Proprotein convertase subtilisin/kexin type 9 (PCSK9) is an enzyme encoded by the PCSK9 gene in humans on chromosome 1. [5] It is the 9th member of the proprotein convertase family of proteins that activate other proteins. [6] Similar genes (orthologs) are found across many species. As with many proteins, PCSK9 is inactive when first synthesized, because a section of peptide chains blocks their activity; proprotein convertases remove that section to activate the enzyme. [7] The PCSK9 gene also contains one of 27 loci associated with increased risk of coronary artery disease. [8]
PCSK9 is ubiquitously expressed in many tissues and cell types. [9] PCSK9 binds to and degrades the receptor for low-density lipoprotein particles (LDL), which typically transport 3,000 to 6,000 fat molecules (including cholesterol) per particle, within extracellular fluid. The LDL receptor (LDLR), on liver and other cell membranes, binds and initiates ingestion of LDL-particles from extracellular fluid into cells and targets the complex to lysosomes for destruction. If PCSK9 is blocked, the LDL-LDLR complex separates during trafficking, with the LDL digested in the lysosome, but the LDLRs instead recycled back to the cell surface and so able to remove additional LDL-particles from the extracellular fluid. [10] [11] Therefore, blocking PCSK9 can lower blood LDL-particle concentrations. [12] [13]
PCSK9 has medical importance because it acts in lipoprotein homeostasis. Agents that block PCSK9 can lower LDL particle concentrations. The first two PCSK9 inhibitors, alirocumab and evolocumab, were approved as once every two week injections, by the U.S. Food and Drug Administration in 2015 for lowering LDL-particle concentrations when statins and other drugs were not sufficiently effective or poorly tolerated. The cost of these new medications, as of 2015 [update] , was $14,000 per year at full retail; judged of unclear cost effectiveness by some. [14] While these medications are prescribed by many physicians, the payment for prescriptions are often denied by insurance providers. [15] [16] [17] As a result, pharmaceutical manufacturers lowered the prices of these drugs. [18]
In February 2003, Nabil Seidah and Jae Byun, a scientist at the Clinical Research Institute of Montreal in Canada, discovered a novel human proprotein convertase, the gene for which was located on the short arm of chromosome 1. [19] Meanwhile, a lab led by Catherine Boileau at the Necker-Enfants Malades Hospital in Paris had been following families with familial hypercholesterolaemia, a genetic condition that, in 90% of cases causes coronary artery disease (FRAMINGHAM study) and in 60% of cases may lead to an early death; [20] they had identified a mutation on chromosome 1 carried by some of these families, but had been unable to identify the relevant gene. The labs got together and by the end of the year published their work, linking mutations in the gene, now identified as PCSK9, to the condition. [21] [19] In their paper, they speculated that the mutations might make the gene overactive. In that same year, investigators at Rockefeller University and University of Texas Southwestern had discovered the same protein in mice, and had worked out the novel pathway that regulates LDL cholesterol in which PCSK9 is involved, and it soon became clear that the mutations identified in France led to excessive PCSK9 activity, and thus excessive removal of the LDL receptor, leaving people carrying the mutations with too much LDL cholesterol. [19] Meanwhile, Helen H. Hobbs and Jonathan Cohen at UT-Southwestern had been studying people with very high and very low cholesterol, and had been collecting DNA samples. [22] With the new knowledge about the role of PCSK9 and its location in the genome, they sequenced the relevant region of chromosome 1 in people with very low cholesterol and they found nonsense mutations in the gene, thus validating PCSK9 as a biological target for drug discovery. [19] [23]
In July 2015, the FDA approved the first PCSK9 Inhibitor drugs for medical use. [24]
The PCSK9 gene resides on chromosome 1 at the band 1p32.3 [25] and includes 15 exons. [26] This gene produces two isoforms through alternative splicing. [27] : Sequence & Isoform
PCSK9 is a member of the peptidase S8 family. [27] : Family & Domains
