Biological or Clinical Significance:
Proprotein convertase subtilisin kexin 9 (PCSK9), also named neural apoptosis-regulated convertase 1 (NARC-1), is a member of the proteinase K subfamily of subtilisin-related serine endoproteases. The full-length protein has 692 amino acids, including a signal peptide, a prodomain and a catalytic domain. PCSK ( is highly expressed in the liver, intestine and kidney. It is initially synthesized as a soluble 74 kDa precursor protein. In the endoplasmic reticulum, it undergoes autocatalytic intramolecular cleavage to generate a 14 kDa pro- domain and a 60 kDa catalytic domain. These two domains remain associated when PCSK9 is secreted outside the cells. The primary physiologic function of PCSK9 is to mediate the degradation of low density lipoprotein receptor (LDLR). Early observations indicated that gain-of-function missense mutations in the PCSK9 gene can cause an autosomal dominant form of hypercholesterolemia. The expression of PCSK9 is also up-regulated by the sterol regulatory element binding proteins (SREBP’s), a family of transcription factors that are responsible for the upregulation of genes involved in cholesterol and fatty acid metabolism, such as the LDLR gene. Further experimental evidence revealed that in mice, when the PCSK9 gene was knocked out, the number of LDLR in the hepatocytes increased; whereas when PCSK9 was over-expressed, the amount of LDLR was reduced in the liver. In humans, genetic analyses have shown that individuals who have nonsense or loss-of-function mutations in the PCSK9 gene have significantly lower plasma LDL cholesterol levels. These investigations clearly indicate that PCSK9 plays a key role in reducing the hepatic LDLR levels. Recently, the underlying mechanism has been uncovered: under normal physiologic conditions, the LDLR is internalized on the cell surface and directed to the endosomes in order to be recycled back to the cell surface. PCSK9 binds to the EGF domain of the LDLR and prevents LDLR from being sorted to the endosomes. Instead, the PCSK9/LDLR complex is redistributed to the lysosomes for degradation. As such, PCSK9 regulates the amounts of LDLR in the circulation and hence, modulates cholesterol levels. Serum PCSK9 concentrations have been found to be directly associated with cholesterol levels. Since individuals with loss-of-function PCSK9 mutations have strikingly reduced risk of coronary heart diseases, PCSK9 has become an attractive drug target in recent years. One approach is to generate small molecules that are able to interfere with PCSK9 autoactivation and its interaction with LDLR. Other approaches aiming to reduce the amounts of PCSK9 in the circulation, such as small interfering RNAs (siRNAs) have also shown promise.
Principle of Test Method:
The PCSK9 assay is a solid phase ELISA designed to measure PCSK9 in cell culture supernates, cell lysates, serum and plasma.