperchylomicronemia.

Clinical characteristics and variant analyses of transient infantile hypertriglyceridemia related to GPD1 gene

Objective: Our study aims to summarize and analyze the clinical characteristics of transient infantile hypertriglyceridemia (HTGTI) and variants in the glycerol-3-phosphate dehydrogenase 1 (GPD1) gene and the effect of HTGTI on the protein structure of GPD1.

Methods: Retrospective analysis, using the general data, symptoms, signs, and auxiliary examinations, was performed on patients with HTGTI, which were confirmed by genetic testing in our hospital and reported cases online. The clinical data were analyzed using statistical and bioinformatic approaches.

Results: A total of 31 genetically confirmed HTGTI patients were collected from our hospital and cases reported in the literature. The clinical manifestations showed the median age of onset was 6.0 (1.9, 12.0) months. All the patients had normal psychiatric status, but 22.6% of them presented growth retardation and short stature, 93.5% had hepatomegaly, and 16.1% had splenomegaly. Just a few children were reported with jaundice, cholestasis, and obesity (3.2–6.5%). The laboratory investigations showed that 96.8% of them had hypertriglyceridemia (HTG) with a median level of 3.1 (2.1, 5.5) mmol/L, but only 30.0% had returned to normal during follow-up. In addition, 93.5% of patients had elevated alanine aminotransferase (ALT) with an average level of 92.1 ± 43.5 U/L, while 38.7% had hypercholesterolemia. Upon abdominal imaging, all patients presented fatty liver and liver steatosis, with 66.7% of patients showing hepatic fibrosis. Statistical differences in triglyceride (TG) level were observed in the ≤6 months group compared with the older groups and in the 13 months to 6 years group with >6 years group (H = 22.02, P < 0.05). The restricted cubic spline model showed that severe HTG decreased in the early stage of infants to the normal level; however, it rebounded again to a mild or moderate level after the following days. The genetic test revealed that the main variant types of the GPD1 gene were missense variants (51.6%), followed by splicing variants (35.5%) and nonsense variants (12.9%). Of patients, 87.1% had homozygous variants, with the most frequent loci being c.361-1G > C and c.895G > A.

Conclusion: The common manifestations of HTGTI were HTG, hepatomegaly, elevated liver transaminases, and hepatic steatosis in early infancy. However, the recurrence of aberrant HTG may pose long-term detrimental effects on HTGTI patients.

Population Genetics and Signatures of Selection in Early Neolithic European Farmers

Human expansion in the course of the Neolithic transition in western Eurasia has been one of the major topics in ancient DNA research in the last 10 years. Multiple studies have shown that the spread of agriculture and animal husbandry from the Near East across Europe was accompanied by large-scale human expansions. Moreover, changes in subsistence and migration associated with the Neolithic transition have been hypothesized to involve genetic adaptation. Here, we present high quality genome-wide data from the Linear Pottery Culture site Derenburg-Meerenstieg II (DER) (N = 32 individuals) in Central Germany. Population genetic analyses show that the DER individuals carried predominantly Anatolian Neolithic-like ancestry and a very limited degree of local hunter-gatherer admixture, similar to other early European farmers. Increasing the Linear Pottery culture cohort size to ∼100 individuals allowed us to perform various frequency- and haplotype-based analyses to investigate signatures of selection associated with changes following the adoption of the Neolithic lifestyle. In addition, we developed a new method called Admixture-informed Maximum-likelihood Estimation for Selection Scans that allowed us test for selection signatures in an admixture-aware fashion. Focusing on the intersection of results from these selection scans, we identified various loci associated with immune function (JAK1, HLA-DQB1) and metabolism (LMF1, LEPR, SORBS1), as well as skin color (SLC24A5, CD82) and folate synthesis (MTHFR, NBPF3). Our findings shed light on the evolutionary pressures, such as infectious disease and changing diet, that were faced by the early farmers of Western Eurasia.

Transethnic meta-analysis of exome-wide variants identifies new loci associated with male-specific metabolic syndrome

BACKGROUND

Metabolic syndrome (MetS) is a group of very common human conditions promoting strong understand the impact of rare variants, beyond exome-wide association studies, to potentially discover causative variants, across different ethnic populations.

OBJECTIVE

We performed transethnic, exome-wide MetS association studies on MetS in men.

METHODS

We analyzed genotype data of 5302 European subjects (2658 cases and 2644 controls), in the discovery stage of the European METabolic Syndrome In Men study, generated from exome chips, and 2481 subjects (714 cases and 1767 controls), in the replication stage, across 6 independent cohorts of 5 ancestries (T2D-GENES consortium), using whole-exome sequencing. We therefore evaluated gene-level and variant-level associations, of rare variants for MetS, using logistic regression (LR) and multivariate analyses (MulA).

RESULTS

Gene-based association found the gene for the cholesteryl ester transfer protein (CETP) (from MulA, p value = 4.67 × 10-9; from LR, p value = 0.009) to well associate with MetS. At two missense variants, from 8 rare variants in CETP, Ala390Pro (rs5880) (from MulA, p value = 1.28 × 10-7; from LR, p value = 1.34 × 10-4) and Arg468Gln (rs1800777) (from MulA, p value = 2.40 × 10-5; from LR, p value = 1.49 × 10-3) significantly associated with MetS across five ancestries.

