Oncogenic FAM131B-BRAF fusion resulting from 7q34 deletion comprises an alternative mechanism of MAPK pathway activation in pilocytic astrocytoma

Activation of the MAPK signaling pathway has been shown to be a unifying molecular feature in pilocytic astrocytoma (PA). Genetically, tandem duplications at chromosome 7q34 resulting in KIAA1549-BRAF fusion genes constitute the most common mechanism identified to date. To elucidate alternative mechanisms of aberrant MAPK activation in PA, we screened 125 primary tumors for RAF fusion genes and mutations in KRAS, NRAS, HRAS, PTPN11, BRAF and RAF1. Using microarray-based comparative genomic hybridization (aCGH), we identified in three cases an interstitial deletion of similar to 2.5 Mb as a novel recurrent mechanism forming BRAF gene fusions with FAM131B, a currently uncharacterized gene on chromosome 7q34. This deletion removes the BRAF N-terminal inhibitory domains, giving a constitutively active BRAF kinase. Functional characterization of the novel FAM131B-BRAF fusion demonstrated constitutive MEK phosphorylation potential and transforming activity in vitro. In addition, our study confirmed previously reported BRAF and RAF1 fusion variants in 72% (90/125) of PA. Mutations in BRAF (8/125), KRAS (2/125) and NF1 (4/125) and the rare RAF1 gene fusions (2/125) were mutually exclusive with BRAF rearrangements, with the exception of two cases in our series that concomitantly harbored more than one hit in the MAPK pathway. In summary, our findings further underline the fundamental role of RAF kinase fusion products as a tumor-specific marker and an ideally suited drug target for PA.