Rare genetic variants with large effect on triglycerides in subjects with a clinical diagnosis of familial vs nonfamilial hypertriglyceridemia

The role of lipase maturation factor 1 in hypertriglyceridaemia and atherosclerosis: An update

Lipase maturation factor 1 is an endoplasmic reticulum-resident transmembrane protein, which acts as a critical chaperone necessary for the folding, dimerisation, and secretion of lipases. In this review, we summarise data about the recently revealed role of lipase maturation factor 1 in endoplasmic reticulum redox homeostasis, its novel interaction partners among oxidoreductases and lectin chaperones, and the identification of fibronectin and the low-density lipoprotein receptor as novel non-lipase client proteins of lipase maturation factor 1. Additionally, the role of lipase maturation factor 1-derived circular RNA in atherosclerosis progression via the miR-125a-3p/vascular endothelial growth factor A\Fibroblast Growth Factor 1 axis is discussed. Finally, we focus on the causative role of lipase maturation factor 1 variants in the development of hypertriglyceridaemia - a type of dyslipidaemia that significantly contributes to the development of atherosclerosis and other cardiovascular diseases via different mechanisms.

Evaluation of apolipoprotein A5 variants: A cohort of patients with severe hypertriglyceridemia from Turkiye

BACKGROUND

This study aims to show the clinical and biochemical features in patients with severe hypertriglyceridemia (HTG) associated with rare variants in the apolipoprotein A-V (APOA5) gene.

MATERIALS AND METHODS

Demographics, blood lipid levels, body mass index (BMI) and APOA5 mutation subtypes were collected from the endocrinology clinic registry and analyzed for a retrospective cohort study of ten patients with severe HTG and APOA5 gene variants.

RESULTS

Of the 10 cases, four were female, and six were male. The median age was 45.0 years (min-max: 21-60 years), the median triglyceride was 2429.5 mg/dL (27.5 mmol/L) (min-max: 1351-4087 mg/dL, 15.3-46.2 mmol/L), and the mean BMI was calculated as 30.4 ± 4.4 kg/m2 (min-max: 24.9-41.0 kg/m2). Four cases had diabetes mellitus (DM); two were on intensive insulin therapy, and two were on basal insulin therapy. The mean hemoglobin A1c was 9.2 ± 1.2 % (min-max: 8.3-11.0 %). Among the study group, eight different APOA5 gene mutations were detected. These variants were heterozygous in 2 patients and homozygous (bi-allelic) in 8 patients. One patient was homozygous for APOA5 p.Ser19Trp, a relatively common polymorphism that is a risk variant for HTG.

CONCLUSION

We report a cohort of patients with biallelic and single copy APOA5 variants, who were diagnosed later in life. Most had secondary factors, such as DM or obesity with increased BMI. Most rare APOA5 variants found in our patients were of uncertain significance. Our results add to the growing evidence that rare variants in certain candidate genes may predispose to developing HTG, together with secondary factors such as obesity. The genetic basis of HTG in many other patients is still unknown and remains the subject of further investigation.

Genetics of Familial Combined Hyperlipidemia (FCHL) Disorder: An Update

Familial combined hyperlipidemia (FCHL) is one of the most common familial lipoprotein disorders of the lipoproteins, with a prevalence of 0.5% to 2% in different populations. About 10% of these patients suffer from cardiovascular disease and this number is increased by up to 11.3% in the young survivors of myocardial infarction and by 40% among all the survivors of myocardial infarction. Although initially thought to be that FCHL has an inheritance pattern of monogenic, the disease's etiology is still not fully understood and it appears that FCHL has a complex pattern related to genetic variants, environmental factors, and lifestyles. Two strategies have been used to identify its complex genetic background: candidate gene and the linkage approach, which have yielded an extensive list of genes associated with FCHL with a variable degree of scientific evidence. Until now, more than 30 different genetic variants have been identified related to FCHL. In this study, we aimed to review the individual genes that have been described in FCHL and how these genes and variants can be related to the current concept of metabolic pathways resulting in familial combined hyperlipidemia.

