New mutations in APOB100 involved in familial hypobetalipoproteinemia

Familial hypolipoproteinemia (FHBL) is characterized by an inherited low plasma level of apolipoprotein B containing lipoproteins. FHBL may be caused by mutations of APOB. Individuals with FHBL typically have intestinal malabsorption and frequently suffer from a deficiency of fat-soluble vitamins. Most mutations that cause FHBL are APOB truncating mutations. Here we describe a patient with FHBL caused by a novel truncating mutation together with a novel missense mutation.     

Acanthocytosis in a patient with homozygous familial hypobetalipoproteinemia due to a novel APOB splice site mutation

We report on acanthocytosis in a 31-year-old woman with homozygous familial hypobetalipoproteinemia due to a mutation affecting the splicing of the APOB gene encoding apolipoprotein B. Treatment with fat-soluble vitamins was associated with arrest of the usually progressive neurological complications of this condition. However, the acanthocytosis - literally 'thorny' erythrocytes that result from abnormal membrane fluidity - persists despite treatment.     

Functional analysis of two novel splice site mutations of APOB gene in familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by reduced plasma levels of low density lipoprotein cholesterol (LDL-C) and its protein constituent apolipoprotein B (apoB), which may be due to mutations in APOB gene, mostly located in the coding region of this gene. We report two novel APOB gene mutations involving the acceptor splice site of intron 11 (c.1471-1G>A) and of intron 23 (c.3697-1G>C), respectively, which were identified in two patients with heterozygous FHBL associated with severe fatty liver disease. The effects of these mutations on APOB pre-mRNA splicing were assessed in COS-1 cells expressing the mutant APOB minigenes.The c.1471-1G>A APOB minigene generated two abnormal mRNAs. In one mRNA the entire intron 11 was retained; in the other mRNA exon 11 joined to exon 12, in which the first nucleotide was deleted due to the activation of a novel acceptor splice site. The predicted products of these mRNAs are truncated proteins of 546 and 474 amino acids, designated apoB-12.03 and apoB-10.45, respectively. The c.3697-1G>C APOB minigene generated a single abnormal mRNA in which exon 23 joined to exon 25, with the complete skipping of exon 24. This abnormal mRNA is predicted to encode a truncated protein of 1220 amino acids, designated apoB-26.89.These splice site mutations cause the formation of short truncated apoBs, which are not secreted into the plasma as lipoprotein constituents. This secretion defect is the major cause of severe fatty liver observed in carriers of these mutations.     

Novel mutations of APOB cause apoB truncations undetectable in plasma and familial hypobetalipoproteinemia

The following error was found in the original version of this article: In Family 98, a novel point mutation in exon 26 (11163T4G) referring to GenBank number XM_036735 should be 11168T4G.     

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.     

Novel mutations of APOB cause ApoB truncations undetectable in plasma and familial hypobetalipoproteinemia

Familial hypobetalipoproteinemia (FHBL) is a genetic disorder characterized by low levels of apoB-100 and LDL cholesterol. Truncation-producing mutations of apoB (chromosome 2) are among several potential causes of FHBL in patients. Ten new families with FHBL linked to chromosome 2 were identified. In Family 8, a 4432delT in exon 26 produces a frame-shift and a premature stop codon predicted to produce a truncated apoB-30.9. Even though this truncation is just 10 amino acid shorter than the well-documented apoB-31, which is readily detectable in plasma, apoB-30.9 is undetectable. Most truncations shorter than apoB-30 are not detectable in plasma. In Family 34, an acceptor splicing mutation at position -1 of exon 14 changes the acceptor splice site AG to AA. Two families (Family 50 and 52) had mutations (apoB-9 and apoB-29) reported previously. In Family 98, a novel point mutation in exon 26 (11163T>G) causes a premature stop codon, and produces a truncated apoB-80.5 readily detectable in plasma. Sequencing of the ApoB gene in families 1, 5, 18, 58, and 59 did not reveal mutations.     

一种新复合杂合突变导致21-羟化酶缺陷症

目的探讨1例单纯男性化型21-羟化酶缺陷症(21-OHD)基因突变的类型和特点及临床表型与基因突变类型之间的关系。方法收集患者的临床资料,提取外周血白细胞DNA,用PCR方法扩增CYP21A2基因的10个外显子及内含子边界,测序鉴定CYP21A2基因突变位点,进一步分析突变位点与临床表型的关系。结果患者的临床表现主要为外阴发育异常。基因测序结果显示为复合杂合突变,其一个等位基因为c.515 T>A,p.I172N,另一个等位基因为c.593 T>G,p.L198X,此种复合杂合突变主要引起单纯男性化表现。p.L198X是至今尚未见报道的一种新突变。结论发现了CYP21A2基因一种新的突变p.L198X,丰富了CYP21A2基因突变数据库。同时从分子遗传学方面证实了对患者的诊断,患者基因型能很好地解释其临床表现。     

High frequency of APOB gene mutations causing familial hypobetalipoproteinaemia in patients of Dutch and Spanish descent

Background: Familial hypobetalipoproteinaemia (FHBL) is an autosomal co-dominant hereditary disorder of lipoprotein metabolism characterised by decreased low density lipoprotein (LDL) cholesterol and apolipoprotein B (APOB) plasma levels. High levels of plasma APOB and LDL cholesterol are strong predictors for risk of cardiovascular disease (CVD), while individuals with low APOB and LDL cholesterol levels are thought to have lower than average risk for CVD, and in fact, heterozygous FHBL patients appear to be asymptomatic.

