The WNT1G177C mutation specifically affects skeletal integrity in a mouse model of osteogenesis imperfecta type XV

The recent identification of homozygous WNT1 mutations in individuals with osteogenesis imperfecta type XV (OI-XV) has suggested that WNT1 is a key ligand promoting the differentiation and function of bone-forming osteoblasts. Although such an influence was supported by subsequent studies, a mouse model of OI-XV remained to be established. Therefore, we introduced a previously identified disease-causing mutation (G177C) into the murine Wnt1 gene. Homozygous Wnt1^G177C/G177C mice were viable and did not display defects in brain development, but the majority of 24-week-old Wnt1^G177C/G177C mice had skeletal fractures. This increased bone fragility was not fully explained by reduced bone mass but also by impaired bone matrix quality. Importantly, the homozygous presence of the G177C mutation did not interfere with the osteoanabolic influence of either parathyroid hormone injection or activating mutation of LRP5, the latter mimicking the effect of sclerostin neutralization. Finally, transcriptomic analyses revealed that short-term administration of WNT1 to osteogenic cells induced not only the expression of canonical WNT signaling targets but also the expression of genes encoding extracellular matrix modifiers. Taken together, our data demonstrate that regulating bone matrix quality is a primary function of WNT1. They further suggest that individuals with WNT1 mutations should profit from existing osteoanabolic therapies.

Mice Carrying a Ubiquitous R235W Mutation of Wnt1 Display a Bone-Specific Phenotype

Since a key function of Wnt1 in brain development was established early on through the generation of non-viable Wnt1-deficient mice, it was initially surprising that WNT1 mutations were found to cause either early-onset osteoporosis (EOOP) or osteogenesis imperfecta type XV (OI-XV). The deduced function of Wnt1 as an osteoanabolic factor has been confirmed in various mouse models with bone-specific inactivation or overexpression, but mice carrying disease-causing Wnt1 mutations have not yet been described. Triggered by the clinical analysis of EOOP patients carrying a heterozygous WNT1 mutation (p.R235W), we introduced this mutation into the murine Wnt1 gene to address the question of whether this would cause a skeletal phenotype. We observed that Wnt1^+/R235W and Wnt1^R235W/R235W mice were born at the expected Mendelian ratio and that they did not display postnatal lethality or obvious nonskeletal phenotypes. At 12 weeks of age, the homozygous presence of the Wnt1 mutation was associated with reduced trabecular and cortical bone mass, explained by a lower bone formation rate compared with wild-type littermates. At 52 weeks of age, we also observed a moderate bone mass reduction in heterozygous Wnt1^+/R235W mice, thereby underscoring their value as a model of WNT1-dependent EOOP. Importantly, when we treated wild-type and Wnt1^+/R235W mice by daily injection of parathyroid hormone (PTH), we detected the same osteoanabolic influence in both groups, together with an increased cortical thickness in the mutant mice. Our data demonstrate the pathogenicity of the WNT1-R235W mutation, confirm that controlling skeletal integrity is the primary physiological function of Wnt1, and suggest that osteoanabolic treatment with teriparatide should be applicable for individuals with WNT1-dependent EOOP. © 2020 American Society for Bone and Mineral Research.

Wnt1 is an Lrp5-independent bone-anabolic Wnt ligand

WNT1 mutations in humans are associated with a new form of osteogenesis imperfecta and with early-onset osteoporosis, suggesting a key role of WNT1 in bone mass regulation. However, the general mode of action and the therapeutic potential of Wnt1 in clinically relevant situations such as aging remain to be established. Here, we report the high prevalence of heterozygous WNT1 mutations in patients with early-onset osteoporosis. We show that inactivation of Wnt1 in osteoblasts causes severe osteoporosis and spontaneous bone fractures in mice. In contrast, conditional Wnt1 expression in osteoblasts promoted rapid bone mass increase in developing young, adult, and aged mice by rapidly increasing osteoblast numbers and function. Contrary to current mechanistic models, loss of Lrp5, the co-receptor thought to transmit extracellular WNT signals during bone mass regulation, did not reduce the bone-anabolic effect of Wnt1, providing direct evidence that Wnt1 function does not require the LRP5 co-receptor. The identification of Wnt1 as a regulator of bone formation and remodeling provides the basis for development of Wnt1-targeting drugs for the treatment of osteoporosis.

