A case of hypocholesterolemia under study

Primary hypocholesterolemia (or hypobetalipoproteinemia) is a rare disorder of lipoprotein metabolism that may be due to a polygenic predisposition or a monogenic disease. Among these, it is possible to differentiate between symptomatic and asymptomatic forms, in which, in the absence of secondary causes, the initial clinical suspicion is plasma ApoB levels below the 5th percentile of the distribution by age and sex. Here we describe the differential diagnosis of a case of asymptomatic hypocholesterolemia. We studied proband's clinical data, the lipid profile of the proband and her relatives and the clinical data of the family relevant to carry out the differential diagnosis. We performed a genetic study as the diagnostic test. The information obtained from the differential diagnosis suggested a heterozygous hypobetalipoproteinemia due to PCSK9 loss-of-function variants. The diagnostic test revealed, in the proband, the presence of a heterozygous PCSK9 frame-shift variant of a maternal origin. Plasma levels of LDL cholesterol and PCSK9 of the patient and her relatives were compatible with the segregation of the variant revealed. In conclusion, the diagnostic test performed confirmed the suspected diagnosis of the proband as asymptomatic familial hypobetalipoproteinemia due to a loss-of-function variant in the PCSK9 gene.

APOB CRISPR-Cas9 Engineering in Hypobetalipoproteinemia: A Promising Tool for Functional Studies of Novel Variants

Hypobetalipoproteinemia is characterized by LDL-cholesterol and apolipoprotein B (apoB) plasma levels below the fifth percentile for age and sex. Familial hypobetalipoproteinemia (FHBL) is mostly caused by premature termination codons in the APOB gene, a condition associated with fatty liver and steatohepatitis. Nevertheless, many families with a FHBL phenotype carry APOB missense variants of uncertain significance (VUS). We here aimed to develop a proof-of-principle experiment to assess the pathogenicity of VUS using the genome editing of human liver cells. We identified a novel heterozygous APOB-VUS (p.Leu351Arg), in a FHBL family. We generated APOB knock-out (KO) and APOB-p.Leu351Arg knock-in Huh7 cells using CRISPR-Cas9 technology and studied the APOB expression, synthesis and secretion by digital droplet PCR and ELISA quantification. The APOB expression was decreased by 70% in the heterozygous APOB-KO cells and almost abolished in the homozygous-KO cells, with a consistent decrease in apoB production and secretion. The APOB-p.Leu351Arg homozygous cells presented with a 40% decreased APOB expression and undetectable apoB levels in cellular extracts and supernatant. Thus, the p.Leu351Arg affected the apoB secretion, which led us to classify this new variant as likely pathogenic and to set up a hepatic follow-up in this family. Therefore, the functional assessment of APOB-missense variants, using gene-editing technologies, will lead to improvements in the molecular diagnosis of FHBL and the personalized follow-up of these patients.

Lipids Responsible for Intestinal or Hepatic Disorder: When to Suspect a Familial Intestinal Hypocholesterolemia?

ABSTRACT

Familial intestinal hypocholesterolemias, such as abetalipoproteinemia, hypobetalipoproteinemia, and chylomicron retention disease, are rare genetic diseases that result in a defect in the synthesis or secretion of lipoproteins containing apolipoprotein B.In children, these conditions present with diarrhoea and growth failure, whereas adults present with neuromuscular, ophthalmological, and hepatic symptoms. Simple laboratory investigations have shown that diagnosis can be made from findings of dramatically decreased cholesterol levels, deficiencies in fat-soluble vitamins (mostly vitamin E), endoscopic findings of the characteristic white intestinal mucosa, and fat-loaded enterocytes in biopsy samples. Genetic analysis is used to confirm the diagnosis. Treatment is based on a low-fat diet with essential fatty acid supplementation, high doses of fat-soluble vitamins, and regular and life-long follow-up.The present study examines cases and literature findings of these conditions, and emphasises the need to explore severe hypocholesterolemia and deficiencies in fat-soluble vitamins to not miss these rare, but easy to diagnose and treat, disorders.

