2020 Background: IDH1 mutations have been indentified in about 70% of WHO grade 2 and 3 astrocytomas and oligodendrogliomas, conferring a better prognosis. The most frequent mutation leads to a specific amino-acid change from arginine to histidine at codon 132 (c.395G>A, p.R132H). We compared mIDH1 R132H immunohistochemistry, allele specific PCR and DNA sequencing for determination of IDH1 status. Methods: We performed a retrospective study of 91 WHO grade 2 (n=43; mean 40.6 years) and grade 3 (n=48; mean 50 years) oligodendrogliomas, diagnosed in the Neurosurgery Department of Nantes. Immunohistochemistry was performed with mouse monoclonal mIDH1 R132H (gift von Dr von Deimling, Heidelberg, Germany). DNA was extracted from FFPE sections following macrodissection using an iPrep system. A fragment of exon 4 (122 bp) spanning the sequence encoding the catalytic domain of IDH1 including codon 132 was amplified and sequenced using standard conditions. Allele specific amplification was performed using forward primers with variations in their 3′ nucleotides such that each was specific for the wild type or the mutated variant. Results: DNA could be amplified in 90 cases. IDH1 mutations were found in 55/90 patients (61.1%) by direct sequencing. Among these patients, R132H mutations was found in 47/55 patients (85.4%). The results of the allele specific PCR was totally correlated to DNA sequencing. Other mutations (p.R132C, p.R132S and pR132G) were found by DNA sequencing in 3, 3 and 2 tumors, respectively (8/55 patients, 14.6%). mIDH1 R132H immunostaining was found in the 47 patients presenting the R132H mutation (sensitivity 47/47 = 100% for this mutation). None of the tumors presenting a wild type IDH1 gene were stained (specificity 35/35 = 100%). Conclusions: Immunohistochemistry with mIDH1 R132H antibody is feasible, even in case of small biopsies, highly sensitive and specific. Because it only recognizes the most frequent mutation of IDH1 gene, immunohistochemical negative cases must then be tested by allele-specific PCR or DNA sequencing.

2002 Background: Recent studies have shown that IDH1 mutations occur frequently in WHO grade 2 and 3 gliomas. However, their prognostic and predictive impact remains unclear. Methods: Search for IDH1 mutation by mIDH1 R132H immunohistochemistry (IHC) and by DNA sequencing were performed in a series of 43 WHO grade 3 oligodendrogliomas. IHC was performed with mouse monoclonal mIDH1 R132H (gift von Dr von Deimling, Heidelberg, Germany). DNA was extracted from FFPE sections following macrodissection using an iPrep system. A fragment of exon 4 (122 bp) spanning the sequence encoding the catalytic domain of IDH1 including codon 132 was amplified and sequenced using standard conditions. The impact of IDH1 mutations on progression free survival (PFS) and overall survival (OS) was analyzed. Results: Mean age at diagnosis was 51 years, ratio male/female (28/15) and Karnovsky Performans Status (KPS): [<70] n=3, [70-100] n=40. Diagnosis was obtained by surgical biopsy (n=13, 30%), suboptimal resection (n=20, 47%) and optimal resection (n=10, 23%). IDH1 R132H mutation was identified by IHC in 20/43 (47%) patients and R132G mutation by DNA sequencing in 2 (9%) of the 23 remaining patients. Finally, IDH1 mutation was identified in 22/43 (51%) patients. First-line treatments following surgery were chemotherapy alone (n=5), radiotherapy alone (n=5) and sequential or concomitant radio-chemotherapy (n=33) according to PCV regimen (n=29) or Stupp regimen (n=4). With a median follow up of 19 months (range 1.4-128), median PFS and OS were 22 and 35 months. IDH1 mutation was statistically associated with prolonged PFS and OS in univariate analysis: 1-year PFS rate 100% vs 31%, 3-years PFS rate 86% vs 6%, 1-year OS rate 100% vs 46% and 3-years OS 92% vs 9% for IDH1 mutated patients versus wild type respectively. Median OS was 108 vs 12 months (p=0.00002, Hazard Ratio 0.1 IC95 0-0.3) for IDH1 mutated patients vs wild type respectively. Conclusions: In this treated group of grade 3 oligodendrogliomas, the presence of IDH1 mutation assessed by IHC and DNA sequencing is found to carry a very strong predictive impact on PFS and OS.