AOP report: Adverse outcome pathway network for developmental androgen signaling inhibition leading to hypospadias

This report summarizes an adverse outcome pathway network (AOPN) describing how reduced androgen signaling during fetal life may lead to hypospadias in male offspring. Hypospadias is a penile malformation caused by disrupted masculinization of the genital tubercle. Under normal physiological conditions, androgen action promotes differentiation of the genital tubercle to a penis in male fetuses. Suboptimal androgen receptor signaling can impair genital tubercle development, causing hypospadias in humans and other mammals. In rodent toxicity studies, including test guideline studies from the Organisation for Economic Co-operation and Development for reproductive toxicity, hypospadias is assessed as an adverse outcome. This report presents three adverse outcome pathways (AOP-Wiki IDs 477, 570, 571), each with distinct upstream events representing antiandrogenic mechanisms. Downstream, the pathways converge at the nodal key event "decrease, androgen receptor activation" (1614) and share the adverse outcome "hypospadias" (2082). This report provides assessments of all three adverse outcome pathways (477, 570, 571), including one new key event (2082) and three new key event relationships (2828, 3350, 3488) not previously reported. The three key event relationships, connecting upstream events nonadjacently with the adverse outcome, were developed using a systematic weight-of-evidence approach. Overall, empirical evidence for the adverse outcome pathways is strong, with few exceptions or uncertainties. The AOPN is considered applicable to male mammals, but supporting data come primarily from rodent and human studies; hence, the applicability domain is currently restricted to these species. While quantitative understanding remains limited, the AOPN establishes robust mechanistic links between antiandrogenic activity and hypospadias, providing a foundation for future efforts to quantify key event relationships and develop predictive methods based on upstream events, often measured in vitro.