Increased catabolism of VLDL-apolipoprotein B and synthesis of bile acids in a case of hypobetalipoproteinemia

Unusual presentation of three siblings with familial heterozygous hypobetalipoproteinaemia

We describe three siblings with the unusual presentation of manifest steatorrhoea and vitamin E deficiency mimicking homozygous familial hypobetalipoproteinaemia (FHBL) but whose lipid profile (cholesterol and ApoB) was consistent with heterozygous FHBL. Upper gastrointestinal endoscopy and small intestinal biopsy were normal. We discuss the diagnosis with reference to the relevant literature.

CONCLUSION

although rare, familial hypobetalipoproteinaemia should be considered among the causes of manifest steatorrhoea in childhood even without evidence of failure to thrive. Dietary restriction of fat and high dose vitamin E supplementation improves quality of life by reducing stool frequency and may prevent or delay neurological complications.

Bile acid regulation of gene expression: roles of nuclear hormone receptors

Bile acids derived from cholesterol and oxysterols derived from cholesterol and bile acid synthesis pathways are signaling molecules that regulate cholesterol homeostasis in mammals. Many nuclear receptors play pivotal roles in the regulation of bile acid and cholesterol metabolism. Bile acids activate the farnesoid X receptor (FXR) to inhibit transcription of the gene for cholesterol 7alpha-hydroxylase, and stimulate excretion and transport of bile acids. Therefore, FXR is a bile acid sensor that protects liver from accumulation of toxic bile acids and xenobiotics. Oxysterols activate the liver orphan receptors (LXR) to induce cholesterol 7alpha-hydroxylase and ATP-binding cassette family of transporters and thus promote reverse cholesterol transport from the peripheral tissues to the liver for degradation to bile acids. LXR also induces the sterol response element binding protein-1c that regulates lipogenesis. Therefore, FXR and LXR play critical roles in coordinate control of bile acid, cholesterol, and triglyceride metabolism to maintain lipid homeostasis. Nuclear receptors and bile acid/oxysterol-regulated genes are potential targets for developing drug therapies for lowering serum cholesterol and triglycerides and treating cardiovascular and liver diseases.

Plasma lipids and lipoproteins response to a dietary challenge: analysis of four candidate genes

The possible role of four candidate genes in lipid and lipoprotein response to diet was examined in 63 male students. Four site polymorphisms (signal peptide insertion/deletion, XbaI, MspI and EcoRI) of the apo B gene, three RFLPs (AvaII, StuI, and HincII) of the LDL receptor gene, two SSCPs of the cholesterol 7 alpha-hydroxylase gene and the common apo E genotypes were determined. The average reductions induced by diet in participants homozygous for the absence of the XbaI restriction site (X-X-) of the apo B gene compared to those harboring this site (X+) were: 14.5 mg/dl and 9.4 mg/dl for total cholesterol (TC) (p < 0.09) and 15.5 mg/dl and 7.9 mg/dl for LDL-C (p < 0.003), respectively. Differences in dietary responsiveness among the apo E, LDL receptor and the cholesterol 7 alpha-hydroxylase genotypes were largely insignificant. Using the four apo B polymorphic sites, six unambiguous haplotypes were constructed and a model for their possible evolutionary relationship is presented. Genetic variation in the apo B gene region, as defined by haplotypes, accounted for 8.7% and 24.3% of the phenotypic variance in TC and LDL-C response to diet, respectively. Sequence analysis of a candidate locus, the putative LDL receptor binding region of apo B and its flanking sequences, was performed in two individuals, one homozygous for an apo B "hyper-responding" and another for the "lower-responding" haplotype, and no differences were found. In conclusion, haplotypes at the apo B gene locus are associated with dietary response of TC and LDL-C in young males. Yet, the sequence variation responsible for these differences is possibly located outside the putative LDL receptor binding domain.

A variant form of hypobetalipoproteinaemia associated with ataxia, hearing loss and retinitis pigmentosa

A six-year-old Japanese boy had ataxia, mental retardation, peripheral neuropathy, proximal myopathy, hearing loss, retinitis pigmentosa and deficiencies in apolipoprotein AI, B, CII and CIII. His clinical features except for hearing loss resembled those of abetalipoproteinaemia or symptomatic hypobetalipoproteinaemia, but his apolipoprotein abnormalities were distinct from these disorders. He had apolipoprotein B-100 with a normal molecular weight. Although most of his neurological manifestations were compatible with those of vitamin E deficiency, their early onset and the presence of hearing loss was unusual for that condition. There has been slight deterioration of ataxia during two years follow-up despite high-dose vitamin E supplementation. Other abnormalities in lipid metabolism might be associated with the neurological damage in this case.