The Genetic Basis of Hypertriglyceridemia

PURPOSE OF REVIEW

Hypertriglyceridemia is a common dyslipidemia associated with an increased risk of cardiovascular disease and pancreatitis. Severe hypertriglyceridemia may sometimes be a monogenic condition. However, in the vast majority of patients, hypertriglyceridemia is due to the cumulative effect of multiple genetic risk variants along with lifestyle factors, medications, and disease conditions that elevate triglyceride levels. In this review, we will summarize recent progress in the understanding of the genetic basis of hypertriglyceridemia.

RECENT FINDINGS

More than 300 genetic loci have been identified for association with triglyceride levels in large genome-wide association studies. Studies combining the loci into polygenic scores have demonstrated that some hypertriglyceridemia phenotypes previously attributed to monogenic inheritance have a polygenic basis. The new genetic discoveries have opened avenues for the development of more effective triglyceride-lowering treatments and raised interest towards genetic screening and tailored treatments against hypertriglyceridemia. The discovery of multiple genetic loci associated with elevated triglyceride levels has led to improved understanding of the genetic basis of hypertriglyceridemia and opened new translational opportunities.

Genetic basis of hypertriglyceridemies

Diagnosis and treatment of triglyceride metabolism disorders: from pathophysiology to clinical practice. Hypertriglyceridaemia (HTG) affects 15%-20% of the world's population, and is frequently discovered as an incidental finding in a laboratory test. Disorders of triglyceride (TG) metabolism have a complex genetic basis. New genetic tools that allow a more precise approach to the disorders have made it possible to redefine and classify HTG into two groups: monogenic HTG (TG>10 mmol/L) and polygenic HTG (2 mmol/L<TG<10 mmol/L) with a milder phenotype, but with a clear genetic influence. In approximately 50% of patients with severe HTG a genetic cause has not yet been found. In addition to the inclusion of ever more genes in studies, statistical models are now also being examined that consider complex gene-environment interactions that could explain why the presence of a set of apparently benign variants may cause HTG in the presence of a triggering factor such as adiposity. Knowledge of the genetic nature of HTG has also helped identify targets for pharmacological treatments, thus avoiding a strict diet with a fat content of less than 20%, which is difficult to maintain. Accurate diagnosis of these disorders is essential for correct treatment according to the inherent risk of each HTG, since, as has been shown in multiple studies, high fasting and postprandial TG concentrations are an independent risk factor for cardiovascular disease.

Variants of Lipid-Related Genes in Adult Japanese Patients with Severe Hypertriglyceridemia

Aim: Hypertriglyceridemia is a type of dyslipidemia that contributes to atherosclerosis and coronary heart disease. Variants in lipoprotein lipase (LPL), apolipoprotein CII (APOC2), apolipoprotein AV (APOA5), glycosylphosphatidylinositol- anchored high-density lipoprotein-binding protein 1 (GPIHBP1), lipase maturation factor 1 (LMF1), and glucokinase regulator (GCKR) are responsible for hypertriglyceridemia. We investigated the molecular basis of severe hypertriglyceridemia in adult patients referred to the Clinical Laboratory at Fukuoka University Hospital.

Methods: Twenty-three adult patients with severe hypertriglyceridemia (> 1,000 mg/dL, 11.29 mmol/L) were selected. The coding regions of candidate genes were sequenced by next-generation sequencing. Forty-nine genes reportedly associated with hypertriglyceridemia were analyzed.

Results: In the 23 patients, we detected 70 variants: 28 rare and 42 common ones. Among the 28 rare variants with < 1% allele frequency, p.I4533L in APOB, p.M490I in MLXIPL, p.L152M in NCAN, and p.S264T in TIMD4 were novel. We did not observe single gene homozygous or compound heterozygous disease-causing rare variants in any of the 23 hypertriglyceridemia cases. However, in silico algorithms and previous reports indicated that five rare variants, APOA5 (p.T184S), GCKR (c.354 + 1G>A), LMF1 (p.G410R), and LRP1 (p.G813R; p.R2173Q), and seven common variants, APOA5 (pG185C), APOE (p.C130R; p.E262K/p.E263K), GCKR (p.V103M), GPIHBP1 (p.C14F), LRP1 (p.Y4054F), and MLXIPL (p.Q241H), can cause hypertriglyceridemia. However, all five disease-causing rare variants detected in this study were heterozygous.

