Fibroblast Activation Protein (FAP) as a Serum Biomarker for Fibrotic Ovarian Aging: A Clinical Validation Study...

Chronological age is an imprecise proxy for reproductive capacity, necessitating biomarkers that reflect the underlying pathophysiology of the ovary. Fibrotic remodeling of the ovarian stroma is a key hallmark of biological ovarian aging, yet it cannot be assessed by current clinical tools. This study aimed to identify and validate a novel serum biomarker for fibrotic ovarian aging by applying supervised machine learning (ML) to human ovarian transcriptomic data. Transcriptomic data from the Genotype-Tissue Expression (GTEx) database were analyzed using ML algorithms to identify candidate genes predictive of ovarian aging, and finally, fibroblast activation protein (FAP) and collectin-11 (COLEC11) were selected for clinical validation. In a cross-sectional study, serum levels of FAP and COLEC11, along with key hormonal indices, were measured in two nested patient cohorts, and their associations with ovarian reserve and clinical parameters were analyzed. Serum FAP levels did not correlate with age but showed a strong inverse correlation with anti-Müllerian hormone (AMH) (r = -0.61, p = 0.001), a finding accentuated in women with decreased ovarian reserve (DOR). While COLEC11 correlated with age, it failed to differentiate DOR status. FAP levels were independent of central hormonal regulation, consistent with preclinical fibrotic models. Circulating FAP reflects age-independent, fibrotic ovarian aging, offering stromal-specific information not captured by conventional hormonal markers. This study provides the first clinical validation of FAP as a biomarker for ovarian stromal aging, holding potential for improved reproductive risk assessment.