CONCLUSIONS

Our findings highlight novel rare variants of genes that confer MetS susceptibility, in Europeans, that are shared with diverse populations, emphasizing an opportunity to further understand the biological target or genes that underlie MetS, across populations.

A Heterozygous LMF1 Gene Mutation (c.1523C>T), Combined With an LPL Gene Mutation (c.590G>A), Aggravates the Clinical Symptoms in Hypertriglyceridemia

Hypertriglyceridemia is an important contributor to atherosclerotic cardiovascular disease (ASCVD) and acute pancreatitis. Familial hypertriglyceridemia is often caused by mutations in genes involved in triglyceride metabolism. Here, we investigated the disease-causing gene mutations in a Chinese family with hypertriglyceridemia and assessed the functional significance in vitro. Whole-exome sequencing (WES) was performed revealing that the severe hypertriglyceridemic proband carried a missense mutation (c.590G > A) in exon 5 of the LPL gene, as well as a missense mutation (c.1523C > T) in exon 10 of the LMF1 gene. Conservation analysis by Polyphen-2 showed that the 508 locus in the LMF1 protein and 197 locus in the LPL protein were highly conserved between different species. I-TASSER analysis indicated that the LMF1 c.1523C > T mutation and the LPL c.590G > A mutation changed the tertiary structure of the protein. A decrease in mRNA and protein expression was observed in 293T cells transfected with plasmids carrying the LMF1 c.1523C > T mutation. Subcellular localization showed that both wild-type (WT) and mutant LMF1 protein were localized at the cell cytoplasm. In the cell medium and cell lysates, these LMF1 and LPL gene mutations both caused a decreased LPL mass. Moreover, the combination of LMF1 and LPL gene mutations significantly decreased LPL levels compared to their individual effects on the LPL concentration. Both the clinical and in vitro data suggest that severe hypertriglyceridemia was of digenic origin caused by LMF1 and LPL mutation double heterozygosity in this patient.

Expression and one-step purification of active lipoprotein lipase contemplated by biophysical considerations

Lipoprotein lipase (LPL) is essential for intravascular lipid metabolism and is of high medical relevance. Since LPL is notoriously unstable, there is an unmet need for a robust expression system producing high quantities of active and pure recombinant human LPL. We showed previously that bovine LPL purified from milk is unstable at body temperature (T_m is 34.8 °C), but in the presence of the endothelial transporter glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) LPL is stabile (T_m increases to 57.6 °C). Building on this information, we now designed an expression system for human LPL using Drosophila S2 cells grown in suspension at high cell density and at an advantageous temperature of 25 °C. We co-transfected S2 cells with human LPL, LMF1 and soluble GPIHBP1 to provide an efficient chaperoning and stabilization of LPL in all compartments during synthesis and after secretion into the conditioned medium. For LPL purification, we used heparin-Sepharose affinity chromatography, which disrupted LPL-GPIHBP1 complexes causing GPIHBP1 to elute with the flow-through of the conditioned media. This one-step purification procedure yielded high quantities of pure and active LPL (4‒28 mg/L). Purification of several human LPL variants (furin-cleavage resistant mutant R297A, active-site mutant S132A, and lipid-binding-deficient mutant W390A-W393A-W394A) as well as murine LPL underscores the versatility and robustness of this protocol. Notably, we were able to produce and purify LPL containing the cognate furin-cleavage site. This method provides an efficient and cost-effective approach to produce large quantities of LPL for biophysical and large-scale drug discovery studies.

GPIHBP1 and ANGPTL4 Utilize Protein Disorder to Orchestrate Order in Plasma Triglyceride Metabolism and Regulate Compartmentalization of LPL Activity

Intravascular processing of triglyceride-rich lipoproteins (TRLs) is crucial for delivery of dietary lipids fueling energy metabolism in heart and skeletal muscle and for storage in white adipose tissue. During the last decade, mechanisms underlying focal lipolytic processing of TRLs along the luminal surface of capillaries have been clarified by fresh insights into the functions of lipoprotein lipase (LPL); LPL's dedicated transporter protein, glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1); and its endogenous inhibitors, angiopoietin-like (ANGPTL) proteins 3, 4, and 8. Key discoveries in LPL biology include solving the crystal structure of LPL, showing LPL is catalytically active as a monomer rather than as a homodimer, and that the borderline stability of LPL's hydrolase domain is crucial for the regulation of LPL activity. Another key discovery was understanding how ANGPTL4 regulates LPL activity. The binding of ANGPTL4 to LPL sequences adjacent to the catalytic cavity triggers cooperative and sequential unfolding of LPL's hydrolase domain resulting in irreversible collapse of the catalytic cavity and loss of LPL activity. Recent studies have highlighted the importance of the ANGPTL3-ANGPTL8 complex for endocrine regulation of LPL activity in oxidative organs (e.g., heart, skeletal muscle, brown adipose tissue), but the molecular mechanisms have not been fully defined. New insights have also been gained into LPL-GPIHBP1 interactions and how GPIHBP1 moves LPL to its site of action in the capillary lumen. GPIHBP1 is an atypical member of the LU (Ly6/uPAR) domain protein superfamily, containing an intrinsically disordered and highly acidic N-terminal extension and a disulfide bond-rich three-fingered LU domain. Both the disordered acidic domain and the folded LU domain are crucial for the stability and transport of LPL, and for modulating its susceptibility to ANGPTL4-mediated unfolding. This review focuses on recent advances in the biology and biochemistry of crucial proteins for intravascular lipolysis.