Chylomicron retention disease: genetics, biochemistry, and clinical spectrum

PURPOSE OF REVIEW

Chylomicron retention disease (CRD) is an autosomic recessive disorder, in which intestinal fat malabsorption is the main cause of diverse severe manifestations. The specific molecular defect was identified in 2003 and consists of mutations in the SAR1B or SARA2 gene encoding for intracellular SAR1B GTPase protein. The aim of this review is first to provide an update of the recent biochemical, genetic and clinical findings, and second to discuss novel mechanisms related to hallmark symptoms.

RECENT FINDINGS

CRD patients present with SAR1B mutations, which disable the formation of coat protein complex II and thus blocks the transport of chylomicron cargo from the endoplasmic reticulum to the Golgi. Consequently, there is a total absence of chylomicron and apolipoprotein B-48 in the blood circulation following a fat meal, accompanied by a deficiency in liposoluble vitamins and essential fatty acids. The recent discovery of Transport and Golgi organization and Transport and Golgi organization-like proteins may explain the intriguing export of large chylomicron, exceeding coat protein complex II size. Hypocholesterolemia could be accounted for by a decrease in HDL cholesterol, likely a reflection of limited production of intestinal HDL in view of reduced ATP-binding cassette family A protein 1 and apolipoprotein A-I protein. In experimental studies, the paralog SAR1A compensates for the lack of the SAR1B GTPase protein.

SUMMARY

Molecular testing for CRD is recommended to distinguish the disease from other congenital fat malabsorptions, and to early define molecular aberrations, accelerate treatment, and prevent complications.

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.

Abetalipoproteinemia and homozygous hypobetalipoproteinemia: a framework for diagnosis and management

Abetalipoproteinemia (ABL; OMIM 200100) and homozygous hypobetalipoproteinemia (HHBL; OMIM 107730) are rare diseases characterized by hypocholesterolemia and malabsorption of lipid-soluble vitamins leading to retinal degeneration, neuropathy and coagulopathy. Hepatic steatosis is also common. The root cause of both disorders is improper packaging and secretion of apolipoprotein (apo) B-containing lipoprotein particles due to mutations either in both alleles of the MTP (alias MTTP) gene encoding microsomal triglyceride transfer protein (MTP) or both alleles of the APOB gene itself in the case of ABL and HHBL, respectively. Clinical diagnosis is based on signs and symptoms, acanthocytosis on blood smear, and virtually absent apo B-containing lipoproteins, including chylomicrons, very low density lipoprotein and low density lipoprotein. Obligate heterozygote parents of ABL patients usually have normal lipids consistent with autosomal recessive inheritance, while heterozygous parents of HHBL patients typically have half normal levels of apo B-containing lipoproteins consistent with autosomal co-dominant inheritance. Definitive diagnosis involves sequencing the MTP and APOB genes, for which >30 and >60 mutations have been described for ABL and HHBL, respectively. Follow-up includes monitoring for ophthalmologic, neurologic, hematologic, and hepatic complications, as well as compliance with treatment. Investigations include lipid profile, serum transaminases, markers for lipid-soluble vitamins, and periodic instrumental assessment of ocular and neurological function. Mainstays of treatment include adherence to a low-fat diet, and supplementation with essential fatty acids and high oral doses of fat soluble vitamins. Prognosis is variable, but early diagnosis and strict adherence to treatment can recover normal neurological function and halt disease progression.

The c.43_44insCTG variation inPCSK9 is associated with low plasma LDL-cholesterol in a Caucasian population

The genetic etiology of familial hypobetalipoproteinemia (FHBL) is unclear in the majority of cases. Mutations in apolipoprotein B (APOB) are the only confirmed causes of FHBL. Recently, loss-of-function mutations of PCSK9 gene have been shown to be associated with the hypocholesterolemia phenotype. Our primary goal was to confirm that mutations in PCSK9 could be another cause of FHBL. Using the sequencing approach, we found that the c.43_44insCTG variation in PCSK9, a common in-frame insertion in both African American and Caucasian populations, is associated with the hypocholesterolemia phenotype in three FHBL families. Then we tested whether this variation could be associated with lower cholesterol levels in the general population. A total of 403 subjects from a Caucasian population, in which hypobetalipoprotein (HBL) and normal groups were classified using standard criteria, were sequenced for this variation. The allele frequency of this variation in the HBL group was 0.186, but was only 0.128 in the normal lipid group. The mean plasma low-density lipoprotein (LDL)-cholesterol level in subjects heterozygous for this variant is significantly lower than that in the normal group (p<0.01). Heterozygous subjects also had higher high-density lipoprotein (HDL)-cholesterol levels (p<0.01). In general, LDL-cholesterol concentration in individuals with PCSK9 c.43_44insCTG variation was approximately 10-15 mg/dL lower than that in normal individuals. We conclude that the c.43_44insCTG variant plays a role in lowering cholesterol in the general population.

