Disease relevance of PCSK9

• Mutations in PCSK9 cause autosomal dominant hypercholesterolemia [1].
• 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 [2].
• Sequence variations in PCSK9, low LDL, and protection against coronary heart disease [3].
• Patients homozygous or compound heterozygous for LDLR mutations or double heterozygous for LDLR and apo B R3500Q mutation have higher LDL-C levels, more extensive xanthomatosis and more severe premature coronary disease (pCAD) than simple heterozygotes for mutations in either these genes or for missense mutations in PCSK9 gene [4].
• Proband L.R. and her sister (LDL-C 11.51 and 10.47 mmol/L, xanthomatosis and carotid atherosclerosis) were heterozygous for an LDLR mutation (p.Y419X) inherited from their mother (LDL-C 6.54 mmol/L) and a PCSK9 mutation (p.N425S) probably from their deceased father [4].
• Mutations of PCSK9 are associated either with hypercholesterolemia or with hypocholesterolemia [5].

Psychiatry related information on PCSK9

• No genetic association between PCSK9 polymorphisms and Alzheimer's disease and plasma cholesterol level in Japanese patients [6].

High impact information on PCSK9

• These mutations are probably gain-of-function mutations, as overexpression of PCSK9 in the liver of mice produces hypercholesterolemia by reducing LDLR number [7].
• Missense mutations in PCSK9, encoding a serine protease in the secretory pathway, also cause hypercholesterolemia [7].
• Low LDL cholesterol in individuals of African descent resulting from frequent nonsense mutations in PCSK9 [7].
• PCSK9 encodes NARC-1 (neural apoptosis regulated convertase), a newly identified human subtilase that is highly expressed in the liver and contributes to cholesterol homeostasis [1].
• Selected missense mutations in the proprotein convertase subtilisin/kexin type 9 serine protease gene (PCSK9) cause autosomal dominant hypercholesterolemia, whereas nonsense mutations in the same gene are associated with low plasma levels of low-density lipoprotein cholesterol (LDL-C) [8].

Chemical compound and disease context of PCSK9

• Proprotein convertase subtilisin/kexin type 9 (PCSK9) is at a locus for autosomal dominant hypercholesterolemia, and recent data indicate that the PCSK9 gene is involved in cholesterol biosynthesis [9].

Biological context of PCSK9

• Association studies with 93 noncoding single-nucleotide polymorphisms (SNPs) at the PCSK9 locus identified 3 SNPs associated with modest differences in plasma LDL-C levels [8].
• To understand the physiological role of PCSK9, we overexpressed human PCSK9 in mouse and cellular models as well as attenuated the endogenous expression of PCSK9 in HuH7 hepatoma cells using RNA interference [10].
• In this study, DNA sequencing of the 12 exons of the PCSK9 gene has been performed in 51 Norwegian subjects with a clinical diagnosis of familial hypercholesterolemia where mutations in the low-density lipoprotein receptor gene and mutation R3500Q in the apolipoprotein B-100 gene had been excluded [9].
• Rare dominant gain-of-function mutations in PCSK9 cosegregate with hypercholesterolemia, and one mutation is associated with a particularly severe FH phenotype [11].
• CONCLUSIONS: These results showed that the effect of the S127R mutation of PCSK9 on plasma cholesterol homeostasis is mainly related to an overproduction of apolipoprotein B100 [12].

Anatomical context of PCSK9

• Wild-type PCSK9 inhibits LDL clearance but does not affect apoB-containing lipoprotein production in mouse and cultured cells [10].
• Lifelong lowered LDL cholesterol associated with PCSK9 point mutations in 2-3% of individuals have been shown to provide marked protection from coronary artery disease (CAD) [13].
• In contrast, the loss-of-function mutations A443T and C679X lead either to the lack of trans-Golgi network/recycling endosome localization and an enhanced susceptibility to furin cleavage (A443T) or to the inability of PCSK9 to exit the endoplasmic reticulum (C679X) [14].
• The exact mechanism of action of PCSK9 is not clear, although it is known to enhance the intracellular degradation of the low density lipoprotein (LDL) receptor in acidic compartments, likely the endosomes/lysosomes [14].
• Because of the extremely low levels of PCSK9 mRNA in EBV-transformed lymphocytes, it is possible that the LDLR-dependent endocytosis of LDL could be more severely affected in hepatocytes from D374Y heterozygotes than in EBV-transformed lymphocytes [15].

