Familial chylomicronemia syndrome.

Familial chylomicronemia syndrome is a rare disorder of lipoprotein metabolism due to familial lipoprotein lipase (LPL) or apolipoprotein C-II deficiency or the presence of inhibitors to lipoprotein lipase. It manifests as eruptive xanthomas, acute pancreatitis, and lipaemic plasma due to marked elevation of triglyceride and chylomicron levels. We report two siblings with this rare disorder and review the literature.

Apolipoprotein CI causes hypertriglyceridemia independent of the very-low-density lipoprotein receptor and apolipoprotein CIII in mice.

We have recently shown that the predominant hypertriglyceridemia in human apolipoprotein C1 (APOC1) transgenic mice is mainly explained by apoCI-mediated inhibition of the lipoprotein lipase (LPL)-dependent triglyceride (TG)-hydrolysis pathway. Since the very-low-density lipoprotein receptor (VLDLr) and apoCIII are potent modifiers of LPL activity, our current aim was to study whether the lipolysis-inhibiting action of apoCI would be dependent on the presence of the VLDLr and apoCIII in vivo. Hereto, we employed liver-specific expression of human apoCI by using a novel recombinant adenovirus (AdAPOC1). In wild-type mice, moderate apoCI expression leading to plasma human apoCI levels of 12-33 mg/dl dose-dependently and specifically increased plasma TG (up to 6.6-fold, P < 0.001), yielding the same hypertriglyceridemic phenotype as observed in human APOC1 transgenic mice. AdAPOC1 still increased plasma TG in vldlr(-/-) mice (4.1-fold, P < 0.001) and in apoc3(-/-) mice (6.8-fold, P < 0.001) that were also deficient for the low-density lipoprotein receptor (LDLr) and LDLr-related protein (LRP) or apoE, respectively. Thus, irrespective of receptor-mediated remnant clearance by the liver, liver-specific expression of human apoCI causes hypertriglyceridemia in the absence of the VLDLr and apoCIII. We conclude that apoCI is a powerful and direct inhibitor of LPL activity independent of the VLDLr and apoCIII.

Characterization of a new mouse model for human apolipoprotein A-I/C-III/A-IV deficiency.

Human data raised the possibility that coronary heart disease is associated with mutations in the apolipoprotein gene cluster APOA1/C3/A4 that result in multideficiency of cluster-encoded apolipoproteins and hypoalphalipoproteinemia. To test this hypothesis, we generated a mouse model for human apolipoprotein A-I (apoA-I)/C-III/A-IV deficiency. Homozygous mutants (Apoa1/c3/a4(-/-)) lacking the three cluster-encoded apolipoproteins were viable and fertile. In addition, feeding behavior and growth were apparently normal. Total cholesterol (TC), high density lipoprotein cholesterol (HDLc), and triglyceride levels in the plasma of fasted mutants fed a regular chow were 32% (P < 0.001), 17% (P < 0.001), and 70% (P < 0.01), respectively, those of wild-type mice. When fed a high-fat Western-type (HFW) diet, Apoa1/c3/a4(-/-) mice showed a further decrease in HDLc concentration and a moderate increase in TC, essentially in non-HDL fraction. The capacity of Apoa1/c3/a4(-/-) plasma to promote cholesterol efflux in vitro was decreased to 75% (P < 0.001), and LCAT activity was decreased by 38% (P < 0.01). Despite the very low total plasma cholesterol, the imbalance in lipoprotein distribution caused small but detectable aortic lesions in one-third of Apoa1/c3/a4(-/-) mice fed a HFW diet. In contrast, none of the wild-type mice had lesions. These results demonstrate that Apoa1/c3/a4(-/-) mice display clinical features similar to human apoA-I/C-III/A-IV deficiency (i.e., marked hypoalphalipoproteinemia) and provide further support for the apoa1/c3/a4 gene cluster as a minor susceptibility locus for atherosclerosis in mice.

Missense mutation Leu72Pro located on the carboxyl terminal amphipathic helix of apolipoprotein C-II causes familial chylomicronemia syndrome.