The Importance of Lipoprotein Lipase Regulation in Atherosclerosis

Lipoprotein lipase (LPL) plays a major role in the lipid homeostasis mainly by mediating the intravascular lipolysis of triglyceride rich lipoproteins. Impaired LPL activity leads to the accumulation of chylomicrons and very low-density lipoproteins (VLDL) in plasma, resulting in hypertriglyceridemia. While low-density lipoprotein cholesterol (LDL-C) is recognized as a primary risk factor for atherosclerosis, hypertriglyceridemia has been shown to be an independent risk factor for cardiovascular disease (CVD) and a residual risk factor in atherosclerosis development. In this review, we focus on the lipolysis machinery and discuss the potential role of triglycerides, remnant particles, and lipolysis mediators in the onset and progression of atherosclerotic cardiovascular disease (ASCVD). This review details a number of important factors involved in the maturation and transportation of LPL to the capillaries, where the triglycerides are hydrolyzed, generating remnant lipoproteins. Moreover, LPL and other factors involved in intravascular lipolysis are also reported to impact the clearance of remnant lipoproteins from plasma and promote lipoprotein retention in capillaries. Apolipoproteins (Apo) and angiopoietin-like proteins (ANGPTLs) play a crucial role in regulating LPL activity and recent insights into LPL regulation may elucidate new pharmacological means to address the challenge of hypertriglyceridemia in atherosclerosis development.

Current Diagnosis and Management of Primary Chylomicronemia

Primary chylomicronemia (PCM) is a rare and intractable disease characterized by marked accumulation of chylomicrons in plasma. The levels of plasma triglycerides (TGs) typically range from 1,000 - 15,000 mg/dL or higher.

PCM is caused by defects in the lipoprotein lipase (LPL) pathway due to genetic mutations, autoantibodies, or unidentified causes. The monogenic type is typically inherited as an autosomal recessive trait with loss-of-function mutations in LPL pathway genes ( LPL , LMF1 , GPIHBP1 , APOC2 , and APOA5 ). Secondary/environmental factors (diabetes, alcohol intake, pregnancy, etc.) often exacerbate hypertriglyceridemia (HTG).

The signs, symptoms, and complications of chylomicronemia include eruptive xanthomas, lipemia retinalis, hepatosplenomegaly, and acute pancreatitis with onset as early as in infancy. Acute pancreatitis can be fatal and recurrent episodes of abdominal pain may lead to dietary fat intolerance and failure to thrive.

The main goal of treatment is to prevent acute pancreatitis by reducing plasma TG levels to at least less than 500-1,000 mg/dL. However, current TG-lowering medications are generally ineffective for PCM. The only other treatment options are modulation of secondary/environmental factors. Most patients need strict dietary fat restriction, which is often difficult to maintain and likely affects their quality of life.

Timely diagnosis is critical for the best prognosis with currently available management, but PCM is often misdiagnosed and undertreated. The aim of this review is firstly to summarize the pathogenesis, signs, symptoms, diagnosis, and management of PCM, and secondly to propose simple diagnostic criteria that can be readily translated into general clinical practice to improve the diagnostic rate of PCM. In fact, these criteria are currently used to define eligibility to receive social support from the Japanese government for PCM as a rare and intractable disease.

Nevertheless, further research to unravel the molecular pathogenesis and develop effective therapeutic modalities is warranted. Nationwide registry research on PCM is currently ongoing in Japan with the aim of better understanding the disease burden as well as the unmet needs of this life-threatening disease with poor therapeutic options.

Familial chylomicronemia and multifactorial chylomicronemia

The accumulation of chylomicrons in plasma beyond the postprandial period is a pathological event secondary to the partial or complete lack of activity of lipoprotein lipase that can lead to recurrent episodes of abdominal pain and acute pancreatitis. This article reviews the pathophysiology of this syndrome and the differential characteristics depending on whether it is due to congenital monogenic causes or acquired on a polygenic basis in which multiple factors may inluence.

Lipid metabolism and classification of hyperlipaemias

This chapter summarises, and updates, lipid metabolism. Both pathways, exogenous metabolisms route via the chylomicrons, and the endogenous pathway of very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). The reverse cholesterol metabolism will also be mentioned. It also includes the current classification of hyperlipidaemias or hyperlipoproteinaemias, with a reminder of the phenotype classification, and further developments of the aetiological classification. Both parts have updated references, with which knowledge of this vast subject can be expanded.

Digenic Inheritance and Gene-Environment Interaction in a Patient With Hypertriglyceridemia and Acute Pancreatitis

The etiology of hypertriglyceridemia (HTG) and acute pancreatitis (AP) is complex. Herein, we dissected the underlying etiology in a patient with HTG and AP. The patient had a 20-year history of heavy alcohol consumption and an 8-year history of mild HTG. He was hospitalized for alcohol-triggered AP, with a plasma triglyceride (TG) level up to 21.4 mmol/L. A temporary rise in post-heparin LPL concentration (1.5–2.5 times of controls) was noted during the early days of AP whilst LPL activity was consistently low (50∼70% of controls). His TG level rapidly decreased to normal in response to treatment, and remained normal to borderline high during a ∼3-year follow-up period during which he had abstained completely from alcohol. Sequencing of the five primary HTG genes (i.e., LPL, APOC2, APOA5, GPIHBP1 and LMF1) identified two heterozygous variants. One was the common APOA5 c.553G > T (p.Gly185Cys) variant, which has been previously associated with altered TG levels as well as HTG-induced acute pancreatitis (HTG-AP). The other was a rare variant in the LPL gene, c.756T > G (p.Ile252Met), which was predicted to be likely pathogenic and found experimentally to cause a 40% loss of LPL activity without affecting either protein synthesis or secretion. We provide evidence that both a gene-gene interaction (between the common APOA5 variant and the rare LPL variant) and a gene-environment interaction (between alcohol and digenic inheritance) might have contributed to the development of mild HTG and alcohol-triggered AP in the patient, thereby improving our understanding of the complex etiology of HTG and HTG-AP.