The solved structure of PCSK9 reveals four major components in the pre-processed protein: the signal peptide (residues 1-30); the N-terminal prodomain (residues 31–152); the catalytic domain (residues 153–425); and the C-terminal domain (residues 426–692), which is further divided into three modules. [29] The N-terminal prodomain has a flexible crystal structure and is responsible for regulating PCSK9 function by interacting with and blocking the catalytic domain, which otherwise binds the epidermal growth factor-like repeat A (EGF-A) domain of the LDLR. [29] [30] [31] While previous studies indicated that the C-terminal domain was uninvolved in binding LDLR, [32] [33] a recent study by Du et al. demonstrated that the C-terminal domain does bind LDLR. [29] The secretion of PCSK9 is largely dependent on the autocleavage of the signal peptide and N-terminal prodomain, though the N-terminal prodomain retains its association with the catalytic domain. In particular, residues 61–70 in the N-terminal prodomain are crucial for its autoprocessing. [29]
PCSK9 is synthesized as a soluble zymogen that undergoes autocatalytic intramolecular processing in the endoplasmic reticulum. [7] It is expressed mainly in liver, intestine, kidney, skin and the central nervous system. [34] After being processed in the ER, PCSK9 co-localizes with the protein sortilin on its way through the Golgi and trans-Golgi complex. A PCSK9-sortilin interaction is proposed to be required for cellular secretion of PCSK9. [35] In healthy humans, plasma PCSK9 levels directly correlate with plasma sortilin levels, following a diurnal rhythm similar to cholesterol synthesis. [36] [37] The plasma PCSK9 concentration is higher in women compared to men, and the PCSK9 concentrations decrease with age in men but increase in women, suggesting that estrogen level most likely plays a role. [38] [39] PCSK9 gene expression can be regulated by sterol-response element binding proteins (SREBP-1/2), which also controls LDLR expression. [36]
As a negative post-translational regulator of the low-density lipoprotein receptor (LDLR), PCSK9 plays a major role in cholesterol homeostasis. Upon binding of low-density lipoprotein (LDL) cholesterol to the LDL receptor, the resulting LDLR-LDL complex is internalized. When exposed to the acidic environment within the resulting endosome LDLR adopts a hairpin conformation. [40] This conformational change in turn induces the dissociation of the LDL-LDLR complex, allowing LDLR to be recycled back to the plasma membrane. Binding of PCSK9 to cell surface LDLR (through the LDLR EGF-A domain) also induces LDLR internalization. However, unlike LDL binding, PCSK9 prevents LDLR from undergoing a conformational change. This inhibition redirects LDLR to a lysosome where it is degraded. [40] Thus, PCSK9 lowers cell surface expression of LDLR and thereby decreases metabolism of LDL-particles, which in turn may lead to hypercholesterolemia. [41] PCSK9 also plays an important role in triglyceride-rich apoB lipoprotein production in small intestine and postprandial lipemia. [42] [43] [44]
ApoB lipoprotein, PCSK9, and the genes involved in cholesterol synthesis are highly expressed in the epidermis. [45] [46] The cutaneous expression of PCSK9 is likely important for proper skin barrier formation as ceramides, free fatty acids, and cholesterol are the three major components of the epidermal lipid barrier. [47] Matching its function in cholesterol homeostasis, there is a gradient of PCSK9 expression in the epidermis. PCSK9 is selectively expressed in basal and spinous layer keratinocytes with little to no expression in granular layer keratinocytes. [45] In contrast to basal layer keratinocytes, granular layer keratinocytes release large amounts of cholesterol and other lipids to form a lipid rich "mortar" in the intracellular space between keratinocytes. [47] In addition to its likely role in epidermal lipid barrier formation, PCSK9 has also been linked to skin inflammation. For example, genetic variants of PCSK9 have been linked psoriasis, [45] and knockdown expression of PCSK9 in keratinocytes results in increase expression of IL-36G and other keratinocyte-derived inflammatory mediators. [45]
PCSK9 may also have a role in the differentiation of cortical neurons. [5]
