Biomedical subjects

Anne K Soutar

Publications and source records attributed to Anne K Soutar.

13 recordsLinked to original sources

PubMed2006

Genetic defects causing familial hypercholesterolaemia: identification of deletions and duplications in the LDL-receptor gene and summary of all mutations found in patients attending the Hammersmith Hospital Lipid Clinic.

Familial hypercholesterolaemia (FH) results from defective catabolism of low density lipoproteins (LDL), leading to premature atherosclerosis and early coronary heart disease. It is commonly caused by mutations in LDLR, encoding the LDL receptor that mediates hepatic uptake of LDL, or in APOB, encoding its major ligand. More rarely, dominant mutations in PCSK9 or recessive mutations in LDLRAP1 (ARH) cause FH, gene defects that also affect the LDL-receptor pathway. We have used multiplex ligation-dependent probe amplification (MLPA) to identify deletions and rearrangements in LDLR, some not detectable by Southern blotting, thus completing our screening for mutations causing FH in a group of FH patients referred to a Lipid Clinic in London. To summarise, mutations in LDLR were found in 153 unrelated heterozygous FH patients and 24 homozygotes/compound heterozygotes, and in over 200 relatives of 80 index patients. LDLR mutations included 85 different point mutations (7 not previously described) and 13 different large rearrangements. The APOB R3500Q mutation was present in 14 heterozygous patients and a mutation in PCSK9 in another 4; LDLRAP1 mutations were found in 4 "homozygous" FH patients. Our data confirm that DNA-based diagnosis provides information that is important for management of FH in a considerable number of families.

Apolipoproteins B↗

PubMed2006

A single point mutation in the low-density lipoprotein receptor switches the degradation of its mature protein from the proteasome to the lysosome.

Pathogenic mutations in the low-density lipoprotein receptor prevent cholesterol uptake and cause familial hypercholesterolemia. In comparison to the biogenesis and endocytic trafficking of this receptor and some of its mutants, their degradation mechanisms are not well understood. Therefore, to gain some insights into this aspect, we analyzed the effects of proteasomal and lysosomal inhibitors on the levels of the wild type low-density lipoprotein receptor and a mutant form, C358Y, which was prevalent in a sample of Spanish familial hypercholesterolemia patients. In transfected cells, the mutant C358Y exhibited lower activity than the wild type receptor, as well as retarded post-translational processing of its precursor to the mature form. Interestingly, about 30% of the mutant precursor was degraded by a lysosomal pathway. Moreover, its mature form was more rapidly degraded than the wild type receptor (half lives of 5.3 and 10.9 h, respectively) and its degradation was exclusively dependent on a lysosomal pathway. In contrast, the mature form of the wild type receptor was mainly degraded by proteasomes and, to a minor extent (30%), by lysosomes. We conclude that a single mutation in the low-density lipoprotein receptor switches the degradation of the mature receptor from a proteasomal to a lysosomal pathway which degrades the protein at a faster rate. This suggests cooperation of proteasomes and lysosomes in the degradation of the low-density lipoprotein receptor and adds an intriguing new aspect to our understanding of receptor-mediated endocytosis.

Acetylcysteine↗

PubMed2005

Severe hypercholesterolemia in four British families with the D374Y mutation in the PCSK9 gene: long-term follow-up and treatment response.

OBJECTIVE: Analysis of long-term (30 years) clinical history and response to treatment of 13 patients with the D374Y mutation of PCSK9 (PCSK9 patients) from 4 unrelated white British families compared with 36 white British patients with heterozygous familial hypercholesterolemia attributable to 3 specific mutations in the low-density lipoprotein (LDL) receptor gene (LDLR) known to cause severe phenotype. METHODS AND RESULTS: The PCSK9 patients, when compared with the LDLR patients, were younger at presentation (20.8+/-14.7 versus 30.2+/-15.7 years; P=0.003), had higher pretreatment serum cholesterol levels (13.6+/-2.9 versus 9.6+/-1.6 mmol/L; P=0.004) that remained higher during treatment with simvastatin (10.1+/-3.0 versus 6.5+/-0.9 mmol/L; P=0.006), atorvastatin (9.6+/-2.9 versus 6.4+/-1.0 mmol/L; P=0.006), or current lipid-lowering therapy, including LDL apheresis and partial ileal bypass in 2 PCSK9 patients (7.0+/-1.6 versus 5.4+/-1.0 mmol/L; P=0.001), and were affected >10 years earlier by premature coronary artery disease (35.2+/-4.8 versus 46.8+/-8.9 years; P=0.002). LDL from PCSK9 patients competed significantly less well for binding to fibroblast LDL receptors than LDL from either controls or LDLR patients. CONCLUSIONS: These British PCSK9 patients with the D374Y mutation have an unpredictably severe clinical phenotype, which may be a unique feature for this cohort, and requires early and aggressive lipid-lowering management to prevent cardiovascular complications.

