Genetic analysis of a kindred with familial hypobetalipoproteinemia. Evidence for two separate gene defects: one associated with an abnormal apolipoprotein B species, apolipoprotein B-37; and a second associated with low plasma concentrations of apolipoprotein B-100

Non-alcoholic fatty liver disease in a pediatric patient with heterozygous familial hypobetalipoproteinemia due to a novel APOB variant: a case report and systematic literature review

Background

Familial hypobetalipoproteinemia (FHBL) is an autosomal semi-dominant disorder usually caused by variants in the APOB gene that frequently interferes with protein length. Clinical manifestations include malabsorption, non-alcoholic fatty liver disease, low levels of lipid-soluble vitamins, and neurological, endocrine, and hematological dysfunction.

Methods

Genomic DNA was isolated from the blood samples of the pediatric patient with hypocholesterolemia and his parents and brother. Next-generation sequencing (NGS) was performed, and an expanded dyslipidemia panel was employed for genetic analysis. In addition, a systematic review of the literature on FHBL heterozygous patients was performed.

Case report

Genetic investigation revealed the presence of a heterozygous variant in the APOB (NM_000384.3) gene c.6624dup[=], which changes the open reading frame and leads to early termination of translation into the p.Leu2209IlefsTer5 protein (NP_000375.3). The identified variant was not previously reported. Familial segregation analysis confirmed the variant in the mother of the subject, who also has a low level of low-density lipoprotein and non-alcoholic fatty liver disease. We have introduced therapy that includes limiting fats in the diet and adding lipid-soluble vitamins E, A, K, and D and calcium carbonate. We reported 35 individuals with APOB gene variations linked to FHBL in the systematic review.

Conclusion

We have identified a novel pathogenic variant in the APOB gene causing FHBL in pediatric patients with hypocholesterolemia and fatty liver disease. This case illustrates the importance of genetic testing for dyslipidemias in patients with significant decreases in plasma cholesterol as we can avoid damaging neurological and ophthalmological effects by sufficient vitamin supplementation and regular follow-ups.

Successful Nutritional Intervention for an Infant with Abetalipoproteinemia: A Novel Modular Formula (AbetaMF)

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Complex genetic architecture in severe hypobetalipoproteinemia

BACKGROUND

Abetalipoproteinemia and homozygous hypobetalipoproteinemia are classical Mendelian autosomal recessive and co-dominant conditions, respectively, which are phenotypically similar and are usually caused by bi-allelic mutations in MTTP and APOB genes, respectively. Instances of more complex patterns of genomic variants resulting in this distinct phenotype have not been reported.

METHODS

A 43 year-old male had a longstanding severe deficiency of apolipoprotein (apo) B-containing lipoproteins and circulating fat soluble vitamins consistent with either abetalipoproteinemia or homozygous familial hypobetalipoproteinemia (FHBL). He also had acanthocytosis, a long term history of fat malabsorption, and mild retinopathy, but was free from coagulopathy, myopathy and neuropathy. He had taken high dose oral fat soluble vitamins since childhood.

RESULTS

Targeted next generation DNA sequencing revealed several rare heterozygous missense variants in both MTTP and APOB genes known or predicted to be deleterious, in addition to a novel heterozygous missense variant in SAR1B, which encodes the gene causing chylomicron retention disease. Evaluation of first degree relatives with mild FHBL clarified the segregation of variants.

CONCLUSIONS

The proband's characteristic phenotype likely resulted from an oligogenic interaction involving multiple rare variants in MTTP and APOB, and related genes, each of which individually was associated with a milder or minimal clinical and biochemical phenotype.

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.

