Researchers at Universite de la Mediterranee, and other centers in Marseille, France, using the microarray technique to compare the transcriptional profiles of five optic pathway, hypothalamo-chiasmatic and six cerebellar pilocytic astrocytomas, found that these 2 tumors are genetically distinct and topography-dependent entities. NOTCH2, a gene expressed in radial glia and involved in gliomagenesis, was upregulated in hypothalamo-chiasmatic pilocytic astrocytomas. A morphological study of the chiasma identified, in the floor of the third ventricle, a unique population of cells resembling radial glial cells from which the tumor originates. 
COMMENT. Pilocytic astrocytomas are grade I gliomas of childhood that share features of astroglia and oligodendroglia, and affect the cerebellum or the optic pathway, especially the hypothalamo-chiasmatic region. The 2 tumors differ in their localization but also in prognosis. The cerebellar pilocytic astrocytoma has a benign course whereas the optic pathway tumor has a poor prognosis. The present study demonstrating distinct differences and the molecular basis responsible for the aggressive behavior of the optic pathway tumor may allow the development of new molecular targeted therapies.