Adolescent↗

PubMed2005

Evidence for effect of mutant PCSK9 on apolipoprotein B secretion as the cause of unusually severe dominant hypercholesterolaemia.

Typically, autosomal dominant familial hypercholesterolaemia (FH) is caused by mutations in the low density lipoprotein (LDL) receptor or apolipoprotein B genes that result in defective clearance of plasma LDL by the liver, but a third gene (PCSK9), encoding a putative proprotein convertase, has recently been implicated. Two independent microarray studies support a role for PCSK9 in sterol metabolism and adenoviral-mediated over-expression of PCSK9 in mouse liver depletes hepatic LDL-receptor protein, but the mechanism by which dominant mutations cause human FH is unclear. We have identified the D374Y mutant of PCSK9 in three FH families of English origin; all 12 affected individuals have unusually severe hypercholesterolaemia and require more stringent treatment than typical FH patients, who are heterozygous for defects in the LDL receptor. We have stably expressed wild-type (WT) and variant PCSK9 in McArdle-7777 rat hepatoma cells and shown by confocal microscopy that all forms of PCSK9 co-localize with protein disulphide isomerase in the ER whether or not they can be autocleaved. Expression of the proposed pathogenic variants, but not of WT, S386A or F216L PCSK9, increases secretion of apolipoprotein B100-containing lipoproteins from the cells by 2-4-fold probably by reducing the degradation of nascent protein; no differences in LDL-receptor content were observed in cells expressing WT, S386A or F216L PCSK9 and only a small reduction in cells expressing the D374Y or S127R mutants. This suggests that the variants of PCSK9 found in FH influence the secretion of apoB-containing lipoproteins, providing an explanation for the marked increase in circulating LDL in heterozygous carriers.

Adult↗

PubMed2004

Two novel missense mutations in ABCA1 result in altered trafficking and cause severe autosomal recessive HDL deficiency.

Extremely low concentrations of high density lipoprotein (HDL)-cholesterol and apolipoprotein (apo) AI are features of Tangier disease caused by autosomal recessive mutations in ATP-binding cassette transporter A1 (ABCA1). Less deleterious, but dominantly inherited mutations cause HDL deficiency. We investigated causes of severe HDL deficiency in a 42-year-old female with progressive coronary disease. ApoAI-mediated efflux of cholesterol from the proband's fibroblasts was less than 10% of normal and nucleotide sequencing revealed inheritance of two novel mutations in ABCAI, V1704D and L1379F. ABCA1 mRNA was approximately 3-fold higher in the proband's cells than in control cells; preincubation with cholesterol increased it 5-fold in control and 8-fold in the proband's cells, but similar amounts of ABCA1 protein were present in control and mutant cells. When transiently transfected into HEK293 cells, confocal microscopy revealed that both mutant proteins were retained in the endoplasmic reticulum, while wild-type ABCA1 was located at the plasma membrane. Severe HDL deficiency in the proband was caused by two novel autosomal recessive mutations in ABCA1, one (V1704D) predicted to lie in a transmembrane segment and the other (L1379F) in a large extracellular loop. Both mutations prevent normal trafficking of ABCA1, thereby explaining their inability to mediate apoA1-dependent lipid efflux.

ATP Binding Cassette Transporter 1↗

PubMed2004

Sorting motifs in the intracellular domain of the low density lipoprotein receptor interact with a novel domain of sorting nexin-17.

The low density lipoprotein (LDL) receptor plays a major role in maintaining human plasma cholesterol levels and mutations in the gene cause familial hypercholesterolemia. The LDL receptor (LDLR) pathway has been well characterized, but little is known of proteins involved in its complex intracellular sorting and trafficking. Sorting nexin 17 (SNX17) has recently been implicated in LDLR intracellular trafficking. We show here that endogenous SNX17 is highly expressed in several cell types and is localized partially in early endosomes. We found that the PX domain of SNX17 is required for its endosomal localization but does not interact directly with the LDL receptor. A novel domain containing a FERM-like domain of SNX17 is needed for its interaction with the LDL receptor. Mutations in the NPXY motif of the LDL-receptor cytoplasmic tail that disrupt internalization also disrupt its interaction with SNX17, whereas mutations elsewhere had little effect. When transiently overexpressed in Chinese hamster ovary cells, SNX17 localized to large vesicular structures and disrupted normal trafficking of the LDL receptor in a PX domain-dependent manner. These results suggest that SNX17 plays a role in the cellular trafficking of the LDL receptor through interaction with the NPVY motif in its cytoplasmic domain and interaction of the PX domain with subcellular membrane compartments.

Amino Acid Motifs↗

PubMed2004

Genetic diagnosis of familial hypercholesterolaemia: a mutation and a rare non-pathogenic amino acid variant in the same family.

