Nicotinamide N-methyltransferase deficiency disrupts endometrial decidualization partly through dysregulated H3K27 trimethylation in recurrent implantation failure.

Embryo implantation critically depends on the establishment of endometrial receptivity, which is governed by progesterone-driven stromal decidualization during the mid-secretory phase. In recurrent implantation failure (RIF), this process is frequently disrupted, yet the molecular mechanisms remain unclear. Here, through integrative analysis of bulk and single-cell transcriptomic datasets, we identified nicotinamide N-methyltransferase (NNMT) as one of the most significantly downregulated genes in the endometrium of RIF patients. Temporal profiling of human endometrial samples across the menstrual cycle revealed progressive upregulation of NNMT from proliferative to secretory phase, while in hormonally induced stromal cell differentiation, NNMT expression rose sharply from day 4 onward. Functional assays demonstrated that NNMT knockdown impaired decidualization, as evidenced by reduced expression of PRL and IGFBP1 and attenuated cytoskeletal reorganization. Mechanistically, NNMT deficiency increased intracellular S-adenosylmethionine levels, enhanced the expression of core PRC2 components, and promoted H3K27me3 enrichment at the promoters of decidual genes, consistent with their transcriptional repression. Pharmacologic inhibition of histone methylation by 3-deazaneplanocin A or selective EZH2 inhibition by GSK126 partially rescued both molecular and morphological defects in NNMT-deficient stromal cells. In vivo, uterine NNMT silencing in pregnant mice also increased H3K27me3 level, reduced the expression of decidualization-related genes, and decreased the number of implantation sites. Collectively, our findings identify NNMT as a metabolic-epigenetic regulator of endometrial receptivity, and support its potential as a candidate biomarker for RIF.