DNA hydroxymethylases Tet1 and Tet2 regulate bone aging and BMSC metabolism through the IGF-1/ mTOR signalling axis.

The Ten-Eleven Translocases (Tet) family of DNA hydroxymethylases have recently been implicated in bone development, with Tet1 and Tet2 mediating Bone Marrow Stromal Cell (BMSC) growth and osteogenic differentiation. The present study investigated the effects of Tet1 and Tet2 deregulation on bone development and age-related bone loss, with respect to BMSC function. Histomorphometric and micro-CT analysis of skeletal parameters found significant reductions to trabecular structure and volume as well as reduced osteoblast numbers within the bone of Prx-1:Cre driven Tet1 and Tet2 double knockout (TetDKO) mice at skeletal maturity. Moreover, these effects were exacerbated with age, particularly in male mice. In vitro studies found a significant reduction in TetDKO BMSC osteogenic potential and a shift towards adipogenesis, as well as changes to DNA repair, proliferation and senescence properties. RNA sequencing of BMSC derived from TetDKO male mice uncovered several differentially expressed genes, and an array of significantly enriched gene set pathways. Notably Pappa2, involved in regulation of IGF-1 signalling, was significantly differentially regulated, leading to reduction in IGF-1 bioavailability and signalling in BMSC and differentiated osteoblasts. Furthermore, changes in mTOR activity in TetDKO animals indicated altered metabolic activity, differentiation and proliferation capabilities of TetDKO BMSC. These findings indicate that Tet1 and 2 regulate the IGF-1 regulatory element, Pappa2, where deregulation of Tet1 and Tet2 in BMSC can disrupt this pathway leading to enhanced bone loss and premature aging. Targeting of these novel regulatory pathways may offer new therapeutic approaches for treatment of age-related bone loss.