The microsomal triglyceride transfer protein (MTP) is a dimeric lipid transfer protein consisting of protein disulfide isomerase and a unique 97-kDa subunit. In vitro, MTP accelerates the transport of triglyceride, cholesteryl ester, and phospholipid between membranes. It was recently demonstrated that abetalipoproteinemia, a hereditary disease characterized as an inability to produce chylomicrons and very low-density lipoproteins in the intestine and liver, respectively, results from mutations in the gene encoding the 97-kDa subunit of the microsomal triglyceride transfer protein. Downstream effects resulting from this defect include malnutrition, very low plasma cholesterol and triglyceride levels, altered lipid and protein compositions of membranes and lipoprotein particles, and vitamin deficiencies. Unless treated, abetalipoproteinemic subjects develop gastrointestinal, neurological, ophthalmological, and hematological abnormalities.

Anderson's disease (AD) or chylomicron retention disease (CMRD) is a rare hereditary lipid malabsorption syndrome linked to SARA2 gene mutations. We report in this study a novel mutation in two sisters for which the Sar1b protein is predicted to be truncated by 32 amino acids at its carboxyl-terminus. Because the SARA2 gene is also expressed in the muscle, heart, liver and placenta, extraintestinal clinical manifestations may exist. For the first time, we describe in this study in the two sisters muscular as well as cardiac abnormalities that could be related to the reported expression of SARA2 in these tissues. We also evaluated six other patients for potential manifestations of the SARA2 mutation. The creatine phosphokinase levels were increased in all patients [1.5-9.4 x normal (N)] and transaminases were moderately elevated in five of the eight patients (1.2-2.6 x N), probably related to muscle disease rather than to liver dysfunction. A decreased ejection fraction occurred in one patient (40%, N: 60%). The muscle, liver and placental tissues that were examined had no specific abnormalities and, in particular, no lipid accumulation. These results suggest that myolysis and other extraintestinal abnormalities can occur in AD/CMRD and that the clinical evaluation of patients should reflect this.

The microsomal triglyceride transfer protein (MTP) is a heterodimer composed of the ubiquitous multifunctional protein, protein disulfide isomerase, and a unique 97-kDa subunit. Mutations that lead to the absence of a functional 97-kDa subunit cause abetalipoproteinemia, an autosomal recessive disease characterized by a defect in the assembly and secretion of apolipoprotein B (apoB) containing lipoproteins. Previous studies of abetalipoproteinemic patient, C.L., showed that the 97-kDa subunit was undetectable. In this report, [35 S]methionine labeling showed that this tissue was capable of synthesizing the 97-kDa MTP subunit. Electrophoretic analysis showed two bands, one with a molecular mass of the wild type 97-kDa subunit and the other with a slightly lower molecular weight. Sequence analysis of cDNAs from additional intestinal biopsies showed this patient to be a compound heterozygote. One allele contained a perfect in-frame deletion of exon 10, explaining the lower molecular weight band. cDNAs of the second allele were found to contain 3 missense mutations: His 297 3 Gln, Asp 384 3 Ala, and Arg 540 3 His. Transient expression of each mutant showed that only the Arg 540 3 His mutant was non-functional based upon its inability to reconstitute apoB secretion in a cell culture system. The other amino acid changes are silent polymorphisms. High level coexpression in a baculovirus system of the wild type 97-kDa subunit or the Arg 540 3 His mutant along with human protein disulfide isomerase showed that the wild type was capable of forming an active MTP complex while the mutant was not. Biochemical analysis of lysates from these cells showed that the Arg to His conversion interrupted the interaction between the 97-kDa subunit and protein disulfide isomerase. Replacement of Arg 540 with a lysine residue maintained the ability of the 97-kDa subunit to complex with protein disulfide isomerase and form the active MTP holoprotein. These results indicate that a positively charged amino acid at position 540 in the 97-kDa subunit is critical for the productive association with protein disulfide isomerase. Of the 13 mutant MTP 97-kDa subunit alleles described to date, this is the first encoding a missense mutation. The microsomal triglyceride transfer protein (MTP)1 is an endoplasmic reticulum resident protein that catalyzes the transfer of lipids between phospholipid surfaces. Although MTP can transfer most classes of lipids, it shows a distinct preference for triglyceride and cholesteryl esters. MTP is a heterodimer composed of the multifunctional protein, protein disulfide isomerase (PDI) and a unique large subunit of 97 kDa (1). PDI is a ubiquitous protein found at high levels in many different tissues (2). The 97-kDa subunit of MTP is expressed primarily in hepatocytes and intestinal enterocytes (3). Thus, the active MTP complex is found predominantly in the liver and small intestine. The subcellular localization, tissue distribution, and activity of MTP all suggest a role for this protein in the assembly and secretio...

