Targeted genetic testing for familial hypercholesterolaemia using next generation sequencing: a population-based study.

BACKGROUND: Familial hypercholesterolaemia (FH) is a common Mendelian condition which, untreated, results in premature coronary heart disease. An estimated 88% of FH cases are undiagnosed in the UK. We previously validated a method for FH mutation detection in a lipid clinic population using next generation sequencing (NGS), but this did not address the challenge of identifying index cases in primary care where most undiagnosed patients receive healthcare. Here, we evaluate the targeted use of NGS as a potential route to diagnosis of FH in a primary care population subset selected for hypercholesterolaemia. METHODS: We used microfluidics-based PCR amplification coupled with NGS and multiplex ligation-dependent probe amplification (MLPA) to detect mutations in LDLR, APOB and PCSK9 in three phenotypic groups within the Generation Scotland: Scottish Family Health Study including 193 individuals with high total cholesterol, 232 with moderately high total cholesterol despite cholesterol-lowering therapy, and 192 normocholesterolaemic controls. RESULTS: Pathogenic mutations were found in 2.1% of hypercholesterolaemic individuals, in 2.2% of subjects on cholesterol-lowering therapy and in 42% of their available first-degree relatives. In addition, variants of uncertain clinical significance (VUCS) were detected in 1.4% of the hypercholesterolaemic and cholesterol-lowering therapy groups. No pathogenic variants or VUCS were detected in controls. CONCLUSIONS: We demonstrated that population-based genetic testing using these protocols is able to deliver definitive molecular diagnoses of FH in individuals with high cholesterol or on cholesterol-lowering therapy. The lower cost and labour associated with NGS-based testing may increase the attractiveness of a population-based approach to FH detection compared to genetic testing with conventional sequencing. This could provide one route to increasing the present low percentage of FH cases with a genetic diagnosis.

The use of next-generation sequencing in clinical diagnosis of familial hypercholesterolemia.

PURPOSE: Familial hypercholesterolemia is a common Mendelian disorder associated with early-onset coronary heart disease that can be treated by cholesterol-lowering drugs. The majority of cases in the United Kingdom are currently without a molecular diagnosis, which is partly due to the cost and time associated with standard screening techniques. The main purpose of this study was to test the sensitivity and specificity of two next-generation sequencing protocols for genetic diagnosis of familial hypercholesterolemia. METHODS: Libraries were prepared for next-generation sequencing by two target enrichment protocols; one using the SureSelect Target Enrichment System and the other using the PCR-based Access Array platform. RESULTS: In the validation cohort, both protocols showed 100% specificity, whereas the sensitivity for short variant detection was 100% for the SureSelect Target Enrichment and 98% for the Access Array protocol. Large deletions/duplications were only detected using the SureSelect Target Enrichment protocol. In the prospective cohort, the mutation detection rate using the Access Array was highest in patients with clinically definite familial hypercholesterolemia (67%), followed by patients with possible familial hypercholesterolemia (26%). CONCLUSION: We have shown the potential of target enrichment methods combined with next-generation sequencing for molecular diagnosis of familial hypercholesterolemia. Adopting these assays for patients with suspected familial hypercholesterolemia could improve cost-effectiveness and increase the overall number of patients with a molecular diagnosis.

Premature senescence in cells from patients with autosomal recessive hypercholesterolemia (ARH): evidence for a role for ARH in mitosis.

OBJECTIVE: The defective gene causing autosomal recessive hypercholesterolemia (ARH) encodes ARH, a clathrin-associated adaptor protein required for low-density-lipoprotein receptor endocytosis in most cells but not in skin fibroblasts. The aim here was to elucidate why ARH fibroblasts grow slowly and undergo premature senescence. METHODS AND RESULTS: Knockdown of ARH by RNA interference in IMR90 cells produces the same phenotype, indicated by increased p16 expression, γ-H2AX-positive foci, and enlarged flattened morphology. We showed that ARH contributes to several aspects of mitosis: it localizes to mitotic microtubules, with lamin B1 on the nuclear envelope and spindle matrix, and with clathrin heavy chain on mitotic spindles. Second, ARH is phosphorylated in G(2)/M phase by a roscovitine-sensitive kinase, probably cdc2. Third, cells lacking ARH show disfigured nuclei and defective mitotic spindles. Defects are most marked in ARH W22X cells, where translation starts at Met46, so the protein lacks a phosphorylation site at Ser14, identified by mass spectrometry of wild-type ARH. CONCLUSIONS: The ARH protein is involved in cell cycle progression, possibly by affecting nuclear membrane formation through interaction with lamin B1 or other mitotic proteins, and its absence affects cell proliferation and induces premature senescence, which may play a role in the development of atherosclerosis in ARH.

