Severe subfertility in Thoroughbred stallions positive for the Impaired Acrosome Reaction susceptible gene (FKBP6) in Argentina

Subfertility in stallions is one of the major causes of reproductive and economic loss in the equine industry. Low pregnancy rates have been reported in stallions with normal semen quality after conventional evaluation. Acrosome reaction (AR) is necessary for fertilization and impaired acrosome reaction (IAR) leads to subfertility or infertility in horses; however, the AR test is not included in routine semen analysis. Failure of AR may affect more than 90% of sperm, although the remaining sperm can perform normal AR during fertilization. Up to date, IAR has been confirmed as a cause of subfertility only in Thoroughbred stallions. A genome-wide association study identified FKBP6 as a strong candidate gene responsible for this failure. The gene encodes for FK506 binding protein 6 (FKBP6) which is involved in sperm development and function. The objective of this study was to evaluate the presence of IAR susceptibility genes in stallions with satisfactory breeding soundness examination (BSE) and acceptable semen parameters but a history of severe subfertility. Thoroughbred stallions (A, B and C), between 6 and 9 years old with satisfactory BSE but with a history of low pregnancy rates during the 2020 (stallion A = 11%) and 2021 breeding seasons (stallion A= 0%; stallion B= 29%; stallion C= 10%) were included. They were referred after several consultations, andrological examinations and non-specific treatments by differentveterinarians. Ejaculates of stallions A, B and C in DSO had the following values of total number of spermatozoa/ejaculate (4.6 × 109, 8.5 × 109, 6.7 × 109); progressive motility (40%, 60%, 60%); hypo-osmotic swelling test (73%, 60%, 72%); and normal sperm morphology (58%, 68%, 76%), respectively. Evaluation of acrosome status by FITC-PSA showed 12%, 84% and 20% intact acrosomes for stallion A, B and C, respectively. Based on history and results of BSE, hair samples of the three stallions were sent to Molecular Cytogenetics Laboratory, Texas A&M University, and all three stallions, tested positive for the IAR susceptibility gene (FKBP6). The IAR is associated with male subfertility in humans and some domestic species including horses. A/A-A/A genotype of FKBP6 gene is considered a susceptibility gene for IAR in thoroughbred stallions with idiopathic subfertility. Hernandez Aviles et al. (Theriogenology. 2022; 186, 40-49) reported a low prevalence (0.7%) of acrosomal dysfunction in the overall population of breeding stallions but a significantly higher rate (38.1%) in idiopathically subfertile stallions. New developments in equine genomics provide tools for mapping fertility traits and discover causes of subfertility by genetic defects. Therefore, we must consider this analysis within the andrological examination in stallions with idiopathic infertility.