Abstract-Anderson's disease is a rare, hereditary hypocholesterolemic syndrome characterized by chronic diarrhea, steatorrhea, and failure to thrive associated with the absence of apo B48 -containing lipoproteins. To further define the molecular basis of the disease, we studied 8 affected subjects in 7 unrelated families of North African origin after treatment with a low-fat diet. Lipid loading of intestinal biopsies persisted, but the pattern and extent of loading was variable among the patients. Electron microscopy showed lipoprotein-like particles in membrane-bound compartments, the densities (0.65 to 7.5 particles/ 2 ) and the mean diameters (169 to 580 nm) of which were, in general, significantly larger than in a normal fed subject (0.66 particles/ 2 , 209 nm mean diameter). There were also large lipid particles having diameters up to 7043 nm (average diameters from 368 to 2127 nm) that were not surrounded by a membrane. Rarely, lipoprotein-like particles 50 to 150 nm in diameter were observed in the intercellular spaces. Intestinal organ culture showed that apo B and apo AIV were synthesized with apparently normal molecular weights and that small amounts were secreted in lipid-bound forms (density Ͻ1.006 g/mL). Normal microsomal triglyceride transfer protein (MTP) and activity were also detected in intestinal biopsies. Segregation analyses of 4 families excluded, as a cause of the disease, significant regions of the genome surrounding the genes for apo AI, AIV, B, CI, CII, CIII, and E, as were the genes encoding 3 proteins involved in intracellular lipid transport, MTP, and fatty acid binding proteins 1 and 2. The results suggest that a factor other than apoproteins and MTP are important for human intestinal chylomicron assembly and secretion.

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Secretion of triglycerides by the liver in ruminants as components of very low density lipoproteins particles is low as compared with that in primates or rodents. The rate-limiting steps for the hepatic export of very low density lipoproteins have been studied in liver slices to determine the origin of the low lipotropic capacity of calf liver compared to that of rat liver. The rates of production of apolipoprotein B (apo B) and albumin as well as the rate of secretion of VLDL-apolipoproteins were measured during 12-h incubation of liver slices in organo-culture using [35S]methionine-cysteine labeling. Hepatic apo B production was similar in the two animal species but the VLDL-apolipoprotein secretion rate for calf liver slices amounted to only 20% of that observed for rat liver slices. Although calf and rat liver slices synthesized similar amounts of total protein, the hepatic production of albumin, measured in cells and media, was much higher in calf than rat liver slices (around 2.7-fold), whereas the rate secretion of albumin was similar in the two species. Our results showed that the slow rate of secretion of VLDL by calf liver cells was not consecutive to a low rate of synthesis of apo B but rather to a defect in VLDL assembly and/or secretion.

Clinical trials have been launched using RNA interference approaches to reduce PCSK9 expression or specific antibodies targeting and inhibiting PCSK9 interaction with the LDL receptor. They constitute very promising approaches to reducing cholesterol levels and coronary heart disease.

This study extends the spectrum of MVID to less severe clinical presentations.