Collagen Type III as a Possible Blood Biomarker of Fibrosis in Equine Endometrium

SIMPLE SUMMARY: In the mare, endometrosis is a disease characterized by excessive collagen fibers deposition in the endometrium (uterus inner layer), which is responsible for infertility. The gold standard method for endometrosis evaluation has been endometrial biopsy histopathological classification. The use of blood biomarkers for endometrosis identification would be less invasive, and could provide additional information regarding endometrosis diagnosis and fertility prognosis. Therefore, this study aimed to identify possible blood biomarkers for endometrosis diagnosis and fertility assessment on mares. Reproductive examination, endometrial biopsy histopathological classification, and blood collection were performed. Endometrium and serum collagen type I (COL1) and type III (COL3), and hydroxyproline concentrations were determined. In conclusion, serum COL3 concentration might be considered as a potential aid for the diagnosis of endometrosis and fertility prognosis in the mare. In contrast, COL1 and hydroxyproline did not prove to be effective as biomarkers of endometrial fibrosis in this species. Although it is very unlikely that a single blood biomarker could replace a histopathological evaluation, serum COL3 may have clinical applications. Thus, it may be useful to evaluate a group of mares as possible recipients in embryo transfer programs, where performing endometrial biopsies of several mares is not feasible. ABSTRACT: Collagen pathological deposition in equine endometrium (endometrosis) is responsible for infertility. Kenney and Doig’s endometrial biopsy histopathological classification is the gold standard method for endometrosis evaluation, whereby blood biomarkers identification would be less invasive and could provide additional information regarding endometrosis diagnosis and fertility prognosis. This study aimed to identify blood biomarkers for endometrosis diagnosis (42 mares were used in experiment 1), and fertility assessment (50 mares were used in experiment 2). Reproductive examination, endometrial biopsy histopathological classification (Kenney and Doig) and blood collection were performed. Endometrium and serum collagen type I (COL1) and type III (COL3), and hydroxyproline concentrations were measured (ELISA). Serum COL3 cut-off value of 60.9 ng/mL allowed healthy endometria (category I) differentiation from endometria with degenerative/fibrotic lesions (categories IIA, IIB or III) with 100% specificity and 75.9% sensitivity. This cut-off value enabled category I + IIA differentiation from IIB + III (76% specificity, 81% sensitivity), and category III differentiation from others (65% specificity, 92.3% sensitivity). COL1 and hydroxyproline were not valid as blood biomarkers. Serum COL3 cut-off value of 146 ng/mL differentiated fertile from infertile mares (82.4% specificity, 55.6% sensitivity), and was not correlated with mares’ age. Only COL3 may prove useful as a diagnostic aid in mares with endometrial fibrosis and as a fertility indicator.