Loss of NKX2-1 predisposes thyroid to neoplasm development through regulation of oxidative stress

Many factors including ionizing radiation and iodine deficiency are known to increase thyroid carcinogenesis risk. Our dataset analysis of The Cancer Genome Atlas (TCGA) showed that lower mRNA expression of NK2 homeobox 1 (NKX2-1) transcription factor, a master regulator of genesis, homeostasis, and function of thyroid, is linked to poor prognosis of papillary thyroid cancer patients. Here we provide the findings that thyroid-specific Nkx2-1 conditional knockout (Nkx2-1{Delta}T) mice develop thyroid adenoma and carcinoma in higher frequency with combined exposure to radiation and iodine deficiency than control Nkx2-1fl/fl mice. Iodine deficiency caused oxidative stress, which subsequently resulted in DNA damage, leading to transformation of thyroid follicular cells. RNA-seq gene set enrichment analysis indicated higher production of reactive oxygen species (ROS) in the thyroids of Nkx2-1{Delta}T as compared to Nkx2-1fl/fl mice with combined exposure to radiation and iodine deficiency. This was accompanied by a feedback induction of SOD3 (superoxide dismutase 3) and GPX2 (glutathione peroxidase 2). These antioxidants were naturally expressed at higher levels in the thyroids of Nkx2-1{Delta}T than Nkx2-1fl/fl mice without iodine deficiency or radiation. Nkx2-1{Delta}T thyroids exhibited abnormal follicle architecture and up-regulation of Acox2 (encoding acyl-CoA oxidase 2), which produces hydrogen peroxide. These results suggest that loss of NKX2-1 may contribute to excess ROS production, which elevates basal oxidative stress resulting in the promotion of ROS-induced carcinogenesis. We propose a role for NKX2-1 as a regulator of ROS production homeostasis in the thyroid. Its disturbance would dispose thyroid follicular cells more vulnerable to the ROS-producing carcinogens.