Conclusions: The prevalence of disease-causing rare variants in candidate genes in severe hypertriglyceridemia patients was low. The major causes of severe hypertriglyceridemia were not single gene abnormalities, but involved multiple gene variations and environmental factors.

Primary Hypertriglyceridemia: A Look Back on the Clinical Classification and Genetics of the Disease

INTRODUCTION

Hypertriglyceridemia (HTG) is one of the most common metabolic disorders leading to pancreatitis and cardiovascular disease. HTG develops mostly due to impaired metabolism of triglyceride-rich lipoproteins. Although monogenic types of HTG exist, most reported cases are polygenic in nature.

AIM

This review article is focused on the classification of Primary HTG and the genetic factors behind its development with the aim of providing clinicians a useful tool for early detection of the disease in order to administer proper and effective treatment.

DISCUSSION

HTG is often characterized by a complex phenotype resulting from interactions between genetic and environmental factors. In many instances, the complexity, perplexing causes, and classification of HTG make it difficult for clinicians to properly diagnose and manage the disorder. Better availability of information on its pathophysiology, genetic factors involved, environmental causes, and their interactions could help in understanding such complex disorders and could support its effective diagnosis and treatment.

CONCLUSION

The current review has summarized the case definition, epidemiology, pathophysiology, clinical presentation, classification, associated genetic factors, and scope of genetic screening in the diagnosis of primary HTG.

Genetic variants in the LPL and GPIHBP1 genes, in patients with severe hypertriglyceridaemia, detected with high resolution melting analysis

INTRODUCTION

Pathogenic variants in lipoprotein lipase (LPL) and glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) have been described in patients with severe hypertriglyceridaemia. We aimed to optimise high resolution melting (HRM) assays to detect the presence of functional variants in these genes.

METHODS

One hundred and sixteen patients with severe hypertriglyceridaemia were studied. HRM assays were optimised to scan exons and splice junctions in LPL and GPIHBP1. Sanger sequencing was the reference method. Next-generation-sequencing (NGS) was performed in five patients, including one with Familial Chylomicronemia syndrome (FCS).

RESULTS

We identified 15 different variants in LPL and 6 in GPIHBP1. The variants revealed with NGS were also detected with HRM, including a rare premature stop codon in LPL (p.Trp421*) and two LPL pathogenic variants in the patient with FCS (p.His80Arg + p.Gly215Glu). Having multiple functional variant alleles was associated with pancreatitis onset at younger ages and higher baseline triglycerides.

CONCLUSIONS

Our HRM assays detected the presence of functional gene variants that were confirmed with Sanger and NGS sequencing. The presence of multiple functional variant alleles was associated with differences in the clinical profile. Therefore, these assays represent a reliable, cost-effective tool that can be used to complement the NGS approach for gene scanning.

Spectrum of Mutations and Long-Term Clinical Outcomes in Genetic Chylomicronemia Syndromes

OBJECTIVE

Familial chylomicronemia syndrome (FCS) and multifactorial chylomicronemia syndrome (MCS) are the prototypes of monogenic and polygenic conditions underlying genetically based severe hypertriglyceridemia. These conditions have been only partially investigated so that a systematic comparison of their characteristics remains incomplete. We aim to compare genetic profiles and clinical outcomes in FCS and MCS. Approach and Results: Thirty-two patients with severe hypertriglyceridemia (triglyceride >1000 mg/dL despite lipid-lowering treatments with or without history of acute pancreatitis) were enrolled. Rare and common variants were screened using a panel of 18 triglyceride-raising genes, including the canonical LPL, APOC2, APOA5, GP1HBP1, and LMF1. Clinical information was collected retrospectively for a median period of 44 months. Across the study population, 37.5% were classified as FCS due to the presence of biallelic, rare mutations and 59.4% as MCS due to homozygosity for nonpathogenic or heterozygosity for pathogenic variants in canonical genes, as well as for rare and low frequency variants in noncanonical genes. As compared with MCS, FCS patients showed a lower age of hypertriglyceridemia onset, higher levels of on-treatment triglycerides, and 3-fold higher incidence rate of acute pancreatitis.