Variants of PCSK9 can reduce or increase circulating cholesterol. LDL-particles are removed from the blood when they bind to LDLR on the surface of cells, including liver cells, and are taken inside the cells. When PCSK9 binds to an LDLR, the receptor is destroyed along with the LDL particle. PCSK9 degrades LDLR by preventing the hairpin conformational change of LDLR. [48] If PCSK9 does not bind, the receptor will return to the surface of the cell and can continue to remove LDL-particles from the bloodstream. [49]
Other variants are associated with a rare autosomal dominant familial hypercholesterolemia (HCHOLA3). [50] [21] [51] The mutations increase its protease activity, reducing LDLR levels and preventing the uptake of cholesterol into the cells. [21]
In humans, PCSK9 was initially discovered as a protein expressed in the brain. [52] [53] However, it has also been described in the kidney, the pancreas, liver and small intestine. [53] Recent evidence indicate that PCSK9 is highly expressed in arterial walls such as endothelium, smooth muscle cells, and macrophages, with a local effect that can regulate vascular homeostasis and atherosclerosis. [54] [55] [56] Accordingly, it is now very clear that PCSK9 has pro-atherosclerotic effects and regulates lipoprotein synthesis. [57]
As PCSK9 binds to LDLR, which prevents the removal of LDL-particles from the blood plasma, several studies have determined the potential use of PCSK9 inhibitors in the treatment of hyperlipoproteinemia (commonly called hypercholesterolemia). [14] [53] [58] [59] [60] [61] [62] [63] Furthermore, loss-of-function mutations in the PCSK9 gene result in lower levels of LDL and protection against cardiovascular disease. [57] [64] [65]
In addition to its lipoprotein synthetic and pro-atherosclerotic effects, PCSK9 is involved in glucose metabolism and obesity, [66] regulation of re-absorption of sodium in the kidney which is relevant in hypertension. [67] [68] Furthermore, PCSK9 may be involved in bacterial or viral infections and sepsis. [69] [70] [71] In the brain the role of PCSK9 is still controversial and may be either pro-apoptotic or protective in the development of the nervous system. [5] PCSK9 levels have been detected in the cerebrospinal fluid at a 50-60 times lower level than in serum. [72]
A multi-locus genetic risk score study based on a combination of 27 loci including the PCSK9 gene, identified individuals at increased risk for both incident and recurrent coronary artery disease events, as well as an enhanced clinical benefit from statin therapy. The study was based on a community cohort study (the Malmo Diet and Cancer study) and four additional randomized controlled trials of primary prevention cohorts (JUPITER and ASCOT) and secondary prevention cohorts (CARE and PROVE IT-TIMI 22). [8]
Several studies have determined the potential use of PCSK9 inhibitors in the treatment of hyperlipoproteinemia (commonly called hypercholesterolemia). [14] [53] Furthermore, loss-of-function mutations in the PCSK9 gene result in lower levels of LDL and protection against cardiovascular disease. [57]
PCSK9 inhibitor drugs are now approved by the FDA to treat familial hypercholesterolemia. [15]
Drugs can inhibit PCSK9, leading to lowered circulating LDL particle concentrations. Since LDL particle concentrations are thought by many experts to be a driver of cardiovascular disease like heart attacks, it is plausible that these drugs may also reduce the risk of such diseases. Clinical studies, including phase III clinical trials, are now underway to describe the effect of PCSK9 inhibition on cardiovascular disease, and the safety and efficacy profile of the drugs. [74] [75] [76] [77] [78] Among those inhibitors under development in December 2013 were the antibodies alirocumab, evolocumab, 1D05-IgG2 (Merck), RG-7652 and LY3015014, as well as the RNAi therapeutic inclisiran. [79] PCSK9 inhibitors are promising therapeutics for the treatment of people who exhibit statin intolerance, or as a way to bypass frequent dosage of statins for higher LDL concentration reduction. [80] [81]
A review published in 2015 concluded that these agents, when used in patients with high LDL-particle concentrations (thus at greatly elevated risk for cardiovascular disease) seem to be safe and effective at reducing all-cause mortality, cardiovascular mortality, and heart attacks. [82] However a 2020 review concluded that while PCSK9 inhibitor treatment provides additional benefits beyond maximally tolerated statin therapy in high-risk individuals, [83] PCSK9 inhibitor use probably produces little or no difference in mortality. [84]
