Leu22_Leu23 Duplication at the Signal Peptide of PCSK9 Promotes Intracellular Degradation of LDLr and Autosomal Dominant Hypercholesterolemia

Abstract

Background: PCSK9 (Proprotein convertase subtilisin/kexin type 9) regulates LDL-C (low-density lipoprotein cholesterol) metabolism by targeting LDLr (LDL receptor) for lysosomal degradation. PCSK9 gain-of-function variants cause autosomal dominant hypercholesterolemia by reducing LDLr levels, the D374Y variant being the most severe, while loss-of-function variants are associated with low LDL-C levels. Gain-of-function and loss-of-function activities have also been attributed to variants occurring in the PCSK9 signal peptide. Among them, L11 is a very rare PCSK9 variant that seems to increase LDL-C values in a moderate way causing mild hypercholesterolemia. Methods: Using molecular biology and biophysics methodologies, activities of L8 and L11 variants, both located in the leucine repetition stretch of the signal peptide, have been extensively characterized in vitro. Results: L8 variant is not associated with increased LDLr activity, whereas L11 activity is increased by ≈20% compared with wt PCSK9. The results suggest that the L11 variant reduces LDLr levels intracellularly by a process resulting from impaired cleavage of the signal peptide. This would lead to less efficient LDLr transport to the cell membrane and promote LDLr intracellular degradation. Conclusions: Deletion of a leucine in the signal peptide in L8 variant does not affect PCSK9 activity, whereas the leucine duplication in the L11 variant enhances LDLr intracellular degradation. These findings highlight the importance of deep in vitro characterization of PCSK9 genetic variants to determine pathogenicity and improve clinical diagnosis and therapy of inherited FH disease.

Low circulating PCSK9 levels in LPL homozygous children with chylomicronemia syndrome in a syrian refugee family in Lebanon

“…In a study conducted on 1,745 apparently healthy individuals, plasma PCSK9 levels were significantly lower in individuals carrying a leucine insertion in exon 1 of the PCSK9 gene (Awan et al, 2013). Moreover, the insertion of two leucines in the signal peptide has been reported in a family with familial combined hyperlipidemia and two patients with FH (Abifadel et al, 2008) and in vitro studies have shown that it causes a reduction in the secretion of the mature form of PCSK9 compared to the wild type PCSK9 (Benito-Vicente et al, 2022). However, these variations might not alone explain the very low levels of PCSK9 observed in homozygous carriers of the LPL variant in our study.…”

“…These polymorphisms occur in the signal peptide region of the PCSK9 protein and are characterized by the insertion of one or two leucines into a stretch of nine leucines (Chen et al, 2005;Yue et al, 2006;Abifadel et al, 2008). They might lead to a structural change in the signal peptide causing an impairment in its cleavage and processing in the endoplasmic reticulum (Yue et al, 2006;Pisciotta et al, 2012;Benito-Vicente et al, 2022). In a study conducted on 1,745 apparently healthy individuals, plasma PCSK9 levels were significantly lower in individuals carrying a leucine insertion in exon 1 of the PCSK9 gene (Awan et al, 2013).…”

“…On one hand, the leucine insertion L10 also designated p.Leu21dup or p.L15_L16insL is a common variation associated with lower levels of LDL-C in populations with normal to low LDL-C levels, but also in patients suffering from FH carrying the same p.(Cys681X) mutation in the LDL receptor (LDLr) gene (Abifadel et al, 2009). On the other hand, the leucine insertion L11 also designated p.Leu21tri or p.L15_L16ins2L is a rare variant that has been associated with familial combined hyperlipidemia and was found at a low frequency in subjects presenting LDL-C levels of 2.96-4.90 mmol/L and coronary lesions in the American population (Chen et al, 2005;Abifadel et al, 2008;Benito-Vicente et al, 2022). In fact, low levels of LDL-C and HDL-C in T1HLP may be explained by the disruption in the activity of the LPL (Hegele et al, 2018).…”