Abstract C31: Molecular profiling in patients with radioactive iodine-refractory thyroid cancers using targeted mutation analysis.

Background: Thyroid cancer is the most common neoplasia of the endocrine system, accounting for 1%-3% of all cancers. Differentiated thyroid cancer (DTC) accounts for ∼93% of all thyroid cancers. While most cases of DTC are treated with surgery followed by radioactive iodine (RAI) ablation, 5-15% of DTC become RAI-refractory (RAI-R) resulting in a poorer prognosis and limited treatment options. In an attempt to use genetic information to predict prognosis in patients RAI-R DTC, we designed and implemented a mass spectrometry based genotyping assay panel encompassing the most significant oncogenes in thyroid cancer. Germline mutations associated with thyroid hormone dysgenesis were also included in the panel to examine their potential role in thyroid cancer prognosis. Methods: Somatic and germline mutations were evaluated in tumor DNA using targeted mutation analysis by Sequenom MassARRAY iPLEX platform. A total of 321 assays interrogating 212 somatic mutations and 113 germline mutations in 44 genes were included in a thyroid-cancer mutational panel. Genotype-calling was manually done by the Sequenom Typer 4.0 software. Statistical significance in categorical data was determined by a χ2 test using SPSS 11.5. Results: Of 80 patients with RAI-R DTC, 37 were male and 43 were female. Median age at diagnosis was 59 years (range 27-86). Somatic mutations were identified in 38 (47.5%) patients; mutated genes included BRAF, RAS, TP53, MET, PIK3CA, GNAS and TPO. The BRAF V600E mutation was the most prevalent somatic mutation (28.8%) and was significantly associated with regional lymph nodes metastasis (P=0.02). RAS mutations were found in 9.9% and TP53 in 6.2% of patients, while somatic mutations in other genes were rare (<2.5%). Germline mutations in 5 genes, including SLC5A5, SLC26A4, IGF1R, RAP1GAP and TG, were identified in tumor DNA. Only the TG polymorphism M1028V was detected at very high allele frequency of 61.9% while mutations in other 4 genes were detected at low allele frequencies (<3%). As expected, homozygous for the TG mutation produced one and heterozygous produced two MassARRAY peaks. Notably the detected allele frequency for TG M1028V in the RAI-R DTC group analyzed is significantly higher (61.9% vs 50.9%, P=0.017) compared to previously published population allele frequency (Collins et al, 2004). Within the group analyzed, 42.5% of patients were homozygous for TG M1028V accounting for its high allele frequency. The TG M1028V was associated significantly with such clinicopathologic parameters, as regional lymph nodes metastasis (P<0.01) and lymphatic invasion in primary tumor (P=0.018). Conclusion: These data suggest that in addition to well characterized activating BRAF V600E mutation, the germline TG allele, M1028V, may have clinical significance in thyroid cancer. This is first time report that a specific TG allele may be prognostic for patients with RAI-R DTC. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C31. Citation Format: Tatyana Appelbaum, Waixing Tang, Stephen Stopenski, Marcia Brose. Molecular profiling in patients with radioactive iodine-refractory thyroid cancers using targeted mutation analysis. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C31.