Metabolic studies in an unusual case of asymptomatic familial hypobetalipoproteinemia with hypolphalipoproteinemia and fasting chylomicronemia

Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) Inhibitors, Reality or Dream in Managing Patients with Cardiovascular Disease

BACKGROUND

Statins have been a major keystone in the management of patients with atherosclerotic cardiovascular disease. The benefits of inhibiting HMG CoA reductase, via statins, were translated into reduction in LDL-c with proportionate decrease in cardiovascular events in response to the magnitude of LDL-c reduction. Despite major advances in pharmacological treatments, including the use of high-dose statins, there are urgent need to further reduce future cardiovascular risk. This is in particularly important since 1 out of 5 high-risk atherosclerotic patients who achieve low LDL-c return with a second cardiovascular event within five years. Although this residual risk post-statin is largely heterogeneous, lowering LDL-c beyond 'normal' or guidelines-recommended level using novel therapies has resulted in further reduction in cardiovascular events.

OBJECTIVE

The current review will discuss the use of PCSK9 inhibitors in patients with atherosclerotic disease. PCSK9 inhibitors are a new class of lipid-lowering drugs that are either fully human monoclonal antibodies (evolocumab and alirocumab) or humanised monoclonal antibodies (bococizumab) that effectively reduce LDL-c to unprecedented level. By blocking circulating PCSK9, these drugs would preserve LDL receptors and prevent them from cellular degradation. This process promotes recycling of LDL receptors back to hepatocytes surface, leading into further reduction of LDL-c. Combining PCSK9 inhibitors with statin have led into lower LDL-c, reduction in plaque volume and more importantly reduction in future cardiovascular events.

CONCLUSION

These drugs are very promising, nonetheless, the unselective approach of applying these monoclonal antibodies may not prove to be cost-effective and potentially exposing some patients to unnecessary side effects.

The Holstein Friesian Lethal Haplotype 5 (HH5) Results from a Complete Deletion of TBF1M and Cholesterol Deficiency (CDH) from an ERV-(LTR) Insertion into the Coding Region of APOB

Background

With the availability of massive SNP data for several economically important cattle breeds, haplotype tests have been performed to identify unknown recessive disorders. A number of so-called lethal haplotypes, have been uncovered in Holstein Friesian cattle and, for at least seven of these, the causative mutations have been identified in candidate genes. However, several lethal haplotypes still remain elusive. Here we report the molecular genetic causes of lethal haplotype 5 (HH5) and cholesterol deficiency (CDH). A targeted enrichment for the known genomic regions, followed by massive parallel sequencing was used to interrogate for causative mutations in a case/control approach.

Methods

Targeted enrichment for the known genomic regions, followed by massive parallel sequencing was used in a case/control approach. PCRs for the causing mutations were developed and compared to routine imputing in 2,100 (HH5) and 3,100 (CDH) cattle.

Results

HH5 is caused by a deletion of 138kbp, spanning position 93,233kb to 93,371kb on chromosome 9 (BTA9), harboring only dimethyl-adenosine transferase 1 (TFB1M). The deletion breakpoints are flanked by bovine long interspersed nuclear elements Bov-B (upstream) and L1ME3 (downstream), suggesting a homologous recombination/deletion event. TFB1M di-methylates adenine residues in the hairpin loop at the 3’-end of mitochondrial 12S rRNA, being essential for synthesis and function of the small ribosomal subunit of mitochondria. Homozygous TFB1M^-/- mice reportedly exhibit embryonal lethality with developmental defects. A 2.8% allelic frequency was determined for the German HF population. CDH results from a 1.3kbp insertion of an endogenous retrovirus (ERV2-1-LTR_BT) into exon 5 of the APOB gene at BTA11:77,959kb. The insertion is flanked by 6bp target site duplications as described for insertions mediated by retroviral integrases. A premature stop codon in the open reading frame of APOB is generated, resulting in a truncation of the protein to a length of only <140 amino acids. Such early truncations have been shown to cause an inability of chylomicron excretion from intestinal cells, resulting in malabsorption of cholesterol. The allelic frequency of this mutation in the German HF population was 6.7%, which is substantially higher than reported so far. Compared to PCR assays inferring the genetic variants directly, the routine imputing used so far showed a diagnostic sensitivity of as low as 91% (HH5) and 88% (CDH), with a high specificity for both (≥99.7%).

