Previous data suggest that apolipoprotein (apo) CIII may inhibit both triglyceride hydrolysis by lipoprotein lipase (LPL) and apo E-mediated uptake of triglyceride-rich lipoproteins by the liver. We studied apo B metabolism in very low density (VLDL), intermediate density (IDL), and low density lipoproteins (LDL) in two sisters with apo CiII-apo AI deficiency. The subjects had reduced levels of VLDL triglyceride, normal LDL cholesterol, and near absence of high density lipoprotein (HDL) cholesterol. Compartmental analysis of the kinetics of apo B metabolism after injection of '"I-VLDL and 131I-LDL revealed fractional catabolic rates (FCR) for VLDL apo B that were six to seven times faster than normal. Simultaneous injection of V3Hjglycerol demonstrated rapid catabolism of VLDL triglyceride. VLDL apo B was rapidly and efficiently converted to IDL and LDL. The FCR for LDL apo B was normal.In vitro experiments indicated that, although sera from the apo CII-apo-AI deficient patients were able to normally activate purified LPL, increasing volumes of these sera did not result in the progressive inhibition of LPL activity demonstrable with normal sera. Addition of purified apo CIII to the deficient sera resulted in 20-50% reductions in maximal LPL activity compared with levels of activity attained with the same volumes of the native, deficient sera. These in vitro studies, together with the in vivo results, indicate that in normal subjects apo CIII can inhibit the catabolism of triglyceride-rich lipoproteins by lipoprotein lipase.
Skin fibroblasts of human males affected with adrenoleukodystrophy (ALD) have previously been shown to be abnormal with respect to C26 fatty acid content. Skin fibroblast clones from heterozygotes in three families segregating this mutation have been analyzed and are oftwo types: clones with normal ratios of C26 to C22 fatty acids and clones with an excess ofC26 fatty acids similar to that found in cells of affected males. This indicates not only that the locus is X linked but also that it is subject to inactivation. In most of the heterozygotes there were significantly more clones of abnormal type than those expressing the normal allele, indicating a proliferative advantage in vitro for skin fibroblasts of mutant type. The increased levels offatty acids in plasma in most heterozygotes and the phenotype of blood cells of women heterozygous for both ALD and glucose-6-phosphate dehydrogenase (G6PD) in one family are evidence that selection favoring the mutant allele may occur in vivo as well as in vitro and may explain why many heterozygotes manifest clinical symptoms of the disease. These studies have also revealed the close linkage between ALD and G6PD loci, because there are no recombinants among 18 informative offspring of doubly heterozygous mothers. Therefore, the ALD locus can be mapped on the human X chromosome near the G6PD locus at Xq28. Adrenoleukodystrophy (ALD), a lipid storage disease, is characterized by adrenal insufficiency and progressive demyelination of the cerebral white matter (1). The onset of symptoms is usually between 4 and 8 years of age, with death in 1-4 years. ALD is believed to be an X-linked mutation, on the basis ofthe pattern of inheritance; usually males are affected and no maleto-male transmission has been reported. ALD has many features in common with a more indolent neurological disorder, adrenomyeloneuropathy (AMN), which affects females as well as males. Characteristic cytoplasmic inclusions and the accumulation of very long chain fatty acids are found in both disorders (2). Stronger evidence for a relationship between the two disorders is that they cosegregate in the same family (3), and skin fibroblasts from individuals affected with either ALD or AMN are abnormal with regard to the quantity ofC26 long-chain fatty acids (4-6).Because the ALD phenotype is demonstrable in cultured fibroblasts, we initiated studies of heterozygous females to determine if this locus is indeed X linked, ifit is subject to X chromosome inactivation (7), and to explore the relationship with AMN. Our strategy was to analyze skin fibroblast clones from heterozygotes to determine if some clones were normal and others expressed the mutant gene, as expected for X-chromosomal loci at which inactivation occurs (8). Studies of families segregating not only the ALD mutation but also electrophoretic variants of glucose-6-phosphate dehydrogenase (G6PD) revealed that the ALD mutation is X linked and that the locus is subject to inactivation and is closely linked to G6PD. These studies also suggest ...
