The R-spondins are a family of four small, secreted agonists of the Wnt signaling pathway. Growing evidence from both in vitro studies and in vivo models supports the major role of these proteins in the skeletal development processes. In humans, common genetic variation in the RSPO3 gene has been associated with BMD in large scale GWAS study.
This study aimed at further investigation of the genetic and functional contributions of the R-spondins in bone biology. We performed an expression study in differentiating osteoblastic KS483 cells revealing that only RSPO1-3 are robustly expressed during this process. After prioritizing RSPO1-3 for genetic testing, we conducted a candidate gene mutation screening in a population of patients suffering from different forms of craniotubular hyperostosis in search for rare, activating mutations in the genes but no potentially pathogenic variants were detected. To further study the relation between common variants in R-spondin genes and BMD we decided to perform a candidate gene association study in two subpopulations of Odense Androgen Study (OAS) stratified based on BMD. This study was performed to confirm the association signals detected in GWAS and search for causal variants with increased resolution. Unfortunately, no significant associations were detected in our cohort. Interestingly, one of the polymorphisms, namely rs140821794 (p.Met16Val) was only detected in a single individual out of the low BMD OAS subpopulation. We performed a dual luciferase reporter assay to test the functional relevance of this variant but could not confirm the causal role of this polymorphism.
Taken together, our data suggest that despite robust expression during bone formation and clear Wnt signaling activation in the presence of the R-spondins, the genetic variation in these genes does not contribute greatly to the genetic determination of bone mass. These findings do not diminish the potential utility of the R-spondins as targets for modulating the Wnt signaling pathway in future treatments of osteoporosis.
14 May 2016 - 17 May 2016