Molecular Evaluation of Oligodendrogliomas Using Next Generation Sequencing: The GCRI Experience

Abstract

Introduction: Oligodendroglioma is a rare, slow-growing glioma originating from oligodendrocytic cells and primarily affects adults. According to the 5th edition of the World Health Organization Classification of Tumors of the Central Nervous System, a definitive diagnosis requires the presence of an Isocitrate Dehydrogenase (IDH) gene mutation and co-deletion of chromosome arms 1p and 19q. Histopathology, immunohistochemistry (IHC), and molecular genetic analyses form the diagnostic foundation. However, the accuracy of IHC alone in distinguishing oligodendrogliomas from astrocytomas remains questionable. Methods: Based on IHC findings, 11 cases were initially diagnosed as oligodendroglioma, they all were found positive for IDH1 R132H mutation and retained ATRX protein expression and were subjected to further molecular testing. Next Generation Sequencing (NGS) was used to identify the single nucleotide variations (SNVs), fusions and copy number variations (CNVs). The gold standard technique Fluorescence In-Situ Hybridization (FISH) technique was used to identify 1p/19q co-deletion detection along with CNVs. Results: Our NGS results demonstrated that out of 11 patients who were positive for IDH mutation by IHC, 4 patients had IDH wild-type glioma tumors. In another result, we found that 1 patient had 1p/19q co-deletion by FISH and had IDH mutation alongwith ATRX retained by IHC, however, by NGS, it showed presence of ATRX mutation and no co-deletion in CNVs. Rest 5 samples showed concordance findings with FISH results proving to be negative for oligodendrogliomas. Conclusion: Based on our findings with limited sample size, we conclude that for Oligodendroglioma tumor classification, IHC alone is insufficient for definitive diagnosis, while FISH remains the diagnostic gold standard. However, NGS offers broader molecular profiling by offering higher throughput, with limited biopsy material, enabling precise sub-typing even in challenging cases aligning with WHO CNS5. Although, assessment in higher number of patients is warranted.