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Uterine fibroids (UFs), benign tumours prevalent in up to 80% of women of reproductive age, are associated with significant morbidity, including abnormal uterine bleeding, pain and infertility. Despite identification of key genomic alterations in MED12 and HMGA2, the pathogenic mechanisms underlying UFs and heavy menstrual bleeding (HMB) remain poorly understood. To correlate systematically genetic, transcriptional and proteomic phenotypes, our study involved an integrative analysis of fibroid, myometrium and endometrium tissues from 137 patients, utilising genome-wide SNP arrays, targeted sequencing, RNA sequencing and proteomics. Our findings reveal 39.7% of UFs possess MED12 mutations, alongside novel variants in genes such as COL4A5 and COL4A6. Multi-omics factor analysis of integrated protein and mRNA highlighted differential regulation related to extracellular matrix remodelling, proteolysis and homeostasis in fibroid versus myometrium tissues, and distinct gene sets associated with RNA splicing in the endometrium of patients with HMB, particularly in MED12-mutated fibroids. Our study proposes a model, which is supported by in vivo evidence, where altered signalling of MED12-mutated fibroids influences RNA transcript isoform expression in endometrium, potentially leading to abnormal uterine bleeding. This integrative approach unravels complex molecular pathways in UF pathogenesis and HMB, offering novel insights for targeted therapeutic development.