Insights from human congenital disorders of intestinal lipid metabolism

The intestine must challenge the profuse daily flux of dietary fat that serves as a vital source of energy and as an essential component of cell membranes. The fat absorption process takes place in a series of orderly and interrelated steps, including the uptake and translocation of lipolytic products from the brush border membrane to the endoplasmic reticulum, lipid esterification, Apo synthesis, and ultimately the packaging of lipid and Apo components into chylomicrons (CMs). Deciphering inherited disorders of intracellular CM elaboration afforded new insight into the key functions of crucial intracellular proteins, such as Apo B, microsomal TG transfer protein, and Sar1b GTPase, the defects of which lead to hypobetalipoproteinemia, abetalipoproteinemia, and CM retention disease, respectively. These "experiments of nature" are characterized by fat malabsorption, steatorrhea, failure to thrive, low plasma levels of TGs and cholesterol, and deficiency of liposoluble vitamins and essential FAs. After summarizing and discussing the functions and regulation of these proteins for reader's comprehension, the current review focuses on their specific roles in malabsorptions and dyslipidemia-related intestinal fat hyperabsorption while dissecting the spectrum of clinical manifestations and managements. The influence of newly discovered proteins (proprotein convertase subtilisin/kexin type 9 and angiopoietin-like 3 protein) on fat absorption has also been provided. Finally, it is stressed how the overexpression or polymorphism status of the critical intracellular proteins promotes dyslipidemia and cardiometabolic disorders.

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.

Mining the genome for lipid genes

Mining of the genome for lipid genes has since the early 1970s helped to shape our understanding of how triglycerides are packaged (in chylomicrons), repackaged (in very low density lipoproteins; VLDL), and hydrolyzed, and also how remnant and low-density lipoproteins (LDL) are cleared from the circulation. Gene discoveries have also provided insights into high-density lipoprotein (HDL) biogenesis and remodeling. Interestingly, at least half of these key molecular genetic studies were initiated with the benefit of prior knowledge of relevant proteins. In addition, multiple important findings originated from studies in mouse, and from other types of non-genetic approaches. Although it appears by now that the main lipid pathways have been uncovered, and that only modulators or adaptor proteins such as those encoded by LDLRAP1, APOA5, ANGPLT3/4, and PCSK9 are currently being discovered, genome wide association studies (GWAS) in particular have implicated many new loci based on statistical analyses; these may prove to have equally large impacts on lipoprotein traits as gene products that are already known. On the other hand, since 2004 - and particularly since 2010 when massively parallel sequencing has become de rigeur - no major new insights into genes governing lipid metabolism have been reported. This is probably because the etiologies of true Mendelian lipid disorders with overt clinical complications have been largely resolved. In the meantime, it has become clear that proving the importance of new candidate genes is challenging. This could be due to very low frequencies of large impact variants in the population. It must further be emphasized that functional genetic studies, while necessary, are often difficult to accomplish, making it hazardous to upgrade a variant that is simply associated to being definitively causative. Also, it is clear that applying a monogenic approach to dissect complex lipid traits that are mostly of polygenic origin is the wrong way to proceed. The hope is that large-scale data acquisition combined with sophisticated computerized analyses will help to prioritize and select the most promising candidate genes for future research. We suggest that at this point in time, investment in sequence technology driven candidate gene discovery could be recalibrated by refocusing efforts on direct functional analysis of the genes that have already been discovered. This article is part of a Special Issue entitled: From Genome to Function.

Whole-exome sequencing identifies rare and low-frequency coding variants associated with LDL cholesterol

Elevated low-density lipoprotein cholesterol (LDL-C) is a treatable, heritable risk factor for cardiovascular disease. Genome-wide association studies (GWASs) have identified 157 variants associated with lipid levels but are not well suited to assess the impact of rare and low-frequency variants. To determine whether rare or low-frequency coding variants are associated with LDL-C, we exome sequenced 2,005 individuals, including 554 individuals selected for extreme LDL-C (>98(th) or <2(nd) percentile). Follow-up analyses included sequencing of 1,302 additional individuals and genotype-based analysis of 52,221 individuals. We observed significant evidence of association between LDL-C and the burden of rare or low-frequency variants in PNPLA5, encoding a phospholipase-domain-containing protein, and both known and previously unidentified variants in PCSK9, LDLR and APOB, three known lipid-related genes. The effect sizes for the burden of rare variants for each associated gene were substantially higher than those observed for individual SNPs identified from GWASs. We replicated the PNPLA5 signal in an independent large-scale sequencing study of 2,084 individuals. In conclusion, this large whole-exome-sequencing study for LDL-C identified a gene not known to be implicated in LDL-C and provides unique insight into the design and analysis of similar experiments.

