Identification of a Variant in APOB Gene as a Major Cause of Hypobetalipoproteinemia in Lebanese Families

Familial hypobetalipoproteinemia (FHBL) is a codominant genetic disorder characterized by reduced plasma levels of low-density lipoprotein cholesterol and apolipoprotein B. To our knowledge, no study on FHBL in Lebanon and the Middle East region has been reported. Therefore, we conducted genetic studies in unrelated families and probands of Lebanese origin presenting with FHBL, in order to identify the causes of this disease. We found that 71% of the recruited probands and their affected relatives were heterozygous for the p.(Arg490Trp) variant in the APOB gene. Haplotype analysis showed that these patients presented the same mutant haplotype. Moreover, there was a decrease in plasma levels of PCSK9 in affected individuals compared to the non-affected and a significant positive correlation between circulating PCSK9 and ApoB levels in all studied probands and their family members. Some of the p.(Arg490Trp) carriers suffered from diabetes, hepatic steatosis or neurological problems. In conclusion, the p.(Arg490Trp) pathogenic variant seems a cause of FHBL in patients from Lebanese origin, accounting for approximately 70% of the probands with FHBL presumably as a result of a founder mutation in Lebanon. This study is crucial to guide the early diagnosis, management and prevention of the associated complications of this disease.

A Case of Hypolipidemia and Hypocholesterolemia; Cause and Consequences

Background: Hypolipidemia and hypocholesterolemia are uncommon and because of the established risk of hypercholesterolemia for cardiovascular disease, reduced lipids and total cholesterol levels are often clinically desired and/or deemed clinically inconsequential. A finding of persistently low lipid levels and total cholesterol may however not be innocuous nor desirable. We describe the case of a 46 yr old man with persistently low total cholesterol levels <70mg/dl and the associated complications and comorbidities identified. Clinical Case: A 46 yr old Hispanic man with non-alcoholic fatty liver disease (NAFLD) was referred for evaluation of hyperhidrosis in the setting of persistent hypolipidemia and hypocholesterolemia. Review of the patient’s clinical and biochemical history showed persistently low total cholesterol (mean 58mg/dl), hypotriglyceridemia, and low LDL-C (mean 13.4mg/dl) over the prior 7 yrs in addition to undetectable serum lipoprotein A. Evaluation for secondary causes of hypolipidemia, such as multiple myeloma, was unremarkable. He was found to have low carotene, borderline vitamin A and low vitamin E levels while the rest of his serum fat-soluble vitamins were normal. His mother who had presumed Alzheimer’s dementia also had a history of very low cholesterol levels. The degree and persistence of his hypolipidemia and hypocholesterolemia raised the possibility of a genetic etiology of his hypolipidemia. Genetic testing confirmed that the patient was heterozygous for a pathogenic variant in the APOB gene, consistent with familial hypobetalipoproteinemia (FHBL) which is autosomal recessive linked. Subsequent close review of his clinical history revealed other potential complications and comorbidities of FHBL including NAFLD with prediabetes, hypogonadism, progressive cognitive and memory decline, peripheral neuropathy and multiple neuropsychiatric syndromes including adult ADHD, borderline personality disorder, bipolar disorder and chronic anxiety. He is presently on vitamin E and A supplementation and being followed by neurology and psychiatry in addition to ongoing endocrine and metabolic clinical surveillance. In addition, in view of his maternal history and several biologic children he has undergone formal genetic and family counselling. Conclusions: While lipid panels are ubiquitous in clinical care, clinicians need to be vigilant in settings of severe persistent hypolipidemia and/or hypocholesterolemia to evaluate for possible genetic basis for this and to also screen for possible associated complications and comorbidities.

Novel zebrafish mutants reveal new roles for Apolipoprotein B during embryonic development and pathological conditions

ABSTRACTApolipoprotein B (ApoB) is the primary protein of chylomicrons, VLDLs and LDLs and is essential for their assembly. Defects in ApoB synthesis and secretion result in several human diseases, including abetalipoproteinemia and familial hypobetalipoproteinemia. Conversely, high levels of APOB in plasma are associated with increased risk for coronary heart disease and atherosclerosis.The involvement of APOB in lipid metabolism and atherogenesis prompted the generation of several mutant mice. However, as APOB is required for supplying nutrients to the developing embryo, ApoB null mice are embryonic lethal, thereby precluding the study of the roles of this protein during development.Here, we established novel zebrafish mutants for two apoB genes: apoBa and apoBb.1. Double-mutant embryos display clear hallmarks of human hypolipidemia-related diseases, including intestinal defects and fatty liver, as well as profound vascular defects. We further use these models to identify the domains within ApoB responsible for its functions. By assessing the ability of different truncated forms of human APOB to rescue the mutant phenotypes, we demonstrate the benefits of this model for prospective therapeutic screens. Overall, our novel zebrafish models uncover new functions of ApoB in organ development and morphogenesis and shed new light on the mechanisms underlying hypolipidemia-related diseases.

Impaired Cytoskeletal and Membrane Biophysical Properties of Acanthocytes in Hypobetalipoproteinemia – A Case Study

Familial hypobetalipoproteinemia is a metabolic disorder mainly caused by mutations in the apolipoprotein B gene. In its homozygous form it can lead without treatment to severe ophthalmological and neurological manifestations. In contrast, the heterozygous form is generally asymptomatic but associated with a low risk of cardiovascular disease. Acanthocytes or thorny red blood cells (RBCs) are described for both forms of the disease. However, those morphological changes are poorly characterized and their potential consequences for RBC functionality are not understood. Thus, in the present study, we asked whether, to what extent and how acanthocytes from a patient with heterozygous familial hypobetalipoproteinemia could exhibit altered RBC functionality. Acanthocytes represented 50% of the total RBC population and contained mitoTracker-positive surface patches, indicating the presence of mitochondrial fragments. While RBC osmotic fragility, calcium content and ATP homeostasis were preserved, a slight decrease of RBC deformability combined with an increase of intracellular free reactive oxygen species were observed. The spectrin cytoskeleton was altered, showing a lower density and an enrichment in patches. At the membrane level, no obvious modification of the RBC membrane fatty acids nor of the cholesterol content were detected but the ceramide species were all increased. Membrane stiffness and curvature were also increased whereas transversal asymmetry was preserved. In contrast, lateral asymmetry was highly impaired showing: (i) increased abundance and decreased functionality of sphingomyelin-enriched domains; (ii) cholesterol enrichment in spicules; and (iii) ceramide enrichment in patches. We propose that oxidative stress induces cytoskeletal alterations, leading to increased membrane stiffness and curvature and impaired lipid lateral distribution in domains and spicules. In addition, ceramide- and spectrin-enriched patches could result from a RBC maturation defect. Altogether, the data indicate that acanthocytes are associated with cytoskeletal and membrane lipid lateral asymmetry alterations, while deformability is only mildly impaired. In addition, familial hypobetalipoproteinemia might also affect RBC precursors leading to disturbed RBC maturation. This study paves the way for the potential use of membrane biophysics and lipid vital imaging as new methods for diagnosis of RBC disorders.