Familial hypercholesterolaemia (FH), a relatively common inherited disorder, is caused by mutations in the gene for the low density lipoprotein (LDL) receptor (LDLR) that result in impaired clearance of LDL. Identification of mutations in patients with the clinical phenotype of FH allows unequivocal diagnosis in potentially affected relatives, but depends critically on distinguishing mutations that affect protein function from variants with no significant effect. A presumed functional mutation in LDLR (G198D in exon 4) was identified in two hypercholesterolaemic English brothers by high throughput screening and was not found in 550 controls. However, a second variant (L458P) was identified separately in their mother that co-segregated with hypercholesterolaemia in the entire pedigree. L458, but not G198, is strongly conserved between species and lies in a region important for beta-propeller stability. G198D was inherited from their normolipidaemic father by two of three siblings heterozygous for L458P; they appeared less severely hypercholesterolaemic and more responsive to statins than the third affected brother and their mother. This study emphasises that apparent co-segregation of an amino acid substitution in a critical region of the protein with hypercholesterolaemia and its absence from a large control population is insufficient evidence that a variant of the LDL receptor is necessarily deleterious to its function.

Adolescent↗

PubMed2004

Autosomal recessive hypercholesterolaemia: long-term follow up and response to treatment.

Autosomal recessive hypercholesterolaemia (ARH) is caused by mutations in ARH on chromosome 1p35-36, encoding a putative adaptor protein. Mutations in the gene prevent normal internalisation of the low density lipoprotein (LDL) receptor by cultured lymphocytes and monocyte-derived macrophages, but not skin fibroblasts. This newly identified disorder is characterised by severe hypercholesterolaemia, large tendon, tuberous and planar xanthomas and premature atherosclerosis. We describe long-term (9-23 years) follow up and response to treatment of eight subjects with ARH from four families (Turkish/Lebanese, Indian-Asian, English and Italian). The clinical phenotype of ARH is similar to that of classical homozygous familial hypercholesterolaemia (FH) caused by mutations in the LDL-receptor gene but is more variable, less severe and is more responsive to lipid-lowering therapy with bile acid sequestrants and/or HMG-CoA reductase inhibitors. The latter reduced total serum cholesterol by up to 60% and the former by 20-35%. The cardiovascular complications of premature atherosclerosis seem to be delayed in some individuals and the involvement of the aortic root and valve are rarer in comparison with homozygous FH.

Adult↗

PubMed2004

Autosomal recessive hypercholesterolemia.

Autosomal recessive hypercholesterolemia (ARH) presents with a clinical phenotype similar to that of classical homozygous familial hypercholesterolemia (FH) caused by defects in the low-density lipoprotein (LDL) receptor gene but is more variable, generally less severe, and more responsive to lipid-lowering therapy than homozygous FH; furthermore, FH is inherited with a dominant pattern. The approximately 50 known affected ARH individuals are mostly of Sardinian or Middle Eastern origin, but rare cases of ARH have occurred worldwide. The physiological defect in ARH is a failure of some, but not all, cell types to mediate LDL receptor-dependent internalization of LDL and is caused by mutations in the gene for a putative adaptor protein called ARH. In affected cells, the LDL receptor gene is normal but LDL receptor protein accumulates at the cell surface; this also occurs in livers of recombinant mice lacking ARH, providing an explanation for the failure of clearance of LDL from plasma in ARH patients. The structural features of the ARH protein and its capacity to interact with the internalization sequence of the LDL receptor, plasma membrane phospholipids, and the clathrin endocytic machinery suggest that it plays a key role in the LDL receptor pathway.

Adaptor Proteins, Signal Transducing↗

PubMed2004

Current management of severe homozygous hypercholesterolaemias.

PURPOSE OF REVIEW: This review focuses on recent advances in the management of patients with homozygous familial hypercholesterolaemia, autosomal recessive hypercholesterolaemia and familial defective apolipoprotein B. RECENT FINDINGS: Autosomal recessive hypercholesterolaemia has been described as a 'phenocopy' of homozygous familial hypercholesterolaemia. Although the clinical phenotypes are similar, autosomal recessive hypercholesterolaemia seems to be less severe, more variable within a single family, and more responsive to lipid-lowering drug therapy. The cardiovascular complications of premature atherosclerosis are delayed in some individuals and involvement of the aortic root and valve is less common than in homozygous familial hypercholesterolaemia. Apheresis is still the treatment of choice in homozygous familial hypercholesterolaemia and in autosomal recessive hypercholesterolaemia patients in whom maximal drug therapy does not achieve adequate control. In addition to the profound cholesterol-lowering effects of apheresis, other potentially beneficial phenomena have been documented: improved vascular endothelial function and haemorheology, reduction in lipoprotein (a) and procoagulatory status, and a decrease in adhesion molecules and C-reactive protein. SUMMARY: Patients with severe homozygous hypercholesterolaemia illustrate the natural history of atherosclerosis within a condensed timeframe. Effective cholesterol-lowering treatment started in early childhood is essential to prevent onset of life-threatening atherosclerotic involvement of the aortic root and valve, and the coronary arteries. Noninvasive methods for regular monitoring of the major sites involved in the atherosclerotic process are necessary in patients with no symptoms or signs of ischaemia. Management of patients with severe homozygous hypercholesterolaemia continues to be a major challenge.