BACKGROUND: Chylomicronemia syndrome can be caused by 2 autosomal recessive disorders - lipoprotein lipase (LPL) deficiency and apolipoprotein C-II (apo C-II) deficiency. METHODS: We described 2 siblings with chylomicronemia syndrome of a consanguineous family. To determine the molecular basis of chylomicronemia syndrome in this family, we performed direct DNA sequencing of the LPL and APOC2 genes of the proband. RESULTS: A novel homozygous mutation, Leu72Pro, in the APOC2 gene was found in both siblings whereas their parents were carriers. No LPL mutations were detected in the siblings. Apo C-II contains 3 amphipathic alpha helices; the C-terminal alpha helix is composed of residues 64 to 74. Substitution of residue 72 from a helix former leucine to a helix breaker, proline, is predicted to change the secondary structure of the C-terminal helix and subsequently alter the interaction between apo C-II and LPL. CONCLUSIONS: To our knowledge, Leu72Pro is the first missense mutation identified in the C-terminal of apo C-II. The result is consistent with the current biochemical and structural findings that the C-terminal helix of apo C-II is important for activation of LPL.

ApoC-III deficiency prevents hyperlipidemia induced by apoE overexpression.

Adenovirus-mediated overexpression of human apolipoprotein E (apoE) induces hyperlipidemia by stimulating the VLDL-triglyceride (TG) production rate and inhibiting the LPL-mediated VLDL-TG hydrolysis rate. Because apoC-III is a strong inhibitor of TG hydrolysis, we questioned whether Apoc3 deficiency might prevent the hyperlipidemia induced by apoE overexpression in vivo. Injection of 2 x 10(9) plaque-forming units of AdAPOE4 caused severe combined hyperlipidemia in Apoe-/- mice [TG from 0.7 +/- 0.2 to 57.2 +/- 6.7 mM; total cholesterol (TC) from 17.4 +/- 3.7 to 29.0 +/- 4.1 mM] that was confined to VLDL/intermediate density lipoprotein-sized lipoproteins. In contrast, Apoc3 deficiency resulted in a gene dose-dependent reduction of the apoE4-associated hyperlipidemia (TG from 57.2 +/- 6.7 mM to 21.2 +/- 18.5 and 1.5 +/- 1.4 mM; TC from 29.0 +/- 4.1 to 16.4 +/- 9.8 and 2.3 +/- 1.8 mM in Apoe-/-, Apoe-/-.Apoc3+/-, and Apoe-/-.Apoc3-/- mice, respectively). In both Apoe-/- mice and Apoe-/-.Apoc3-/- mice, injection of increasing doses of AdAPOE4 resulted in up to a 10-fold increased VLDL-TG production rate. However, Apoc3 deficiency resulted in a significant increase in the uptake of TG-derived fatty acids from VLDL-like emulsion particles by white adipose tissue, indicating enhanced LPL activity. In vitro experiments showed that apoC-III is a more specific inhibitor of LPL activity than is apoE. Thus, Apoc3 deficiency can prevent apoE-induced hyperlipidemia associated with a 10-fold increased hepatic VLDL-TG production rate, most likely by alleviating the apoE-induced inhibition of VLDL-TG hydrolysis.

Apolipoprotein C3 deficiency results in diet-induced obesity and aggravated insulin resistance in mice.

Our aim was to study whether the absence of apolipoprotein (apo) C3, a strong inhibitor of lipoprotein lipase (LPL), accelerates the development of obesity and consequently insulin resistance. Apoc3(-/-) mice and wild-type littermates were fed a high-fat (46 energy %) diet for 20 weeks. After 20 weeks of high-fat feeding, apoc3(-/-) mice showed decreased plasma triglyceride levels (0.11 +/- 0.02 vs. 0.29 +/- 0.04 mmol, P < 0.05) and were more obese (42.8 +/- 3.2 vs. 35.2 +/- 3.3 g; P < 0.05) compared with wild-type littermates. This increase in body weight was entirely explained by increased body lipid mass (16.2 +/- 5.9 vs. 10.0 +/- 1.8 g; P < 0.05). LPL-dependent uptake of triglyceride-derived fatty acids by adipose tissue was significantly higher in apoc3(-/-) mice. LPL-independent uptake of albumin-bound fatty acids did not differ. It is interesting that whole-body insulin sensitivity using hyperinsulinemic-euglycemic clamps was decreased by 43% and that suppression of endogenous glucose production was decreased by 25% in apoc3(-/-) mice compared with control mice. Absence of apoC3, the natural LPL inhibitor, enhances fatty acid uptake from plasma triglycerides in adipose tissue, which leads to higher susceptibility to diet-induced obesity followed by more severe development of insulin resistance. Therefore, apoC3 is a potential target for treatment of obesity and insulin resistance.