Associations of PCSK9 with chemical compounds

• A spectrum of PCSK9 alleles contributes to plasma levels of low-density lipoprotein cholesterol [8].
• 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 [2].
• METHODS AND RESULTS: In vivo kinetics of apolipoprotein B100-containing lipoproteins using a 14-hour primed constant infusion of [2H3] leucine was conducted in 2 subjects carrying the mutation S127R in PCSK9, controls subjects, and FH subjects with known mutations on the low-density lipoprotein (LDL) receptor gene (LDL-R) [12].
• By cueing into the genomic basis for low serum LDL cholesterol levels, much work has been done to advance the importance of the serine protease PCSK9, which modulates LDL receptor function [13].
• The PCSK9-induced degradation of the LDLR was not affected by inhibitors of the proteasome, lysosomal cysteine proteases, aspartic acid proteases, or metalloproteases [16].
• This study demonstrates a more general effect of PCSK9 on the degradation of the LDLR family that emphasizes its major role in cholesterol and lipid homeostasis as well as brain development [17].
• The authors' findings suggest that evolutionary dynamics may underlie the 'gain-of-function' mutations in PCSK9 that are associated with higher LDL cholesterol levels [18].

Physical interactions of PCSK9

• LDL from PCSK9 patients competed significantly less well for binding to fibroblast LDL receptors than LDL from either controls or LDLR patients [19].
• Structure-function analyses demonstrated that the C-terminal cysteine-histidine-rich domain of PCSK9 interacts specifically with the N-terminal repeat R1 of AnxA2 [20].

Enzymatic interactions of PCSK9

• Furthermore, we report the presence of both native and furin-like cleaved forms of PCSK9 in circulating human plasma [14].

Regulatory relationships of PCSK9

• PCSK9 mutation dramatically increased the production rate of apolipoprotein B100 (3-fold) compared with controls or LDL-R mutated subjects, related to direct overproduction of VLDL (3-fold), IDL (3-fold), and LDL (5-fold) [12].
• In FAO-1 cells PCSK9 expression is downregulated by cholesterol and 25-hydroxycholesterol and upregulated in the absence of sterols following the same pattern of expression as HMG-CoA reductase, synthase, and LDLR [21].

Other interactions of PCSK9

• This pattern was not affected by the cholesterogenic transcription factor sterol regulatory element-binding protein-2, which normally up-regulates NARC-1/PCSK9 mRNA levels in liver [22].
• APOB (p.R3500Q) and PCSK9 (p.D374Y) were detected by specific assays [23].
• PCSK9 encodes proprotein convertase subtilisin/kexin type 9a (PCSK9), a member of the proteinase K subfamily of subtilases [24].
• Purified PCSK9 had no effect on cell-surface LDLRs in hepatocytes lacking autosomal recessive hypercholesterolemia (ARH), an adaptor protein required for endocytosis of the receptor [25].
• We conclude that PCSK9 acts primarily as a secreted factor to cause LDLR degradation [26].

Analytical, diagnostic and therapeutic context of PCSK9

• Immunoprecipitation and immunoblotting of plasma for PCSK9 provided direct evidence that the serine protease is present in the circulation and identified the first known individual who has no immunodetectable circulating PCSK9 [27].
• Subcellular fractionation, combined with Western blotting, showed that PCSK9 is localized in the ER and intermediate vesicular compartment of the cell but not in Golgi cisternae [21].
• Radiation hybrid mapping located four candidate genes at 1p34.1-p32, outside the critical region, showing no identity with FH3 [28].
• In situ hybridization and Northern blot analyses of NARC-1 expression during development in the adult and after partial hepatectomy revealed that it is expressed in cells that have the capacity to proliferate and differentiate [29].
• In this study we re-created the FH3 mutation (Asp154-->Asn) in exon 4 by site-directed mutagenesis and analyzed the expression of the mutant receptors in Chinese hamster ovary cells [30].

References