Triglyceride-rich lipoproteins and atherosclerotic cardiovascular disease risk: current status and treatments

PURPOSE OF REVIEW

The role of triglyceride-rich lipoproteins (TRLs) in the development of atherosclerotic cardiovascular disease (ASCVD) is at the forefront of current research and treatment development programs. Despite extreme lowering of LDL-cholesterol there remains a high risk of cardiovascular disease and mortality. Recent large epidemiological, genomic wide association studies and Mendelian randomization studies have identified novel mechanisms and targets regulating TRL. This review will focus on recent and ongoing clinical trials that aim to reduce cardiovascular risk by decreasing plasma levels of TRL.

RECENT FINDINGS

Ongoing efforts of basic and clinical scientist have described novel TRL regulating mechanism. The concentration on lifestyle changes is key to prevention and treatment guidelines. There is continue evidence that supports previous guidelines using fibrates alone and in combination with niacin to reduce TRLs, in special cases. The recent results from the REDUCE-IT study support the use of eicosapentaenoic acid (EPA) for risk reduction and ASCVD, but recently presented data from the Long-Term Outcome Study to Assess Statin Residual Risk Reduction With Epanova in High Cardiovascular Risk Patients with Hypertriglyceridemia and Omega-3 Fatty Acids in Elderly Patients With Acute Myocardial Infarction studies do not support the use of combination EPA/docosahexaenoic acid. The latter highlights the need for further studies into the pathways regulating ASCVD risk reduction after EPA administration. The identification of novel targets, such as apolipoprotein C3 and angiopoietin-like protein-3, are driving the development of novel treatments, and is the focus of this review.

SUMMARY

The current management of elevated triglyceride levels and the effect on cardiovascular outcomes is an emerging area of research. New data from fish oil studies suggest differences in EPA vs. EPA/docosahexaenoic acid cardio protection outcomes. The preliminary data from ongoing clinical trials of novel triglyceride-lowering therapeutics are promising. These programs will ultimately provide foundations for future triglyceride-lowering guidelines.

A novel GPIHBP1 mutation related to familial chylomicronemia syndrome: A series of cases

BACKGROUND AND AIMS

GPIHBP1 is an accessory protein of lipoprotein lipase (LPL) essential for its functioning. Mutations in the GPIHBP1 gene cause a deficit in the action of LPL, leading to severe hypertriglyceridemia and increased risk for acute pancreatitis.

METHODS

We describe twelve patients (nine women) with a novel homozygous mutation in intron 2 of the GPIHBP1 gene.

RESULTS

All patients were from the Northeastern region of Brazil and presented the same homozygous variant located in a highly conserved 3' splicing acceptor site of the GPIHBP1 gene. This new variant was named c.182-1G > T, according to HGVS recommendations. We verified this new GPIHBP1 variant's effect by using the Human Splicing Finder (HSF) tool. This mutation changes the GPIHBP1 pre-mRNA processing and possibly causes the skipping of the exon 3 of the GPIHBP1 gene, affecting almost 50% of the cysteine-rich Lys6 GPIHBP1 domain. Patients presented with severe hypertriglyceridemia (2351 mg/dl [885-20600]) and low HDL (18 mg/dl [5-41). Four patients (33%) had a previous history of acute pancreatitis.

CONCLUSIONS

We describe a novel GPIHBP1 pathogenic intronic mutation of patients from the Northeast region of Brazil, suggesting the occurrence of a founder effect.

Non-Alcoholic Fatty Liver in Patients with Chylomicronemia

Non-alcoholic fatty liver disease (NAFLD) is frequent in patients with features of the metabolic syndrome (MetS), obesity, or type 2 diabetes. Lipoprotein lipase (LPL) is the main driver of triglyceride (TG) hydrolysis in chylomicrons and very-low density lipoproteins (VLDL). In some patients with MetS, dysfunction of this pathway can lead to plasma TG values > 10 mmol/L (multifactorial chylomicronemia or MCS). Chylomicronemia also characterizes LPL deficiency (LPLD), a rare autosomal recessive disease called familial chylomicronemia syndrome (FCS), which is associated with an increased risk of recurrent pancreatitis. This study aims to investigate the expression of NAFLD, as assessed by transient elastography, in MCS and FCS subjects. Data were obtained from 38 subjects with chylomicronemia; 19 genetically confirmed FCS and 19 sex- and age-matched MCS. All participants underwent liver ultrasonography and stiffness measurement after a 4-h fast using transient elastography (FibroScan®, Echosens, Waltham, MA, USA). NAFLD (controlled attenuation parameter (CAP) > 280 dB/m) was observed in 42.1% of FCS and 73.7% of MCS subjects (p = 0.05). FCS subjects had lower body mass index (BMI) than MCS. Only 25% of FCS subjects with NAFLD had a BMI ≥ 30 compared to 64.3% in MCS (p = 0.004). In FCS, NAFLD occurred even in the presence of very low (≤18 kg/m2) BMI. In both FCS and MCS, CAP was negatively associated with acute pancreatitis risk. In this study, NAFLD was commonly observed in both FCS and MCS subjects and occurred independently of the BMI and fasting glucose values in FCS; NAFLD was associated with a lower occurrence of acute pancreatitis episodes.

