Combination therapy in the Col1a2G610C mouse model of Osteogenesis Imperfecta reveals an additive effect of enhancing LRP5 signaling and inhibiting TGFβ signaling on trabecular bone but not on cortical bone

Enhancing LRP5 signaling and inhibiting TGF beta signaling have each been reported to increase bone mass and improve bone strength in wild-type mice. Monotherapy targeting LRP5 signaling, or TGF beta signaling, also improved bone properties in mouse models of Osteogenesis Imperfecta (OI). We investigated whether additive or synergistic increases in bone properties would be attained if enhanced LRP5 signaling was combined with TGF beta inhibition. We crossed an Lrp5 high bone mass (HBM) allele (Lrp5(A214V)) into the Col1a2(G610C/+) mouse model of OI. At 6-weeks-of-age we began treating mice with an antibody that inhibits TGF beta 1, beta 2, and beta 3 (mAb 1D11), or with an isotype-matched control antibody (mAb 13C4). At 12-weeks-old, we observed that combining enhanced LRP5 signaling with inhibited TGF beta signaling produced an additive effect on femoral and vertebral trabecular bone volumes, but not on cortical bone volumes. Although enhanced LRP5 signaling increased femur strength in a 3-point bending assay in Col1a2(G610C/+) mice, femur strength did not improve further with TGF beta inhibition. Neither enhanced LRP5 signaling nor TGF beta inhibition, alone or in combination, improved femur 3-point-bending post-yield displacement in Col1a2(G610C/+) mice. These pre-clinical studies indicate combination therapies that target LRP5 and TGF beta signaling should increase trabecular bone mass in patients with OI more than targeting either signaling pathway alone. Whether additive increases in trabecular bone mass will occur in, and clinically benefit, patients with OI needs to be determined.