Abstract A008-PR002: Membrane localization mediated by BRAF fusion partners is necessary for transformation of human neural stem cells

Pediatric cancers are commonly driven by fusion oncogenes between BRAF, a proto-oncogene, and a protein-coding gene that cannot confer oncogenesis on its own. In most of these fusion onco-proteins, the BRAF regulatory domains are truncated off while the kinase domain is maintained. Expression of the BRAF kinase domain is thought to be the cause of tumor growth, while the fusion partners have largely been labeled as passenger genes and their effects on oncogenicity remain understudied. We have discovered that many of these fusion partners contain domains that facilitate subcellular localization to membranes, where truncated BRAF can activate the MAPK pathway. KIAA1549::BRAF (K::B) is the most common BRAF fusion in brain tumors, and is particularly prevalent in pediatric low-grade gliomas (pLGGs). Though the canonical function of KIAA1549 is unknown, the wild-type conformation has a predicted transmembrane domain that is maintained in the K::B fusion. Using super-resolution microscopy, we observed that deletion of the transmembrane domain restricted K::B exit from the endoplasmic reticulum. Additionally, deletion of the K::B transmembrane domain blocked transformation of human neural stem cells (hNSC). Also, we found that the second most common BRAF fusion in pLGG is dependent on its fusion partner, FAM131B, for transformation. This fusion consists of the BRAF kinase domain and nine amino acids from the N-terminus of FAM131B which includes a myristoylation site. Myristoylation is a post-translational modification where fatty acids are covalently bonded to proteins, localizing them to membranes. Mutation of the myristoylation site in FAM131B:BRAF displaced its membranous localization to the cytosol and inhibited the fusion protein’s ability to transform hNSCs. These findings reveal a novel role of oncogenically inert fusion partners in tumorigenesis driven by BRAF fusions. Citation Format: Alexander Zhang, Sean A Misek, Anna M Borgenvik, Kelly Y Cai, Daniel Christen, Sangita Pal, Timothy J Ragnoni, Esteban Miglietta, Beth Cimini, Anne Carpenter, Jesse Boehm, Keith L Ligon, David T Jones, Rameen Beroukhim, Pratiti Bandopadhayay. Membrane localization mediated by BRAF fusion partners is necessary for transformation of human neural stem cells [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Discovery and Innovation in Pediatric Cancer— From Biology to Breakthrough Therapies; 2025 Sep 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_2):Abstract nr A008-PR002.