Abstract 55: Disruption of PCSK9 Using a Targeted Base Editing Strategy

Proprotein convertase subtilisin/kexin type 9 (PCSK9) regulates low density lipoprotein receptor protein levels by diverting it to lysosomes. Monoclonal antibody therapeutics aimed to neutralize PCSK9 have been shown to successfully lower serum LDL levels; however, we previously found that such therapeutic antibodies are subject to PCSK9-mediated clearance. In this study, we discovered that PCSK9 interacts via its C-terminal domain directly and in a pH-dependent manner with amyloid precursor protein as well as its closely related family member, amyloid precursor protein-like protein 2. Furthermore, we determined that amyloid precursor protein-like protein-2, but not amyloid precursor protein, is involved in mediating postendocytic delivery of PCSK9 to lysosomes and is therefore important for PCSK9 function. Based on our data, we propose a model for a lysosomal transport complex by which a soluble protein can target another protein for degradation from the luminal side of the membrane by bridging it to a lysosomally targeted transmembrane protein.

Proprotein convertase subtilisin kexin type 9 (PCSK9) is in the focus of cardiovascular research due to its role in hepatic low density lipoprotein (LDL) clearance. However, extrahepatic expression of PCSK9 such as in cardiomyocytes and its regulation by oxidized LDL (oxLDL) put notion on extrahepatic effects of PCSK9 as well. This study was aimed to reveal the role of PCSK9 in oxLDL-dependent regulation of cardiomyocyte function. Adult rat and mouse ventricular cardiomyocytes and isolated perfused hearts were used. OxLDL was applied to increase PCSK9 expression in cardiomyocytes. Cell function was analyzed by load-free cell shortening as well as left ventricular developed pressure of isolated hearts. OxLDL decreased shortening in wild-type-derived mouse cardiomyocytes but not in those isolated from PCSK9 knockout mice. Overexpression of human PCSK9 in rat cardiomyocytes reduced shortening in the absence of oxLDL. Addition of recombinant PCSK9 mimicked these effects. In cardiomyocytes, oxLDL induced PCSK9 release into the supernatant. Inhibition of PCSK9 by Pep 2–8 or alirocumab attenuated the oxLDL-induced loss of cardiomyocyte shortening. Cardiomyocytes express surfeit locus protein 4 (SURF-4), a protein required for PCSK9 secretion in human embryonic kidney cells (HEK 293 T), and silencing of SURF-4 reduced the oxLDL effects on cardiomyocytes. In isolated perfused rat hearts PCSK9 inhibition by alirocumab improved the function. In addition, left ventricular function of isolated hearts from PCSK9 knockout mice was increased under basal conditions as well as at 10 min and 120 min of reperfusion following 45 min of ischemia. Collectively, the data show that cardiomyocytes express and release PCSK9 that acts in an autocrine way on cardiomyocytes and impairs their function.

The Framingham Heart Study has contributed importantly to understanding of the causes of coronary heart disease (CHD), stroke, and other cardiovascular diseases. Framingham research has helped define the quantitative and additive nature of these causes or, as they are now called, “cardiovascular risk factors.”1 The National Cholesterol Education Program (NCEP)2 3 has made extensive use of Framingham data in developing its strategy for preventing CHD by controlling high cholesterol levels. The NCEP guidelines2 3 adjust the intensity of cholesterol-lowering therapy with absolute risk as determined by summation of risk factors. The National High Blood Pressure Education Program (NHBPEP) has set forth a parallel approach for blood pressure control. In contrast to the NCEP,2 however, earlier NHBPEP reports issued through the Joint National Committee4 did not match the intensity of therapy to absolute risk for CHD. “Normalization” of blood pressure is the essential goal of therapy regardless of risk status. Blood pressure–lowering therapy is carried out as much for prevention of stroke and other cardiovascular complications as for reduction of CHD risk. Nonetheless, risk assessment could be important for making decisions about type and intensity of therapy for hypertension. Thus, the most recent Joint National Committee report5 gives more attention to risk stratification for adjustment of therapy for hypertension. Although Framingham data have already been influential in the development of national guidelines for risk factor management, the opportunity may exist for both cholesterol and blood pressure programs to draw more extensively from Framingham results when formulating improved risk assessment guidelines and recommending more specific strategies for risk factor modification. The American Heart Association has previously used Framingham risk factor data to prepare charts for estimating CHD risk. Framingham investigators of the National Heart, Lung, and Blood Institute prepared the original charts and have now revised …