CONCLUSIONS

Our data indicate that the genetic architecture and natural history of FCS and MCS are different. FCS expressed the most severe clinical phenotype as determined by resistance to triglyceride-lowering medications and higher incidence of acute pancreatitis episodes. The most common genetic abnormality underlying FCS was represented by biallelic mutations in LPL while APOA5 variants, in combination with high rare polygenic burden, were the most frequent genotype of MCS.

Intensive genetic analysis for Chinese patients with very high triglyceride levels: Relations of mutations to triglyceride levels and acute pancreatitis

BACKGROUND

Severe hypertriglyceridemia (SHTG, TG ≥5·65 mmol/L), a disease, usually resulting from a combination of genetic and environmental factors, may increase the risk of acute pancreatitis (AP). However, previous genetic analysis has been limited by lacking of related observation of gene to AP.

METHODS

The expanding genetic sequencing including 15 TG-related genes (LPL, LMF1, APOC2, GPIHBP1, GCKR, ANGPTL3, APOB, APOA1-A4-C3-A5, TRIB1, CETP, APOE, and LIPI) was performed within 103 patients who were diagnosed with primary SHTG and 46 age- and sex-matched normal controls.

FINDINGS

Rare variants were found in 46 patients and 12 controls. The detection rate of rare variants in SHTG group increased by 19·5% via intensive genetic analysis. Presence of rare variants in LPL, APOA5, five LPL molecular regulating genes and all the sequenced genes were found to be associated with SHTG (p < 0·05). Of noted, patients with history of AP presented higher frequency of rare variants in LPL gene and all the LPL molecular regulating genes (27·8% vs.4·7% and 50·0% vs. 20·0%). The risk scores for SHTG determined by common TG-associated variants were increased in subgroups according to the extent of SHTG when they were compared with that of controls. Finally, patients without rare variants within SHTG group also presented higher risk scores than control group (p < 0·05).

INTERPRETATION

Expanding genetic analysis had a higher detection rate of rare variants in patients with SHTG. Rare variants in LPL and its molecular regulating genes could increase the risk of AP among Chinese patients with SHTG. FUND: This work was partially supported by the Capital Health Development Fund (201614035) and CAMS. Major Collaborative Innovation Project (2016-I2M-1-011) awarded to Dr. Jian-Jun Li, MD, PhD.

Characterization of two novel pathogenic variants at compound heterozygous status in lipase maturation factor 1 gene causing severe hypertriglyceridemia

BACKGROUND

Severe hypertriglyceridemia is a rare disease characterized by triglyceride levels higher than 1000 mg/dL (11.3 mmol/L) and acute pancreatitis. The disease is caused by pathogenic variants in genes encoding lipoprotein lipase (LPL), apolipoprotein A5, apolipoprotein C2, glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1, and lipase maturation factor 1 (LMF1).

OBJECTIVE

We aim to identify the genetic cause of severe hypertriglyceridemia and characterize the new variants in a patient with severe hypertriglyceridemia.

METHODS

The proband was a male showing severe hypertriglyceridemia (triglycerides 1416 mg/dL, 16.0 mmol/L); proband's relatives were also screened. Genetic screening included direct sequencing of the above genes and identification of large rearrangements in the LPL gene. Functional characterization of mutant LMF1 variants was performed by complementing LPL maturation in transfected LMF1-deficient mouse fibroblasts.

RESULTS

The proband and his affected brother were compound heterozygotes for variants in the LMF1 gene never identified as causative of severe hypertriglyceridemia c.[157delC;1351C>T];[410C>T], p.[(Arg53Glyfs*5)];[(Ser137Leu)]. Functional analysis demonstrated that the p.(Arg53Glyfs*5) truncation completely abolished and the p.(Ser137Leu) missense variant dramatically diminished the lipase maturation activity of LMF1.

CONCLUSIONS

In addition to a novel truncating variant, we describe for the first time a missense variant functionally demonstrated affecting the lipase maturation function of LMF1. This is the first case in which compound heterozygous variants in LMF1 were functionally demonstrated as causative of severe hypertriglyceridemia.

Chronic pancreatitis: an update on genetic risk factors

PURPOSE OF REVIEW

Genetic mutations in genes within and outside of the trypsin-dependent pathologic pathway have been found to be associated with chronic pancreatitis. This review highlights recent developments.

