Papillary Thyroid Carcinoma (PTC) represents 80% of all thyroid cancers. Though the vast majority of PTC are indolent tumors, around 15% behave aggressively, developing distant metastases (DM), which cause patient´s death. The molecular mechanisms underlying DM are poorly understood. Little it is known about the contribution of intratumor heterogeneity to DM. Dynamic changes in mutation distribution through space and time have not been in deep characterized.
By genotyping 13 cases of matched primary tumors (PrT) and DM, we sought to determine the prevalence of mutations in genes that have been associated with tumor progression and aggressiveness in follicular thyroid carcinogenesis. To assess the contribution of intratumor heterogeneity and clonal evolution to DM, 54 tumor areas, including different areas across space and time within the PrT and the DM were analyzed. Genotyping was approached by PCR and SSCP or DS.
Twelve cases (92%) were mutated in at least one of the genes screened [TERT 69%, BRAF 54%, KRAS 23%, NRAS 23%, HRAS 15.4%, PIK3CA 15.4%]. No mutations were seen at MED12 or EIF1AX. Among the mutated cases 67% exhibited more than 1 gene activated. Three mutated genes co-existed in 62.5% of the cases. Concurrent activation of TERT+RAS or TERT+BRAF was seen in 5 cases each event (62.5%). Simultaneous activation of BRAF and RAS was found in 4 cases (50%). In all the cases in which more than 1 area of DM, across space and time, was analyzed, the mutations at TERT, KRAS and HRAS resulted clonal. De novo mutations at DM were seen in 6 cases. Within PrT, subclonality was as follows TERT 33%; RAS 20%; BRAF 40% and PIK3CA 100%.
The number of mutational events in PTC with DM is strikingly higher than in PTC without DM. While TERT and RAS mutations tend to be clonal within PrT and DM, BRAF mutations tend to be subclonal. TERT and RAS mutations may appear the novo at DM. PTC with DM display a much higher rate of genetic heterogeneity (67%). The coexistence of mutations in different genes is in agreement with the hypothesis that tumor progression relies on progressive accumulation of genetic alterations. MED12 and EIF1AX do not play a role in aggressive PTCs. Concurrent mutations at TERT, and different PI3K/AKT and MAPK pathway genes are common in metastatic PTC.
Ginesa García Rostán.
Head of Group "Pathobiology of Cancer: Inter-, Intra-tumoral heterogeneity and Molecular Targets at Institute of Molecular Biology and Genetics (IBGM) of Valladolid University.
All authors have declared no conflicts of interest.