Targeted deletion of Sost distal enhancer increases bone formation and bone mass

Abstract

The Wnt antagonist Sost has emerged as a key regulator of bone homeostasis through the modulation of Lrp4/5/6 Wnt coreceptors. In humans, lack of Sclerostin causes sclerosteosis and van Buchem (VB) disease, two generalized skeletal hyperostosis disorders that result from hyperactive Wnt signaling. Unlike sclerosteosis, VB patients lack SOST coding mutations but carry a homozygous 52 kb noncoding deletion that is essential for the transcriptional activation of SOST in bone. We recently identified a putative bone enhancer, ECR5, in the VB deletion region, and showed that the transcriptional activity of ECR5 is controlled by Mef2C transcription factor in vitro. Here we report that mice lacking ECR5 or Mef2C through Col1-Cre osteoblast/osteocyte-specific ablation result in high bone mass (HBM) due to elevated bone formation rates. We conclude that the absence of the Sost-specific long-range regulatory element ECR5 causes VB disease in rodents, and that Mef2C is the main transcription factor responsible for ECR5-dependent Sost transcriptional activation in the adult skeleton. osteocytes S everal rare genetic disorders that interfere with Wnt signaling have provided strong evidence that the "canonical" Wnt signaling pathway is critical in bone (1). The Wnt coreceptor LRP5 has been described as a modulator of bone mass where loss-offunction mutations cause osteoporosis-pseudoglioma syndrome (OPPG) (2), an autosomal recessive disorder characterized by low bone mass and skeletal fragility; conversely, gain-of-function Lrp5 alleles cause high bone mass (HBM) (3). Similar hyperactive osteoblast activity due to elevated Wnt signaling was observed when Sost, a secreted Wnt inhibitor, was mutated in knockout (KO) mice or in sclerosteosis patients who suffer from generalized hyperostosis (4-6). Lrp5 gene targeting or SOST overexpression in transgenic (TG) mice causes osteopenia (3, 7), whereas TG overexpression of G171V Lrp5 allelic variant causes HBM, similar to the Sost KO phenotypes (4,8,9). The recapitulation of the human phenotypes in mouse models supports the conclusion that canonical Wnt signaling plays a critical role in bone metabolism, and points to Sost and Lrp5 as key regulators of bone mass.The skeletal phenotype describing sclerosteosis patients is similar to what has been documented for van Buchem (VB) disease. Although both VB and sclerosteosis map to the same locus on human chromosome 17 that includes the SOST transcript, the Sclerostin transcription unit was not affected in VB. All VB patients examined to date carry a 52-kb noncoding deletion, 35 kb downstream of SOST that results in the absence of postnatal SOST transcript and protein (10, 11). Although both sclerosteosis and VB are caused by sclerostin deficiency, the VB phenotype is a result of dysregulated SOST transcription. To identify the potential transcriptional regulatory elements responsible for Sost transcription in bone, we have characterized the expression of a human SOST transgene or an engineered allele corresponding to VB in mice. Only the wi...

Determining the Role of Sost and Sostdc1 During Fracture Healing

“…We have previously shown that conditionally knocking out a transcription factor, Mef2C, of Sost in osteoblasts results in a high bone mass phenotype similar to Sost -/-mice [83], which we would also expect to observe in Sost coinko; Col1 cre+.…”

“…3). Consistent with the human SOST null phenotype, the high bone mass phenotype is recapitulated in Sost knockout mice (Sost -/-) with 3-4 times more bone mineral density (BMD) [89]. In contrast, transgenic mice with elevated levels of human SOST develop osteopenia [81] and limb defects [90].…”

“…The Sclerostin (SOST) gene is located on human chromosome 17q12-q21, where its expression is regulated by myocyte enhancer factor 2 (MEF2C) transcription factor, which binds to the distal enhancer ECR5 in the 52kb noncoding region [83]. The Sost protein is a potent WNT antagonist secreted by osteocytes that normally functions to inhibit bone formation in osteoblasts by inhibiting WNT signaling through binding to the LRP5/6 co-receptors [72] [84] [74] with a preference in binding to LRP6 over LRP5 [85].…”

“…Though SOST antibody is a promising treatment for fracture repair, there are still many unknowns in terms of SOST antibody's mechanism of action, impaired healing in T2DM patients, and whether Sost acts in an endocrine or paracrine manner. The current understanding of Sost is that osteocytes predominately secrete Sost protein, however Sost has also been shown to be expressed in the aorta branches of the adult heart, cerebellum, kidney, and significant protein is detected in circulation, suggesting the possibility of Sost acting in an endocrine manner instead what is understood to be a paracrine manner [83]. Sost is also expressed in some osteoblasts and osteocytes thus, understanding cell-type specific contributions of Sost to bone metabolism would be beneficial in understanding bone metabolism and fracture healing.…”

“…In humans, lack of sclerostin causes sclerosteosis, a generalized skeletal hyperostosis disorder that results from elevated Wnt signaling/osteoblast activity [108,135], while non-coding deletions of gene regulatory regions that control SOST expression result in similar phenotypes [83,88]. In animal models, overexpression of human SOST causes osteopenia and limb defects [136,137], while lack of Sost in knockout mice causes 3-4 times more bone mass, consistent with human phenotypes [83]. Type 2 diabetic patients have elevated circulating SOST levels compared to nondiabetic patients [138], suggesting that Wnt signaling is altered in diabetic patients and may contribute to the observed osteopenia and delayed healing phenotype.…”