Unexpected roles for PCSK9 in lipid metabolism.

PURPOSE OF REVIEW: To consider the evidence that PCSK9 has effects on lipoprotein metabolism that are in addition to its role in promoting the degradation of the LDL receptor. RECENT FINDINGS: Transgenic mice expressing human PCSK9 under physiological control have recently been described. As well as the expected effects on LDL-receptor protein levels in the liver, mice expressing the gain-of-function mutant D374Y secrete more triglyceride than control mice or mice expressing wild-type PCSK9, supporting earlier suggestions that apoB synthesis is increased in hepatocytes expressing D374Y PCSK9 and that patients heterozygous for PCSK9 mutations have increased apoB synthesis. No increase in triglyceride secretion was observed in LDLR mice, suggesting that the effect of PCSK9 on triglyceride secretion is to some extent independent of the LDL receptor. Other recent studies have shown an association between serum PCSK9 concentration and serum triglyceride, but care has to be taken in interpretation of these results as it has also been shown that the level of PCSK9 in human serum shows strong diurnal variation. SUMMARY: Understanding the physiology of PCSK9 is important because this protein has become a major new target for lipid lowering therapy.

Increased secretion of lipoproteins in transgenic mice expressing human D374Y PCSK9 under physiological genetic control.

OBJECTIVE: To produce transgenic mice expressing the D374Y variant of the human proprotein convertase subtilisin/kexin type 9 (PCSK9) gene at physiological levels to investigate the mechanisms causing hypercholesterolemia and accelerated atherosclerosis. METHODS AND RESULTS: A bacterial artificial chromosome containing PCSK9 and its flanking regions was modified to introduce the D374Y mutation and a C-terminal myc(2) tag. Transgenic mice that expressed 1 copy of the mutant or wild-type (WT) PCSK9 bacterial artificial chromosome were produced. Human PCSK9 mRNA was expressed at levels comparable to endogenous pcsk9 and with the same tissue specificity. The expression of D374Y or WT human PCSK9 increased the serum cholesterol level and reduced hepatic low-density lipoprotein receptor protein levels in the transgenic mice compared with bacterial artificial chromosome-negative controls; however, the effects were more marked in D374Y mice. The effect of a high-cholesterol diet on increasing serum cholesterol level was greater in D374Y mice, and atherosclerotic plaques after 15 weeks were more extensive in mice expressing D374Y than in WT PCSK9. D374Y mice secreted more triglyceride-rich lipoproteins into the circulation than WT mice. CONCLUSIONS: The expression of human D374Y PCSK9 at physiological levels produced a phenotype that closely matched that found in heterozygous D374Y patients and suggested that reduced low-density lipoprotein receptor activity is not the sole cause of their hypercholesterolemia.

Rare genetic causes of autosomal dominant or recessive hypercholesterolaemia.

Familial hypercholesterolaemia (FH) is a human inherited disorder of metabolism characterised by increased serum low-density lipoprotein (LDL) cholesterol. It is caused by defects in the LDL-receptor pathway that impair normal uptake and clearance of LDL by the liver. The commonest cause of FH is mutations in LDLR, the gene for the LDL receptor, but defects also occur in APOB that encodes its major protein ligand. More recently, defects in two other genes, LDLRAP1 and PCSK9, have been found in patients with FH and investigation of these has shed new light on the functioning and complexity of the LDL receptor pathway. Cells from patients with autosomal recessive hypercholesterolaemia (ARH) fail to internalise the LDL receptor because they carry two defective alleles of LDLRAP1, a gene that encodes a specific clathrin adaptor protein. PCSK9 encodes proprotein convertase subtilisin kexin type 9, a secreted protein that binds to the LDL receptor and promotes its degradation. Gain-of function mutations in PCSK9 are autosomal dominant and cause hypercholesterolaemia because they increase the affinity of PCSK9 protein for the LDL receptor, whereas loss-of-function mutations reduce serum cholesterol because LDL-receptor protein is exposed to reduced PCSK9-mediated degradation. Thus, PCSK9 has become a new target for cholesterol-lowering drug therapy.

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.

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.

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.

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.

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.

Improved cardiovascular outcomes following temporal advances in lipid-lowering therapy in a genetically-characterised cohort of familial hypercholesterolaemia homozygotes.

