Transcriptomic analysis reveals a critical role for activating Gsa mutations in spontaneous feline hyperthyroidism
Feline hyperthyroidism (FHT) is a debilitating disease affecting >10% of elderly cats. It is generally characterised by chronic elevation of thyroid hormone in the absence of circulating TSH. Understanding of the molecular pathogenesis of FHT is currently limited. However, FHT shares clinical and histopathological similarities with human toxic multinodular goitre, which has been associated with activating mutations in TSH receptor (TSHR) and Gsa encoding genes. Using RNA-seq transcriptomic analysis of thyroid tissue from hyperthyroid and euthyroid cats, we identified differentially expressed genes and dysregulated pathways in FHT, many of which are downstream of TSHR. In addition, we detected missense variants in thyroid RNA-seq reads that alter the structure of both TSHR and Gsa. All FHT-associated mutations were absent in germline sequence from paired blood samples. Only a small number of hyperthyroid cats demonstrated TSHR variation, however all thyroids from advanced cases of FHT carried at least one missense variant affecting Gsa.  The activating nature of the acquired Gsa mutations was demonstrated by increased cAMP production in vitro. These data indicate that constitutive activation of signalling downstream of TSHR is central to the TSH-independent production of thyroid hormone in FHT, offering a novel therapeutic target pathway in this common disease. Overall design: To investigate the thyroid transcriptome in feline hyperthyroidism, we used RNA-seq to profile gene expression in thyroid tissue samples collected from 4 euthyroid (ET) and 11 hyperthyroid (HT) cats. We then identified differentially expressed genes between HT and ET samples.