👤 Sebastiano Calandra

We aimed to compare the molecular and clinical characteristics of patients identified in Italy as affected by either familial chylomicronemia syndrome (FCS) or multifactorial chylomicronemia syndrome (MCS) and to assess the overall benefit of novel triglyceride-lowering therapies prescribed to these patients within the routine clinical care. From the national LIPIGEN-sHTG (Lipid Transport Disorders Italian Genetic Network-Severe Hypertriglyceridemia) registry, 169 patients (57 FCS, 51 MCS, 61 variant-negative, variant-negative MCS) were retrospectively analyzed. Data on clinical and genetic characteristics, medical history, and medications were collected. Peak triglyceride levels were used to define untreated lipid phenotypes. In FCS, 72% exhibited biallelic As compared with MCS, patients with FCS showed a more severe phenotype and higher prevalence of Show less

Three members of the angiopoietin-like (ANGPTL) protein family-ANGPTL3, ANGPTL4 and ANGPTL8- are important regulators of plasma lipoproteins. They inhibit the enzyme lipoprotein lipase, which plays a key role in the intravascular lipolysis of triglycerides present in some lipoprotein classes. This review focuses on the role of ANGPTL3 as emerged from the study of genetic variants of Angptl3 gene in mice and humans. Both loss of function genetic variants and inactivation of Angptl3 gene in mice are associated with a marked reduction of plasma levels of triglyceride and cholesterol and an increased activity of lipoprotein lipase and endothelial lipase. In humans with ANGPTL3 deficiency, caused by homozygous loss of function (LOF) variants of Angptl3 gene, the levels of all plasma lipoproteins are greatly reduced. This plasma lipid disorder referred to as familial combined hypolipidemia (FHBL2) does not appear to be associated with distinct pathological manifestations. Heterozygous carriers of LOF variants have reduced plasma levels of total cholesterol and triglycerides and are at lower risk of developing atherosclerotic cardiovascular disease, as compared to non-carriers. These observations have paved the way to the development of strategies to reduce the plasma level of atherogenic lipoproteins in man by the inactivation of ANGPTL3, using either a specific monoclonal antibody or anti-sense oligonucleotides. Show less

The incidental finding of severe hypertriglyceridemia (HyperTG) in a child may suggest the diagnosis of familial chylomicronemia syndrome (FCS), a recessive disorder of the intravascular hydrolysis of triglyceride (TG)-rich lipoproteins. FCS may be due to pathogenic variants in lipoprotein lipase (LPL), as well as in other proteins, such as apolipoprotein C-II and apolipoprotein A-V (activators of LPL), GPIHBP1 (the molecular platform required for LPL activity on endothelial surface) and LMF1 (a factor required for intracellular formation of active LPL). Molecular characterization of 5 subjects in whom HyperTG was an incidental finding during infancy/childhood. We performed the parallel sequencing of 20 plasma TG-related genes. Three children with severe HyperTG were found to be compound heterozygous for rare pathogenic LPL variants (2 nonsense, 3 missense, and 1 splicing variant). Another child was found to be homozygous for a nonsense variant of APOA5, which was also found in homozygous state in his father with longstanding HyperTG. The fifth patient with a less severe HyperTG was found to be heterozygous for a frameshift variant in LIPC resulting in a truncated Hepatic Lipase. In addition, 1 of the patients with LPL deficiency and the patient with APOA-V deficiency were also heterozygous carriers of a pathogenic variant in LIPC and LPL gene, respectively, whereas the patient with LIPC variant was also a carrier of a rare APOB missense variant. Targeted parallel sequencing of TG-related genes is recommended to define the molecular defect in children presenting with an incidental finding of HyperTG. Show less

