DDX21 Links BRAF^V600E and TERT to Promote Thyroid Cancer Progression

The coexistence of BRAF^V600E and telomerase reverse transcriptase (TERT) promoter alterations has been reported to cooperatively upregulate TERT expression and is well recognized as a biomarker for aggressive thyroid cancer. However, the mechanism underlying their synergistic effect remains elusive.

TERT-interacting proteins were identified via immunoprecipitation (IP) and mass spectrometry and validated by Co-IP. DDX21 expression was detected in 60 paired thyroid cancer and adjacent normal tissues, and the function of DDX21 in thyroid cancer was explored in vitro and in vivo. Integrated analyses of RNA-seq, ChIP-seq, and Ribo-seq assays were conducted to elucidate the molecular mechanisms. BRAF/MEK inhibitors and BRAF^V600E overexpression were used to investigate the regulation of DDX21 by BRAF mutation. In silico analysis, ChIP, and luciferase reporter assays were used to identify the transcription factors binding to DDX21 promoter.

DDX21 interacted with TERT to regulate ribosomal RNA (rRNA) transcription by affecting the binding of Pol I to rDNA. DDX21 is upregulated in thyroid cancer, and DDX21 knockdown decreased the expression of multiple cancer hallmark genes and suppressed cell proliferation, colony formation, cell migration, and invasion of thyroid cancer cells. GABPA bound to the promoter of DDX21, and knockdown of GABPA or GABPB1 suppressed DDX21 promoter activity and transcription. Pharmacological inhibition of the MAPK pathway in BRAF-mutant cancer cells decreased the expression of GABPA, GABPB1, and DDX21.

BRAF^V600E upregulates expression of DDX21 through the MEK/ERK/GABP axis. DDX21, in turn, interacts with TERT to promote rRNA transcription and modulate the expression of multiple cancer hallmark genes, driving the malignant progression of thyroid cancer. These findings suggest that DDX21 acts as a mediator to bridge the crosstalk between BRAF^V600E and TERT and may serve as a therapeutic target for patients with thyroid cancer harboring BRAF and TERT alterations.