Abstract
Constitutive activation of the cAMP pathway causes both, increased growth and function of thyroid follicular cells resulting in the development of toxic thyroid nodules (TTNs). It is known that the aberrant activation of this signalling cascade in the thyroid is mainly due to somatic point mutations that constitutively activate the TSHR or the Gs-protein. However, in different studies examining TSHR exons 9 and 10, the prevalence of TSHR- and Gs-mutations in TTNs has been reported to vary from 38 to 82% and 8 to 75%, respectively. 25 TTNs were screened for somatic TSHR mutations in the exons 9 and 10 and for Gs- mutations in the exons 7 and 8 using DGGE. Determination of X-chromosome inactivation was used for clonality analysis. TSHR mutations were detected in only eight (32%) out of 25 TTNs. A Gs- mutation (Q227H) was detected in a single TTN. However, in 85% of the mutation negative TTNs a clonal origin was shown. The prevalence of TSHR mutations in the present study is within the range of reported frequencies in studies examining TSHR exons 9 and 10 by sequencing. However, a comparison with two previous studies in our laboratory detecting TSHR mutations in 57% and 70% of samples by DGGE reveals a significantly lower TSHR mutation prevalence in the present study (2 = 3.8, p=0.05; and 2 = 9.9, p=0.002, respectively). Since most of the mutation-negative TTNs show a clonal origin in the clonality analysis, which is less sensitive than the mutation screening by DGGE, a contamination of the nodular tissue e.g. by surrounding tissue and blood that might influence the mutation screening is very unlikely. Therefore, the high percentage of clonal mutation-negative TTNs suggests alternative molecular aberrations leading to the development of these TSHR mutation negative nodules.