Lipoprotein Lipase and Its Regulators: An Unfolding Story

Lipoprotein lipase (LPL) is one of the most important factors in systemic lipid partitioning and metabolism. It mediates intravascular hydrolysis of triglycerides packed in lipoproteins such as chylomicrons and very-low-density lipoprotein (VLDL). Since its initial discovery in the 1940s, its biology and pathophysiological significance have been well characterized. Nonetheless, several studies in the past decade, with recent delineation of LPL crystal structure and the discovery of several new regulators such as angiopoietin-like proteins (ANGPTLs), glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), lipase maturation factor 1 (LMF1) and Sel-1 suppressor of Lin-12-like 1 (SEL1L), have completely transformed our understanding of LPL biology.

Involvement of a homozygous exon 6 deletion of LMF1 gene in intermittent severe hypertriglyceridemia

Severe hypertriglyceridemia (HTG), characterized by triglycerides (TG) permanently over 10 mmol/L, may correspond to familial chylomicronemia syndrome (FCS), a rare disorder. However, hypertriglyceridemic patients more often present multifactorial chylomicronemia syndrome (MCS), characterized by highly variable TG. A few nonsense variants of LMF1 gene were reported in literature in FCS patients. In this study, we described a woman with an intermittent severe HTG. NGS analysis and the sequencing of a long range PCR product revealed a homozygous deletion of 6507 base pairs in LMF1 gene, c.730-1528_898-3417del, removing exon 6, predicted to create an in-frame deletion of 56 amino acids, p.(Thr244_Gln299del). Despite an exon 6 homozygous deletion of LMF1, the patient's highly variable lipid phenotype was suggestive of MCS diagnosis.

Management of hypertriglyceridemia

Hypertriglyceridemia is one of the most common lipid abnormalities encountered in clinical practice. Many monogenic disorders causing severe hypertriglyceridemia have been identified, but in most patients triglyceride elevations result from a combination of multiple genetic variations with small effects and environmental factors. Common secondary causes include obesity, uncontrolled diabetes, alcohol misuse, and various commonly used drugs. Correcting these factors and optimizing lifestyle choices, including dietary modification, is important before starting drug treatment. The goal of drug treatment is to reduce the risk of pancreatitis in patients with severe hypertriglyceridemia and cardiovascular disease in those with moderate hypertriglyceridemia. This review discusses the various genetic and acquired causes of hypertriglyceridemia, as well as current management strategies. Evidence supporting the different drug and non-drug approaches to treating hypertriglyceridemia is examined, and an easy to adopt step-by-step management strategy is presented.

A CLN6-CLN8 complex recruits lysosomal enzymes at the ER for Golgi transfer

Lysosomal enzymes are synthesized in the endoplasmic reticulum (ER) and transferred to the Golgi complex by interaction with the Batten disease protein CLN8 (ceroid lipofuscinosis, neuronal, 8). Here we investigated the relationship of this pathway with CLN6, an ER-associated protein of unknown function that is defective in a different Batten disease subtype. Experiments focused on protein interaction and trafficking identified CLN6 as an obligate component of a CLN6-CLN8 complex (herein referred to as EGRESS: ER-to-Golgi relaying of enzymes of the lysosomal system), which recruits lysosomal enzymes at the ER to promote their Golgi transfer. Mutagenesis experiments showed that the second luminal loop of CLN6 is required for the interaction of CLN6 with the enzymes but dispensable for interaction with CLN8. In vitro and in vivo studies showed that CLN6 deficiency results in inefficient ER export of lysosomal enzymes and diminished levels of the enzymes at the lysosome. Mice lacking both CLN6 and CLN8 did not display aggravated pathology compared with the single deficiencies, indicating that the EGRESS complex works as a functional unit. These results identify CLN6 and the EGRESS complex as key players in lysosome biogenesis and shed light on the molecular etiology of Batten disease caused by defects in CLN6.

Aging and plasma triglyceride metabolism

The risk for metabolic disease, including metabolic syndrome, insulin resistance, and diabetes, increases with age. Altered plasma TG metabolism and changes in fatty acid partitioning are also major contributors to metabolic disease. Plasma TG metabolism itself is altered by age in humans and rodents. As discussed in this review, the age-induced changes in human TG metabolism include increased plasma TG levels, reduced postprandial plasma TG clearance rates, reduced postheparin LPL activity, decreased adipose tissue lipolysis, and elevated ectopic fat deposition, all of which could potentially contribute to age-associated metabolic diseases. Similar observations have been made in aged rats. We highlight the limitations of currently available data and propose that mechanistic studies are needed to understand the extent to which age-induced alterations in TG metabolism contribute to metabolic disease. Such mechanistic insights could aid in therapeutic strategies for preventing or managing metabolic disease in older individuals.

