PCSK9, from gene to protein and plasma. M. Abifadel, B. Jessica, A. Marques, M. Devillers, D. Erlich, J.P. Rabès, C. Boileau, M. Varret. INSERM U781, INSERM U781 hôpital Necker, France, Paris cedex 15, France.
   Autosomal Dominant Hypercholesterolemia (ADH) is one of the most frequent human inherited disorders. Until 2003, mutations in two major genes, LDLR and APOB, had been clearly implicated in the disease. We were the first to identify a third gene involved in ADH by positional cloning in French non LDLR/non APOB families. The PCSK9 gene (Proprotein Convertase Subtilisin Kexin 9) encodes an enzyme Narc-1 (Neural Apoptosis Regulated Convertase). Several hypercholesterolemic mutations of PCSK9 have been reported: S127R, F216L, D374Y, R218S and R357H and three variations have been associated with LDLR mutations in patient with severe hypercholesterolemia. Two non sense variations Y142X and C679X (frequency of 2% in black Americans) were associated with a reduction of LDL-cholesterol levels and of CHD. The R46L variation (frequency of 3.2% in white Americans) is associated with a reduction of LDL-cholesterol of 15% and 47% of CHD. We studied the frequency of 2 variations in 600 Caucasian (400 hypercholesterolemic and 200 normolipemic) subjects. R46L was found only in the control population with a frequency of 2%. A443T was found in a woman aged 50 who presented with mild hypercholesterolemia and no mutation in LDLR and APOB genes. This variation was not found in 340 French Caucasians controls. This variation turns out to be a rare polymorphism in whites, more frequent in blacks, associated with lower plasma levels of LDL-C but which has been found in both low and high LDL-C subjects in the Dallas Heart Study. We have also studied PCSK9 protein in the plasma of two S127R carrier patients by western blot analysis after resolution by SDS-PAGE (4-12%). The profile found for these plasmas was not different from the control. The S127R mutation doesnt seem to modify the molecular weight of the secreted PCSK9 forms. But modifications of its interaction with other proteins or of the amount of PCSK9 secreted cannot be ruled out and are under investigation. PCSK9 is an attractive therapeutic target for LDL-C lowering but further investigations are required to understand its precise role in cholesterol homeostasis and to identify its substrates and inhibitors.