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

Current Diagnosis and Management of Familial Hypobetalipoproteinemia 1

Familial hypobetalipoproteinemia (FHBL) 1 is a rare genetic disorder with an autosomal codominant mode of inheritance and is caused by defects in the apolipoprotein (apo) B (APOB) gene that disable lipoprotein formation. ApoB proteins are required for the formation of very low-density lipoproteins (VLDLs), chylomicrons, and their metabolites. VLDLs transport cholesterol and triglycerides from the liver to the peripheral tissues, whereas chylomicrons transport absorbed lipids and fat-soluble vitamins from the intestine. Homozygous or compound heterozygotes of FHBL1 (HoFHBL1) are extremely rare, and defects in APOB impair VLDL and chylomicron secretion, which result in marked hypolipidemia with malabsorption of fat and fat-soluble vitamins, leading to various complications such as growth disorders, acanthocytosis, retinitis pigmentosa, and neuropathy. Heterozygotes of FHBL1 are relatively common and are generally asymptomatic, except for moderate hypolipidemia and possible hepatic steatosis. If left untreated, HoFHBL1 can cause severe complications and disabilities that are pathologically and phenotypically similar to abetalipoproteinemia (ABL) (an autosomal recessive disorder) caused by mutations in the microsomal triglyceride transfer protein (MTTP) gene. Although HoFHBL1 and ABL cannot be distinguished from the clinical manifestations and laboratory findings of the proband, moderate hypolipidemia in first-degree relatives may help diagnose HoFHBL1. There is currently no specific treatment for HoFHBL1. Palliative therapy including high-dose fat-soluble vitamin supplementation may prevent or delay complications. Registry research on HoFHBL1 is currently ongoing to better understand the disease burden and unmet needs of this life-threatening disease with few therapeutic options.

Diagnosis and management of secondary causes of steatohepatitis

The term non-alcoholic fatty liver disease (NAFLD) was originally coined to describe hepatic fat deposition as part of the metabolic syndrome. However, a variety of rare hereditary liver and metabolic diseases, intestinal diseases, endocrine disorders and drugs may underlie, mimic, or aggravate NAFLD. In contrast to primary NAFLD, therapeutic interventions are available for many secondary causes of NAFLD. Accordingly, secondary causes of fatty liver disease should be considered during the diagnostic workup of patients with fatty liver disease, and treatment of the underlying disease should be started to halt disease progression. Common genetic variants in several genes involved in lipid handling and metabolism modulate the risk of progression from steatosis to fibrosis, cirrhosis and hepatocellular carcinoma development in NAFLD, alcohol-related liver disease and viral hepatitis. Hence, we speculate that genotyping of common risk variants for liver disease progression may be equally useful to gauge the likelihood of developing advanced liver disease in patients with secondary fatty liver disease.

Normal plasma apoB48 despite the virtual absence of apoB100 in a compound heterozygote with novel mutations in the MTTP gene

"Normotriglyceridemic abetalipoproteinemia (ABL)" was originally described as a clinical entity distinct from either ABL or hypobetalipoproteinemia. Subsequent studies identified mutations in APOB gene which encoded truncated apoB longer than apoB48. Therefore, "Normotriglyceridemic ABL" can be a subtype of homozygous familial hypobetalipoproteinemia 1. Here, we report an atypical female case of ABL who was initially diagnosed with "normotriglyceridemic ABL", because she had normal plasma apoB48 despite the virtual absence of apoB100 and low plasma TG level. Next generation sequencing revealed that she was a compound heterozygote of two novel MTTP mutations: nonsense (p.Q272X) and missense (p.G709R). We speculate that p.G709R might confer residual triglyceride transfer activity of MTTP preferentially in the intestinal epithelium to the hepatocytes, allowing production of apoB48. Together, "normotriglyceridemic ABL" may be a heterogenous disorder which is caused by specific mutations in either APOB or MTTP gene.