We studied two sisters 29 and 31 years old who had skin and tendon xanthomas, corneal clouding, and severe coronary atherosclerosis. Histologic examination showed collections of lipid-laden histiocytes in the skin. The patients' plasma cholesterol concentrations were 177 and 135 mg per deciliter (4.58 and 3.49 mmol per liter). Levels of high-density-lipoprotein cholesterol were 4 and 7 mg per deciliter (0.1 and 0.2 mmol per liter). Only traces of apolipoprotein A-I were detected in whole plasma. The plasma density fraction from 1.06 to 1.21 g per milliliter contained no high-density lipoprotein on high-pressure liquid chromatography, no apolipoprotein A-I on sodium dodecyl sulfate electrophoresis, and only traces of apolipoprotein A-I on radioimmunoassay. Apolipoprotein C-III was also not detectable. The activity of lecithin-cholesterol acyltransferase was 40 per cent of normal. The half-life of infused normal high-density lipoprotein was three days (normal, 5.8 days). The parents and children of these two patients had low levels of high-density-lipoprotein cholesterol and apolipoprotein A-I. These cases support the hypothesis that low concentrations of high-density lipoprotein promote atherosclerosis.
The X chromosome in mammalian somatic cells is subject to unique regulation--usually genes on a single X chromosome are expressed while those on other X chromosomes are inactivated. The X-locus for steroid sulphatase (STS; EC 3.1.6.2), the microsomal enzyme that catalyses the hydrolysis of various 3 beta-hydroxysteroid sulphates, is exceptional because it seems to escape inactivation. Evidence for this comes from fibroblast clones in females heterozygous for mutations that result in a severe deficiency of this enzyme in affected males; all clones from these heterozygotes have STS activity, and enzyme-deficient clones that are expected if the locus were subject to inactivation, have not been found. Further evidence that the STS locus escapes inactivation is that the human inactive X chromosomes contributes STS activity to mouse-human hybrid cells. On the basis of these hybrid studies the STS locus has been mapped to the distal half of the short arm (p22-pter) of the human X chromosome. Although the STS locus on both X chromosomes in human female cells is expressed, quantitative measurements of STS activity in males and females do not accurately reflect the sex differences in number of X chromosomes (Table 1). The ratio of mean values for normal females to that of normal males is greater than 1:1 but less than the ratio of 2:1 expected if STS loci on all X chromosomes were equally expressed. The incomplete dosage effect suggests that the STS locus on the inactive X chromosome might not be fully expressed. To test this hypothesis, we examine two heterozygotes for X-linked STS deficiency who were also heterozygous for the common electrophoretic variants of glucose-6-phosphate dehydrogenase (G6PD A and B). Studies of fibroblast clones from these females provide evidence, presented here, for differential expression of STS loci on the active and inactive X chromosome.
Epidemiological studies have identified elevated low density lipoprotein (LDL) and diminished high density lipoprotein (HDL) cholesterol levels as risk factors for coronary artery disease. The major protein component of HDL is apoprotein A-I (apo A-I), a polypeptide of 243 amino acids of known primary amino acid sequence. This apoprotein serves as a cofactor for the plasma lecithin-cholesterol acyltransferase (LCAT) enzyme responsible for the formation of most cholesteryl esters in plasma, and also promotes cholesterol efflux from cells. The primary translation product of apo A-I contains both a pre and a pro segment, and post-translational processing of apo A-I may be involved in the formation of the functional plasma apo A-I isoproteins. Defective apo A-I processing may be the underlying problem in Tangier disease, in which patients have low plasma HDL and apo A-I levels despite normal apo A-I synthesis. Patients have been reported with conditions distinct from Tangier disease in whom severe deficiency or absence of apo A-I has been associated with very low HDL levels and severe coronary artery disease. We have now examined the apo A-I gene in two such patients and their first degree relatives. These patients have been reported to have skin and tendon xanthomas, corneal clouding and severe premature coronary atherosclerosis associated with very low HDL levels and deficiencies of two apoproteins, apo A-I and apo C-III. We show that both probands are homozygous for a defect in the apo A-I gene locus.
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