Mutation analysis of the Epac--Rap1 signaling pathway in cold thyroid follicular adenomas

The cyclic AMP (cAMP) cascade is the main regulatory pathway in thyrocytes. Whilst activating mutations in the TSH receptor or in the Gs alpha-subunit, which increase cAMP levels, have been shown to be responsible for 80% of the autonomous adenomas, no such mutations have been observed in the other types of thyroid tumors, suggesting that other mechanisms exist. The discovery of Epac ('exchange nucleotide protein directly activated by cAMP'), a novel cAMP-binding protein, which is strongly expressed in the thyroid, raised the possibility of a role for this protein in the generation of the unexplained cold thyroid follicular adenomas. Thus, we investigated whether activating mutations in either Epac or Rap (the downstream target of Epac) could be responsible for the generation of these thyroid nodules.

Epac and Rap1 (Rap1A and Rap1B) cDNAs were sequenced in 10 patients. The sequencing of the cDNAs was realized on both strands in the cold nodule and the juxtanodular tissue of each patient.

No mutations in either Epac or Rap1 cDNAs were found. For five patients, a polymorphism in Epac at codon 332 (Gly--Ser) was observed.

In this report, we show that the cAMP--Epac--Rap1 signaling pathway in the thyroid gland does not play a major role in the generation of cold thyroid follicular adenomas, since no mutations in either Epac or Rap1 could be observed in the 10 nodules studied.