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

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.