Rosette-forming glioneuronal tumor of the cerebellum in statu nascendi: an incidentally detected diminutive example indicates derivation from the internal granule cell layer.

Rosette-forming glioneuronal tumor (RGNT) is a recently introduced, indolent neoplasm composed of diminutive circular aggregates of neurocytic-like cells on a noninfiltrative astrocytic background, typically located in the cerebellar midline The traded concept of RGNT being derived from site-specific periventricular precursors may be questioned in the face of extracerebellar examples as well as ones occurring in combination with other representatives of the glioneuronal family. We describe a hitherto not documented example of asymptomatic RGNT discovered during autopsy of a 74-year-old male. Located in the tuberal vermis, this lesion of 6 mm diameter consisted of several microscopic nests of what were felt to represent nascent stages of RGNT, all of them centered on the internal granular layer, and ranging from mucoid dehiscences thereof to fully evolved - if small - tumor foci. Molecular genetic analysis revealed a missense mutation in Exon 20 of the PIK3CA gene involving an A→G transition at Nucleotide 3140. On the other hand, neither codeletion of chromosomes 1p/19q nor pathogenic mutations of IDH1/2 were detected. By analogy with in situ paradigms in other organs, we propose that this tumor is likely to have arisen from the internal granular layer, rather than the plate of the 4th ventricle. A suggestive departure from the wholesale argument of "undifferentiated precursors", this finding also indirectly indicates that a subset of non-classical RGNTs - in particular extracerebellar examples, whose origin cannot be mechanistically accounted for by either of the above structures - may possibly reflect an instance of phenotypic convergence, rather than a lineage-restricted entity.