gms | German Medical Science

Rosette-forming glioneuronal tumors (RGNT) of the fourth ventricle are rare mixed glio-neuronal tumors included in the revised WHO classification of central nervous system tumors. The histogenesis of RGNT is largely unknown and little data on the molecular features of this rare tumor entity are available, but include mutations of PIK3CA. Due to the presence of pilocytic-like areas in most RGNT, an analysis of KIAA1549-BRAF, as a molecular hallmark of pilocytic astrocytoma, in a representative series of RGNT is important to elucidate the potential molecular similarities between these tumors. To date, only two separate cases have been investigated for the KIAA1549-BRAF fusion and both were found to be negative. In this study, we analysed a series of 9 RGNTs for the presence of KIAA1549-BRAF fusions using interphase fluorescence in situ hybridisation (FISH). An additional analysis of BRAF ^V600E mutational status was performed using a pyrosequencing-based method in five cases. The series included 4 male and 5 female patients. The mean age at diagnosis was 28.3 years (9–54 years). Eight cases were localized in the fourth ventricle and 1 case in the third ventricle. The analysis revealed no cases showing KIAA1549-BRAF gene fusion or BRAF ^V600E mutations. The absence of KIAA1549-BRAF fusions in the RGNT of our series, together with the two cases previously reported, suggests it is unlikely that RGNT are part of the pilocytic astrocytoma family. Moreover, the presence of the KIAA1549-BRAF fusion in a case of ambiguous histology would support a diagnosis of pilocytic astrocytoma and could be a useful tool for distinguishing KIAA1549-BRAF fusion positive pilocytic astrocytoma from RGNT.