Genetically-achieved disturbances to the expression levels of TNFSF11 receptors modulate the effects of zoledronic acid on growing mouse skeletons.

Zoledronic acid (ZOL), a nitrogen bisphosphonate (N-BP), is currently used to treat and control pediatric osteolytic diseases. Variations in the intensity of the effects and side effects of N-BPs have been reported with no clear explanations regarding their origins. We wonder if such variations could be associated with different levels of RANKL signaling activity in growing bone during and after the treatment with N-BPs. To answer this question, ZOL was injected into neonate C57BL/6J mice with different genetically-determined RANKL signaling activity levels (Opg+/+\RankTg-, Opg+/+\RankTg+, Opg+/-\RankTg-, Opg+/-\RankTg+, Opg-/-\RankTg- and Opg-/-\RankTg+ mice) following a protocol (4 injections from post-natal day 1 to 7 at the dose of 50 µg/kg) that mimics those used in onco-pediatric patients. At the end of pediatric growth (1 and half months) and at an adult age (10 months), the bone morphometric and mineral parameters were measured using µCT in the tibia and skull for the different mice. A histologic analysis of the dental and periodontal tissues was also performed. At the end of pediatric growth, a delay in long bone and skull bone growth, a blockage of tooth eruption, some molar root alterations and a neoplasia-like structure associated with incisor development were found. Interestingly, the magnitude of these side effects was reduced by Opg deficiency (Opg-/-) but increased by Rank overexpression (RankTg). Analysis of the skeletal phenotype at ten months confirmed respectively the beneficial and harmful effects of Opg deficiency and Rank overexpression. These results validated the hypothesis that the RANKL signaling activity level in the bone microenvironment is implicated in the modulation of the response to ZOL. Further studies will be necessary to understand the underlying molecular mechanisms, which will help decipher the variability in the effects of N-BPs reported in the human population. SIGNIFICANT STATEMENTS: The present study establishes that in mice the RANKL signaling activity level is a major modulator of the effects and side-effects of bisphosphonates on the individual skeleton during growth. However, the modulatory actions are dependent on the ways in which this level of activity is increased. A decrease in OPG expression is beneficial to the skeletal phenotype observed at the end of growth, while RANK overexpression deteriorates it. Far removed from pediatric treatment, in adults, the skeletal phenotypes initially observed at the end of growth for the different levels of RANKL signaling activity were maintained, although significant improvement was associated only with reductions in OPG expression.