BACKGROUND AND AIMS: There is a paucity of data concerning the influence of lipid-lowering therapy on cardiovascular (CV) outcomes in patients with homozygous familial hypercholesterolaemia (FH). To redress this a retrospective analysis was undertaken of the demographic features, lipid levels, low density lipoprotein receptor and Autosomal Recessive Hypercholesterolaemia gene mutations, CV outcomes and vital status of 44 FH homozygotes referred to a single centre in the UK between 1964 and 2014. METHODS: Data were obtained from past publications, case records and death certificates. Differences in categorical and continuous variables between living and dead patients were analysed using Fisher's exact test and an independent t-test respectively. RESULTS: During the 50 years covered by this survey 13 patients have died, 30 are still alive and 1 was lost to follow up. The mean age of Alive patients was 32.6 ± 11.5 versus 28.3 ± 14.9 years in Dead ones (P = 0.31) and they were born 18 years later (P = 0.0001). Pre-treatment serum total cholesterol (TC) was similar in Alive and Dead (20.2 ± 5.1 v 21.3 ± 4.4 mmol/l, P = 0.52) but on-treatment TC was lower in Alive than Dead (8.1 ± 2.8 v 14.5 ± 6.0 mmol/l, P = 0.0001) and CV adverse events were far less frequent (eg aortic stenosis, 33% v 77%, P = 0.02). CONCLUSIONS: The lower on-treatment TC and fewer CV adverse events in FH homozygotes still living reflect advances in apheresis and drug therapy since the 1990s. Further improvements in prognosis can be expected with the impending introduction of novel lipid-lowering agents.

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.

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.

Identification and biochemical analysis of a novel APOB mutation that causes autosomal dominant hypercholesterolemia.

Patients with autosomal dominant hypercholesterolemia (ADH) have a high risk of developing cardiovascular disease that can be effectively treated using statin drugs. Molecular diagnosis and family cascade screening is recommended for early identification of individuals at risk, but up to 40% of families have no mutation detected in known genes. This study combined linkage analysis and exome sequencing to identify a novel variant in exon 3 of APOB (Arg50Trp). Mass spectrometry established that low-density lipoprotein (LDL) containing Arg50Trp APOB accumulates in the circulation of affected individuals, suggesting defective hepatic uptake. Previously reported mutations in APOB causing ADH have been located in exon 26. This is the first report of a mutation outside this region causing this phenotype, therefore, more extensive screening of this large and highly polymorphic gene may be necessary in ADH families. This is now feasible due to the high capacity of recently available sequencing platforms.

Degradation of LDLR protein mediated by 'gain of function' PCSK9 mutants in normal and ARH cells.

Dominant gain-of-function mutations in proprotein convertase subtilisin kexin type 9 (PCSK9) cause familial hypercholesterolaemia (FH) and result in accelerated atherosclerosis and premature coronary heart disease. It is believed that PCSK9 binds to LDL-receptor (LDLR) protein and prevents its recycling to the cell surface; gain-of-function PCSK9 mutants enhance LDLR degradation. Several new variants of PCSK9 have been identified, but their effect on PCSK9 activity has not been determined. We describe a new procedure for assessing the activity of four putative gain-of-function mutations identified in FH patients (D129N, D374H, N425S, R496W). All four mutant proteins were secreted normally from transfected HEK293T cells. Immortalized lymphocytes from normolipaemic controls were incubated with conditioned medium from transfected cells and cell-surface LDLR protein was determined by FACS. D374H was as potent as D374Y in reducing cell-surface LDLR, while the other three mutations were more potent than wild type, but less so than the D374 mutants; this correlated with total serum cholesterol in the patients. Substitution of different amino acids at 374 showed that aspartate in this position was critical; even glutamate at residue 374 increased LDLR degradation. When the assay was carried out with ARH-negative lymphocytes that are unable to internalise the LDLR, D374Y-PCSK9 was able to reduce cell-surface LDLR by 35%, compared with approximately 70% for normal lymphocytes. Thus, PCSK9-mediated LDLR degradation is not entirely dependent on ARH function. We propose a novel ARH-independent pathway for PCSK9 activity on LDLR.

Effects of ezetimibe and/or simvastatin on LDL receptor protein expression and on LDL receptor and HMG-CoA reductase gene expression: a randomized trial in healthy men.