Apolipoprotein CIII deficiency prevents the development of hypertriglyceridemia in streptozotocin-induced diabetic mice.

To explore the role of apolipoprotein (apo) CIII in the development of hypertriglyceridemia associated with diabetes mellitus, we examined triglyceride (TG) kinetics in apo CIII - deficient mice (apo CIII - null) and wild-type (WT) (C57BL/6J) mice with diabetes induced by the injection of streptozotocin (STZ). Plasma TG levels increased significantly in WT mice after diabetes was induced (102 +/- 29 v 65 +/- 33 mg/dL, P <.01). Apo CIII-null mice had a significantly lower TG level (35 +/- 9 mg/dL) that remained unchanged even when diabetes was induced (35 +/- 8 mg/dL). The TG secretion rate (TGSR) measured by the Triton WR1339 method tended to decrease in diabetic WT, indicating that catabolism of TG was impaired. Apo CIII-null mice showed 2-fold higher TG production than WT mice, indicating markedly faster clearance of TG. The high TGSR was halved when diabetes was induced in apo CIII-null mice, and the fractional catabolic rate (FCR) of TG was also halved, although it was still significantly higher than in WT mice. Lipoprotein lipase (LPL) activity in postheparin plasma was not significantly altered in WT or apo CIII-null mice regardless of the presence or absence of diabetes. [(3)H] very-low-density lipoprotein (VLDL)-TG from WT or apo CIII-null mice showed similar clearance by WT recipients, and this was also observed when VLDL was obtained from diabetic counterparts. In contrast, VLDL-TG was cleared faster by apo CIII-null recipients compared with WT recipients, regardless of the VLDL donors. These results suggest that apo CIII deficiency prevents the development of hypertriglyceridemia associated with diabetes by stimulating TG removal, possibly by promoting the interaction of VLDL with the TG removal system.

Genetic disorders of the pancreas.

The venues opened to all by the remarkable studies of the genome are just starting to become manifest; they can now distinguish different variants of a disease; they are given the tools to better understand the pathophysiology of illness; they hope to be able to provide better treatment alternatives to our patients. The examples described in this review demonstrate the applicability of these concepts to pancreatic disorders. Researchers may be just scratching the surface at this time, but the potential is enormous. Many philosophic and ethical questions need to be answered as physicians move along: Should all family members of an index case be screened? Who should pay for testing? Who should get results? But, without the participation of so many patients, their family members, and numerous volunteers, researchers would not have witnessed the bridging of so many gaps as they have so far. All of us may now look forward to the application of this incredible knowledge to the therapeutic solutions so eagerly awaited.

A novel type hypertriglyceridemia observed in FLS mice.

The unique inborn hypertriglyceridemia seen in FLS (fatty liver Shionogi) mice was relieved by the administration of purified apolipoprotein (apo) C-II. Lipoprotein lipase (LPL) and its cofactor, apoC-II, play a pivotal role in VLDL metabolism. Therefore, we investigated the genetic background involved in this hypertriglyceridemia. Plasma levels of TG and total cholesterol as well as LPL activity were measured in male FLS mice and C57/BL6J mice. Agarose gel electrophoresis and fast protein liquid chromatography were used to analyze the lipoprotein profile. A cross experiment was done to determine the genetic background of hypertriglyceridemia observed in FLS mice. cDNA sequences of apoC-II and apoC-III of FLS mice were determined. Prealpha-lipoprotein was the predominant lipoprotein class in FLS mouse plasma. LPL activity remained in the range observed in C57/BL6J mice, and purified apoC-II transiently relieved FLS mice from hypertriglyceridemia. Prealpha-lipoproteinemia was inherited in an autosomal recessive manner. ApoC-III appeared to be a causal factor for this unique hypertriglyceridemia. Microsatellite analysis, however, revealed that the responsible chromosome was not 7; rather, apoC-III mapped onto chromosome 9. Therefore, we suggest apoC-III as a candidate causative factor for the hypertriglyceridemia observed in FLS mice because an excessive amount of apoC-III attenuates LPL activity in vivo and in vitro.

