Mutations of FH and IDH may induce gliomagenesis by similar mechanisms

To the Editor,

Loss of the Krebs cycle enzyme fumarate hydratase (FH) results in the accumulation of fumarate and consequent inhibition of α-ketoglutarate-dependent histone and DNA demethylases (1). Biallelic inactivation of FH has been established in the development of tumors arising in the setting of the autosomal dominant hereditary leiomyomatosis and renal cell carcinoma (HLCRCC) syndrome, including cutaneous and visceral leiomyomas and FH-deficient renal cell carcinoma (2). No known association between diffuse glioma with the HLCRCC syndrome has yet been established.

We encountered the case of a 53-year-old woman presenting with progressive headaches and a seizure. Head magnetic resonance imaging studies revealed a mass centered in the right temporal lobe, harboring regions of postcontrast enhancement, and associated with mass effect. The histological sections showed a hypercellular infiltrating glioma, composed of cells with pleomorphic, elongated, and irregular nuclei, harboring up to 8 mitoses in a single high-power field, and lacking unequivocal microvascular proliferation and tumor necrosis (Fig. 1A). A neuro-oncology targeted 187-gene next-generation sequencing (NGS) panel revealed concurrent ATRX (p.L1134Gfs*12), TP53 (p.N200Ifs*47), and NF1 (p.F2316Lfs*10) mutations with variant allele frequencies of 44.3%, 72.8%, and 23.6%, respectively (approximately 60% tumor concentration), while no IDH1, IDH2, TET1, TET2, TERT promoter, or H3-3A mutation was identified. Oncoscan chromosomal microarray analysis revealed apparent tetraploidy with complex changes, including duplications of EGFR, PDGFRA and MET, amplification of MYC, loss of 9p without homozygous deletion of CDKN2A/B, loss of 10p, and 17p copy neutral loss of heterozygosity including TP53. No copy number changes of 2q34 (IDH1), 15q26.1 (IDH2), 10q21.3 (TET1), or 4q24 (TET2) were observed. The overall copy number profile was considered consistent with a glioma harboring a mutation in the IDH-, DNA repair, or homologous recombination pathways. Whole genome DNA methylation tumor classification indicated a high-confidence match to the methylation class IDH-mutant glioma, subclass astrocytoma, with a calibrated score of 0.997. Subsequent evaluation by an expanded 523-gene-targeted NGS panel additionally revealed an FH (p.M195V) mutation at a variant allele frequency of 91%. No alteration of the 1q43 region harboring FH was identified by chromosomal microarray analysis. FH inactivation results in nuclear accumulation of S-(2-succinyl)-cysteine (2SC), and the glioma cells were found to show widespread overexpression of 2SC (polyclonal; Discovery antibodies, Cleveland, UK), with loss of expression of fumarate hydratase (FH; J-13 clone, Santa Cruz Biotechnology, Inc., Dallas, TX) identifiable in focal regions (Fig. 1B, C, respectively). Given the unusual genomic profile, a “Not Elsewhere Classified (NEC)” designation was assigned, in accordance with the 2021 WHO Classification. This diffuse glioma was found to be indeterminate for MGMT promoter methylation. The patient received standard radiation therapy and oral temozolomide after surgery and, at the last follow-up 7 months after the procedure, was performing her normal activities of daily living.

Figure 1.

This hypercellular diffuse glioma ((A); 200× magnification) shows nuclear and cytoplasmic accumulation of S-(2-succinyl)-cysteine ((B); 400× magnification) and loss of fumarate hydratase ((C); 400× magnification) by immunohistochemistry.

Mutations of the isocitrate dehydrogenase genes (IDH1, IDH2) result in intracellular accumulation of 2-hydroxyglutarate (3), and subsequent evolution of a diffuse glioma with a characteristic CpG island methylator phenotype (4). Notably, 2SC overexpression has not been described in IDH-mutant diffuse gliomas and was not detected in a cohort of 5 retrospectively evaluated IDH-mutant diffuse gliomas, including 4 IDH-mutant astrocytomas and one oligodendroglioma. Exploration of the publicly available TCGA database (portal.gdc.cancer.gov; accessed 15 August 2022) revealed a single case, histologically diagnosed as anaplastic astrocytoma harboring an FH (p.R343Q) missense variant, while also IDH- and H3-wildtype (TCGA-DU-6392). Additionally, concurrent mutations of ATRX and TP53 were present, as have been frequently detected in IDH-mutant astrocytoma (5). However, a total of 12 709 sequence variants were detected in this case, and the significance of the FH mutation in this context remains uncertain. Recent demonstration of a CpG island methylator phenotype in a large subset of FH-deficient renal cell carcinomas (6) may, in part, explain the match of the current diffuse glioma to methylation class IDH-mutant glioma. The accumulation of 2-hydroxyglutarate, fumarate, and succinate appears to deregulate homology-dependent DNA repair via similar mechanisms (7). This novel case raises consideration for fumarate accumulation leading to gliomagenesis by an alternative biochemical mechanism akin to 2-hydroxyglutarate, and suggests an alternate potential therapeutic target.

CONFLICT OF INTEREST

The authors have no duality or conflicts of interest to declare.