Conclusion

With the availability of direct genetic tests it will now be possible to more effectively reduce the carrier frequency and ultimately eliminate the disorders from the HF populations. Beside this, the fact that repetitive genomic elements (RE) are involved in both diseases, underline the evolutionary importance of RE, which can be detrimental as here, but also advantageous over generations.

Homozygous familial hypobetalipoproteinemia: two novel mutations in the splicing sites of apolipoprotein B gene and review of the literature

OBJECTIVE

Familial hypobetalipoproteinemia (FHBL) is autosomal codominant disorder of lipoprotein metabolism characterized by low plasma levels of total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C) and apolipoprotein B (apoB) below the 5(th) percentile of the distribution in the population. Patients with the clinical diagnosis of homozygous FHBL (Ho-FHBL) are extremely rare and few patients have been characterized at the molecular level. Here we report the medical history and the molecular characterization of one paediatric patient with clinical features of Ho-FHBL.

METHODS

A one month old infant with failure to thrive, severe hypocholesterolemia and acanthocytosis was clinically and genetically characterized. Molecular characterization of the proband and her parents was performed by direct sequencing of the APOB gene and functional role of the identified mutations was assessed by the minigene methodology.

RESULTS

The proband was found carrying two novel splicing mutations of the APOB gene (c.3696+1G > C and c.3697-1G > A). CHOK1H8 cells expressing minigenes harbouring the mutations showed that these two mutations were associated with the retention of intron 23 and skipping of exon 24, resulting in two truncated apoB fragments of approximate size of 26-28 % of ApoB-100 and the total absence of apoB.

CONCLUSION

We describe the first case of Ho-FHBL due to two splicing mutations affecting both the donor and the acceptor splice sites of the same intron of the APOB gene occurring in the same patient. The clinical management of the proband is discussed and a review of the clinical and genetic features of the published Ho-FHBL cases is reported.

Homozygous MTTP and APOB mutations may lead to hepatic steatosis and fibrosis despite metabolic differences in congenital hypocholesterolemia

BACKGROUND & AIMS

Non-alcoholic steatohepatitis leading to fibrosis occurs in patients with abetalipoproteinemia (ABL) and homozygous or compound heterozygous familial hypobetalipoproteinemia (Ho-FHBL). We wanted to establish if liver alterations were more frequent in one of both diseases and were influenced by comorbidities.

METHODS

We report genetic, clinical, histological and biological characteristics of new cases of ABL (n =7) and Ho-FHBL (n = 7), and compare them with all published ABL (51) and Ho-FHBL (22) probands.

RESULTS

ABL patients, diagnosed during infancy, presented mainly with diarrhea, neurological and ophthalmological impairments and remained lean, whereas Ho-FHBL were diagnosed later, with milder symptoms often becoming overweight in adulthood. Despite subtle differences in lipid phenotype, liver steatosis was observed in both groups with a high prevalence of severe fibrosis (5/27 for Ho-FHBL vs. 4/58 for ABL (n.s.)). Serum triglycerides concentration was higher in Ho-FHBL whereas total and HDL-cholesterol were similar in both groups. In Ho-FHBL liver alterations were found to be independent from the apoB truncation size and apoB concentrations.

CONCLUSIONS

Our findings provide evidence for major liver abnormalities in both diseases. While ABL and Ho-FHBL patients have subtle differences in lipid phenotype, carriers of APOB mutations are more frequently obese. These results raise the question of a complex causal link between apoB metabolism and obesity. They suggest that the genetic defect in VLDL assembly is critical for the occurrence of liver steatosis leading to fibrosis and shows that obesity and insulin resistance might contribute by increasing lipogenesis.