Integrin β3 Deficiency Results in Hypertriglyceridemia via Disrupting LPL (Lipoprotein Lipase) Secretion

OBJECTIVE

Integrin β3 is implicated in numerous biological processes such as its relevance to blood triglyceride, yet whether β3 deficiency affects this metabolic process remains unknown. Approach and Results: We showed that the Chinese patients with β3-deficient Glanzmann thrombasthenia had a 2-fold higher serum triglyceride level together with a lower serum LPL (lipoprotein lipase) level than those with an αIIb deficiency or healthy subjects. The β3 knockout mice recapitulated these phenotypic features. The elevated plasma triglyceride level was due to impaired LPL-mediated triglyceride clearance caused by a disrupted LPL secretion. Further analysis revealed that β3 directly bound LPL via a juxtamembrane TIH (threonine isoleucine histidine)^720-722 motif in its cytoplasmic domain and functioned as an adaptor protein by interacting with LPL and PKD (protein kinase D) to form the PKD/β3/LPL complex that is required for β3-mediated LPL secretion. Furthermore, the impaired triglyceride clearance in β3 knockout mice could be corrected by adeno-associated virus serotype 9 (AAV9)-mediated delivery of wild-type but not TIH^720-722-mutated β3 genes.

CONCLUSIONS

This study reveals a hypertriglyceridemia in both β3-deficient Chinese patients and mice and provides novel insights into the molecular mechanisms of the significant roles of β3 in LPL secretion and triglyceride metabolism, drawing attention to the metabolic consequences in patients with β3-deficient Glanzmann thrombasthenia.

Genetic and functional studies of the LMF1 gene in Thai patients with severe hypertriglyceridemia

Severe hypertriglyceridemia (HTG) due to chylomicronemia is associated with acute pancreatitis and is related to genetic disturbances in several proteins involved in triglyceride (TG) metabolism. Lipase maturation factor 1 (LMF1) is a protein essential for the maturation of lipoprotein lipase (LPL). In this study, we examined the genetic spectrum of the LMF1 gene among subjects with severe HTG and investigated the functional significance of 6 genetic variants in vitro. All 11 exons of the LMF1 gene were sequenced in 101 Thai subjects with severe HTG. For an in vitro study, we performed site-directed mutagenesis, transient expression in cld cells, and measured LPL protein and LPL activity. We identified 2 common variants [p.(Gly36Asp) and p.(Pro562Arg)] and 12 rare variants [p.(Thr143Met), p.(Asn249Ser), p.(Ala287Val), p.(Met346Val), p.(Thr395Ile), p.(Gly410Arg), p.(Asp433Asn), p.(Asp491Asn), p.(Asn501Tyr), p.(Ala504Val), p.(Arg523His), and p.(Leu563Arg)] in 29 patients. In vitro study of the p.(Gly36Asp), p.(Asn249Ser), p.(Ala287Val), p.(Asn501Tyr), p.(Pro562Arg) and p.(Leu563Arg) variants, however, revealed that both LPL mass and LPL activity in each of the transfected cells were not significantly different from those in the wild type LMF1 transfected cells, suggesting that these variants might not play a significant role in severe HTG phenotype in our subjects.

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Among 101 subjects with severe hypertriglyceridemia (HTG), 2 common and 12 rare variants in the LMF1 gene were identified

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None of the 6 missense variants studied were associated with a reduction in lipoprotein mass or activity

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These rare variants in the LMF1 gene may not play an important role in severe HTG phenotypes in the Thai population

Critical Role of SREBP-1c Large-VLDL Pathway in Environment-Induced Hypertriglyceridemia of Apo AV Deficiency

Objective- APOA5 variants are strongly associated with hypertriglyceridemia, as well as increased risks of cardiovascular disease and acute pancreatitis. Hypertriglyceridemia in apo AV dysfunction often aggravates by environmental factors such as high-carbohydrate diets or aging. To date, the molecular mechanisms by which these environmental factors induce hypertriglyceridemia are poorly defined, leaving the high-risk hypertriglyceridemia condition undertreated. Previously, we reported that LXR (liver X receptor)-SREBP (sterol regulatory element-binding protein)-1c pathway regulates large-VLDL (very low-density lipoprotein) production induced by LXR agonist. However, the pathophysiological relevance of the finding remains unknown. Approach and Results- Here, we reconstitute the environment-induced hypertriglyceridemia phenotype of human APOA5 deficiency in Apoa5^-/- mice and delineate the role of SREBP-1c in vivo by generating Apoa5^-/- ;Srebp-1c^-/- mice. The Apoa5^-/- mice, which showed moderate hypertriglyceridemia on a chow diet, developed severe hypertriglyceridemia on high-carbohydrate feeding or aging as seen in patients with human apo AV deficiency. These responses were nearly completely abolished in the Apoa5^-/- ;Srebp-1c^-/- mice. Further mechanistic studies revealed that in response to these environmental factors, SREBP-1c was activated to increase triglyceride synthesis and to permit the incorporation of triglyceride into abnormally large-VLDL particles, which require apo AV for efficient clearance. Conclusions- Severe hypertriglyceridemia develops only when genetic factors (apo AV deficiency) and environmental effects (SREBP-1c activation) coexist. We demonstrate that the regulated production of large-sized VLDL particles via SREBP-1c determines plasma triglyceride levels in apo AV deficiency. Our findings explain the long-standing enigma of the late-onset hypertriglyceridemia phenotype of apo AV deficiency and suggest a new approach to treat hypertriglyceridemia by targeting genes that mediate environmental effects.

