Biological and clinical significance of anti-Müllerian hormone determination in blood serum of the mare

Anti-Müllerian hormone (AMH), a member of the transforming growth factor β superfamily of growth and differentiation factors, is expressed in granulosa cells of preantral and small antral ovarian follicles. In humans, AMH appeared to regulate recruitment and growth of small ovarian follicles. Furthermore, circulating AMH concentrations were elevated in women with granulosa-cell tumors (GCT). In the horse, GCTs are the most common tumor of the ovary, and a variety of endocrine assays have been used to diagnose presumptive GCTs. The objectives of the present study were to validate a heterologous enzyme immunoassay for determination of serum AMH in the horse, and to determine concentrations of AMH in the blood of mares during the estrous cycle, pregnancy, and in mares with granulosa-cell tumors. Mares with normal estrous cycles (n = 6) and pregnant mares (n = 6) had blood samples collected throughout one interovulatory period and monthly throughout gestation, respectively. Mares diagnosed with GCT had blood samples taken before (n = 11) and after ovariectomy (n = 5). Tumors were sectioned and fixed for immunohistochemistry and snap frozen for immunoblot analyses and RT-qPCR. In normal cyclic mares and in pregnant mares, there was no effect of cycle stage or month of gestation on serum AMH concentrations. In GCT mares, serum concentrations of AMH (1901.4 ± 1144.6 ng/mL) were higher than those in cyclic (0.96 ± 0.08 ng/mL) or pregnant (0.72 ± 0.05 ng/mL) mares and decreased after tumor removal. Both AMH and AMH receptor (AMHR2) immunolabeling and expression were detected by immunohistochemistry in the tumor and cyst fluid obtained from mares with GCTs. Therefore, we concluded that AMH was a useful biomarker for detection of granulosa-cell tumors in mares.