Anderson's disease: genetic exclusion of the apolipoprotein-B gene in two families

Guidelines for the diagnosis and management of chylomicron retention disease based on a review of the literature and the experience of two centers

Familial hypocholesterolemia, namely abetalipoproteinemia, hypobetalipoproteinemia and chylomicron retention disease (CRD), are rare genetic diseases that cause malnutrition, failure to thrive, growth failure and vitamin E deficiency, as well as other complications. Recently, the gene implicated in CRD was identified. The diagnosis is often delayed because symptoms are nonspecific. Treatment and follow-up remain poorly defined.

The aim of this paper is to provide guidelines for the diagnosis, treatment and follow-up of children with CRD based on a literature overview and two pediatric centers 'experience.

The diagnosis is based on a history of chronic diarrhea with fat malabsorption and abnormal lipid profile. Upper endoscopy and histology reveal fat-laden enterocytes whereas vitamin E deficiency is invariably present. Creatine kinase (CK) is usually elevated and hepatic steatosis is common. Genotyping identifies the Sar1b gene mutation.

Treatment should be aimed at preventing potential complications. Vomiting, diarrhea and abdominal distension improve on a low-long chain fat diet. Failure to thrive is one of the most common initial clinical findings. Neurological and ophthalmologic complications in CRD are less severe than in other types of familial hypocholesterolemia. However, the vitamin E deficiency status plays a pivotal role in preventing neurological complications. Essential fatty acid (EFA) deficiency is especially severe early in life. Recently, increased CK levels and cardiomyopathy have been described in addition to muscular manifestations. Poor mineralization and delayed bone maturation do occur. A moderate degree of macrovesicular steatosis is common, but no cases of steatohepatitis cirrhosis.

Besides a low-long chain fat diet made up uniquely of polyunsaturated fatty acids, treatment includes fat-soluble vitamin supplements and large amounts of vitamin E. Despite fat malabsorption and the absence of postprandial chylomicrons, the oral route can prevent neurological complications even though serum levels of vitamin E remain chronically low. Dietary counseling is needed not only to monitor fat intake and improve symptoms, but also to maintain sufficient caloric and EFA intake.

Despite a better understanding of the pathogenesis of CRD, the diagnosis and management of the disease remain a challenge for clinicians. The clinical guidelines proposed will helpfully lead to an earlier diagnosis and the prevention of complications.

Chylomicron retention disease--the role of ultrastructural examination in differential diagnosis

Three children with malabsorption presumably caused by celiac disease had undergone jejunal biopsy. While a histological examination revealed microvacuolization of enterocytes in the absence of celiac lesions, an ultrastructural investigation disclosed numerous chylomicrons and larger lipid vacuoles inside the cytoplasm of enterocytes, mostly in the supranuclear region. No chylomicrons were evident in the interstitium between adjacent enterocytes, as observed in normal subjects. These ultrastructural findings allowed for the diagnosis of "Chylomicron retention disease" (CRD). CRD was described for the first time by Anderson in 1961, and it is included in the group of disorders of biosynthesis and secretion of B apolipoproteins (apoB). This disease, in particular, appears to result from a specific defect involving the secretion of lipoproteins containing apoB-48 from the gut, with the complete absence of post prandial chylomicrons in the sera. CRD needs to be recognized early because of its adverse effects on growth and its potential for neurological and ocular complications, and the ultrastructural identification of chylomicron-size lipid droplets clustered in the enterocytes, with the absence of fat outside the cells, represents the gold standard to identify CRD. together with clinical aspects and laboratory measurements. In this study, we describe the histological and ultrastructural aspects observed in three pediatric cases of CRD.

APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation

Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small intestine of all mammals secretes apoB48, which arises following C-to-U deamination of a single cytidine base in the nuclear apoB transcript, introducing a translational stop codon. This process, referred to as apoB RNA editing, operates through a multicomponent enzyme complex that contains a single catalytic subunit, apobec-1, in addition to other protein factors that have yet to be cloned. ApoB RNA editing also exhibits stringent cis-acting requirements that include both structural and sequence-specific elements-specifically efficiency elements that flank the minimal cassette, an AU-rich RNA context, and an 11-nucleotide mooring sequence-located in proximity to a suitably positioned (usually upstream) cytidine. C-to-U RNA editing may become unconstrained under circumstances where apobec-1 is overexpressed, in which case multiple cytidines in apoB RNA, as well as in other transcripts, undergo C-to-U editing. ApoB RNA editing is eliminated following targeting of apobec-1, establishing that there is no genetic redundancy in this function. Under physiological circumstances, apoB RNA editing exhibits developmental, hormonal, and nutritional regulation, in some cases related to transcriptional regulation of apobec-1 mRNA. ApoB and the microsomal triglyceride transfer protein (MTP) are essential for the assembly and secretion of apoB-containing lipoproteins. MTP functions by transferring lipid to apoB during its translation and by transporting triglycerides into the endoplasmic reticulum to form apoB-free lipid droplets. These droplets fuse with nascent apoB-containing particles to form mature, very low-density lipoproteins or chylomicrons. In cultured hepatic cells, lipid availability dictates the rate of apoB production. Unlipidated or underlipidated forms of apoB are subjected to presecretory degradation, a process mediated by retrograde transport from the lumen of the endoplasmic reticulum to the cytosol, coupled with multiubquitination and proteasomal degradation. Although control of lipid secretion in vivo is primarily achieved at the level of lipoprotein particle size, regulation of apoB production by presecretory degradation may be relevant in some dyslipidemic states.

A proposed model for the assembly of chylomicrons

The intestine synthesizes very low density lipoproteins (VLDL) and chylomicrons (CM) to transport fat and fat-soluble vitamins into the blood. VLDL assembly occurs constitutively whereas CM assembly is a characteristic property of the enterocytes during the postprandial state. The secretion of CM is specifically inhibited by Pluronic L81. CM are very heterogeneously-sized particles that consist of a core of triglycerides (TG) and cholesterol esters and a monolayer of phospholipids (PL), cholesterol and proteins. The fatty acid composition of TG, but not PL, in CM mirrors the fatty acid composition of fat in the diet. CM assembly is deficient in abetalipoproteinemia and CM retention disease. Abetalipoproteinemia results due to mutation in the mttp gene and is characterized by the virtual absence of apoB-containing lipoproteins in the plasma. Patients suffer from neurologic disorders, visual impairment, and exhibit acanthocytosis. CM retention disease, an inherited recessive disorder, is characterized by chronic diarrhea with steatorrhea in infancy, abdominal distention and failure to thrive. It is caused by a specific defect in the secretion of intestinal lipoproteins; secretion of lipoproteins by the liver is not affected. Besides human disorders, mice that do not assemble intestinal lipoproteins have been developed. These mice are normal at birth, but defective in fat and fat-soluble vitamin absorption, and fail to thrive. Thus, fat and fat-soluble vitamin transport by the intestinal lipoproteins is essential for proper growth and development of neonates. Recently, differentiated Caco-2 cells and rabbit primary enterocytes have been described that synthesize and secrete CM. These cells can be valuable in distinguishing between the two different models proposed for the assembly of CM. In the first model, the assembly of VLDL and CM is proposed to occur by two 'independent' pathways. Second, CM assembly is proposed to be a product of 'core expansion' that results in the synthesis of lipoproteins of different sizes. According to this model, intestinal lipoprotein assembly begins with the synthesis of 'primordial' lipoprotein particles and involves release of the nascent apoB with PL derived from the endoplasmic reticulum (ER) membrane. In addition, TG-rich 'lipid droplets' of different sizes are formed independent of apoB synthesis. The fusion of lipid droplets and primordial lipoproteins results in the formation of different size lipoproteins due to the 'core expansion' of the primordial lipoproteins.