Familial hypobetalipoproteinemia-induced nonalcoholic steatohepatitis

Familial hypobetalipoproteinemia (FHBL) is a rare genetic disorder of lipid metabolism that is associated with abnormally low serum levels of low-density lipoprotein (LDL) cholesterol and apolipoprotein B. It is an autosomal co-dominant disorder, and depending on zygosity, the clinical manifestations may vary from none to neurological, endocrine, hematological or liver dysfunction. Nonalcoholic fatty liver disease is common in persons with FHBL, however progression to nonalcoholic steatohepatitis is unusual. We describe here a patient with a novel APOB mutation, V703I, which appears to contribute to the severity of the FHBL phenotype. He had liver enzyme abnormalities, increased echogenicity of the liver consistent with steatosis, very low LDL cholesterol at 0.24 mmol/l (normal 1.8–3.5 mmol/l) and an extremely low apolipoprotein B level of 0.16 g/l (normal 0.6–1.2 g/l). APOB gene sequencing revealed him to be a compound heterozygote with two mutations (R463W and V703I). APOB R463W has previously been reported to cause FHBL. Genetic sequencing of his first-degree relatives identified the APOB V703I mutation in his normolipidemic brother and father and the APOB R463W mutation in his mother and sister, both of whom have very low LDL cholesterol levels. These results suggest that the APOB V703I mutation alone does not cause the FHBL phenotype. However, it is possible that it has a contributory role to a more aggressive phenotype in the presence of APOB R463W.

Pediatric gallstone disease in familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is an monogenic co-dominant disorder characterized by reduced plasma levels of cholesterol, low density lipoproteins (LDL) and apolipoprotein B (apoB) often associated with non-alcoholic fatty liver disease (NAFLD). It has been suggested that FHBL might predispose to gallstone disease (GD). We report a hypocholesterolemic 10 year old girl with obstructive jaundice due to cholesterol stones in gallbladder and common bile duct which required cholecistectomy. The analysis of patient's plasma lipoproteins revealed a marked reduction of LDL and apoB, a lipid profile consistent with the clinical diagnosis of heterozygous FHBL. The same profile was found in her mother who had severe NAFLD. The analysis of apoB gene, the main candidate gene in FHBL, revealed that the patient and her mother were heterozygotes for a novel nonsense mutation (Y1220X) predicted to cause the formation of a short truncated apoB (apoB-26.87) not secreted into the plasma. The presence of cholesterol stones could result from increased biliary cholesterol secretion as a compensatory mechanism for the reduced capacity of the liver to export cholesterol incorporated into apoB-containing lipoproteins. FHBL should be considered as a possible predisposing factor for cholesterol gallstones in children (190).

A gene-targeted mouse model for familial hypobetalipoproteinemia. Low levels of apolipoprotein B mRNA in association with a nonsense mutation in exon 26 of the apolipoprotein B gene

Familial hypobetalipoproteinemia, a syndrome characterized by abnormally low plasma levels of low density lipoprotein cholesterol, is caused by mutations in the apolipoprotein (apo) B gene that interfere with the synthesis of a full-length apoB100. In many cases of familial hypobetalipoproteinemia, nonsense or frameshift mutations result in the synthesis of a truncated apoB protein. To understand why these mutations result in low plasma cholesterol levels, we used gene targeting in mouse embryonic stem cells to introduce a nonsense mutation (N1785Stop) into exon 26 of the mouse Apob gene. The sole product of this mutant Apob allele was a truncated apoB, apoB39. Mice homozygous for this "apoB39-only" (Apob39) allele had low plasma levels of apoB39 and markedly reduced plasma levels of very low density lipoprotein and low density lipoprotein cholesterol when fed a high fat diet. Analysis of liver and intestinal RNA from heterozygous apoB39-only mice revealed that the Apob39 mRNA levels were 60-70% lower than those from the wild-type allele. Interestingly, apoB39 was not cleared as rapidly from the plasma as apoB48. The apoB39-only mice provide new insights into the mechanisms of familial hypobetalipoproteinemia and the structural features of apoB that are important for lipoprotein metabolism.

