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 Randomisation (MR). A genetic instrument for circulating sclerostin, derived from a genome wide association study (GWAS) meta-analysis of serum sclerostin in 10,584 Europeandescent individuals, was examined in relation to femoral neck bone mineral density (BMD; n= 32,744) in GEFOS, and estimated BMD by heel ultrasound (eBMD; n=426,824), and fracture risk (n=426,795), in UK Biobank. Our GWAS identified two novel serum sclerostin loci, B4GALNT3 (standard deviation (SD)) change in sclerostin per A allele (β=0.20, P=4.6x10 -49 ), and GALNT1 (β=0.11 per G allele, P=4.4x10 -11 ). B4GALNT3 is an N-acetylgalactosaminyltransferase, 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 SNPs as genetic instruments, MR revealed an inverse causal relationship between serum sclerostin and femoral neck BMD (β= -0.12, 95%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.

Sclerostin Downregulation Globally by Naturally Occurring Genetic Variants, or Locally in Atherosclerotic Plaques, Does Not Associate With Cardiovascular Events in Humans

“…We assessed two further variants remote from SOST that are associated with reduced circulating sclerostin protein levels in trans ( 35 ) (rs215226, located on chromosome 12 in B4GALNT3 ; rs7241221, located on chromosome 18 next to GALNT1 ); no cis variants were associated with circulating protein levels of sclerostin. These alleles were also associated with increased BMD and other musculoskeletal and anthropometric phenotypes but were not associated with any cardiovascular phenotypes (Supplemental Fig.…”

“…Two additional strategies were used to select variants related to reduced sclerostin levels. The first are variants that are associated with circulating sclerostin protein levels at genomewide significance level (rs215226 and rs7241221) ( 35 ) and the second are two variant sets associated with BMD levels in the SOST region (rs7209826 and rs188810925; rs2741856 and rs7217502) found in recent large‐scale genomewide association studies. ( 32,36 ) The correlations between the variants in these variant sets are displayed in Supplemental Fig.…”

Sclerostin and Cardiovascular Disease

“…However, rather than implying that sclerostin inhibition is likely to have a beneficial (as opposed to negative) influence on these outcomes, the direction of associations seen observationally may not necessarily reflect causal effects. This is exemplified by relationships between sclerostin and BMD, since whereas sclerostin levels are considerably elevated in those with high bone mass [15], sclerostin acts causally to lower BMD [16]. The positive association between sclerostin and BMD in observational studies might also contribute to the positive observational association between sclerostin levels and CVD risk through confounding.…”

“…Presumably, cis SNPs within the SOST gene influence sclerostin production in tissues via effects on transcriptional efficiency and mRNA expression. In terms of trans SNPs, B4GALNT3 was the strongest genetic signal for circulating sclerostin, and contributed to the relationship between with sclerostin levels, eBMD and fracture risk [16]. Precisely how the protein product beta-1,4-N-acetylgalactosaminyltransferase 3 (B4GALNT3), a glycosylation enzyme, affects circulating sclerostin levels is unclear.…”