Anderson's disease: exclusion of apolipoprotein and intracellular lipid transport genes

Anderson's disease is a rare, hereditary hypocholesterolemic syndrome characterized by chronic diarrhea, steatorrhea, and failure to thrive associated with the absence of apo B48-containing lipoproteins. To further define the molecular basis of the disease, we studied 8 affected subjects in 7 unrelated families of North African origin after treatment with a low-fat diet. Lipid loading of intestinal biopsies persisted, but the pattern and extent of loading was variable among the patients. Electron microscopy showed lipoprotein-like particles in membrane-bound compartments, the densities (0.65 to 7.5 particles/mu(2)) and the mean diameters (169 to 580 nm) of which were, in general, significantly larger than in a normal fed subject (0.66 particles/mu(2), 209 nm mean diameter). There were also large lipid particles having diameters up to 7043 nm (average diameters from 368 to 2127 nm) that were not surrounded by a membrane. Rarely, lipoprotein-like particles 50 to 150 nm in diameter were observed in the intercellular spaces. Intestinal organ culture showed that apo B and apo AIV were synthesized with apparently normal molecular weights and that small amounts were secreted in lipid-bound forms (density <1.006 g/mL). Normal microsomal triglyceride transfer protein (MTP) and activity were also detected in intestinal biopsies. Segregation analyses of 4 families excluded, as a cause of the disease, significant regions of the genome surrounding the genes for apo AI, AIV, B, CI, CII, CIII, and E, as were the genes encoding 3 proteins involved in intracellular lipid transport, MTP, and fatty acid binding proteins 1 and 2. The results suggest that a factor other than apoproteins and MTP are important for human intestinal chylomicron assembly and secretion.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Nutritional effects and management of diarrhoea in infancy

The interactions between diarrhoeal disease and nutritional status are complex and synergistic. These are serious issues globally because they affect hundreds of millions of young children and annually cause > 3 million deaths in children aged under 5 y. Despite intensive field-based and laboratory studies over three decades, many questions remain unanswered about the causes, pathophysiology and best approaches to management and prevention of this "diarrhoea-malnutrition" syndrome. Oral rehydration therapy (ORT) has been a major advance and has saved many lives from acute diarrhoea. However, persistent diarrhoea is now a major problem and is very significant because of its strong negative impacts on nutritional status and because persistent diarrhoea and dysentery are now major causes of infant and young child deaths. ORT provides clear and practical methods for replacement of fluid and electrolyte losses during diarrhoea. Rehydration salts can be made available as (i) a simple, easy-to-use package, complete with user instructions; (ii) cereal-based formulae based on widely available ingredients that can be prepared domestically or commercially; and (iii) home-made mixtures of sugar and salt which should be simple to prepare but are risky because of inadequate understanding about their preparation at home and the chance of mixing the ingredients inaccurately and giving them wrongly. Continuation and encouragement of breastfeeding is an important strategy to prevent and control diarrhoea and as part of its management. Early refeeding during diarrhoea is another important principle to help to reduce its duration, severity and its nutritional impacts. Supplementation with specific dietary ingredients, such as vitamin A, zinc and folate, is rather contentious and drug therapy is of little value unless specifically indicated. Some patients may require enteral nutrition or parenteral nutrition but these require specialized equipment and skills that are usually beyond the reach of developing countries and infants and children who live in remote areas.

