Mendelian Randomization Analysis Reveals a Causal Influence of Circulating Sclerostin Levels on Bone Mineral Density and Fractures

Abstract

In bone, sclerostin is mainly osteocyte-derived and plays an important local role in adaptive responses to mechanical loading. Whether circulating levels of sclerostin also play a functional role is currently unclear, which we aimed to examine by two-sample Mendelian randomization (MR). A genetic instrument for circulating sclerostin, derived from a genomewide association study (GWAS) meta-analysis of serum sclerostin in 10,584 European-descent individuals, was examined in relation to femoral neck bone mineral density (BMD; n = 32,744) in GEFOS and estimated bone mineral density (eBMD) by heel ultrasound (n = 426,824) and fracture risk (n = 426,795) in UK Biobank. ◼This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.Our GWAS identified two novel serum sclerostin loci, B4GALNT3 (standard deviation [SD]) change in sclerostin per A allele (β = 0.20, p = 4.6 × 10 −49 ) and GALNT1 (β = 0.11 per G allele, p = 4.4 × 10 −11 ). B4GALNT3 is an N-acetyl-galactosaminyltransferase, adding a terminal LacdiNAc disaccharide to target glycocoproteins, found to be predominantly expressed in kidney, whereas GALNT1 is an enzyme causing mucin-type O-linked glycosylation. Using these two single-nucleotide polymorphisms (SNPs) as genetic instruments, MR revealed an inverse causal relationship between serum sclerostin and femoral neck BMD (β = -0.12, 95% confidence interval [CI] -0.20 to -0.05) and eBMD (β = -0.12, 95% CI -0.14 to -0.10), and a positive relationship with fracture risk (β = 0.11, 95% CI 0.01 to 0.21). Colocalization analysis demonstrated common genetic signals within the B4GALNT3 locus for higher sclerostin, lower eBMD, and greater B4GALNT3 expression in arterial tissue (probability >99%). Our findings suggest that higher sclerostin levels are causally related to lower BMD and greater fracture risk. Hence, strategies for reducing circulating sclerostin, for example by targeting glycosylation enzymes as suggested by our GWAS results, may prove valuable in treating osteoporosis. Fig. 2. Forest plot of putative causal relationship between serum sclerostin and bone phenotypes using Mendelian randomization. The x axis represents the causal estimates and 95% confidence intervals of SD change in BMD/eBMD and OR for fracture, per SD increase in sclerostin, as calculated by inverse variance-weighted method. The y axis lists the four bone phenotypes used in the MR analysis. Journal of Bone and Mineral Research ■ 1830 ZHENG ET AL.In murine gene expression studies, B4galnt3 mRNA was expressed at highest levels in kidney, with relatively high levels of expression ◼ 8 ZHENG ET AL.

Use of Mendelian Randomization to Examine Causal Inference in Osteoporosis

“…For example, in their study examining causal relationships between blood lipids and eBMD, Cherny et al found broadly similar inverse associations between LDL-C and eBMD as assessed by inverse-variance weighted (IVW), MR-Egger, weighted median and weighted mode estimates (53). Similarly, in our recent study, MR-Egger, IVW and weighted median estimates showed similar causal effects of BMD on sclerostin (54). That said, the statistical evidence against the null for the MR-Egger estimates was somewhat lower, in both these papers, reflecting the lower statistical power of this test (48, 49).…”

“…It's a reasonable assumption that correlated variables as a result of shared biology show equivalent “causal” effects on bidirectional MR, whereas for a true causal relationship, an effect is just observed in one direction. However, we recently observed a further pattern in our study exploring the relationship between circulating sclerostin levels on eBMD, namely bidirectional causal pathways in opposite directions (54). We found that higher levels of serum sclerostin were causally related to lower FN BMD, lower eBMD and higher fracture risk.…”

“…For example, we recently performed an MR study to examine the causal relationship between sclerostin levels and eBMD, based on results of a sclerostin GWAS where we identified just two loci. However, we were able to establish a causal relationship between sclerostin and eBMD using co-localization analysis, which interrogates LD structure at a single locus, in this case the gene encoding B4GALNT3 (54).…”

“…We found that higher levels of serum sclerostin were causally related to lower FN BMD, lower eBMD and higher fracture risk. In contrast, greater BMD was causally related to higher sclerostin levels, using BMD SNPs identified in the GEFOS BMD GWAS (54). This finding aligned with the observational relationship between BMD and sclerostin we reported in the same paper and may be a reflection of a previously unsuspected negative control feedback mechanism for BMD (see Figure 3).…”

Cytokines CCL2 and CXCL1 may be potential novel predictors of early bone loss

“…other biomarkers were also indicated to be associated with BMd. For instance, early bone loss is frequently accompanied by changes in the body microenvironments, in particular, changes in serum cytokine levels, such as cc motif ligand 4 (CCL4)/macrophage inflammatory protein-1β (MiP-1β) (19), sclerostin (20) and neuropeptide vasoactive intestinal peptide (21). Based on this serum screening method, the present study explored which of the various cytokines may be used as predictors of early bone loss.…”