Inhibiting WNT secretion reduces high bone mass caused by Sost loss-of-function or gain-of-function mutations in Lrp5

Proper regulation of Wnt signaling is critical for normal bone development and homeostasis.Mutations in several Wnt signaling components,which increase the activity of the pathway in the skeleton,cause high bone mass in human subjects and mouse models.Increased bone mass is often accompanied by severe headaches from increased intracranial pressure,which can lead to fatality and loss of vision or hearing due to the entrapment of cranial nerves.In addition,progressive forehead bossing and mandibular overgrowth occur in almost all subjects.Treatments that would provide symptomatic relief in these subjects are limited.Porcupine-mediated palmitoylation is necessary for Wnt secretion and binding to the frizzled receptor.Chemical inhibition of porcupine is a highly selective method of Wnt signaling inhibition.We treated three different mouse models of high bone mass caused by aberrant Wnt signaling,including homozygosity for loss-of-function in Sost,which models sclerosteosis,and two strains of mice carrying different point mutations in Lrp5(equivalent to human G171V and A214V),at 3 months of age with porcupine inhibitors for 5–6 weeks.Treatment significantly reduced both trabecular and cortical bone mass in all three models.This demonstrates that porcupine inhibition is potentially therapeutic for symptomatic relief in subjects who suffer from these disorders and further establishes that the continued production of Wnts is necessary for sustaining high bone mass in these models.

Bone loss from Wnt inhibition mitigated by concurrent alendronate therapy

Dysregulated Wnt signaling is associated with the pathogenesis of cancers, fibrosis, and vascular diseases. Inhibition of Wnt signaling has shown efficacy in various pre-clinical models of these disorders. One of the key challenges in developing targeted anti-cancer drugs is to balance efficacy with on-target toxicity. Given the crucial role Wnts play in the differentiation of osteoblasts and osteoclasts, acute inhibition of Wnt signaling is likely to affect bone homeostasis. In this study, we evaluated the skeletal effect of small molecule inhibitor of an o-acyl transferase porcupine（PORCN） that prevents Wnt signaling by blocking the secretion of all Wnts. Micro-computed tomography and histomorphometric evaluation revealed that the bones of mice treated with two structurally distinct PORCN inhibitors LGK974 and ETC-1922159（ETC-159） had loss-of-bone volume and density within 4 weeks of exposure. This decreased bone mass was associated with a significant increase in adipocytes within the bone marrow. Notably,simultaneous administration of a clinically approved anti-resorptive, alendronate, a member of the bisphosphonate family,mitigated loss-of-bone mass seen upon ETC-159 treatment by regulating activity of osteoclasts and blocking accumulation of bone marrow adipocytes. Our results support the addition of bone protective agents when treating patients with PORCN inhibitors.Mitigation of bone toxicity can extend the therapeutic utility of Wnt pathway inhibitors.