Differential effects of PCSK9 loss of function variants on serum lipid and PCSK9 levels in Caucasian and African Canadian populations

Lipids in Health and Disease, May 2013

Objectives Variants of the secreted glycoprotein, proprotein convertase subtilisin/kexin 9 (PCSK9), associate with both hypo- and hyper-cholesterolemic phenotypes. Herein, we carried out full exonic sequencing of PCSK9 documenting the frequency of single and multiple PCSK9 variations and their effects on serum lipoprotein and PCSK9 levels in Caucasian Canadians. Methods The 12 exons of PCSK9 were sequenced in 207 unrelated Caucasian Canadians. Minor allele frequencies of PCSK9 variants were compared amongst LDL cholesterol (LDLC) quintiles. Serum PCSK9 levels were measured by ELISA and lipoproteins by enzymatic methods. Comparisons were made with a Caucasian family cohort (n = 51) and first generation African Canadians (n = 31). Results In Caucasians, but not African Canadians, the c.61_63insCTG (denoted L10Ins) and A53V PCSK9 variations were linked and their frequency was significantly higher among Caucasian Canadians with LDLC levels in the <25th percentile. In both the unrelated and family Caucasian cohorts those carrying the L10A53V PCSK9 variant had significantly lower LDLC without reduction in plasma PCSK9. The I474V PCSK9 variant associated with significantly lower serum PCSK9 and LDLC. A novel PCSK9 variant was identified; E206K. We found that the frequency of multiple PCSK9 variations was higher in first generation African Canadians. Conclusions We showed that the L10A53V and I474V PCSK9 variants were significantly associated with lower LDLC levels in Caucasian Canadians but differed in their effect on serum PCSK9 concentrations, illuminating differences in their mechanism of inaction and indicating that that PCSK9 measurement alone may not always be a good indicator of PCSK9 function. Full exonic sequencing of PCSK9 pointed to factors that may contribute to L10Ins PCSK9 variant loss of function in Canadians of Caucasian but not those of African descent. These included; (1) its tight linkage with the A53V variant in Caucasians and/or (2) for both the L10 and I474V, the combined (and negating) effect of multiple, differing phenotypic PCSK9 variants within individuals of African ancestry for which combinations of PCSK9 variations and their overall frequency was higher. No population studies, to our knowledge, have addressed or accessed the effect of multiple PCSK9 variants on cholesterol profiles. Our results indicate that this should be considered.

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Differential effects of PCSK9 loss of function variants on serum lipid and PCSK9 levels in Caucasian and African Canadian populations

