Rethinking the Estrogen Receptor Beta Dominance Hypothesis in Endometriosis: Insights from Single Cell RNA Sequencing Meta-analysis.

Background: Endometriosis is a chronic, estrogen-dependent disease characterized by the presence of endometrial-like tissue growing outside the uterus. The molecular and clinical heterogeneity of endometriosis complicate diagnostic and treatment options -- diagnostic delays of seven to ten years are common and therapies often lack long-term efficacy. Estrogen signaling and estrogen receptor beta (ERβ) expression is thought to be increased in endometriosis, contributing to increased cell proliferation in lesions. The "ERβ dominance hypothesis" is a prevailing hypothesis in the field, setting ERβ as a high-priority therapeutic target. If effectively modulated, ERβ could be the first therapy to directly target lesion biology, rather than only managing symptoms. Objectives: We aimed to characterize ERβ's expression in endometriosis by cell type and evaluate its therapeutic relevance, primarily assessing the validity of the ERβ dominance hypothesis. Study Design: We reanalyzed scRNAseq data from eight previously published studies. Our final filtered dataset included 557,061 cells, the largest endometriosis single cell atlas ever constructed. We quantified gene expression levels of ESR1 and ESR2, which encode ERɑ and ERβ respectively, across each tissue and cell type, to identify cell-type specific drivers of ESR2/ERβ expression across diseased and healthy tissues. To characterize the differences between cells that uniquely express ESR1 versus those that uniquely express ESR2, we performed differential gene expression and pathway enrichment analyses. Results: Count and distribution analyses revealed no significant ESR2/ERβ dominance in any cell or tissue type by Fisher's Exact Tests and Wilcoxon Rank Sum Tests. Differential gene expression and pathway enrichment analyses suggest distinct roles of each estrogen receptor isoform. Conclusions: Overall, our results argue against a simplified model of ERβ dominance and instead propose a dual-isoform and cell and tissue-specific framework for understanding estrogen receptor signaling in endometriosis. These findings hold important implications for future therapeutic strategies. Specifically, treatments that target ERβ alone may fail to account for the functional role and relative abundance of ERα. In the future, therapeutic approaches that consider isoform-specific, tissue-specific, and cell-specific expression patterns may prove most effective in reducing recurrence and improving outcomes for patients.