Chinese hamster ovary cells require the coexpression of microsomal triglyceride transfer protein and cholesterol 7alpha-hydroxylase for the assembly and secretion of apolipoprotein B-containing lipoproteins

Due to the absence of microsomal triglyceride transfer protein (MTP), Chinese hamster ovary (CHO) cells lack the ability to translocate apoB into the lumen of the endoplasmic reticulum, causing apoB to be rapidly degraded by an N-acetyl-leucyl-leucyl-norleucinal-inhibitable process. The goal of this study was to examine if expression of MTP, whose genetic deletion is responsible for the human recessive disorder abetalipoproteinemia, would recapitulate the lipoprotein assembly pathway in CHO cells. Unexpectedly, expression of MTP mRNA and protein in CHO cells did not allow apoB-containing lipoproteins to be assembled and secreted by CHO cells expressing apoB53. Although expression of MTP in cells allowed apoB to completely enter the endoplasmic reticulum, it was degraded by a proteolytic process that was inhibited by dithiothreitol (1 mM) and chloroquine (100 microM), but resistant to N-acetyl-leucyl-leucyl-norleucinal. In marked contrast, coexpression of the liver-specific gene product cholesterol 7alpha-hydroxylase with MTP resulted in levels of MTP lipid transfer activity that were similar to those in mouse liver and allowed intact apoB53 to be secreted as a lipoprotein particle. These data suggest that, although MTP-facilitated lipid transport is not required for apoB translocation, it is required for the secretion of apoB-containing lipoproteins. We propose that, in CHO cells, MTP plays two roles in the assembly and secretion of apoB-containing lipoproteins: 1) it acts as a chaperone that facilitates apoB53 translocation, and 2) its lipid transfer activity allows apoB-containing lipoproteins to be assembled and secreted. Our results suggest that the phenotype of the cell (e.g. expression of cholesterol 7alpha-hydroxylase by the liver) may profoundly influence the metabolic relationships determining how apoB is processed into lipoproteins and/or degraded.

Normal intestinal dietary fat and cholesterol absorption, intestinal apolipoprotein B (ApoB) mRNA levels, and ApoB-48 synthesis in a hypobetalipoproteinemic kindred without any ApoB truncation

The purpose of this study was to characterize intestinal apolipoprotein B (apoB) metabolism in subjects with familial hypobetalipoproteinemia (FHBL), where segregation analysis supports linkage to the apoB gene but no apoB truncations are present. We investigated cholesterol and fat absorption, intestinal apoB mRNA synthesis and editing, as well as apoB-48 synthesis. Plasma triglycerides (TG) and retinyl palmitate in the chylomicron fractions were analyzed after 12 hours of fasting and then repeatedly for 14 hours after ingestion of a vitamin A-containing high-fat meal. Cholesterol absorption was assessed using a dual stable-isotope method. Mean peak times and concentrations and areas under the curve (AUCs) for fat absorption and mean percentages of cholesterol absorption were comparable in affected and nonaffected family members. Intestinal biopsies were extracted for total RNA and also incubated with 35S-methionine for measurements of apoB synthesis. Similar quantities of apoB mRNA were found to be expressed in the intestine in affected and control subjects by RNase protection assay. ApoB mRNA editing assay showed that the majority of apoB-100 mRNA was edited to the apoB-48 form to a similar extent in both groups. Virtually no apoB-100 protein was synthesized by the intestine in any subject, and apoB-48 protein synthesis was not significantly different in the affected individuals. These data are consistent with in vivo metabolism data that show normal production rates for liver-derived apoB-100 but increased apoB-100 fractional catabolic rates in affected members of this family. Thus, the molecular defect probably does not affect transcription, translation, or secretion of apoB-containing lipoproteins, but may instead affect their clearance.