Homozygous familial hypercholesterolemia is a rare clinical phenotype with a variable expression, which is characterized by extremely elevated plasma low-density lipoprotein (LDL), tendon and skin xanthomas, and a progressive atherosclerosis. In 95% of patients, homozygous familial hypercholesterolemia is due to mutations in low-density lipoprotein receptor (LDLR) gene, which abolish (receptor-negative) or greatly reduce (receptor-defective) LDLR function. The objective of the study was the molecular and phenotypic characterization of 4 siblings with severe hypercholesterolemia. The major LDL-related genes (LDLR, APOB, PCSK9, ANGPTL3, APOE, and APOC3) were sequenced. LDLR messenger RNA, isolated from leukocytes, was reverse transcribed and sequenced. The index cases were 24-year-old identical twin sisters with long-standing tendon xanthomas and high low-density lipoprotein cholesterol (LDL-C ∼10 mmol/L) but no coronary heart disease. They were carriers of 2 LDLR mutations: (1) a previously reported mutation [p.(G335S)] inherited from the mother who had LDL-C level within normal range; (2) a novel 24 bp deletion in exon 8/intron 8 junction inherited from the hypercholesterolemic (LDL-C 6.1 mmol/L) father. The deletion allele encodes an messenger RNA with a partial deletion of exon 8, whose translation product has an in-frame deletion of 17 amino acids [p.(Glu380_Gly396del)]. Family screening revealed that the 2 siblings of the twin sisters were also compound heterozygotes but had much lower LDL-C levels (8.2 and 7.1 mmol/L). The sequence of potential modifying genes showed that the 2 siblings and the mother of the twin sisters were heterozygous for a rare missense variant of apoB [p.(S2429T)], which might have an LDL-lowering effect. We report a rare event of 4 siblings found to be compound heterozygotes for 2 LDLR gene mutations but showing a different phenotype severity. The less severely affected siblings were carriers of a rare apoB missense variant. Show less

We describe a kindred with high-density lipoprotein (HDL) deficiency due to APOA1 gene mutation in which comorbidities affected the phenotypic expression of the disorder. An overweight boy with hypertriglyceridemia (HTG) and HDL deficiency (HDL cholesterol 0.39 mmol/L, apoA-I 40 mg/dL) was investigated. We sequenced the candidate genes for HTG (LPL, APOC2, APOA5, GPIHBP1, LMF1) and HDL deficiency (LCAT, ABCA1 and APOA1), analyzed HDL subpopulations, measured cholesterol efflux capacity (CEC) of sera and constructed a model of the mutant apoA-I. No mutations in HTG-related genes, ABCA1 and LCAT were found. APOA1 sequence showed that the proband, his mother and maternal grandfather were heterozygous of a novel frameshift mutation (c.546₅₄₇delGC), which generated a truncated protein (p.[L159Afs*20]) containing 177 amino acids with an abnormal C-terminal tail of 19 amino acids. Trace amounts of this protein were detectable in plasma. Mutation carriers had reduced levels of LpA-I, preβ-HDL and large HDL and no detectable HDL-2 in their plasma; their sera had a reduced CEC specifically the ABCA1-mediated CEC. Metabolic syndrome in the proband explains the extremely low HDL cholesterol level (0.31 mmol/L), which was half of that found in the other carriers. The proband's mother and grandfather, both presenting low plasma low-density lipoprotein cholesterol, were carriers of the β-thalassemic trait, a condition known to be associated with a reduced low-density lipoprotein cholesterol and a reduced prevalence of cardiovascular disease. This trait might have delayed the development of atherosclerosis related to HDL deficiency. In these heterozygotes for apoA-I truncation, the metabolic syndrome has deleterious effect on HDL system, whereas β-thalassemia trait may delay the onset of cardiovascular disease. Show less

Common variants of APOA5 gene affect plasma triglyceride (TG) in the population and a number of rare variants APOA5 have been reported in individuals with hypertriglyceridemia (HTG). APOA5 was analysed in 98 HTG individuals (plasma TG >9 mmol/L) in whom no mutations in LPL and APOC2 had been found. Two patients were found to be heterozygous for two novel APOA5 variants. The first variant (p.L253P) was identified in an obese male who consumed a diet rich in fat and simple sugars. He was also a carrier in trans of the common TG-raising p.S19W SNP (5*3 haplotype). The second variant (c.295-297 del GAG, p.E99 del) was found in a lean male with no life style or metabolic factors known to affect plasma TG. He was a carrier in trans of the TG-raising 5*2 haplotype and was homozygous for the rare c.1337T allele of a SNP of GCKR gene. No mutations in other genes affecting plasma TG (LMF1 and GPIHBP1) were found in these patients. These APOA5 variants, resulted to be deleterious in silico, were not found in 350 control subjects. These novel APOA5 variants predispose to HTG in combination with other genetic or nutritional factors. Show less