Apolipoprotein C-II deficiency presenting as a lipid encephalopathy in infancy.

An infant presented with massive hyperchylomicronemia and a severe encephalopathy. MRI showed marked lipid deposition throughout the brain. Despite the normalization of the biochemistry, there was little clinical improvement, and at 18 months of age she has severe developmental delay, a strikingly abnormal MRI. Apolipoprotein C-II, the lipoprotein on chylomicrons responsible for the activation of lipoprotein lipase, was not detectable in blood. Analysis of the APO C-II gene revealed a novel homozygous point mutation, 1118C-->A. Subsequently, another sibling has been born with the same homozygous mutation and similar biochemistry but, perhaps because of early treatment, a normal neurological outcome.

Endothelial-derived lipoprotein lipase is bound to postprandial triglyceride-rich lipoproteins and mediates their hepatic clearance in vivo.

Lipoprotein lipase (LPL) is the key enzyme in the intravascular hydrolysis of triglyceride-rich lipoproteins (TRL). Furthermore, it has been shown that inactive LPL can mediate cellular binding and uptake of TRL in vitro. This study investigated whether LPL is bound to postprandial human TRL in vivo, and whether it plays a role in the hepatic clearance of these particles independent of its catalytic activity. LPL was found to bind to postprandial TRL in preheparin plasma of healthy young men. To study the effect of inactive LPL on particle uptake, TRL isolated from patients with inactive LPL (LPL or apoC-II mutations) were used before and after heparin administration. These model particles allow one to study the bridging effect of LPL independent of its enzymatic activity. Organ uptake studies with these particles in mice revealed that inactive LPL increases the hepatic clearance of TRL significantly while uptake into other organs remains largely unaffected. Further evidence that endothelial-derived LPL directs TRL to the liver in vivo was gained with transgenic mice that express inactive LPL exclusively in muscle, revealing greater hepatic uptake than in wild-type mice. In conclusion, these data demonstrate for the first time that LPL is a structural component of postprandial TRL which facilitates hepatic TRL clearance from the circulation independent of its catalytic function.

A case of apolipoprotein C-II deficiency with coronary artery disease.

A 56-year-old male with apolipoprotein C-II deficiency experienced a myocardial infarction without pancreatitis. A coronary angiogram showed complete occlusions of both the right and circumflex coronary arteries. His serum lipid levels were as follows: fasting total cholesterol 3.15 mmol/l; postprandial total cholesterol 3.62 mmol/l; fasting triglycerides 1.46 mmol/A; postprandial triglycerides 6.14 mmol/l; fasting high-density lipoprotein-cholesterol 0.47 mmol/l; and postprandial high-density lipoprotein cholesterol 0.36 mmol/l. His fasting level of plasma apolipoprotein C-II was 0.005 g/l, but his plasma levels of other apolipoproteins were within normal ranges. A DNA sequence analysis of the apolipoprotein C-II gene showed no mutations in exon 1, 2, 3, or 4, where most gene mutations related to apolipoprotein C-II deficiency occur. We report this patient's very rare heterozygous apolipoprotein C-II deficiency with coronary artery disease. Although this patient had some risk factors for coronary artery disease, coronary atherosclerosis in this patient might have occurred as a result of lipoprotein abnormalities caused by at least one mutation in the apolipoprotein C-II gene.

Apolipoprotein CI deficiency markedly augments plasma lipoprotein changes mediated by human cholesteryl ester transfer protein (CETP) in CETP transgenic/ApoCI-knocked out mice.

