Long-term clinical outcomes and management of hypertriglyceridemia in children with Apo-CII deficiency.

BACKGROUND AND AIM: APO CII, one of several cofactors which regulate lipoprotein lipase enzyme activity, plays an essential role in lipid metabolism. Deficiency of APO CII is an ultra-rare autosomal recessive cause of familial chylomicronemia syndrome. We present the long-term clinical outcomes of 12 children with APO CII deficiency. METHODS AND RESULTS: The data of children with genetically confirmed APO CII deficiency were evaluated retrospectively. Twelve children (8 females) with a mean follow-up of 10.1 years (±3.9) were included. At diagnosis, the median age was 60 days (13 days-10 years). Initial clinical findings included lipemic serum (41.6%), abdominal pain (41.6%), and vomiting (16.6%). At presentation, the median triglyceride (TG) value was 4341 mg/dL (range 1277-14,110). All patients were treated with a restricted fat diet, medium-chain triglyceride (MCT), and omega-3-fatty acids. In addition, seven patients (58.3%) received fibrate. Fibrate was discontinued in two patients due to rhabdomyolysis and in one patient because of cholelithiasis. Seven (58.3%) patients experienced pancreatitis during the follow-up period. One female experienced recurrent pancreatitis and was treated with fresh frozen plasma (FFP). CONCLUSIONS: Apo CII deficiency is an ultra-rare autosomal recessive condition of hypertriglyceridemia associated with significant morbidity and mortality. Low-fat diet and MCT supplementation are the mainstays of therapy, while the benefit of TG-lowering agents are less well-defined.

ApoC2 deficiency elicits severe hypertriglyceridemia and spontaneous atherosclerosis: A rodent model rescued from neonatal death.

RATIONALE: ApoC2 is an important activator for lipoprotein lipase-mediated hydrolysis of triglyceride-rich plasma lipoproteins. ApoC2-deficient patients display severe hypertriglyceridemia (sHTG) and recurrent acute pancreatitis. However, due to embryonic lethality in ApoC2 deleted mouse extensive understanding of ApoC2 function is limited in mammalian species. OBJECTIVE: We sought to generate an animal model with ApoC2 deficiency in a rodent with some human-like features and then study the precise effects of ApoC2 on lipid and glucose homeostasis. METHODS AND RESULTS: Using CRISPR/Cas9, we deleted Apoc2 gene from golden Syrian hamster and the homozygous (-/-) pups can be born in matured term but exhibited neonatal lethality. By continuous iv administration of normal hamster serum the ApoC2-/- pups could survive till weaning and displayed severe HTG in adulthood on chow diet. A single iv injection of AAV-hApoC2 at birth can also rescue the neonatal death of ApoC2-/- pups. Adult ApoC2-/-hamsters exhibited a unique phenotype of sHTG with hypoglycemia, hypoinsulinemia and spontaneous atherosclerosis. The sHTG in ApoC2-/- adult hamsters could not be corrected by various lipid-lowering medications, but partially ameliorated by medium chain triglyceride diet and completely corrected by AAV-hApoC2. CONCLUSIONS: Our study provides a novel ApoC2-deleted mammalian model with severe hypertriglyceridemia that was fully characterized and highlights a potential therapeutic approach for the treatment of ApoC2 deficient patients.

Apolipoprotein C-II: New findings related to genetics, biochemistry, and role in triglyceride metabolism.

Apolipoprotein C-II (apoC-II) is a small exchangeable apolipoprotein found on triglyceride-rich lipoproteins (TRL), such as chylomicrons (CM) and very low-density lipoproteins (VLDL), and on high-density lipoproteins (HDL), particularly during fasting. ApoC-II plays a critical role in TRL metabolism by acting as a cofactor of lipoprotein lipase (LPL), the main enzyme that hydrolyses plasma triglycerides (TG) on TRL. Here, we present an overview of the role of apoC-II in TG metabolism, emphasizing recent novel findings regarding its transcriptional regulation and biochemistry. We also review the 24 genetic mutations in the APOC2 gene reported to date that cause hypertriglyceridemia (HTG). Finally, we describe the clinical presentation of apoC-II deficiency and assess the current therapeutic approaches, as well as potential novel emerging therapies.

A Novel APOC2 Missense Mutation Causing Apolipoprotein C-II Deficiency With Severe Triglyceridemia and Pancreatitis.

Context: Familial chylomicronemia syndrome (FCS) is a rare heritable disorder associated with severe hypertriglyceridemia and recurrent pancreatitis. Lipoprotein lipase deficiency and apolipoprotein C-II deficiency are two well-characterized autosomal recessive causes of FCS, and three other genes have been described to cause FCS. Because therapeutic approaches can vary according to the underlying etiology, it is important to establish the molecular etiology of FCS. Case Description: A man originally from North Africa was referred to the University of Pennsylvania Lipid Clinic for severe hypertriglyceridemia and recurrent pancreatitis, consistent with the clinical diagnosis of FCS. Molecular analyses of FCS-associated genes revealed a homozygous missense variant R72T in APOC2. Molecular modeling of the variant predicted that the apolipoprotein C-II R72T peptide has reduced lipid binding affinity. In vitro studies of the patient's plasma confirmed the lack of functional apoC-II activity. Moreover, the apoC-II protein was undetectable in the patient's plasma, quantitatively as well as qualitatively. Conclusions: We identified a missense APOC2 variant causing apoC-II deficiency in a patient with severe hypertriglyceridemia and recurrent pancreatitis. Beyond dietary management and usual pharmacologic therapies, an apoC-II mimetic peptide may become an optional therapy in patients with apoC-II deficiency in the future.

Apoc2 loss-of-function zebrafish mutant as a genetic model of hyperlipidemia.