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.

Endoplasmic reticulum quality control in lipoprotein metabolism

Lipids play a critical role in energy metabolism, and a suite of proteins is required to deliver lipids to tissues. Several of these proteins require an intricate endoplasmic reticulum (ER) quality control (QC) system and unique secondary chaperones for folding. Key examples include apolipoprotein B (apoB), which is the primary scaffold for many lipoproteins, dimeric lipases, which hydrolyze triglycerides from circulating lipoproteins, and the low-density lipoprotein receptor (LDLR), which clears cholesterol-rich lipoproteins from the circulation. ApoB requires specialized proteins for lipidation, dimeric lipases lipoprotein lipase (LPL) and hepatic lipase (HL) require a transmembrane maturation factor for secretion, and the LDLR requires several specialized, domain-specific chaperones. Deleterious mutations in these proteins or their chaperones may result in dyslipidemias, which are detrimental to human health. Here, we review the ER quality control systems that ensure secretion of apoB, LPL, HL, and LDLR with a focus on the specialized chaperones required by each protein.

Parenchyma Preserving Surgery for Idiopathic Chronic Calcific Pancreatitis in Children: A Report of Three Cases

Background: Idiopathic chronic calcific pancreatitis is a rare entity. Early surgical intervention and a parenchyma sparing procedure should be advocated to prevent further decay of the pancreas and the occurrence of cancer.

Case Presentations: Case 1: A 14-year-old boy presented with a 3-year history of right upper abdominal pain that has been aggravated in the last 2 months. Imaging revealed a dilated pancreatic duct of 6 mm with pancreatic duct stones in the head of pancreas. He underwent a Frey's procedure. Unfortunately, he was discharged with grade B pancreatic fistula. Case 2: A 12-year-old boy presented with a 1-year history of dull and recurring epigastric pain. Imaging studies showed multiple stones in a 12 mm dilated pancreatic duct. The patient underwent a modified Puestow procedure. Up to the 42th month follow-up, the patient had no pain complaints. Case 3: A 12-year-old boy with a 1-week history of a dull epigastric pain presented with with multiple stones in a 10 mm duct. He underwent a modified Puestow procedure and was discharged with alleviated pain.

Conclusions: “Conservative” surgery allows satisfactory pancreatic duct drainage, reduced rehospitalizations, and longer pain relief than alternative endoscopic procedures.

Very Severe Hypertriglyceridemia in a Large US County Health Care System: Associated Conditions and Management

Context

Patients with very severe hypertriglyceridemia (triglyceride levels ≥2000 mg/dL; 22.6 mmol/L) require aggressive treatment. However, little research exists on the underlying etiologies and management of very severe hypertriglyceridemia.

Objective

We hypothesized (i) very severe hypertriglyceridemia in adults is mostly associated with secondary causes and (ii) most patients with very severe hypertriglyceridemia lack appropriate follow-up and treatment.

Design

We queried electronic medical records at Parkland Health and Hospital Systems for lipid measurements in the year 2016 and identified patients with serum triglyceride levels ≥2000 mg/dL (22.6 mmol/L). We extracted data on demographics, underlying causes, lipid-lowering therapy, and follow-up.

Results

One hundred sixty-four serum triglyceride measurements were ≥2000 mg/dL (22.6 mmol/L) in 103 unique patients. Of these, 60 patients were admitted to the hospital (39 for acute pancreatitis). Most were Hispanic (79%). The major conditions associated with very severe hypertriglyceridemia included uncontrolled diabetes mellitus (74%), heavy alcohol use (10%), medication use (7%), and hypothyroidism (2%). Two patients were known to have monogenic causes of hypertriglyceridemia. After the index measurement of triglycerides ≥2000 mg/dL (22.6 mmol/L), the use of triglyceride-lowering drugs increased, most prominently the use of fish oil supplements, which increased by 80%. However, in follow-up visits, hypertriglyceridemia was addressed in only 50% of encounters, and serum triglycerides were remeasured in only 18%.

Conclusion

In summary, very severe hypertriglyceridemia was quite prevalent (∼0.1% of all lipid measurements) in our large county health care system, especially in Hispanic men. Most cases were related to uncontrolled diabetes mellitus, and follow-up monitoring was inadequate.

Structure of lipoprotein lipase in complex with GPIHBP1

Lipoprotein lipase (LPL) plays a central role in triglyceride (TG) metabolism. By catalyzing the hydrolysis of TGs present in TG-rich lipoproteins (TRLs), LPL facilitates TG utilization and regulates circulating TG and TRL concentrations. Until very recently, structural information for LPL was limited to homology models, presumably due to the propensity of LPL to unfold and aggregate. By coexpressing LPL with a soluble variant of its accessory protein glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) and with its chaperone protein lipase maturation factor 1 (LMF1), we obtained a stable and homogenous LPL/GPIHBP1 complex that was suitable for structure determination. We report here X-ray crystal structures of human LPL in complex with human GPIHBP1 at 2.5-3.0 Å resolution, including a structure with a novel inhibitor bound to LPL. Binding of the inhibitor resulted in ordering of the LPL lid and lipid-binding regions and thus enabled determination of the first crystal structure of LPL that includes these important regions of the protein. It was assumed for many years that LPL was only active as a homodimer. The structures and additional biochemical data reported here are consistent with a new report that LPL, in complex with GPIHBP1, can be active as a monomeric 1:1 complex. The crystal structures illuminate the structural basis for LPL-mediated TRL lipolysis as well as LPL stabilization and transport by GPIHBP1.

