Insufficient NNMT promotes autophagy and disrupts progesterone signaling in endometrial stromal cells in recurrent implantation failure by modulating the H3K9me3-ALDH1A3 pathway.

Defective endometrial receptivity represents an important factor in recurrent implantation failure (RIF), though its precise regulatory mechanisms remain unclear. While nicotinamide N-methyltransferase (NNMT) is abundantly expressed in human endometrial tissues, its role in endometrial receptivity and RIF pathogenesis has not been defined. This study demonstrated that NNMT expression was significantly downregulated in midluteal-phase endometrium from RIF patients relative to fertile controls. Functional analyses in human endometrial stromal cells (ESCs) revealed that NNMT knockdown enhanced autophagy flux and disrupted progesterone signaling. Mechanistically, NNMT deficiency elevated H3K9me3 enrichment at the Aldh1a3 promoter, suppressing its expression. Notably, knockdown of ALDH1A3 resulted in similar effects with NNMT downregulation, and exogenous rhALDH1A3 reversed the autophagy alterations and rescued progesterone signaling in NNMT-knockdown cells. In vivo, NNMT inhibition in a murine model reduced embryo implantation rates and decreased ALDH1A3 expression. Collectively, these findings indicate that reduced NNMT impairs endometrial receptivity through H3K9me3-mediated ALDH1A3 repression, leading to aberrant autophagy and disrupted progesterone signaling in decidualized ESCs. This study identifies the NNMT-H3K9me3-ALDH1A3 axis as a key epigenetic-metabolic pathway underlying RIF, offering novel diagnostic and therapeutic targets.