Novel anabolic drug targets are needed to treat osteoporosis. Having established a large national cohort with unexplained high bone mass (HBM), we aimed to identify a novel monogenic cause of HBM and provide insight into a regulatory pathway potentially amenable to therapeutic intervention. We investigated a pedigree with unexplained HBM in whom previous sequencing had excluded known causes of monogenic HBM. Whole exome sequencing identified a rare (minor allele frequency 0.0023), highly evolutionarily conserved missense mutation in SMAD9 (c.65T>C, p.Leu22Pro) segregating with HBM in this autosomal dominant family. The same mutation was identified in another two unrelated individuals both with HBM. In silico protein modeling predicts the mutation severely disrupts the MH1 DNA-binding domain of SMAD9. Affected individuals have bone mineral density (BMD) Z-scores +3 to +5, mandible enlargement, a broad frame, torus palatinus/mandibularis, pes planus, increased shoe size, and a tendency to sink when swimming. Peripheral quantitative computed tomography (pQCT) measurement demonstrates increased trabecular volumetric BMD and increased cortical thickness conferring greater predicted bone strength; bone turnover markers are low/normal. Notably, fractures and nerve compression are not found. Both genome-wide and gene-based association testing involving estimated BMD measured at the heel in 362,924 white British subjects from the UK Biobank Study showed strong associations with SMAD9 (PGWAS = 6 × 10-16 ; PGENE = 8 × 10-17 ). Furthermore, we found Smad9 to be highly expressed in both murine cortical bone-derived osteocytes and skeletal elements of zebrafish larvae. Our findings support SMAD9 as a novel HBM gene and a potential novel osteoanabolic target for osteoporosis therapeutics. SMAD9 is thought to inhibit bone morphogenetic protein (BMP)-dependent target gene transcription to reduce osteoblast activity. Thus, we hypothesize SMAD9 c.65T>C is a loss-of-function mutation reducing BMP inhibition. Lowering SMAD9 as a potential novel anabolic mechanism for osteoporosis therapeutics warrants further investigation. © 2019 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

中文翻译：

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SMAD9 中与高骨量相关的罕见突变将依赖 SMAD 的 BMP 信号通路确定为骨质疏松症的潜在合成代谢靶点。

需要新的合成代谢药物靶点来治疗骨质疏松症。在建立了一个具有无法解释的高骨量 (HBM) 的大型国家队列后，我们旨在确定 HBM 的一种新的单基因原因，并提供对可能适合治疗干预的调节途径的洞察。我们调查了一个具有无法解释的 HBM 的谱系，其中先前的测序已经排除了单基因 HBM 的已知原因。全外显子组测序鉴定了一个罕见的（次要等位基因频率为 0.0023）、高度进化保守的 SMAD9 错义突变（c.65T>C，p.Leu22Pro），在这个常染色体显性家族中与 HBM 分离。在另外两个与 HBM 无关的个体中发现了相同的突变。计算机蛋白质模型预测该突变严重破坏了 SMAD9 的 MH1 DNA 结合域。受影响的个体有骨矿物质密度 (BMD) Z 评分 +3 至 +5、下颌骨扩大、宽框架、腭环/下颌肌、pes planus、鞋码增加以及游泳时下沉的趋势。外周定量计算机断层扫描 (pQCT) 测量显示骨小梁体积 BMD 增加和皮质厚度增加，从而提供更大的预测骨强度；骨转换标志物低/正常。值得注意的是，没有发现骨折和神经受压。来自英国生物库研究的 362,924 名英国白人受试者的脚后跟测量的全基因组和基于基因的关联测试均显示与 SMAD9 密切相关（PGWAS = 6 × 10-16；PGENE = 8 × 10-17）。此外，我们发现 Smad9 在小鼠皮质骨来源的骨细胞和斑马鱼幼虫的骨骼成分中都高度表达。我们的研究结果支持 SMAD9 作为一种新型 HBM 基因和用于骨质疏松症治疗的潜在新型骨合成代谢靶点。SMAD9 被认为可抑制骨形态发生蛋白 (BMP) 依赖性靶基因转录，从而降低成骨细胞活性。因此，我们假设 SMAD9 c.65T>C 是一种功能丧失突变，可降低 BMP 抑制。降低 SMAD9 作为骨质疏松症治疗的潜在新合成代谢机制值得进一步研究。© 2019 作者。美国骨与矿物质研究学会出版的骨与矿物质研究杂志。SMAD9 被认为可抑制骨形态发生蛋白 (BMP) 依赖性靶基因转录，从而降低成骨细胞活性。因此，我们假设 SMAD9 c.65T>C 是一种功能丧失突变，可降低 BMP 抑制。降低 SMAD9 作为骨质疏松症治疗的潜在新合成代谢机制值得进一步研究。© 2019 作者。美国骨与矿物质研究学会出版的骨与矿物质研究杂志。SMAD9 被认为可抑制骨形态发生蛋白 (BMP) 依赖性靶基因转录，从而降低成骨细胞活性。因此，我们假设 SMAD9 c.65T>C 是一种功能丧失突变，可降低 BMP 抑制。降低 SMAD9 作为骨质疏松症治疗的潜在新合成代谢机制值得进一步研究。© 2019 作者。美国骨与矿物质研究学会出版的骨与矿物质研究杂志。