SAT-573 Characterization of the Gene Expression Program of a Mutant Thyroid Hormone Receptor in Anaplastic Thyroid Cancer

Abstract

Thyroid cancer is the most commonly diagnosed endocrine malignancy. Although survival rates for the majority of thyroid cancer subtypes are high compared to other cancers, the de-differentiated anaplastic thyroid cancer (ATC) is particularly aggressive, with a median survival time of less than 6 months. Intriguingly, thyroid hormone receptor beta (TRβ) has been demonstrated to repress tumor growth in thyroid cancer models, but the mechanism is not well established. Additionally, phosphorylation of a tyrosine residue (Y406) is critical for the tumor suppressive phenotype of TRβ; mutation of this residue ablates TRβ-mediated tumor suppression (1). We hypothesized that the signaling mediated by wild type, but not Y406 TRβ defines the critical pathways for tumor suppression. We restored WT or Y406F TRβ expression by lentiviral transduction in an authenticated ATC cell line, SW1736. WT TRβ decreased cell proliferation, an effect amplified by T3. Y406F did not significantly reduce ATC cell proliferation and addition of T3 did not restore the antiproliferative effect implicating differential signaling. As we have previously identified TRβ suppression of the oncogene RUNX2 in thyroid cells (2), we determined the impact of Y406F TRβ on this signaling pathway. While Y406F TRβ reduced RUNX2 expression, the effect was less pronounced than noted with WT. Of importance, we previously demonstrated that the chromatin ATPase BRG1 cooperates with TRβ to reduce RUNX2 levels (3). Mutation of TRβ at Y406 eliminated this synergy. Finally, we determined the ligand-dependent and independent transcriptomes of WT and Y406F TRβ by RNA-sequencing. Critically, WT and Y406F TRβ both responded to T3 treatment, but exhibit different transcriptional profiles. Importantly, we were able to identify several tumor suppressor genes and putative tumor suppressor genes that were upregulated only in the WT-T3 treated cells including BRCA2, CHD1, and SEPT7. Utilizing Ingenuity Pathway Analysis, we determined repression of the epigenetic regulator KDM5B (a putative oncogene) and upregulation of CD24 activity (low in CD44+/CD24- cancer stem cells). These results define the TRβ transcriptome in aggressive thyroid cancer cells and reveal novel signaling in ATC. 1. Park JW, Zhao L, Webb P, Cheng S-y. Src-dependent phosphorylation at Y406 on the thyroid hormone receptor β confers the tumor suppressor activity. Oncotarget 2014;5:10002-16 2. Carr FE, Tai PW, Barnum MS, Gillis NE, Evans KG, Taber TH, et al. Thyroid Hormone Receptor-beta (TRbeta) Mediates Runt-Related Transcription Factor 2 (Runx2) Expression in Thyroid Cancer Cells: A Novel Signaling Pathway in Thyroid Cancer. Endocrinology 2016;157:3278-92 3. Gillis NE, Taber TH, Bolf EL, Beaudet CM, Tomczak JA, White JH, et al. Thyroid Hormone Receptor beta Suppression of RUNX2 is Mediated by Brahma Related Gene 1 Dependent Chromatin Remodeling. Endocrinology 2018