Unveiling a new SORBS2::BRAF fusion in papillary thyroid carcinoma: insights from molecular diagnostics

Papillary thyroid carcinoma (PTC) is the most common form of thyroid cancer, with the majority of cases driven by genetic alterations that activate the MAPK signaling pathway. The BRAF V600E mutation is the most frequent alteration, while BRAF fusions are relatively rare but increasingly recognized as oncogenic drivers. These fusions typically involve the loss of BRAF's autoinhibitory N-terminal domain, leading to constitutive MAPK pathway activation. Here, we report a novel SORBS2::BRAF fusion in a case of PTC, further expanding the spectrum of BRAF alterations in thyroid cancer. A 32-year-old male was incidentally found to have a left thyroid nodule during a routine physical examination. Follow-up examinations revealed changes in the nodule's characteristics, prompting fine-needle aspiration biopsy, which identified atypical follicular epithelial cells suggestive of papillary thyroid carcinoma. Histopathological examination confirmed the diagnosis, and next-generation sequencing (NGS) revealed a novel in-frame fusion between SORBS2 exon 18 and BRAF exon 9. The resulting fusion protein retains the BRAF kinase domain while replacing its autoinhibitory domains with those of SORBS2. RT-PCR and Sanger sequencing confirmed the presence of the SORBS2::BRAF fusion. Quantitative PCR profiling of MAPK transcriptional output genes (DUSP6, CCND1, ETV4, c-Myc, and c-FOS) revealed marked upregulation in the tumor versus adjacent normal tissue, providing functional evidence for pathway activation. The SORBS2::BRAF fusion has not been previously reported in PTC or any other tumor type. Given the deletion of BRAF's inhibitory domain, this fusion likely acts as a tumor driver through constitutive activation of the MAPK pathway. This case underscores the importance of molecular diagnostics in identifying rare genetic alterations and highlights the need for further research into targeted therapies for BRAF fusion-driven cancers. The discovery of this novel fusion expands our understanding of the molecular landscape of PTC and provides a foundation for future therapeutic development.