Reading-frame restoration by transcriptional slippage at long stretches of adenine residues in mammalian cells

We previously characterized a mutant apoB allele (the apoB86 allele) that produces both a truncated apoB (apoB86) and a full-length apoB100. The mutant allele contained a deletion of a single cytosine in exon 26, creating a stretch of eight consecutive adenines in the -1 reading frame. The altered reading-frame allele was restored, with approximately 10% efficiency, by the transcriptional insertion of an extra adenine into the stretch of eight consecutive adenines, thereby accounting for the synthesis of the full-length apoB100. Here, we demonstrate that this reading-frame restoration does not occur when the long stretch of adenines is interrupted by a cytosine. To assess whether reading-frame restoration is unique to a single site in the apoB gene, the same mutation (eight consecutive adenines in the -1 reading frame) was inserted into another site within the apoB gene. Reading-frame restoration occurred at the second site and was abrogated when the stretch of adenines was interrupted by another base. Of note, a computerized analysis of human cDNA sequences revealed that long stretches of adenines in protein-coding sequences occur at a lower than predicted frequency, suggesting that evolution has selected against these sequences.

Apoprotein B-100 production is decreased in subjects heterozygous for truncations of apoprotein B

Among individuals who are heterozygous for familial hypobetalipoproteinemia (FHBL) and who have various truncations of apoprotein (apo) B (ie, FHBL with apoB truncation/apoB-100 genotypes), the plasma concentrations of apoB-100 are typically approximately 30% rather than the expected approximately 50% of those in unaffected family members. The metabolic basis for the low apoB-100 levels is unknown. Therefore, we compared the metabolism of apoB-100 in 8 subjects with heterozygous FHBL (2 apoB-89/apoB-100, 2 apoB-75/apoB-100, 2 apoB-54.8/apoB-100, 1 apoB-52/apoB-100, and 1 apoB-31/apoB-100) with the metabolism of apoB-100 in 8 apoB-100/apoB-100 control subjects who were paired with the heterozygotes by gender, age, height, weight, and race. Endogenous labeling of apoB-100 with [13C]leucine and a multicompartmental kinetic model were used to obtain kinetic parameters. FHBL heterozygotes had significantly reduced VLDL apoB-100 production rates (7.7 +/- 3.7 versus 21.2 +/- 6.2 mg.kg-1.d-1, P = .002) and LDL apoB-100 production rates (4.5 +/- 3.12 versus 15.3 +/- 1 mg.kg-1.d-1, P = .05) compared with control subjects. Fractional conversion rates of VLDL to LDL were not significantly different (0.67 +/- 0.36 versus 0.77 +/- 0.17 pools/d), and the respective fractional catabolic rates of apoB-100 in VLDL, IDL, and LDL also were similar in both groups. Thus, FHBL heterozygotes produced apoB-100 at about 30% of the rates of control subjects. We believe these reduced production rates largely account for the lower than expected levels of apoB-100 and LDL cholesterol in the plasma of FHBL heterozygotes.

Carboxyl-terminal truncation of apolipoproteinB-100 inhibits lipoprotein(a) particle formation

Recombinant expression systems for both apo(a) and apoB were used to identify sequences in apoB which are required for Lp(a) formation. Incubation of a [35S]Cys-labelled 17-kringle form of apo(a) with supernatants from rat hepatoma (McA-RH7777) cells expressing apoB-88, apoB-94 and apoB-100 resulted in covalent r-Lp(a) formation only with apoB-100. Additionally, apoB-86 present in the LDL of a hypobetalipoproteinemic subject did not associate with a 12-kringle form of recombinant apo(a) to form r-Lp(a) complexes. Our data suggest that sequences within the C-terminal 6% of apoB-100 are essential for Lp(a) assembly.

Familial hypobetalipoproteinaemia: a rare presentation to the lipid clinic

OBJECTIVE

To report a case of familial hypobetalipoproteinaemia in a woman who presented after the incidental finding of marked hypocholesterolaemia during laboratory tests.

CLINICAL FEATURES

An asymptomatic 37-year-old Lebanese woman presented to the lipid clinic with a serum total cholesterol concentration of 1.1 mmol/L, high density lipoprotein (HDL) cholesterol of 1.0 mmol/L, and triglycerides of 0.28 mmol/L. No secondary cause for the hypocholesterolaemia was established. INVESTIGATION AND OUTCOME: Her serum apolipoprotein B (apo B) levels were markedly reduced at 0.07 g/L. Except for one daughter (IV-4), all other family members including her husband (her first cousin) had apo B levels about 25% of normal. Daughter IV-4 had undetectable apo B levels. Family studies confirmed an autosomal dominant pattern of inheritance consistent with familial hypobetalipoproteinaemia.

