Examining the altered immunological and molecular landscape of the jaw following bisphosphonate treatment to define mechanisms of bone homeostasis

Bisphosphonates (BPs) are central drugs of the orthopaedic clinic, used for treating a variety of diseases of the bone. Through targeting osteoclasts, they have strong anti-resorptive effects, and improve bone density in skeletal diseases including osteogenesis imperfecta and osteoporosis. However, despite well documented reduction in fracture rates in patients, all bisphosphonates suffer from adverse clinical complications and side effects. Commonly, bisphosphonates induce osteonecrosis of the jaw (ONJ), presenting as non-healing exposed bone in maxillofacial areas. ONJ poses significant complications to clinical treatment regimes. Bisphosphonate related ONJ (BRONJ) is not understood mechanistically. Proposed cellular and molecular mechanisms proposed to explain BRONJ are highly discordant and lack consensus. It is unclear what cellular effect bisphosphonates are having at the jaw and why they specifically target this region. We have recently established a robust model of BRONJ using adult zebrafish, which, unlike current animal models, is highly consistent, rapid, and permits easy assaying of the relevant tissues. This offers a completely novel and unique method to interrogate the cellular, genetic, and molecular changes underlying BRONJ. We have established transgenic lines able to label relevant cell types and have already identified a mutant strain which is resistant to BRONJ, giving us direct and testable hypotheses. This proposal has 3 major aims. 1. We will apply microCT and genetic cellular labelling approaches to define the tissue defect and the cells present in the jaw contributing to BRONJ onset and determine their origin. 2. We will use molecular genetic tools and RNA-Seq approaches to identify molecular alterations over the course of disease progression, leading to relevant cell changes. 3. Test the implicated cell and molecular culprits through cell ablation and pharmacological interventions to define pathomechanism in our zebrafish BRONJ model. Our proposal will thus identify how cellular resorptive behaviour in the jaw is constrained under steady-state conditions, furnish new understanding about how this regulation becomes deranged in the BRONJ patient, and offer molecular pathways for future clinical testing, confirmation, and pharmacological targeting. Ultimately this proposal exploits a novel preclinical model for understanding BRONJ pathomechanism.