The effects of lipid-lowering drugs [including statins, ezetimibe, and proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors] on hyperlipidaemia have been established. Some may have treatment effects beyond their reported properties, offering potential opportunities for drug repurposing. Epidemiological studies have reported conflicting findings on the relationship between lipid-lowering medication use and sarcopenia risk. We performed a two-sample Mendelian randomization (MR) study to investigate the causal association between the use of genetically proxied lipid-lowering drugs (including statins, ezetimibe, and PCSK9 inhibitors, which use low-density lipoprotein as a biomarker), and sarcopenia risk. The inverse-variance weighting method was used with pleiotropy-robust methods (MR-Egger regression and weighted median) and colocalization as sensitivity analyses. According to the positive control analysis, genetically proxied inhibition in lipid-lowering drug targets was associated with a lower risk of coronary heart disease [PCSK9 (OR, 0.67; 95% CI, 0.61 to 0.72; P=7.7E-21); 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR; OR, 0.68; 95% CI, 0.57 to 0.82; P=4.6E-05), and Niemann-Pick C1-like 1 (NPC1L1; OR, 0.53; 95% CI, 0.40 to 0.69; P=3.3E-06)], consistent with drug mechanistic actions and previous trial evidence. Genetically proxied inhibition of PCSK9 (beta, -0.040; 95% CI, -0.068 to -0.012; P=0.005) and circulating PCSK9 levels (beta, -0.019; 95% CI, -0.033 to -0.005; P=0.006) were associated with reduced appendicular lean mass (ALM) with concordant estimates in terms of direction and magnitude. Validation analyses using a second instrument for PCSK9 yielded consistent results in terms of direction and magnitude [(PCSK9 to ALM; beta, -0.052; 95% CI, -0.074 to -0.032; P=7.1E-7); (PCSK9 protein to ALM; beta, -0.060; 95% CI, -0.106 to -0.014; P=0.010)]. Genetically proxied inhibition of PCSK9 gene expression in the liver may be associated with reduced ALM (beta, -0.013; 95% CI, -0.035 to 0.009; P=0.25), consistent with the results of PCSK9 drug-target and PCSK9 protein MR analyses, but the magnitude was less precise. No robust association was found between HMGCR inhibition (beta, 0.048; 95% CI, -0.015 to 0.110; P=0.14) or NPC1L1 (beta, 0.035; 95% CI, -0.074 to 0.144; P=0.53) inhibition and ALM, and validation and sensitivity MR analyses showed consistent estimates. This MR study suggested that PCSK9 is involved in sarcopenia pathogenesis and that its inhibition is associated with reduced ALM. These findings potentially pave the way for future studies that may allow personalized selection of lipid-lowering drugs for those at risk of sarcopenia.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) interferes with the recycling of low-density lipoprotein (LDL) receptor (LDLR). This leads to LDLR degradation and reduced cellular uptake of plasma LDL. Naturally occurring human PCSK9 loss-of-function mutations are associated with low levels of plasma LDL cholesterol and a reduced risk of coronary heart disease. PCSK9 gain-of-function mutations result in lower LDL clearance and increased risk of atherosclerosis. The exact mechanism by which PCSK9 disrupts the normal recycling of LDLR remains to be determined. In this study, we have assembled homologs of human PCSK9 from 20 vertebrates, a cephalochordate and mollusks in order to search for conserved regions of PCSK9 that may be important for the PCSK9-mediated degradation of LDLR. We found a large, conserved protrusion on the surface of the PCSK9 catalytic domain and have performed site-directed mutagenesis experiments for 13 residues on this protrusion. A cluster of residues that is important for the degradation of LDLR by PCSK9 was identified. Another cluster of residues, at the opposite end of the conserved protrusion, appears to be involved in the physical interaction with a putative inhibitor of PCSK9. This study identifies the residues, sequence segments and surface patches of PCSK9 that are under strong purifying selection and provides important information for future studies of PCSK9 mutants and for investigations on the function of this regulator of cholesterol homeostasis.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is associated with familial autosomal dominant hypercholesterolemia and is a natural inhibitor of the LDL receptor (LDLr). PCSK9 is degraded by other proprotein convertases: PC5/6A and furin. Both PCSK9 and the LDLr are up-regulated by the hypocholesterolemic statins. Thus, inhibitors or repressors of PCSK9 should amplify their beneficial effects. In the present study, we showed that PPARalpha activation counteracts PCSK9 induction by statins by repressing PCSK9 promoter activity and by increasing PC5/6A and furin expression. Quantification of mRNA and protein levels showed that various fibrates decreased PCSK9 and increased PC5/6A and furin expression. Fenofibric acid (FA) reduced PCSK9 protein content in immortalized human hepatocytes (IHH) as well as its cellular secretion. FA suppressed PCSK9 induction by statins or by the liver X receptor agonist TO901317. PCSK9 repression is occurring at the promoter level. We showed that PC5/6A and furin fibrate-mediated up-regulation is PPARalpha-dependent. As a functional test, we observed that FA increased by 30% the effect of pravastatin on the LDLr activity in vitro. In conclusion, fibrates simultaneously decreased PCSK9 expression while increasing PC5/6A and furin expression, indicating a broad action of PPARalpha activation in proprotein convertase-mediated lipid homeostasis. Moreover, this study validates the functional relevance of a combined therapy associating PCSK9 repressors and statins.