CONCLUSION

Familial hypobetalipoproteinaemia is a rare condition that should be considered in the differential diagnosis of hypocholesterolaemia. Absence of clinical features, autosomal dominant pattern of inheritance, and reduced apo B levels suggest the diagnosis.

Targeted modification of the apolipoprotein B gene results in hypobetalipoproteinemia and developmental abnormalities in mice

Familial hypobetalipoproteinemia is an autosomal codominant disorder resulting in a dramatic reduction in plasma concentrations of apolipoprotein (apo) B, cholesterol, and beta-migrating lipoproteins. A benefit of hypobetalipoproteinemia is that mildly affected individuals may be protected from coronary vascular disease. We have used gene targeting to generate mice with a modified Apob allele. Mice containing this allele display all of the hallmarks of human hypobetalipoproteinemia: they produce a truncated apoB protein, apoB70, and have markedly decreased plasma concentrations of apoB, beta-lipoproteins, and total cholesterol. In addition, the mice manifest several characteristics that are occasionally observed in human hypobetalipoproteinemia, including reduced plasma triglyceride concentrations, fasting chylomicronemia, and reduced high density lipoprotein cholesterol. An unexpected finding is that the modified Apob allele is strongly associated with exencephalus and hydrocephalus. These mice should help increase our understanding of hypobetalipoproteinemia, atherogenesis, and the etiology of exencephalus and hydrocephalus.

Reading-frame restoration with an apolipoprotein B gene frameshift mutation

We examined a mutant human apolipoprotein B (apoB) allele that causes hypobetalipoproteinemia and has a single cytosine deletion in exon 26. This frameshift mutation was associated with the synthesis of a truncated apoB protein of the predicted size; however, studies in human subjects and minigene expression studies in cultured cells indicated that the mutant allele also yielded a full-length apoB protein. The 1-base-pair deletion in the mutant apoB allele created a stretch of eight consecutive adenines. To understand the mechanism whereby the mutant apoB allele yielded a full-length apoB protein, the cDNA from cells transfected with the mutant apoB minigene expression vector was examined. Splicing of the mRNA was normal; however, 11% of the cDNA clones had an additional adenine within the stretch of eight adenines, yielding nine consecutive adenines. The insertion of the extra adenine, presumably during apoB gene transcription, is predicted to restore the correct apoB reading frame, thereby permitting the synthesis of a full-length apoB protein.

Apolipoprotein B gene mutations affecting cholesterol levels

In the past 5 years, many different mutations in the apolipoprotein (apo) B gene have been described that affect plasma cholesterol levels. More than 20 different mutations in the apoB gene have been shown to cause familial hypobetalipoproteinaemia, a condition characterized by abnormally low plasma concentrations of apoB and LDL cholesterol. Almost all of the mutations are nonsense or frameshift mutations that interfere with the translation of a full-length apoB100 molecule. Many, but not all, of these apoB gene mutations result in the synthesis of a truncated species of apoB that can be detected within the plasma lipoproteins. Familial hypobetalipoproteinaemia heterozygotes are almost always asymptomatic and have LDL cholesterol levels about one-quarter to one-third of those of unaffected family members. Several homozygotes and compound heterozygotes for familial hypobetalipoproteinaemia have been described. In these individuals, the LDL cholesterol levels are extremely low, usually less than 5 or 10 mg dl-1, and the clinical phenotype is variable, ranging from completely asymptomatic to severe problems related to intestinal fat malabsorption. One missense mutation in the apoB gene (an Arg----Gln substitution at apoB amino acid 3500) is associated with very poor binding of apoB100 to the cellular LDL receptor. This syndrome has been designated familial defective apolipoprotein B (FDB). The amino-acid substitution at residue 3500 delays the clearance of LDL from the plasma and results in hypercholesterolaemia. In some Western populations, the frequency of FDB heterozygotes appears to be as high as 1 in 500 individuals.

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.