RECENT FINDINGS

CTRB1-CTRB2 has been identified as a new risk locus for chronic pancreatitis and the disease mechanism may involve trypsin degradation. Misfolding mutations in PRSS1, CPA1, and CEL, as well as environmental stress factors like tobacco and alcohol can trigger endoplasmic reticulum stress (ER-Stress).

SUMMARY

Protein misfolding as well as enzyme activity changes due to altered autoactivation, intracellular degradation, or enzyme inhibition represent the most important pathological mechanisms of chronic pancreatitis to date. Analysis of composite risk patterns by next-generation sequencing will help elucidate complex gene interactions and identify new potential therapeutic targets.

New rare genetic variants of LMF1 gene identified in severe hypertriglyceridemia

BACKGROUND

The LMF1 (lipase maturation factor 1) gene encodes a protein involved in lipoprotein lipase and hepatic lipase maturation. Homozygous mutations in LMF1 leading to severe hypertriglyceridemia (SHTG) are rare in the literature. A few additional rare LMF1 variants have been described with poor functional studies.

OBJECTIVE

The aim of this study was to assess the frequency of LMF1 variants in a cohort of 385 patients with SHTG, without homozygous or compound heterozygous deleterious mutations identified in lipoprotein lipase (LPL), apolipoprotein A5 (APOA5), apolipoprotein C2 (APOC2), glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1) genes, and to determine their functionality.

METHODS

LMF1 coding variants were screened using denaturing high-performance liquid chromatography followed by direct sequencing. In silico studies of LMF1 variants were performed, followed by in vitro functional studies using human embryonic kidney 293T (HEK-293T) cells cotransfected with vectors encoding human LPL and LMF1 cDNA. LPL activity was measured in cell culture medium after heparin addition using human VLDL-TG as substrate.

RESULTS

Nineteen nonsynonymous coding LMF1 variants were identified in 65 patients; 10 variants were newly described in SHTG. In vitro, p.Gly172Arg, p.Arg354Trp, p.Arg364Gln, and p.Arg537Trp LMF1 variants decreased LPL activity, and the p.Trp464Ter variant completely abolished LPL activity. We identified a young girl heterozygote for the p.Trp464Ter variant and a homozygote carrier of the p.Gly172Arg variant with a near 50% decreased LPL activity in vitro and in vivo.

CONCLUSION

The study confirms the rarity of LMF1 variants in a large cohort of patients with SHTG. LMF1 variants are likely to be involved in multifactorial hyperchylomicronemia. Partial LMF1 defects could be associated with intermittent phenotype as described for p.Gly172Arg homozygous and p.Trp464Ter heterozygous carriers.

GPIHBP1 autoantibodies in a patient with unexplained chylomicronemia

BACKGROUND

GPIHBP1, a glycolipid-anchored protein of capillary endothelial cells, binds lipoprotein lipase (LPL) in the interstitial spaces and transports it to the capillary lumen. GPIHBP1 deficiency prevents LPL from reaching the capillary lumen, resulting in low intravascular LPL levels, impaired intravascular triglyceride processing, and severe hypertriglyceridemia (chylomicronemia). A recent study showed that some cases of hypertriglyceridemia are caused by autoantibodies against GPIHBP1 ("GPIHBP1 autoantibody syndrome").

OBJECTIVE

Our objective was to gain additional insights into the frequency of the GPIHBP1 autoantibody syndrome in patients with unexplained chylomicronemia.

METHODS

We used enzyme-linked immunosorbent assays to screen for GPIHBP1 autoantibodies in 33 patients with unexplained chylomicronemia and then used Western blots and immunocytochemistry studies to characterize the GPIHBP1 autoantibodies.

RESULTS

The plasma of 1 patient, a 36-year-old man with severe hypertriglyceridemia, contained GPIHBP1 autoantibodies. The autoantibodies, which were easily detectable by Western blot, blocked the ability of GPIHBP1 to bind LPL. The plasma levels of LPL mass and activity were low. The patient had no history of autoimmune disease, but his plasma was positive for antinuclear antibodies.

CONCLUSIONS

One of 33 patients with unexplained chylomicronemia had the GPIHBP1 autoantibody syndrome. Additional studies in large lipid clinics will be helpful for better defining the frequency of this syndrome and for exploring the best strategies for treatment.