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.

Pathway-Wide Association Study Implicates Multiple Sterol Transport and Metabolism Genes in HDL Cholesterol Regulation

Pathway-based association methods have been proposed to be an effective approach in identifying disease genes, when single-marker association tests do not have sufficient power. The analysis of quantitative traits may be benefited from these approaches, by sampling from two extreme tails of the distribution. Here we tested a pathway association approach on a small genome-wide association study (GWAS) on 653 subjects with extremely high high-density lipoprotein cholesterol (HDL-C) levels and 784 subjects with low HDL-C levels. We identified 102 genes in the sterol transport and metabolism pathways that collectively associate with HDL-C levels, and replicated these association signals in an independent GWAS. Interestingly, the pathways include 18 genes implicated in previous GWAS on lipid traits, suggesting that genuine HDL-C genes are highly enriched in these pathways. Additionally, multiple biologically relevant loci in the pathways were not detected by previous GWAS, including genes implicated in previous candidate gene association studies (such as LEPR, APOA2, HDLBP, SOAT2), genes that cause Mendelian forms of lipid disorders (such as DHCR24), and genes expressing dyslipidemia phenotypes in knockout mice (such as SOAT1, PON1). Our study suggests that sampling from two extreme tails of a quantitative trait and examining genetic pathways may yield biological insights from smaller samples than are generally required using single-marker analysis in large-scale GWAS. Our results also implicate that functionally related genes work together to regulate complex quantitative traits, and that future large-scale studies may benefit from pathway-association approaches to identify novel pathways regulating HDL-C levels.

Familial hypobetalipoproteinemia: early neurological, hematological, and ocular manifestations in two affected twins responding to vitamin supplementation

Familial hypobetalipoproteinemia is a disorder of lipid metabolism characterized by extremely low plasma levels of apolipoprotein B as well as low levels of total and low-density lipoprotein cholesterol. We report the case of impairment of retinal function and diffuse pain in both legs often related to physical activity, as well as the presence of acanthocytosis on peripheral blood smear. Neurophysiological studies suggested dysfunction of the thin myelinated (A) and unmyelinated (C) fibers, in spite of preserved A fiber function, which has not been previously described in this condition. All clinical symptoms and the neurophysiological abnormalities improved after high-dose vitamin E and A supplementation. These findings suggest that this syndrome may have a wide spectrum of manifestations and an early appearance of symptoms in the pediatric age group.

Identification of patients with abetalipoproteinemia and homozygous familial hypobetalipoproteinemia in Tunisia

BACKGROUND

Abetalipoproteinemia (ABL) and Homozygous Familial Hypobetalipoproteinemia (Ho-FHBL) are rare monogenic diseases characterised by very low plasma levels of cholesterol and triglyceride and the absence or a great reduction of apolipoprotein B (apoB)-containing lipoproteins. ABL results from mutations in the MTP gene; Ho-FHBL may be due to mutations in the APOB gene.

METHODS

We sequenced MTP and APOB genes in three Tunisian children, born from consanguineous marriage, with very low levels of plasma apoB-containing lipoproteins associated with severe intestinal fat malabsorption.

RESULTS

Two of them were found to be homozygous for two novel mutations in intron 5 (c.619-3T>G) and in exon 8 (c.923 G>A) of the MTP gene, respectively. The c.619-3T>G substitution caused the formation of an abnormal mRNA devoid of exon 6, predicted to encode a truncated MTP of 233 amino acids. The c.923 G>A is a nonsense mutation resulting in a truncated MTP protein (p.W308X). The third patient was homozygous for a novel nucleotide deletion (c.2172delT) in exon 15 of APOB gene resulting in the formation of a truncated apoB of 706 amino acids (apoB-15.56).

CONCLUSIONS

These mutations are expected to abolish the apoB lipidation and the assembly of apoB-containing lipoproteins in both liver and intestine.