Hepatocyte nuclear factor 1 and C/EBP are essential for the activity of the human apolipoprotein B gene second-intron enhancer

The tissue-specific transcriptional enhancer of the human apolipoprotein B gene contains multiple protein-binding sites spanning 718 bp. Most of the enhancer activity is found in a 443-bp fragment (+621 to +1064) that is located entirely within the second intron of the gene. Within this fragment, a 147-bp region (+806 to +952) containing a single 97-bp DNase I footprint exhibits significant enhancer activity. We now report that this footprint contains four distinct protein-binding sites that have the potential to bind nine distinct liver nuclear proteins. One of these proteins was identified as hepatocyte nuclear factor 1 (HNF-1), which binds with relatively low affinity to the 5' half of a 20-bp palindrome located at the 5' end of the large footprint. A binding site for C/EBP (or one of the related proteins that recognize similar sequences) was identified in the center of the 97-bp footprint. This binding site is coincident or overlaps with the binding sites for five other proteins, two of which appear to be distinct from the C/EBP-related family of proteins. The binding site for a nuclear factor designated protein I is located between the HNF-1 and C/EBP binding sites. Finally, the 3'-most 15 bp of the footprinted sequence contain a binding site for another nuclear protein, which we have called protein II. Mutations that abolish the binding of either HNF-1, protein II, or the C/EBP-related proteins severely reduce enhancer activity. However, deletion experiments demonstrated that neither the HNF-1-binding site alone, nor the combination of binding sites for HNF-1, protein I, and C/EBP, nor the C/EBP-binding site plus the protein II-binding site is sufficient to enhance transcription from a strong apolipoprotein B promoter. Rather, HNF-1 and C/EBP act synergistically with protein II to enhance transcription of the apolipoprotein B gene.

Hepatocyte nuclear factor 1 and C/EBP are essential for the activity of the human apolipoprotein B gene second-intron enhancer

The tissue-specific transcriptional enhancer of the human apolipoprotein B gene contains multiple protein-binding sites spanning 718 bp. Most of the enhancer activity is found in a 443-bp fragment (+621 to +1064) that is located entirely within the second intron of the gene. Within this fragment, a 147-bp region (+806 to +952) containing a single 97-bp DNase I footprint exhibits significant enhancer activity. We now report that this footprint contains four distinct protein-binding sites that have the potential to bind nine distinct liver nuclear proteins. One of these proteins was identified as hepatocyte nuclear factor 1 (HNF-1), which binds with relatively low affinity to the 5' half of a 20-bp palindrome located at the 5' end of the large footprint. A binding site for C/EBP (or one of the related proteins that recognize similar sequences) was identified in the center of the 97-bp footprint. This binding site is coincident or overlaps with the binding sites for five other proteins, two of which appear to be distinct from the C/EBP-related family of proteins. The binding site for a nuclear factor designated protein I is located between the HNF-1 and C/EBP binding sites. Finally, the 3'-most 15 bp of the footprinted sequence contain a binding site for another nuclear protein, which we have called protein II. Mutations that abolish the binding of either HNF-1, protein II, or the C/EBP-related proteins severely reduce enhancer activity. However, deletion experiments demonstrated that neither the HNF-1-binding site alone, nor the combination of binding sites for HNF-1, protein I, and C/EBP, nor the C/EBP-binding site plus the protein II-binding site is sufficient to enhance transcription from a strong apolipoprotein B promoter. Rather, HNF-1 and C/EBP act synergistically with protein II to enhance transcription of the apolipoprotein B gene.

Low density lipoprotein kinetics in a family having defective low density lipoprotein receptors in which hypercholesterolemia is suppressed

Heterozygous familial hypercholesterolemia (FH) usually presents with severe elevations of low density lipoprotein (LDL) cholesterol. Recently, a family with FH was described in which several members heterozygous for a mutation in the LDL receptor gene had normal LDL cholesterol levels. Kinetic studies of LDL apolipoprotein B (apo B) were conducted to determine the metabolic differences between the normolipidemic and hypercholesterolemic FH heterozygotes in the family. Studies were performed in 14 family members including the proband (who has homozygous FH), four FH heterozygotes with high LDL levels, four FH subjects with normolipidemia, and five healthy relatives without FH. The proband had a very low fractional catabolic rate (FCR) for LDL (0.15 pool/day). All the FH and non-FH subjects studied, excluding the FH homozygote, had higher than expected FCRs for LDL. The average FCRs for LDL of hypercholesterolemic and normocholesterolemic subjects were not significantly different (0.39 +/- 0.06 versus 0.37 +/- 0.02 pool/day), and these values were 70-80% of those in unaffected relatives. Compared with hypercholesterolemic FH heterozygotes, normolipidemic heterozygotes had much lower input rates for LDL (17.1 +/- author query macros2.6 versus 8.7 +/- 0.9 pools/day, respectively). These low input rates, together with the higher than usual FCRs for LDL, are responsible for the normal concentrations of LDL cholesterol in some of the FH heterozygotes. The low input of LDL could be due to either a decreased secretion of apo B-containing lipoproteins or an enhanced clearance of LDL precursor lipoproteins.

