TOPORS Deficiency Impairs TET2 SUMOylation leading to aberrant m5C Hypermethylation that promotes adipogenic differentiation of mesenchymal stem cells in osteoporosis

Osteoporosis is characterized by reduced bone mass and increased bone marrow adiposity, largely resulting from an imbalance between the osteogenic and adipogenic differentiation of mesenchymal stem cells (MSCs). Although posttranslational protein modification and RNA epitranscriptomic regulation are increasingly recognized as key determinants of MSC fate, their mechanistic involvement in osteoporosis remains unclear. Here, we identify the dual ubiquitin/SUMO E3 ligase TOPORS as a critical regulator of MSC lineage commitment and bone–fat homeostasis. TOPORS expression was markedly reduced in MSCs from patients with osteoporosis and in ovariectomized (OVX) and aged mice and was positively correlated with bone mineral density. The results of functional studies revealed that TOPORS deficiency promoted the adipogenic differentiation of MSCs, whereas its overexpression suppressed this process. Mechanistically, TOPORS interacts with TET2 and stabilizes it through SUMOylation at lysine 1055, thereby preventing its proteasomal degradation. Loss of TOPORS or TET2 increased m5C modification of FABP4 mRNA and elevated FABP4 expression. In vivo, rAAV9-mediated TOPORS overexpression improved trabecular bone microarchitecture, increased bone formation activity, and decreased bone marrow adiposity in OVX mice. Collectively, these findings establish TOPORS as a key modulator of MSC fate and reveal a mechanistic link between protein SUMOylation and RNA m5C regulation in skeletal homeostasis, supporting a previously unrecognized regulatory role of TOPORS in osteoporosis.