Relationship of apolipoprotein E phenotypes to hypocholesterolemia

PURPOSE

Persons with total cholesterol (TC) levels less than 130 mg/dL (less than 3.26 mmol/L) make up less than 1% of a healthy population. Causes of hypocholesterolemia include a diet very low in cholesterol and saturated fat, disease, genetic factors (including low apolipoprotein B-100 [apo B-100] and the apo E allele), and drug therapy. The purpose of this study was to determine the causes of hypocholesterolemia in a healthy Kaiser Foundation Health Plan (KFHP) population.

PATIENTS AND METHODS

We conducted a dietary and health survey of 201 healthy hypocholesterolemic adults (range: 2.04 to 3.88 mmol/L [79 to 150 mg/dL]) and 200 matched control subjects with TC levels in the middle quintile of the population (range: 5.0 to 5.61 mmol/L [194 to 217 mg/dL]) who had routine health screening from 1983 through 1985. We did apo E phenotyping studies and lipid and apo A-1 and B-100 measurements in a subgroup of 45 hypocholesterolemic subjects (mean TC level: 3.26 mmol/L [126 mg/dL]) and in a comparison group of 49 unmatched volunteers (mean TC level: 5.04 +/- 0.75 mmol/L [195 +/- 29 mg/dL]).

RESULTS

We found no differences in dietary intake or clinically significant medical illness between hypocholesterolemic and control subjects. In the hypocholesterolemic subgroup, we found an increased frequency of the apo E2 allele (epsilon 2) and a decreased frequency of the apo E4 allele (epsilon 4); the frequencies of the epsilon 2, epsilon 3, and epsilon 4 alleles were 33.3%, 63.3%, and 3.3%, respectively. The corresponding apo E allele frequencies in the comparison subgroup were 8.2%, 73.5%, and 18.4%, similar to those previously reported for the general population and significantly different from those found in the hypocholesterolemic subgroup (p < 0.0001). One hypocholesterolemic subject (a 46th patient) had a mutation in the apo B gene that resulted in the synthesis of a truncated species of apo B (apo B-46).

CONCLUSION

Our study indicates that hypocholesterolemia in our KFHP urban population is usually not caused by diet or disease. Biochemical factors, including the increased frequency of the apo E-2 phenotype and the decreased frequency of the apo E-4 phenotype, are more important.

Cloning of the human cholesterol 7 alpha-hydroxylase gene (CYP7) and localization to chromosome 8q11-q12

Cholesterol 7 alpha-hydroxylase (7 alpha-hydroxylase) is a microsomal cytochrome P450 that catalyzes the first step in bile acid synthesis. In this paper, we describe the cloning, characterization, and chromosomal mapping of the human 7 alpha-hydroxylase gene (CYP7). The gene spans 10 kb and contains six exons and five introns. The exon-intron boundaries are completely conserved between the human and rat genes. Sequencing of the 5' flanking region revealed consensus recognition sequences for a number of liver-specific transcription factors. The human CYP7 gene was mapped to chromosome 8q11-q12 using both mouse-human somatic cell hybrids and in situ chromosomal hybridization studies. A total of four single-stranded conformation-dependent DNA polymorphisms and an Alu sequence-related polymorphism were identified. Of the individuals analyzed, 80% were heterozygous for at least one of these five polymorphisms. The localization and characterization of the human 7 alpha-hydroxylase gene, as well as the identification of polymorphisms, provide the molecular tools necessary to investigate the role of this gene in disorders of cholesterol and bile acid metabolism.

