Pathology and genetics of diffuse gliomas in adults

The current World Health Organization (WHO) classification of tumors of the central nervous system (CNS) is essentially a lineage-oriented classification based on a presumable developmental tree of CNS. A four-tiered WHO grading scheme has been successfully applied to a spectrum of diffusely infiltrative astrocytomas, but it is not fully applicable to other gliomas, including oligodendrogliomas and ependymomas. Recent genetic studies have revealed that the major categories of gliomas, such as circumscribe astrocytomas, infiltrating astrocytomas/oligodendrogliomas, and glioblastoma, roughly correspond to major genetic alterations, including isocitrate dehydrogenases (IDHs) 1/2 mutations, TP53 mutations, co-deletion of chromosome arms 1p/19q, and BRAF mutation/fusion. These genetic alterations are clinically significant in terms of the response to treatment(s) and/or the prognosis. It is, thus, rational that future classification of gliomas should be based on genotypes, rather than phenotypes, although the genetic features of each tumor are not sufficiently understood at present to draw a complete map of the gliomas, and genetic testing is not yet available worldwide, particularly in Asian and African countries. This review summarizes the current concepts of the WHO classification, as well as the current understanding of the major genetic alterations in glioma and the potential use of these alterations as diagnostic criteria.