Hypoxia-driven circ-TBC1D1/miR-520h/EPHB4 impairs trophoblast function in unexplained recurrent spontaneous abortion

Hypoxia during early pregnancy critically regulates trophoblast function, and its dysregulation has been implicated in unexplained recurrent spontaneous abortion (URSA). This study investigated the circ-TBC1D1/miR-520 h/EPHB4 regulatory axis in HTR-8/SVneo trophoblast function under hypoxic conditions (2% O₂) and its association with URSA. Clinical analysis of chorionic tissues from 14 URSA patients and 14 controls revealed upregulated EPHB4 expression in URSA samples. EPHB4 suppressed proliferation, migration, and invasion while increasing apoptosis and altering angiogenesis markers (VEGFA, CD31). Mechanistically, circ-TBC1D1 acted as a competitive endogenous RNA by sequestering miR-520 h, thereby elevating EPHB4 expression and modulating HTR-8/SVneo trophoblast function under hypoxia. Additionally, downregulated miR-520 h and a non-significant trend toward elevated circ-TBC1D1 were observed in chorionic tissues from URSA patients. These findings demonstrate that the circ-TBC1D1/miR-520 h/EPHB4 axis may impair extravillous trophoblast cell functionality, and dysregulation of this pathway in URSA patients may correlate with compromised uterine spiral artery remodeling. This study highlights potential molecular targets involved in URSA pathogenesis associated with hypoxic stress.