Receptor-dependent and -independent catabolism of low-density lipoprotein in a kindred with familial hypobetalipoproteinemia

Three affected members of a kindred with asymptomatic hypobetalipoproteinemia (HBL) were injected intravenously with 125I-labeled native low-density lipoproteins (LDL) and 131I-labeled cyclohexanedione (CHD)-treated LDL. Plasma and urine radioactivity data were collected for 15 days at regular intervals. A compartmental model using the SAAM program was built to fit simultaneously 125I and 131I plasma radioactivity decay and urine excretion data. This model allows precise calculation of the kinetic parameters of both receptor-independent (NR) and receptor-dependent (R) pathways. Compared with normal subjects, HBL patients show a 90% increased fractional catabolic rate (FCR) of LDL by both routes, more marked for the R pathway (215% increase), and an approximately 50% reduced production rate (PR). Structural analysis did not show significant abnormalities of apolipoprotein (apo) B in HBL patients compared with normal. These data suggest that the very reduced, LDL-apo B plasma levels result from a combination of two processes: (1) an increased activity of all catabolic routes, and (2) a reduced "synthesis" rate. The latter may result from a decreased conversion of very-low-density lipoprotein (VLDL) to LDL secondary to an increased direct removal of large VLDL, suggested by apo C-II and C-III turnover studies previously reported.

A form of familial hypobetalipoproteinaemia not due to a mutation in the apolipoprotein B gene

Familial hypobetalipoproteinaemia (FHBL) is a dominant disorder of lipoprotein metabolism characterized by levels of apolipoprotein B-carrying lipoproteins (VLDL, IDL and LDL) which are 50% of the normal levels in the heterozygotes and almost absent in the homozygotes. Several reports have recently shown that the underlying defect in FHBL involves different mutations in the apo B gene which lead to reduced levels of apo B mRNA or to the production of truncated forms of apo B having either a lower synthetic rate or a higher catabolic rate than normal apo B. We here present a three-generation family with several FHBL members in which the linkage analysis shows absence of co-segregation between apo B gene alleles and the hypocholesterolaemic phenotype. We conclude that a dominantly transmitted mutation in a gene other than that for apo B is responsible for the low plasma cholesterol levels.

Apolipoprotein C-II and C-III metabolism in a kindred of familial hypobetalipoproteinemia

Three affected members of a kindred with asymptomatic hypobetalipoproteinemia (HBL) showed low levels of triglycerides, low-density lipoprotein (LDL)-cholesterol, and apolipoproteins (apo) B, C-II, and C-III. Turnover of iodine-labeled apo C-II and apo C-III associated in vitro to plasma lipoproteins was studied after intravenous injection. Radioactivity in plasma and lipoproteins (95% recovered in high-density lipoprotein [HDL] density range) and in 24-hour urine samples was observed for 16 days. A parallelism of the slowest slopes of plasma decay curves was observed between apo C-II and apo C-III, indicating a partial common catabolic route. Urine/plasma radioactivity ratio (U/P) varied with time, suggesting heterogeneity of metabolic pathways. A new compartmental model using the SAAM program was built, not only fitting simultaneously plasma and urine data, but also taking into account the partial common metabolism of lipoprotein particles (LP) containing apo C-II and apo C-III. The low apo C-II and C-III plasma concentrations observed in HBL compared with normal resulted from both an increased catabolism and a reduced synthesis, these changes being more marked for apo C-III. The modifications in the rate constants of the different pathways calculated from the new model are in favor of an increased direct removal of particles following the fast pathway, likely in the very-low-density lipoprotein (VLDL) density range.

Plasma lipids, lipoproteins and apolipoproteins in two kindreds of hypobetalipoproteinemia

Plasma lipids and apolipoproteins were quantified in two kindreds of hypobetalipoproteinemia. All affected members were asymptomatic but showed a decrease of 75% in apolipoprotein B and of 69% in LDL-cholesterol. There were no major changes in apo A-I and A-II but all affected family members had reduced levels of apo C-II (by 58%) and C-III (by 59%) without significant decrease in apo C-I and no specific decrease of apo C-III1. Apolipoprotein E is decreased in SDS-PAGE. The plasma level and phenotype of Lp(a) are not affected by HBL, suggesting that a catabolic rather than a synthetic mechanism is responsible for the disease. As shown by density gradient ultracentrifugation, HDL2 particles that contain essentially apolipoprotein A-I, cholesterol and phospholipids represent in affected subjects the major part of HDL. Due to the net reduction of apolipoprotein B-containing particles (VLDL and LDL) as acceptors of lipids in HBL, there is an accumulation of large particles rich in cholesteryl esters.