Absence of fatty liver in familial hypobetalipoproteinemia linked to chromosome 3p21

Our aim was to ascertain whether fatty liver may be present in the genetic form of familial hypobetalipoproteinemia (FHBL) linked to a susceptibility locus on chromosome 3p21. Three genetic forms of FHBL exist: (a) FHBL caused by truncation-specifying mutations of apolipoprotein B (apoB), (b) FHBL linked to chr3p21, and (c) FHBL not linked either to APOB or to chr3p21. Fatty liver is common in apoB-defective FHBL. Hepatic fat contents were quantified by magnetic resonance spectroscopy in 16 subjects with 3p21-linked FHBL, 32 subjects with apoB-defective FHBL, and 39 sex- and age-matched controls. Mean liver fat of 3p21 subjects was similar to controls and approximately 60% lower than apoB-defective FHBL subjects ( P = .0012). Indices of adiposity (body mass index, waist/hip ratio) and masses of abdominal subcutaneous, retroperitoneal, and intraperitoneal adipose tissue (IPAT) were quantified by MR imaging. Mean measures of adiposity were similar in the 3 groups, suggesting that adiposity per se was not responsible for differences in liver fat. Liver fat content was positively correlated with IPAT. The intercepts of regression lines of IPAT on liver fat content were similar in controls and 3p21, but higher in apoB-defective FHBL subjects. The slopes of the lines were steepest in apoB-defective, intermediate in 3p21, and flattest in controls. Lipoprotein profiles and very low density lipoprotein-apoB100 kinetics of 3p21 and apoB-defective groups also differed. Thus, 2 genetic subtypes of FHBL also differ in several phenotypic features.

Unusual presentation of three siblings with familial heterozygous hypobetalipoproteinaemia

We describe three siblings with the unusual presentation of manifest steatorrhoea and vitamin E deficiency mimicking homozygous familial hypobetalipoproteinaemia (FHBL) but whose lipid profile (cholesterol and ApoB) was consistent with heterozygous FHBL. Upper gastrointestinal endoscopy and small intestinal biopsy were normal. We discuss the diagnosis with reference to the relevant literature.

CONCLUSION

although rare, familial hypobetalipoproteinaemia should be considered among the causes of manifest steatorrhoea in childhood even without evidence of failure to thrive. Dietary restriction of fat and high dose vitamin E supplementation improves quality of life by reducing stool frequency and may prevent or delay neurological complications.

A truncated species of apolipoprotein B (B-38.7) in a patient with homozygous hypobetalipoproteinemia associated with diabetes mellitus

Familial hypobetalipoproteinemia is caused by mutations in the apolipoprotein (apo) B gene. We identified a 57-year-old woman whose plasma total cholesterol and apoB levels were 2.17 mmol/L and 0.03 g/L, respectively. Separation of plasma lipoproteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the absence of apoB-100 and the presence of a faster-migrating form of apoB with an apparent Mr of 195 kDa. Direct sequencing of a polymerase chain reaction-amplified fragment of the patient's apoB gene DNA revealed a single C-->T transition at nucleotide 5472 that converts glutamine 1755 (CAA) to a stop codon (TAA). We predict this novel nonsense mutation of the apoB gene to produce a truncated protein that contains 1754 amino-terminal amino acid residues of apoB-100. We designated this mutant form of apoB apoB-38.7 by following the centile nomenclature of the apoB species. The same mutation was found in both of her children. The proband revealed clinical findings of retinitis pigmentosa, acanthocytosis, and loss of deep tendon reflexes that are characteristic of severe hypobetalipoproteinemia. In addition, the proband had type II diabetes mellitus with nephropathy, anemia, cholelithiasis, hepatic hemangioma, bronchiectasis, and extensive calcification of major arteries including, the celiac, splenic, and renal. In summary, we have found a novel truncated apoB, apoB-38.7, in a patient with an unusual presentation of hypobetalipoproteinemia that includes diabetes mellitus and extensive arterial calcification.

