Comparative multi-omics analysis uncovers contrasting molecular profiles of canine and human thyroid carcinomas

Thyroid tumors represent 1-3% of canine cancers, with most tumors classified as follicular carcinomas and less frequently as medullary carcinomas. In comparison, only 10-15% of human thyroid cancers are follicular and 2% are medullary, with prevalent activating mutations in BRAF and NRAS, or RET, respectively. A cohort of canine thyroid carcinomas underwent histopathological (n=60 and paired molecular (n=30, WES and/or RNAseq) exploration. Clustering of tumor transcriptomes produced 2 groups; T1 and T2 clusters comprised of follicular thyroid carcinomas (FTC) and medullary thyroid carcinomas (MTC), respectively. Tumors were histologically typed as follicular, compact, and follicular-compact on blinded review, with most MTC classified as compact with rare follicular-compact appearance, while FTC displayed all 3 patterns. FTC samples had significantly elevated levels of ERBB2 and HER2 protein, and RET signaling was up-regulated in MTC. Recurrent somatic mutations in DNMT1, STAT2, SALL4, HSP90AA1, MEN1, MUC4, THRAP3, CDK4, NOTCH2, and THRAP3 were identified in at least 10% of samples. Individual variants were also identified in KRAS, ARAF, GNAS, and ERBB2. Additionally, we identified fusion genes that included TG, FGFR2, and PAX8. These data suggest that canine MTC, like their human counterparts, may be driven by RET signaling. In contrast, FTC show limited reliance on RAS/RAF signaling, prevalent in human TC, for oncogenic progression. Across the analyzed samples, 60% of tumors had mutations in at least one DNA repair-related pathway, suggesting that the accumulation of DNA damage may drive cancer progression in canine thyroid tumors. Elevated HER2 protein staining was associated with shorter progression free survival (PFS). Thyroid tumor size (>4.25 cm) was also associated with shorter overall survival and PFS, consistent with previous reports; however, metastasis at diagnosis was not correlated with outcome. Additional studies are warranted to explore the utility of these biomarkers to improve diagnosis and treatment of thyroid carcinoma in dogs.