Methods: Rather than identifying truncated APOB proteins in plasma fractions separated by gel electrophoresis, which will miss any mutations in proteins smaller than 30 kb, we analysed the APOB gene directly, using PCR.

Results: We identified nine different mutations, six of which are novel. Each mutation showed complete co-segregation with the FHBL phenotype in the families, and statistically significant differences between carriers and non-carriers were found for plasma total, LDL, and HDL cholesteroll, triglycerides, and APOB levels, but not for APOA1 levels. All carriers of an APOB mutation were completely free from CVD.

Conclusions: Prolonged low levels of LDL cholesterol and elevated levels of HDL cholesterol may reduce the progression of atherosclerotic disease, but this has not been unequivocally shown that this is indeed the case in individuals with FHBL, and is the subject of a current study.

     

Novel APOB nonsense variant related to familial hypobetalipoproteinemia and hepatic steatosis: A case report and review

Hereditary familial hypobetalipoproteinemia (FHBL) is a syndrome caused by variants in the APOB gene, that cause a defect in the secretion and mobilization of liver lipids to peripheral tissues, associated with the synthesis of truncated ApoB100 apolipoproteins. This condition causes significant reduction in total cholesterol (TC), low-density lipoproteins (LDL), very low-density proteins (VLDL) and serum triglyceride levels, with unchanged high-density lipoprotein (HDL) cholesterol levels. Herein we present the case of a middle-aged woman diagnosed with FHBL and hepatic steatosis, heterozygous for c.4698C>A; (p.Tyr1566Ter) variant in APOB. The variant presented herein showed high expressiveness in the two generations of individuals analyzed and has not yet being described in the medical literature. Early diagnosis and screening for associated metabolic comorbidities such as metabolic fatty liver disease and its subsequent progression to fibrosis are the two main goals in the treatment of this condition, in order to prevent medium to long term potential complications.     

Two novel missense mutations in the LDL receptor gene causing familial hypercholesterolemia

We have employed analysis of single-strand conformation polymorphisms to identify mutations in the low density lipoprotein receptor gene causing familial hypercholesterolemia. Two familial hypercholesterolemia heterozygotes had abnormal single-strand conformation polymorphism patterns of exons 4 and 8. DNA sequencing revealed that the abnormal pattern of exon 4 was due to heterozygosity (G/T) at nucleotide 502. Nucleotide 502 is the first base of codon 147, and the G→T mutation (D147Y) changes this codon from Asp_GAC to Tyr_UAC. The abnormal pattern of exon 8 was due to heterozygosity (A/G) at nucleotide 1097. Nucleotide 1097 is the second base of codon 345, and the A→G mutation (Q345R) changes this codon from Gln_CAG to Arg_CGG- Based upon screening of 437 unrelated familial hypercholesterolemia heterozygotes, both D147Y and Q345R account for about 0.5% of the mutations causing familial hypercholesterolemia in Norway.     

Functional relationships between three novel homozygous mutations in the ACTH receptor gene and familial glucocorticoid deficiency

Familial glucocorticoid deficiency (FGD) is an autosomal recessive disorder characterized by a glucocorticoid adrenal insufficiency without mineralocorticoid deficiency. Mutations of the ACTH receptor (MC2-R) gene have been reported in some FGD cases, but only a few of them have been functionally studied. We reported clinical features and MC2-R gene analysis in three families. For each proband, an homozygous mutation was identified after amplification and sequencing of the whole intronless MC2-R gene. One mutation converted Val-142 located in the second intracellular loop to Leu. Another mutation in the sixth transmembrane domain converted Ala-233 to Pro. The last mutation converted the negatively charged Asp-103 in the first extracellular loop to an uncharged Asn. Functional studies of these mutations as well as the S120R mutation were performed after stable transfection of M3 cells and measurement of ACTH-induced cAMP production. For the S120R, V142L, and A233P mutated MC2-R, cAMP production curves were similar to that obtained with M3 parental cells, confirming that these mutations are responsible for the FGD in the affected patients. The D103N-mutated MC2-R had an impaired cAMP response to physiological doses of ACTH, but the maximal response at very high concentrations of ACTH was similar to that obtained for the wild-type MC2-R. All these results demonstrated clear relationships based on functional studies between MC2-R homozygous mutations and FGD phenotype.     

Brothers with novel compound heterozygous mutations in COL27A1 causing dental and genital abnormalities

COL27A1 encodes a collagen type XXVII alpha 1 chain. It is the product of this gene that provides the structural support of connective tissue and is reported to be the causative gene of Steel syndrome (OMIM #615155). The primary symptoms of patients with this defect are consistent with systemic bone disease; however, recent reports note findings of intellectual disability and hearing loss. In this study, we identified novel COL27A1 compound heterozygous variants in two brothers with rhizomelia and congenital hip dislocation as well as dental and genital abnormalities that have not yet been reported in Steel syndrome. This variant, of maternal origin, caused an amino acid substitution of arginine for glycine, c.2026G>C or p.G676R, in the collagen helix domain, which is assumed to damage the structure of the helix. The paternally transmitted variant, c.2367G>A, is located at the 3′ end of exon 12, and cDNA analysis revealed a splicing alteration. These novel, compound heterozygous COL27A1 variants might indicate an association of the gene with tooth and genital abnormalities.