Metachronous oligodendroglial tumours with different IDH mutational profile in a young patient
Authors: Pohl U , Nagaraju Santhosh 
Published: October 2021

Abstract

Aims
Oligodendroglioma is molecularly defined by mutation of isocitrate dehydrogenase (IDH) and 1p19q codeletion. IDH mutation is an early driver of tumorigenesis, via its oncometabolite 2-hydroxyglutarate, regardless of the exact mutational subtype in homologues IDH1 or IDH2. IDH mutant cells then acquire 1p19q codeletion, with haploinsufficiency likely to contribute to oncogenesis by reduced expression of genes on 1p and 19q, as well as mutations in TERT, FUBP1 (on 1p31.1) in ~30% and CIC (on 19q13.2) in ~&gt;60% of 1p19q-codeleted gliomas.
We present a case of a young patient with metachronous oligodendroglial tumours, initially thought to represent contralateral recurrence of the same disease. However, IDH mutation analysis in each tumour revealed distinct types of mutations, involving both IDH1 and IDH2, indicating different cellular lineages of tumorigenesis. We aim to present this unusual combination by illustrating the histology and molecular profile, and review the literature with regards to multifocal but molecularly distinct glioma.

Method
Case: The patient is a 33 year old man initially presenting with seizures, who was found to have a frontal lobe lesion (hence called tumour 1) with focal radiological enhancement, followed by a contralateral lesion in the parietal lobe 6 months later (hence designated as tumour 2). He underwent separate surgical debulking, and each time, tumour tissue was histologically and genetically examined. Testing included targeted mutation screening by immunohistochemistry and PCR based methods, pyrosequencing for MGMT methylation analysis, FISH for chromosomal LOH analysis of 1p and 19q, immunohistochemistry for mismatch repair enzymes and next generation sequencing.

Results
Histology of tumour 1 revealed a neoplasm with uniform cells, round nuclei and oligodendroglioma-like clear cell change, without mitoses, microvascular proliferation or necrosis. Immunohistochemistry showed absence of IDH1 R132H mutation, retained expression of ATRX and no altered p53 staining. The ki-67 index reached 6%. Sequencing of IDH1/2 mutations revealed a rare IDH2 mutation (non-/R172K). FISH confirmed codeletion of 1p19q, and the integrated diagnosis was oligodendroglioma, IDH mutant and 1p19q codeleted, WHO grade II.
Histology of tumour 2 demonstrated oligodendroglioma morphology in areas, but more cellular and nuclear pleomorphism and focally brisk mitotic activity (7 mitoses in 10 hpf; ki67 index 20%), while both microvascular proliferation and necrosis were absent.
Immunohistochemistry showed IDH1 R132H mutation and retained ATRX, while p53 was not expressed. FISH studies confirmed codeletion of 1p19q, and the integrated diagnosis was anaplastic oligodendroglioma, IDH mutant and 1p19q codeleted, WHO-2016 grade III. NGS data and MMR results are compared.

Conclusion
We present a patient with two histologically similar, but molecularly distinct oligodendroglial tumours affecting both cerebral hemispheres. Apart from the grade, the important difference is the presence of different IDH mutations, 1) a rare IDH2 mutation (non-R172K) and 2) the common IDH1 (R132H) mutation.
While both types of IDH mutations identified are known to occur in oligodendroglioma, the difference clearly indicates two distinct lineages of tumorigenesis, especially as IDH mutation is considered an early event in gliomagenesis. IDH2 mutations are often associated with oligodendrogliomas, while IDH1 R132H is recognised to be frequent in both diffuse oligodendroglial and astroglial neoplasms.
Multifocal divergent gliomas have been described previously but oligodendrogliomas with differing IDH mutations in the same patient have not knowingly been reported yet. Importantly, though therapeutically irrelevant here, multicentric gliomas do not automatically imply relatedness. However, a common origin or predisposition (here, even predating IDH mutation) may not be ruled out.