Transcriptomic Profiling Reveals Thyroid Hormone-Mediated and Compound-Specific Effects of Methimazole and Amitrole on Testis Development

Thyroid hormones (THs) influence testis development, with early life hypothyroidism resulting in smaller testes in neonates. Developmental exposure to thyroperoxidase (TPO)-inhibiting drugs such as propylthiouracil (PTU) and methimazole (MMI) also impair testis development in rodents by reducing TH levels, leading to smaller testes due to for instance disrupted Sertoli cell proliferation and maturation. Comparable effects are seen for exposure to the TPO-inhibiting pesticide amitrole, one of many environmental chemicals with TH-disrupting properties. Despite this described phenotype, the molecular underpinnings of hypothyroid-induced testis effects are less clear, complicating mechanics-based chemical toxicity testing relying on alternative test methods and omics approaches. Overall design: Here, we report on transcriptomics profiling of testes from hypothyroid rats induced by chemical exposures. Pregnant Sprague-Dawley rat dams were exposed by oral gavage to two doses of MMI (8 or 16 mg/kg bw/day) or amitrole (25 or 50 mg/kg bw/day) from gestational day (GD) 7 to pup day (PD) 16, with BRB-seq performed for both life stages. Both MMI and amitrole caused significant changes to the testis transcriptome, seen particularly at PD16, with 313 differentially expressed genes (DEGs) defining a shared TH-mediated profile. Additionally, amitrole exposure resulted in a distinct profile of 1,517 DEGs, suggesting compound specific effects beyond TH disruption. Despite some discrepancies between transcriptomic and protein-level data, this study highlights value of establishing comprehensive transcriptomic profiling to improve predictive models in chemical risk assessment.