Persistent transaminase elevation due to heterozygous (familial) apolipoprotein B deficiency

Homozygous apolipoprotein B deficiency can present with fatty liver and raised levels of transaminases. Subjects with heterozygous deficiency are almost always asymptomatic. We report an asymptomatic 26-year-old man with persistently raised transaminases, in whom the diagnosis of heterozygous (familial) apolipoprotein B deficiency was made on the basis of characteristic lipid profile.

[Familial hypobetalipoproteinemia]

BACKGROUND

Familial hypobetalipoproteinaemia (FHBL) is a relatively rare inborn error of metabolism which must be considered in the differential diagnosis of hypocholesterolaemia which cannot be explained by secondary causes (severe malnutrition, generalization of neoplastic disease etc.).

METHODS AND RESULTS

In the submitted paper the authors present the results assembled in a family with four heterozygotes with FHBL. The proband is a 27-year-old woman (total cholesterol (TCh) 1.70, triacylglycerols (TG) 0.20, HDL-cholesterol (HDL-ch.) 1.38, LDL-cholesterol (LDL-ch.) 0.34 all in mmol/l, apolipoprotein B (apo B) 0.25, lipoprotein(a)(Lp(a) 0.09 all in g/l, isoforms of apolipoprotein E/E3 (iso apo E). Mother (age 53 years) of the proband (TCh 3.06, TG 0.37, HDL-ch. 1.99, LDL-ch. 0.90 all in mmol/l, Apo B 0.37, Lp(a) 0.14 all in g/l, iso apo E3/E3)). Two of the proband's sisters (23 and 20 years) (TCh 3.92 and 2.55 resp., TG 0.57 and 0.23 resp. HDL-ch. 1.86 and 1.63 resp., LDL-ch. 1.80 and 0.82 resp. all in mmol/l, Apo B 0.73 and 0.37 resp., Lp(a) 0.47 and 0.63 resp. all in g/l, iso apo E2/E3 and E2/E3 resp.). The diagnosis confirms the autosomal dominant transmission. During the proband's pregnancy (during the 38th week), contrary to normocholesterolaemic women the TCh did not rise (1.43 mmol/l, the TG level (0.62 mmol/l), Apo B (0.43 and Lp(a) 0.18 all in g/l rose.

CONCLUSIONS

According to the available literature this is the first description of FHBL in our literature and a priority investigation of plasma lipid concentrations, lipoproteins and apolipoproteins in a women with a heterozygous form of FHBL during pregnancy.

A variant form of hypobetalipoproteinaemia associated with ataxia, hearing loss and retinitis pigmentosa

A six-year-old Japanese boy had ataxia, mental retardation, peripheral neuropathy, proximal myopathy, hearing loss, retinitis pigmentosa and deficiencies in apolipoprotein AI, B, CII and CIII. His clinical features except for hearing loss resembled those of abetalipoproteinaemia or symptomatic hypobetalipoproteinaemia, but his apolipoprotein abnormalities were distinct from these disorders. He had apolipoprotein B-100 with a normal molecular weight. Although most of his neurological manifestations were compatible with those of vitamin E deficiency, their early onset and the presence of hearing loss was unusual for that condition. There has been slight deterioration of ataxia during two years follow-up despite high-dose vitamin E supplementation. Other abnormalities in lipid metabolism might be associated with the neurological damage in this case.

[Cryptogenetic liver cirrhosis with hypobetalipoproteinemia. Typing of the HLA histocompatibility system]

One hundred and forty-six patients with chronic liver disease have been studied. Some of them (15) showed a clinical picture characterized by cryptogenetic liver cirrhosis associated with hypotriglyceridemia and hypobetalipoproteinemia. These patients had compensated cirrhosis and no history of alcohol abuse; they did not suffer major illness in the past and had no signs of portosystemic encephalopathy. The pathogenetic mechanism of this association and the possible role of genetic factors are discussed. The HLA system has been studied and the A2 antigen found with high frequency, raising the possibility that patients with this syndrome may represent a particular subgroup among these with liver cirrhosis.