Regeneron Pharmaceuticals (in collaboration with Sanofi) became the first to market a PCSK9 inhibitor, with a competitor Amgen reaching market slightly later. Prices were very high, inhibiting adoption. [15] The drugs are approved by the FDA for treatment of hypercholesterolemia, notably the genetic condition heterozygous familial hypercholesterolemia which causes high cholesterol levels and heart attacks at a young age. [20] These drugs were later approved by the FDA for the reduction of cardiovascular events including a reduction in all-cause mortality. [85]
In a meta-analysis involving data from 3 randomized controlled trials, early initiation of PCSK9 inhibitors within 72 hours of acute coronary event along with high dose statin was associated with a more rapid decline in cholesterol levels 4 weeks after the cardiac event, which translated into a significant reduction hospital readmission post-acute cardiac event. [86]
An FDA warning in March 2014 about possible cognitive adverse effects of PCSK9 inhibition caused concern, as the FDA asked companies to include neurocognitive testing into their Phase III clinical trials. [87]
A number of monoclonal antibodies that bind to and inhibit PCSK9 near the catalytic domain were in clinical trials as of 2014 [update] . These include evolocumab (Amgen), bococizumab (Pfizer), and alirocumab (Sanofi/Regeneron Pharmaceuticals). [73] As of July 2015 [update] , the EU approved these drugs including Evolocumab/Amgen according to Medscape news agency report. A meta-analysis of 24 clinical trials has shown that monoclonal antibodies against PCSK9 can reduce cholesterol, cardiac events and all-cause mortality. [82] The most recent guidelines for cholesterol management from the American Heart Association and American College of Cardiology now provide guidance for when PCSK9 inhibitors should be considered, particularly focusing on cases in which maximally tolerated statin and ezetimibe fail to achieve goal LDL reduction. [88]
A possible side effect of the monoclonal antibody might be irritation at the injection site. Before the infusions, participants received oral corticosteroids, histamine receptor blockers, and acetaminophen to reduce the risk of infusion-related reactions, which by themselves will cause several side effects. [89]
Peptides that mimick the EGFA domain of the LDLR that binds to PCSK9 have been developed to inhibit PCSK9. [90]
The PCSK9 antisense oligonucleotide increases expression of the LDLR and decreases circulating total cholesterol levels in mice. [91] A locked nucleic acid reduced PCSK9 mRNA levels in mice. [92] [93] Initial clinical trials showed positive results of ALN-PCS, which acts by means of RNA interference. [78] [94] [95]
In 2021, scientists demonstrated that CRISPR gene editing can decrease blood levels of LDL cholesterol in vivo in Macaca fascicularis monkeys for months by 60% via knockdown of PCSK9 in the liver. [96] [97]
In 2023, a clinical trial demonstrated that VERVE-101 gene therapy, which works via CRISPR gene editing, could reduce LDL cholesterol by as much as 55% in human volunteers with heterozygous familial hypercholesterolemia. [98] [99]
A vaccine that targets PCSK9 has been developed to treat high LDL-particle concentrations. The vaccine uses a VLP (virus-like particle) as an immunogenic carrier of an antigenic PCSK9 peptide. VLPs consist of the outer shell of a virus particle but lack a viral genome and are unable to replicate; they can induce immune responses without causing infection. Mice and macaques vaccinated with bacteriophage VLPs displaying PCSK9-derived peptides developed high-titer IgG antibodies that bound to circulating PCSK9. Vaccination was associated with significant reductions in total cholesterol, free cholesterol, phospholipids, and triglycerides. [100]
The plant alkaloid berberine inhibits the transcription of the PCSK9 gene in immortalized human hepatocytes in vitro, [101] and lowers serum PCSK9 in mice and hamsters in vivo. [102] It has been speculated [102] that this action contributes to the ability of berberine to lower serum cholesterol. [103] Annexin A2, an endogenous protein, is a natural inhibitor of PCSK9 activity. [104]
Cholesterol is the principal sterol of all higher animals, distributed in body tissues, especially the brain and spinal cord, and in animal fats and oils.