Mouse germ line mutations due to retrotransposon insertions

Transposable element (TE) insertions are responsible for a significant fraction of spontaneous germ line mutations reported in inbred mouse strains. This major contribution of TEs to the mutational landscape in mouse contrasts with the situation in human, where their relative contribution as germ line insertional mutagens is much lower. In this focussed review, we provide comprehensive lists of TE-induced mouse mutations, discuss the different TE types involved in these insertional mutations and elaborate on particularly interesting cases. We also discuss differences and similarities between the mutational role of TEs in mice and humans.

The Forgotten Lipids: Triglycerides, Remnant Cholesterol, and Atherosclerotic Cardiovascular Disease Risk

Atherosclerotic cardiovascular disease (ASCVD) remains the leading cause of death worldwide. Low-density lipoprotein cholesterol (LDL-C) is a well-established mediator of atherosclerosis and a key target for intervention for the primary and secondary prevention of ASCVD. However, despite substantial reduction in LDL-C, patients continue to have recurrent ASCVD events. Hypertriglyceridemia may be an important contributor of this residual risk. Observational and genetic epidemiological data strongly support a causal role of triglycerides (TGs) and the cholesterol content within triglyceride-rich lipoproteins (TGRLs) and/or remnant cholesterol (RC) in the development of ASCVD. TGRLs are composed of hepatically derived very low-density lipoprotein and intestinally derived chylomicrons. RC is the cholesterol content of all TGRLs and plasma TGs serve as a surrogate measure of TGRLs and RC. Although lifestyle modification remains the cornerstone for management of hypertriglyceridemia, many novel drugs are in development and have shown impressive efficacy in lowering TG levels. Several ongoing, randomized controlled trials are underway to examine the impact of these novel agents on ASCVD outcomes. In this comprehensive review, we provide an overview of the biology, epidemiology, and genetics of TGs and ASCVD; we discuss current and novel TG-lowering therapies under development.

Mitochondrial function in immature bovine oocytes is improved by an increase of cellular cyclic AMP

Although in vitro maturation (IVM) of oocytes is important for assisted reproduction, the rate of development of embryos from IVM oocytes is lower than from their in vivo counterparts. It has been shown that an artificial increase of intracellular cAMP before culture significantly improves oocyte developmental competence in cattle and mice. Here, we revealed that forskolin and 3-isobutyl-1-methylxanthine treatment of prophase-stage oocytes induced the expression of genes required for glycolysis, fatty acid degradation, and the mitochondrial electron transport system and improved mitochondrial functions and ATP levels in oocytes without involving nuclear maturation. We propose the existence of a comprehensive energy-supply system in oocytes under follicle-stimulating hormone stimulation as a potential explanation of how oocytes acquire developmental competence.

Identification of a novel and heterozygous LMF1 nonsense mutation in an acute pancreatitis patient with severe hypertriglyceridemia, severe obesity and heavy smoking

BACKGROUND

Hypertriglyceridemia (HTG) is one of the most common etiologies of acute pancreatitis (AP). Variants in five genes involved in the regulation of plasma lipid metabolism, namely LPL, APOA5, APOC2, GPIHBP1 and LMF1, have been frequently reported to cause or predispose to HTG.

METHODS

A Han Chinese patient with HTG-induced AP was assessed for genetic variants by Sanger sequencing of the entire coding and flanking sequences of the above five genes.

RESULTS

The patient was a 32-year-old man with severe obesity (Body Mass Index = 35) and heavy smoking (ten cigarettes per day for more than ten years). At the onset of AP, his serum triglyceride concentration was elevated to 1450.52 mg/dL. We sequenced the entire coding and flanking sequences of the LPL, APOC2, APOA5, GBIHBP1 and LMF1 genes in the patient. We found no putative deleterious variants, with the exception of a novel and heterozygous nonsense variant, c.1024C > T (p.Arg342*; rs776584760), in exon 7 of the LMF1 gene.

CONCLUSIONS

This is the first time that a heterozygous LMF1 nonsense variant was found in a HTG-AP patient with severe obesity and heavy smoking, highlighting an important interplay between genetic and lifestyle factors in the etiology of HTG.

Coexpression of novel furin-resistant LPL variants with lipase maturation factor 1 enhances LPL secretion and activity

LPL is a secreted enzyme that hydrolyzes triglycerides from circulating lipoproteins. Individuals lacking LPL suffer from severe hypertriglyceridemia, a risk factor for acute pancreatitis. One potential treatment is to administer recombinant LPL as a protein therapeutic. However, use of LPL as a protein therapeutic is limited because it is an unstable enzyme that is difficult to produce in large quantities. Furthermore, these considerations also limit structural and biochemical studies that are needed for large-scale drug discovery efforts. We demonstrate that the yield of purified LPL can be dramatically enhanced by coexpressing its maturation factor, LMF1, and by introducing novel mutations into the LPL sequence to render it resistant to proteolytic cleavage by furin. One of these mutations introduces a motif for addition of an N-linked glycan to the furin-recognition site. Furin-resistant LPL has previously been reported, but is not commonly used. We show that our modifications do not adversely alter LPL's enzymatic activity, stability, or in vivo function. Together, these data show that furin-resistant LPL is a useful reagent for both biochemical and biomedical studies.