IDH, a new biomarker for gliomas

Diatech Pharmacogenetics is pleased to announce the release of the “IDH 1/2 status®” kit (code UP045), which allows to determine the main variants of the IDH1 and IDH2 genes.

The Isocitrate dehydrogenases family in human cells is represented by three different isoforms: IDH1, IDH2 and IDH3. All three of these enzymes catalyze the same reaction: oxidative decarboxylation of isocitrate with production of ?-ketoglutarate (?-KG). IDH1 is located in the cytoplasm and the peroxisomes, while IDH2 and IDH3 are located in the mitochondria. IDH1 and IDH2 use NADP+, while IDH3 uses NAD+ as electron acceptor, to produce NADPH and NADH, respectively. NADPH is involved in many cellular processes such as defense against oxidative stress and biosynthesis of fatty acids and cholesterol. Since cell division requires both the reduction of oxidative stress and the increase of of fatty acids synthesis, NADPH represents an important metabolite for the proliferation of either normal or tumor cells. So far only the IDH1 and IDH2 genes have been found mutated in human cancers. IDH1 and, less frequently, IDH2 are mutated in more than 75% of grade 2 to 3 gliomas and secondary glioblastomas, in about 20% of acute myeloid leukemias (cytogenetically normal subtypes only), in thyroid carcinomas (16%), cartilaginous tumor (75%), intrahepatic cholangiocarcinoma (10-23%), and occasionally in prostate cancer, B-ALL and paraganglioma. The IDH1 and IDH2 mutations are mainly somatic and have been found always in the heterozygous status. In almost all cases they are mutually exclusive and cause a single amino acid substitution, arginine is in position 132 in IDH1, or the same analogue residue R172 in IDH2. Both The localization of both these aminoacids in the active site of the respective enzymes suggests a direct impact of mutations on the catalytic properties of IDH1 and IDH2. In fact, IDH1 and IDH2 mutations determine on one hand the loss of their normal catalytic activity in the oxidative decarboxylation of isocitrate into ?-ketoglutarate and NADPH and on the other hand the acquisition of a new function: the ability to catalyze the NADPH-dependent reduction of ?-KG to hydroxyglutarate 2-(2-HG). Although the accumulation of 2-HG can have a toxic effect on IDH1/2 mutated cells, it seems promoting tumorigenesis through the stabilization of some proteins (HIF1?) that activate the transcription of genes involved in angiogenesis, in cell survival, glucose metabolism and invasion. Moreover, the inhibition of histone demethylases by 2-HG, together with the reduction of ?-KG, would contribute to the alteration of methylation patterns with consequent variation of the expression of oncogenes, tumor suppressor genes and other key components of metabolic and signal transduction pathways. In fact, the presence of mutations in IDH1 and IDH2 is strongly associated with HIF1? upregulation and with the promoter hypermethylation of the DNA repair gene MGMT. IDH1 and IDH2 mutations are also very useful also for the differential diagnosis of gliomas, the most common brain tumors, its whose histological classification is influenced by the variability among observers. The absence of mutations in pilocytic astrocytomas and in ependymomas allows us to distinguish these gliomas from diffuse astrocytomas, since the cathegorization of these lesions based only on histopathological criteria may be difficult. Mutational analysis of IDH1/2 can also be used to discriminate between primary and secondary glioblastomas, that are indistinguishable on a histopathological basis, but clinically they represent two distinct subtypes of grade IV glioma, which develop in different ways and show different prognosis. Mutations in IDH1/2 also facilitate the distinction of oligodendrogliomas from morphological mimics such as dysembryoplastic neuroepithelial tumors, ,infiltrative gliomas from non-neoplastic reactive gliosis or other non-infiltrative neoplasms like gangliogliomas. All studies regarding biomarkers in gliomas have confirmed, in addition to the diagnostic value of IDH1 and IDH2 mutations, also their prognostic value. All patients with mutated IDH1 or IDH2, in fact, showed a longer survival compared to IDH wild-type patients, probably because of the sensitization of glioma cells to chemo- or radiotherapy, caused by the reduction of intracellular pools of NADPH. Therefore, the presence of mutations in IDH1 /2 can be considered as the main favorable prognostic factor for survival in gliomas, especially in IV grade gliomas, and therefore it is as important as the promoter methylation of MGMT. However, the scientists opininons about the predictive value of IDH mutations are still discordant, although it seems prevailing the idea that these genetic changes are positively associated with a better response rate and a longer progression-free survival in patients with secondary glioblastoma or with low-grade gliomas treated with temozolomide, especially when the MGMT promoter is hypermethylated. However, mutations in IDH1 and IDH2 are a good biomarker complementary to the MGMT methylation analysis in the differential diagnosis of gliomas, in a more objective and fine classification of these tumors, for a more accurate prognosis and for patients stratification in clinical trials concerning both anticancer drugs already in use as temozolomide, or treatments able of reversing the DNA methylation status, and eventually for new chemotherapeutic agents that will be able to selectively inhibit 2-HG or the mutated dehydrogenases.

The “IDH 1/2 status®” test allows to identify, using Pyrosequencing, the main somatic mutations of IDH1 gene: R132H, R132L, R132C, R132G, R132S, and IDH2 gene: R172M, R172T, R172K, R172W, R172G, R172S .

The combined use of “IDH 1/2 status®” kit (code UP045) for mutational analysis of IDH1 and IDH2 and of the “MGMT plus®” kit (code UP050) for the analysis of the degree of MGMT methylation allows to obtain important information about the diagnosis and prognosis of glioma.

Riferimenti bibliografici

1. Yang H et al. IDH1 and IDH2 Mutations in Tumorigenesis: Mechanistic Insights and Clinical Perspectives. Clin Cancer Res 18:5562-5571 (2012).
2. Dang L et al. IDH mutations in glioma and acute myeloid leukemia. Trends Mol Med 16(9):387-397 (2010).
3. Yan H et al. IDH1 and IDH2 Mutations in Gliomas. N Engl J Med 360(8):765-773 (2009)
4. Riemenschneider MJ et al. Molecular diagnostics of gliomas: state of the art. Acta Neuropathol 120:567-584 (2010).
5. Christensen BC et al. DNA Methylation, Isocitrate Dehydrogenase Mutation, and Survival in Glioma. J Natl Cancer Inst 103:143-153 (2011).
6. SongTao Q et al. IDH mutations predict longer survival and response to temozolomide in secondary glioblastoma. Cancer Sci 103(2):269-273 (2012).
7. Houillier C et al. IDH1 or IDH2 mutations predict longer survival and response to temozolomide in low-grade gliomas. Neurology 75(17):1560-1566 (2010).
8. Guo C et al. Isocitrate dehydrogenase mutations in gliomas: mechanisms, biomarkers and therapeutic target. Curr Opin Neurol 24(6):648-652 (2011).