Telomerase reactivation induces progression of mouse Braf -driven thyroid cancers without telomere lengthening.

Mutations in the promoter of the telomerase reverse transcriptase ( ) gene are the paradigm of a cross-cancer alteration in a non-coding region. promoter mutations (TPMs) are biomarkers of poor prognosis in several tumors, including thyroid cancers. TPMs enhance transcription, which is otherwise silenced in adult tissues, thus reactivating a oncoprotein. To study deregulation and its downstream consequences, we generated a mutant promoter mouse model via CRISPR/Cas9 engineering of the murine equivalent (Tert ) and crossed it with thyroid-specific Braf -mutant mice. We also employed an alternative model of overexpression (K5-Tert). Whereas all Braf animals developed well-differentiated papillary thyroid tumors, 29% and 36% of Braf +Tert and Braf +K5-Tert mice progressed to poorly differentiated thyroid cancers at week 20, respectively. Braf+Tert tumors showed increased mitosis and necrosis in areas of solid growth, and older animals from these cohorts displayed anaplastic-like features, i.e., spindle cells and macrophage infiltration. Murine promoter mutation increased transcription and , but temporal and intra-tumoral heterogeneity was observed. RNA-sequencing of thyroid tumor cells showed that processes other than the canonical Tert-mediated telomere maintenance role operate in these specimens. Pathway analysis showed that MAPK and PI3K/AKT signaling, as well as processes not previously associated with this tumor etiology, involving cytokine and chemokine signaling, were overactivated. Braf+Tert animals remained responsive to MAPK pathway inhibitors. These models constitute useful pre-clinical tools to understand the cell-autonomous and microenvironment-related consequences of Tert-mediated progression in advanced thyroid cancers and other aggressive tumors carrying TPMs.