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.

Recent progress in understanding apolipoprotein B

For the past 5 years, investigators from many different laboratories have contributed to a greatly increased understanding of two very important lipid-carrying proteins in plasma--apo B-100 and apo B-48. Apo B-100, an extremely large protein composed of 4,536 amino acids, is synthesized by the liver and is crucial for the assembly of triglyceride-rich VLDL particles. Apo B-100 is virtually the only protein of LDL, a cholesteryl ester-enriched class of lipoproteins that are metabolic products of VLDL. The apo B-100 of LDL serves as a ligand for the LDL receptor-mediated uptake of LDL particles by the liver and extrahepatic tissues. The LDL receptor-binding region of apo B-100 is located in the carboxyterminal portion of the molecule, whereas its lipid-binding regions appear to be broadly dispersed throughout its length. Apo B-48 contains the amino-terminal 2,152 amino acids of apo B-100 and is produced by the intestine as a result of editing of a single nucleotide of the apo B mRNA, which changes the codon specifying apo B-100 amino acid 2,153 to a premature stop codon. Apo B-48 has an obligatory structural role in the formation of chylomicrons; therefore, its synthesis is essential for absorption of dietary fats and fat-soluble vitamins. Both apo B-48 and apo B-100 are encoded on chromosome 2 by a single gene that contains 29 exons and 28 introns. An elevated level of apo B-100 in the plasma is a potent risk factor for developing premature atherosclerotic disease. In the past 3 years, many different apo B gene mutations that affect the concentrations of both apo B and cholesterol in the plasma have been characterized. A missense mutation in the codon for apo B-100 amino aid 3,500 is associated with hypercholesterolemia. This mutation results in poor binding of apo B-100 to the LDL receptor, thereby causing the cholesteryl ester-enriched LDL particles to accumulate in the plasma. This disorder is called familial defective apo B-100, and it is probably a cause of premature atherosclerotic disease. Familial hypobetalipoproteinemia is a condition associated with abnormally low levels of apo B and cholesterol; affected individuals may actually have a reduced risk of atherosclerotic disease.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Familial hypobetalipoproteinemia caused by a mutation in the apolipoprotein B gene that results in a truncated species of apolipoprotein B (B-31). A unique mutation that helps to define the portion of the apolipoprotein B molecule required for the formation of buoyant, triglyceride-rich lipoproteins

Apolipoprotein B-100 has a crucial structural role in the formation of VLDL and LDL. Familial hypobetalipoproteinemia, a syndrome in which the concentration of LDL cholesterol in plasma is abnormally low, can be caused by mutations in the apo B gene that prevent the translation of a full-length apo B-100 molecule. Prior studies have revealed that truncated species of apo B [e.g., apo B-37 (1728 amino acids), apo B-46 (2057 amino acids)] can occasionally be identified in the plasma of subjects with familial hypobetalipoproteinemia; in each of these cases, the truncated apo B species has been a prominent protein component of VLDL. In this report, we describe a kindred with hypobetalipoproteinemia in which the plasma of four affected heterozygotes contained a unique truncated apo B species, apo B-31. Apolipoprotein B-31 is caused by the deletion of a single nucleotide in the apo B gene, and it is predicted to contain 1425 amino acids. Apolipoprotein B-31 is the shortest of the mutant apo B species to be identified in the plasma of a subject with hypobetalipoproteinemia. In contrast to longer truncated apo B species, apo B-31 was undetectable in the VLDL and the LDL; however, it was present in the HDL fraction and the lipoprotein-deficient fraction of plasma. The density distribution of apo B-31 in the plasma suggests the possibility that the amino-terminal 1425 amino acids of apo B-100 are sufficient to permit the formation and secretion of small, dense lipoproteins but are inadequate to support the formation of the more lipid-rich VLDL and LDL particles.