Adolescent↗

PubMed2003

Genetics, clinical phenotype, and molecular cell biology of autosomal recessive hypercholesterolemia.

The recent characterization of a rare genetic defect causing autosomal recessive hypercholesterolemia (ARH) has provided new insights into the underlying mechanism of clathrin-mediated internalization of the LDL receptor. Mutations in ARH on chromosome 1p35-36.1 prevent normal internalization of the LDL receptor by cultured lymphocytes and monocyte-derived macrophages but not by skin fibroblasts. In affected cells, LDL receptor protein accumulates at the cell surface; this also occurs in the livers of recombinant mice lacking ARH, thereby providing an explanation for the failure of clearance of LDL from the plasma in subjects lacking ARH. The approximately 50 known affected individuals are mostly of Sardinian or Middle Eastern origin. The clinical phenotype of ARH is similar to that of classic homozygous familial hypercholesterolemia caused by defects in the LDL receptor gene, but it is more variable, generally less severe, and more responsive to lipid-lowering therapy. Structural features of the ARH protein and its capacity to interact simultaneously with the internalization sequence of the LDL receptor, plasma membrane phospholipids, and the clathrin endocytic machinery suggest how ARH can play a pivotal role in gathering the LDL receptor into forming endocytic carrier vesicles.

Cholesterol, LDL↗

PubMed2003

The transmembrane domain and PXXP motifs of ApoE receptor 2 exclude it from carrying out clathrin-mediated endocytosis.

The low density lipoprotein (LDL) receptor family comprises several proteins with similar structures including the LDL receptor and apoE receptor 2 (apoER2). The human brain expresses two major splice variants of apoER2 mRNA, one of which includes an additional exon that encodes 59 residues in the cytoplasmic domain. This exon is absent from the LDL receptor and contains three proline-rich (PXXP) motifs that may allow apoER2 to function as a signal transducer. To investigate the role of this insert, we took advantage of the well characterized low density lipoprotein receptor pathway. Chimeras comprising the ectodomain and transmembrane domain of the LDL receptor fused to the cytoplasmic domain of apoER2 lacking the PXXP-containing residues are able to mediate clathrin-dependent endocytosis of LDL as effectively as cells expressing the LDL receptor but not if the PXXP insert is present in the protein. Although expressed on the cell surface, the PXXP-containing chimeric receptor is excluded from clathrin vesicles as judged by its failure to co-localize with adaptor protein-2 possibly due to interaction with intracellular adaptors or scaffolding proteins. Chimeras with the transmembrane domain of apoER2, predicted to be longer than that of the LDL receptor by several residues, fail to mediate endocytosis of LDL or to co-localize with adaptor protein-2 regardless of the presence or absence of the PXXP insert. Thus features of apoER2 that distinguish it as a signaling receptor, rather than as an endocytosis receptor like the LDL receptor, reside in or near the transmembrane domain and in the proline-rich motifs.

Amino Acid Motifs↗

PubMed2002

Restoration of LDL receptor function in cells from patients with autosomal recessive hypercholesterolemia by retroviral expression of ARH1.

Familial hypercholesterolemia is an autosomal dominant disorder with a gene-dosage effect that is usually caused by mutations in the LDL receptor gene that disrupt normal clearance of LDL. In the homozygous form, it results in a distinctive clinical phenotype, characterized by inherited hypercholesterolemia, cholesterol deposition in tendons, and severe premature coronary disease. We described previously two families with autosomal recessive hypercholesterolemia that is not due to mutations in the LDL receptor gene but is characterized by defective LDL receptor-dependent internalization and degradation of LDL by transformed lymphocytes from the patients. We mapped the defective gene to chromosome 1p36 and now show that the disorder in these and a third English family is due to novel mutations in ARH1, a newly identified gene encoding an adaptor-like protein. Cultured skin fibroblasts from affected individuals exhibit normal LDL receptor activity, but their monocyte-derived macrophages are similar to transformed lymphocytes, being unable to internalize and degrade LDL. Retroviral expression of normal human ARH1 restores LDL receptor internalization in transformed lymphocytes from an affected individual, as demonstrated by uptake and degradation of (125)I-labeled LDL and confocal microscopy of cells labeled with anti-LDL-receptor Ab.

Adaptor Proteins, Signal Transducing↗