Identification of Retinol Binding Protein 1 (RBP1) Methylation as a Marker of IDH1 and IDH2 Mutation in Gliomas

Keywords: gene, glioma, glioblastoma multiforme, molecular biology, neuropathology

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     Mutations in isocitrate dehydrogenase 1 (IDH1) are found in a large percentage of gliomas and may represent an early event in their development. However, it remains unknown how IDH1 mutations trigger gliomagenesis.
     One hypothesis is that IDH1 mutations may lead to aberrant promoter hypermethylation and inactivation of tumor suppressor genes.
     DNA methylation patterns associated with IDH1 mutations were analyzed with massively-parallel reduced representation bisulfite sequencing (RRBS) and compared with data obtained from a methylation sensitivity micro-array based technique (MSRE). Differentially methylated genes were also compared with methylation-array and gene-expression data from The Cancer Genome Atlas (TCGA). Methylation in the gene RBP1 was identified in IDH1 mutant tumors and was further analyzed with primer-based bisulfite sequencing (BiSEQ). Gene-expression was analyzed with real-time RT-PCR.
     The retinol transport protein, cellular retinol binding protein 1 (CRBP1, encoded by RBP1), is important in retinoic acid metabolism and was found to be significantly hypermethylated in 31/32 IDH1 mutant (MUT), 3/3 IDH2 MUT, and 1/107 IDH1/IDH2 wild-type (WT) glioma tumor samples. Data from the TCGA (124WT:23MUT) showed IDH1 MUT tumors to be RBP1-hypermethylated with decreased CRBP1 expression. Preliminary data suggest RBP1-methylated patients may have improved survival when treated with retinoids such as isotretinoin.
     This is a retrospective study.
     RBP1 promoter hypermethylation is found in nearly all IDH1 mutant gliomas and is associated with decreased CRBP1 gene expression.
     Since CRBP1 is involved in retinoic acid synthesis, our results raise the possibility that dysregulation of retinoic acid metabolism may contribute to glioma formation along the IDH1-mutant pathway and affect response to retinoic acid based therapies.


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