A Rare Mutation in The APOB Gene Associated with Neurological Manifestations in Familial Hypobetalipoproteinemia

Clinical phenotypes of familial hypobetalipoproteinemia (FHBL) are related to a number of defective apolipoprotein B (APOB) alleles. Fatty liver disease is a typical manifestation, but serious neurological symptoms can appear. In this study, genetic analysis of the APOB gene and ophthalmological diagnostics were performed for family members with FHBL. Five relatives with FHBL, including a proband who developed neurological disorders, were examined. A sequencing analysis of the whole coding region of the APOB gene, including flanking intronic regions, was performed using the next-generation sequencing (NGS) method. Electrophysiological ophthalmological examinations were also done. In the proband and his affected relatives, NGS identified the presence of the pathogenic, rare heterozygous splicing variant c.3696+1G>T. Two known heterozygous missense variants-c.2188G>A, p.(Val730Ile) and c.8353A>C, p.(Asn2785His)-in the APOB gene were also detected. In all patients, many ophthalmologic abnormalities in electrophysiological tests were also found. The identified splicing variant c.3696+1G>T can be associated with observed autosomal, dominant FHBL with coexisting neurological symptoms, and both identified missense variants could be excluded as the main cause of observed clinical signs, according to mutation databases and the literature. Electroretinography examination is a sensitive method for the detection of early neuropathy and should therefore be recommended for the care of patients with FHBL.

Anti-PCSK9 treatment: is ultra-low low-density lipoprotein cholesterol always good?

Anti-PCSK9 (proprotein convertase subtilisin kexin 9) monoclonal antibodies (Mab) are novel, potent lipid-lowering drugs. They demonstrated to improve the lipid profile in high cardiovascular risk patients. Anti-PCSK9 Mab inhibit the targeted low-density lipoprotein (LDL)-receptor degradation induced by PCSK9 protein and are able to reduce LDL cholesterol (LDL-C) levels on top of conventional lipid-lowering therapy. Though these drugs proved to be very safe in the short-term, little is known about the possible long-term effects, due to the short period of their marketing. The genetic low cholesterol syndromes (LCS) represent the natural models of the lipid-lowering anti-PCSK9 therapy, and a valuable opportunity to predict the long-term effects of these drugs. By looking at the clinical features of such models, we could be able to foresee possible drug-induced side effects. In the present review, the correspondences and discordances between the side effects of anti-PCSK9 therapy and the corresponding LCS models will be examined in the attempt to forecast possible long-term consequences of these novel lipid-lowering agents.

In vitro functional characterization of splicing variants of the APOB gene found in familial hypobetalipoproteinemia

BACKGROUND

Familial hypobetalipoproteinemia type 1 (FHBL-1) is a codominant disorder characterized by greatly reduced plasma levels of total cholesterol, low-density lipoprotein cholesterol, and apolipoprotein B. Rare exonic pathogenic variants of APOB gene (nonsense variants, minute deletions/insertions and nonsynonymous variants) have been frequently reported in subjects with FHBL-1. Also, rare intronic variants of APOB located at intron/exon junctions and assumed to affect splicing have been reported. However, the pathogenicity of most of these intronic variants remains to be established.

OBJECTIVE

The objective of this study was the in vitro functional characterization of six splicing variants of APOB gene identified in seven putative FHBL-1 heterozygotes.

METHODS

ApoB minigenes harboring each variant were expressed in COS-1 cells and their transcripts were sequenced.

RESULTS

Four novel variants (c.237+1G>A, c.818+5G>A, c.3000-1G>T, and c.3842+1G>A), predicted in silico to obliterate splice site activity, were found to generate abnormal transcripts. The abnormal transcripts were generated by the activation of cryptic splice sites or exon skipping. All these transcripts harbored a premature termination codon and were predicted to encode truncated apoBs devoid of function. The predicted translation products were: i) p.(Lys41Serfs*2) and p.(Val80Ilefs*10) for c.237+1G>A; ii) p.(Asn274*) for c.818+5G>A; iii) p.(Leu1001Alafs*10) for c.3000-1G>T, and iv) p.(Ser1281Argfs*2) for c.3842+1G>A. Two previously annotated rare variants (c.905-15C>G and c.1618-4G>A) with uncertain effect in silico were found to generate only wild-type transcripts.