Transgenic mice expressing human cholesteryl ester transfer protein (HuCETPTg mice) were crossed with apolipoprotein CI-knocked out (apoCI-KO) mice. Although total cholesterol levels tended to be reduced as the result of CETP expression in HuCETPTg heterozygotes compared with C57BL6 control mice (-13%, not significant), a more pronounced decrease (-28%, p < 0.05) was observed when human CETP was expressed in an apoCI-deficient background (HuCETPTg/apoCI-KO mice). Gel permeation chromatography analysis revealed a significant, 6.1-fold rise (p < 0.05) in the cholesteryl ester content of very low density lipoproteins in HuCETPTg/apoCI-KO mice compared with control mice, whereas the 2.7-fold increase in HuCETPTg mice did not reach the significance level in these experiments. Approximately 50% decreases in the cholesteryl ester content and cholesteryl ester to triglyceride ratio of high density lipoproteins (HDL) were observed in HuCETPTg/apoCI-KO mice compared with controls (p < 0.05 in both cases), with intermediate -20% changes in HuCETPTg mice. The cholesteryl ester depletion of HDL was accompanied with a significant reduction in their mean apparent diameter (8.68 +/- 0.04 nm in HuCETPTg/apoCI-KO mice versus 8.83 +/- 0.02 nm in control mice; p < 0.05), again with intermediate values in HuCETPTg mice (8.77 +/- 0.04 nm). In vitro purified apoCI was able to inhibit cholesteryl ester exchange when added to either total plasma or reconstituted HDL-free mixtures, and coincidently, the specific activity of CETP was significantly increased in the apoCI-deficient state (173 +/- 75 pmol/microg/h in HuCETPTg/apoCI-KO mice versus 72 +/- 19 pmol/microg/h in HuCETPTg, p < 0.05). Finally, HDL from apoCI-KO mice were shown to interact more readily with purified CETP than control HDL that differ only by their apoCI content. Overall, the present observations provide direct support for a potent specific inhibition of CETP by plasma apoCI in vivo.

Apolipoprotein C-III deficiency accelerates triglyceride hydrolysis by lipoprotein lipase in wild-type and apoE knockout mice.

Previous studies with hypertriglyceridemic APOC3 transgenic mice have suggested that apolipoprotein C-III (apoC-III) may inhibit either the apoE-mediated hepatic uptake of TG-rich lipoproteins and/or the lipoprotein lipase (LPL)-mediated hydrolysis of TG. Accordingly, apoC3 knockout (apoC3(-/-)) mice are hypotriglyceridemic. In the present study, we attempted to elucidate the mechanism(s) underlying these phenomena by intercrossing apoC3(-/-) mice with apoE(-/-) mice to study the effects of apoC-III deficiency against a hyperlipidemic background. Similar to apoE(+/+) apoC3(-/-) mice, apoE(-/-)apoC3(-/-) mice exhibited a marked reduction in VLDL cholesterol and TG, indicating that the mechanism(s) by which apoC-III deficiency exerts its lipid-lowering effect act independent of apoE. On both backgrounds, apoC3(-/-) mice showed normal intestinal lipid absorption and hepatic VLDL TG secretion. However, turnover studies showed that TG-labeled emulsion particles were cleared much more rapidly in apoC3(-/-) mice, whereas the clearance of VLDL apoB, as a marker for whole particle uptake by the liver, was not affected. Furthermore, it was shown that cholesteryl oleate-labeled particles were also cleared faster in apoC3(-/-) mice. Thus the mechanisms underlying the hypolipidemia in apoC3(-/-) mice involve both a more efficient hydrolysis of VLDL TG as well as an enhanced selective clearance of VLDL cholesteryl esters from plasma. In summary, our studies of apoC3(-/-) mice support the concept that apoC-III is an effective inhibitor of VLDL TG hydrolysis and reveal a potential regulating role for apoC-III with respect to the selective uptake of cholesteryl esters.

Apoprotein C-III deficiency markedly stimulates triglyceride secretion in vivo: comparison with apoprotein E.

Apoprotein (apo) C-III plays an important role in the development of hypertriglyceridemia by inhibiting triglyceride (TG) removal. However, the effect of apo C-III on TG production remains unclear. We measured TG secretion rate (TGSR) in apo C-III gene-disrupted (apo C-III-null) mice to investigate the influence of this protein on TG turnover. TGSR measured by the Triton WR-1339 method was increased twofold in these mice compared with wild-type (WT) mice. Obesity was induced by the injection of gold-thioglucose (GTG), which made the WT mice hypertriglyceridemic due to a threefold increase of TGSR. However, GTG-induced obesity failed to increase TG in apo C-III-null mice, although TGSR was increased 10-fold, suggesting substantial stimulation of TG removal. Apo E-null mice were severely hypercholesterolemic but were not hypertriglyceridemic, and TGSR was rather decreased. GTG-induced obesity made these mice hypertriglyceridemic because of TG overproduction to an extent similar to that seen in WT mice. These results suggest that apo C-III deficiency potently enhances TG turnover, especially when TG production is stimulated, and that apo E deficiency is not always rate limiting for TG production.