破骨细胞分化和功能异常激活是骨质疏松症发生的重要原因。Msx1基因在骨骼发育和细胞分化中发挥重要作用，最新研究提示Msx1基因与骨质疏松相关，但其具体作用和机制未明。我们前期研究发现Msx1基因在破骨细胞中表达下调，破骨细胞条件性Msx1敲除小鼠表现为骨量丢失。进一步研究发现Msx1基因表达下调与转录因子PU.1和Dnmt3a介导的DNA甲基化修饰有关，同时骨质疏松患者Msx1基因启动子处于高甲基化状态。由此我们提出假说：PU.1结合到Msx1基因启动子上并招募Dnmt3a促进其甲基化并抑制Msx1基因转录，从而促进破骨细胞分化导致骨质疏松。本项目拟通过临床样本研究Msx1基因启动子甲基化与骨质疏松的临床相关性，同时借助基因敲除小鼠、动物模型和多种体外实验技术阐明Msx1基因启动子甲基化的分子机制及Msx1基因启动子去甲基化对骨质疏松的治疗作用，为骨质疏松症的诊治提供新的理论和实验依据。

Dysfunction of osteoclast and its differentiation is an important cause of osteoporosis. Homeobox gene Msx1 plays an essential role on the skeletal development and cell differentiation. The latest study revealed that Msx1 might relate to osteoporosis, but the specific role and underlying mechanisms remain largely uncharacterized. Our preliminary experiments revealed that the Msx1 expression was down-regulated in osteoclasts, and Msx1 gene osteoclast conditioned knockout mice performed a bone loss phenotype. Further study found that the transcription factor PU.1 and DNA methyltransferase 3a (Dnmt3a) were involved in the methylation of the Msx1 promoter, which led to a decrease in Msx1 expression. Meanwhile, the Msx1 promoter hypermethylation was found in patients with osteoporosis. Thus, we hypothesized that the Msx1 promoter methylation occurred in response of the PU.1 recruited Dnmt3a via binding to the Msx1 promoter, and inhibited the Msx1 transcription, thereby regulating osteoclasts to induce osteoporosis. This project intends to clarify the clinic relevance of the methylation level of the Msx1 promoter and osteoporosis via clinical specimens. We will, subsequently, systematically elucidate molecular mechanisms of the Msx1 promoter methylation, and further study the therapeutic effect of the demethylation of Msx1 promoter on osteoporosis in vivo and in vitro. The results will provide a new diagnostic and therapeutic strategy of osteoporosis.