BackgroundPlasma concentration of low-density lipoprotein (LDL) cholesterol is a well-established risk factor for cardiovascular disease. Inhibition of proprotein convertase subtilisin/kexin type 9 (PCSK9), which regulates cholesterol homeostasis, has recently emerged as an approach to reduce cholesterol levels. The development of humanized animal models is an important step to validate and study human drug targets, and use of genome and base editing has been proposed as a mean to target disease alleles.ResultsTo address the lack of validated models to test the safety and efficacy of techniques to target human PCSK9, we generated a liver-specific human PCSK9 knock-in mouse model (hPCSK9-KI). We showed that plasma concentrations of total cholesterol were higher in hPCSK9-KI than in wildtype mice and increased with age. Treatment with evolocumab, a monoclonal antibody that targets human PCSK9, reduced cholesterol levels in hPCSK9-KI but not in wildtype mice, showing that the hypercholesterolemic phenotype was driven by overexpression of human PCSK9. CRISPR-Cas9-mediated genome editing of human PCSK9 reduced plasma levels of human and not mouse PCSK9, and in parallel reduced plasma concentrations of total cholesterol; genome editing of mouse Pcsk9 did not reduce cholesterol levels. Base editing using a guide RNA that targeted human and mouse PCSK9 reduced plasma levels of human and mouse PCSK9 and total cholesterol. In our mouse model, base editing was more precise than genome editing, and no off-target editing nor chromosomal translocations were identified.ConclusionsHere, we describe a humanized mouse model with liver-specific expression of human PCSK9 and a human-like hypercholesterolemia phenotype, and demonstrate that this mouse can be used to evaluate antibody and gene editing-based (genome and base editing) therapies to modulate the expression of human PCSK9 and reduce cholesterol levels. We predict that this mouse model will be used in the future to understand the efficacy and safety of novel therapeutic approaches for hypercholesterolemia.