Two distinct truncated apolipoprotein B species in a kindred with hypobetalipoproteinemia

Two novel, distinct truncated forms of apolipoprotein B (apo B) designated as apo B-90 and apo B-40 were found in five members of a kindred with hypobetalipoproteinemia. Sodium dodecyl sulfate gels and immunoblots of plasma or low density lipoprotein (LDL) (d = 1.019 to 1.063 g/ml) of the affected members demonstrated the presence of one or both of the truncated apo B bands. Employing four monoclonal anti-LDL antibodies with defined regional specificities, we demonstrated that amino terminal epitopes of the truncated apo Bs were intact, but that 10% and 60%, respectively, of the carboxyl terminal regions were absent. Thrombin digestion of apo B-90 generated an abnormally small T2 fragment, confirming that approximately 550 amino acids had been deleted from the carboxyl terminus of apo B-100. Restriction fragment length polymorphism analysis and variable number of tandem repeat typing of the 3' flanking hypervariable region of the apo B gene made it possible to distinguish all four parental alleles and therefore to follow the inheritance of the apo B variants through the family. This pedigree analysis confirmed the inheritance of the apo B-90 and apo B-40 identified by monoclonal antibody binding studies. Siblings heterozygous for apo B-90 or apo B-40 exhibited greater than 65% lower concentrations of apo B-90 or apo B-40 relative to apo B-100 and had 5th percentile LDL cholesterol concentrations. Compound heterozygotes (apo B-90/apo B-40) had the lowest LDL levels, and their LDL particles were small in size.(ABSTRACT TRUNCATED AT 250 WORDS)

Homozygous hypobetalipoproteinemia with spared chylomicron formation

Thirteen members of a family carrying a gene for pedigree of hypobetalipoproteinemia were analyzed for lipoprotein compositions, apolipoprotein (apo) B levels, and apo B isoforms. Judging from low density lipoprotein (LDL)-cholesterol (Chol) and apo B levels, a 75-year-old proband, a father who died of unknown fever, thrombopenia, and anemia, and his wife were heterozygous for hypobetalipoproteinemia. The proband had ataxic movement of hands and gait disturbance in later life. Three of four living siblings had extremely low levels of LDL-Chol (6 mg/dL) and LDL-apo B (2 mg/dL), and were postulated to have homozygous hypobetalipoproteinemia. Electrophoresis revealed marked deficiency of apo B-100, although trace amounts were noted in LDL. In contrast, apo B-48 was present in chylomicrons obtained after a fatty meal in the two patients with homozygous hypobetalipoproteinemia, indicating a selective deficiency of apo B-100 but not apo B-48. The defect in these patients seemingly is different from abnormal apo B-37 reported recently for a family with hypobetalipoproteinemia. Clinically, acanthocytotic red blood cells (8% to 12%), fatty liver, and low levels of serum lipid-soluble vitamins A and D were noted in homozygotes. One heterozygous sibling had 26 mg/dL LDL-Chol and 5 mg/dL LDL-apo B levels. All seven subjects in the third generation had low levels of Chol (85 to 140 mg/dL), LDL-Chol (40 to 63 mg/dL) and LDL-apo B (10 to 20 mg/dL). They also showed mild acanthocytosis (0.5% to 2%) and a decrease of fat-soluble vitamins in plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Low plasma cholesterol levels caused by a short deletion in the apolipoprotein B gene

Familial hypobetalipoproteinemia is a syndrome in which the plasma levels of apolipoprotein B (apo-B) and cholesterol are abnormally low. A truncated species of apo-B was identified in the plasma lipoproteins of members of a kindred with familial hypobetalipoproteinemia. DNA sequencing studies on genomic clones and enzymatically amplified genomic DNA samples revealed a four-base pair deletion in the apo-B gene. This short deletion, which results in a frameshift and a premature stop codon, accounts for the truncated apo-B species and explains the low apo-B and low cholesterol levels in this family.