High-density lipoprotein (HDL) is one of the five major groups of lipoproteins. Lipoproteins are complex particles composed of multiple proteins which transport all fat molecules (lipids) around the body within the water outside cells. They are typically composed of 80–100 proteins per particle. HDL particles enlarge while circulating in the blood, aggregating more fat molecules and transporting up to hundreds of fat molecules per particle.
Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL delivers fat molecules to cells. LDL has been associated with the progression of atherosclerosis.
Lipid-lowering agents, also sometimes referred to as hypolipidemic agents, cholesterol-lowering drugs, or antihyperlipidemic agents are a diverse group of pharmaceuticals that are used to lower the level of lipids and lipoproteins, such as cholesterol, in the blood (hyperlipidemia). The American Heart Association recommends the descriptor 'lipid lowering agent' be used for this class of drugs rather than the term 'hypolipidemic'.
In pharmacology, the fibrates are a class of amphipathic carboxylic acids and esters. They are derivatives of fibric acid. They are used for a range of metabolic disorders, mainly hypercholesterolemia, and are therefore hypolipidemic agents.
Hypercholesterolemia, also called high cholesterol, is the presence of high levels of cholesterol in the blood. It is a form of hyperlipidemia, hyperlipoproteinemia, and dyslipidemia.
Dyslipidemia is a metabolic disorder characterized by abnormally high or low amounts of any or all lipids or lipoproteins in the blood. Dyslipidemia is a risk factor for the development of atherosclerotic cardiovascular diseases (ASCVD), which include coronary artery disease, cerebrovascular disease, and peripheral artery disease. Although dyslipidemia is a risk factor for ASCVD, abnormal levels don't mean that lipid lowering agents need to be started. Other factors, such as comorbid conditions and lifestyle in addition to dyslipidemia, is considered in a cardiovascular risk assessment. In developed countries, most dyslipidemias are hyperlipidemias; that is, an elevation of lipids in the blood. This is often due to diet and lifestyle. Prolonged elevation of insulin resistance can also lead to dyslipidemia. Likewise, increased levels of O-GlcNAc transferase (OGT) may cause dyslipidemia.
HMG-CoA reductase is the rate-controlling enzyme of the mevalonate pathway, the metabolic pathway that produces cholesterol and other isoprenoids. HMGCR catalyzes the conversion of HMG-CoA to mevalonic acid, a necessary step in the biosynthesis of cholesterol. Normally in mammalian cells this enzyme is competitively suppressed so that its effect is controlled. This enzyme is the target of the widely available cholesterol-lowering drugs known collectively as the statins, which help treat dyslipidemia.
Apolipoproteins are proteins that bind lipids to form lipoproteins. They transport lipids in blood, cerebrospinal fluid and lymph.
Hyperlipidemia is abnormally high levels of any or all lipids or lipoproteins in the blood. The term hyperlipidemia refers to the laboratory finding itself and is also used as an umbrella term covering any of various acquired or genetic disorders that result in that finding. Hyperlipidemia represents a subset of dyslipidemia and a superset of hypercholesterolemia. Hyperlipidemia is usually chronic and requires ongoing medication to control blood lipid levels.
The low-density lipoprotein receptor (LDL-R) is a mosaic protein of 839 amino acids that mediates the endocytosis of cholesterol-rich low-density lipoprotein (LDL). It is a cell-surface receptor that recognizes apolipoprotein B100 (ApoB100), which is embedded in the outer phospholipid layer of very low-density lipoprotein (VLDL), their remnants—i.e. intermediate-density lipoprotein (IDL), and LDL particles. The receptor also recognizes apolipoprotein E (ApoE) which is found in chylomicron remnants and IDL. In humans, the LDL receptor protein is encoded by the LDLR gene on chromosome 19. It belongs to the low density lipoprotein receptor gene family. It is most significantly expressed in bronchial epithelial cells and adrenal gland and cortex tissue.
Apolipoprotein B (ApoB) is a protein that in humans is encoded by the APOB gene. It is commonly used to detect risk of atherosclerotic cardiovascular disease.