High Bone Turnover in Mice Carrying a Pathogenic Notch2 Mutation Causing Hajdu-Cheney Syndrome

Hajdu-Cheney syndrome (HCS) is a rare autosomal-dominant disorder primarily characterized by acro-osteolysis and early-onset osteoporosis. Genetically, HCS is caused by nonsense or deletion mutations within exon 34 of the NOTCH2 gene, resulting in premature translational termination and production of C-terminally truncated NOTCH2 proteins that are predicted to activate NOTCH2-dependent signaling. To understand the role of Notch2 in bone remodeling, we developed a mouse model of HCS by introducing a pathogenic mutation (6272delT) into the murine Notch2 gene. By μCT and undecalcified histology, we observed generalized osteopenia in two independent mouse lines derived by injection of different targeted embryonic stem (ES) cell clones, yet acro-osteolysis did not occur until the age of 52 weeks. Cellular and dynamic histomorphometry revealed a high bone turnover situation in Notch2^+/HCS mice, since osteoblast and osteoclast indices were significantly increased compared with wild-type littermates. Whereas ex vivo cultures failed to uncover cell-autonomous gain-of-functions within the osteoclast or osteoblast lineage, an unbiased RNA sequencing approach identified Tnfsf11 and Il6 as Notch-signaling target genes in bone marrow cells cultured under osteogenic conditions. Because we further observed that the high-turnover pathology of Notch2^+/HCS mice was fully normalized by alendronate treatment, our results demonstrate that mutational activation of Notch2 does not directly control osteoblast activity but favors a pro-osteoclastic gene expression pattern, which in turn triggers high bone turnover. © 2017 American Society for Bone and Mineral Research.

Osteoblast-specific Notch2 inactivation causes increased trabecular bone mass at specific sites of the appendicular skeleton

Notch signaling is a key pathway controlling various cell fate decisions during embryogenesis and adult life. It is activated by binding of specific ligands to four different Notch receptors that are subsequently cleaved by presenilins to release an intracellular domain that enters the nucleus and activates specific transcription factors. While the skeletal analysis of various mouse models with activated or inactivated Notch signaling has demonstrated a general impact of this pathway on bone remodeling, the more recent identification of NOTCH2 mutations in individuals with Hajdu-Cheney syndrome (HCS) has highlighted its human relevance. Since HCS is primarily characterized by skeletal defects, these latter findings led us to analyze the specific role of Notch2 in skeletal remodeling. After observing Notch2 expression in osteoblasts and osteoclasts, we utilized Runx2-Cre and Lyz2-Cre mice to inactivate Notch2 in cells of the osteoblast or osteoclast lineage, respectively. Whereas Notch2(fl/fl)/Lyz2-Cre mice did not display significant alterations of skeletal growth, bone mass or remodeling, Notch2(fl/fl)/Runx2-Cre mice progressively developed skeletal abnormalities in long bones. More specifically, these mice displayed a striking increase of trabecular bone mass in the proximal femur and the distal tibia at 6 and 12months of age. Whereas undecalcified sectioning of the respective regions did not reveal impaired osteocyte differentiation as a potential trigger for the observed phenotype, ex vivo experiments with bone marrow cells identified an increased osteogenic capacity of Notch2(fl/fl)/Runx2-Cre cultures. Collectively, our findings demonstrate that Notch2 physiologically regulates bone remodeling by inhibiting trabecular bone formation in the appendicular skeleton. Understanding the underlying mechanisms may help to improve diagnosis and therapy of HCS.