Variable phenotypic expression of homozygous familial hypobetalipoproteinaemia due to novel APOB gene mutations

Homozygous familial hypobetalipoproteinaemia (Ho-FHBL) is a rare co-dominant disorder characterized by extremely low levels of low-density lipoprotein cholesterol (LDL-C) and apolipoprotein B (apoB). Most patients with Ho-FHBL have mutations in APOB gene resulting in truncated apoBs. Some patients are asymptomatic, while others have fatty liver, intestinal fat malabsorption and neurological dysfunctions. We investigated three adult subjects with severe hypobetalipoproteinaemia and a family history of FHBL. Proband FHBL-47 had liver cirrhosis with hepatocarcinoma and a renal carcinoma but no clinical manifestations related to FHBL. He was a compound heterozygote for a 7-bp deletion in exon 21 and a base insertion in exon 26 resulting in truncated apoBs (apoB-22.46/apoB-66.51). Proband FHBL-53, with severe hepatic steatosis and fibrosis, had a nonsense mutation in exon 19 resulting in a truncated apoB (apoB-20.61) and a rare nucleotide substitution in intron 14 (c.2068-4T>A). The latter was also present in her daughter, found to have low plasma LDL-C and apoB. Proband FHBL-82 had chronic diarrhoea and steatorrhoea. She was found to be homozygous for a nonsense mutation in exon 24 resulting in a truncated apoB (apoB-26.65). In adult subjects, the presence of chronic liver disease and chronic diarrhoea, when associated with severe hypobetalipoproteinaemia, may lead to the diagnosis of Ho-FHBL.

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 single in vitro point mutation in the first non-translated exon silences transcription of the human apolipoprotein B gene in HepG2 cells

Hepatic cell-specific expression of the human apolipoprotein B (apoB) gene is controlled by at least four cis-acting elements located within the -128 to +122 promoter region (S.S. Chuang, H.K. Das, Identification of trans-acting factors that interact with cis-acting elements present in the first non-translated exon of the human apolipoprotein B gene, Biochem. Biophys. Res. Commun. 220 (1996) 553-562). Two cis-acting positive elements (-104 to -85; -84 to -60) are located upstream from the start of transcription. A negative element (+20 to +40) and a strong positive element (+43 to +53) are located in the first non-translated exon of the human apolipoprotein B gene. Trans-acting factors BRF-2, BRF-1, BRF-3, and BRF-4 interact with the above four cis-acting elements respectively. In this study, we examine the roles of the upstream positive elements -104 to -85 and -84 to -60 in modulating transcriptional regulation of the apoB gene by downstream elements +20 to +40 and +43 to +53. Using in vitro mutagenesis and transient transfection experiments in HepG2 cells, the cis-acting element -84 to -60 has been found to be absolutely necessary for the function of the upstream element -104 to -85 and downstream elements +20 to +40 and +43 to +53. In vitro mutagenesis of the downstream positive element +43 to +53 and transfection of the mutant promoter constructs in HepG2 cells reveal that nucleotide G at position +51 is essential for the strong positive activity of the element +43 to +53. A single substitution point mutation of nucleotide G to either A or T at position +51 reduces apolipoprotein B gene transcription substantially in HepG2 cells. These results suggest that a single substitution mutation in vivo, of nucleotide G to either A or T at position +51 in the downstream positive promoter element +43 to +53 may potentially cause hypobetalipoproteinemia, a heterozygous from of an autosomal-dominant disorder.

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.

Identification and molecular analysis of two apoB gene mutations causing low plasma cholesterol levels

BACKGROUND

Familial hypobetalipoproteinemia (FHB) is an autosomal codominant disorder characterized by abnormally low plasma levels of apoB and LDL cholesterol. Heterozygotes for FHB almost always have plasma LDL cholesterol levels < 70 mg/dL and are asymptomatic. Because the low cholesterol levels may protect FHB heterozygotes from coronary heart disease, the mechanisms for FHB are of considerable interest.