Interactions between microsomal triglyceride transfer protein and apolipoprotein B within the endoplasmic reticulum in a heterologous expression system

When apolipoprotein B (apoB) is expressed in heterologous cells, it is not secreted but retained and degraded within the endoplasmic reticulum (ER). We have previously characterized carboxyl-terminal truncated forms of apoB expressed in COS cells and have shown that these proteins were readily synthesized but retained within the ER and degraded, if the size of the truncated protein was larger than apoB 29. Below this size, the smaller the size of the apoB truncates, the greater the extent of secretion, although >50% of these smaller proteins were also degraded within the ER. In the present study, we demonstrate that this secretory defect can be overcome by coexpression with microsomal triglyceride transfer protein (MTP); moreover, this complementation is inversely related to the size of apoB. Secretion of apoBs larger than B29 required the coexpression of MTP and, in the presence of MTP, was oleate-responsive. MTP, in the presence or absence of oleate supplementation, had little or no effect on the secretion of the shorter truncates. We discovered, however, that MTP was physically associated with all forms of apoB intracellularly (B13-B41). The association of MTP with apoB 41 was stable to high salt washing, as well as to low pH, suggesting that these interactions may be hydrophobic in nature. In addition to the interaction with MTP, apoB was also found to be associated with calnexin, confirming previous studies, and with proteins bearing the KDEL retention signal. However, studies on overexpression of human calnexin and tunicamycin inhibition of glycosylation showed that interaction with calnexin was not necessary for the formation or secretion of apoB 41-containing lipoproteins; moreover, in the presence of MTP, the association of calnexin with apoB 41 was transient or absent. These data suggest that for apoB to attain a folded state sufficient to escape the quality control of the ER, it needs to obtain neutral lipid (supplied by MTP), as well as its ability to keep it packaged as a rudimentary lipoprotein, dependent on its size being larger than B29.

Studies on lipoprotein metabolism in a family with jejunal chylomicron retention

We describe two siblings with fat malabsorption and jejunal chylomicron retention. Plasma lipoproteins were studied in the patients and their first-degree relatives. The patients were a 14-year-old girl and her 8-year-old brother. Compared to healthy controls, they both had low fasting plasma concentrations of plasma total, HDL, and LDL cholesterol, as well as of apolipoproteins A-I and B. No increase in plasma lipoprotein levels or detectable apo B-48 was observed following an oral fat load. Histological studies of jejunal biopsy specimens obtained during fasting and 1 h postprandially showed severe steatosis, and an apparent block of chylomicron secretion from the endoplasmic reticulum into the Golgi apparatus was observed by electron microscopy. Liver biopsy specimens showed moderate steatosis and ultrastructural changes similar to those in the enterocytes. One healthy sister had a normal plasma lipoprotein pattern, and showed increased plasma triglyceride levels as well as the presence of apo B-48 following an oral fat load. Both parents had normal plasma total cholesterol levels, but clearly reduced fasting concentrations of HDL cholesterol and apo A-I. At least in this family, determination of plasma apo A-I levels might thus prove useful in the identification of heterozygotes.

Chylomicron retention disease: exclusion of apolipoprotein B gene defects and detection of mRNA editing in an affected family

Chylomicron retention disease (CRD) is a rare autosomal recessive disorder characterized by the absence of post-prandial chylomicrons and apolipoprotein (apo) B-48 in sera from affected individuals. Apo B-100 is synthesized, and apo B-100-containing lipoproteins are present in sera. A crucial difference between the synthesis and secretion of apo B-containing lipoproteins from the liver and gut in man is the generation of apo B-48 by editing of apo B mRNA in the gut to create a premature stop-translation codon. In this study the hypothesis that CRD may represent an absence of editing of apo B mRNA in the gut was investigated. Two affected sisters were identified as having low cholesterol levels and an absence of post-prandial chylomicronemia. Segregation analysis in the family showed that the apo B locus is not the site of the defect. Using reverse transcription-polymerase chain reaction (RT-PCR), duodenal biopsy-mRNA from the affected sisters was isolated and analyzed. The apo B editing site was amplified after cDNA synthesis, and the products analyzed by the primer extension assay. The results show that editing of apo B mRNA is normal in patients with CRD. The data provides strong confirmation that the primary defect in CRD is not in the synthesis, or editing of apo B mRNA in the gut. More likely, the disease arises from a defect in a gene crucial to the assembly and/or secretion of the chylomicron particle.