Intracranial myxoid mesenchymal tumour with EWSR1-ATF1 fusion mimicking high grade glioma
Authors: Nagaraju Santhosh , Boiangiu Ion , Brown Ian , El-Maghraby Hussien , Pohl U 
Published: October 2021

Abstract

Aims
Molecular profiling is increasingly used in the diagnosis of CNS and non-CNS neoplasms. More than a quarter of all soft tissue tumours are characterized by specific recurrent chromosomal translocations which can be used as molecular signatures. With increasing frequency, EWSR1 rearrangements are found on both mesenchymal tumours and primary glial/neuronal tumours.
Here we present a case of intracranial myxoid mesenchymal tumour (IMMT), a rare tumour which is becoming more recognised in recent years, affecting mainly children and young adults, and rarely older adults. It can be found in intraaxial and extraaxial location, with frequent dural connection. The tumour is defined by the genetic hallmark of EWSR1-CREB family gene fusion. Including our case, 16 intracranial tumours with this gene fusion have been reported to date.
Our goal is to contribute further to the characterisation of the morphological spectrum, fusion partners and biological behaviour of rare EWSR1-CREB (non-ETS)-rearranged tumours of the CNS.

Method
Case: The patient is a 27 year old woman with a frontal lobe lesion, radiologically described as a tumour with dural attachment. She underwent surgical debulking, and tumour tissue was histologically examined with conventional immunohistochemistry. Additional genetic testing included targeted mutation screening, FISH, EPIC (Illumina BeadChip) methylation array and next generation sequencing. Histology showed a mitotically active neoplasm with relatively uniform cells, round nuclei and oligodendroglioma-like clear cell change, but no myxoid change. Glomeruloid microvascular proliferation and large areas of tumour necrosis were present. Immunohistochemistry was focally positive for GFAP, and negative or normal for synaptophysin, IDH1 R132H mutation, ATRX and p53. The ki-67 index reached ~20%. Sequencing of IDH1 and IDH2 did not reveal rare IDH mutations, and FISH did not show 1p19q codeletion. Testing for BRAF V600 mutation was negative.

Results
Although the histology initially suggested a diagnosis of oligodendroglioma, the integrated diagnosis was compatible with glioblastoma, IDH wildtype. Methylation array analysis by EPIC array did not result in classification of currently known entities, neither confirming glioblastoma, nor providing a new diagnosis, when analysed on both brain tumour and sarcoma classifier. This suggested a novel tumour entity not yet represented in the classifier algorithm. Additional testing including next generation sequencing revealed EWSR1 gene rearrangement with fusion partner ATF1 (EWSR1-ATF1 fusion). Based on this, the diagnosis was revised to the emerging new entity of ‘intracranial myxoid mesenchymal tumor’ (IMMT) characterised by EWSR1 fusion with members of the cAMP response element binding protein (CREB) family (ATF1, CREB1 and CREM). Subsequent immunohistochemistry demonstrated positive staining for CD99 and EMA but not desmin. The patient underwent various oncological treatments and is recurrence-free 3 years after initial diagnosis.

Conclusion
Histologically, IMMT demonstrates a spectrum of features that overlaps with other tumours, but often displays circumscribed growth, uniform cellularity, cytoplasmic clearing and variable myxoid change. The clinical behaviour of these tumours is not fully understood, however provisionally considered intermediate grade.
EWSR1-CREB family fusion is not specific but shared with a diverse group of extracranial tumours including soft tissue, salivary gland, odontogenic and myoepithelial tumours. Therefore, clinico-radiologico-pathological correlation is essential to achieve the final diagnosis, and ensure the absence of a primary tumour elsewhere.
Familiarisation with IMMT, its characteristic genetic profile and its as yet underreported natural course is crucial, as it can clinically mimic other intracranial tumours such as malignant meningioma or glioma but appears to behave less aggressively than high grade glioma.
It is also important to further our understanding of its optimal treatment through review of larger case series and global comparison of patient management.