The impact of menstrual cycle phase and BMI on mitochondrial dysfunction in PMOS endometrium

STUDY QUESTION: Does the endometrium of oligo-ovulatory women with polyendocrine metabolic ovarian syndrome (PMOS) exhibit menstrual cycle phase-specific transcriptomic alterations compared to women without PMOS? SUMMARY ANSWER: Compared to healthy controls, women with PMOS exhibit endometrial transcriptomic dysregulations which are menstrual cycle phase-specific and, most notably, related to mitochondrial function. WHAT IS KNOWN ALREADY: Infertility and gestational dysfunction in women with PMOS are increasingly attributed to endometrial abnormalities, yet the underlying mechanisms behind PMOS-related endometrial dysfunction remain unclear. Previous studies have focused on anovulatory phenotypes, resulting in a poor understanding of cycle phase-specific defects in the endometria of women with PMOS. STUDY DESIGN, SIZE, DURATION: Endometrial biopsies were obtained from oligo-ovulatory women with PMOS and healthy controls at four distinct menstrual cycle phases (proliferative, early-, mid-, and late-secretory phases) and subjected to RNA sequencing. Parallel in vitro experiments were conducted using cultured endometrial stromal cells (ESCs) and epithelial organoids (EEOs) derived from an additional endometrial sample cohort. PARTICIPANTS/MATERIALS, SETTING, METHODS: RNA sequencing was performed on biopsies from 78 (oligo-)ovulatory women, age- and BMI-matched between PMOS (n = 39) and control (n = 39) groups. Phase-specific transcriptomic profiles and pathway enrichment analyses were performed. For functional assays, endometrial cells were isolated from four PMOS and four control donors per BMI group (lean: < 25 kg/m2; obese: ≥30 kg/m2). Mitochondrial function, including respiration, ATP and reactive oxygen species production, and membrane potential, was assessed in both cell types before and after hormonal exposure. MAIN RESULTS AND THE ROLE OF CHANCE: Endometrial transcriptomes clustered primarily by cycle phase; however, PMOS-specific dysregulation was observed related to extracellular matrix organization, estrogen receptor (ESR) signaling, ion transport, and mitochondrial respiratory function. Gene trajectory analyses revealed phase-specific alterations in progesterone-, insulin signaling-, and mitochondria-related gene expression during the early-secretory phase, and epithelial function during the mid-secretory phase. Functionally, decidualized ESCs from obese PMOS women displayed reduced mitochondrial respiration, whereas EEOs from lean PMOS patients exhibited increased mitochondrial activity. LARGE SCALE DATA: Sequencing data are available from the corresponding author upon request. LIMITATIONS, REASONS FOR CAUTION: Women with PMOS were oligo-ovulatory while controls exhibited regular cycles. The sample size for in vitro studies was modest, warranting validation in larger cohorts. Additionally, the use of bulk RNA sequencing may obscure cell-type-specific transcriptomic signatures, emphasizing the need for future single-cell approaches to dissect cellular heterogeneity in PMOS endometrium. WIDER IMPLICATIONS OF THE FINDINGS: This study uncovers dynamic, cycle phase-dependent molecular and bioenergetic alterations in the endometrium of oligo-ovulatory women with PMOS. Mitochondrial dysfunction, particularly during the implantation window, emerges as a key pathological feature. The observed BMI-dependent divergence in mitochondrial function suggests that personalized therapeutic strategies may be necessary to improve reproductive outcomes in women with PMOS. FUNDING: Funded by the Jusélius Foundation, Novo Nordisk Foundation, Research Council of Finland, and the Horizon 2020 Marie-Curie MATER Innovative Training Network (all to T.T.P.), with additional support from the University of Oulu Scholarship Foundation (L.L.), the Estonian Research Council (E.V.M. and I.R.), and the Estonian Research Council grant no. PRG1076, Swedish Research Council grant no. 2024-02530, Novo Nordisk Foundation grant no. NNF24OC0092384 and Horizon Europe grant NESTOR, grant no. 101120075 (A.S.). DISCLOSURES: The authors declare no competing interests.