Identification of Hedgehog pathway responsive glioblastomas by isocitrate dehydrogenase mutation

Abstract

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.

Primary Orthotopic Glioma Xenografts Recapitulate Infiltrative Growth and Isocitrate Dehydrogenase I Mutation

“…Relevant benefits of primary orthotopic glioma xenografts include infiltrative growth that typifies diffuse gliomas and the retention of genetic alterations and important signaling mechanisms that can be exceedingly difficult to maintain in cultured glioma cells. For example, isocitrate dehydrogenase mutations and Sonic hedgehog ligand production can be maintained in primary orthotopic xenografts 15,16 but only rarely in cultured glioma cells 14,[17][18][19] . It is important to consider significant disadvantages of primary orthotopic xenografts, however, depending upon experimental needs.…”

“…Either source of cells is suitable for successful tumor engraftment in our experience 15,16 , though the use of CD133-enriched cells requires more starting material.…”

Expression of Hedgehog ligand and signal transduction components in mutually distinct isocitrate dehydrogenase mutant glioma cells supports a role for paracrine signaling

“…Total RNA was extracted from one portion of each specimen to generate cDNA libraries as previously described [10, 12, 33]. Another portion of each specimen was formalin-fixed and paraffin-embedded (FFPE) for analysis of Hh ligand and pathway component expression.…”

“…As such, sequence analysis for IDH mutations and immunohistochemistry with an IDH1 antibody specific for R132H mutant protein [31] are specific and sensitive assays that are gaining routine use in adult glioma histopathology and prognostication [21, 22, 32]. Notably, the Hh pathway is operational and activated in the same glioma subtypes that commonly harbor IDH mutations [33]. Therefore, we analyzed IDH1 R132H staining in conjunction with SHH, PTCH1 and GLI1 expression in glioma specimens.…”

Glial Progenitors as Targets for Transformation in Glioma

“…The inhibitor blocked the ability of IDH1 R132H mutation to produce 2-HG, demethylation of histone H3K9me3, and changes in global methylation signatures. The SHH pathway has been proposed as a possible target downstream of IDH1 R132H mutation (Valadez et al, 2013). Since IDH1 R132H mutation is associated with a CIMP phenotype, researchers are evaluating hypomethylating agents or epigenetic modifiers (Fathi & Abdel-Wahab, 2012).…”