GWAS meta-analysis followed by Mendelian randomization revealed potential control mechanisms for circulating α-Klotho levels

Ingrid Gergei,Vincent Brandenburg,Jie Zheng,Bertram Müller-Myhsok,Winfried März,Claes Ohlsson,Jakob Voelkl,Daniel Richard,Sofia Movérare-Skrtic,Jonathan H Tobias,Louise Falk,Nazanin Mirza-Schreiber,George Davey Smith,Bernhard K Krämer,Till F M Andlauer

doi:10.1093/hmg/ddab263

Abstract

The protein α-Klotho acts as transmembrane co-receptor for fibroblast growth factor 23 (FGF23) and is a key regulator of phosphate homeostasis. However, α-Klotho also exists in a circulating form, with pleiotropic, but incompletely understood functions and regulation. Therefore, we undertook a genome-wide association study (GWAS) meta-analysis followed by Mendelian randomization (MR) of circulating α-Klotho levels.Plasma α-Klotho levels were measured by enzyme-linked immunosorbent assay (ELISA) in the Ludwigshafen Risk and Cardiovascular Health and Avon Longitudinal Study of Parents and Children (mothers) cohorts, followed by a GWAS meta-analysis in 4376 individuals across the two cohorts.Six signals at five loci were associated with circulating α-Klotho levels at genome-wide significance (P < 5 × 10−8), namely ABO, KL, FGFR1, and two post-translational modification genes, B4GALNT3 and CHST9. Together, these loci explained >9% of the variation in circulating α-Klotho levels. MR analyses revealed no causal relationships between α-Klotho and renal function, FGF23-dependent factors such as vitamin D and phosphate levels, or bone mineral density. The screening for genetic correlations with other phenotypes followed by targeted MR suggested causal effects of liability of Crohn’s disease risk [Inverse variance weighted (IVW) beta = 0.059 (95% confidence interval 0.026, 0.093)] and low-density lipoprotein cholesterol levels [−0.198 (−0.332, −0.063)] on α-Klotho.Our GWAS findings suggest that two enzymes involved in post-translational modification, B4GALNT3 and CHST9, contribute to genetic influences on α-Klotho levels, presumably by affecting protein turnover and stability. Subsequent evidence from MR analyses on α-Klotho levels suggest regulation by mechanisms besides phosphate-homeostasis and raise the possibility of cross-talk with FGF19- and FGF21-dependent pathways, respectively. Significance statement: α-Klotho as a transmembrane protein is well investigated along the endocrine FGF23-α-Klotho pathway. However, the role of the circulating form of α-Klotho, which is generated by cleavage of transmembrane α-Klotho, remains incompletely understood. Genetic analyses might help to elucidate novel regulatory and functional mechanisms. The identification of genetic factors related to circulating α-Klotho further enables MR to examine causal relationships with other factors. The findings from the first GWAS meta-analysis of circulating α-Klotho levels identified six genome-wide significant signals across five genes. Given the function of two of the genes identified, B4GALNT3 and CHST9, it is tempting to speculate that post-translational modification significantly contributes to genetic influences on α-Klotho levels, presumably by affecting protein turnover and stability.

Highlights

Introduction αKlotho is a transmembrane protein that serves with fibroblast growth factor receptors as a co-receptor for fibroblast growth factor 23 (FGF23) [1]
The top-associated single-nucleotide polymorphism (SNP), rs12607664 [standard deviations difference relative to mean in plasma α-Klotho per minor allele (MA) T: β = 0.24, standard error (SE) = 0.02, P = 2.3 × 10−27], mapped to the second intron of the gene CHST9 on chromosome 18
Having performed a genome-wide association study (GWAS) for circulating levels of α-Klotho, we identified six GWAS significant signals, mapping to five genes, which together explained over 9% of the variance

Summary

Introduction

Klotho is a transmembrane protein that serves with fibroblast growth factor receptors as a co-receptor for fibroblast growth factor 23 (FGF23) [1]. The endocrine FGF23-α-Klotho pathway plays a critical role in regulating vitamin D metabolism and phosphate balance [2]. Though the role of circulating αKlotho remains incompletely understood, it is thought to share functional similarities with the membrane-bound form and to contribute to the actions of its ligand, FGF23 [5]. Circulating α-Klotho mediates effects independent of FGF23 [6]. Previous studies have investigated the clinical utility of serum α-Klotho as a prognostic marker. A recent metaanalysis found that serum levels of circulating α-Klotho are positively related to the estimated glomerular filtration rate (eGFR) in patients with chronic kidney disease (CKD) [7], suggesting a role of α-Klotho as a biomarker for CKD progression. Α-Klotho supplementation is under investigation as a possible drug target for treatment in CKD [5,8]

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Conclusion