Proprotein convertase subtilisin kexin type 9 (PCSK9) promotes the degradation of the low-density lipoprotein (LDL) receptor (LDLR), and its deficiency in humans results in low plasma LDL cholesterol and protection against coronary heart disease. Recent evidence indicates that PCSK9 also modulates the metabolism of triglyceride-rich apolipoprotein B (apoB) lipoproteins, another important coronary heart disease risk factor. Here, we studied the effects of physiological levels of PCSK9 on intestinal triglyceride-rich apoB lipoprotein production and elucidated for the first time the cellular and molecular mechanisms involved. Treatment of human enterocytes (CaCo-2 cells) with recombinant human PCSK9 (10 μg/mL for 24 hours) increased cellular and secreted apoB48 and apoB100 by 40% to 55% each (P<0.01 versus untreated cells), whereas short-term deletion of PCSK9 expression reversed this effect. PCSK9 stimulation of apoB was due to a 1.5-fold increase in apoB mRNA (P<0.01) and to enhanced apoB protein stability through both LDLR-dependent and LDLR-independent mechanisms. PCSK9 decreased LDLR protein (P<0.01) and increased cellular apoB stability via activation of microsomal triglyceride transfer protein. PCSK9 also increased levels of the lipid-generating enzymes FAS, SCD, and DGAT2 (P<0.05). In mice, human PCSK9 at physiological levels increased intestinal microsomal triglyceride transfer protein levels and activity regardless of LDLR expression. PCSK9 markedly increases intestinal triglyceride-rich apoB production through mechanisms mediated in part by transcriptional effects on apoB, microsomal triglyceride transfer protein, and lipogenic genes and in part by posttranscriptional effects on the LDLR and microsomal triglyceride transfer protein. These findings indicate that targeted PCSK9-based therapies may also be effective in the management of postprandial hypertriglyceridemia.

Establishing new approaches for the prevention and treatment of stroke relies on identifying modifiable risk factors that contribute to the development of this complex disease. Mendelian randomization (MR) studies, analogous to naturally occurring randomized trials, can assess causality of potentially modifiable biomarkers and offer new insights into biological pathways. Stroke is the second leading cause of death worldwide and the chief determinant of long-term disability. Stroke is a heterogeneous disease arising from several distinct underlying pathologies and is typically classified as ischemic or hemorrhagic, and further subclassified using imaging data. Ischemic stroke (IS), including its 3 main subtypes: small vessel disease, large vessel disease, and cardioembolic stroke, accounts for ≈80% of stroke and is the result of an interrupted blood supply, leading to localized areas of ischemia in the brain. Small vessel disease may be a consequence of nonatherosclerotic, as well as atherosclerotic, mechanisms that result in an occlusion of the small perforating arteries, whereas large vessel disease results from occlusions or emboli from plaque rupture in larger vessels, such as a carotid artery. Cardioembolic stroke arises typically from emboli from the heart. By contrast, hemorrhagic stroke is a consequence of intracerebral hemorrhage (bleeding into the brain) or subarachnoid hemorrhage (bleeding into the subarachnoid space). These diverse stroke subtypes have distinct underlying pathologies reflecting different risk factor distributions. MR studies, using genetic variants as instrumental variables, afford a powerful approach to assessing causality of risk factors and avoid biases inherent in observational studies, including confounding and reverse causation. This review considers the contribution of MR studies to stroke epidemiology and their relevance to understanding risk factors and new therapeutic targets for stroke. Meta-analyses of large prospective studies have enhanced our knowledge of classical and emerging risk factors for stroke.1–4 Classical risk factors for stroke include nonmodifiable characteristics, …

