Identification of Oncogenic Alterations in 124 Cases of Pediatric Papillary Thyroid Carcinoma: BEND7::ALK, DLG5::RET, and CCDC30::ROS1 Fusions Induce MAPK Pathway Activation.

Pediatric papillary thyroid carcinoma (PPTC; under age 18 at the time of diagnosis) is relatively uncommon. There are few studies concerning the genetic background of PPTC in Asian countries. In this study, we reviewed 124 cases of PPTC from a single medical center in China. DNA-based next-generation sequencing (NGS) was performed to identify genetic alterations, with receptor tyrosine kinase (RTK) fusions further validated by RNA-based NGS. DICER1 mutations and TERT promoter mutations were detected by Sanger sequencing. We also explored the relationship between these genetic alterations and the clinicopathologic features, as well as the prognostic factors. Three recombinant plasmids expressing V5/HIS-tagged RTK fusion proteins (BEND7::ALK, DLG5::RET, CCDC30::ROS1) were constructed to investigate the in vitro effects. We found that the two most common subtypes were the classic subtype (77.4%) and the diffuse sclerosing subtype (17.7%). Hashimoto's thyroiditis was observed in 42.3% of cases, and regional lymph node metastasis was present in 82.9% of patients. The most frequent genetic alteration was the BRAF c.1799 T > A (p.V600E) mutation (63 patients, 50.8%), followed by RTK fusions (31 patients, 25.0%). A DICER1 mutation was detected in two cases, and TERT promoter mutations were not observed in any of the patients. RTK fusions were associated with the diffuse sclerosing subtype, Hashimoto's thyroiditis, and extrathyroidal extension. Adverse prognostic factors identified included RTK fusion, age under 14 years, and tumor size over 2 cm. Additionally, a significant proportion of these patients had actionable molecular alterations. Three rare kinase fusions, BEND7::ALK, DLG5::RET, and CCDC30::ROS1, were shown to induce phosphorylation of the MAPK pathway and promote cell proliferation in vitro. The specific RTK inhibitors could counteract the fusion-induced cell proliferation. Our data highlights the genetic landscape of Chinese PPTC patients, with RTK fusions being associated with aggressive clinicopathologic features. The rare fusions BEND7::ALK, DLG5::RET, and CCDC30::ROS1 may contribute to PPTC development with a BRAF-like effect.