The very-low-density-lipoprotein receptor (VLDLR) is a transmembrane lipoprotein receptor of the low-density-lipoprotein (LDL) receptor family. VLDLR shows considerable homology with the members of this lineage. Discovered in 1992 by T. Yamamoto, VLDLR is widely distributed throughout the tissues of the body, including the heart, skeletal muscle, adipose tissue, and the brain, but is absent from the liver. This receptor has an important role in cholesterol uptake, metabolism of apolipoprotein E-containing triacylglycerol-rich lipoproteins, and neuronal migration in the developing brain. In humans, VLDLR is encoded by the VLDLR gene. Mutations of this gene may lead to a variety of symptoms and diseases, which include type I lissencephaly, cerebellar hypoplasia, and atherosclerosis.
Cholesterol absorption inhibitors are a class of compounds that prevent the uptake of cholesterol from the small intestine into the circulatory system. Most of these molecules are monobactams but show no antibiotic activity. An example is ezetimibe Another example is Sch-48461. The "Sch" is for Schering-Plough, where these compounds were developed. Phytosterols are also cholesterol absorption inhibitors.
Familial hypercholesterolemia (FH) is a genetic disorder characterized by high cholesterol levels, specifically very high levels of low-density lipoprotein cholesterol, in the blood and early cardiovascular diseases. The most common mutations diminish the number of functional LDL receptors in the liver or produce abnormal LDL receptors that never go to the cell surface to function properly. Since the underlying body biochemistry is slightly different in individuals with FH, their high cholesterol levels are less responsive to the kinds of cholesterol control methods which are usually more effective in people without FH. Nevertheless, treatment is usually effective.
Lipoprotein(a) is a low-density lipoprotein variant containing a protein called apolipoprotein(a). Genetic and epidemiological studies have identified lipoprotein(a) as a risk factor for atherosclerosis and related diseases, such as coronary heart disease and stroke.
Nabil G. Seidah, is a Canadian Québécois scientist. Born in Egypt, he was educated at Cairo University, and subsequently at Georgetown University where he obtained his Ph.D. in 1973. He emigrated to Canada and has been working at the Clinical Research Institute of Montreal (IRCM) since 1974. He is the director of the laboratory of Biochemical Neuroendocrinology. He discovered and cloned seven of the nine known enzymes belonging to the convertase family. During this period, he also greatly contributed to demonstrating that the proteolysis by the proprotein convertases is a wide mechanism that also concerns “non-neuropeptide” proteins such as growth factors, α-integrins, receptors, enzymes, membrane-bound transcription factors, and bacterial and viral proteins. In 2003, he discovered PCSK9 and showed that point mutations in the PCSK9 gene cause dominant familial hypercholesterolemia, likely because of a gain of function related to the ability of PCSK9 to enhance the degradation of cell surface receptors, such as the low-density lipoprotein receptor (LDLR). He has since worked on the elucidation of the functions and mechanisms of action of PCSK9 and PCSK7 both in cells and in vivo, and is developing specific PCSK9 and PCSK7 inhibitors/silencers.
Alirocumab, sold under the brand name Praluent, is a medication used as a second-line treatment for high cholesterol for adults whose cholesterol is not controlled by diet and statin treatment. It is a human monoclonal antibody that belongs to a novel class of anti-cholesterol drugs, known as PCSK9 inhibitors, and it was the first such agent to receive FDA approval. The FDA approval was contingent on the completion of further clinical trials to better determine efficacy and safety.
Evolocumab, sold under the brand name Repatha, is a monoclonal antibody that is an immunotherapy medication for the treatment of hyperlipidemia.
Inclisiran, sold under the brand name Leqvio, is a medication used for the treatment of high low-density lipoprotein (LDL) cholesterol and for the treatment of people with atherosclerotic cardiovascular disease (ASCVD), ASCVD risk-equivalents, and heterozygous familial hypercholesterolemia (HeFH). It is a small interfering RNA (siRNA) that acts as an inhibitor of a proprotein convertase, specifically, inhibiting translation of the protein PCSK9.