Tumors originating from thyroid follicular cells are the most common endocrine tumors, with rising incidence. Despite a generally good prognosis, up to 20% of patients experience recurrence and persistence, highlighting the need to identify the underlying molecular mechanisms. Dicer1 has been found to be altered in papillary thyroid cancer (PTC). Studies suggest that Dicer1 functions as a haploinsufficient tumor suppressor gene: partial loss promotes tumorigenesis, while complete loss prevents it. To investigate the effects of partial or total Dicer1 loss in PTC in vitro, we generated stable Dicer1 (+/−) cell lines from TPC1 using CRISPR-Cas9, though no Dicer1 (−/−) lines could be produced. Therefore, siRNA against Dicer1 was transfected into Dicer1 (+/−) cell lines to further decrease its expression. Transcriptomic analysis revealed changes in proliferation and cell locomotion. BrdU staining indicated a slow-down of the cell cycle, with fewer cells in S phase and more in G0-G1-phase. Additionally, transwell assays showed decreased invasion and migration after Dicer1 knockdown by siRNA. Moreover, Dicer1 overexpression led to decreased proliferation, invasion, and increased apoptosis. Our findings deepen the understanding of Dicer1’s role in thyroid cancer, demonstrating that both complete elimination and overexpression of Dicer1 inhibit thyroid oncogenesis, highlighting Dicer1 as a promising target for novel therapeutic strategies.

Beyond well-known genetic drivers, microRNA dysregulation has emerged as a key contributor to thyroid tumorigenesis. Central to this process is Dicer1, a ribonuclease essential for microRNA maturation, whose expression is often reduced in papillary thyroid carcinoma (PTC). Evidence from previous studies suggest Dicer1 functions as a context-dependent haplo-insufficient tumor suppressor gene: partial loss may promote tumor development, whereas complete loss may disrupt essential cellular functions, causing cell death and tumor suppression. However, the effects of partial or complete Dicer1 loss in thyroid cancer remain unclear. To explore this, we genetically inactivated one (heterozygous) or both (homozygous) Dicer1 alleles specifically in thyroid follicular cells of a RET/PTC3 transgenic mouse model using an inducible Cre-Lox system. Our findings deepen the current understanding of the RET/PTC3-driven PTC model by revealing an increased number of vimentin-positive cells and disruption in redox homeostasis. Additionally, whereas heterozygous Dicer1 loss did not alter tumor progression in RET/PTC3 mice, total loss reduced tumor growth and led to accumulated DNA damage and cell death. These findings highlight the crucial role of Dicer1 dosage in thyroid cancer progression and underscore its potential as a therapeutic target for aggressive PTC and other malignancies characterized by aberrant Dicer1 expression.