Molecular subtypes of glioma identified by genome-wide methylation profiling

Recent studies have indicated a prognostic role for genome-wide methylation in gliomas: Tumors that show an overall increase in DNA methylation at CpG sites (CIMP+; CpG island methylator phenotype) have a more favorable prognosis than CIMP- gliomas. Here, we have determined whether methylation profiling can identify more and clinically relevant molecular subtypes of glioma by performing genome-wide methylation profiling on 138 glial brain tumors of all histological diagnosis. Hopach (Hierarchical ordered partitioning and collapsing hybrid) clustering using the 1,000 most variable CpGs identified three distinct glioma subtypes (C+1p19q, C+wt, and C-) and one adult brain subtype. All "C+1p19q" and "C+wt" tumors were CIMP+ whereas most (50/54) "C-" tumors were CIMP-. The C- subtype gliomas contained many glioblastomas and all pilocytic astrocytomas. 1p19q LOH was frequent in the C+1p19q subtype. Other genetic changes (IDH1 mutation and EGFR amplification) and gene-expression based molecular subtypes also segregated in distinct methylation subtypes, demonstrating that these subtypes are also genetically distinct. Each subtype was associated with its own prognosis: median survival for C-, C+1p19q, and C+wt tumors was 1.18, 5.00, and 2.62 years, respectively. The prognostic value of these methylation subtypes was validated on an external dataset from the TCGA. Analysis of recurrences of 14 primary tumors samples indicates that shifts between some C+wt and C+1p/19q tumors can occur between the primary and recurrent tumor, but CIMP status remained stable. Our data demonstrate that methylation profiling identifies at least three prognostically relevant subtypes of glioma that can aid diagnosis and potentially guide treatment for patients.

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