LGG-33. ISOMORPHIC DIFFUSE GLIOMA HAS RECURRENT GENE FUSIONS OF MYBL1 OR MYB AND CAN BE DISTINGUISHED FROM OTHER MYB/MYBL1 ALTERED GLIOMAS BASED ON A DISTINCT MORPHOLOGY AND DNA METHYLATION PROFILE

Isomorphic diffuse glioma (IDG) was first described in 2004 as an epilepsy-associated supratentorial diffuse glioma with low cellularity, low proliferation and very monomorphic tumour cells. Most patients had seizures since childhood but were operated on as adults. To study the position of these lesions among brain tumours we histologically, molecularly and clinically analysed 26 histologically typical IDGs. Tumour cells were GFAP-positive, MAP2-, OLIG2- and CD34-negative and the nuclear ATRX-expression was retained. Proliferation was very low. Sequencing of 24 cases revealed an IDH-wildtype status. Cluster analyses of DNA methylation data showed that IDG has a DNA methylation profile distinct from those of different glial/glio-neuronal brain tumours and normal hemispheric tissue. About half of IDGs had copy number alterations of MYBL1 or MYB (13/25) and half of the cases analysed by RNA-sequencing had gene fusions of MYBL1 or MYB with various gene partners (11/22), often associated with an increased RNA-expression of the respective MYB-family gene. Integrating all data available, 77% of IDGs had either MYBL1 (54%) or MYB (23%) alterations. All patients had a good outcome and most were seizure-free after surgery. In summary, we show that isomorphic diffuse glioma is a distinct benign tumour in the family of MYB/MYBL1-altered gliomas. DNA methylation analysis is very helpful for their identification. More recent analyses of a large cohort of MYB/MYBL1-altered brain tumours suggest the presence of a third methylation group that primarily contains paediatric cases and seems to be distinct from IDG and angiocentric gliomas. Further histological, molecular and clinical analyses are ongoing.