Canonical and non-canonical Wnt signaling for bone formation and maintenance have been well delineated in animal models. In humans, several skeletal dysplasias as well as disorders of bone mass, result from perturbation of Wnt signaling. Although it is known that Wnt signaling is important in skeletal formation, the role of specific human Wnt proteins and the protein interactions required for proper signaling transmission are not fully understood. Individuals with rare bone diseases inform function of various Wnt-related proteins in human development, provide insights into common diseases of bone formation/maintenance (e.g. osteoporosis) and have the potential to guide therapeutic approaches and personalized clinical interventions. Robinow syndrome (RS) is a skeletal dysplasia characterized by mesomelic limb shortening that results from lack or malfunctioning of specific Wnt non-canonical components. Recent data in a subset of individuals have revealed increased bone mineral density, particularly cranial, indicating that some features of RS may also result from disturbed canonical signaling. In line with this observation, a key mediator of both the non-canonical and canonical Wnt pathway, dishevelled (DVL), has been recently associated with RS. Intriguingly, pathogenic mutations arise from -1 frameshifting variants resulting in a truncated DVL protein with its C-terminal tail replaced with a highly basic tail. Remarkably, this unusual disease mechanism has been observed in both DVL1 and DVL3, and recurs independently in at least 20% of dominant RS families. Preliminary data revealed that other relevant components of the Wnt pathway such as Frizzled 2, the main Wnt receptor, and Nucleoredoxin, a DVL regulator, are also mutated in RS. Our data support the contention that phenotypic and genetic heterogeneity can be partially explained by variants in key proteins of the same biochemical pathway. We hypothesize that studying subjects with Robinow syndrome will reveal proteins and interactions required for proper temporal, directional and tissue-specificity expression of the Wnt signaling pathway that contribute to abnormal bone patterning, bone growth and bone mineralization. This hypothesis will be tested by (1) defining the contribution of known and new genes causative of RS by whole exome sequencing; (2) investigating the expression and stability of the mutant proteins to gain insights into RS mutational mechanism; (3) performing clinical characterization of the skeletal phenotype of individuals with RS to elucidate the roles of these genes in bone development and growth. Skeletal phenotypes will be characterized in patients with distinct genetic etiologies using standardized anthropometric measurements, complete skeletal survey by X-rays and dual-energy X-ray absorptiometry scanning (DEXA). This application will inform the human function of various Wnt-related proteins and shed light into the underlying biology of bone signaling during development. Future plans for a R01 includes creation of mouse models in collaboration with the Knockout Mouse Project (KOMP) to test the impact of the RS variants for bone development.
Public health relevance: Gene discovery and functional characterization of abnormally-functioning proteins in Robinow syndrome will shed light onto the role of Wnt-related proteins in skeletal abnormalities and bone mineralization, which can lead to treatment interventions for common bone disorders, such as post-menopausal osteoporosis.