CONCLUSIONS

These in vitro minigene expression studies support the assignment of pathogenicity to four novel splice site variants of APOB gene found in FHBL-1.

Rapid Communication: Cholesterol deficiency-associated APOB mutation impacts lipid metabolism in Holstein calves and breeding bulls

During the last months, the number of reports on Holstein calves suffering from incurable idiopathic diarrhea dramatically increased. Affected calves showed severe hypocholesterolemia and mostly died within days up to a few months after birth. This new autosomal monogenic recessive inherited fat metabolism disorder, termed cholesterol deficiency (CD), is caused by a loss of function mutation of the bovine gene. The objective of the present study was to investigate specific components of lipid metabolism in 6 homozygous for the mutation (CDS) and 6 normal Holstein calves with different genotypes. Independent of sex, CDS had significantly lower plasma concentrations of total cholesterol (TC), free cholesterol (FC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), very-low-density lipoprotein cholesterol (VLDL-C), triacylglycerides (TAG), and phospholipids (PL) compared with homozygous wild-type calves ( < 0.05). Furthermore, we studied the effect of the genotype on cholesterol metabolism in adult Holstein breeding bulls of Swissgenetics. Among a total of 254 adult males, the homozygous mutant genotype was absent, 36 bulls were heterozygous carriers (CDC), and 218 bulls were homozygous wild-type (CDF). In CDC bulls, plasma concentrations of TC, FC, HDL-C, LDL-C, VLDL-C, TAG, and PL were lower compared with CDF bulls ( < 0.05). The ratios of FC:cholesteryl esters (CE) and FC:TC were higher in CDC bulls compared with CDF bulls, whereas the ratio of CE:TC was lower in CDC bulls compared with CDF bulls ( < 0.01). In conclusion, the CD-associated mutation was shown to affect lipid metabolism in affected Holstein calves and adult breeding bulls. Besides cholesterol, the concentrations of PL, TAG, and lipoproteins also were distinctly reduced in homozygous and heterozygous carriers of the mutation. Beyond malabsorption of dietary lipids, deleterious effects of apolipoprotein B deficiency on hepatic lipid metabolism, steroid biosynthesis, and cell membrane function can be expected, which may result in unspecific symptoms of reduced fertility, growth, and health.

GSK-3-mediated phosphorylation couples ER-Golgi transport and nuclear stabilization of the CREB-H transcription factor to mediate apolipoprotein secretion

CREB-H plays a key role in regulating secretion in metabolic pathways, particularly triglyceride homeostasis. Phosphorylation at a conserved serine motif, the P-motif, provides an integrated control mechanism of CREB-H function, coupling intercompartmental transport in the cytoplasm with stabilization of the active form in the nucleus.

CREB-H, an ER-anchored transcription factor, plays a key role in regulating secretion in metabolic pathways, particularly triglyceride homeostasis. It controls the production both of secretory pathway components and cargoes, including apolipoproteins ApoA-IV and ApoC-II, contributing to VLDL/HDL distribution and lipolysis. The key mechanism controlling CREB-H activity involves its ER retention and forward transport to the Golgi, where it is cleaved by Golgi-resident proteases, releasing the N-terminal product, which traffics to the nucleus to effect transcriptional responses. Here we show that a serine-rich motif termed the P-motif, located in the N-terminus between serines 73 and 90, controls release of the precursor transmembrane form from the ER and its forward transport to the Golgi. This motif is subject to GSK-3 phosphorylation, promoting ER retention, while mutation of target serines and drug inhibition of GSK-3 activity coordinately induce both forward transport of the precursor and cleavage, resulting in nuclear import. We previously showed that for the nuclear product, the P-motif is subject to multiple phosphorylations, which regulate stability by targeting the protein to the SCF^Fbw1a E3 ubiquitin ligase. Thus phosphorylation at the P-motif provides integrated control of CREB-H function, coupling intercompartmental transport in the cytoplasm with stabilization of the active form in the nucleus.