Apolipoprotein C-II (APOC2) is an obligatory activator of lipoprotein lipase. Human patients with APOC2 deficiency display severe hypertriglyceridemia while consuming a normal diet, often manifesting xanthomas, lipemia retinalis and pancreatitis. Hypertriglyceridemia is also an important risk factor for development of cardiovascular disease. Animal models to study hypertriglyceridemia are limited, with no Apoc2-knockout mouse reported. To develop a genetic model of hypertriglyceridemia, we generated an apoc2 mutant zebrafish characterized by the loss of Apoc2 function. apoc2 mutants show decreased plasma lipase activity and display chylomicronemia and severe hypertriglyceridemia, which closely resemble the phenotype observed in human patients with APOC2 deficiency. The hypertriglyceridemia in apoc2 mutants is rescued by injection of plasma from wild-type zebrafish or by injection of a human APOC2 mimetic peptide. Consistent with a previous report of a transient apoc2 knockdown, apoc2 mutant larvae have a minor delay in yolk consumption and angiogenesis. Furthermore, apoc2 mutants fed a normal diet accumulate lipid and lipid-laden macrophages in the vasculature, which resemble early events in the development of human atherosclerotic lesions. In addition, apoc2 mutant embryos show ectopic overgrowth of pancreas. Taken together, our data suggest that the apoc2 mutant zebrafish is a robust and versatile animal model to study hypertriglyceridemia and the mechanisms involved in the pathogenesis of associated human diseases.

[Primary hyperchylomicronemia].

Primary hyperchylomicronemia is characterized by a marked hypertriglyceridemia due to an increase in chylomicrons, which may cause acute pancreatitis and eruptive xanthomas. This entity includes familial lipoprotein lipase (LPL) deficiency, familial apolipoprotein C-II deficiency, primary type V hyperlipoproteinemia, and idiopathic hyperchylomicronemia. Idiopathic hyperchylomicronemia is caused by an LPL inhibitor or autoantibody against LPL. More recently, patients with primary hyperchylomicronemia caused by mutations in the gene for glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1(GPIHBP1) or lipase maturation factor 1(LMF1). For the treatment of primary hyperchylomicronemia, a strict restriction of dietary fat is essential to avoid acute pancreatitis.

Apolipoprotein C-II deficiency with no rare variant in the APOC2 gene.

AIM: Familial apolipoprotein C-II (apoC-II) deficiency is a rare autosomal recessive disorder with marked hypertriglyceridemia resulting from impaired activation of lipoprotein lipase. In most cases of apoC-II deficiency, causative mutations have been found in the protein-coding region of APOC2; however, several atypical cases of apoC-II deficiency were reported to have markedly reduced, but detectable levels of plasma apoC-II protein (hereafter referred to as hypoapoC-II), which resulted from decreased promoter activity or improper splicing of apoC-II mRNA due to homozygous mutations in APOC2. Here we aim to dissect the molecular bases of a new case of hypoapoC-II. METHODS: We performed detailed biochemical/genetic analyses of our new case of hypoapoC-II, manifesting severe hypertriglyceridemia (plasma triglycerides, 3235 mg·dL(-1)) with markedly reduced levels of plasma apoC-II (0.6 mg·dL(-1)). RESULTS: We took advantage of a monocyte/macrophage culture system to prove that transcription of apoC-II mRNA was decreased in the patient's cells, which is compatible with the reported features of hypoapoC-II. Concomitantly, transcriptional activity of the minigene reporter construct of the patient's APOC2 gene was decreased; however, no rare variant was detected in the patient's APOC2 gene. Fifty single nucleotide variants were detected in the patient's APOC2, but all were common variants (allele frequencies ＞35%) that are supposedly not causative. CONCLUSIONS: A case of apoC-II deficiency was found that is phenotypically identical to hypoapoC-II but with no causative mutations in APOC2, implying that other genes regulate apoC-II levels. The clinical entity of hypoapoC-II is discussed.

Mutation of conserved cysteines in the Ly6 domain of GPIHBP1 in familial chylomicronemia.

We investigated a family from northern Sweden in which three of four siblings have congenital chylomicronemia. LPL activity and mass in pre- and postheparin plasma were low, and LPL release into plasma after heparin injection was delayed. LPL activity and mass in adipose tissue biopsies appeared normal. [(35)S]Methionine incorporation studies on adipose tissue showed that newly synthesized LPL was normal in size and normally glycosylated. Breast milk from the affected female subjects contained normal to elevated LPL mass and activity levels. The milk had a lower than normal milk lipid content, and the fatty acid composition was compatible with the milk lipids being derived from de novo lipogenesis, rather than from the plasma lipoproteins. Given the delayed release of LPL into the plasma after heparin, we suspected that the chylomicronemia might be caused by mutations in GPIHBP1. Indeed, all three affected siblings were compound heterozygotes for missense mutations involving highly conserved cysteines in the Ly6 domain of GPIHBP1 (C65S and C68G). The mutant GPIHBP1 proteins reached the surface of transfected Chinese hamster ovary cells but were defective in their ability to bind LPL (as judged by both cell-based and cell-free LPL binding assays). Thus, the conserved cysteines in the Ly6 domain are crucial for GPIHBP1 function.

Encephalopathy in type I hyperlipidemia.

Familial chylomicronemia syndrome is a group of rare genetic disorders characterized by deficient activity of an enzyme lipoprotein lipase or apo-protein C-II deficiency. In this paper we present an infant with massive hyperchylomicronemia and severe pancreatitis. Exchange transfusion for controlling hypertriglyceridemia and pancreatitis led to an increase in hyperviscosity which resulted in encephalopathy.