APOA5 encodes a novel apolipoprotein (apo A-V) which appears to be a modulator of plasma triglyceride (TG). In apoA5 knock out mice plasma TG level increases almost fourfold, whereas in human APOA5 transgenic mice it decreases by 70%. Some SNPs in the APOA5 gene have been associated with variations in plasma TG in humans. In addition, hypertriglyceridaemic (HTG) patients have been identified who carried rare nonsense mutations in the APOA5 gene (Q139X and Q148X), predicted to result in apo A-V deficiency. In this study we report a 17-year-old male with high TG and low high density lipoprotein cholesterol (HDL-C), who at the age of two had been found to have severe HTG and eruptive xanthomas suggesting a chylomicronaemia syndrome. Plasma postheparin LPL activity, however, was normal and no mutations were found in LPL and APOC2 genes. The sequence of APOA5 gene revealed that the patient was homozygous for a point mutation (c.289 C>T) in exon 4, converting glutamine codon at position 97 into a termination codon (Q97X). Apo A-V was not detected in patient's plasma, indicating that he had complete apo A-V deficiency. The administration of a low-fat and low-oligosaccharide diet, either alone or supplemented with omega-3 fatty acids, started early in life, reduced plasma TG to a great extent but had a negligible effect on plasma HDL-C. Loss of function mutations of APOA5 gene may be the cause of severe HTG in patients without mutations in LPL and APOC2 genes. Show less

In this review we compare the phenotype and lipoprotein abnormalities of some patients who were found to carry mutations in the APOA5 gene predicted to result in apolipoprotein A-V deficiency. The sequencing of the APOA5 gene in patients with primary hypertriglyceridemia, in whom mutations of the LPL and APOC2 genes had been excluded, led to the identification of four families with two different mutations in this gene predicted to result in truncated apolipoprotein A-V. The first mutation (Q148X) was found in a homozygous state in a child with severe type V hyperlipidemia, some clinical manifestations of chylomicronemia syndrome and a slight reduction in plasma postheparin lipoprotein lipase activity. Carriers of a different mutation (Q139X) were recently reported. Four Q139X heterozygotes had type V hyperlipidemia and markedly reduced plasma postheparin lipoprotein lipase activity. The hypertriglyceridemic Q139X heterozygote had other factors that could have contributed to hypertriglyceridemia. ApoB-100 kinetic studies in hypertriglyceridemic Q139X heterozygotes revealed an impairment of very low-density lipoprotein catabolism. Mutations in the APOA5 gene, leading to truncated apolipoprotein A-V devoid of lipid-binding domains located in the carboxy-terminal end of the protein, if present in the homozygous state, are expected to cause severe type V hyperlipidemia in patients with no mutations in LPL or APOC2 genes. If present in the heterozygous state, these mutations predispose to hypertriglyceridemia in combination with other genetic factors or pathological conditions. Show less

Mutations in LPL or APOC2 genes are recognized causes of inherited forms of severe hypertriglyceridemia. However, some hypertrigliceridemic patients do not have mutations in either of these genes. Because inactivation or hyperexpression of APOA5 gene, encoding apolipoprotein A-V (apoA-V), causes a marked increase or decrease of plasma triglycerides in mice, and because some common polymorphisms of this gene affect plasma triglycerides in humans, we have hypothesized that loss of function mutations in APOA5 gene might cause hypertriglyceridemia. We sequenced APOA5 gene in 10 hypertriglyceridemic patients in whom mutations in LPL and APOC2 genes had been excluded. One of them was found to be homozygous for a mutation in APOA5 gene (c.433 C>T, Q145X), predicted to generate a truncated apoA-V devoid of key functional domains. The plasma of this patient was found to activate LPL in vitro less efficiently than control plasma, thus suggesting that apoA-V might be an activator of LPL. Ten carriers of Q145X mutation were found in the patient's family; 5 of them had mild hypertriglyceridemia. As predicted from animal studies, apoA-V deficiency is associated with severe hypertriglyceridemia in humans. This observation suggests that apoA-V regulates the secretion and/or catabolism of triglyceride-rich lipoproteins. Mutations in APOA5 gene might be the cause of severe hypertriglyceridemia in subjects in whom mutations in LPL or APOC2 genes have been excluded. We detected a nonsense mutation in APOA5 gene (Q145X) in a boy with hyperchylomicronemia syndrome. This is the first observation of a complete apoA-V deficiency in humans. Show less