METHODS AND RESULTS

The plasma lipoproteins of 29 subjects with LDL cholesterol levels < 70 mg/dL were examined by SDS-PAGE. One subject who had virtually undetectable levels of LDL cholesterol had a truncated apoB, apoB-44.4, in his lipoproteins; a second subject with an LDL cholesterol level of 44 mg/dL had apoB-55 in his lipoproteins. The apoB-44.4 (2014 amino acids in length) resulted from a frameshift caused by an 11-bp insertion in exon 26 of the apoB gene; the apoB-55 (2494 amino acids) was caused by a nonsense mutation in exon 26 of the apoB gene. The apoB-55 mutation occurred at a CpG dinucleotide pair, a mutational hot spot, and was identical to a mutation described previously in a subject with hypobetalipoproteinemia. Our subject with apoB-55, however, had a different haplotype than the subject described previously, suggesting that the two apoB-55 mutations may have arisen independently. Of note, the apoB-55 proband's father, who had very low cholesterol levels and who probably carried the apoB-55 mutation, had significant coronary and aortic atherosclerosis at autopsy.

CONCLUSIONS

In a study of adults with low LDL cholesterol levels, we discovered two subjects with truncated apoB proteins and identified the responsible mutations. ApoB gene mutations causing truncated apoB are not particularly rare in subjects with low cholesterol levels. The role of these mutations in preventing atherosclerosis deserves further study.

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.

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.

Transcriptional regulation of the apolipoprotein B100 gene: purification and characterization of trans-acting factor BRF-2

Apolipoprotein B100 (apoB), the only protein of low-density lipoprotein, is produced primarily in the liver and serves as a ligand for the low-density lipoprotein receptor. Hepatic cell-specific expression of the human apoB gene is controlled by at least two cis-acting positive elements located between positions-128 and -70 (H. K. Das, T. Leff, and J.L. Breslow, J. Biol. Chem. 263:11452-11458, 1988). The distal element (-128 to -85) appears to be liver specific since it shows positive activity in HepG2 cells and negative activity in HeLa cells. The proximal element (-84 to -70) acts as a positive element in both these cell lines, and two rat liver nuclear proteins, BRF-1 and C/EBP, bind to two overlapping sites (-84 to -60 and -70 to -50, respectively). By gel mobility shift assay, we have identified a rat liver nuclear protein (BRF-2) which binds to the distal element (-128 to -85) of the apoB gene. This putative trans-acting factor has been purified to apparent homogeneity by DEAE-cellulose, heparin-agarose, and DNA-specific affinity chromatography. The purified BRF-2 has an apparent molecular mass of 120 kDa and was found to specifically recognize sequence -128 to -85; BRF-2 also produced a strong hypersensitive site at nucleotide position -95 with copper-orthophenanthroline reagent. A double-stranded oligonucleotide (-128 to -85) containing a 3-nucleotide (TTC) insertion between position -95 and -94 was found to abolish DNA binding by BRF-2. This result suggests that the region surrounding the hypersensitive site -95 is important for protein-DNA interaction. By using apoB promoter fragments containing various internal deletions as templates for gel mobility shift assay, the region between -104 and -85 was identified to be crucial for binding by BRF-2. We propose that BRF-2 may play an important role in the tissue-specific regulation of apoB gene transcription.

Transcriptional regulation of the apolipoprotein B100 gene: purification and characterization of trans-acting factor BRF-2

Apolipoprotein B100 (apoB), the only protein of low-density lipoprotein, is produced primarily in the liver and serves as a ligand for the low-density lipoprotein receptor. Hepatic cell-specific expression of the human apoB gene is controlled by at least two cis-acting positive elements located between positions-128 and -70 (H. K. Das, T. Leff, and J.L. Breslow, J. Biol. Chem. 263:11452-11458, 1988). The distal element (-128 to -85) appears to be liver specific since it shows positive activity in HepG2 cells and negative activity in HeLa cells. The proximal element (-84 to -70) acts as a positive element in both these cell lines, and two rat liver nuclear proteins, BRF-1 and C/EBP, bind to two overlapping sites (-84 to -60 and -70 to -50, respectively). By gel mobility shift assay, we have identified a rat liver nuclear protein (BRF-2) which binds to the distal element (-128 to -85) of the apoB gene. This putative trans-acting factor has been purified to apparent homogeneity by DEAE-cellulose, heparin-agarose, and DNA-specific affinity chromatography. The purified BRF-2 has an apparent molecular mass of 120 kDa and was found to specifically recognize sequence -128 to -85; BRF-2 also produced a strong hypersensitive site at nucleotide position -95 with copper-orthophenanthroline reagent. A double-stranded oligonucleotide (-128 to -85) containing a 3-nucleotide (TTC) insertion between position -95 and -94 was found to abolish DNA binding by BRF-2. This result suggests that the region surrounding the hypersensitive site -95 is important for protein-DNA interaction. By using apoB promoter fragments containing various internal deletions as templates for gel mobility shift assay, the region between -104 and -85 was identified to be crucial for binding by BRF-2. We propose that BRF-2 may play an important role in the tissue-specific regulation of apoB gene transcription.