Introduction Up to now, limited and contradictory results have been published on the role of prognostic values of lipid profile molecules including: HDL (High Density Lipoprotein), LDL (Low Density Lipoprotein), TG (Triglyceride), Total Cholesterol and PCSK9 (Proprotein Convertase SubtilisinKexin type 9) molecule in occurrence and development of Parkinson’s disease (PD). The aim of this study was to investigate the role of lipid profile and PCSK9 in patients with PD and to compar it with healthy individuals. Methods and Results In the present case-evidence study, 32 individuals diagnosed with PD were compared with 32 healthy individuals. After receiving the participant's consent forms, 5 ml blood was taken from vein and the level of HDL(High -Density Lipoprotein), LDL (Low-Density Lipoprotein), TG (Triglyceride),Total Cholesterol and PCSK9 in the blood samples were measured. The Elisa method was used for measuring PCSK9 level in blood serum. Data were analyzed using SPSS17 software. The P values smaller than 0.05 were considered significant. The mean age of participants in the PD and control group was 56.9±8.8 and 53.7±10.1 years respectively (P>0.05). Twenty seven individuals (87.1%) and 13 individuals (41.9%) in the PD group and control group were men, respectively. The remaining participants were women (P=0.000). LDL level (84.2±24.9 ml/dl vs. 105.5±16.8, P=0.000), HDL (45.5±8.7 ml/dl vs. 51.1±9.5 ml/dl, P=0.000), total cholesterol (155.3±31.2 ml/dl vs. 192.8±32.5 ml/dl P=0.000) were lower and TG level was higher in the PD group (133.3±79.3 ml/dl vs. 131.2±58.6 ml/dl, P=0.9) compared with the control group. PCSK9 level was higher in the PDgroup, but no significant difference was found (141.6±70 vs. 129.7±51 ng/ml, P=0.5). Conclusions Our findings showed that individuals with PD have lower level of HDL, LDL and total cholesterol compared with the control group, but PCSK9 levels were same in both groups. Key words TG (Triglyceride), LDL (Low Density Lipoprotein), HDL (High Density Lipoprotein), PCSK9 (Proprotein Convertase SubtilisinKexin type 9, Total Cholesterol, Parkinson’s disease.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) modulates low-density lipoprotein (LDL) receptor (LDLR) degradation, thus influencing serum cholesterol levels. On the other hand, LDLR binds to and clears PCSK9 from the circulation, thus modulating its serum levels. To study the global and reciprocal effects of PCSK9 and LDLR on serum cholesterol, we developed transgenic mice expressing human (h) PCSK9 and characterized its activity, serum levels, and tissue distribution. Serum hPCSK9 concentration in transgenic mice was 2181±423 ng/ml, about 10 times higher than normal level in human serum. Although hPCSK9 was expressed mainly in the kidney, LDLR degradation activity was most evident in the liver and small intestine. In contrast, LDLR levels were not affected by hPCSK9 expression in the adrenals and large intestine. On a chow diet, hPCSK9 transgenic mice on either C57BL/6 (wild-type, WT) or LDLR -/- background had higher cholesterol levels than their non-transgenic counterparts. Human PCSK9 transgenic mice had over a 4-fold increase in murine (m) PCSK9 serum levels compared to WT controls. However, transgenic expression of hPCSK9 in LDLR -/- mice did not affect the already elevated levels of mPCSK9. On the other hand, induction of hLDLR expression in transgenic mice caused a dramatic decrease in mPCSK9 levels. In addition hPCSK9 levels were increased by 2 fold in transgenic mice under LDLR -/- compare to WT background. Turnover studies with native PCSK9 showed rapid serum clearance in WT mice (half-life 5.2 min), whereas clearance was much slower in LDLR -/- recipient mice (50.5 min), and faster in hLDLR transgenic mice (2.9 min). In WT mice the injected PCSK9 accumulated in the liver and kidney but not in the adrenal gland. Ultracentrifugation and FLPC analysis showed that approximately one quarter of circulating hPCSK9 is associated with LDL, and that the LDL-associated PCSK9 is mainly in monomeric form. Our results show a reciprocal regulation between LDLR and PCSK9, which determines serum PCSK9 levels, hepatic LDLR expression, and serum LDL levels. Understanding these interactions will increase our knowledge of serum cholesterol homeostasis and should provide the basis for an intelligent design of anti-PCSK9 therapies.