New Era of Lipid-Lowering Drugs

There are several established lipid-modifying agents, including statins, fibrates, niacin, and ezetimibe, that have been shown in randomized clinical outcome trials to reduce the risk of having an atherosclerotic cardiovascular event. However, in many people, the risk of having an event remains unacceptably high despite treatment with these established agents. This has stimulated the search for new therapies designed to reduce residual cardiovascular risk. New approaches that target atherogenic lipoproteins include: 1) inhibition of proprotein convertase subtilisin/kexin type 9 to increase removal of atherogenic lipoproteins from plasma; 2) inhibition of the synthesis of apolipoprotein (apo) B, the main protein component of atherogenic lipoproteins; 3) inhibition of microsomal triglyceride transfer protein to block the formation of atherogenic lipoproteins; 4) inhibition of adenosine triphosphate citrate lyase to inhibit the synthesis of cholesterol; 5) inhibition of the synthesis of lipoprotein(a), a factor known to cause atherosclerosis; 6) inhibition of apoC-III to reduce triglyceride-rich lipoproteins and to enhance high-density lipoprotein (HDL) functionality; and 7) inhibition of cholesteryl ester transfer protein, which not only reduces the concentration of atherogenic lipoproteins but also increases the level and function of the potentially antiatherogenic HDL fraction. Other new therapies that specifically target HDLs include infusions of reconstituted HDLs, HDL delipidation, and infusions of apoA-I mimetic peptides that mimic some of the functions of HDLs. This review describes the scientific basis and rationale for developing these new therapies and provides a brief summary of established therapies.

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.

Gene scanning and heart attack risk

Coronary heart disease remains the leading cause of death in the Western World. The advent of microarray and next-generation sequencing technologies has generated multi-dimensional data sets, allowing for new pathophysiological insights into this complex disease. To date, genome-wide association studies (GWAS) have identified 152 associated loci and 320 candidate genes, contributing to the genetic risk of coronary artery disease (CAD) and acute myocardial infarction (AMI). The majority of single nucleotide polymorphisms (SNPs) mediate their risk by unknown mechanisms. A functional analysis based on Gene Ontology and KEGG pathways of candidate genes that are associated with CAD/AMI-SNPs showed the strongest evidence for genes regulating cholesterol metabolism. Additional clusters were significantly enriched for pathways, which play prominent roles during AMI and the development of atherosclerotic plaques in vascular tissue, including focal adhesion/extracellular matrix interaction, TGF-β signaling, apoptosis, regulation of vascular smooth muscle contraction, angiogenesis, calcium ion binding, and transcription factors. A systems genetics approach, which incorporates genetic risk with gene expression data, metabolomic data, and protein biochemistry into genome-wide network studies, holds promise to elucidate the complex interplay between genetic risk and environmental factors for coronary artery disease.

The pathophysiology of intestinal lipoprotein production

Intestinal lipoprotein production is a multistep process, essential for the absorption of dietary fats and fat-soluble vitamins. Chylomicron assembly begins in the endoplasmic reticulum with the formation of primordial, phospholipids-rich particles that are then transported to the Golgi for secretion. Several classes of transporters play a role in the selective uptake and/or export of lipids through the villus enterocytes. Once secreted in the lymph stream, triglyceride-rich lipoproteins (TRLs) are metabolized by Lipoprotein lipase (LPL), which catalyzes the hydrolysis of triacylglycerols of very low density lipoproteins (VLDLs) and chylomicrons, thereby delivering free fatty acids to various tissues. Genetic mutations in the genes codifying for these proteins are responsible of different inherited disorders affecting chylomicron metabolism. This review focuses on the molecular pathways that modulate the uptake and the transport of lipoproteins of intestinal origin and it will highlight recent findings on TRLs assembly.