Infantile refsum disease: gastrointestinal presentation of a peroxisomal disorder

This article describes two siblings with infantile Refsum disease (IRD) whose initial presentation was that of malabsorption and mimicked a-beta- or homozygous hypo-beta-lipoproteinemia. Failure to recognize IRD in the first-born child precluded proper genetic counseling and prenatal diagnosis in subsequent pregnancies and also caused considerable delay in diagnosing IRD in the second child. The clinical heterogeneity of peroxisomal disorders constitutes a diagnostic challenge, which demands a high degree of awareness from the part of the clinician. This is particularly the case with IRD, where protracted diarrhea with low serum cholesterol levels appears to be a frequently occurring initial feature during the 1st months of life.

[Familial hypobetalipoproteinemia: description of a heterozygous form with important biochemical alterations]

We describe a case of a patient affected of familial hypobetalipoproteinemia with marked biochemical alterations (low cholesterol and triglyceride levels) which are characteristic of the homocygote form and with no apparent clinical manifestations. The determination of Apoprotein B and the family study permitted the diagnosis of a heterocygote form.

[Extreme degree familial hypobetalipoproteinemia caused by hypothetic double heterozygosity in a subject with severe mental deficiency]

We described the case of an adult male patient, seriously mentally deficient, hospitalised in Psychiatric Hospital for a period of many years, suffering from a familial hypobetalipoproteinemia with extremely low levels of plasmatic betalipoproteins. The patient has been followed and tested several times over a period of six years. Numerous members of his family, which is part of a restricted ethnic nucleus in a locality (Iolo) of the Comune of Prato in the Provincia of Florence, were examined and tested too. Consanguinity between his parents is not demonstrable. The diagnosis of homozygous hypobetalipoproteinemia is discarded, but it does not seem that the heterozygous one is to be accepted as weel. On the ground of the existence of two syndromes which are quite unlike each other, but both explainable as form of familial heterozygous hypobetalipoproteinemia, one of them present in his father, the other one in his mother and in the maternal relatives as in the patient's brother respectively, a hypothesis of a double heterozygosis could be formulated. Extant is the support of the recent literature data, depending on them the possibility of making the hypothesis of a multiplicity of the genes regulating the apolipoprotein B synthesis. We do not exclude that the peculiarity of the event of a double heterozygosis can also be directly responsible of the patient's serious mental deficiency, being at the same time more supportable the hypothesis of a encephalopathy in his early childhood.

Familial hypobetalipoproteinaemia in 9 children diagnosed as the result of cord blood screening for hypolipoproteinaemia in 10 000 Danish newborns

Out of 10 440 children, 266 with low (less than 2.5 centile) values for very low-low density lipoproteins in cord serum were chosen to be followed up to find out how many came from families with familial hypobetalipoproteinaemia (FHBL). In 176 families (66% of 266 families), FHBL was diagnosed in 9 children and their families.

A clinical and neurophysiological investigation of a Danish kindred with heterozygous familial hypobetalipoproteinemia

A three-generation transmission of under five percentile values for serum low density lipoprotein and low density lipoprotein cholesterol typical of heterozygous familial hypobetalipoproteinemia was demonstrated in a Danish family. Slight clinical signs of CNS abnormality were found in 4 of the 8 subjects with heterozygous familial hypobetalipoproteinemia, but did not resemble the neurological findings in abetalipoproteinemia nor in the previously described patients with familial hypobetalipoproteinemia. There were no signs of myelin dysfunction in the central nervous system as judged from the normal latency of visual and somatosensory evoked potentials.

Hypobetalipoproteinaemia--a variant of the Bassen-Kornzweig syndrome

A patient is reported with hypobetalipoproteinaemia and clinical features resembling the Bassen-Kornzweig syndrome (abetalipoproteinaemia) more completely than previously described. This supports a link between hypobetalipoproteinaemia and abetalipoproteinaemia and it is suggested that the Bassen-Kornzweig syndrome has a wide spectrum with serum betalipoprotein ranging from absent to normal. It is likely that there are different genetic entities with similar end results.