Pravastatin therapy in primary moderate hypercholesterolaemia: changes in metabolism of apolipoprotein B-containing lipoproteins

This study examined the actions of pravastatin on the metabolism of apolipoprotein B (apo B) in very low-, intermediate-, and low-density lipoproteins (VLDL, IDL, and LDL) in 10 patients with primary moderate hypercholesterolaemia. 131I-VLDL apo B was used as a tracer, and appearance of label was followed into IDL apo B and LDL apo B. Compared to placebo, pravastatin therapy reduced levels of cholesterol in total plasma. LDL, VLDL, and IDL cholesterol by 25%, 29%, 31%, and 47%, respectively. Pravastatin treatment also significantly decreased concentrations of apo B in LDL, IDL, and VLDL. The drug significantly reduced the mean production rate for VLDL apo B by 40%, and decreased production rates for LDL apo B in eight of 10 patients. In contrast, fractional catabolic rates (FCRs) were not altered significantly in any of the three lipoprotein fractions on pravastatin therapy. Further, pravastatin produced no consistent changes in LDL particle size, composition, or LDL subclass pattern. Thus pravastatin seemingly reduced input rates for all apo B-containing lipoproteins. Consistent with previous studies, this response was most likely the result of enhanced removal of nascent lipoproteins by increased activity of LDL receptors, although decreased synthesis of apo B in the liver is a possible second action.

Severe acquired hypocholesterolemia: two case reports

We present two case reports from different institutions that emphasize two distinct aspects of severe acquired hypocholesterolemia. The first case report discusses the issue of malabsorption in a man with systemic lupus erythematosus and bacterial overgrowth of the small bowel. The second case report examines the data on metabolic abnormalities associated with severe illness that appear to be independent of digestion and absorption. These two cases serve to alert the physician that severe hypocholesterolemia is a portentous finding that may be associated both with a wide variety of diseases and with a high mortality rate. Successful therapy of the underlying diseases may lead to correction of the hypocholesterolemia.

Lovastatin therapy in nephrotic hyperlipidemia: effects on lipoprotein metabolism

The nephrotic syndrome is characterized by proteinuria, hypoalbuminemia, and hypercholesterolemia. Hypertriglyceridemia often is present as well. In this study, the kinetics of plasma lipoproteins were investigated in four patients with nephrotic hyperlipidemia, and repeat studies were carried out in three of these patients during therapy with lovastatin. Before lovastatin therapy, the patients had an extremely delayed catabolism of very low density lipoproteins (VLDL) without evidence of overproduction of lipoproteins in this fraction. Three of four patients had elevated levels of low density lipoprotein (LDL) that were due mainly to increased production rates for LDL. In the three patients treated with lovastatin, the drug therapy lowered plasma concentrations of total cholesterol, triglycerides, VLDL-cholesterol, and LDL-cholesterol, and raised high density lipoprotein (HDL)-cholesterol. Lovastatin therapy decreased VLDL-triglycerides primarily by enhancing their catabolism, and lowered LDL-cholesterol levels mainly by reducing input rates for LDL. Overall, lovastatin appears to be an effective drug for the treatment of hyperlipidemia in the nephrotic syndrome.

Lovastatin therapy in familial dysbetalipoproteinemia: effects on kinetics of apolipoprotein B

Familial dysbetalipoproteinemia is characterized by hyperlipidemia, increases in beta-migrating, very low density lipoproteins (beta-VLDL), and homozygosity for apolipoprotein E2 (apo E2). In this study, 3 patients with familial dysbetalipoproteinemia were treated with lovastatin, and kinetics for apolipoprotein B (apo B) were determined in control and drug treatment periods. Multicompartmental analyses of apo B kinetics in VLDL and in low density lipoproteins (LDL) were carried out. Lovastatin therapy generally lowered plasma concentrations of apo B and cholesterol in VLDL and LDL. The reductions in concentrations were due mainly to a decrease in transport (production) rates for these fractions. Indeed, the fractional clearance rate (FCR) for LDL-apo B was reduced during lovastatin therapy. The decreased transport rate for VLDL-apo B and LDL-apo B could have been due to an inhibition of the synthesis of lipoproteins containing apo B. An alternate explanation is that lovastatin promoted direct removal of a rapidly-catabolized fraction of VLDL-apo B that is a precursor for longer-lived lipoproteins in the circulation; this mechanism could decrease input rates of identifiable lipoprotein species and retard their clearance because of "saturation" of LDL receptors by more rapidly removed lipoproteins. Finally, both mechanisms, i.e., decreased production and increased clearance of lipoproteins, may have contributed to the fall in VLDL-apo B and LDL-apo B concentrations during lovastatin therapy.