Purpose: Patients with anaplastic thyroid cancer (ATC) have a very high death rate. In contrast, deaths from non-anaplastic thyroid (NAT) cancer are much less common. The genetic alterations in fatal NAT cancers have not been reported.Experimental Design: We performed next-generation sequencing of 410 cancer genes from 57 fatal NAT primary cancers. Results were compared with The Cancer Genome Atlas study (TCGA study) of papillary thyroid cancers (PTCs) and to the genomic changes reported in ATC.Results: There was a very high prevalence of TERT promoter mutations, comparable with that of ATC, and these co-occurred with BRAF and RAS mutations. A high incidence of chromosome 1q gain was seen highlighting its importance in tumor aggressiveness. Two novel fusion genes DLG5-RET and OSBPL1A-BRAF were identified. There was a high frequency of mutations in MED12 and these were mutually exclusive to TERT promoter mutations and also to BRAF and RAS mutations. In addition, a high frequency of mutations in RBM10 was identified and these co-occurred with RAS mutations and PIK3CA mutations. Compared with the PTCs in TCGA, there were higher frequencies of mutations in TP53, POLE, PI3K/AKT/mTOR pathway effectors, SWI/SNF subunits, and histone methyltransferases.Conclusions: These data support a model, whereby fatal NAT cancers arise from well-differentiated tumors through the accumulation of key additional genetic abnormalities. The high rate of TERT promoter mutations, MED12 mutations, RBM10 mutations, and chromosome 1q gain highlight their likely association with tumor virulence. Clin Cancer Res; 23(19); 5970-80. ©2017 AACR.

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