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.

Phenotypes of apolipoprotein B and apolipoprotein E after liver transplantation

Apolipoprotein (apo) E and the two B apolipoproteins, apoB48 and apoB100, are important proteins in human lipoprotein metabolism. Commonly occurring polymorphisms in the genes for apoE and apoB result in amino acid substitutions that produce readily detectable phenotypic differences in these proteins. We studied changes in apoE and apoB phenotypes before and after liver transplantation to gain new insights into apolipoprotein physiology. In all 29 patients that we studied, the postoperative serum apoE phenotype of the recipient, as assessed by isoelectric focusing, converted virtually completely to that of the donor, providing evidence that greater than 90% of the apoE in the plasma is synthesized by the liver. In contrast, the cerebrospinal fluid apoE phenotype did not change to the donor's phenotype after liver transplantation, indicating that most of the apoE in CSF cannot be derived from the plasma pool and therefore must be synthesized locally. The apoB100 phenotype (assessed with immunoassays using monoclonal antibody MB19, an antibody that detects a two-allele polymorphism in apoB) invariably converted to the phenotype of the donor. In four normolipidemic patients, we determined the MB19 phenotype of both the apoB100 and apoB48 in the "chylomicron fraction" isolated from plasma 3 h after a fat-rich meal. Interestingly, the apoB100 in the chylomicron fraction invariably had the phenotype of the donor, indicating that the vast majority of the large, triglyceride-rich apoB100-containing lipoproteins that appear in the plasma after a fat-rich meal are actually VLDL of hepatic origin. The MB19 phenotype of the apoB48 in the plasma chylomicron fraction did not change after liver transplantation, indicating that almost all of the apoB48 in plasma chylomicrons is derived from the intestine. These results were consistent with our immunocytochemical studies on intestinal biopsy specimens of organ donors; using apoB-specific monoclonal antibodies, we found evidence for apoB48, but not apoB100, in donor intestinal biopsy specimens.

A truncated species of apolipoprotein B (B67) in a kindred with familial hypobetalipoproteinemia

We describe a kindred in which the proband and 6 of his 12 children have hypobetalipoproteinemia. The plasma lipoproteins of the affected subjects contained a unique species of apolipoprotein (apo) B, apo B67, in addition to the normal species, apo B100 and apo B48. The size of apo B67 and immunochemical studies with a panel of apo B-specific antibodies indicated that apo B67 was a truncated species of apo B that contained approximately the amino-terminal 3,000-3,100 amino acids of apo B100. Sequencing of genomic apo B clones revealed that affected family members were heterozygous for a mutant apo B allele containing a single nucleotide deletion in exon 26 (cDNA nucleotide 9327). This frameshift mutation is predicted to result in the synthesis of a truncated apo B containing 3,040 amino acids. Apo B67 is present in low levels in the plasma but is easily detectable within the very low density lipoprotein and low density lipoprotein fractions. Examination of the proband's immediate family revealed seven normolipidemic subjects and seven subjects with hypobetalipoproteinemia. In the affected subjects, the mean total and low density lipoprotein cholesterol levels were 120 and 42 mg/dl, respectively. A significantly higher mean high density lipoprotein cholesterol level was found in the affected subjects (75 vs. 55 mg/dl). We hypothesize that the elevated high density lipoprotein cholesterol levels in subjects heterozygous for the apo B67 mutation may be metabolically linked to the low levels of apo B-containing lipoproteins in their plasma.

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.