RSPO3 is important for trabecular bone and fracture risk in mice and humans

Abstract

With increasing age of the population, countries across the globe are facing a substantial increase in osteoporotic fractures. Genetic association signals for fractures have been reported at the RSPO3 locus, but the causal gene and the underlying mechanism are unknown. Here we show that the fracture reducing allele at the RSPO3 locus associate with increased RSPO3 expression both at the mRNA and protein levels, increased trabecular bone mineral density and reduced risk mainly of distal forearm fractures in humans. We also demonstrate that RSPO3 is expressed in osteoprogenitor cells and osteoblasts and that osteoblast-derived RSPO3 is the principal source of RSPO3 in bone and an important regulator of vertebral trabecular bone mass and bone strength in adult mice. Mechanistic studies revealed that RSPO3 in a cell-autonomous manner increases osteoblast proliferation and differentiation. In conclusion, RSPO3 regulates vertebral trabecular bone mass and bone strength in mice and fracture risk in humans.

Estradiol and RSPO3 regulate vertebral trabecular bone mass independent of each other

“…Mice with cell-specific inactivation of RSPO3 were generated by breeding Rspo3 flox/flox mice ( Rspo3 tm1.1Jcob /J, JAX stock 027313, Jackson Laboratories) ( 24 ) with the Tg(Runx2-icre)1Jtuc mice, expressing the cre construct under the Runx2 promotor ( 11 , 25 ), to generate the Runx2-creRspo3 flox/flox mice with a conditional inactivation of RSPO3 in osteoblast-lineage cells, as previously described ( 22 ). Tg(Runx2-icre)1Jtuc mice have an unchanged skeletal phenotype compared with wild-type mice ( 11 ).…”

“…The binding of RSPOs to the LGR receptor reduces the degradation of Frizzled receptor, leading to an increase in WNT signaling ( 17 , 19 – 21 ). Osteoblast-derived RSPO3 increases trabecular bone mass and bone strength in the vertebrae of mice ( 22 ). In addition, certain genetic variations in the RSPO3 locus are associated with trabecular volumetric bone mineral density (vBMD) and risk of distal forearm fractures in humans ( 22 ).…”

“…Osteoblast-derived RSPO3 increases trabecular bone mass and bone strength in the vertebrae of mice ( 22 ). In addition, certain genetic variations in the RSPO3 locus are associated with trabecular volumetric bone mineral density (vBMD) and risk of distal forearm fractures in humans ( 22 ). Previous large-scale human genetic studies reported that the second (after the WNT16 locus) strongest genetic determinant for fractures at any bone site is located at the RSPO3 locus ( 23 ).…”

Body mass index and adipose distribution have opposing genetic impacts on human blood traits

“…Interpretation of SNPs at the RSPO3 locus also dramatically changed ( Figure 2b and d ). RSPO3, a Wnt pathway modulator that directs development of bone and other tissues ( Nilsson et al, 2021 ), has been linked with adipose distribution ( Pulit et al, 2019 ) and blood trait variation ( Vuckovic et al, 2020 ). Similar effects were seen in adjusted HCT data ( Figure 2a and b and Supplementary file 1 — Tables 6–7) and quantitative traits across blood lineages ( Figure 2—figure supplement 1 ).…”

Bone Phenotyping Approaches in Human, Mice and Zebrafish – Expert Overview of the EU Cost Action GEMSTONE (“GEnomics of MusculoSkeletal traits TranslatiOnal NEtwork”)

“…In mouse models, most studies describe changes in material properties and histology at a certain timepoint ( 37 ). Therefore, fracture history for an individual mouse will not be evaluated.…”