Abstract 2992: The role of FOXD1 in the anaplastic transformation of thyroid cancer

Abstract Background: Anaplastic thyroid carcinoma (ATC) is a rare and highly aggressive malignancy. ATC can occur de novo, but accumulating pathological and genomic evidence suggests that a subset may arise through progressive dedifferentiation of pre-existing differentiated thyroid carcinoma (DTC), most commonly papillary thyroid carcinoma (PTC). Supporting this concept, coexistent PTC has been reported in approximately 20-30% of ATC cases, and genomic studies indicate that additional somatic alterations—such as TERT promoter and TP53 mutations—may drive this transition. However, the molecular mechanisms underlying this dedifferentiation process remain poorly understood. To address this gap, we compared gene expression profiles between well-differentiated PTC regions and undifferentiated ATC regions within the same tumors. Methods: RNA was extracted from well-differentiated and undifferentiated regions of the same tumor specimens (FFPE) obtained from six ATC cases. After RNA extraction, RNA libraries were prepared and sequenced using the Illumina NovaSeq 6000 platform, and comparative gene expression analysis was performed. To evaluate the impact of FOXD1 (Forkhead Box D1) on chemosensitivity, siRNA-mediated knockdown was performed, and cell viability after doxorubicin or paclitaxel treatment was assessed using the CCK-8 assay in two ATC cell lines (OCUT-1C and OCUT-1F). To investigate the regulatory mechanism of FOXD1 expression, methylation analysis of the FOXD1 promoter region was performed by bisulfite sequencing in a PTC cell line (KTC-1), OCUT-1C, and OCUT-1F. Results: Gene expression and KEGG pathway analyses showed EMT activation in ATC. Among transcription factors upregulated in the undifferentiated regions, FOXD1 was consistently overexpressed and therefore selected for further investigation. TGF-β-induced EMT of KTC-1 cells increased the expression of FOXD1 and the EMT-related transcription factors SNAI1 and SNAI2. qRT-PCR analysis revealed that siRNA suppression of FOXD1 altered the expression of EMT-related genes (CDH1, CDH2, SNAI1, SNAI2), whereas knockdown of SNAI1/2 did not affect FOXD1 expression. In both OCUT-1C and OCUT-1F cell lines, FOXD1 suppression restored sensitivity to doxorubicin and paclitaxel. Bisulfite sequencing revealed that the FOXD1 promoter was demethylated in OCUT-1C and OCUT-1F compared to KTC-1. Conclusions: These findings suggest that FOXD1 contributes to the dedifferentiation process from PTC to ATC. Moreover, restoration of chemosensitivity upon FOXD1 knockdown indicates that FOXD1 could be a promising therapeutic target for ATC. Future studies will further validate FOXD1 expression in additional clinical samples and perform functional analyses using FOXD1-knockout cell lines. Citation Format: Aki Inase, Mitsuyoshi Hirokawa, Miyoko Higuchi, Naoyoshi Onoda, Takuya Higashiyama, Taiji Koyama, Shiro Kimbara, Masanori Teshima, Hironobu Minami, Ken-Ichi Nibu, Mitch A. Phelps, Naomi Kiyota. The role of FOXD1 in the anaplastic transformation of thyroid cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2992.