Molecular characterization of loss-of-function mutations in PCSK9 and identification of a compound heterozygote

Abstract

Elevated levels of circulating low-density lipoprotein cholesterol (LDL-C) play a central role in the development of atherosclerosis. Mutations in proprotein convertase subtilisin/kexin type 9 (PCSK9) that are associated with lower plasma levels of LDL-C confer protection from coronary heart disease. Here, we show that four severe loss-of-function mutations prevent the secretion of PCSK9 by disrupting synthesis or trafficking of the protein. In contrast to recombinant wild-type PCSK9, which was secreted from cells into the medium within 2 hours, the severe loss-of-function mutations in PCSK9 largely abolished PCSK9 secretion. This finding predicted that circulating levels of PCSK9 would be lower in individuals with the loss-of-function mutations. 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. This healthy, fertile college graduate, who was a compound heterozygote for two inactivating mutations in PCSK9, had a strikingly low plasma level of LDL-C (14 mg/dL). The very low plasma level of LDL-C and apparent good health of this individual demonstrate that PCSK9 plays a major role in determining plasma levels of LDL-C and provides an attractive target for LDL-lowering therapy.

Figures

Figure 1.

Effects of loss-of-function mutations on the synthesis and secretion of PCSK9. A, PCSK9, a protein of 692 aa that contains a signal sequence (SS), a 122-aa prodomain (Pro), a catalytic domain, and a C-terminal domain. The locations of the catalytic triad (D186, H226, and S386), oxyanion hole residue (N317), site of attachment of the N-linked sugar (533), and loss-of-function mutations– are shown. B, Expression of recombinant PCSK9 in HEK-293 cells. Wild-type (WT) and mutant forms of PCSK9 were expressed in HEK-293 cells, and immunoblotting was performed on the cells and medium with use of an anti-FLAG M2 mAb, as described in the “Material and Methods” section. This experiment was repeated three times, with similar results. Lane C = control; P = precursor; M = mature; S = secreted.

Figure 2.

Pulse-chase analysis of the synthesis, processing, and secretion of PCSK9-253F and PCSK9-679X. A, Cells expressing PCSK9-WT (WT) or mutant PCSK9 were incubated with 0.1 mCi/ml S35-Cys/Met for 10 min and then were chased for the indicated time periods. PCSK9 was immunoprecipitated from cell lysates and medium with use of antibody 6389 and then was subjected to 8% SDS-PAGE. Proteins were visualized by exposure to XOMAT film. B, PCSK9-WT (WT) and mutant forms of PCSK9 were expressed in HEK-293 cells, and immunoblotting was performed on the cells and medium as described in the figure 1 legend. P = precursor; M = mature.

Figure 3.

Effect of the C679X mutation on the maturation of N-linked sugars in PCSK9. Cell lysate and medium were collected from HEK-293 cells expressing PCSK9-WT (WT) and mutant PCSK9 and were treated with PNGaseF or EndoH as described in the “Material and Methods” section. Analysis of recombinant PCSK9 containing an alanine rather than asparagine at the site of attachment of the N-linked sugar was also performed. Deglycosylated samples were subjected to 10% SDS-PAGE, and immunoblotting was performed using anti-FLAG M2 mAb. P = precursor; M = mature; S = secreted.

Figure 4.

DTT reduction of PCSK9-679X and secretion of mutant forms of PCSK9. A, Aliquots of the cell lysates and medium from HEK-293 cells expressing wild-type (WT) or PCSK9-679X were treated with increasing concentrations of DTT at RT for 1 h. Samples were then subjected to 8% SDS-PAGE before immunoblotting. B, The highly conserved cysteine C679 (top left) was mutated (top right), and the constructs were expressed in HEK-293 cells (bottom). Immunoblotting was performed on the cells and medium as described in the figure 1 legend. Z. Fish = zebrafish; P = precursor; M = mature; S = secreted.

Figure 5.

Pedigree of a 32-year-old African American woman (II.2) who is compound heterozygous for loss-of-function mutations in PCSK9. A, The proband (I.2) of the family is a participant in the Dallas Heart Study who was found to be heterozygous for the Y142X allele in PCSK9. Fasting blood samples were obtained from additional family members. Plasma and serum were isolated, and the lipids and lipoprotein levels were measured using commercial reagents (table 1). The LDL-C and age- and sex-adjusted percentiles are provided for each family member. PCSK9 was immunoprecipitated from the plasma of selected family members with use of a polyclonal anti-PCSK9 antibody (295A), was size-fractionated by SDS-PAGE, and then was immunoblotted as described in the “Material and Methods” section. Individual II.3 was sampled, but the analysis of circulating PCSK9 was not performed on this subject. B, PCSK9-WT (WT) and mutant PCSK9 (ΔR97) were expressed in HEK-293 cells. After 2 d, the cell lysates and the medium were collected and subjected to immunoblotting as described in the figure 1 legend. NA = not available; P = precursor; M = mature; S = secreted.

Figure 6.

Structural model (A) and alignment of residues 31–450 of PCSK9 with fervidolysin (B). A, A model of PCSK9, generated using coordinates from the crystal structure of fervidolysin (Protein Database ID 1r6v). The N-terminal prodomain (purple) is connected to the catalytic domain (yellow) by a long, thickened coil. The prodomain cleavage site (color boundary) is near the catalytic triad (red). PCSK9 mutations that map to the prodomain or catalytic domain are labeled and numbered according to the human sequence and are depicted in blue (gain of function) or green (loss of function). Those mutations that reside in less accurately mapped regions of the protein are represented by a range of residues (see the “Material and Methods” section). Three predicted C-terminal domains of PCSK9 are represented as a space-fill that is based on the two fervidolysin C-terminal domains. B, An alignment between PCSK9 and the 1r6v N-terminal and catalytic domain sequences, generated with a fold-recognition program (Meta-BASIC). Compared with an alignment made by BLAST (not shown), the residues whose positions in the alignment were in agreement with those obtained using BLAST are in bold type, whereas those that differ are italicized. Noncapitalized residues in 1r6v represent positions where sequence was deleted. Residues with predicted secondary structure are red (helix), blue (strand), and black (coil) and are compared with the observed 1r6v secondary structure (cylinders and arrows shown above the alignment and colored as in panel A). Residues corresponding to the catalytic triad, the gain-of-function mutations, and the loss-of-function mutations are highlighted with the colors and ranges described for panel A.