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.

先前已描述破坏硬化素（由SOST编码）与其受体 LRP4 之间结合的致病变异会导致硬化症，这是一种罕见的高骨量疾病。硬化素-LRP4 复合物抑制经典 WNT 信号传导，这是调节成骨细胞骨形成的关键途径，也是骨质疏松症等常见骨病的有希望的治疗靶点。在当前的研究中，我们将缺乏Sost ( Sost ^−/− ) 的小鼠与我们的p.Arg1170Gln Lrp4敲入( Lrp4 ^KI/KI ) 小鼠模型杂交，以产生双突变体Sost ^−/− ;Lrp4 ^KI/KI小鼠。我们将Sost ^−/−小鼠的表型与Sost ^−/− ;Lrp4 ^KI/KI小鼠的表型进行比较，以研究Lrp4中致病的p.Arg1170Trp 变体对Sost缺陷的可能协同作用。有趣的是， Lrp4 ^KI等位基因的存在部分减轻了Sost ^−/−表型。细胞和动态组织形态计量学没有揭示观察到的表型差异的机制。因此，我们通过对Lrp4 ^{KI /KI}原代成骨细胞进行批量 RNA 测序来确定Lrp4 KI 等位基因的分子效应。出乎意料的是，大多数与骨吸收或重塑相关的基因（Acp5、Rankl、Mmp9 ）在Lrp4 ^KI/KI原代成骨细胞中上调。在Lrp4 ^KI/KI、Sost ^−/−和Sost ^−/− ;Lrp4 ^KI/KI小鼠中验证这些标记物表明，硬化素缺乏会抵消Sost ^−/− ;Lrp4 ^KI/KI小鼠中的Lrp4 ^KI/KI效应。因此，我们假设具有两个失活Lrp4 ^KI等位基因的模型会激活骨重塑，从而使骨量净增加，而硬化素缺乏对骨形成具有更强大的合成代谢作用。此外，硬化素和 Lrp4 的这些作用在雌性小鼠中更强，导致比雄性小鼠更严重的表型，并且不同基因型之间更容易检测到表型差异。