Sex differences in thyroid aging and their implications in thyroid disorders: insights from gene regulatory networks.

Objective: Most, if not all, thyroid disorders are more prevalent in females than males. However, sex differences in thyroid disease risk varies with age, e.g. although the risk of anaplastic thyroid carcinoma (ATC), the most aggressive form of thyroid cancer, increases with age for both sexes, age of diagnosis for most thyroid cancers is lower for females than males. In contrast, the risk of Hashimoto's thyroiditis (HT), an autoimmune condition, is higher in ages 30-50 than older age groups, and females have a higher risk than males at any given age. These age- and sex-dependent variations suggest that thyroid aging is a sex-biased process, where gene regulatory patterns evolve with age differentially between sexes. Yet the underlying molecular mechanisms remain poorly understood. Methods: To characterize sex-specific aging-related changes in gene regulation in healthy thyroid and disease, we constructed individual-specific gene regulatory networks using a two-step approach. First, we estimated gene-gene co-expression networks for each individual using BONOBO. Second, we integrated these networks with sex-specific transcription factor (TF) motif data and protein-protein interaction priors using PANDA to infer individual-level TF-driven gene regulatory networks. Results: We found that within normal thyroid, regulatory patterns of cancer-related genes and biological pathways involved in cell proliferation, immune response, and metabolic processes vary by age in a nonlinear manner. However, the direction and rate of age-related changes differ between sexes. In females we detect two inflection points around ages 40 and 60, when the gene regulatory patterns in healthy thyroid show significant change for most pathways. This drastic change in gene regulatory networks is mostly driven by estrogen and androgen receptor TFs. To understand how aging-related changes in gene regulation drive risk of thyroid disorders, we investigated two diseases that have known sex- and age-specific differences: HT and ATC. We observed that in ATC, disease-relevant immune and metabolic pathways change with age in the same direction as they change in disease. In contrast, in HT, disease-related pathway targeting patterns are in the opposite direction of those in aging. Moreover, in age groups where HT is most diagnosed, TF-targeting patterns of disease-associated immune, metabolic and cell proliferation pathways in females were closer towards the disease state than in males, emphasizing the influence of sex-biased regulatory patterns in increasing thyroid disease risk in females. Conclusion: In thyroid tissue, genes related to immune response and metabolic processes, are regulated by TFs in an age- and sex-biased manner. These age- and sex-specific gene regulatory variations may contribute to the variation in risk of thyroid conditions with age and an overall higher risk of diseases in females compared to males, thus emphasizing the need for tailored screening and prevention strategies.