Telomerase reactivation induces progression of mouse BrafV600E-driven thyroid cancers without telomere lengthening

Abstract

ABSTRACTMutations in the promoter of the telomerase reverse transcriptase (TERT) gene are the paradigm of a cross-cancer alteration in a non-coding region.TERTpromoter mutations (TPMs) are biomarkers of poor prognosis in several tumors, including thyroid cancers. TPMs enhanceTERTtranscription, which is otherwise silenced in adult tissues, thus reactivating abona fideoncoprotein. To studyTERTderegulation and its downstream consequences, we generated aTertmutant promoter mouse model via CRISPR/Cas9 engineering of the murine equivalentlocus(Tert-123C>T) and crossed it with thyroid-specific BrafV600E-mutant mice. We also employed an alternative model ofTertoverexpression (K5-Tert). Whereas all BrafV600Eanimals developed well-differentiated papillary thyroid tumors, 29% and 36% of BrafV600E+Tert-123C>Tand BrafV600E+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. MurineTertpromoter mutation increasedTerttranscriptionin vitroandin vivo, 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.