Identification of Hedgehog pathway responsive glioblastomas by isocitrate dehydrogenase mutation.

The Hedgehog (Hh) pathway regulates the growth of a subset of adult gliomas and better definition of Hh-responsive subtypes could enhance the clinical utility of monitoring and targeting this pathway in patients. Somatic mutations of the isocitrate dehydrogenase (IDH) genes occur frequently in WHO grades II and III gliomas and WHO grade IV secondary glioblastomas. Hh pathway activation in WHO grades II and III gliomas suggests that it might also be operational in glioblastomas that developed from lower-grade lesions. To evaluate this possibility and to better define the molecular and histopathological glioma subtypes that are Hh-responsive, IDH genes were sequenced in adult glioma specimens assayed for an operant Hh pathway. The proportions of grades II-IV specimens with IDH mutations correlated with the proportions that expressed elevated levels of the Hh gene target PTCH1. Indices of an operational Hh pathway were measured in all primary cultures and xenografts derived from IDH-mutant glioma specimens, including IDH-mutant glioblastomas. In contrast, the Hh pathway was not operational in glioblastomas that lacked IDH mutation or history of antecedent lower-grade disease. IDH mutation is not required for an operant pathway however, as significant Hh pathway modulation was also measured in grade III gliomas with wild-type IDH sequences. These results indicate that the Hh pathway is operational in grades II and III gliomas and glioblastomas with molecular or histopathological evidence for evolvement from lower-grade gliomas. Lastly, these findings suggest that gliomas sharing this molecularly defined route of progression arise in Hh-responsive cell types.