OBJECTIVE: The combination of simvastatin, an HMG-CoA reductase inhibitor, and ezetimibe, an inhibitor of Niemann-Pick C1-like 1 protein, decreases cholesterol synthesis and absorption and reduces circulating LDL-cholesterol concentrations. The molecular mechanisms underlying the pronounced lipid-lowering effects of this combination have not been fully elucidated in humans. METHODS AND RESULTS: One center, prospective, randomized, parallel three-group study in 72 healthy men (mean age 32+/-9 years, mean body mass index 25.7+/-3.2 kg/m(2)). Each group of twenty-four subjects received a 14-day treatment with either ezetimibe (10mg/day), simvastatin (40 mg/day) or their combination. Lipid levels, the ratio of non-cholesterol sterols to cholesterol concentrations (used as markers of cholesterol synthesis and absorption), cell surface LDL receptor (LDLR) protein as well as LDLR and HMG-CoA reductase gene expression in mononuclear blood cells were measured at baseline and at the end of the study. LDL-C decreased in all groups. Simvastatin decreased, ezetimibe increased and their combination had no effect on HMG-CoA reductase activity. Simvastatin and the combination of ezetimibe and simvastatin increased the HMG-CoA reductase and LDLR gene expression while ezetimibe had no effect. The cell surface LDLR protein expression remained unchanged in all groups. The combination of ezetimibe and simvastatin increased the expression of the serine protease proprotein convertase subtilisin/kexin 9 (PCSK9), an enzyme shown to down-regulate LDLR protein levels. CONCLUSIONS: The co-administration of ezetimibe and simvastatin abrogates the ezetimibe-induced increase in cholesterol synthesis and up-regulates the LDLR gene but not protein expression, an effect possibly mediated through a parallel upregulation of PCSK9 expression.

Mechanisms of disease: genetic causes of familial hypercholesterolemia.

Familial hypercholesterolemia (FH) is characterized by raised serum LDL cholesterol levels, which result in excess deposition of cholesterol in tissues, leading to accelerated atherosclerosis and increased risk of premature coronary heart disease. FH results from defects in the hepatic uptake and degradation of LDL via the LDL-receptor pathway, commonly caused by a loss-of-function mutation in the LDL-receptor gene (LDLR) or by a mutation in the gene encoding apolipoprotein B (APOB). FH is primarily an autosomal dominant disorder with a gene-dosage effect. An autosomal recessive form of FH caused by loss-of-function mutations in LDLRAP1, which encodes a protein required for clathrin-mediated internalization of the LDL receptor by liver cells, has also been documented. The most recent addition to the database of genes in which defects cause FH is one encoding a member of the proprotein convertase family, PCSK9. Rare dominant gain-of-function mutations in PCSK9 cosegregate with hypercholesterolemia, and one mutation is associated with a particularly severe FH phenotype. Expression of PCSK9 normally downregulates the LDL-receptor pathway by indirectly causing degradation of LDL-receptor protein, and loss-of-function mutations in PCSK9 result in low plasma LDL levels. Thus, PCSK9 is an attractive target for new drugs aimed at lowering serum LDL cholesterol, which should have additive lipid-lowering effects to the statins currently used.

Adaptor protein disabled-2 modulates low density lipoprotein receptor synthesis in fibroblasts from patients with autosomal recessive hypercholesterolaemia.

Autosomal recessive hypercholesterolaemia (ARH), characterized clinically by severe inherited hypercholesterolaemia, is caused by recessive null mutations in LDLRAP1 (formerly ARH). Immortalized lymphocytes and monocyte-macrophages, and presumably hepatocytes, from ARH patients fail to take up and degrade plasma low density lipoproteins (LDL) because they lack LDLRAP1, a cargo-specific adaptor required for clathrin-mediated endocytosis of the LDL receptor. Surprisingly, LDL-receptor function is normal in ARH patients' skin fibroblasts in culture. Disabled-2 (Dab2) has been implicated previously in clathrin-mediated internalization of LDL-receptor family members, and we show here that Dab2 is highly expressed in skin fibroblasts, but not in lymphocytes. SiRNA-depletion of Dab2 profoundly reduced LDL-receptor activity in ARH fibroblasts as a result of profound reduction in LDL-receptor protein, but not mRNA; heterologous expression of murine Dab2 reversed this effect. In contrast, LDL-receptor protein content was unchanged in Dab-2-depleted control cells. Incorporation of 35S-labelled amino acids into LDL receptor protein revealed a corresponding apparent reduction in accumulation of newly synthesized LDL-receptor protein on depletion of Dab2 in ARH, but not in control, cells. This reduction in LDL-receptor protein in Dab2-depleted ARH cells could not be reversed by treatment of the cells with proteasomal or lysosomal inhibitors. Thus, we propose a novel role for Dab2 in ARH fibroblasts, where it is apparently required to allow normal translation of LDL receptor mRNA.

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.