Janice Mayne 1 Teik Chye Ooi 0 Angela Raymond 1 Marion Cousins 0 Lise Bernier Thilina Dewpura 1 Francine Sirois Majambu Mbikay Jean Davignon Michel Chrtien 1 0 Clinical Research Laboratory, Division of Endocrinology and Metabolism, Department of Medicine, The Ottawa Hospital, University of Ottawa , 1967 Riverside Drive, Ottawa, ON K1H 7W9 , Canada 1 Ottawa Institute of Systems Biology, Department of Biochemistry , Microbiology and Immunology , University of Ottawa , 451 Smyth Road, Ottawa, ON K1H 8M5 , Canada Objectives: Variants of the secreted glycoprotein, proprotein convertase subtilisin/kexin 9 (PCSK9), associate with both hypo- and hyper-cholesterolemic phenotypes. Herein, we carried out full exonic sequencing of PCSK9 documenting the frequency of single and multiple PCSK9 variations and their effects on serum lipoprotein and PCSK9 levels in Caucasian Canadians. Methods: The 12 exons of PCSK9 were sequenced in 207 unrelated Caucasian Canadians. Minor allele frequencies of PCSK9 variants were compared amongst LDL cholesterol (LDLC) quintiles. Serum PCSK9 levels were measured by ELISA and lipoproteins by enzymatic methods. Comparisons were made with a Caucasian family cohort (n = 51) and first generation African Canadians (n = 31). Results: In Caucasians, but not African Canadians, the c.61_63insCTG (denoted L10Ins) and A53V PCSK9 variations were linked and their frequency was significantly higher among Caucasian Canadians with LDLC levels in the <25th percentile. In both the unrelated and family Caucasian cohorts those carrying the L10A53V PCSK9 variant had significantly lower LDLC without reduction in plasma PCSK9. The I474V PCSK9 variant associated with significantly lower serum PCSK9 and LDLC. A novel PCSK9 variant was identified; E206K. We found that the frequency of multiple PCSK9 variations was higher in first generation African Canadians. Conclusions: We showed that the L10A53V and I474V PCSK9 variants were significantly associated with lower LDLC levels in Caucasian Canadians but differed in their effect on serum PCSK9 concentrations, illuminating differences in their mechanism of inaction and indicating that that PCSK9 measurement alone may not always be a good indicator of PCSK9 function. Full exonic sequencing of PCSK9 pointed to factors that may contribute to L10Ins PCSK9 variant loss of function in Canadians of Caucasian but not those of African descent. These included; (1) its tight linkage with the A53V variant in Caucasians and/or (2) for both the L10 and I474V, the combined (and negating) effect of multiple, differing phenotypic PCSK9 variants within individuals of African ancestry for which combinations of PCSK9 variations and their overall frequency was higher. No population studies, to our knowledge, have addressed or accessed the effect of multiple PCSK9 variants on cholesterol profiles. Our results indicate that this should be considered. - Background Elevated low density lipoprotein cholesterol (LDLC) levels are recognized as a major risk factor for coronary artery disease [1,2]. Cell surface liver low density lipoprotein receptors (LDLR) clear LDL particles from circulation by binding LDL particles through its protein component, apolipoprotein B100 (ApoB100), followed by endocytosis [3]. LDL is released from the LDLR in the endosome and travels to the lysosome for degradation, while the LDLR recycles to the cell surface. Gene variations in LDLR (low density lipoprotein receptor) and APOB (apolipoprotein B100) were identified as causes of autosomal dominant hypercholesterolemia (ADH) and named familial hypercholesterolemia (FH) 1 and FH2, respectively [2]. They account for approximately 80% of all FH cases [4,5]. The FH3 locus was identified in 2003 by Abifadel et al. as PCSK9 (Proprotein Convertase Subtilisin/kexin type 9) [6]. PCSK9 is the ninth member of a family of endoproteolytic enzymes important in development and normal physiology due to their regulated proteolytic maturation of proproteins including neuropeptides, hormones, cytokines, growth factors, receptors, cell surface and serum proteins [7,8]. The 12 exons of PCSK9 encode a 692 amino acid secreted glycoprotein [9]. PCSK9, like its family members, is synthesized as a preproprotein containing several well-defined motifs including a pre-signal, proinhibitory, catalytic and c-terminal domain [10]. Following autocatalytic cleavage, PCSK9 and its inhibitory pro domain are secreted as a heterodimer [9]. Therefore, unlike its family members, PCSK9 does not function as an enzyme and instead plays a moonlighting role as an escort protein [11,12]. PCSK9 binds to LDLR at the cell surface [13] and escorts it from the endosomal recycling pathway toward the lysosomal compartment for degradation, thereby modifying circulating LDLC levels [14-17]. Regions within PCSK9s catalytic, pro and c-terminal domains have been implicated in the equilibrium between PCSK9:LDLR binding at the cell surface and in the endosome [13,18-21]. PCS (...truncated)


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Janice Mayne, Teik Chye Ooi, Angela Raymond, Marion Cousins, Lise Bernier, Thilina Dewpura, Francine Sirois, Majambu Mbikay, Jean Davignon, Michel Chrétien. Differential effects of PCSK9 loss of function variants on serum lipid and PCSK9 levels in Caucasian and African Canadian populations, Lipids in Health and Disease, 2013, pp. 70, 12, DOI: 10.1186/1476-511X-12-70