An Additional Lrp4 High Bone Mass Mutation Mitigates the Sost-Knockout Phenotype in Mice by Increasing Bone Remodeling.

Pathogenic variants disrupting the binding between sclerostin (encoded by SOST) and its receptor LRP4 have previously been described to cause sclerosteosis, a rare high bone mass disorder. The sclerostin-LRP4 complex inhibits canonical WNT signaling, a key pathway regulating osteoblastic bone formation and a promising therapeutic target for common bone disorders, such as osteoporosis. In the current study, we crossed mice deficient for Sost (Sost -/- ) with our p.Arg1170Gln Lrp4 knock-in (Lrp4 KI/KI ) mouse model to create double mutant Sost -/- ;Lrp4 KI/KI mice. We compared the phenotype of Sost -/- mice with that of Sost -/- ;Lrp4 KI/KI mice, to investigate a possible synergistic effect of the disease-causing p.Arg1170Trp variant in Lrp4 on Sost deficiency. Interestingly, presence of Lrp4 KI alleles partially mitigated the Sost -/- phenotype. Cellular and dynamic histomorphometry did not reveal mechanistic insights into the observed phenotypic differences. We therefore determined the molecular effect of the Lrp4 KI allele by performing bulk RNA sequencing on Lrp4 KI/KI primary osteoblasts. Unexpectedly, mostly genes related to bone resorption or remodeling (Acp5, Rankl, Mmp9) were upregulated in Lrp4 KI/KI primary osteoblasts. Verification of these markers in Lrp4 KI/KI , Sost -/- and Sost -/- ;Lrp4 KI/KI mice revealed that sclerostin deficiency counteracts this Lrp4 KI/KI effect in Sost -/- ;Lrp4 KI/KI mice. We therefore hypothesize that models with two inactivating Lrp4 KI alleles rather activate bone remodeling, with a net gain in bone mass, whereas sclerostin deficiency has more robust anabolic effects on bone formation. Moreover, these effects of sclerostin and Lrp4 are stronger in female mice, contributing to a more severe phenotype than in males and more detectable phenotypic differences among different genotypes.