Rare human genetic variations at the proprotein convertase subtilisin/kexin type 9 (PCSK9) locus revealed the role of this protein in regulating plasma levels of low-density lipoprotein-cholesterol (LDL-C), and its promise as a therapeutic target.1,2 PCSK9 promotes the internalization and degradation of the hepatic LDL receptor, thereby reducing the liver’s ability to clear LDL from the circulation and causing LDL-C levels to rise.3,4 Monoclonal antibodies against PCSK9 that disrupt the interaction of this protein with the LDL receptor have been studied in numerous clinical trials, and these agents have been shown to safely reduce LDL-C by 50% to 60%.5 Although large cardiovascular outcomes trials are still being conducted, 2 of these agents were recently approved by regulatory authorities on the basis of their LDL-C reductions alone, for the treatment of patients with familial hypercholesterolemia or those with established cardiovascular disease who require additional lipid-lowering therapy despite maximally tolerated doses of statins. See accompanying article on page 2517 of the December 2015 issue The rapid development of a novel therapeutic can often outpace our understanding of the underlying biology, and this is becoming apparent in the case of PCSK9 function. Although most intensely studied for its interaction with the LDL receptor, it is now evident that PCSK9 can promote the degradation of other cell surface receptors, including close structural homologues of the LDL receptor (eg, the very low-density lipoprotein …

Joint statement from the European Atherosclerosis Society and European Society of Vascular Medicine focuses on patients with peripheral arterial disease

Proprotein convertase subtilisin/kexin type 9 (PCSK9) down-regulates the low-density lipoprotein (LDL) receptor, elevating LDL cholesterol and accelerating atherosclerotic heart disease, making it a promising cardiovascular drug target. To achieve its maximal effect on the LDL receptor, PCSK9 requires autoproteolysis. After cleavage, PCSK9 retains its prodomain in the active site as a self-inhibitor. Unlike other proprotein convertases, however, this retention is permanent, inhibiting any further protease activity for the remainder of its life cycle. Such inhibition has proven a major challenge toward a complete biochemical characterization of PCSK9's proteolytic function, which could inform therapeutic approaches against its hypercholesterolemic effects. To address this challenge, we employed a cell-based, high-throughput method using a luciferase readout to evaluate the single-turnover PCSK9 proteolytic event. We combined this method with saturation mutagenesis libraries to interrogate the sequence specificities of PCSK9 cleavage and proteolysis-independent secretion. Our results highlight several key differences in sequence identity between these two steps, complement known structural data, and suggest that PCSK9 self-proteolysis is the rate-limiting step of secretion. Additionally, we found that for missense SNPs within PCSK9, alterations in both proteolysis and secretion are common. Last, we show that some SNPs allosterically modulate PCSK9's substrate sequence specificity. Our findings indicate that PCSK9 proteolysis acts as a commonly perturbed but critical switch in controlling lipid homeostasis and provide a new hope for the development of small-molecule PCSK9 inhibitors.

Despite significant advances in prevention and treatment, cardiovascular diseases (CVDs) remain the most common cause of death in the United States.1–3 In part, and compared with treatment developments in oncology, this is because of the lack of current generation CVD precision therapies.4–6 Most CVDs are caused by atherosclerosis, a disease process that causes thickening of the arterial wall because of inflammation and lipid accumulation, resulting in plaque formation.7,8 Macrophage dynamics plays a central role in atherosclerosis progression, vessel wall destabilization, and—as has been recently discovered—plaque aggravation because of myocardial infarction.9–11 Therefore, and despite current standardized treatments, ≈25% of patients who had myocardial infarction or stroke will experience secondary major adverse cardiovascular events, often more harmful than the primary event.12–14 To break this vicious cycle, innovative and tailored therapeutic strategies are needed. Various targeted immunotherapies that showed anticancer potential in in vitro screens also displayed in vivo potential in mouse models. Such proof-of-concept studies in validated syngeneic mouse tumor models not only provide information on in vivo efficacy but also disclose the targeted immunotherapies’ underlying mechanism of action and safety profile. However, immunotherapies’ high costs and the identification of amendable patients compromise clinical translation.15,16 Continued and focused efforts are needed to understand patient responsiveness and reduce high dropout rates in trials. A parallel effort focused on the development of novel imaging tools that can guide patient selection may offer a potential solution. Positron emission tomography (PET) can be used to trace immunotherapies. So-called immunoPET uses antibodies, developed for cancer treatment, that selectively recognize a specific epitope on target cells. The antibodies are labeled with radioactive isotopes, such as zirconium-89 or iodine-124, with relatively long decay half-lives of 3.27 and 4.18 days, respectively, to …